Twin Diesel Repower with lower HP

[DareDevil]

Hi All!
We have a 71 Uniflite 42 with 555 Cummins in dire need of a repower. First off, I should mention I have no experience with diesel engines, nor motorboats for that matter, so this may very well be the dumbest question ever posted. One of the engines in this boat is incomplete, the other has a rear seal leak but is running. I'm told the 555 was meant for running high rpm's and the boat is built for getting on a plane. However, with gas prices the way they are and me being used to little sailboats, I'd be perfectly happy replacing these engines with say twin 50 hp diesels and puttering along slowly.
Of course, ideally, we'd repower around the same HP's with brand new engines, however, 2 running engines could get me places instead of sitting tied up to the dock all the time with my leaking one engine and it's gotta be cheap.
So what do you all think? Would repowering with smaller engines be feasible/cheaper? The other option would be to try and rebuild my one engine, buy another 555, or set of them, and rebuild those... Any advice would be greatly appreciated, i'm getting fed up with not going anywhere!! (and yes, i do understand that i could, theoretically, just limp along on my one leaking engine, but just getting in and out of the slip is enough to give me a heart attack every time, not being able to back up in a straight line..)
Thanks!!

[CDK]

It all depends on your willingness to forget about planing.
Two small but good diesels will reduce fuel consumption dramatically, but it takes some time to get used to this feeling of impotence when you open the throttles.

I made the change from planing with gasoline engines to displacement with small diesels, reducing fuel costs by more than 85% (consumption by 65%).

[DareDevil]

CDK, Thanks for your reply! So glad to hear I am not to only one thinking of treating my planing hull boat like a trawler! I'm completely willing to give up the planing. We've never had this boat over 8 knots, and that's fine with me. How much of a redesign goes with repowering with that much less hp though? I imagine I'd have to change just about everything in my engine room, from brackets to starter, shafts, props, controls? I've asked several yards and diesel mechanics for a quote, but so far none have been willing to even consider the idea of repowering with that much less hp, at least not for less than 100k. Several mechanics insisted we should replace our Cummins 555 with a running pair, but since we will not be going full throttle and from reading the forums that is not what they were made for, that just sounds like an extension of our current problems.
If I want to get out on the water before the turn of the century, I really need to repower cheaply...either that or invest in a rowboat

[FAST FRED]

"Uniflite 42, We've never had this boat over 8 knots, and that's fine with me."

Your long range cruise speed may be closer to 6 or 6.5 for economy with 8K as flank, if you get there.

How much does it weigh?

FF

[CDK]

Because the Cummins 555 turned at only 2800 rpm at full throttle it may be difficult to repower with just 50 hp without changing props or gearboxes. Such small engines generally have a higher rpm but lack the torque to spin your big props.
Maybe you should look for a pair of diesels without turbo chargers between 50 and 100 hp and install smaller props, or engines with gearboxes suitable for the props you have now.

[DareDevil]

Fast Fred, the boat weighs 19 tons and around 6 knots sounds great to me.

CDK, those are some good tips. We'll start looking around and see if we can find something that will work for a reasonable price. Thanks for the feedback, all the negative replies from mechanics around here were starting to get me to think we'll never get this done, but if we can go smaller we might actually be able to get this done before next summer, which is great news! Thanks again!

[Submarine Tom]

If you're going that far, perhaps you should consider dropping to one, centre-line engine...

I'm not familiar with that boats keel, it may not be practical.

-Tom

[FAST FRED]

Fast Fred, the boat weighs 19 tons and around 6 knots sounds great to me.

With the usual "rule of thumb " of 3 hp per ton at normal cruise , 60 hp at a low nice and quiet cruise RPM should be easy to do.

I would suggest Dave Geer's Propeller handbook to work out prop gearing requirements.

You might find that the 50 hp engines with a 6-1 reduction gear might spin the props just fine at 6-7K cruise.

