Re: Need prop advice due to gear change
Oldseaox, speed is directly proportional to shaft RPM but indirectly proportional to Motor RPM because of gear ratio. So that brings us to the change of a 1.87:1 gear ratio to a 2:1 gear ratio, the only thing that really matters is how many RPM the prop turns.
Formula 1 shows your old theoretical speed of about 36 MPH with a prop slip of 10% calculated.
By appearances a change from 1.87:1 to a 2:1 gear ratio is only a loss of about 6.5% in theoretical tach RPM and that is exactly what your change in RPM from 5,300 to 4,950 RPM reflects, but since you have a 2:1 gear ratio it actually doubles the shaft RPM percentage change to 13%. It also encompasses a change in speed of about 4 MPH with a 10% Slip Factor, you are actually losing 359 Shaft RPM as well as 350 Tach RPM, it is mathematically proportional,
Formula 1 and 2 shows this.
When you change from a 15" pitch to a 13" pitch you will gain about 550 RPM with the new 2:1 gear ratio and thus your Tach RPM will increase to about 5,500 RPM and you will have a gear shaft RPM increase of about 381, but you are now only turning 13" with each revolution of the prop shaft.
Formula 3 shows the increase in Tach and Shaft RPM from the 15" to the 13".
It also shows a WOT Top speed loss of about 4 to 5 MPH because of the lower gear ratio change that requires a lower pitch prop to reach satisfactory RPM. This is the way your boat has reacted and will react to the change in your gear ratio.
You need to purchase a 13" prop in the same manufacturer and model that you have now. By raising the motor a little it may help or hurt your performance, depending on how much prop slip you encounter with the aluminum prop versus a stainless steel that is more likely designed for increased prop height because of blade geometry, it just depends on how high your motor is now.
While I am at it, I will also state that the blade flex most people talk about with aluminum props at speeds lower than at least 45 MPH is not a viable statement, if it was then aluminum props would attain more Prop Slip as the speed and RPM increased because of the water pressure exerted on the blades at these speeds in a boat, and in fact that is NOT the case if you will study the prop slips of most boats with aluminum props. Because as the speed increases you will notice that the Prop Slip decreases all the way to WOT RPM, and if
aluminum prop flex was true you would see Prop Slip go UP as the RPM went up not DOWN. The MAIN difference in aluminum and stainless steel props is in thinner
blade thickness because of the difference in material strengths, which makes for less disturbance in the water flow between blades as they turn with stainless props, as the stainless blades are much thinner. And the
MAJOR difference is in blade GEOMETRY of propellers built in stainless steel that is NOT incorporated into most aluminum propellers. The only props I know of in aluminum that have a LOT of blade geometry changes in them is the Turning Point Props and they certainly act more like stainless props than aluminum props.
There is a lot of talk here about the differences in stainless and aluminum propellers, but there is little talk about blade geometry other than the amount of rake that either increases bow lift or zero rake that is used for stern lifting. There is some talk about cupping as used for theoretical increases in pitch and for increasing stern lift, but hardly anything is ever mentioned about progressive pitch and rake and cupping at the prop tip to increase the rake and dynamic thrust.
H
