20hp Suzuki EFI Prop Question

[SmoothOperator]

Hello all, I just got through the break in with my new Suzuki on my 16' 72" beam semi-v hull alumacraft.

Here are my performance #s 20mph 5400 rpm 11"pitch 3 blade aluminum prop 2.08 gear ratio 26% slip if my calculations are correct.

Motor is mounted on a Vance mfg breakaway plate 3" rise 6" setback.

What can I do for better performance? Less 9 or 10 pitch 4 blade or 10" pitch 3 blade?

26% slip is a lot of slip correct?

 

[SmoothOperator]

I forgot to add a few bits of information in my first post above.

The Ideal WOT RPM is 5300 to 6300 for the 20hp Suzuki EFI.

I'd like to see if I can bump up my WOT RPM from 5400 and reduce prop slip percentage while not losing any of the top end speed.

The boat does realy well fully loaded with two people, gear and the live well full.
It gets on plane easily after about 4500 rpm and skips along after that.

Please advise what I should do if anything or if I should just leave it as it is.

Thank you.

 

[Sea Rider]

What's the max HP that boat can handle ? If OB is underpowered can go for a prop maximization to say a 3 blade 9 pitch prop to pull wot revs towards max wot range. With current prop combo is only 100 wot revs over min factory wot rpm range which is bad. If you add more weight rpm will enter the extreme lugging side. Too much mechanical stress for the OB.

Will be a wild guess guessing how much wot rpm OB will gain as loaded going for 1 less pitch. Could be any number from 200 to 500 wot revs per 1 less pitch increase.. As the combo planes fast at reduced throttle, don't need a 4 blade prop.

Going full revs will better hole shot with slight less top end speed loss. OTOH if running middle wot revs will achieve slight better top end speed with less hole shot, but much better performance than with current 11 pitch prop on both scenarios.

You decide going max or middle wot revs as loaded...

Happy Boating

 

[SmoothOperator]

Thanks for your reply,
The boat is rated for up to a 60hp outboard.

From your post above I would say my next test would be to go from a 11" pitch 3 blade prop to a 10" pitch 3 blade prop and see how the RPMs respond as well as the top speed.

 

[Sea Rider]

What's the boat's weight and max number of souls on board ? Depending on your answer...

A 10 pitch prop is too close to a eleven one. OB is lacking 900 wot revs from max wot rpm range to safely play with. A 10 would probably dial around middle wot rpm range if like going that route. If like going per increasing more rpm range a 9 pitch is better.

Be aware that you're runing 1/3 of the max rated HP-OB transom rated for, boating with a near fixed load on board all the time favours the combo performance.

Happy Boating

 

[SmoothOperator]

My Manufacturer's label plate says 4 persons max, 860lbs for persons gear and motor. 60hp Max.

The normal amount or souls on board is 2 as there isn't much room for more than two people casting at a time and I want to make sure everyone is safe and doesnt catch a hook to the face or worse.

Theres also two batteries on board a medium sized starter battery for the outboard and one Deep Cycle battery for the bow mount trolling motor.

Weight break down:
2 People- 400
Outboard- 100
Deep cycle- 40
Starting- 30
Gas tank 6gal- 50
Trolling motor- 35
Fishing gear- 40
Cooler w/ice- 40

I've tried to research on the boats dry weight and i think it might be around 900lbs.

Its the 16ft Pro Crappie model by Alumacraft.

Serial# ACBZ4251E899.

Hope this helps.

 

[Sea Rider]

Having 2 up and near a fixed loadi is much easier to dial best top prop. I guess you're not 100 % satisfied with the combo performance as of now and want to go for a prop maximization.

The issue is : want better top end speed or better hole shot ? for me best hole shot as fixed loaded and just losing 1-2 max MPH compared to a prop that achieves medium wot rpm range.

Happy Boating

 

[SmoothOperator]

So hypothetically what would the results be with the above data for the following props? RPM and Speed compared to my 9.25"D x 11"P - 3 blade Alum.

