thoughts on repowering, smaller not bigger.

[MarioG]

Right off the bat, lets assume resale value is not a factor or a care for this discussion.

Lets take my boat as an example,

1972 58' Hatteras Yachtfisherman, DD 8v71tis 435 HP. aprox. 3000 hours SMO. Semi displacement hull. boat is about 75,000 lbs currently.

We have had our boat for about 3 years now. we use it extensively but mostly for bay cruising and the keys. when able, we cruise the Bahamas. our boat has hardly ever been above 12 knots and rarely on plain. Simply put we use her mostly like a trawler. a good reason for this is because of the high price of fuel and because this boat in reality isn't very fast even on plain by today standards. But mainly we prefer a slower pace. Our future but far away plans of our boat (god willing) is to retire and become permanant cruisers. Our boat was designed during a era of really cheap fuel which was measured in cents and not dollars a gallon. the people that were able to own these boats back then and which still applies to some today, could afford to go through fuel and overhauls with little care.

I have always wondered about what it would be like to install smaller engines into our boat. mind you that I understand that it is cheaper to overhaul what I currently have. but lets look at it from efficiency and application point of view. our engines are rated at 435 HP but I operate the boat at about 8 to 10 kts on average. I don't know exactly how much HP I may be usuing but it coulden't be much more than 150 to 200 hp at best to cruise at 8 to 10 kts. Its is well known and argued to exaustion that operating these engines at low power is not good. Some say powering up once in a while makes sense. But what if the proper engines rated for slow cruise application was installed like 6/71 or 4/71 naturals or any other simple low maintanance engine rated to lets say 250 HP of continous duty and bigger props and proper gear ratios. would this lower fuel consumption and extend engine life? how about ease of maintanance? removing a giant engine like an 8v71 and replacing it with a smaller 4 or 6 inline opens up massive real estate in the engine room. Although reducing overall engine weight would help, I guess it would be a wash because ballast weight would have to be added due to stability issues. it seems that a boat like ours would benefit in our application but What am I missing?

 

[MikeP]

" Its is well known and argued to exaustion that operating these engines at low power is not good."

I think this view is inaccurate and dramatically overstated. Can anyone find me an example of any engine that blew up/died/rotted away/whatever by idling for it's entire life?

I would not hesitate to run those Tis at hull speed RPM (whatever that is) for the rest of their lives - which will probably be longer than yours! ;

Forget the smaller engine repower - there is nothing to gain and just a lot of money to lose.

 

[SKYCHENEY]

A few thoughts:

1) Less weight--Do you have stabilzers? This repower could make for a higher center of gravity and more roll, even to the point of becoming dangerous.

2) Electrical--New engines will require a 12v or 24v system, not 32v. This means changing out many other components.

If I wanted to go slow all of the time, I think I'd look at an LRC or trawler. If I were to repower, it would be to increase hp/speed, not decrease it. You can always run them slower, but if you go smaller, you'll never be able to outrun that storm.

 

[piripucha]

You can also take the middle road, instead of going smaller, or bigger you can repower with modern engines in the 450 to 500 hp range.You will probably shed a lot of weight (those 8v71TI are heavy!!) and have extra power when you need it. I also have a 58YF that I rarely push it more than 1900 rpm and 12 Kts, since we tow a 21 ft wellcraft and the sea does not cooperates.. But it would be nice to have a clean engine room and gain a pair of kts. Fuel price down here is not an issue( 2cts a galon!!!) only its availability and quality. Just some thoughts.

 

[petohazy]

Going smaller may leave room for a SeaGyro (no affiliation). This will add some weight back but you will also gain greatly increased stability even at anchor. At slower/trawler speeds they are very effective at roll stabilization. An addition like this might actually raise the value of your boat even with the negative of smaller mains. I believe the size for your boat is listed at around $30K.

 

[GJH]

I'm sympathetic to what Mario is getting at, and if I could wave a wand I'd probably have a set of Luggers around 300HP, for a whole passel of reasons. But by the time it was all done in the real world, even using old smaller or lower HP Detroits, you might as well just go out and sell your boat and buy a 58LRC.

 

[davidwigler]

I think the belief that smaller engines lower the value may be obsolete. There are several 58 MYs on Yachtworld in the 200K range. Try finding a 58 LRC at less than double that. I know the hull and beam are different, but people are paying more for an efficient trawler these days. There seems to be no premium for the V92 engines over the V71 engines.

 

[MikeP]

"I think the belief that smaller engines lower the value may be obsolete. "

I don't know...the problem, as I see it, is that you are basically making the boat unable to do what it was designed to do - plane. Repowering with equal or more powerful engines is one thing but going to lower power means you are taking the primary cruising design away from the boat.

