Engine Room Ventilatio

[mtnest]

I've gutted my salon on my 38' Sportfih Hatteras. While I'm at it I decided to change out the ventilation tubing. The configuration that exists is to say the least 39 years of God knows what. Question is there any science to the venting tubing and its placement? There are threee five inch diameter holes on the port and starbaord. I had palnned on placing the vent tubing (five inch in diameter) flush at the floor and run each tube downto the floor of the engine room. Is there any benefit to running the tubing all to the bottom on the engine room all to the top or split with 1/2 on the bottom, 1/2 on the top?

Dooley :confused:

[Captned]

Say what? :confused:

[Starman]

Talk about timing.

I come in for a few minutes to check some emails and stuff and see this thread.

I am working in my shop today and am actually working on the new engine room vents.

Chris did the homework on this and I am building, but this is what we have found. Most engine room vent systems are much smaller and allow much less air than the engines require for optimum performance.

We increased our vents from 48 X 10 to 68 1/2 X 12 1/2 opening size. Under the vent on the starboard side we have two 24volt fans that together will blow enough air into the engine room to surpass the engine requirements.

On the port side I cut open every bit of floor I could behind the salon wall to allow the air to flow out.

We have designed our own vent system as well to keep the water out and let the air flow in.

I will be posting pics on this as I build them.

I would suggest you first find out what air flow your engines require for the optimum performance and go from there.

fresh coffee made and I am heading back out side :cool:

[SKYCHENEY]

In a diesel boat the vents don't have to extend all the way to the bilge, but in a gas powered application it is required.

[Starman]

This is one of the vents / boxes I am building to install on Boss Lady.





http://img.photobucket.com/albums/v313/starcarpentry/DSCN3447.jpg

[Boss Lady]

In my research and many discussions on engine vent sizing, it appears that Hatteras never has provided enough airflow in their design. This appears to be true even on the newest Hatteri, the 68C did not hit the speed numbers that Hatteras expected, and I, along with some people who design engine vents for a living think that they did not provide enough ventilation, so that the engines cannot make rated horse power. Unless you are running Briggs and Stratton engines in your Hat, those tiny vents are nowhere near what you need to feed even low powered Detroits. For reference, each of the DD 8v92ti in my boat need 2000 cubic feet per minute at max rpm, for a total of 4000 cubic feet. This is equivalent to roughly a complete change of air in a 20 x 20 foot room eight times per minute. There is no way that much air can pass through the dryer vent hoses that Hatteras used on these old boats! This is one area you may find some free speed, since Hatteras is into restrictor plate racing. :D

[jim rosenthal]

I would make the hoses as short as you can, since the longer they are, the less air they flow. I would also, as he suggested, calculate what your engines need in the way of air flow. Then add twenty percent or so for cooling and incidentals, and figure out if you have enough square inches of intake area.
You may have to go looking for more, especially if you have 2cycle turbo Detroits which at least in theory need more air than 4 strokes. I agree with what Chris said; most of our boats were not designed with enough air intake for the engines we have in them, let alone the engines we WANT in them.

[mtnest]

Can't thank all of you enough. With that said, I had refurbished the hull and put in tear shaped air boxes to vent the engine room . It is done and there is no going back. So... I'm
going to put in some blowers one port and one starboard, run my vent tubes short and hope fpor the best.

I have 8V53s naturals, low horsepower, guess when I repower I'll be going into a new design issue!!

Dooley :)

[Starman]

I think you will be better served if you put both blowers on one side and let the air flow out the other side.

If you have a blower on both sides, the air is "crashing together" and the air flow in the ER actually becomes restricted, IMO .

[Traveler 45C]

Straighten me out here…

The engines will be sucking HUGE amounts of air into the eng rm. How will air be flowing ‘out’ the blowerless vent with the engines trying to pull it in?

It seems to me that while underway both vents will be feeding the eng’s, blower or not. If air is flowing into the eng rm how is heat removed while underway?

[Trojan]

You guys need to decide what you are discussing. You have two topics going at the same time. Are you discussing air input to the engines or are you talking exhausting air from the bilge?

BILL

[Boss Lady]

I will jump in and answer that one. My engines pull 4000 cfm at full tilt. I put two 2000 cfm blowers on the starboard side for 4000 cfm of forced air, the two air boxes will both flow more than 4000 cfm. one for intake air, one for exhaust air. If the engines are running hard all the imput air gets sucked into the engines along with the radiant heat it picks up along the way and goes out the tail pipe. If the engines are not pulling max air, then the forced air not consumed gets blown out the other air vent along with the heat. After shutdown, I can run one or both blowers to remove engine room heat. You could use smaller vents and put a blower on each one, but there would not be any exhaust, so the engine room would stay hot, unless you shut one of the blowers off. Like I have said before, Hatteras got this half right. To see what I am talking about just try to open your engine room up with those Detroits wound up to high rpm. The door is held tight by the vacumm. Hatteras invented restrictor plate racing! Not NASCAR. LOL :D

[Traveler 45C]

To see what I am talking about just try to open your engine room up with those Detroits wound up to high rpm. The door is held tight by the vacumm. Hatteras invented restrictor plate racing! Not NASCAR. LOL :D
Yup, I've experienced that. Now I've got more surgery to do...on Traveler's vents.
Thanks for the reply!:)

[Blaine Thorpe]

Looks like I might be a little late with my comments, but here are some things to think about when considering engine room ventilation.

