Experience with a 9.9 HP 4 stroke on a CP 23

[NateD]

Good luck! Let us know how it goes.

[maynard]

Great choice!  I've had good luck with mine, plenty of power, even though I got the 4 hp, and the more I used it the better it ran. Vibration has been mentioned. Hopefully
you have a good motor mount already. I put on a new heavy duty mount and epoxyed in a big 3/4" plywood backer plate. This had made a difference by taking care of most of the motor
vibration.  I'm not sure if the sail pro comes with the big prop already, if not, that'd be good to get, it makes a big difference too.

[Shawn]

"I'm not sure if the sail pro comes with the big prop already, if not, that'd be good to get, it makes a big difference too."

Yes, it comes with the elephant ears high thrust prop.

Shawn

[bob lamb]

What is the proper prop size for a 9.9 on a cp23?

Bobl

[skip1930]

At 104 lb, that 9.9 four stroke is a bunch of weight. I'm old enough to shake lifting a 56 lb, 5 hp two stroke up onto the mount once a season.
It's a shame two strokes were legislated out of existence, they inefficiently burn such a small percentage of the fuel and oil cracked.
The motor mount says good for 20 hp.
I don't have any good ideas for an O/B on a 23 foot.
When I bought my 1996 CP-19 the boat next to it was a 23 with a diesel in it. Pretty rare.

...I just grabbed a book off the shelve, flipped a few pages to find what I was looking for-->"Looking ahead to times when Guba II would land and maneuver on unknown lakes, Archbold had a nine-horsepower outboard motor installed in a rear tunnel hatch. This gave the big plane the ability to slowly approach the shoreline with it's Pratt & Whitneys switched off, to turn on a dime, and even to back up..."<-- PBY: The Catalina Flying Boat, Roscoe Creed, author. The PBY built in 1935 weighs empty 21,000 lb. and 9 hp is good enough to move that aeroplane.

skip.

[skip1930]

The common question. " What's the maximum horsepower I can use on my boat? "

On single hulled vessels simply take your boats length [not including pulpits, transom brackets, swim platforms and the like] and multiply it by her beam at the transom. [Don't use maximium beam at mid ship]. An 11 foot dinghy with a 3'-3" beam at the transom is 35.75 square foot. Say 36 square foot. For a displacement boat this gives a 5 hp [3.7 kw] which would drive her at a good 4.5 knots. On my CP-19 with the 5 hp two cycle, I can see 4.5 knots and sometimes almost 5 knot. With the sails and the motor 6.3 knots is possible.

I spared us of the tedious calculations, I found a chart. If it is too small hold Ctrl key down and tap + to enlarge chart.

skip.



For maximum people on board= length of boat x beam divided by 15.

[skip1930]

Other common question; " What size prop? "

Many people don't realize that the propeller's barely a hundred years old. John Ericsson~of Monitor fame~made one of the first attempts at using propellers instead of paddle wheels. At the time primitive propellers were about half efficient as paddle wheels. But at least they didn't get shot away or rammed to pieces.

Your prop will drive forward a certain distance for each complete turn. If it moves forward 10 inches, it has a 10 inch pitch. Since the prop is attached to a shaft and thrust bearing, it moves forward 10 inches, and pushes against the hull and that moves the boat forward 10 inches. A excellent reason for strong engine mounts and engine/mount beds.

Prop blades are twisted as you look at them on edge. When rotating say a 16 inch prop the inner part close to the hub travels 11 inches, the outer circumference at the tip travels 50 inches. You don't want the tip to be racing ahead of the inner part of the prop so you have to give the tips an angle so that the tip ends up at the same place as the blade root every turn. Carring this principle all the way down the length of the blade to the root gives props a typical looking twist. Remember that the pitch of a prop is not the same as its blade angles. These angles vary all along the length of the blade to keep the pitch constant.

Propeller blades when driving the boat forward and rotating clockwise when viewed from the stern is right-hand, counter clockwise when viewed from the stern is left-hand.

Propeller blades are narrowest at the root, and broadest about 2/3 of the way out. The amount of blade area that can be driven by a given power is limited so you have to distribute the area where it will do the most work. [I have wondered for years why engines don't have an automatic variable speed automotive type transmission attached to them driving a propeller. Power is limited.]

How many blades? Best is one! only because there are no other blades distributing the water flow ahead of it. But they can not be balanced. Two blades are reduse drag but need huge diameter to get all the blade to drive efficiently. So its three blade is the logical choice. This configuration has proved best for balance, area, and efficiency. Four blades yield more area with less diameter but the extra blade makes things closer together and scramble up each other's water flow. Efficiency drops.

Less blades reduce vibration. A three blade at 1000 rpm passes under the hull or strut 3000 times a minute or 50 times a second. A four blade at 1000 rpm makes 4,000 'bangs' a second or 66 cycles of vibration.

more later.





working on the answer

[Shawn]

"What is the proper prop size for a 9.9 on a cp23?"

I would think a 6 or 7" pitch prop would be about right. The 6hp Tohatsu Sailpro prop is a 6" pitch. See if there is a "high thrust" version of whatever engine you have and grab the prop it uses.

Shawn

[LConrad]

PropCalc suggested 8.6 Diameter, 5.5 Pitch for a three blade and 8.1 D and 5.4 P at 5,700 RPM 4 blade using my assumptions on the two stroke.  It also showed 9.8 horsepower is more than the boat needs, but that is what came with the boat. I chose the 8.7 D, 5P four blade 3B2W645110 high thrust and am happy with it. 

I am not sure about the gear ratio of the 4 stroke, but shorter pitch is better for displacement hulls. The short pitch allowed the boat to go slower at idle speed - minimum was too fast with a long pitch.  Acceleration is better with short pitch when you need to avoid somebody's navigational mistake.  RPMs are a little high wide open, but there is little reason to do full throttle.  9.8 HP is more than we need for the hull speed of the boat, so trying to go faster wastes lots of power and fuel.  I back off a little and don't see much difference in boat speed.  The short pitch has been also helpful pushing into a heavy wind and when towing racers to the course. 

[jcatkeson]

My C23 came with a 8 hp Honda. Aside from being overpowered, it does affect the balance. With the rudder lifted as high as it will go a portion remains in the water. When the motor is removed the rudder blade clears the water by about four to five inches. No idea as to how it affects the balance when under sail but it must do so.

[gfspencer]

I ended up with the Tohatsu 6 hp Four-Stroke Outboard (Sail Pro).  Will be shaking it down tomorrow.

How did that motor work out for you?  I'm thinking about getting one.