How much horsepower would I need?

I'm thinking of buying an older glass trawler. It is currently powered with twin Detroit 8V92 engines.
If I buy it, I'd like to repower it with something more efficient. The Detroits are supposedly rated at 475 hp each. The vessel is: 56' loa with an 18' beam and 58" draft, which would put her theoretical hull speed at right around 10 kts. I would estimate she is in the 20-25 ton range. I would like to be able to use her as a coastal cruiser and a liveaboard.
I am fond of the John Deere 6068 series, but I'm not sure if that's the best route or not...ANY helpful insight would be MUCH appreciated.
Thanks in advance.

 

With an understanding that the boat will be a displacement boat , and no dreams of getting up on the plane very little power is required.

First most power boats do not attempt "hull speed" SQ RT LWL times about 1.34 as they don't want to spend the diesel fuel.

Usually SQ RT LWL times .9 to times 1.1 or so is 1/2 or 1/3 the fuel bill of "hull speed".

AS a rule of thumb 2HP per ton of displacement to 3hp is used to cruise with about 5HP per ton of rated engine power.

TO power the boat , first weigh it as over the years boats gain loads of weight.

One ton of displacement is 2240 lbs.

Then see if your favorite engine can produce that power at cruise RPM usually 1500-1800 or so.

SKEENS" eliments of yacht design" will have what is needed to see if the current props will be OK , and what reduction gear will be required to be efficient at cruise.

 

This nomogram will help

 

The art of how successful the refitted boat will be will be determined by how well you match the engine to the boat , using the existing drive.

The cost of all new engine , tranny and shafts & props will easily out weigh the value of the finished boat.

IF you can match the engine of your choice to the shafts and existing props it will be far cheaper.

The JD engines can be had factory rebuilt from the FARM guys at far less than the marine folks.

Every year the have a "sale" where a core engine is NOT required and there are dozens of sources for rebuilt trannys , look in Boats & Harbors a resale magazine.

I would only purchase a Twin Disc , for its long term reliability.

It will be a challenge to either find marine manifolds and heat exchangers for the JD used .

I would look to install a keel cooler and dry stack exhaust.

No winterizing and far less expense.

While the JD are nice engines you might consider the International DT 360 or DT 466.

These are very heavy duty engines and dirt cheap with low mileage as they are used in skool buses.

When crashed -even with 5,000 miles-they are scrapped as the feds pay for 90% of the bus.

Good hunting!!

 

The DDs in the boat now are to go fast. The same engines will use at least 2/3 less fuel at 10 knots. How much will you save in fuel over the cost of new engines and all the hassle of matching up the shaft, fuel, water and so on? The props will need changing, too. You'll probably never save what you spend in the change out.

 

DD engines have a problem in that to be efficient they need about 60% of the engine build load while operating.

That's why DD built so many engines of different size 1,2,3,4,6,8,12,16 cylinders could be chosen with about 20HP per cylinder , up to 30hp per cylinder to chose from .

Rebuilt injectors are cheap , about $30 each so an engine can be de powered , if required , but your 400+hp can never go to 60--80-HP and be efficient.

Smaller injectors and modest 1200-1500 RPM may get a reasonable fuel burn for little hassle and few bucks , compared to a swop.

Find out the RPM and injector size for a long term generator (not an emergency stand by set) for your V8 engine and see if that would work.

http://ecomodder.com/wiki/images/e/ee/Ford_2.0l_zetec_bsfc.JPG

This is a fuel map so you get the idea,

 

After WWII many people bought war surplus DDs and often put them in boats needing smaller engines. But the engines didn't know this and ran fine for 20-30 years anyway. Many commercial fishing boats had Detroit 671's when a 471 or 371 would have been better. (Detroit numbering: 671 is 6 cylinder, 71 cu. in. per cylinder) But they bought the engine for 1/4 of the price of the proper new diesel. In a trawler or any boat not trying to plane, running the engines conservatively at lower rpm, will go several times the normal overhaul hours rate. This isn't scientific, but seen in my lifetime dozens of times over and over. I t never pays to swap out good working engines.
The new electronic engines are unreliable. In time some or all of the components will fail. Usually without warning. People I know operating commercial vessels, carry hundreds if not thousands of dollars in electronic spares. The people running 24/7 are full of stories of electronic problems and outright failure. The one thing you need on the ocean from your engine is reliability.

 

I disagree with your assessment of electronically controlled diesels. The lower numbers in warranty claims and the longer running life are facts.

 

"The lower numbers in warranty claims and the longer running life are facts.

Come on over and rewire the De Dec in my Series 50 DD and tell me about electrics reliability as you do.

Eng mfg are working very hard to pass the various Air Police demands with newer better all MECHANICAL engines.

The one that succeeds will own the diesel world for a while.

 

I will rewire if you pay my rates.

 

Im a big fan of the detroit diesels myself.. It can be made very efficient with the right combination of gear reduction... Prop size and injector size.. I have had many commercial fishing vessels myself.. Shrimp boats mainly... And the detroit diesels have proved themselves very reliable and efficient .. Almost bullet proof .. And keep in mind.. In some cases... More power is cheaper to run.. And you dont have to push it to its limits.. Making it much more reliable... Sometimes reduction in injector size can create more torque at a specific rpm range making it a lot more efficient ...

 

I live aboard an 83' x 17' boat powered by 2 Detroit Diesels (actually Gray Marine) 671 naturals. I cruise at 10 knots and burn a little less than 8.5 gallons an hour. The engines had way over the 20,000 hours I could document before they were overhauled by me. Since I did the labor, the 2 engines cost about $5000 for parts, gaskets and head work. Engines were made in 1947. So I guess sometime about 2085 they'll need another overhaul. Try that with your electronic engines.