You need to supply combustion air and ventilation air (already mentioned in previous posts). Maximum engine room temperature should be around 120 F.

Need to size the ducts for total installed horsepower, engines and generators. Generally it takes a total of .5 to .75 square inches of duct cross sectional area for combined engine horsepower for combustion air and ventilation. Keep bends and restrictions to a minimum. Inlet restriction should be less than the manufacturers spec for engine inlet restriction.

It is a good idea to have one intake and one exhaust duct on each side of the engine room, but at different ends. If all ducting is all at the rear of the engine room, anything at the front of the engine room will be breathing very hot air. In my case, the generator is mounted at the front of the engine room. It would also be a good idea to have the intake and exhaust ducts at different levels in the engine room to allow for flow across the engine, i.e. introduce the air into the engine room low and exhaust at the top. Since heat rises, it makes sense to have the exhaust at the top. Although for gas engines you will still need blowers to pick up lower in the bilge.

Having said all of this, it is very difficult to set up the ductwork to accomplish all of this. Some sacrifices have to be made to fit everything in.

Blaine

[TedZ]

I don't think anyone wants the engine room pressure to go positive unless you have the cleanest burning Detroits in history...and you don't!

I am surprised not one person has suggested measuring the engine room pressure in various situations. We did a whole thread on this. Manometers are available for either hand held or permanent installation. There are lots of fancy ways to provide the right amount of air at any speed if you drive variable speed fans with manometer info. This is trivial technology today...not so a few years ago.

Obviously, when you go faster, the ram effect should reduce the need for forced air and the engine speed increases that need. How can you guess at these numbers...it is not possible.

If you are going through the trouble of "fixing" what Hatteras did, throwing holes and fans at the problem is not going to work except in a very narrow speed band. The problem with being wrong is either not enough air for the engines or fumes in the boat. Both yucks!

Ted

[Boss Lady]

Ted, the system I described is self regulating. There is no way for the pressure to go positive since the inlet and outlet airflows are equal even with the engines off. What I do have is positive ventillation from off to full rpm. No manometer needed. With the fans off, there is twice the airflow available passively from the vent system than needed. I do not understand what you meant by "you don't want it to go positive unless you have clean burning engines". As far as positive pressure in the engine room, I don't know if you could actually get the pressure to go very positive unless you use some exotic blower system, especially if the engines are running. I will say it again, the three dryer vent hoses per side on my boat were about 1/3 the size needed for the original Naturals, so did Hat get this right? I couldn't open the ER access hatch if the engines were running above 1500 without using both hands and a foot against the bulkhead!

[jim rosenthal]

I think what he meant by not wanting the ER pressure to go positive is that CO from the running engines could end up in the passenger spaces of the boat, and poison people on board. I agree with this, but I also agree that the ventilation area is barely sufficient on most of these boats for the power they were built with, and when you start cranking up the horsepower, no way is there enough area for the air that needs to get in. It is very ahrd to find additional area, though.

My backup plan if I need more intake area is to retrofit the same style vent from a larger contemporary Hatteras- it will appear the same, unless my boat is next to another 36C from the same era. A 45C vent is longer, deeper, and taller than mine, but it can be reworked to be longer only, not taller, and since all the air intake area is up at the top of the box, more air will flow, but it will look more or less the same. Although I hope I don't have to do all that...

[Traveler 45C]

Hell, I can feel the suction on the ER hatch at idle...

[TedZ]

What i was trying to point out - and may not have done it very well - is that with high volume fans and the boat at low rpm.s, it seemed the engine room could be pressured. We are always taking inches of water here,another words 1/20th of a psi + or - when talking pressure. Of course quite a volume of air.

That is why, as always, i like to take measurements of the actual ER pressure at different throttle settings.

With both air and water, every time we guessed at a logical high or low pressure area on race boats, we were invariable wrong...and surprised how off our intuition was when we made measurements.

Ted

[Boss Lady]

Ted you would be correct if there is not a way to exhaust the incomming air. This also something that Hatteras did not do very well, in fact with the inadequate ventilation coming in from the hull side vents and dryer hose arrangement, most of the air gets sucked through cracks coming from other places. Hatteras did not provide any exhaust vents on my boat.

[Captned]

Sorry to jump in kinda off track, but won't big blowers be sucking in salty mist and blowing it all over your engines? Just curious... I need to increase my air flow also...

Captned
78 46 CON

[Boss Lady]

Ned, I designed an air impingment scrubber for the vent system. Any solids, including mist literally has to change direction several times before entering the blowers. Contact Paul if you would like to see a picture. You can also use additional filters if desired, these are easily removable for cleaning.

[TedZ]

The ability of the exhaust vents to exhaust is dependent on there size obviously. The air passing through those vents does so with a pressure drop, i.e. the pressure on the inside of the vent is greater than on the outside. That is true of every fluid going through every orifice, not only compressible fluids.