As noted, I suppose there are some people who would like that but I would think it would dramatically reduce the customer pool. The LRC is a displacement hull but I'd bet that if you run a 58LRC and a 58MY at hull speed , the "mileage" difference would not be significant. In fact, I wouldn't be surprised if the MY's was better - it's a much lighter boat. Remember, when the LRC was designed, it ran at hull speed or less while the "regular" 58 normally ran at planing speeds. So in that situation, there was a huge difference in "mileage." Bring them both to hull speed and that difference is gone.

 

[partgypsy]

I posted a related question in a different section and got no response, so I will try over here.
I have a 1971 Twin Cabin 41'. It has two 454's. The engines are running but not running great. I have the original fiberglass tanks which leaves most of the Chesapeake Bay off limits due to the ethanol mandate. If I found a couple of smaller diesels... e.g. DD4v53's... at a reasonable price, I would be very tempted to do the swap. They will put out less power and be slower than the gas guzzlers, but I find that going about 8-10 knots is best in the chop of the waters where I cruise. The 43 LRC's of the era used these engines and I expect they would be adequate for my smaller and lighter 41'. I like to enjoy the ride and not feel fatigued by bashing through the chop. (I am also lucky to have a 29' Cobalt that runs about 38 kn, if I want excitement.)
If I did the swap, I would also consider removing the genset in favor of high-output alternators and a larger battery bank and a second inverter.
I would appreciate any comments on the pro's or con's of this thinking.

 

[Angela]

As noted, I suppose there are some people who would like that but I would think it would dramatically reduce the customer pool. The LRC is a displacement hull but I'd bet that if you run a 58LRC and a 58MY at hull speed , the "mileage" difference would not be significant. In fact, I wouldn't be surprised if the MY's was better - it's a much lighter boat. Remember, when the LRC was designed, it ran at hull speed or less while the "regular" 58 normally ran at planing speeds. So in that situation, there was a huge difference in "mileage." Bring them both to hull speed and that difference is gone.

I calculated our fuel burn for the trip to the Bahamas we just did in our 58MY with 8v92TI motors. The fuel burn for the mains (genny excluded) was 16 gph, for both engines, combined, and we ran between 9.2 and 10.4 knots. I was hoping for about 12 gph. Perhaps if we slowed down to around 9 knots or just under, we might attain that efficiency. Apparently, 10 knots is not our sweet spot. What does the LRC get for gph, just out of curiosity? Anybody know?

 

[MikeP]

According to the Hatt brochure on the 58LRC, it has a 3000 mile range at 9K. Doing the math with the 2300 gal fuel tank, it ends up at around 1.3 miles per gallon - I ASSUME all of the data is in nautical miles but the brochure only mentions the cruise speed of 9k and states "miles" for the range, not "nautical miles.". So it's possible that the mileage could be either 1.3 statute miles or 1.3 nautical miles per gallon at 9k. Our 53MY will easily do that at 9k though I realize the 58MY is heavier. But the 58 MY's additional water line should help.

I'm ASSUMING that for advertising purposes Hatteras did not include any reserve in their range figures. But I could be wrong.

 

[MarioG]

Thats what keeps poping up in my mind. a LRC moving 9kts at about 1.3 miles per gal. but with a 4 cyl? but at what fuel burn? I agree with Mike in the sense that if I slowed a bit more I may obtain close to 1.3 mpg. Also Mike I agree to a point with the your statement about proof of a destroyed engine do to running it at low power but there is something to be said about wet stacking. Also isn't the product of incomplete combustion in a diesel engine (in this case) wasted fuel? which leads to a high gph? Could this explain that a smaller HP engine has a more complete fuel burn resulting in a lower gph rate. Similarly on the subject of power, there is a calulation of how much HP is needed for a given speed. I think part of equation is boat weight and length.

The easy way is replacing our boat with a LRC but that not the subject of this discussion.

 

[MikeP]

"Also isn't the product of incomplete combustion in a diesel engine (in this case) wasted fuel? which leads to a high gph? Could this explain that a smaller HP engine has a more complete fuel burn resulting in a lower gph rate."

Yes, it could. And it would lead me to the conclusion of replacing lower temp tstats with higher temp ones. For example, replace the 160s in my 8V71TIs with 180's. In theory, this would be better all the way around because the cooling system SHOULD be able to keep the temps in the correct range for a 180 degree tstat at wot all day long. But we all know that the cooling systems on these marine engines are on the marginal side. Also, according to many experts, the DDs shouldn't be run at 195 which is the upper variation limit for full open on a 180 Tstat.

HOWEVER, the DD service manual does not cite that 195 degree limitation as I recall (don't have the manual here). Seems to me they say 210 is the safe limit. BUT I could be remembering those figures wrong. I'll be back on the boat on the 18th so I can look them up then.

I don't care to get in an argument about the safe temps - I'm just quoting what I remember from the DD manual for the 71 series. With 160 stats and clean system, our engs run at 165 at our normal cruise. IMO that is much too cool but I have never done anything about it. At WOT they will hold around 188.

 

[MikeP]

Just found the temp specs for the 71 series - converted to F it is 171 to 195.

I think I've just convinced myself to change the 4 160 tstats to 170s when I get to the boat next week.

