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A little confused and hoping for some propeller help...

Oxbow26

New member
My boat Penn yan tournament 210 currently has a 15x17 alum 3 blade prop. It runs ok for a mildly heavy boat but I’ve heard others with the same boat claim they see 40mph out of their boat. I’m closer to 35 mph, however at WOT I’m teetering at just above 4100 rpm. I’m being told that I should be looking for closer to 4400 rpm. I’m considering a 15.75”x15” SS 3 blade prop.

recap: (have). 15x17, 4100 rpm, 35 mph

(considering). 15.75x15, rpm???, mph???

can anyone tell me if I’m heading in the right direction for slight increase in rpm and possibly top speed? i know that a change in pitch and a change in diameter have a certain affect but not sure if the change I want to make gets me a little of each benefit
 
fact-----Did a test years ago 1970 model 115 HP Evinrude.------Same 16' ski boat, same trim angle, same afternoon.-----all 3 blade same style stainless props.------19" pitch , great acceleration max RPM of 5800 and speed 54 MPH.------21" pitch, good acceleration max RPM 5400 and speed 54 MPH.----23" pitch poor acceleration max RPM 4900 and max speed 54 MPH -----Your results may vary.
 
I have the same engine/drive in a 1988 Four Winns H-200 which is about the same size (20’ 9”). Stock was an aluminum 15x17 and that gave me approx 4600 rpm, I recently went to a 15.5x15 aluminum and that is giving me approx 4900 at wide open throttle. These #s are with me alone in the boat so if loaded it will result in better performance the closer you get to the redline (5,000).
 
Thank you! And did you see a difference in acceleration or top speed? I mean this isn’t a ski boat so I’m not worried about going 40plus but I want to get whatever the boat is capable of. And like I said currently I’m getting roughly 4100 rpm at WOT that’s why I wanted to 2” drop in pitch to gain the Rpm’s and thought although slight, a 3/4” increase in diameter would help cancel out any lost speed I may trade with the pitch change...
 
Quicker acceleration and less strain on the engine pulling the boat up on plane, slightly lower top speed. Acceleration and reduced strain on the engine are more important to me than top speed. To prop so you are as close to the redline is actually better for engine life. Under propping (wide open rom too low) causes combustion chamber temps to rise and exhaust valves to burn.
The bottom line in my opinion boats this size should have a V8 for both good acceleration and top speed. With the V6 you do save gas but have to give up one thing (top speed) for another (acceleration and reduced engine strain),
 
The OEM has established a WOT RPM range for your boat/engine/drive.
WOT = wide open throttle.
This is the RPM range that the engine should be able to reach with the Engine healthy and in full tune, hull clean and with the boat loaded as you normally would, etc.

WOT RPM is a test RPM ONLY, and is not an RPM to be sustained for any duration, unless you want to shorten the life span of your engine.

I would suggest finding your OEM WOT RPM specs and follow them.

An engine that won't make WOT RPM will be over-burdened at any RPM.
An engine that makes above WOT RPM, will not be over-burdened, but it may not give you the speed that you may want.
An engine that makes the WOT RPM range, will be happy.


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For the 4.3 2bbl OMC listed 4200-4600 as the wot RPM
For the 4.3 4bbl OMC listed 4400-4800 as the wot RPM

I think for a boat your size, with with the 4.3 (keep in mind it is lacking in torque compared to a 5.7) I'd shoot for 4800-4900. You want a nice comfortable cruise RPM of about 3500 or so, if you go with the higher wot RPM, it gets you on plane easier and less strain on the engine. Like I said, the 4.3 is small for boats in the 20' and slightly larger size range. It is best mated to boats from 18-19'.
The 4bbl Quadrajet works very well on these engines, the small primaries give excellent fuel economy and the huge secondaries give power when you want/need it.
 
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