If there is too much air, the exhaust vents restrict its flow to some degree...may be very slight but there is no free lunch going through an orifice. If there is a lot too much air on one side of the vent, the pressure on the pressured side gets greater. This is basic fluid flow...no big revelation here! If simply providing an exhaust solved this problem...well you all know that there is a tremendous amount of technology that goes into building ventilation, engine exhaust flow etc. etc.

All i said was you don't really want the ER pressure to go positive...may not be a big deal depending on if, when you go into your ER, it smells like an early morning walk in a spring garden!!!

Ted

[MikeP]

I don't know about calculations - Wouldn't a simple test determine if there is an vent air-flow problem? Run the boat at loaded WOT and have someone open the eng room doors; does anything change for the better (RPM goes up, black smoke [if any] reduces/disappears, etc)? If not, seems to me there's no point in bothering with enlarged vents regardless of how tight the doors are being held by the vacuum.

[Boss Lady]

I suspect that a lot of the black soot a lot people experince is caused by lack of adequate airflow. I am simply presenting my findings, depending on how you use your boat this may not be an issue. I have the rare opportunity to correct some difficiencies and it would be shame on me if we simply copied the same mistakes. If you happen to find yourself doing some updates this one area you should definitely take a look at and see if your boat has adequate airflow and ventilation. If you have a hot engine room, then I would suggest you should do some calculations on available vent area and airflow down to the ER and then see if there is an air exhaust. My boat did not.

[TedZ]

I agree with Boss Lady...good advice.

MikeP Do you run at WOT? Do you want to check an unrealistic condition and make decisions? Granted, if you don't have a problem at WOT under way - very unlikely - then you can forget the whole thread, but we are talking fuel efficiency here also.

The notion here is that if you are making modifications to your boat, why not give this area some thought. The venting necessary is not intuitive...that's all. Can't hurt and some of the impingement ideas to trap water are superb.

Ted

[MikeP]

Sorry if I managed to offend - I'm just trying to help folks avoid spending a lot of time and money fixing something that may not be broken.

If the symptoms indicate there is a lack of air to the engine, fine - it needs correction. If they don't, and that's what you are attempting to fix, then the results won't be what you expect them to be.

Several folks have noted that the eng room doors in some cases are harder to open at WOT - they are on our 53MY. Some intrepret this as proving there is insufficient air flow to the engines. But depending on the setup, the airflow in the eng room will create vacuum against the doors that is unrelated to whether the engines are receiving sufficient air for the throttle setting.

When I built racing engines you could fit an engine with an appropriate air cleaner, run it up on the dyno to WOT/max power and, at that time, remove the air cleaner. There was always considerable vacuum holding the aircleaner in place. One might assume that this vacuum indicated the engine wasn't getting enough air. But there was no change in any of the engine monitoring - power, exh gas composition, airflow. The flow of air created the vacuum, not the engine starving for air. We did this on virtually every engine to ensure we were properly sizing the airfilter/airbox.

Certainly, if the air vents on the boat are too small for the engines, the engines will exhibit the symptoms of insufficient air - black smoke and/or the inability to turn the RPM they can turn with the doors open. But if this doesn't happen, then there is no issue with the size of the air vent from an air supply point of view.

[Boss Lady]

Good Point Mike, how much air does a 4" dryer hose flow? My original airboxes were close to being adequate, but the real restriction was the three per side dryer hoses that went from the airboxes to the engine room. There was two of those toy sized blowers, that blow about as much air as hair dryer connected on each side as well. I chose to calculate the needed openings to provide the Detroit Diesel specified air volumes, and made them large enough to allow for any losses. The airbox size is just one part of the equation.

[Trojan]

Mike is correct. If your not blowing black smoke there is not likely an air flow problem. On a properly running engine there will always be a negative air pressure in the engine room. You can make it positive. But there is no guarantee that you will gain any performance for the price. If anything you will make the turbo produce more air and a higher output pressure. In doing so you could over charge your engine and blow it up. But not likely. Being positive is not always good. That's just one more thing to go wrong.
BILL

[Traveler 45C]

Mike that was helpful and really put the issue in perspective. It also took some of my worries away.:)

[Starman]

Mike,

I think you mentioned something before about the door pressure . Did you have a thread about fixing hinges and gasket ?

I know this guy who has a 72' hatt ( was a 60-61 and then extended ) and his doors were the same way. It was near impossible to open them up when running.

I want to say he even left them part open when running. But in his ER you could eat dinner off the floor, even directly under the engines.

In any case, when we think about it, that is a lot of pressure created by engines to basically vacu-seal a room .

In the next few days, I will post some pics of the old ER vents, and a side by side comparison of the new.

As Chris stated earlier, for us, this was the time to address this issue. Another Hatt owner I know who is installing some Yanmars is also increasing his ER vents, just to be safe and because now is also a good time for him considering the scope of his job.

