Twin Diesel Repower with lower HP

Good point Aliboy but I disagree in that I think the most important consideration for a re-power (or power) is the power required by the hull design. Light loading is'nt the end of the world but but when one has the chance to get it right one should choose an engine that will get loaded about 60 to 70% most of the time. When I re-powered my boat I discovered there is a very narrow range of power that could be considered "ideal". By my commitment to this I rejected my favorite engine for another but I'm good w my decision as I really do have excellent hull loading and engine loading.

 

CDK,
I'd be interested in the boat you re-powered. Do you have before and after speed, gph, or nmpg?

Thanks
Rubenova

 

As I wrote in post #2, consumption is down 65%. That means: to get from A to B, if the Mercruisers burned 10 gallons of unleaded gasoline, the diesels cover the same distance with 3.5 gallons of diesel fuel or heating oil. In both cases the speed is 5-6 knots.

I am confident in obtaining even better mileage once the port engine gets the overhaul it deserves, something I've planned for the next winter.
The savings in $$ are even more impressive, but apply only to the local circumstances (taxation).

 

Those are great numbers. I ask because, until the last boat I chartered, I was a hard core traditionalist/trawler guy. When the economical single diesel ruptured an oil line 4 hours into the first day, we were offered a Sea Ray 370 as a replacement...with twin 454's and a Floscan. Clearing the breakwater I was shocked to see 20kts and 40gph. Pulling back to 1400rpm only raised the 0.5 nmpg to 0.8 nmpg at about 7kts. To make things worse 1400rpm was not enough to keep the batteries charged so on went the generator, probably bringing overall fuel consumption back closer to to the original figure. Fuel budget woes aside, the boat turned out to be pretty comfortable and got my traditionalist mind to thinking about the title of this post, why not pull the big blocks and replace them with small diesels. From what little I know and reading other posts, planing hulls at displacement speeds are no where near optimized. But doubling or tripling a comfortable boats range seems like a compromise worth exploring further. In short, I'm very interested in hearing more about your boat and results. Thanks in advance.

 

In the interest of performing an economical change, I would toss the 555's. It would be foolish to put money and effort into those white elephants. look to replace them with something that will still be compatible with your existing transmissions, shafts, and possibly even props.
I think the low hp Cummins 6bt (180hp?) would work nicely for your boat, lots of torque on a slow turning, fuel sipping motor.
Take a look at boatdiesel.com, they have extensive calculation tools that allow you to toss around the alternatives, as well as forums staffed with genuine experts to answer your questions. It costs $25 to join, well worth it in my opinion. (No, I dont work for them, but have saved many thousands of dollars by educating myself there)
Cummins offers re manufactured diesels at roughly half the cost of new, and they are, for all practical purposes, new motors.

 

There is an interesting problem with fuel economy. There is a diminishing return for any effort. Take the Sea Ray charter... under best conditions a gasolene engine makes 10 horsepower per every gph, usually near the peak torque and rpm. A boat is propped so this would never happen, so this number is "impossible" to reach. As the throttle(s) are pulled back engines are less and less efficient. The 370 was only makeing 3 or 4 hp per gph, NOT the 10 under best conditions. Combine that with the hull and propeller being mismatched for the single digit cruise speed and the nmpg barely changed. So repower with small diesels yes? Ok, but the smaller the engine the higher bsfc, and less reserve power. Get a top notch NA to run the calcs for repower (good luck with that for a speed and power compromise that would suit your time and range constraints and idle into the sunset right? Well and good until conditions change (a stiff head wind for example). Suddenly the exhaust temps are beyond continuous ratings at max torque rpm and reduced speeds are needed. Controllable pitch props would be the next step. It could go on and on! I'm trying to find a compromise between a factory boat and total custom designed boat. Maybe pulling the 454's and running 3.0 Mercruisers with different props (solid not CPP)? Are there any opinions concerning balancing room, mpg/ton, etc, cost? Take the room and comfort of a Sea Ray 370, the fuel use of a single diesel trawler at single digit speeds, and combine them for a cheap? Let the games begin!
Kind regards,
Rubenova

 

I have a similar case; an older powercat with two Chrysler 440's. The present owner reports economy cruise is 18 mph at 2800 rpm, getting one mpg. I want to replace these engines with lighter, less thirsty diesels.* I am interested in the boat for long distance cruising, with an 800 mile range. I am NOT interested in going any faster than 20 mph, as I currently cruise at less than 10 with a range of 900 miles.
How much does a typical 440 weigh excluding transmission, shaft and prop?
How much power is a healthy 440 developing at 2800 rpm? Torque?
What horse power range should I be shopping for if I want to get 18 mph at 75% power? I understand that full throttle operation will net me no more than a couple knots, and a lot higher fuel consumption.
It should be noted that this @15,000# power cat appears to be a displacement design with very narrow hulls (2'?) with a sharp entry, hard chines and flat bottoms.

* "YOU CAN MAKE A FAST BOAT GO SLOW, BUT YOU CAN'T MAKE A FAST BOAT GO FAR"

 

Dou you know the speed and rpm at full throttle? I'm also curious about the reported fuel consumption at cruise. Was it measured with Floscan?

 

I've asked for WOT numbers. I doubt a 50 year old boat would have floscans; do you remember how cheap gas was back then?