Logo

Mercruiser Prop size

I have a mercruiser mcm120 in

I have a mercruiser mcm120 in a 21 foot 1983 Galaxy cuddy cabin.I just put it in the water and I tried a 16X15 prop and I am only reaching 3000 R.P.M. and 1/2 the speed I was getting 2 years ago the last time I used the boat I do not remember the prop size and this is not the original prop.Any ideas on what the original prop might have been.This boat is pretty heavy.
 
"John,

No prop issue is exa


"John,

No prop issue is exactly the same as the other but ill help the best i can. your current prop is a 16x15. (16"diameter 15 Pitch)More diameter means more thrust(lower planing speed, better holeshot).This is usually better for a bigger boat. More pitch means more speed. Usually more pitch is better than more diameter in a smaller boat.

However your boat is not reaching recommended WOT RPM with your current prop. My estimate guessing it should be around 4300-4800 WOT RPM!!! Every inch of diameter you DECREASE should give you 200 or so RPM MORE!!! Being your boat is a 21 footer you shouldnt need 16" of diameter to get it out of the water or keep plane at a lower speed. Your biggest issue is being able to get recommended WOT RPM with a prop. Overloading your engine(not enough RPM) will do just as much damage as over revving your engine(not enough propeller).

All this being said:

I would try to look for a 13 5/8 x 17 or a 14x17 and see how that works for you. By doing guesstimates in my head that seems like a sensible size to demonstrate. But like i said before prop sizing is a trial and error game and no boat even of same size engine and length are the same. EVER!! Good Luck

FYI: If anybody wants to chime in and tell me I'm outta my mind or I'm giving incorrect info please do so as I'm just trying to help with the knowledge I have. Thanks"
 
J.C.H. is steering you in the

J.C.H. is steering you in the right direction. Before changing your prop make sure the engine is in top running condition. Check the timing at WOT to see if the timing advance is working.
 
"As you haven't used it in

"As you haven't used it in 2 yrs, I don't think your issue is the prop.

You may want to do a compression test on the engine.

I hope your not using 2yr old fuel ??

I would remove all of the fuel(and water from the tank), put some fresh stuff in

Remove the carb and get it cleaned and rebuilt."
 
"Sorry I should have gave more

"Sorry I should have gave more info.Just had head replaced by a mechanic and all new fuel.When I stored boat 2 years ago I misplaced the prop and don't remember the size.My mechanic said that it is trial and error to get the prop matched to the boat size and weight and engine size which is a 4 cyl,with a 1;98 outdrive.I was just hoping maybe someone out there might have the same boat so that I know where to begin.I am learning some things just from all the responses I have received. Thank you Thank You Thank You.

John"
 
"Mercruiser 2.5L and 3.0L 140h

"Mercruiser 2.5L and 3.0L 140hp 4cyls use a 1.98r drive a typical prop is 13.5-14x17 (if at sea level)
Inline 6's and the 470 series type inline 4's use a 1.65r now a 1.62r drive V6's and 5.0L(sometimes) use a 1.80r, 5.7L use 1.5r, I think Big Blocks, use the same to 1.42r? (not my forte')


I would still do a compression test, check the distributor weights(get rusty-underneath the points plate), and have the carb rebuilt."
 
big blocks (454) used

big blocks (454) used 1.32 to 1
350 and 305 used 1.50:1
v6 used I believe a 1.84:1 ???
No manual here to review for exact.

inline 6 used 1.65:1

4 bangers used 1.98:1

Some time in the late '90's or 2000's they went to a new ratio for the v8's 305 and 350. I am not sure but I believe it may be 1.42 to 1. Again dont have a manual here at work.......
 
"I was thinking the 14X17 prop

"I was thinking the 14X17 prop would be the right one for a small motor/big boat combo. 21' footer with a 4 banger in it isn't going too fast anyway. Getting on plane and maintaining a decent and efficient cruising speed will be your main concern. Currently you are way over-propped. I'm not sure how you got a 16" in there in the first place, it wouldn't fit on an Alpha 1. I think 15" is as big as it gets.

Yes the V6 and 470 use a 1.84:1. The 305's at one time used the 1.62:1 and the 350 use the 1.50:1. I'm sure as HP ratings increased things changed."
 
sorry about my posted gear rat

sorry about my posted gear ratio I'm used to Volvo's is guess. Im glad i was close as far as prop size goes. Good luck John. and I think Pugetsounder and I are on the right track as far as prop size for ya
 
Thanks to all.For the record t

Thanks to all.For the record the carb was rebuilt when they changed the head.I will take all of this info and try to drop my diameter down to a 14x17 and take it from there.
Once again thank You All.
John
 
"Puget Sound, A 16 inch prop w

"Puget Sound, A 16 inch prop will fit an alpha one fine. I have a 24 foot (fairly heavy) boat with a 351 Ford v8 and have a 16 in x 16 pitch on now. It clears the cav plate by a big 1/2 inch."
 
"Barry, after doing some resea

"Barry, after doing some research I stand corrected as they do make a 16" diam prop for the Alpha. When I changed my prop over it just didn't look like I had enough room for anything bigger than 15". My 14" only has very little room and with 21P I had to modify the speed prop rudder zinc quite a bit so it wouldn't hit. I did notice the pitch on the 16" is very little for torque more than anything."
 
I have a 5.0 mercruiser and ne

I have a 5.0 mercruiser and need to know what i have to do to winterize and get it ready for the cold weather we will be getting soon. Live in Indiana..
 
Back
Top