So much good information here from so many :D

[MikeP]

Re dryer hose:

Any corrugated hose for any purpose reduces flow compared to smooth hose. I don't have any personal experience in actually testing it but at one point in my engine-building life I worked with Racing Head Service in Memphis, TN. They did some flow bench work that showed a fairly substantial flow restriction using dryer hose for an airbox application. I don't remember the specific figures or the diameter hose tested but I think it was on the order of 20% flow reduction at high flow rates compared to smooth hose. These would have been on engines in the 500-700HP range; obviously the flow restriction would be less at lower power with the same size hose.

Whatever the actual figure was, it impressed all of us enough to never recommend any type of corrugated hose if it was possible to use something smooth.

Interestingly enough, back then everyone knew that corrugated exhaust tubing was death on exhaust flow but the assumption was - this was the early 70's - that the intake flow was not affected by the corugated hose. I don't know why everybody (in our area, anyway) thought that but they did until RHS did the flow tests.

Thought I'd add this: Despite knowing that corugated-type hose is not as efficient - I haven't replaced it anywhere it's used on the boat - eng room blower fans, bilge pump hoses, etc. So I can certainly be declared guilty of not practicing what I preach! I have to admit I HAVE thought about replacing it for bilge pump use. Wonder if I ever will? Probably not.:rolleyes:

[Trojan]

There is always going to be the exception. Hatteras engineers are not perfect. It can easily happen in a boat that was designed for a gas engine and they decided to make the same boat a diesel model.

BILL

[Boss Lady]

I don't think that Hat ever put gas engines in 48 footer.

[Starman]

This is the starboard ER box. ( blue ) with the old box on top for a comparison.

We will have a stainless steel cover with many slots so as to not block the air flow.

Inside this box are 29 fins made of corrugated fiberglass panel. This will make it hard for water to pass through the box. On the backside , it is wide open with only a 1/2 lip on the top, and a 1" lip on the bottom . These lips are for water control and as a back brace fro the fins.

Notice also that the port side old vent box, and the limited air flow with the vent cover and dryer hose size ( 3) and 1 longer slot.

http://img.photobucket.com/albums/v313/starcarpentry/DSCN3495.jpg

http://img.photobucket.com/albums/v313/starcarpentry/DSCN3498.jpg

http://img.photobucket.com/albums/v313/starcarpentry/DSCN3499.jpg

[TedZ]

Starman, usual gorgeous work.

I'm glad Bill (Trojan) pushed back.

If your test on ER ventilation is whether the engine throws black smoke and it is ok if the ER room door is "welded" shut at high output, there is not a lot anyone can say because it is likely you can't be convinced.

Suffice it to say there is a long way from lousy ventilation to black smoke and in between you burn a lot of extra fuel and do damage to the engine...slowly i'm sure, but damage just the same.

Think about it...the manufacturer goes crazy improving air flow and you destroy it - for whatever reason - with bad venting...

Ted

[the kuz]

Guys, after reading all this, thought I would pass on my single data point. Durring instrumented sea trials after putting 330Bs into our 77 36C (which has open style rectangular vents different from Dr Jim's) we did two runs with everything closed up, and then another two with everything (cabin door, salon floor hatches, ports, side windows, etc.) open. No measurable changes in performance, EGT, turbo boost, or other readings except intake temps which were a few degrees cooler across all RPMs tested (it was a cool day - 63 F). Everything was well within Cummin's installation specs on the closed up run, so I guess the ER was getting enough air. OBTW, have not had any issues so far with Cummins risers, but will ask Wes Denton, the Northern Chesapeake Bay Cummins rep about them. I have a lot of confidence in Wes, he is a straight shooter with many years at Cummins and also has a pair of 330Bs in his own boat. The advice he has given me so far is great, and has not always been the path that would make the most money for Cummins. I intend to re-run the sea trial tests at 600 hrs on a hot day this summer. I have lost about 50 turns @ WOT since new, which I think is a combination of increased weight, year old bottom paint, and warmer temps. Would be nice if more air would help performance. So, I may run her up open and closed like the original sea trial runs to see what happens @ WOT. If I get any real impact I'll post again.

Regards, Bob K

[Starman]

That sounds good Bob,

Which means that extra air did not hurt anything, but would at least keep the ER air fresher.

I wonder if cummings are like the old ford 302. Seemed the colder it was the better that engine ran.

[MikeP]

Cooler engine intake air is denser and so makes more power on any engine, whether it's an old 302, an old DD, or a new whatever. Ideally you want the engine hot and the intake air cold for max efficiency/power. THis is one reason that all cars nowadays have an airbox intake somewhere to the "outside" of the engine compartment. I don't think anyone pulls the intake air from under the hood any more.

