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Prop for BF20 D. 4 blade to 3 blade Q?

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nzbay

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Have a BF 20 on a 16ft plywood dory. Prop is the standard 9.25 x 10.

I get 5000rpm at WOT.

It takes noting to get the flat hull plaining.

I was wondering if I would be better off with a 3 blade prop for more top end speed possibly?

If so, what Solas prop should I be looking at? Is it just a matter of getting a 9.25 x 10 prop of should I change pitch also?
 
Sorry, cant find the edit button.

There is a part in the specification area of my manual that says my prop is a 9.25 x 8 pitch.. (not 9.25 x 10 above). BUT, further down the manual (page 426) it says the standard that comes with my model is 9.25 x 10. Little confusing.

Am i correct in saying that an 11" pitch is coarser than a 10" pitch?

Recommended WOT 5000-6000 rpm. I am aiming for 5500 rpm at WOT.
 
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I work with this motor and the last time I checked, 9.25 X 10 was indeed stock. I'm guessing the 8 pitch reference is a typo. I say "guessing" because I don't have the documentation with me at the moment.

I've tried both the 4 and 3 blade versions Honda has for that outboard and don't see much difference between the two at all.

Pitch is defined as the distance, in inches, the propeller will advance through the water while turning one revolution. This is subject to load and water density of course. But, generally speaking, yes, the 11 pitch is more coarse than the 10 and, theoretically, should provide a bit more "thrust" for the same RPM and that "should" give more speed over a given distance. I'm no guru here and the key words are "theoretical" and "should".

I think you could achieve more RPM and possibly more speed with a SMALLER DIAMETER prop. But, there is no "free lunch" as they say. What you gain in RPM and possible top speed you may lose in "bite" or "hole shot" and ease of planing the hull. It's all a trade off at best and finding a "sweet spot" for prop size and pitch matched with hull and horsepower is a trial and error sort of game.

The other part of the equation is the limited power you are working with. With the 20, you just don't have much to begin with and a BIG gain in speed might only be 3 or 4 MPH. When you consider the money a new prop costs, you have to decide if the money is worth the small gain available.

On the other hand, it is ALWAYS good to carry a spare prop on board so if you wanted to try a change, even if you don't meet your goal, you end up with a spare prop that could get you out of a jam someday.
 
Thanks for the reply.

I will tinker a bit more with trim also when I get the chance. If 5000rpm is the most I can get at WOT I think that means I have to go finer pitch, maybe 2 inches to achieve another 400 rpm roughly.

All very interesting stuff.
 
Have tried this setup out again, adjusting the trim. I have trimmed in up the the second hole from the transom on the mount.

I still cant acheive 5500rpm at WOT and engine feels like its lugging with 2 people and full tank of gas on baord. Max I can get is 5000 rpm.

I have read that on small engines (up to 20hp) reducing the pitch has a greater impact on rpm than larger engines. More like 10%. This a 4 blade 9.25x9 should give me another 500rpm.

Are the Solas 4 blade and Honda 4 blade the same prop? I have also read the Solas make Honda blades. Would like to use a Solas blade as they are priced right.
 
The solas and Honda 4 blades are different part numbers. I do not know is there is any difference in design. Have used solas to replace several OEM props on this size motor. Customers have not noticed any difference in performance.

Just one size difference can make a lot of difference. I would try the 9 like you are considering. Then experiment again with different trim angles on the motor.

Mike
 
I am going to try a 3 blade Solas 9.25x8". I could get one at short notice to try out. This was from a propeller spe******t who sells Solas blades. If this bring it up to close to 6000 rpm I will prob stick with it. He didn't have a 3 blade 9.25 x 9" this time of year, would have to wait until people get back to work next week. He was not a fan of 4 blade props in general, particularly for small motors. He did say the Solas 3 blade do have quite are large surface area by comparison to other 3 blade brands and added a 9" pitch may only give me 500 rpm extra. We will see.

It is just going to take some time to sort this which isn't a problem (it is actually quite interesting). I knew when I built the boat I would be right on the edge of performance with a 20, but I was aiming for economy. I will get there yet.
 
Rules of thumb....

Typically a three blade will propel a boat faster than a four blade...

Typically a four blade will get lift the back end to help planning....

Keep in mind...in your test that you are making two changes. # of blades and pitch.

It will be hard to actually tell what did what....but if you get lucky with the results you want, that is good.

Mike
 
hondadude said:
Sometimes it is better to be lucky than to be good!

Let us know what happens.

Mike
Click to expand...


I was not lucky.

Engine went over 6K rpm then tach needle dropped down to below 5K. I am thinking there is a rev limiter of the engine or the over reving gave false pulses maybe. In any event, over 6K with the 3 x 9.25x8" prop. Speed was low too.

I have sent prop back to exchange/re pitch for a 9" pitch. Also ordered a 4 blade 9.25x9" Solas which is possibly what I could have done to start with.

Interesting.
 
4 blade 9.25 x 9" goes the best of the 4 props I hav tried.

Gives 21-22mph about 5400rpm. Mod range is also good, stays on plane.

3 blade props tried are okay top end but if I button off boat slows. 4 blade give better speed at 4500rpm which is where I run it most, 3 blade is poor speed at that rpm area.

So, 4 blade it is.

Tried different height setting and found out I was spot on the way I had it first off.
 
Wow! That is some pretty thorough experimentation! Thanks for the report. Glad you found your best "compromise".
 
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