[Easy Rider]

Fred,
Dare Devil's not going to be efficient enough to go 6 or 7 knots with 3hp per ton. The stern will need to be reshaped to a full disp form to have reasonable performance w only 100hp. 120hp is considered marginal on a 36 GB and even it has a better form for going slow than the Uniflite.

Easy Rider

[thudpucker]

While I was still working, I had a 10 Knot boat. I'd gladly have traded it for a 20 Knot boat.
I'm guessing you'd need a Tide book to make sure you dont motor along backwards at 6 Knots.

[FAST FRED]

"The stern will need to be reshaped to a full disp form to have reasonable performance w only 100hp. "

A hi speed stern that is underwater and dragging a 5 to10 ft wide wake will only require 10% to 15% more HP to operate at modest speeds like 6-8K.

If 60 hp needs 70 hp of fuel burn to go 6K the difference 3 gph up to 3.5 gph would take the normal pleasure boater 100-200 years to justify a stern rebuild.

FF

[keysdisease]

Maybe you should think about working the problem backwards. Shop for a pair of replacement engines and don't limit yourself to 50 or 100 HP. You may find a pair of engines in 150 or 200HP for the right price and that might fit your needs better.

Just because an engine is only 50 HP doesn't mean the price will be as low as the HP.

Steve

Originally Posted by DareDevil

and it's gotta be cheap.

[FAST FRED]

Shop for a pair of replacement engines and don't limit yourself to 50 or 100 HP. You may find a pair of engines in 150 or 200HP for the right price and that might fit your needs better.


The problem with oversized engines (very oversized) is the huge loss in fuel efficiency (at 10% to 25% power use) as well as the loss of engine service life.

Bigger ain't better with diesels , a good setup will use 80% of rated Hp at 90% of rated RPM at cruise.

OF course if there cheap enough installed , who cares?

FF

[keysdisease]

The 555's are typically 300+ HP engines. My point was if daredevil is on a budget and shopping for a used pair if he limits himself to 100HP and down the field shrinks, while if he shops for 200HP down he has a larger field to shop from and may find a pair over 100HP in his price where he may not find something under 100.

I understand the loss of fuel efficiency, however as you said, "if there cheap enough installed , who cares?"

It could take decades and thousands of hours for the upfront savings to offset the loss of efficiency for an occassional user.

Steve

Originally Posted by FAST FRED

The problem with oversized engines (very oversized) is the huge loss in fuel efficiency (at 10% to 25% power use) as well as the loss of engine service life.

Bigger ain't better with diesels , a good setup will use 80% of rated Hp at 90% of rated RPM at cruise.

OF course if there cheap enough installed , who cares?

FF

[Aliboy]

I run my 43ft, 13.5tonne planing hull at 8.0 - 8.5knts (trolling) at 15lph with a pair of twin 330hp engines. According to the engine power curves I am making less than 80hp (x2), although it could quite a bit less than this as the power curves are the generic ones rather than the ones for my hull loading. At a guess (based on fuel burn) I suspect that I am closer to 50 - 60hp (x2) in my hull at ~8.0knts.

Looking at the fuel consumption curves my 330hp engines are making hp at ~ 225gpkwh, whilst a 100hp engine from the same manufacturer is making the ~same hp at ~ 245gpkwh. A similar 150hp engine is making the ~same hp at ~ 230gpkwh. Bottom line is that my larger engines are as efficient as the smaller ones. Only problem is that cost more to buy up front and a little more to maintain (but not much).

For the OP's boat I would be looking for an engine that gave me max torque at my normal desired cruising speed. The problem with small engines in larger boats can be a lack of 'control' through lack of engine torque and hence lack of ability to counter the push of waves etc. You might get away with running a 100hp (or smaller) engine at 90% rpm's, but how much torque have you got behind you. A slightly larger hp engine turning a bit slower and closer to peak torque would probably be a better choice. Since he probably has the space, a pair of larger displacement, naturally aspirated slow turning, high torque engines would be my suggestion. They are generally easier and cheaper to maintain as well. For HP I wouldn't personally be looking at much under 100hp (x2).