Propping a 20 HP honda for a Zodiac

[Jake Windemere]

I have a 2008 Honda 20 horse outboard (BF20DKSH). It came with a 9 1/4 x 10 (235 x 254 mm) four blade aluminum propeller.

I put the engine on a 14 foot Zodiac MK II Grand Raid inflatable.

The boat was slow out of the hole so I replaced the 9 1/4 x 10 prop with a Solas 9 1/4 x 9 aluminum prop.

The boat is still a bit of a dog out of the hole. It does okay with a light load but takes a while to get on step with a full load.

I have a Tiny Tach induction tachometer on the engine and I find that the engine tops out at about 4800 RPM and between 15 and 17 mph (load dependent) when running at WOT.

I'd like to tweak the system further but don't know where to invest. Do I put a three blade prop on the engine? Do I switch to stainless? Do I further reduce the pitch? Do I add a Stingray Hydrofoil? Should I try to get the top end (5500 RPM)?

What would you do?

 

[pvanv]

Re: Propping a 20 HP honda for a Zodiac

This may possibly be better answered in the Prop forum, but here's my $0.02...

Is the Solas 9 pitch also a 4-blade? If so, you may find that dropping down to 4-blade x 7 or 8 (because the HT props have less slippage), or a 3-blade x 9 will get you both better hole shot and allow more WOT RPM.

Sometimes, if in a slightly underpowered rig, a Solas High-Thrust 4-blade unit of suprising low pitch will help -- especially with hole shot. We have found that to be the case both on sailboats (under 10 knots) and Walker Bay Genesis 11-foot RIBs, when powered by Tohatsu 15's at up to about 18 knots.

FWIW, we have found that SS props act as though they are steeper-pitched, as they don't flex as much (if at all) during full thrust situations. I.E. a 12" SS prop of identical dimensions often behaves like a 14" aluminum model, especially in hole shot.

 

[Jake Windemere]

Re: Propping a 20 HP honda for a Zodiac

This may possibly be better answered in the Prop forum, but here's my $0.02...

Is the Solas 9 pitch also a 4-blade? If so, you may find that dropping down to 4-blade x 7 or 8 (because the HT props have less slippage), or a 3-blade x 9 will get you both better hole shot and allow more WOT RPM.

Oops, sorry about not using the prop forum. I have this forum bookmarked and usually do all of my reading here.

Yes, the Solas is a 4 blade.

I was wondering what performance advantage or disadvantage is afforded by using a 4 blade rather than a 3 blade (or vice versa).

Now, do I go "power thrust" 4 blade aluminum 10 x 7 5/8 inch pitch, 3 blade aluminum 9 1/4 x 8 inch pitch, 3 blade steel 9 1/4 x 8 pitch, or 3 blade steel 9 1/4 x 7 pitch?

 

[lepar5]

Re: Propping a 20 HP honda for a Zodiac

I have the same engine mount on a zodiac fastroller 3.40m (11.2 ft)
Your engine is running too low rpm. It is supposed to run at wot from 5000 to 6000 rpm. For example i have tried 4 different propellers on it

9.25x10 4 blades wot 5600 rpm speed 22 mph ok
9x10 3 blades wot 6200 rpm speed 21 mph but rpm to high
9.25x11 4 blades wot 5200 rpm speed 20-21 mph but poor acceleration
9.25x11 3 blades wot 5825 rpm speed 24.2 mph perfect for me
with the same pitch a 3 blades propeller give you higher rpm

hope it will help you.... sorry about my english it is not my language

 

[Jake Windemere]

Re: Propping a 20 HP honda for a Zodiac

Thanks. Any other ideas from anyone?

 

[Jake Windemere]

Re: Propping a 20 HP honda for a Zodiac

I discovered that a high thrust four blade aluminum prop is a step backward. The larger surface area of the four 10 inch blades reduced the slippage considerably. This loss of slippage prevented the engine from gaining RPMs until the boat was moving. The effect was like trying to start your car in second gear. Then once the boat finally got into the power band (torque band) I found the top end to be quite limited. I couldn't get much faster than 11 MPH at WOT. Dog out of the hole. Dog at cruising.

I realized that this little engine doesn't get up to power until the engine has some RPMs.

I then bought and installed a THREE BLADE 9 1/4 x 9 inch stainless prop.

That seems to be the best of both worlds. The prop slips enough when I nail the throttle from rest. First the engine accelerates then the boat comes up to speed with sufficient authority. I get a pretty decent top end with that prop. My hand held GPS wasn't working during my initial test run due to dead batteries, but the speed seems to be pretty good for a 20 horse.