Getting the most MPH (prop question)

[DaRy0362]

Hello all,

First time poster, thanks in advance for replies. I have a 1996, 35hp, 3 cylinder Evinrude with tilt and trim. Currently running a 4 blade aluminum prop (guessing OMC...came with the motor when I bought it used) at 10.5x14. Runs approximately 28 MPH at 5800 RPM (shop manual states that WOT range is 5200-5800 RPM for this motor). Boat weighs approx 700lbs (Model: 1996 Sea Nymph TX165, 16' long), over 1000 lbs when full. I am trying to get 30 MPH or better at WOT, and was thinking about trying a 3 blade aluminum prop to accomplish this. Will a 3 blade prop get the most MPH at WOT? Any recommendations on what type of prop will get me the most MPH at WOT? Thanks!

 

[steelespike]

Re: Getting the most MPH (prop question)

While a 4 blade should help hole shot and midrange performance there is only one prop listed here with higher pitch.
It surprises me that the 4bld.14 will turn 5800.And the calculations say 21% slip odd for a 4 blade. 6-12% more typical
We may want to verify the tach before any prop selection.You might borrow a shop tach or get something like a Tinytach .
It does seem that a 13 is probably closer to what that setup would use.
It appears that if the motor did 5800 with a 13" prop speed would be about 29.9.
The calculation doesn't take into consideration the weight of the boat.

 

[DaRy0362]

Re: Getting the most MPH (prop question)

Thanks for the info. I did install the tach myself...it's a teleflex model, and you're right, I have not verified the actual RPMs. When you say, "It does seem that a 13 is probably closer to what that setup would use", do you mean with a 3 blade or 4 blade prop?

 

[steelespike]

Re: Getting the most MPH (prop question)

I'm assuming your going to find the tach reading a ltitle high and your looking for more speed.
I was thinking 3 blade for speed, the reduction in pitch may help hole shot in place of the 4 blade.
But wait on the tach. before any prop decision.

 

[DaRy0362]

Re: Getting the most MPH (prop question)

Steelespike-Got the Tinytach in the other day, and was able to try it out on the boat. Took three different trial runs: 1)27.1 MPH WOT (clocked on GPS) @ 5590 RPM, 2) 27 MPH WOT @ 5600 RPM, 3) 27 MPH WOT @ 5590RPM. The lake was like glass, and there was little, if any, wind, and the motor was trimmed to hit maximum MPH. What would you recommend for a prop to get the most MPH?

 

[DaRy0362]

Re: Getting the most MPH (prop question)

Forgot to mention that the boat had a normal load at the time of the trial runs-myself (200lbs) and 100lbs of equipment, so total weight 1000 lbs. I occasionally have someone fishing with me, so running weight may be pushed to 1200-1300lbs during those times.

 

[steelespike]

Re: Getting the most MPH (prop question)

I don't think we discussed motor height.
The anti vent plate(just above the prop) should be about even with the bottom of the transom.
If you have no venting issues you should be able to raise the motor. I always say to a point just before venting becomes a problem.Its not unusual to be high enough to have to trim down for a serious turn.
Your Speed and rpm were pretty consistent.Slip indicated 21% It would seem that a 3 blade 13" pitch would put you close to 5800
and my improve the slip.If we could get 10%; speed would be about 29 15% about 28+.

 

[DaRy0362]

Re: Getting the most MPH (prop question)

The anti vent plate is aligned with the bottom of the boat. I usually trim it to get the most RPM, but of course if I trim just beyond this point the prop starts to ventilate. I previously had the motor mounted higher than I do now, but I think I had a bow planing problem at full speed...so I lowered the engine roughly 2 inches. It seemed to have helped.

I see your reasoning here, and it makes sense. I do have a question though: Most of the props I've seen (including on this website) are around 10" in diameter, not 10.5" like my current prop. Could changing to a 10" x13" may increase the RPM's above the recommended RPM range because of the change in pitch AND the relatively large difference in prop diameter?

The iboats propeller store info says that changing from a 4-3 blade prop, one should increase 1" in pitch. What if I got a 10x15? I would hit in RPM range I am hitting now (~5600RPM), but the change in blade diameter (10.5" to 10") may increase my RPMs slightly above 5600, say around 5700. Thoughts? Any idea why the prop may be slipping so much?

 

[steelespike]

Re: Getting the most MPH (prop question)

I had forgotten about the 1" going back to a 3 blade as we usually are going the other way.
Your logic makes sense.
But I am puzzled a 15" prop is the highest pitch available here.It doesn't seem that you would be using a 15 to push
900 to 1000 lbs. And expect it to turn close 5600 to 5700.
My 50 Evinrude only uses a 19" to push 1000 lbs. 18ft bare utility, motor and me.
Do you have good hole shot? Can you reduce the throttle and it's till easy to stay planed?
does it seem to handle adding weight well?
If it does then maybe a 3 blade 15 will work.

 

[DaRy0362]

Re: Getting the most MPH (prop question)

I'm not entirely sure about my hole shot, I never really timed it. I can say it goes relatively fast from 0 to 27 MPH. It can definitely stay on plane at lower speeds, and I know that with another person in the boat+gear the top end speed doesn't change. It really performs well with little or lots of weight. This motor is an oddball because it has three cylinders, I really have trouble finding information about it...doesn't seem like very many people own this type of motor. May explain the slip issue and higher pitch prop usage.

Next question-would stainless make a difference here instead of aluminum? I assume the prop doesn't flex much since it is such a small HP motor, and that stainless wouldn't have that much of an added benefit, but I could be wrong. I figured on buying aluminum first since its cheap, to try and find the right pitch and diameter combo before I went and blow my money on stainless.

 

[steelespike]

Re: Getting the most MPH (prop question)

From your description it seems the prop and motor are comfortable.
Going from 4 to 3 should cushion the increase in pitch as far as wot rpm is concerned.
A 15 May slow hole shot a little and weight may slow speed some.But hopefully you will be starting from a higher speed.
As far as a ss prop its results will partially depend on your success with the al prop.

 

[DaRy0362]

Re: Getting the most MPH (prop question)

Awesome, thanks steelespike. I'll post results when I get the prop in a few weeks.

 

[DaRy0362]

Re: Getting the most MPH (prop question)

Got the new prop in (3 blade 10"x15") and went 28.1MPH at 5740 RPM, so about +1 MPH increase. Could feel the difference in holeshot. Using the propeller calculator, I am getting 25% slip, a bit higher than my previous 4 blade prop (10.5"x14", 21%). Guess I want to know, is this amount of slip normal for this outboard+boat setup? Also, I feel like my current boat setup (see first post) should be going faster than it is now...does anyone feel the same about that notion? I do have a lot of weight in the back of the boat (3 batteries, onboard battery charger and 6 gallons of gas, altogether about 200lbs in the back), and I am going to move heavy objects around to see how the boat performs. Could this weight be an issue, and how would it effect my boats performace? Also, I've posted a pic of the anti-ventilation plate, to see if anyone has an opinion of whether or not its in the right location.

 

[steelespike]

Re: Getting the most MPH (prop question)

You said it seemed to help when you lowered it. With the high slip lets lower it another hole or half a hole.
A heavy boat in relation to the motor might produce higher slip.Though 25 seems excessive.
Three batteries,6 gallons of gas and the motor and yourself thats about 500 lbs in the stern that's about half your total
weight.It would seem that getting some of that forward should help even if just the hole shot.
Maybe move the gas and 2 batteries forward for a test maybe midway then various spots.
Any changes do one at a time and test.If you move stuff forward it may make the motor act like it was raised.