WOT varies - why......

cocobob

Recruit
Joined
Jun 6, 2006
Messages
4
Got a little over 100 hrs on my new 115 4stroke Merc outboard. After my break in period :On identical water, wind and passenger load conditions I get a different WOT. Sometimes I get 5100 RPM and other times I have goten 5700 rpm. My trim is adjusted for each situation to get the most without cavitating the prop. Anybody know what causes this...<br /><br />Cheers
 

sport15

Petty Officer 3rd Class
Joined
Jun 1, 2006
Messages
95
Re: WOT varies - why......

Does the throttle linkage/cable have slop in it? Just 'cause the remote lever is in the same position every time does not mean that the engine end is wide open.. EXACT weight? was fuel load included? The smaller the displacement of the boat the bigger the differance a small weight change effects it. 600 RPM is alot but a 200 lb differance (10 gal of gas, a average size Lab and a full 20 qt cooler..) could do it.. <br /><br />Trim will make a differance too. With a load in the bow, the engine can be trimmed up to the "sweet spot", the weight forward will keep the bow down and this will increase the hulls wetted surface and cause more drag. More drag=more power required=less turns. To much weight in the stern in the same running condition will cause the stren to squat, angle of attack to plain increases, with the AOA increase drag increases...<br /><br />A good demo of all this is to set the throttle, note the turns then have someone move around. <br />BE CAREFUL!! Only do this at a safe speed on smooth water and remember, one hand for you and one for the boat!<br /><br />Poster takes no responsibility for any acts insanity or injuries caused by flatheads trying this at wide open throttle! :) !!
 
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