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Engine height and porpoising
panalukes	#1  Posted on 08/01/13 - 7:36 PM
Member Personal Page Posts: 12 Comments: 0 Joined: 09/12/10	Experts, Please view the pictures of my motor on my personal pages. My concern is that my motor is too low. I have a 1989 outrage 18, 1989 Yamaha 130 25 inch leg, suspected 17 inch stainless prop manufacturer unknown (not sure b/c 17-K is the only marking I can find on the used prop), T-top, 2 people, half tank gas, WOT 34 knots at 5500 RPM. My issue is porpoising and initially I searched the forums for input on the stingray type hydrofoil thinking that might solve my problem. But I read more and more about engine height and think that might be an issue. Currently the CAV plate sits about 1.5 inches below the keel. Seems like the recommendation is for 1.5 inches above the keel. But I don't necessarily want to re-prop for the sake of better speed, I'll take any improvements of course. But if I can raise the motor and get rid of the porpoise it would be worth my trouble. Regards, Luke
Joe Kriz	#2  Posted on 08/01/13 - 7:38 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Yes, I commented on your personal page. The motor is too low. Mine is mounted one hole up but should go up more.
panalukes	#3  Posted on 08/01/13 - 7:48 PM
Member Personal Page Posts: 12 Comments: 0 Joined: 09/12/10	Joe. Thanks. Do you think I can use the same propeller? and do you think it will have any affect on the porpoise?
Joe Kriz	#4  Posted on 08/01/13 - 7:52 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Not sure if you can use the same prop or not. I have 3 props. 15p, 17p, 19p. For my use and boat/motor combination, I settled on the 17p OMC prop for now. There are many other options. My suggestion, raise the motor two holes up and see what WOT RPM's you get. Test it.
Phil T	#5  Posted on 08/01/13 - 8:45 PM
Administrator Personal Page Personal Album Project Albums Posts: 7043 Comments: 6 Joined: 03/26/05	For each bolt hole the engine is raised, you can expect a 150-200 rpm increase at WOT if the same prop is the same. I am thinking the prop is a Yamaha painted stainless steel 17" pitch. The"k" is for intermediate gearcase. Edited by Phil T on 08/01/13 - 8:50 PM
Tom W Clark	#6  Posted on 08/01/13 - 9:30 PM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Raising the motor two bolt holes (1-1/2") will, by itself, go along way to reduce the porpoising. Your propeller is a 13" x 17" Yamaha Painted Stainless Steel model (part # 688-45930-02-98) and is not a bad prop at all. It may not be optimal for an Outrage 18 though. Take Joe's advice. Raise the motor and retest. You can expect to gain 50-100 RPM or each bolt hole you raise it, so you'l likely end up at 5600-5700 RPM at WOT.
panalukes	#7  Posted on 08/18/13 - 8:35 AM
Member Personal Page Posts: 12 Comments: 0 Joined: 09/12/10	All, Wanted to close the loop on this one. I raised the motor two holes. Porpoising problem solved. I can't say that I've gained any RPM or Knots out of it but that's not really what I was chasing anyway. Thanks for the advice. Cheap fix and I learned something doing it myself. Luke
Joe Kriz	#8  Posted on 08/18/13 - 11:55 AM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Thanks for the follow up and letting us know. Glad you got the porpoising problem fixed.