Question for DHadley

[tbeaun]

DHadley, I see you frequently recommend raising an engine to improve performance. My boat has 2 holes above where the engine is set now & I believe I'm getting good performance, judging by what I'm learning here & by my tach reads. I'm wondering if I need to raise the engine anyway. Is this something you do only if you are not satisfied with your engines performance or should the engine be raised as high as possible as part of its set-up. Also, at the risk of becoming an example of my signature, what is a cavitation (anti-cavitation?) plate?

 

[Cricket Too]

Re: Question for DHadley

tbeaun...I'm no Dhadley, but if you are satisfied with your performance or actually if you have proper performance, then there really is no need to change your setup. If you are turning high enough WOT RPM's and have a hole shot that you are satisfied with, then you have no reason to raise your engine. A cavitation plate, which is really called an anti-ventilation plate I think, but I have heard both, is the plate directly above your prop, that keeps the prop from ventilating, so if you have a motor mounted too high and that plate is out of the water you will probably vetilate (spin out) your prop once you plane off or on turns. Dhadley will probably explain it much better.

 

[Dhadley]

Re: Question for DHadley

With any set up you can run the numbers (prop pitch, top rpm & accurate speed) and calculate the slip percentage. Its very possible that you may be completely satisfied with any given set up and find out your efficiency could be better.<br /><br />Lets say youre happy with your set up but the numbers say youre in the 16-18% slip range. You dial in the set up and -- without changing props -- get in the 12% range. Your holeshot improves as does top speed, cruise speed, fuel economy -- everything. The idea is to lose drag.<br /><br />Each set up depends on your intended use, what style prop you chose to run and how it reacts with any given hull. If you chose to run an aluminum prop it may not run as high as an SST II. A Raker will run higher. A Stiletto higher and a Turbo even higher. And stay hooked up. <br /><br />Look at it this way -- it only costs time to raise or lower a motor. If you try it and like it, it didnt cost anything except the couple of beverages you had while doing it. But you were gonna have those anyway.<br /><br />Post your numbers and we'll see where youre at now. Maybe youre already there.<br /><br />Oh -- there isnt any set height that works for all rigs all the time. We've run boats that the prop stayed hooked up even when the low water pick up nosecone was so far up that it lost water.

 

[tbeaun]

Re: Question for DHadley

DHadley, numbers are: 6250 RPM @ WOT turning an aluminum, 13 1/4 x 17 prop. Top MPH (according to the onboard speedo) is 43. Motor is a 1974 115hp Johnson pushing a 1974 Crestliner Apollo 16, no idea of its weight but looks like its heavy.

 

[Dhadley]

Re: Question for DHadley

That comes out to about 15.5% slip. Not too bad but I think it could be a bit better. Now, boat speedo's are not known for accuracy unless its something like a liquid filled Lavorsi or Gafferig. But its a reference point.<br /><br />If I were doing it, I'd try a 19. Something like a 19 Stiletto. We may be able to swing it. It'll have more bite which drops the r's but it also has more lift (less drag) which will help get r's back. The extra bite should drop the r's to about 5800, which is still OK, and if the slip improved to only 12% youd gain over 3 mph.<br /><br />Those numbers, if accurate, tell us the prop must be in good shape. However, if the speed is off considerably that might indicate the tach is off. We'd like to do 2 things before we test new props. 1- verify the tach is set on position 6. 2- verify the speed over a measured mile or with a GPS.

 

[G DANE]

Re: Question for DHadley

I think you would gain speed by jumping to a 19", at best a good stainless prop, but pay with a little loss of hole shot. By then you can start experimenting with raising motor. Seems like it runs fine though. Depending of if 6250 is with you alone or average load ??

 

[tbeaun]

Re: Question for DHadley

Tach is set to 6 & I have a GPS coming sometime this week so I can post numbers as soon as I take it out. I have a SST Renegade 13 1/4 x 21 I would like to use when not pulling tubers. Your thoughts on that prop, please.