Prop help 115 yamaha

[MAKO72]

I have a new to me 1971 aquasport 19 with a 1995 yamaha 115. I am currently running a 19 pitch alum. I can not go to WOT as I will over rev. This boat is light around 1300 lb and has a very flat bottom almost no dead rise. At 5500 rpm I am only around 27 mph and the throttle still has a bit of room to go. Could I possibly need so much pitch 21 or 23? My worry is could my tach be wrong? The boat has an after market analog tach not the yamaha digital? Any thoughts? Do flat bottom boats need a lot of pitch? Thanks for any help.

 

[Silvertip]

Re: Prop help 115 yamaha

A short burst of WOT won't hurt as long as you don't get into the 6500 RPM territory but you can figure "roughly" that each inch of pitch increase will lower engine rpm 150 - 200 rpm at the same speed. The actual amount will depend on the new prop. Understand that a prop from manufacturer "A" will not and generally does not, perform exactly the same as a prop with the same pitch from manufacturer "B". Until you know what the "actual WOT" throttle RPM is, you won't know how much pitch to add.

 

[Dhadley]

Re: Prop help 115 yamaha

Ditto. It's important to know the exact top rpm with an average load. It sounds like the Turbo Lightning Jr in a 20 pitch might work pretty well. Too bad they quit making them.

I wonder what a Turbo 1 with Lightning style rake and blades would be worth in a 20 pitch with a Yamaha hub?

 

[DC20]

Re: Prop help 115 yamaha

When I purchased my boat it came with the wrong prop that someone had just stuck on there. I could hit the rev limiter. What I did was go to Yamaha's web site, find a boat that they tested similiar to mine with the same motor and see what prop they run.

 

[MAKO72]

Re: Prop help 115 yamaha

I ran the boat today and gave it a quick WOT and it reached around 6300 remember this is an analog tach. That only gave me a top speed of 31mph. Like I said the boat is light and I was alone I think I should be getting better speed.

 

[Silvertip]

Re: Prop help 115 yamaha

With a proper prop and proper setup it will.

 

[steelespike]

Re: Prop help 115 yamaha

I ran the boat today and gave it a quick WOT and it reached around 6300 remember this is an analog tach. That only gave me a top speed of 31mph. Like I said the boat is light and I was alone I think I should be getting better speed.

Usually a flatishbottom will get up easier and run faster than a deep V.
I don't know your motors max rated rpm I would guess about 5,500.
It appears you need to knock off about 800 rpm that would put you at
about 23".Fine tuning the vertical height could gain you a little more performance.The anti vent plate(just above the prop) even with the bottom
is considered a starting point If you have no ventilation issues you could surely raise the motor.

 

[Dhadley]

Re: Prop help 115 yamaha

Are you sure your tach is set correctly? Or is the prop worn considerably? A 19 or 20 is about right for that combo generally.

 

[MAKO72]

Re: Prop help 115 yamaha

Like I said I do have concerns with the tach since it is not a Yamaha tach but it is set on 6 p and seems to reading fine normal at idle. ?Could the tach be wrong at higher rpm?

 

[Dhadley]

Re: Prop help 115 yamaha

Yes, it can be close at lower readings and way off on higher readings. And vise-versa.

What condition is the prop in?