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  1. #1
    Join Date
    Jun 2009
    Location
    Warwick, R.I., U.S.A.
    Posts
    35

    Default 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.

  2. #2
    Join Date
    Oct 2008
    Location
    Lakewood, NY USA
    Posts
    317

    Default "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"

  3. #3
    Join Date
    Aug 2006
    Location
    SW Ohio, USA
    Posts
    10,396

    Default 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.
    Fix minor things (replace wear items for preventative maintenance.) that cause major problems before they break.

  4. #4
    Join Date
    Mar 2009
    Location
    Lowell, MA
    Posts
    1,217

    Default "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."

  5. #5
    Join Date
    Jun 2009
    Location
    Warwick, R.I., U.S.A.
    Posts
    35

    Default "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"

  6. #6
    Join Date
    Oct 2008
    Location
    Lakewood, NY USA
    Posts
    317

    Default 1.98 gear ratio on a 4cyl??? u

    1.98 gear ratio on a 4cyl??? usually they are up around 2.15 or 2.30 to 1?? Is this right?

  7. #7
    Join Date
    Mar 2009
    Location
    Lowell, MA
    Posts
    1,217

    Default "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."

  8. #8
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
    Posts
    4,793

    Default 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.......
    Jack
    Northborough Ma
    Helping others sometimes with an attitude....haha..

  9. #9
    Join Date
    Apr 2007
    Location
    Seattle, WA,
    Posts
    902

    Default "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."

  10. #10
    Join Date
    Oct 2008
    Location
    Lakewood, NY USA
    Posts
    317

    Default 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

  11. #11
    Join Date
    Jun 2009
    Location
    Warwick, R.I., U.S.A.
    Posts
    35

    Default 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

  12. #12
    Join Date
    Sep 2006
    Location
    Coquitlam, British Columbia, Canada
    Posts
    193

    Default "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."

  13. #13
    Join Date
    Apr 2007
    Location
    Seattle, WA,
    Posts
    902

    Default "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."

  14. #14
    Join Date
    Apr 2009
    Location
    Mishawaka, In, USA
    Posts
    3

    Default 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..

  15. #15
    Join Date
    Aug 2006
    Location
    SW Ohio, USA
    Posts
    10,396

    Default look above for "search"

    look above for "search" and do one for winterize engine.
    Fix minor things (replace wear items for preventative maintenance.) that cause major problems before they break.

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