WOT Range For Prop Determination

[sc_shane]

Hey guys. I've got a 1991 25 foot pontoon with a 1989 90HP Yamaha 2 stroke on the back. I haven't been able to identify the prop that is on it, but I think I am on the right track (unfortunately I keep the boat in storage about an hour away and haven't had a chance to get down there in about 2 months). I think there are 5 numbers stamped on the prop itself, which, after a little research, makes sense. The OEM props part numbers have similair numbers on each end of a 5 number combonation (if that makes sense). Anywho, here's the deal.

The aluminum prop that is on there (which I believe is the original) has a small dent in one of the blades. I'd say about the size of your pinky fingernail, but it's probably smaller, and its right on the edge of the blade. There's probably a few very small knicks on this and maybe other blades too. I've owned the boat 2 years, and it runs like a top. If I remember correctly, current WOT is around 4400 RPMs, maybe less, with my average load, which is probably pretty heavy. She's not a speed demon, but she'll top out around 20-22mph. I'd like to get this number up if possible, but I do like to pull intertubes and kneeboarders and don't want to sacrifice performance for them.

What is the rated WOT for this engine? Looks like the most common OEMs are 3x13 with a 14 - 23 pitch. What do you guys think? Oh, and attached is a picture of the motor and prop, in case that might help. Thanks for your help and advice in advance.

 

[jay_merrill]

Re: WOT Range For Prop Determination

I don't know the specs for that engine but 4400 at WOT seems very low. I would think that it should be more like 5000 to 5500. If you have a manual on the motor, it should tell you what the acceptable range is.

 

[sc_shane]

Re: WOT Range For Prop Determination

I was thinking the same thing (low rpms at WOT). I know out of gear, she'll rev much higher. I just ordered a manual for it but it hasn't made it's way here yet.

One thing I was thinking is maybe because it's got the dent and a few knicks it's affecting performance and low WOT?

 

[jay_merrill]

Re: WOT Range For Prop Determination

Low rpm at WOT is more likely to be because you have then engine overpropped, than a result from minor nicks, etc. I think the acceptable rpm range for that motor at WOT is between 4500 and 5500. By running it under the recommended range, you are stressing the motor in much the same way that trying to drive a manual shift car slowly up a steep hill in high gear, would be.

Since rpm change is generally about 200 revs for each inch of prop pitch change, I would say that you need to go down in pitch by at least 3 to 4". I don't think you are going to get your pontoon boat to go alot faster, if any faster. And, while there are now pontoon boats with stabilizing strakes on the pontoons, which are meant to accept much larger engines and go much faster, your boat may not be one that will handle well at higher speeds.

 

[Silvertip]

Re: WOT Range For Prop Determination

Try a 13-5/8 x 13P Yamaha prop (part number 6E5-45949-00EL). Their tests of a 90HP motor on a 3000# 24 foot pontoon produces 23.3 MPH at 5400 RPM. You are about 1000 RPM under the recommended wide open throttle rpm range and severely lugging that engine.

 

[sc_shane]

Re: WOT Range For Prop Determination

Thanks for the info guys. I think I am going to see if I can cross reference the number stamped on the prop so I know what is on there now. That way I can move down in pitch 3 or 4. If I can't reference it, I may just go with your recommendation Silvertip.

I'm not that worried about going faster, but it's always an added plus when cruising over an hour to get back to the lauch! Ha! Thanks so much for the info guys. As always, your help is GREATLY appreciated!

 

[alanaexall]

Re: WOT Range For Prop Determination

I was thinking the same thing (low rpms at WOT). I know out of gear, she'll rev much higher.

rev it out of gear that high and something may go BANG!!!!!!!!!!!!!!!

 

[sc_shane]

Re: WOT Range For Prop Determination

Oh no - if she got above 4500 it was only for about 1/2 a second, tops!

 

[jay_merrill]

Re: WOT Range For Prop Determination

My guess is that you will find out that your current prop is a 17" pitch. I think Silvertip's advice is good, regardless of what is on it now. Your boat is probably very similar in weight and configuration to the one mentioned in the test, so it may be expected to perform similarly.

Check with your local Yammy dealer. He/she may have a used 13" wheel that they will let you try before buying a new one. As long as you come back with an undamaged loaner prop, most dealers are pretty good about letting a good customer try different pitch wheels to nail down what they need.

 

[sc_shane]

Re: WOT Range For Prop Determination

Sounds like a great idea - I'll call my mechanic tomorrow and see if he's up for it. I really appreciate everyone's help on this!