V-King YT Slip & Trim

Randy,

I am currently running an 87 V-King YT hull with 2.4 EFI Bridgeport 240 horsepower.
The propshaft is about 1\2 inch below the bottom of the boat and I have 10 inches of setback. I'm running a 30 offshore cleaver prop.  The question I have is with two people in the boat about 170 and 200 pounds it will run 89.7 GPS at about 7500 RPM. To me that adds up to a lot of prop slip so we ran the boat the other day, stopped, did not move trim, pulled boat out of the water leveled boat and trailer and checked the angle of the prop shaft. It was up about 3 or 4 degrees above zero trimmed out.  Do you think that the weight would cause more prop slip or is that too much trim? Any help you could offer would be a big help.

Thank You
Tony Hall

Tony,

Sounds like you have a great running YT. With that much weight up front, and running a cleaver, I would expect a little positive trim angle and more slip. To run 90 with 2 guys is very good. Unless, you have a prop that is faster than the cleaver, do not worry too much about the slip. Prop slip is relative and not a bad thing. Without slip, there would be no thrust. And we need more thrust with more weight. Also the slip calculation only works if you know what pitch the prop is (as opposed to what it is stamped) and what part of the prop is doing the most work. Most props have varying pitch throughout the blade. The part of the blade near the hub is mostly used at slow speed and during acceleration, with the center of pressure moving out toward the top due to blade speed and centrifugal force, the faster you go. Running negative trim causes the slip numbers to approach zero because the effective pitch is higher at that angle of attack. Running positive trim causes the opposite effect. The prop acts small (or seems to have a lot of slip) because the effective pitch is smaller in relation to the angle of attack. Also at positive trim angles, the prop is pulled higher out of the water and has less "grip" so hence slightly more real slip. Most props on performance outboard are most efficient in the 3 o'clock to 8 o'clock positions in terms of blade swing.

Hope all that makes sense, but more importantly, if I have a prop that runs faster than all my other props, but it has 20% slip, it is still my best prop. Slip is irrelevant if it works better. And who knows when the boat has 2 people in, it may be using the 29 or 28 pitch part of the blade more, then your numbers are better.

Randy

 

 

Please submit whatever questions you have to IHRTechTalk@yahoo.com.  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Thanks,
Mark C.

 

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