prop changes vs performance

[laybackman]

I own a 1998 Force (by Mercury) 40HP outboard that has been re-carbed with a 50HP carb. I posted about that earlier today It currently runs a Michigan Match 10.375 x 13 prop. The original prop was marked 48-7313-13 but I don't know who the manufacturer is, do you? I cannot detect any differences in these two props visually. The MM prop runs 3-5 knot slower and 200 less prms than the original prop did. <br /><br />The original prop needs repair again.<br /><br />1.) Would this prop still be balanced after being <br />repaired by a prop shop a second time?<br /><br />2.) Would a 10.625 x 12 prop be better? or would <br />it be a "wash" performance wise?<br /><br />3.) Would a prop with a larger exhaust port be <br />the best choice and leave the pitch at 13 and <br />the diameter as close to 10.375" as possible?<br /><br />Any help is appreciated

 

[walleyehed]

Re: prop changes vs performance

Generally when you see a drop in RPM and MPH, you are running an inefficient prop over-all.<br />The 48- P/N prop is Merc, and 13" pitch. <br />I should ask, beings you know you had a 200RPM drop, what was your RPM (max) with the old prop?<br />p.s. What was the reason for going to a 50hp carb??

 

[laybackman]

Re: prop changes vs performance

Thanks for the head up on the prop manufacturer. I thought it was a Merc prop but I can't cross reference it with anything being sold by anyone. I guess I would have to deal directly with a Merc stealership.<br /><br />Before the carb upgrade the max rpm was 5200 now it's only 5000. Both props I have are supposed to be identical but I think the original prop has a larger diameter exhaust port and thinner blades.<br />I recarbed the 40 with the 50hp carb to grab the 10 HP. Basically they detune a 50hp by sending less fuel to the combustuion chambers. You can also restrict HP by strangling the exhaust. I should get up to 5500 rpm with this motor and the expect top end speed from 50 hp motor. In my case that is about 30 knots instead of 25 knots.<br />On top of all this I remember the 50hp version of this motor new was $1100 more than the 40hp version! The carb costs 137 bucks. (Do the math.)I have the factory manual and the only differences between the motors are the carburator and the exterior decals. The 50HP carb has a larger throat and a larger jet. EVERYTHING else about these two carbs are identical all the parts are the same ecxept the carb body and the jet which is larger on the 50hp carb. I think they do the same with the 75 and 90 hp versions also. Same motor different carbs.

 

[laybackman]

Re: prop changes vs performance

I have to correct something I said. I stated that "Before the carb upgrade the max rpm was 5200 now it's only 5000." It was all before I changed the carb. The change in rpm occured when I changed the props.