4bt in a boat

[BJMarshall]

Anybody got any experience repowering a small boat with a diesel?

 

[BKilbury]

hi, about 10 yrs ago a freind removed twin 351 gas fords from his 28 ft bertram and replaced them with 4bt's. top end was considerably slower but the cruise speed of 22kts was very effecient. we would run off shore 50 miles... troll all day and run back in 10-12 hrs later and burn 70 to 80 gallons, it would be twice that with gas motors.

 

[JLEONARD]

4 years ago I repowered my boat. . removed a wimpy old Perkins T6. 354 and put in a Cummins 6bta 270.

I have also helped several others replace diesels and also helped my neighbor replace twin chevvy gassers with Yanmar diesels.

There are some items that will have to be changed out and modified...

for example yiou will need to flush the fuel tank(s) and add in a return line, you MAY need a larger fuel lines, exhaust, water intake. You will have to install a decent fuel filtering system, you may need additional air ventilation to the engine room. Some gages may have to change (tachs for sure).

The prop shaft may not be large enough and the gear ratio will likely need to change.

What specifically are you planning? You may want to check out this excellent website www.boatdiesel.com

Jay

 

[BJMarshall]

Jay,

I'm looking for a 20 to 24 foot bowrider. I'm trying to find one in decent shape that currently has a IO V-8 in it. I'm not looking for high speed or the ability to pull a skier. We just like to cruise. The boats I have had in the past whether they were 4,6,or 8cyl gas were terrible on fuel. I'd like to be able to run around at 20 or 25 and not run through 3 tanks of fuel a day. I also prefer the reliability of a diesel. I figured a gear change and a prop change would be in order. Would an alpha drive hold up to the 4bt?

 

[JLEONARD]

I'm not sure about outdrive compatability and strength. I've always worked on straight inboards.

The issue with diesels and marine transmission ratings and strength is that they all have rpm vs strength ratings. While thay may appear to be able to hold up, you need to look at the rpm you will be running at rated hp ... which will be lower than the gas engine. That is what may tear it up.

Of course taking it easy will make it last longer.

I would say get the biggest capacity outdrive you can find.

BJ,

What HP 4bt are you considering... something like a 130 from a bread truck that you will marinize, or a marine rated engine like a 150?

One thing that is pretty critical in a marine diesel is to make sure you gear/prop it so that you get the goverend rpm (plus a little) out of it at WOT when it's loaded up. Otherwise it's life will be severely shortened.

A marine engine is always under a load and that's tough on them if you are overpropped.

My marina neighbor removed twin 350 Chevy based gassers rated at 250 (or 260) hp each and replaced them with 4 cyl Yanmars rated at 170 hp ea. His comfortable cruise speed dropped maybe 2 knots, but his fuel consumption dropped drastically... he uses about 1/3 the fuel he used to at cruise speed.

This may act a a guideline for you in planning.

Hope this helps. Sounds like a fun project.

Feel free to e-mail me at -- email address removed -- if you have specific questions that I may be able to offer some help on.

 

[CumminsBu]

The other thing to think about is cooling. Some marine engine have raw water flow through the engine, others have a heat exchanger? I would go with the heat exchanger that way your are not entering very cool water to the engine. The exhaust manifold are also cooled by the engines coolant, that well help reduse the chance of fire in your engine compartment and keep temp down. If you are good to use an I/O I would use the Bravo series made by Mercuriser for speed the Bravo I, For curising and heavy work the Bravo II and for counter rotating props the Bravo III. I have never liked the Alpha drives I think they are two weak for a diesel.