Lots of old high performance "tricks" are common on cars now - cold air intake, airboxes, roller cams/lifters, etc. :D

[the kuz]

Paul, Mike, you are right, they definately run better in cool weather - wind up faster, and turn up 25 -50 more RPM @ WOT. In the winter, the boat seems to jump up out of the hole and you can really feel the acceleration as the turbos kick in. Sea trial time to plane was 9 seconds, so I can check that again also. Once we get a 90 degree day and the water warms up, I'll repeat the boat/hatches open & closed WOT runs to see what if any difference it makes on a hot day (after a lift and clean). Any of you other 36C series one owners have good weights for the boat? I'll ask the travel lift guy to check it when I clean after the barnacle set is done, but that is not always real accurate. Would be interested to compare with other similar boats. Repower to 330Bs/ZF trans from original Cummins 555 V-8s/Twin Disc trans took about 2200-2400 lbs out of the boat. Some of that may have been paint build up, because I had the bottom shaved clean also. I still have the manuals for my old motors/trans, and will check the listed weights vs the specs for the 330Bs/ZFs. If anyone can use these old manuals, let me know and I will send them to you. Also, weight and dirty shafts/props/bottoms really impact performance for this motor/boat combo. (more sensitive than some other boats I have run). Regards, Bob K

[luckydave215]

It's all about charge density, that's where power is made. The serious (crazy?) tournament guys with their Garlingtons and Paul Manns all do their high speed running with the hatches open.
It helps (a little).

[MikeP]

Even better is cool, DAMP air! Water injection has been around at least since WWII for supercharged/turbocharged engines to increase density, cool the charge air and thereby eliminated detonation.

Hmmm....

[luckydave215]

Even better is cool, DAMP air!

WRONG!!
Water vapor is less dense than air.
Water injection introduces LIQUID water into the intake charge, as it evaporates, it absorbs heat, cooling the intake charge, increasing charge density. The presence of residual water droplets in the cumbustion chamber also slows (stabilizes, the same as more octane) the combustion process, allowing those old supercharged engines with the primitive combustion chamber designs to tolerate more boost before the onset of detonation.
Damp air has the water already in solution (it's already a gas) so no evaporation takes place. Humid air is less dense than the same temperature dry air, resulting in less charge density and therefore less power production.
In addition, any air (oxygen) displaced by water vapor means less oxygen to burn fuel, as WATER DOESN'T BURN!!.

[MikeP]

Dave,

Yes, you are absolutely correct - higher humidity of the charge air makes less HP at the same air temp as any HP calculator (electronic or old-fashioned formula on paper), or dyno run will show. I apologize for my unclear statement - I was referring to the water injection use when I said "damp air." I didn't mean to suggest that air of higher humidity was better but in re-reading it I see that it sounds that way.

Sorry for any confusion I might have created and thanks for making it clear. I certainly didn't!

[Trojan]

High humidity does increase HP. Hot air does not. Water injection was used in the war to increase the HP of the airplanes that were shot up. Any engine using water injection will eventually destroy its self. It was only used in an emergency. Hot humid air will make a plane hard to fly because of it's density. Gaining altitude in the summer is harder than the winter. Your car will have more HP in the rain than sunshine. The cylinder compression will be greater with moist air increasing HP.

BILL

[luckydave215]

High humidity does increase HP.

Nope. Look it up in any pilot training manual.
Water vapor is LESS dense than air, and doesn't burn, so any air displaced by water vapor has the same effect as increasing altitude. (less air density, less air to burn, less power)

[Trojan]

I did not say it would burn. I said it would increase cylinder compression increasing HP.

BILL

[luckydave215]

I did not say it would burn. I said it would increase cylinder compression increasing HP.

Still wrong.
Water ingested with the intake air will only increase combustion chamber pressure if it's a LIQUID when it reaches the combustion chamber.
Water increases volume 1700 fold when it changes phase from liquid into gas (like in a steam engine) but not any great amount when the gas is heated, no more than air, which burns (water doesn't).
And water vapor is LESS LESS LESS dense than air!!!!!!!!! reducing charge density.
Get it yet?

[Capten]

Check out these custome made vents. They are supoosed to let air in without water and salt.

http://www.sjeffect.com/detail.php?group_id=1&project_id=6

[Traveler 45C]

To me it looks kind of small, proportional to the boat. I don't see how air and water will separate or how the drains work.

[Capten]

To me it looks kind of small, proportional to the boat. I don't see how air and water will separate or how the drains work.
I'll try to find the info the designer and owner shared with me. I thought it was online somewhere. From what I remember it seemed to make sense......

[Traveler 45C]

I just finished (I mean 2hrs ago) reading about this project in The Big Game Fishing Journal. Interesting. Have they finished the interior?

[MikeP]

Re opening necessary...the photo below is of a 1350+ HP Pro Stock drag racer. The sealed airbox opening is not any larger than the airhorn area of the two 4-barrel carbs. The car must make full HP on the starting line for launches; there is no "ram" air effect at launch. It makes that HP with that air inlet opening. Granted that a two stroke DD uses more air per HP than does a 4 stroke but even if they use twice as much, a 600HP DD still doesn't need as much air as that pro stock car.

So an unrestricted opening the size of the airhorn area, an 8V71TI would only need to be around 25 Sq In (two 4" diameter turbo inlets).

My opinion (and that's ALL it is!) is that boat vents in general are as big as they are for ventilation when the boat is NOT running to keep air circulating in the closed compartment. If you had 25sq in openings in our boat's eng rooms, there wouldn't be any circulation at all and it would get very nasty in there very quickly.

[Trojan]

Your only filling 1 or 2 cylinder with air at a time, depending on the engine.

