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Author Topic:   Prop Questions for Twins
Tom Byrum posted 07-22-2000 06:21 PM ET (US)   Profile for Tom Byrum   Send Email to Tom Byrum  
Is there any performance difference regarding counter-rotating versus similar-rotating props? Does the lower unit control the direction of rotation or does the engine rotate differently? The reason I am asking is I might be able to make a deal on a counter-rotating motor like mine with a blown powerhead. My twins are similar-rotating 1989 Yamaha 70 hp.
Tom Byrum posted 07-22-2000 06:28 PM ET (US)     Profile for Tom Byrum  Send Email to Tom Byrum     
My props are stamped '17-K' Anyone know what it means? I suspect they are stock Yamaha props. They are stainless steel but they do rust slighty.
Alioop posted 07-22-2000 09:46 PM ET (US)     Profile for Alioop  Send Email to Alioop     
The 17 indicates 17" pitch,the K indicates Steel (black painted)as in accessory catalog
lhg posted 07-23-2000 12:12 PM ET (US)     Profile for lhg    
Tom: Many people don't realize that stainless steel means "stain" "less" (than regular steel. It can exhibit surface rust, but this can usually be polished off the surface.

Regarding counter rotation, the lower unit gear case is where it happens, but different shift controls are also needed. Mercury, for example, makes two twin engine control boxes - one for twin regular rotation, and one for twin counter rotation. I have a Whaler with both setups, and the counter rotation is far superior, eliminating the steering and port lean characterists of
conventional rotation, which is magnified with twins. I've never heard of it being available below about 115hp, however. On my twin engine 18, which is non-counter rotating, getting rid of the standard twin cable mechanical steering in favor of hydraulic, helped a lot with the steering torque problems.

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