4 blade for skiing

[metriccrescentwrench]

Re: 4 blade for skiing

...... The 2-1 lower unit allows higher engine RPM while delivering slower prop speed. Turning a higher pitched prop at slower speed reduces the parasitic drag of the prop blades through the water allowing more horsepower available to drive the hull.

Frank, I wonder if there have been any real-life tests of this. I have a background in aeronautics (Commercial Pilot and Flight Instructor certificates), and you're absolutely correct on this. I just wonder how much of an effect it's really gonna' be on a boat of this size and speed. And the other thing that comes into play if you take this too far, a higher pitched prop will stall at low forward speeds and you'd lose hole-shot big time.

 

[Frank Acampora]

Re: 4 blade for skiing



Well, Metric: There are two sides to this story. Some racing engines use an overdriven gear ratio with a small diameter very high pitched prop. You don't really care about efficiency or prop slip. ALL you care about is top speed. You simply can not get top speeds with low pitches. (Similarly: There 'aint no replacement for displacement)

However, in this case, I am describing a recreational engine that was never properly geared, don't ask me why. Switching to the 2 to 1 ratio lower unit enables a 2 inch pitch increase, not much more. There are no other lower gear ratios available so swinging a much higher pitch prop is not an option--depending upon hull size.

Given the increased efficiency of the thinner blades on stainless props allowing about a 200 RPM gain, It is my logical assumption that the lower gear ratio turning a higher pitched prop is also more efficient. Drag your hand sideways through the water at a slow speed then at a higher speed. Feel the difference in force against the leading finger. Same principal with the prop leading edge. I can not actually test parasitic drag but my actual results always were a significant increase in top speed with the 2 to 1 lower unit.

Getting back to pitch and hull size, the 55 MPH boat above is powered (actually, WAS powered. I sold the engine) by a 1990 Force 125 that was swinging a 17 pitch prop on my 21 foot cuddy. With the 1.7 ratio gears this hull only achieved 33 MPH. I never tested the engine under different conditions on the cuddy. HOWEVER: When I mounted it on the 14 footer (with 85 decals) with a 19 pitch prop, initially it delivered about 47 MPH. Switching to a 2 to 1 lower and a 21 pitch prop increased speed to 55 at somewhere around 5900-6000 engine RPM and hole shot was breathtaking: Hit the throttle with full down trim and the boat would jump up, bounce once and be doing an immediate 45. Then, as up trim was applied both tach needle and speedometer needle would rise together until the boat was at top speed.

 

[Starcraft5834]

Re: 4 blade for skiing

20ft cuddy here, 3.0 GM, 4 blade Solas 15P, on plane in 5 seconds, 33mph 4600 rpm... had a 3 blade 21p,, that was a mistake,,would not get on plane quickly,, a bit faster,,, hole shot with 4 blade excellent now,,,that's the ticket for my boat