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What to expect by switching props.

archerxx

archerxx

New member
I have 20 ft Pontoon with 75 hp Honda four stroke. It currently has 4 blade prop with 13 x 15 pitch. This gets me 5100 RPM and 20 to 21 mph at WOT with 3 people.

The 5100 is only at top of RPM range, however I was afraid to go to lower pitch in fear I might lose some speed.

I was thinking of going to 13 X 15 .. 3 blade prop. Does anyone know the differences I could encounter with 3 bade versus 4 blade prop.

Would that give me any differences as far as RPM ?
Would there be handling differences or vibration differences 3 versus 4 ?
 
If you go to the 13 x15 your rpms will go down and you will continue to bogg down your engine. Not good...

It appears that if you go to a 13 3/4 x 13, your rpms will go up...your take off will be better...and the speed will be just as good, if not better than now. Check out the following test report. You can decide how close it fits your situation.


View attachment ml.reraw21a.ptSMOKERCRAFT_2.pdf

Mike
 
In addition, it's better for your engine that your WOT rpm's are around 6000.

Addressing your other issue, I have never been able to detect any difference in vibration between a 3-blade and 4-blade prop.
 
? I probably have to re-prop also

I have a BF 90 year 2000 with 1100 hrs. Original owner. I dont know what my prop is my cdealer put it on in 2000 and its stainless.

I have my Honda on a 23' Tolman skiff. First couple seasons it turned up fine to 6000 rpm but each year I add more weight (deck, hardtop, more fuel etc)

Last trip we were somewhat heavy going offshore and had 80 gals fuel, #400 ice, and #200 bait / chunks for a tuna chunking trip.

Going out we could only make 19-20 kts, and coming in less most of the fuel, ice, bait, and a couple fish on board we hit 24-25kts.

Can this much difference in speed be due to weight?

details below

I think the delta outbound to coming in near the end of the trip was 70 gals fuel (430 lbs), 350 ice (kept #50 for one BFT, a 48" mahi, and some chic mahis), and (#200 bait).

The skiff is light about 1500 lbs and shallow deadrise (8 degrees aft, about 15 midship, and more forward) and 7.5 max beam, about 6 ft at waterline)

Used 69 gallons on 230 ocean miles
 
Yep - it could be due to the weight alone. But I have found over the years that variations in wind and tides have as much influence as variations in weight. The key to prop size is to make sure your WOT is near or at 6000 RPM. When you are there, you are getting your best efficiency and doing the most good for your motor.
 
I appreciate all the feedback. Wanted to share that in talking to a Honda mechanic he told me they put 4 blade props on pontoon boats because they cut into water better. A pontoon has issues with cavalation when loaded and nose gets heavy.
 
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