Engine Room Forced Air Ventilation CFM Needed?

[Fair Well]

I have searched the forum and could not find a good for sure answer to how many CFM air flow fan or fans I would need. I did find this formula on Boat Diesel:
Multiply the max rpms by the cc displacement of the engine and divide by 1,000,000 and multiply by 34.3. That is how many cfm you need to provide for combustion. Triple it and provide a place for exhaust, and you´re good to go.
Using this I come up with 201.64284 CFM per engine. (I am not a math whiz) This seems low to me when I look at the vents in the sides of the boat now. They are big enough for me to to crawl into and I'm a BIG boy. This is something I'm just kicking around since we are doing such a major refit. I promise I will try to start posting pic's just so busy with the boat and so tired at the end of the day. Seems like all I do boat boat boat eat drink a beer or six and sleep.

1988 65' MY Extended to 79' with 12-71 TA's ( I used 700 HP. I believe they are 675 HP)

 

[bobk]

I think ventilation blowers are for cool down after running. Mine at least blow out, not in. But the weird thing is they draw from the lower hull rather than the ceiling of the ER. Sounds like maybe a vestige from gas power where fume removal was needed.

It might be worth replacing while you are at it. Those in my 1981 48 had plastic squirrel cages and they had disintegrated. I was not aware of it until the bearings became noisy and found there were no blowers left.

Bobk

 

[MikeP]

As noted...if you are talking about ER ventilation - to cool it down after a run - that's a different thing than what the engines require for WOT operation. It is extremely unlikely that there is insufficient engine air unless the outside vents are covered/closed. Engines with FAR more HP than any of our Hatt motors get plenty of air through openings much smaller than the typical ER external vents.

 

[GJH]

The ones down low are to vent the batteries before start. My 56 also had ceiling "out" blowers to cool things down post running.

As for the OP, the boat is 28 years old. So now it needs something for this purpose?

 

[Boatsb]

Blowers were usually installed pulling from the bottom of the area. Diesel engines do not require them but like chicken soup they don't hurt.

It sounds like you have over thought the issue. Relax and enjoy. I haven't had blowers on my 41c in the 10 years ive owe'd it.

 

[Fair Well]

I'm sorry I didn't make it clear what I'm try to do. I'm kicking around the idea of doing away with the engine room side vents and going to the forced air system like in a lot of the newer MY's. This would help eliminate salt air especially while sitting dock side. I saw quite a few at the Miami boat show. Doesn't look like it would be that complicated.

 

[MikeP]

Seems like a LOT of trouble/expense. If your OEM intakes have dorade-type boxes, there really isn't much spray getting in. I don't know if all Hatts had them or if they had them beginning in specific years. Our '80 53MY has them. But if it has Dorade boxes, I don't think there would be much difference, certainly I would question whether whatever improvement would even remotely justify the work.

OTOH, it's your boat and if you want forced-air induction, you can do it just because you want to!

 

[Pete]

Another opinion. The one thing you do not want in an engine room not designed for it, is positive pressure. The entire boat will smell engine room in short order.

Pete

 

[MVCaprice]

If you are looking at a new ventilation system take a look at Delta T. I have been really impressed with the quality of their fans and demisters. In general the larger fans they have are quieter and deliver more air volume. They also have a temperature controller which would be very nice.

 

[Cricket]

Look into adding Delta T or Livos (now Dometic) demisters to your vent box holes or possibly even as an insert to replace the factory louvers. We have the Slane vents with the Livos demisters as inserts from th top into the box which is hidden by the fiberglass louver.

 

[MVCaprice]

Look into adding Delta T or Livos (now Dometic) demisters to your vent box holes or possibly even as an insert to replace the factory louvers. We have the Slane vents with the Livos demisters as inserts from th top into the box which is hidden by the fiberglass louver.

I was thinking the same thing. I think Delta T can make them to spec so you could probably get them to fit into the factory hatteras boxes.

 

[Fair Well]

Thanks all I well look into it.

 

[tonytrakovich]

Not sure what the actual cf, is but the air seps for 1271's are rated at 1245 CFO each. I'm sure that is way more than the engine uses. The displacement method for calculating should have a huge safety factor because the turbos will compress the air to some extent.

I agree that you do not want positive pressure in the er.