Engine Mounting Height

[JasonM1955]

I checked my engine height last night and the cavitation plate is even with the botom of the boat. I have heard some people say to measure from the shaft to the bottom of the boat also. I'm about 8 inches from bottom of boat to centerline of shaft. Am I close to where I need to be?<br />Thanks:<br />Jason<br />BTW It's a 1985 Sunbird with a 135 Evinrude on it.

 

[Dhadley]

Re: Engine Mounting Height

Your prop and hull type will dictate what the X dimension will be. If your plate is even with the running surface of the hull, you should be close.<br /><br />As you buy props with more technology (cup angle, cup length, rake, progressive pitch, blade area etc) you may be able to run the motor higher (reduce gearcase drag) and increase lift (reduce the wetted surface which reduces hull drag).<br /><br />Try raising the motor. If you start to ventilate, you are too high for your prop / hull combination. Its real easy, almost free and if you dont like it you can always lower the motor. Just keep an eye on the water pressure.<br /><br />Good luck!

 

[JasonM1955]

Re: Engine Mounting Height

Dhadley,<br />How is the best way in determining what prop to go with? There are so many to choose from that it gets real intimidating. I'm looking for a prop now with top end speed in mind. I have enough props now that I can ski behind. I'm wanting speed out of this thing now. Do you have any suggestions? Here are a few specifics on what I have now.<br />1985 17' Sunbird with a 135 Evinrude cross flow design, 20" shaft lenght. The prop is an older S.S. so I'm not sure what name brand it is but it is a 13-1/4"x19" cupped. My rpm's are low as we talked about in another post. I'm running about 47mph with one person in it turning 4900-5000 rpm's. I would like to try to get this thing in the low to mid 50's. Do you think there is a chance in that? If there is any more info you need, just let me know.<br />Thanks again:<br />Jason

 

[Dhadley]

Re: Engine Mounting Height

I would start by raising the motor and see how the hull responds. I'm sure you will pick up some RPM. At 5000 RPM with a V4 gear ratio your theoretical top speed is 45.5 mph. With 12% slip you are at 40 mph. <br /><br />Raise the motor and let us know. Watch your water pressure!

 

[JasonM1955]

Re: Engine Mounting Height

Ok, I raised the motor up last night as high as it will go and now my cavitation plate is 1" above the bottom of the boat. Am I close to where I need to be now? It's going to be at least a week before I can try it because I sent my S.S. prop off yesterday to have it trued, balanced and polished.<br />Thanks:<br />Jason

 

[Dhadley]

Re: Engine Mounting Height

Oops! You forgot to mention that you will be testing with another unknown. When testing it is vital you make only one change at a time. I know the prop is supposed to come back in "new" condition. It may or may not. Some shops trim the diameter. Some dont. Some don't have the correct pitch blocks. Some do. <br /><br />You have two choices; 1- leave the motor where it is and try the "new" prop. 2- lower the motor back to the original position and confirm or re-establish your baseline. <br /><br />Keep us informed!

 

[JasonM1955]

Re: Engine Mounting Height

Dhadley,<br />I got you!! I will probally try it both ways once it comes back in. I'll keep you posted on the results.<br />Thanks again for all your help.<br />Jason

 

[Dhadley]

Re: Engine Mounting Height

Good plan!<br /><br />I have some ideas on props but we need to know how your hull responds to X dimension. I have a feeling it will respond nicely. <br /><br />When you get the prop back, look for any numbers that may give us a clue as to the manufacturer. <br /><br />Good luck!