Correct propeller pitch for 10 hp kicker

Ken O

Hello,

I've acquired a 9.8 Nissan/Tohatsu 4-stroke for use as the kicker on my C-22 Cruiser. The prop and spare that came with it are identical: Diameter 8.5, 3-blade, Pitch: 9. The original owner used it to push an inflatable. Does this sound like an OK prop for use as a kicker on a C-22? Thanks!

DayBreak · Posted: Sat Sep 26, 2020 7:32 pm Post subject:

I am concerned that you may be lugging your kicker with a 9 inch prop at full throttle. If you are just using the kicker for slow troll it may be OK. Check with the manufacturer on your motor for prop recommendations. Go with the lowest pitch 3 blade prop (maybe 7 inch?) that you can go with for your motor. A small motor trying to push a large vessel at full throttle like a C-Dory should have a smaller pitch prop to help keep the rpm's in the upper range.

Ken O

Thanks Gary.

Using the calculator at

http://www.csgnetwork.com/marinepropcalc.html

I input this info:
Speed: 6.1 mph (C-22 hull speed)
Gear Ratio: 2.08:1
Prop Pitch: 5 (lowest I can find at Tohatsu)
Slip: 30% (a guess)

And I get 3828 RPM. The 9.8 owner's manual says that the motor tops out between 5K and 6K. I'll go for 5K. 3828 RPM still sounds a bit low. Even though we will use the motor for occasional trolling, the main purpose is as a backup to move the boat if the main engine fails. Should I look for a lower pitch, or does this sound good enough?

DayBreak · Posted: Sun Sep 27, 2020 6:55 pm Post subject:

3828 rpm is still pretty low for a constant full throttle operation. If the 5 inch Tohatsu prop is what they have listed for the model of your motor then go with that. You will definately get closer to that 5 to 6K range than with the 9 inch prop. Check the full-throttle operation speed out on your current prop and compare it to the speed under the same water conditions with the 5 inch prop and give us a report. I have a feeling both props will push your C-Dory at about the same speed but you will not harm the motor as much with long-term use using the lower pitch prop.

thataway · Posted: Sun Sep 27, 2020 10:06 pm Post subject:

Props are relatively cheap. Go for the 5" to 6" with the largest diameter which is available. You have to allow 10% tip clearance. Any of the smaller props are like "egg beaters"--but that is what is available. A 4 blade would be better if you can find it.

Looking at what is available there are some which are 8.9" in diameter, in 6" pitch. Also a 7" pitch with 4 blades, but you will not get the RPM with that...in the $70 or less. range.

A bit more expensive Quicksilver 8.7" diameter 4. blade 5" pitch for about $112. I would probably go for that one.

Ken O · Posted: Sun Sep 27, 2020 11:29 pm Post subject:

Thanks guys. I see that Tohatsu makes a prop with those specs:

Tohatsu Part No. 3B2B645110 PROPELLER (8.7d X 5p) 4-BLADE HIGH THRUST

for $72. I have choices.

DayBreak · Posted: Mon Sep 28, 2020 6:56 pm Post subject:

Dr. Bob makes an important point in keeping a 10% tip clearance. Our Suzuki 9.9 doesn't list the 4 blade prop as an option for us from the manufacturer. As a result, we went with the lowest pitch 3 blade prop. A 4 blade prop will typically have a larger diameter, so be careful in that you are able use a 4 blade prop recommended by Tohatsu for your specific model.

Ken O · Posted: Mon Sep 28, 2020 7:12 pm Post subject:

Thanks. By 10% clearance do we mean 10% of the prop diameter, or 10% of the distance from shaft center to tip of skeg? If I measure from shaft center to the bottom of the anti-cavitation plate, I get 4 5/8". That would mean that this 8.7" prop would have close to 9/16" clearance from blade tip to plate. Would that work?

thataway · Posted: Mon Sep 28, 2020 9:08 pm Post subject:

Shaft center to bottom of antiventillation plate. Double check the clearance.. There are some difference in various model year blocks. Like prior to 2005, after 2005 etc...