CD 28 W/ 22 HP Volvo?

[John Vigor]

John, the only practical way to find out is to do a sea trial.

Twenty-two horses ought to be plenty for a CD28. In calm water, at right angles to the wind, your engine at full throttle should be able to reach within 50-100 rpm of the manufacturer's stated top revs. At that stage, your boat should be doing at least hull speed.*

If your engine is lugging, and won't get within 100 rpm of maximimum rated rpm, your propeller is either too large in diameter or (more likely) too coarse in pitch. (It's like too high a gear on a bike pedalling uphill.) A prop shop can change the pitch downward for you.

If your engine reaches top manufacturer revs too easily, but your boat speed is still below hull speed by half a knot or more, your propeller is either too small in diameter or (more likely) not pitched enough. (It's like too low a gear on a bike pedalling downhill.) A prop shop can increase the pitch for you.

Something to note is the fact that a prop that is overpitched will let you run the engine at lower revs and still make good speed with great fuel economy IN CALM WATER and no wind. If sounds great and feels great but it actually puts more stress on the engine, just as you would feel the stress on your knees if you were trying to pedal uphill in top gear.

Furthermore, to reach maximum power to push your boat against a wind and chop, your engine needs to be able to reach top-rated prm. It's neither as quiet nor as economical, but the power is there when you need it, and I think that's more important in an auxiliary sailboat when you need to get out of trouble.

*Hull speed in knots, for this purpose, is calculated by taking the square root of your waterline length in feet and multiplying the result by 1.34. A GPS will give you an approximate idea of your speed as long as there is no current running, but a more accurate speed through the water is given by a well-calibrated speed log.

I don't know offhand what the waterline length of a CD28 is, but your hull speed should be something over 6 knots. Perhaps CD28 owners can chime in here.

Cheers,

John Vigor

[The Patriot]

... Twenty-two horses ought to be plenty for a CD28. In calm water, at right angles to the wind, your engine at full throttle should be able to reach within 50-100 rpm of the manufacturer's stated top revs. At that stage, your boat should be doing at least hull speed.*

If your engine is lugging, and won't get within 100 rpm of maximimum rated rpm, your propeller is either too large in diameter or (more likely) too coarse in pitch. (It's like too high a gear on a bike pedalling uphill.) A prop shop can change the pitch downward for you.

If your engine reaches top manufacturer revs too easily, but your boat speed is still below hull speed by half a knot or more, your propeller is either too small in diameter or (more likely) not pitched enough. (It's like too low a gear on a bike pedalling downhill.) A prop shop can increase the pitch for you. ...

I don't know offhand what the waterline length of a CD28 is, but your hull speed should be something over 6 knots. Perhaps CD28 owners can chime in here ...

John Vigor describes precisely the sequence of events I went through when re-powering my CD 28. The original 13 HP MD7a was replaced with an 18HP Volvo 2002. The original 13LH13 prop was _eventually_ replaced with a 14LH14, but not until I had tried several other pitch and diameter combinations. The local prop shop was a key player in this sequence.

Incidentally some of the early CD 28's featured a 25 HP engine, although I don't recall the details just now. In my case the 13 HP MD7a seemed a bit under-powered (and had several other troubling issues), while the Volvo 2002 was a big improvement, both design- and performance-wise. Also, the LWL for the CD28 is roughly 22 feet.

[Jim Davis]

Get Michigan Wheel to compute a prop for you. There is no obligation, but this will be in the ball park. Make sure you specify the apeture size correctly.
http://www.miwheel.com/MIWheel/PropAnal ... nboOne.asp

I have no ties to them, but have used this numerous times and been happy.

[Ben Thomas]

I can run the numbers for you and give you a ball park prop size. I have the waterline and displacement for the 28, you say the engine size is 22 Hp.
I need max. RPM of engine and the gear ratio on transmission (forward) to run the numbers on my prop. cal. program.
Ben

[Julian]

That seems like a lot of horses to me. Our CD30C only has a MD7A with 13 hp of which 9hp come out as sound energy. It seems to move the boat around OK and we had no trouble maintaining 6 knots on Lake Ontario on a smooth day. On rough days we usually sail. It's certainly plenty for moving the boat around in the marina, so I would think that 22hp would be plenty for a 28 footer, I guess it depends how much motoring you like to do. Remember that more horse power means more fuel for the same distance.

Interestingly James Baldwin on his 28 foot Trition uses a 3.5hp outboard. A Trition is about 8000 lbs and the CD28 about 9000 lbs.

I am seriously thinking of pulling the engine out and putting a 9.9 outboard on the back. The reason; inboard servicing is tricky because I am not as flexible as I used to be and there's something about having all those holes in the boat to let water in, gases out and a propellor that make me nervous.

Julian

[The Patriot]

... How many attempts did it take? Can someone dive and replace a prop or do you have to lift her each time? I recently got a quote of $7.00 a foot to lift a boat for survey ...

I don't recall how many changes we made until we got the revs and the speed we were looking for. We pulled the boat for each prop change. I suppose there are divers with gear to use a prop puller under water and make the change, but it could happen that the cost would be quite close to that for pulling the boat. Not sure of this.