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Hi All I just splashed my 1978 15' sport after converting her to a SS and repowering with a Merc. 2003 60HP Bigfoot. I have the engine mounted so that the cav plated sits above the bottom of the hull by about 1/2". After her first sea trial I was happy with the height and the cav plate was flush with the water at speed. The prop that is on her now is a 13,3/4" X 15" and got her on plane very quickly. The engine is rated at 6000 RPM WOT and at that she runs at 33Kts GPS, not bad but I would like more speed. What size prop should I be looking for?
Cheers
Why are you happy with the current engine height? - because it performs optimally?
You have nothing to lose by trialing the engine a hole higher (all the way up) except some elbow greese and possibly much to gain in performance improvement.
You are currently running at max engine rpm wot so if you lower the pitch of the prop you will over rev and if you increase the pitch you will rev lower. Assuming your engine is producing max hp at 6000 rpm raising the pitch and lowering rpms will likely result in worse performance.
If you raise the engine (resulting is less drag) the rpms will rise (over rev). Then you could increase the pitch of the prop to again reduce the rpm back down (by ~250rpm per 1 degree pitch increase) into the normal wot operating range and achieve higher top speed.
Also can you tell us what your lower unit gear case ratio is? Best I can tell it should be 2.33:1. You can use the following link to calculate your prop slip which will give you an idea of where you are: http://www.mercuryracing.com/prop-sli...alculator/.
If I go up any higher the cav plate would be 1,1/4" above the bottom and as I mentioned at speed the cav plate is flush with the water. I don't see any benefit of spending a day raising the engine.
I only went up to 6000 rpm and still had throttle.
I only went up to 6000 rpm and still had throttle.
You don't have the correct prop size. If the throttle will advance past the WOT range, you need more pitch. How far past 6k will the throttle go?
You need to test the boat with the same weight/passengers etc. Don't change the motor height quite yet. Raising the engine 1 hole adds 100-200 rpm's at WOT.
For the exact same brand and model prop, stepping up the pitch (2" increments) will drop your RPM's.
If they tell you you can turn a 19" pitch prop, get the Laser II. It's the fastest prop they make for your size gearcase. If you have to use 17", consider the brand new aluminum 4-blade Sptifire (also offered in the Quicksilver line as a Nemesis). Mercury makes remarkably fast aluminum props. I think Boston Whaler recommends an SS Mercury Vengeance prop ,most often for that engine, either 16" or 18" pitch.
