Prop decision help?

[Dukester]

My 1986 20' Osprey is powered with a 2002 Yamaha 115 four stroke.  I have to check out the current prop pitch on her now but it is not a SS prop and I want to replace it with one.  My question is based on the following observations.  With two people on board I run about 31 mph (gps) at about 5600 - 5800 rpm and that is not wide open.  It gets on plane really well with the hydrafoil installed.  What would one suggest in prop pitch to get better speed or performance out of her?  Once I check the current prop pitch (which I believe it is a 19 pitch) should I go up or down or with a SS prop stay the same?  I can't afford purchasing three props to find out the best performance.  Thanks in advance for your help!

[Aswaff400]

im running a13x17" ss prop on my 112 johnnyrude and WOT i reach 38 mph @ 5800rpm. sounds like you may have a 15" pitch prop some boat dealers/service shops may let you borrow a prop or 2 to test and tune...

[MarshMarlowe196]

You guys actually have about the same gear ratio in your engines -

2.15:1

Hard to think you could be running a 19" prop and still be capable of doing more than 5800-6000 RPM's with that motor.  Aswaff is probably right- sounds more like a 14-15" prop.

Fishcole has a F115 on his 19-6 (same hull) and runs in the high 30's/low 40's with a 17" prop.  May want to PM him.

[Dukester]

Thank you!  I'll have to pull the prop when I get home and check it to see.  I'm sure I should be getting more speed out of my boat with the right prop.  Now I'm really curious and will have to check mine out and will let you know.

[Dukester]

Checked the prop and it is a 13 X 17 aluminum prop.  Should I go to a 19 SS prop or stay with a 17 in a SS prop?  Hmmmmm.

[MarshMarlowe196]

When I had a 17" prop, I had about the same performance as you around 5800RPM's.  I'm running a 21" now and @ 5800RPM's, I do over 40mph.

Based on that, I could say you should go with a 21" prop, but there's really no way to be sure what your performance will be like until you try it.

The ideal prop will not allow your motor to do over 5800-5900RPM's WOT.  With my 21", trimmed out, I can not go over 5900RPM's WOT.  

Hopefully, as Aswaff mentioned, you can find a shop that will allow you to try a 19" and a 21".  The 21" is perfect for me, but I'm running more HP and a different gear ratio.

[hwsiii]

Bad could you tell me which exact make, model and diameter of 17" and 21" props you had on your boat that run the same WOT 5,800 RPM. I am curious as to the differences in the blade geometry that caused this.


H

[MarshMarlowe196]

Bad could you tell me which exact make, model and diameter of 17" and 21" props you had on your boat that run the same WOT 5,800 RPM. I am curious as to the differences in the blade geometry that caused this.


H

I'm not quite sure I understand the question...

The 17" prop I had on my DF140 would run the boat around 30MPH @ 5800-5900RPM's, but that was not WOT, as I could push the motor over 6000RPM's w/ the 17" prop.  The motor shuts down quickly once you start creeping over 6K RPM's, so I'm not really sure how many more RPM's I could have turned w/ the 17".

The 21" I have now runs the boat over 40mph @ 5800-5900 RPM's, and I cannot turn any more RPM's than this.  The 21" has proven to be the sweet spot for my rig.

So, if you're asking if I had the same performance w/ the 17" as I do the 21", then no I don't.

I'm not sure the make and diameter; the pitch is really what makes a difference.  I know that I had to go with a Suzuki prop (A Johnzuki prop is what I actually have) because that's what would fit my outboard, and happened to be what was lying around the KW warehouse.