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Old 13-03-2015, 01:18 AM   #1
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Join Date: May 2009
Posts: 496
Advise on propeller sizing

Can anyone give me some advise on propeller sizing

As a starter base line I have a Speed Zone turning point 14 3/4 x 25 in propeller on my Cougar 27.

"SpeedZone propellers take the next step in high performance to best harness the power of high performance bass boats and powerful sterndrives. Greater blade area and more aggressive design features deliver optimum acceleration and top end speed".

GPS speed in both directions with approx 2 ft chop sea conditions was 60mph at 3500rpm with 1:1.37 gear ratio.

with two people, full tank of fuell and all equipment, accelerating from standstill there is almost no cavitation and acceleration on the plane is probably 5 seconds, which for a diesel I am quite happy with.

Coming out of Cows harbor there was a 3ft chop and had no problems getting on the plane, although there was some vibration from the steering during acceleration.

According to the dyno readout for my engine max power is 3750rpm and drops off after this.

I have used a online prop slip calculator with the following data

Prop pitch 25 Gear Ratio 1.37 Engine RPM 3500 Actual speed 60mph

This gives a result of 0.80% slip which seems wrong, or could this be correct.

Is my prop over size diameter wise and would I gain more speed by dropping down to 24in prop pitch.

Or should I just leave as is, I am quite happy with the performance and on a flat calm sea would I probably hit max rpm of 3750?

Also from initial testing it seems incredibly economical on fuel and cruses quite happy around 45 mph 2500 rpm.

look forward to any comments.
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