Another prop ??

[WaveWacker]

Great post in the past on prop/pitch questions. Here is another quick one. A friend of mine is running an older 1890 Lund Pro V (Just had it refurbed, it looks great), and has that coupled with a new Yamaha 115hp 4 stroke. I believe he is running a 13" pitch Alum. prop on it (not sure of dia.). I believe the suggested "high end" rpm's is around the 5000-6000 range and he feels that it is over rev-ing as it is running over 6,000rpm when opened up (about 6300 I would guess). He also gas the same pitch SS prop. From what I can remember last fall the SS had even higher rpm's. That makes sense to me because there would be less flex in the SS, right? If a person is running to high rpm's with a 13" pitch Alum. prop would going to a 15" pitch SS be right or would one have to go even more (17") to get the high end rpm's to be right?

[upnorth]

An 1890, I would say that this a older one

The rule of thumb for pitch is, for every two inches of pitch increase you will loose 200 rpm. If he is running a 13 on that combo and getting 6300 rpm, I would really consider a 17 or maybe a 19 inch pitch.

I have a 70 on the back of a 17 foot crestliner and am pulling 5300 with a 17" pitch prop and that seems to just about right. Comes out of the water and on plane right now and cruises to 35 mph without any bogging or lugging.

[wastewaterguru]

The 115 four strke yamaha is supposed to operate at 6000 rpm at wide open throttle. I am actually surprised the stainless has higher rpm. Check the diameter of the two props and I bet the stainless has a smaller diameter but same pitch.

All other things equal a stainless should run lower rpm then an aluminum because of the increased cupping and greater resistance due to the fact that the blades don't flatten under heavy load like aluminum does.

Based on the info, I think a 15 would be fine. You want to get the 6000 rpm's at full throttle with the boat loaded as it would normally be...(full tank of gas, 3 or 4 people, gear, etc.)

Otherwise, lets say you get a 17 and run 5900 rpm at full throttle with just one guy in the boat and no gear at a faster top speed than the 15......All is good right.

Then you fill the tank and add 2 or three guys to the boat and suddenly at full throttle you can only get 5000 rpm and the boat doesn't plane out worth a crud.

You just have to weigh your options.

[WaveWacker]

Oh I see, it is a matter of the alum flattening out more and not "grabbing" as much water. I was under the impression that the alum. would "open up" an grab more water at higher rpm's. I may be wrong on the SS running higher. I'll have to check with him on the diameter's. Makes perfect sense. I think what he is looking for is to not over rev. under normal weight conditions but still not go too large of a pitch that it speeds up his min. troll speed. Keep any more imputs coming in. Thanks.

WW

[Starman]

Wavewacker,
I also have a new Yamaha 115hp Four stroke. It's fitted with a Hustler 13.25" with a 17" pitch. I posted similar questions regarding dia. and pitch. Yamaha-motor.com under the accessories section answered a lot of my basic questions. They have a nice chart that shows some average speeds you can expect with different sizes and pitches of props. However I have yet to find out for sure if you can lower the dia. of a prop while increasing the pitch to try and increase top end speed. Or if that defeats the purpose. Anyway, just wanted to pass the website along.
STARMAN*