 

[Seabrooke]

What about installing shaft locks then running one engine?



For economy running this should be a viable option.

In theory at least as I have no actual experience with these boats - this would keep the load and temperature efficiency, keep the fuel burn down - to what level I could not say but Bobk was saying on another thread that he belives it would be significant, give you full power from both engines if required, leave the original layout intact and leave a MASSIVE amount of money in your hip pocket.

By using a regular program of changing over to each engine you would have even wear, even load (to a point) and keep your engine hours down.

I bow to experienced Hatt owners to shoot me down here if it is not a good idea.

 

[MikeP]

Locking down one shaft puts a considerable drag on the boat. imposed by the non-turning prop. There have been some past threads that, as I recall, indicate that the drag actually reduces efficiency as opposed to both engines running to push the boat at the same speed through the water.

 

[Seabrooke]

Locking down one shaft puts a considerable drag on the boat. imposed by the non-turning prop. There have been some past threads that, as I recall, indicate that the drag actually reduces efficiency as opposed to both engines running to push the boat at the same speed through the water.

Thanks Mike,

I completely agree that the locked shaft will inrease drag over a freewheeling prop.

However, if the options are either pull the old engines out for smaller units to slow down then slowing the original engines makes more sense to me. Then there is the suggested problem of the DD's being inefficient at lower revs.

There is feedback from a Hatteras owner on the website of www.shaftlok.com (no affiliation) "A 1977 cruising Hatteras 42 footer with twin diesel 165hp engines and 24 x 25 inch 3 blade propellers at 2200 RPM on both engines uses 15 GPH producing a boat speed of 11.5 MPH which is .76 MPG. With one engine shut down the GPH drops to 7.5 and speed drops to 9.43 MPH which is an 18% speed loss with a 50% fuel savings at 1.25 MPG. Still with one engine off and reducing the RPM to 1800 the speed is now 8.05 MPH with the GPH at 4 or 73% reduction results in a 2.01 MPG. Thank you for the information Captain Bob. 1/15/2010"

Obviously slowing down makes the major difference as we have discussed before. Locking the shaft is not to produce the efficiency but to prevent damage to the transmission and keeping the other engine running at peak efficiency but at lower boat speed and lower fuel burn.

It's a hellovalot cheaper and easier than an engine changeout or an engine rebuild or transmission rebuild at a later date.

 

[Boatsb]

Sounds like an LRC but thats neither here nor there. I get over 1 MPG at about 7.5-8 Kts in fact probably 1.5 MPG without the genny and a clean bottom on my 41C. that is with 289 HP detroits running both engines. If a boat was not designed to run on a single engine it's not going to give you that much better performance. now if you pulled one engine and ran a PTO to the transmission from the missing engine maybe it would make sense but these boats need to run 2 props for the best performance. I have seen a single engine running 2 props so I know it can be done. Its just not the best solution to drive a boat from one side or the other as the loss in correcting for the uneven thrust is probably as much as running the second engine.

 

[Seabrooke]

Fair call Scott,

I suppose it also depends on the style of use of the boat.

If mostly day cruising and laying at anchor then there is probably not much to be gained by saving engine hours and a good wide open throttle "Italian Tune Up" on the way home would blow the soot out if required.

Just kidding.

Still, I'd be reluctant to change engines to save money on fuel when it would take 300 years to make up the difference in fuel cost savings.

Different I suppose for a charter or commercial usage boat where the engine change could be financed and the fuel cost offset with continuous use would make a viable business case. Especially with the cost of fuel over here in Australia.

There is a great video on youtube (search big yacht repower) about a complete repower of a 53 classic with new Yanmars, new gen, new controls, etc. etc. More power, less wieght (only about 1000lbs) AND 30% more fuel efficiency.

It's a great video.

In the mean time, my research continues.......

 

[Pascal]

"the problem, as I see it, is that you are basically making the boat unable to do what it was designed to do - plane"

Mike, not sure I can agree with that since the early 53/58 had naturals and will never get on plane. Even the later 8v71TI will barely get them on plane anyway...

Angela, there is something very wrong with your numbers, there is no way Sanctuary should burn 16 GPH if ran at/just below hull speed. Either your tanks were not really full when you left or you ran most if the time at 10+ not 9.3.

A single knot above hull speed will cost you 30 to 40% in fuel... It s that dramatic. I know the 8v92s will burn a little more, but even the Viking 63 with 12v71 I ran earlier this year from st Pete to the abacos didn't burn that much (with a wider beam). At 9.5kts charmer burns 17gph and while the 3412Es are more efficient they re pushing 130k lbs and almost 20' of beam

Again, that single knot above hull speed is a killer

Back to Mario's question, I find it to be an interesting idea especially for the natural boats... Those big DD make very little sense. I don't think the weight saving will be a big issue handling wise, for instance there isn't a huge difference when the tanks are nearly empty and between fuel and water you re dealing with 5 000lbs or more