Re: Prop gurus please help
I am on the road this week for work but I will try to help the best I can.
You are not going to see much of any more speed. The load of your boat and the design of the hull are your limits.
How I know this is by your current pitch and RPM. So lets do the math the simple way
RPM/2.42 (your gear ratio)=Y
Y* Pitch of prop /12/5280 *60= your speed
The rule on the water is this
To Double your Speed you Need Four times the Horse Power
That does not mean that if your boat is rated for a 90 hp and you drop a 135 on it that you might not see twice the speed. Why, because your boat is now illegal and very unsafe.
Bass boats that run 60 MPH are running in most cases 150 HP or more. They are able to turn a 24 pitch prop because of Horse Power and gear Ratio. In most cases, that I have experience with, as the Horse Power goes up the Gear Ratio can go down, but there is a limit.
Other boat motor companies that use lower ratios also have to use lower pitch props. Example is the Yamaha 70 HP carbed built today.
The Yamaha motor is almost the same motor as the old evinrude 3 looper 70 hp. Yamaha guys will disagree but the stats on the Yamaha site are almost the same. Yamaha runs their motor at a 2.0 ratio which means in most cases where the older evinrude would run a 17 to 19 pitch prop the Yamaha runs a 13 to 15 pitch prop.
Goto the Yamaha web site and look up the preformance reports for Carbed two strokes in the 70 hp range. That will give you a good idea of what I mean. Find a boat that is very close to yours and you will see that the performance will be very close to the same.
70 HP in the water is 70 HP in the water. It will only do so much.
4 Blades will slow you down and the 21 pitch will destroy your Motor.
Your RPM will go down to maybe 4200 and that will be the end of it. The motor will be under so much stress that the cylinders will oval and the bearings will wear out.
Here is the Math
4200/2.42*21/12/5280*60= 34.51 with no slip.
With slip 30 to 31 at best.
I would Put a 13.25/17 Stainless that would put you at about 5100 but look on your motor plate on the side of the transom bracket to see what the WOT needs to be.
My motor is an 84 and it is rated for 4500 to 5500. I run it at 5450. and with just me in the boat I go 36 on a cool day and 35 on a day were it is 85 degrees or hotter, (GPS)
Kyle