BILL

[MikeP]

I'm not sure I understand what you mean - filling only one or two cylinders at a time:confused:

As far as the engine requirements for combustion air, you still come back to the fact that the total air inlet opening to the engine on the (8v71ti) DD is about 25 sq in. So if the 1350 HP prostock motor at 10,000 RPM doesn't need an inlet bigger than its airhorn area, why would a 435HP DD at 2350 RPM need an air inlet bigger than ITS airhorn area? Even if it was bigger, how could the engine "use" it anyway?

Airflow is determined by the most restrictive portion of the inlet system. The airhorns are the most restrictive portion if the hole in the side of the engine room is larger than the airhorn opening (and isn't restricted for other reasons).

Here's a pic of a 5000+ HP top fueler - again, look at the size of the air intake...

[Traveler 45C]

Mike, so your saying those racing eng’s could breathe just as easily in a boat’s eng rm as they can outside? It seems to me that if the air required for those eng’s couldn’t get into the eng rm fast enough a vacuum would develop restricting air to the eng as well as making the hatches heavier.

The air vents must be large enough so the eng ‘thinks’ its outside. Otherwise it’s like putting your hands over those big intakes, choking them.

COOL Pics!

[garyd]

I think you guys are on to something. The engine manufactureres today are not saying too much about the size of the openings any longer.

In fact the new drum beat is they are requiring a temperature difference of a maximum of 30 degrees higher than the outside air temps. So you could have the corecct size opening in the side of the boat to give you plenty of airflow but still small enough that the air is killer hot.

Then with the killer hot air you don't make the HP then you push it up burns more fuel gets the air even hotter etc etc etc.

I went with the Slane vents and am very happy there is enough air flow for the engines and enough air turnover without really trying to keep the temp differential less than 30 degrees.

I have 1250hp at WOT - 2400RPM. I cruise all day long at 1900 RPM 500 off the pegs. I don't know exactly what the difference in air flow requirements from 2400 to 1900. It seems it would be linear. If it was then I would only require about 80% of the WOT airflow. The vents easily allow the WOT airflow without going crazy. NBD at cruise RPM.

The problem to me seems to be getting the whole darn thing cooled off after ya get done cruising. That is a harder problem to solve then getting the correct size hole and flow for WOT airflow.

The Hat blowers take air about mid bilge level and blows it out. It seems to make more sense to me to bring cold air in low and let the hot air naturally go high and out. But if ya get get fumes/smoke/fire/fire retardent agents, then maybe you want to blow them out. When the Hatt blowers are trying to blow air out they are trying to bring cold air in through the vents that are trying to let hot air out, not very effiecient at all.

I'm going to change the direction of air flow from out to in and add a short lenght of hose to bring the cold air to the bottom of the bilge and the let hot air raise up and out the side vents naturally and with a little push.

garyd

Hey Mike is that a skirt driving that 5000HP thang, WOW what an admiral LOL

[luckydave215]

The car must make full HP on the starting line for launches; there is no "ram" air effect at launch. It makes that HP with that air inlet opening.
Well, no.
The car doesn't need (or want) full power at launch, it's hard enough for the team to find traction with only a portion of full power. Full power only occurs near the end of the run, when ram air is helping as much as possible - it's like free supercharging.
One thing you guys are all ignoring is the difference in intake velocity between the general engine room volume and the intake horn of the engine itself. Once the air is actually in the intake ports the velocity can be as high as thousands of feet per second, a flow rate that would generate outrageous flow restriction in the aerodynamically messy engine room, where the air flow must be fairly slow.
Think of arranging the intake flow through an endless tube the same diameter as the intake ports. The engine would be choked to death by the friction of all that wetted area exposed to high velocity flow.
The point of all this is to illustrate why the engine room vents need to be so large, the flow needs to be low speed, high volume, and unrestricted.

[MikeP]

Traction nowadays in the pro drag classes is controlled by automated clutches - these are programmed based on the track conditions/wx, etc before the run. Assuming the racer doesn't have to back off for some catestrophic issue, the engine is at max power from the instant of launch to the end.

BUT ANYWAY...we still get back to the need for these big vents on boats...

I agree that IF the flow went through a long tube of the same diameter it would reduce flow due to turbulence. But it doesn't go through a long tube, it enters the airbox (engine room) and then becomes, just as you said, high volume, low speed. The air doesn't rush into the eng room and continue in a rush into the engine. The airbox intake size ONLY needs to flow the air the engine needs. A 25 SQ in opening will do that for an 8v71ti. I AGREE that a large opening is good for lots of other reasons but to sell it as a device to increase engine power or efficiency - nothing supports that claim except marketing and wishful thinking.

The marketing folks probably think they struck gold with the "Are your doors stuck shut at WOT? Why then sir, you need our ultra-super-maxo ventoLator to let your poor engine find all that missing horsepower."

Unbolt the airbox on that Prostock car while it's at WOT on a dyno. You wouldn't be able to lift it off because of the vacuum. But none of those guys are looking for bigger vents. The door vacuum thing is a red herring and has no relevance to the issue.

[TedZ]

"Unbolt the airbox on that Prostock car while it's at WOT on a dyno. You wouldn't be able to lift it off because of the vacuum. But none of those guys are looking for bigger vents. The door vacuum thing is a red herring and has no relevance to the issue."

