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Thread: Prop madness

  1. #1

    Default Prop madness

    I need a new prop for my boat, a 1978 Sea Ray with a pre alpha 260 outdrive. The prop I was running came with the boat and has no markings on it other then the hub which is stamped with 48-79576-21 and across from that the letter U. But I can't find anything online other then more unanswered questions.

    Anyone familiar with this stuff? There's got to be other options out there that will fit.
    Last edited by 805HD; 08-20-2017 at 11:56 AM.

  2. #2
    Join Date
    Sep 2008
    Maryland - USA

    Default Re: Prop madness

    the last two digits give you the pitch - 21" for the prop you have. the other variable left is diameter and there isn't much choice there due to the outdrive design....the splines are fixed. the other variables are material and blade count. for those outdrives, three blade props are standard....and most opt for aluminum due to $$...stainless holds up pretty well but costs more $$$.

    one typically selects the prop's pitch to allow the engine to get to the WOT rpm band specified....and due it with a 'full and normal' load in the boat. if you are 'in the band' then the prop's pitch is acceptable. If the rpm band is exceeded, you need more pitch to load the engine more; if it wont get into the WOR rpm band, there's too much load on the engine (lugging) and a smaller pitched prop is in order. due all of this with the engine in proper working order.

    Finally, any decent dealer or prop shop can answer all your questions - they are usually worth a visit just for that...

    PS the owner's manual and the shop manual both have decent discussions on prop selection....may be worth the read.....
    Last edited by makomark; 08-20-2017 at 02:46 PM.

  3. #3
    Join Date
    Sep 2008
    Hague, Virginia, USA

    Default Re: Prop madness

    If the current prop was 21 pitch, you can get a close approximation of the diameter by placing the prop on a piece of cardboard and tracing the outer edge of one of the blades as you turn it through 360 degrees.

    Be careful - many of the Volvo props are left turning, long hub props. For comparison, on a previous C-Hawk I owned with the pre-alpha drive powered by a Crusader 220 (which was basically a GM 305.) The WOT band was 4400 - 4800 rpm. This was a 6000 lb boat when fully loaded. I experimented with several props. The best setup was a 15 X 17. So, in addition to Makomark's good advice, you need to consider the engine horsepower and the weight of the boat. And, as he said, a good prop shop will likely get you what you need.

    There are also several Sea Ray forums out there. If you find someone that has the same setup as you have, they may know the prop size and pitch that works for them.
    CHawk 25 DLX

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