4 cyl or 6cyl Yanmar

[sisubob]

I am looking to repower my CD 28 Fly Bridge from a 360 Chrysler to a Yanmar diesel. I have spoke with several yards that do repowering and some are recommending the 240 HP (4 Cyl) others are recommending the 315 HP (6 Cyl).

When the boats were originally built, it looks like Cape Dory offered a 275 HP gas Chrysler or a 200 HP diesel Volvo. The hull speed on this boat is 14-15 knots, do I need the power to go 20 knots?

I would like some advise from Cape Dory owners who have repowered. Having never road on the boat, (because I purchased it knowing it had a bad engine) I do not know where to turn.
I only want to do this job once...

Any advise or comments would be appreciated.

Thanks for your time,

Bob
Elephant Shoes
Huntington, NY
Hull # 157

[bill2]

Bob

Even though I don't have anything more than some general uninformed comments I thought I'd jump in till others more knowledgeable about such things see your question.

FWIW - ( and please check out all I say because I'm certainly not an expert here ). My gut reaction is to go with the four cylinder diesel. here's why . . .

first gas versus diesel is a nobrainer. Diesels are cheaper to run, reliable and last longer - proof - look at our over the road trucks - all diesels.

now harder diesel versus diesel. Since diesels make their power in the form of torque ( not HP like gas motors ) at lower rpms they don't need the horsepower nor higher rpms of gas motors as they can use lower gearing ( slower engine revolutions ) to achieve the same thrust as a gas engine. So bottom line is that you don't need to duplicate the HP ( and rpm's ) of a gas engine to get the same performance. Compare torque to torque to see what it takes to "turn" ( torque to twist ) the prop shaft. You probably will also find that the mfg's diesel option used different trans gearing and/or prop ( pitch and such ) to equal the gas engine at lower rpm's. So what's this all mean. The smaller Yanmar four cyl w/240 HP would seem to be well over Cape Dory's original diesel engine's power which should be more than sufficient if Cape Dory got their diesel choice right - but check the torque output and verify the trans gearing and prop used as I suspect they may also need to be tweaked. For example - my VW diesel uses lower gearing as I can run at 65 mph at 2400 rpm's while the gas engined VW's need to turn 3000 rpm's to get similar torque by using higher gearing. I run slower and more economically ( even more than the hybrids but thats another story ) but can pretty much keep pace with the non-turbo gas engined cars though I have less HP.

Now all this makes sense to me but its not my hard earned $$$$ on the line. You need to check it out yourself to make your own decision.

Good Luck . . .

Feel free to send me a PM if you think I can help

[Bill - Dulcie]

The CD 28 hull is semi-displacement, and it tends to "keel walk" at much over 20 kts. In effect, the keel lifts the stern out of the water, and at a certain point, steering becomes a problem. Thus, 300+ hp may be too much for the hull. Mine had a TAMD41A Volvo diesel (200hp), cruised comfortably at 14 kts and, with a clean bottom and half-empty tanks, could touch 20 kts at WOT. Your 4 cyl Yanmar sounds quite adequate at its rated hp. 6 cyl/310 hp may be a bit much, and certainly would be less efficient.

Critical with any repowering is the right prop! Rule of thumb: if WOT will give you within 50 rpm of engine max rpm specs, you've got the right prop.

[Parfait's Provider]

If you really want to save money, keep the speed down to hull speed, which is a lot closer to 7 knots than 14, which is a semi-planing speed compatible with the semi-displacement hull.

Our CD 36 is adequately powered with a 3 cylinder Yanmar which burns about 3/4 gal of diesel an hour at hull speed. If you like boring temporary holes in the water, go for the several hundred HP option and watch your wallet.

[AnnapolisMark]

You might want to consider dropping an email to Clive Dent at TPI Composites. Since he designed the boat, he might have some thoughts on options for repowering.

Mark Cline
Power CD28 FB #219
"Brandywine"
Annapolis, MD