Respectfully disagree...a lot!

Ted

[Traveler 45C]

If I run both my eng’s up to 1200rpm with the ER hatches closed, my saloon door is harder to close from the extra air rushing in (its getting to the ER somehow). Is that telling me that not enough air is getting into the ER through the side vents?

I’ll be so happy if this is normal.:)

[TedZ]

There are a 100 links that say that inlet air pressure (and temperature) are critical.

Here's one

http://in3.dem.ist.utl.pt/labcombustion/EMEEcourse/presentations/trab8.pdf

there are lots more.

If you have an engine that is pulling negative pressure (reducing atmospheric pressure in a NA engine) anywhere...anywhere, eliminating it to whatever degree possible helps the volumetric efficiency and the combustion process. Picture starting an engine in a box and slowly reducing the pressure in the box. Eventually, the engine will die. Between running and dying there is a lot of inefficient operation!

This is so basic...are we suppose to throw out every MAF sensor in all of the cars.

Give me a break!!! How can you possibly believe anything to the contrary??

Ted

[luckydave215]

Give me a break!!! How can you possibly believe anything to the contrary??

Head...in....sand.

[Traveler 45C]

Eng in a shoe box, Hmmm…

So, with the shoe box lid off there really isn’t a problem.

Putting it on with no other way for air to get in kills it. Ok, with the lid on (ER) holes must be punched in the sides of the box (like a boat) to let air in or it’ll kill the hamster. If the holes are not large enough, not enough air will get in and the hamster will have a hard time breathing. The holes must be large enough for 2 main hamsters and a baby (generator) to breathe easily.

I get it now… So as long as those hamsters are running the boat and not out of breath (black smoke) all is well.

Does that put it all in proper perspective?

[MikeP]

My experience with performance engines on dynos, airbox design for those engines, and every bit of evidence that I can find fails to indicate the need for these large air vents JUST for engine performance. There's no science to support it at all. Plug the numbers into any engine airflow calculator (or look at real world applications) and you will come up with a miniscule air inlet size compared to what is being touted as necessary for these boat engines to breathe. Heck, look at semis that used to use these DDs. They don't have 500+ Sq In of inlet area like the ones on either side of our 53! The trucks used the same engines with air intake openings probably less than 1/20th that size.

Look at any airbox - the inlet is fairly small - approximately the size of the throttle body opening while the box is fairly large. The Porsche 911 turbo is a perfect example of a current airbox design and it also has an airbox inlet that is approximately the same size as the throttle body opening.

The big boat vents are GREAT for general ventilation and for engine room cooling, etc and, as I said, I wouldn't want just a couple of 25 Sq in holes in the sides of the boat for the eng rooms. I believe that when these boats were designed originally, THAT"S why the vents were made as big as they were.

But the great thing is we can (or not) buy vents as big as we want for whatever reason we want and that's really all that matters since owning a boat is, by definition, kinda stupid anyway!:D

[luckydave215]

The intake plenums on those truck DD's and Porsche turbo's are scientifically designed resonating chambers, our engine rooms are not.
Cool unrestricted air flow is always better than hot restricted airflow.

[MikeP]

:confused:

OK, for my last shot at this...I'll abandon everthing I know about engine airflow and ask this question: where is ANY data showing that these larger vents improve engine performance?

I guess I'm repeating myself for maybe the 3rd or 4th time and I apologise but the simple test is:

Does the engine pull spec WOT under load numbers with the doors closed?


IF Yes, we're done. Save your time and money and FORGET the fact that the eng room doors may be sucked against their jambs.
It Doesn't Matter!!!!!!!!

If No, open the doors. Any change for the better? If yes, a bigger vent is needed.

If you want to get really scientific about it, buy a digital flow meter and mount it to the engine air horn and MEASURE the airflow difference to the engines with doors open/closed.

That's really all that is necessary to determine if there is or is not enough air for the engines to run properly. Doing anything else just a waste of time and money. But it's your money and your time.

I promise I won't post anymore on this subject. It's worse than religion and politics though it shouldn't be because it's all easily measurable.;)

[luckydave215]

Think of the engine room doors as a primitive, but scientific none the less airflow meter. If engine vacuum at wot makes them hard to open, the vents aren't big enough.
My last post too :D

[Traveler 45C]

Hey you guys, where’d you go???

In Nigel Calder’s Boatowner's Mechanical and Electrical Manual, there is a specific procedure for measuring ER pressure/vacuum. This’ll tell ya’ for sure whether the ER is in spec for your eng’s or not.

Is it possible to have reduced performance (not making WOT) due to insufficient air flow into the ER without making black smoke?

