prop pitch question. Xpress H50, Yamaha 50hp, need to gain 800-900 rpm!

[squealer]

Little 16' Xpress H50 with Yamaha 50 hp 2 stroke I bought has an 11"D x 15"P stainless prop, that has been cupped a little and worked on some, so there is actually no real way to tell what it actually is any more. Motor will only turn 4500rpm with my son and I (thats an easy 500 lbs!), 3 big batteries in back, and no water in livewell. Motor should run 5500 RPM, I would like to get there with just my son (240), so I looking to get another 800 - 900 RPM. Rule of thumb is 200 RPM per inch of pitch, which says I need and 11" pitch prop. My question is really more about the "rule of thumb". If I am trying to change RPM this much, is it still pretty accurate? Most of the calculators I have used are steering me toward a 12"P. Would be nice to get it right the first time if possible...

Also, want to keep porpoising to a minimum. It is OK now, but I think some props will add bow lift and I know this little boat is pretty notorious for porpoising. Do you think I will have more of a problem if I get the boat to the correct RPM by changing props?

Thanks!!

 

[steelespike]

Re: prop pitch question. Xpress H50, Yamaha 50hp, need to gain 800-900 rpm!

What is the speed at 4500, By gps if possible?I wouldn't be surprised if it porpoised with 3 big batteries in the back.
Probably slow to plane as well.
Tell us about your boat. Weight,tiller or remote?etc.Year of the boat and motor.
Near as I can tell the dry weight is about 700lbs But I couldn't find w weight for a H50 in particular.
Further research indicates a 12" pitch should be close to what you need.Assuming there isn't some underlying fault for your poor performance.
You will likely pick up some speed and the prop will have more effect on the boat.
So very likely you will need to move some of that battery weight forward.
As far as the rule of thumb it should hold pretty true though but keep in mind 200 rpm per inch
is not carved in stone.

 

[squealer]

Re: prop pitch question. Xpress H50, Yamaha 50hp, need to gain 800-900 rpm!

Probably should have included that info!! Guy I bought the boat from said it would run 35...LOL. The speedo does indeed say 35, but it is probably running 28-30 (diidnt have the GPS on me). With our fat butts in the boat, it would not plane at all (its a long story, but there are other suspected motor problems going on). The boat had a whale tail on it, but I took that off while I am troubleshooting everything, and will leave off if I can get away with it. I believe with the tail it will plane, but that is not the answer I am looking for. The prop was grossly over pitched for this little boat, and I think that the correct prop may actually help or fix some of the motor issues I am experiencing.
With all the weight in the boat and the fact that it was only getting to 4500rpm, I am actually impressed by the speed. If I can get 29-31 out of it at the correct rpm, I will be tickled. 12"P seems the norm, but with us big guys, a 15 gallon fuel tank, the 3 big (27 & 29 series) batteries, I am wondering if shoud go with an 11". I plugged in the numbers on the Mercury prop finder site, and included all the weights, and it still said 12" though.

 

[steelespike]

Re: prop pitch question. Xpress H50, Yamaha 50hp, need to gain 800-900 rpm!

Yamaha tests; G3 1782 lbs all up weight 12" al prop 6.51 to plane,Express, 1679 lbs 12" al prop 4.22 to plane,Starcraft 15'11" 1560 12"al prop 5.08 to plane. Assuming a 700 lb dry weight I get about 1679 all up. 15 gal fuel and 189 for the motor.
I think the rpm was about 5800 on these tests. So it would seem a 12" is the right size. Sure would help to get at least 2 of those batteries forward even if only in the mid section.Just stick em there for a test.
Things to check; Is it firing on all cylinders?Is the throttle opening all the way?Is there any water absorbed in the foam under the floor?How is the compression?

 

[jestor68]

Re: prop pitch question. Xpress H50, Yamaha 50hp, need to gain 800-900 rpm!

That 16 ft Express did 5625 rpm @ 30.9 mph using a 10-5/8 X 12 aluminum 3 blade Yamaha prop.

Your results should be similar if using a similar(aluminum) prop. I suspect your "reworked" SS prop is rending some unusual results. The extra cupping could make it act like a 16 or 17 pitch.

Interestingly; when I run your numbers through the Merc calculator, it comes up with a list of 4 blade props in 12 pitch; saying 5300 rpm @33 mph. Since you need a little more rpm, it looks like an 11 pitch is in order.

Solas Amita aluminum 4 blade props for your motor come in pitches ranging from 10 to 14 in one inch increments, so it shouldn't be hard to find the right one. Because Solas aluminum props are lightly cupped, I suggest the 11 pitch Amita 4.

 

[squealer]

Re: prop pitch question. Xpress H50, Yamaha 50hp, need to gain 800-900 rpm!

Well, I took a gamble and ordered a prop. I got a Michigan Wheel Apollo, in 10.5"D x 12"P. Was very tempted to get the 11"P, and I still have my doubts. Because the prop I had was cupped and had been worked on, I could not really rely on it being a 15", so I kind of averaged this based on actual prop and several of the online prop finders. Assuming the rule of thumb is reliable, (and the 15" prop I have is really a 15"), going down to a 12" should give me 600rpm, and dropping 1/2 in dia should add 150 or so). I am hoping that the cupping that the existing prop has will account for another 100, but I think the 12" will get me pretty close. I didn't want to get too much, as I would like to to be able to get around 5500 with the boat empty, and not go over that. I'll let you know how it works out!