Re-Powering the 38X...

[cshreeve]

Has anyone had any experience re-powering one of these hulls
with a larger more powerful engine? What was your original
engine, and what did you replace it with? How much increase in
speed did you get? Any increase/decrease in fuel economy for
the same speeds? Is there a theoretical (or practical)limit to
this kind of a power swap? What about props?
Certainly these 38Xs should be able to handle 50hp or more.
(I found a Columbia 39' with 60hp) In the Gulfcoast region
thunderstorms can come on quite rapidly. An extra knot or two
could make the difference in any kind of emergency. Anyone out
there with any rumors or hard data on this?

 

[aurora]

Eddie: I have never repowered my boat but I offer you this thought: If you take the time to determine the hull speed of the boat once you have an engine that will drive the boat to hull speed and beyond all you are doing is wasting power and fuel to drive the boat and drag the inevitable water off the stern of the boat. I have lost the formula for hull speed determination but I am sure you can find it in some reference book. The hull speed of my CM38 is 7.35. With a heavy spinnaker up I have sailed well beyond that speed but I was dragging 3 feet of wake off the stern. JH.

 

[bobclark]

<div>On a true displacement hull the formula for maximum hull speed is 1.4 X the square root of the waterline. On a more modern hull that formula can be 1.3 X. Simply put, our hulls are a surpurb blend of old and new. The square root of 30 (our waterline) is 5.48 (adjusted). 1.4 X 5.48 = 7.67 maximum hull speed. In reality 71/2 to 73/4 sailing in a good breeze. Yes, you will hit over 8knts at times, while digging a deep hole in the water.
Southerly is original, powered by a Perkins 4-108 52hp, and swinging the standard 3 blade 16 X 11 prop. She maxis out at 3000 rpm doing 7.6 knts on a flat sea. We cruise at 2100 rpm doing 6.2 to 6.4 with a clean bottom, on an average sea.
The fact is, in a rough sea we do not try to do our maximum speed. We slow down to be in accord with the wave conditions, resulting in a comfortable ride. The M38 has a great hull design. If you use it the way it is intended (let it find it's own comfort zone) you will always be safe. I know that, that is not the answer you were looking for, but perhaps you might consider thinking about it. Fast power-boats have to run from the weather, their hulls are not safe in those conditions. Then there are trawler hulls, slow, but safe.
Larry
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[aurora]

Larry: Thanks for refreshing my memory on the hull speed formula. The CM38's waterline is 27.5 that's why my hull speed is 7.35 as mentioned earlier. So true your comment about "digging a hole in the water" ! JH

 

[captrob]

<div>Nigle Calder's "Electrical & Mechanical" book has a great section on re-powering complete with formular for calculating the HP needed for a specific design. I used it when I re-powered my old Ranger and it was spot on. I would also add that I agree with the others. There is such a thing as too much power espically with desiels. Desiels like to run at about 80% max RPMs. Running with too little or too much load can reduce engine life. Sizing your engine, transmision & prop to your boat is important. Engine, transmission & prop are a system and all three must be considered to do the job right.Fairwinds & Rum Drinks,Vic C.
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