[Trojan]

ENGINE ROOM DOORS NOT. Unless you plum the intakes to the vent, you will always have a vacuum in the engine room. In my power house the door always were sucked shut and the boilers and steam engines ran just fine. I think it is just a wast of time. Just measure the intake diameter figure the area. Make the vent area twice as big and you can't do any better. I don't buy it. Do you really think that for one minute that Hatteras engineers didn't figure this out before they build any boat. Its been fun. Everyone back to there corner. :D


BILL

[yachtsmanbill]

...AND IN THIS CORNER...
When I'm running at my modest 1500 and ready for sea, i.e. ports and windows closed, theres just enough vacuum to hold the lower salon door
closed, just. I will admit the engine room is about 4-5 degrees above ambient though. Even the manifolds are about 100 F. Shut them down and it rolls up to 150 fast !! I need to find one or two more 32V blowers for a cool down helper. The one is barely adequate. One will be hosed low for fumes, but the others for hot air.
BTW, we always ran the boiler room at a negative pressure. It keeps the fire "in" where its supposed to be :D Even with forced draft fans on the boiler, it too was under neg. pressure. If you'd open an inspection port (gun port) it would upset the fuel/air mix enough to warrant an adjustment... and these guys were 20 stories tall. ws

[Boss Lady]

I do love this forum! Ok guys, I posted this to show what we did to my boat and the reasons for it. I brought up some food for thought, on area that I, and a lot of airflow "engineers" discussed at great length. The concensus was that the original vents were in fact way undersized for the original power, along with the restrictions of the inefficient dryer hose ducting. The blowers were rated at like 175 cfm each, which is no where near what Detroit specifies as max airflow required of 2000 cfm for each engine at wot for a total of 4000 cfm. Any a/c duct calculator will do, just figure up the amount of vent area to flow 4000 cfm. I wanted positive cross flow ventilation of outside air across the engine room to the exhaust vent in addition to eliminating air restrictions to feed the engines. We did some overkill but not much, about 10 square inches. Negative pressure in the ER "is" caused by lack of vent size (or duct size). Your air filter is rated to flow x at so many inches of water (suction pressure). This gets the job done but it is not the most efficient, if you want that then you need a much larger filter that will flow the same amount of air at near zero negative pressure. We live in a world of compromises. size and cost being the two that normally drive the train on design. Most engines never reach their volumetric efficiency due to restrictions in the induction system. A tuned runner induction system will give the most volumetric efficiency at a given rpm or resonance, and will suck at all other rpms. A naturally aspirated gas engine operates at around 32 inches of vacumm and wide open throttle is around 10 inches. there in lies your answer. A turbo engine has a mechanical pump so the restrictions are not as noticed, so you wind up having to use instruments to measure these parameters. I paid for 735 HP per side and I intend to get them. If the airflow is restricted then there is no way that I can burn all the fuel since the engines will not have enough airflow(oxygen) to do so. What to test your theory, then just put some cardboard over one vent and then see what happens to performance. You can't feed one of these babies with a soda straw. :D

[Trojan]

Tomorrow I'm getting out my ax and going to the boat. Going to check this out. I want that extra 3 MPH :D If you can't open your engine room doors. Then yes you do have a problem. Through all this crap there has been 3 different objectives. I think that if the 3 original objectives were spelled out. As requested in an early post. There would not have been all this discussion. Air feed, air exchange and air cooling have been all rolled into one. All of the info has been discussed in general. Everyone seamed to be on a different track. Damn I hope we all got it straight. :D

BILL

[luckydave215]

A tuned runner induction system will give the most volumetric efficiency at a given rpm or resonance, and will suck at all other rpms.

That's exactly right, and why most engines have different intake runner length for each cylinder. This spreads out the torque peak, giving a wide usable powerband.
Casual observation would suggest ease of packaging and cost reduction is the real reason, but it's usually not. Think of what the typical intake manifold looks like for a gas V-8. The "middle" intake runners are short, and the "outer" ones long. It's done on purpose.
When peak torque over a small rpm band is desired, (a generator comes to mind) the intake runners may all be the same length, and sharply tuned for the target rpm. This increases the power and efficeincy at that rpm by optimizing for it.

[MikeP]

I know I said I wouldn't post again on this subject. I lied. Just this one little post...;)

For those of you reading all the way to this point on this thread - Congratulations on your persistence!

But please, PLEASE, PLEASE, ignore everything you have read here - Yes, ignore mine as well - and do your own research on this subject SOMEWHERE ELSE.

Just do some searches, find some sites that discuss engines and CFM airflow, inlet opening sizes, and decide from there. You might also look at inlet/exhaust tuning to further your knowledge. Subjects such as equal length headers/ equal length intake runners might also reveal some interesting facts. Try to find sites that inform re these issues, not sites that want to sell you something.

As far the opposing views on this thread, as the diplomats correctly say, we will have to agree to disagree.:rolleyes:

[luckydave215]

Mike, equal length intake and exhaust runners are an engineering choice you would typically see on a high performance engine, where maximum output is more important than the wide (but weak!) powerband you'd like to have pulling, say, a family station wagon driven by a soccer mom talking on a cell phone.
Engineering choices are always a compromise. You give up one thing to gain another.
I've spent an awful lot of dyno hours building motorcycle roadracing engines (thousands of hours, I raced for 20 years) and won 7 national championships to show for it.
For my money, the best tome on engine resonance is still "The High Speed Internal Combustion Engine" by Sir Harry Ricardo. I recommend it to anyone interested in engines.
It deals with fighter plane engines (written in the 30's) but engines are engines, the physics of things like sonic wave activity aren't going to change anytime soon.