Outboard for CD 25

[casampson]

I've decided to replace my 6 HP Tohatsu Sailpro because it is a little eccentric, and I just don't love it. I'm thinking of getting a Honda or a Yamaha in the 4 - 6 hp range. A friend of mine assures me that a 4 HP motor would be more than sufficient, but I can't help but think that there would be times when I would want the extra power of a 6 HP. I would appreciate your thoughts on this.

Thanks!

Chris

[wikakaru]

I think I have posted this previously, but I can't find it when I searched the board, so I will post it again here.

How much power you need depends on what you want the motor to be able to do. On the low-power end of the spectrum is just getting home when the wind dies; on the high-power end of the spectrum is motoring directly against strong winds and large, steep seas.

The January 1965 issue of the magazine Motor Boating published an article called "Auxiliary Power by Rule of Thumb":
https://books.google.com/books?id=Cjlsx ... &q&f=false

The rule of thumb is this:

• To propel a vessel at its S/L 1, allow 1 hp per ton.
• To propel a vessel at hull speed, allow 2 hp per ton.
• For very heavy cruisers with blunt ends, allow 3 hp per ton.

By "S/L 1," the author means the speed in knots equal to one times the square root of the waterline length in feet; for “hull speed” the author uses 1.5√LWL, though many naval architects calculate hull speed as 1.34√LWL. I think the calculated horsepower is intended to propel the boat at those speeds in calm winds and smooth water.

For your CD25, an S/L of 1 is about 4.2 knots; hull speed is about 5.7-6.4 knots depending on which formula you use. Using the design displacement of 4,000 lbs, a 4 HP outboard will get you to 4.2 knots, an 8 hp outboard will get you to hull speed, and you might consider a 9.9 or 15 if you really want to punch straight into nasty seas.

I have found this rule of thumb to be reasonably accurate. On my Typhoon and CD22 I have used a variety of engines of different horsepower from 2.5 to 9.8, and I eventually settled on a 2.5 HP for the 2,000 lb Typhoon (2.5 HP per ton) and a 6 HP for the 3,200 CD22 (3.75 HP per ton). Though I typically sail the CD22 without an engine at all, when I do mount the engine I want to be able to be able to, among other things, motor through the falls at the entrance to The Basin in Vinalhaven, Maine, a place where you do not want to be underpowered.

For what it's worth, I've owned outboards by Tohatsu, Yamaha, Johnson and Evinrude, and I personally prefer the Yamahas without question.

Smooth sailing (and motoring),

Jim

[mgphl52]

I'm sure I have replied to a prior question re CD25 outboards also. The last CD25 I had came with a 9.9 Evinrude which I quickly got rid off... Too damn heavy and too much drag when not needed. I replaced it with "modern" (at that time) Tohatsu that performed perfectly, when it was in use. Since I could usually sail in and out of my marina slip, the motor spent most of the time on the cabin sole because I could easily handle getting it in and out of the well (of course, that was twenty years younger ago!). Keeping the motor in the cabin was also good since the marina was in brackish water and growth on exposed lower units was a big problem.

Lastly, something I haven't seen mentioned in this thread is prop size & pitch. That can be the most critical factor for our heavier displacement CDs.