9.25"D x 10"P - 3 blade Alum.
9.25"D x 9"P - 4 blade Alum.
9.25"D x 9"P - 3 blade Alum.
9.25"D x 9"P - 3 blade Stainless

 

[Sea Rider]

Tough answer, only work with 3 blade alum props, why complicate myself with SS and 4 blade props. The issue is that if nobody lends test props you'll will be dumping lots of money into the deep blue buying props for testing.

Can say from experience maximizing prop pitches for different applications using under powered OB's is that if going for a 3 blade 10 pitch prop, current wot revs will increase way more than with 11 pitch.

If going for same 3 blade 9 pitch prop current wot revs will increase way more than achieved with 10 pitch. How much, impossible to know, would not dare go for the Rule of Thumb that states + 200 wot revs for every 1 pitch decrease. One pitch drcrease can dial anything from 200 to 500 + wot revs. Will depend on many factors as boat lenght, hull drag, overall load, HP, engine height and trim.

Have not answered yet which scenario you want to better : improve hole shot, or improve top end speed. BTW, can't have both worlds in one prop, it's a give or take issue, you decide.

Happy Boating

 

[SmoothOperator]

My answer to the question "Hole Shot or Top Speed?" My answer is top speed while taking advantage of the power band of the higher RPM range.

When using the Prop calculators I have some questions. Will the Prop slip percentage be reduced if the prop pitch is reduced but the RPMs are increased? Or is Prop slip constant with the same boat, load, and Engine HP?

 

[Sea Rider]

The issue here is that isn't known how much wot rpm will OB gain with say one less pitch as to bump rpm into the middle wot range or slight more to improve a bit top end speed.

Higher revs improves hole shot but lessens top end speed, less revs betters top end speed sacrificing best hole shot. Can't speak for prop calculators, don't ever use them. Being a Tohatsu junkie know excatly how much wot rpm will gain playing with just less pitch OEM props runing under powered OB's while powering light boats.

Personally would try a 10 pitch to improve top end speed and hole shot as well compared to current 11 pitch prop, just don't hallucinate thinking that you will turn your combo being a speed demon LOL!!

Happy Boating

 

[SmoothOperator]

Well, I ordered two props tonight... One each 9.25Dx10"P 3 Blade aluminum and one each 9.25Dx9"P 4 blade aluminum prop. I will keep you all updated with the results of both. Both new props are Solas brand and I am hopeful that they perform better than the Suzuki 9.25Dx11P 3 bladed aluminum prop that is on the motor now. With the 10"P 3 blade I am hoping for better hole shot and be higher in the optimum power band RPM range which may lead to higher Speed over all. With the 9"P 4 blade I will utilize if I have heavier loads than normal like a 3rd person or an extra cooler aboard. Wish me luck!

 

[Sea Rider]

Nice prop experimentation, report which one achieved better max wot rpm, hole shot, top end speed. Definitely will be much better than with current 11 pitch prop.

Happy Boating

 

[SmoothOperator]

Ok here are my results... Only loaded with myself aboard so it was not loaded as it normally would be.

Original: 11" pitch 3 blade = 5400-5500 rpm @ 20mph
New: 10" pitch 3 blade = 5800-6000 rpm @ 20mph
New: 9" pitch 4 blade = 6000+ rpm @ 9mph

I'm happy with the results with the 10" pitch 3 blade. It get on plane quicker and didn't lose any speed on the top end.
The 9" pitch 4 blade has me baffled at how terrible it did compared to the 11" and 10" pitch 3 blade props.
The 9" pitch 4 blade ventilated at even the lowest trim position.

 

[Sea Rider]

Interesting, with a 10 pitch prop OB has gained 400-450 wot revs with just one less in pitch. You are confirming that the rule of thumb doesn't apply for this combo and that most OB's factory delivered with a prop is usually over pitched. Next time run it at neutral trim along normal load evenly distributed on deck.

OEM 3 blade props performs very nice, don't know why boaters like going for 4 blade props without trying 3 blade ones playing with just pitches. Thanks for sharing..

Happy Boating