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Thread: Prop Question

  1. #1

    Prop Question

    How close can you get to the hull of your boat when sizing up props? I have 20x18's and still have 2.5 inches to the hull from the blade when its closest to my hull. I figure maybe 22" props would be my max?

  2. #2

    Re: Prop Question

    From Dave Gerr's book, "Propeller handbook". "Generally there should be a tip clearance of at least 15% of the overall propeller diameter between the blade tips and the hull. The ideal tip clearance is 20%or more..."

    Jackman, see if you can find Gerr's book in the library. The math is a little rough but if you take your time you can get through it. I did and barely passed my second class of calculus! Great information. I can send you my copy if you like, but I want it back!
    Last edited by sammidog; 03-31-2007 at 04:40 PM.

  3. #3

    Re: Prop Question

    Sammi,

    I probably will take you up on that offer. Where are from? I know that I should probably look for the biggest prop my boat can handle and that will tell me what HP to look for, I'm assuming the 330B's are the optimum engines but I'd like to prove it, being a scientist and all.....

  4. #4

    Re: Prop Question

    I live in Erie, Pa. The book won't cost much to ship. 8X11 and 152 pages. Excellent reference.

  5. #5

    Re: Prop Question

    Jackman,

    Your hull and mine are very much the same design, the later model 36 and your 32 both have tunnels for the props allowing for a much closer tip clearance. My propellers are 26x25 with a 3.125 inch or 12% tip clearance. It's OK to be a little under with a tunneled hull.

    Tony D

  6. #6

    Re: Prop Question

    From the old timers in the boatyards where I worked they looked at approx 2 x shaft diameter as optimal. 1.5 x shaft diameter as functional and any less as dangerous.

    Doing the basic math I have 1 1/2 shafts refitted to 1 3/4 and about 2 1/2 -3" clearance I am ok with 23 x 24 3 blades

    Remember the length of the shaft also makes a difference. Adding an inch or so to the length if you are still in tolerance with the distance behind the strut gains another 1/2" or so. You do not want to be too close to the hull or it becomes an issue as the water will not flow over and around the prop correctly.

    Add a blade if you need because the diameter and the pitch and the # of blades all work together.
    Scott
    41C117 "Hattatude"
    Port Canaveral Florida.


    Marine Electronics and Electrical Products Distributor.

  7. #7

    Re: Prop Question

    I have 1.5" shafts A22. From what I've been hearing that is in the middle of the safety range for sizing, but please correct me if I am wrong. I figure that with the torque of the diesels I should be able to swing the biggest props my boat can swing, but with safety in mind. That's what I am trying to do...fit the shaft to the prop to gear to the engine...in no particular order, before I make more moves on this conversion.

  8. #8

    Re: Prop Question

    Again from Gerr's book: multiply 321000 x shp x sf/ St x rpm. Take the cube root of that number and you have your shaft dia.
    shp= shaft hp
    sf=safety factor (gerr recommends 3 for yachts and light commercial craft)
    St= Yield strength in torsional shear, in psi. For aquamet 22 it is 70,000
    RPM= revolutions per minute of the shaft.
    You should be fine with 1.5 inch shafts if you know you have A22.

  9. #9

    Re: Prop Question

    You can go up to 22" wheels, but not any bigger than that. That would still give you 1.5". That's 7 percent. A bit tight, but I think permissible. 21" with a larger disc area ratio and/or more pitch would be better, probably. The shaft size is fine with A22 shafts.

  10. #10

    Re: Prop Question

    Gerr's Propeller Handbook is $13.57 from Amazon. A bargain for the amount of info. in it.

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