Prop for a 27’ uniflite. (Big hp increase)

Discussion in 'Props' started by crankshaft, Jul 14, 2019.

  1. crankshaft
    Joined: Apr 2011
    Posts: 26
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Big Lake, Alaska

    crankshaft Junior Member

    I have been reading through the confusing prop info, for quite a while now, trying to figure out what would work best for my new set up. My boat came with a single RH 220hp 318, 1.5:1 reduction, 16x14.5, 3 blade.

    It is getting an upgrade.

    It will be getting a 350-375hp LH drive power plant. Going from RH to LH, a prop change is a must so, here is what I have.

    27’3”WL, 70 uniflite hardtop express. Hard to find the weight on it, but I am figuring 9000 with fuel, people, and a weeks worth of supply’s. 350-375hp chrysler 440, 1.5-1 72c inboard, LH rotation. The uni is a pretty heavy built semi displacement, not the most efficient on the water, but I love it.

    Right now, with the 220hp, the “planing” cruise is 15-16kts, at 3600-3800rpm. Needless to say, it does not spend much time there, way too hard on the motor I built out of 3 junk truck engines and a RH camshaft. Lol.

    Torque speaking, the 440 will put torque the 318 at its peak, from 1500rpm on.

    I am hoping, to get the 16kts rpm under 3000 with the 440.

    Now, the boats use. When it hits the water, it is for days. It spends hours on hours running at 7kts saving fuel, (1800-2000rpm on the 318), but, once on a mission, I want to be able to run 80+ miles at 16kts plus. Only, and I mean ONLY time I would expect to run any faster than a comfortable plane, would be if someone was in danger. If it will skip along 16 kts at 2800-3000, i’ll Be a happy boater. Theoretically it should have a top speed of 24 plus. (A fellow boat guy has a newer 28’ uni, with an outdrive, and it had a 440, it ran 28kts top speed).

    So, my question (s)

    Factory it came with a 16x14.5.
    I have 3 3/4” between the prop and the hull.
    Would stepping up to an 18” help?
    Would a-four blade work better for my desired (relatively slow) cruising speeds?

    Prop calculator came out at a 16x16.5”. Would the added power really justify an inch and a half more pitch? Seems like a ton.

    I read a lot of different opinions, etc on props. It gets pretty confusing.
  2. missinginaction
    Joined: Aug 2007
    Posts: 1,098
    Likes: 251, Points: 83, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Prop selection is an art as well as a science IMO. I'm no expert. I'd suggest that you find a good propeller shop near you. They might be able to give you an opinion. There is also a book out there by Dave Gerr called The Propeller Handbook. I'll post a link to Amazon but you don't need to pay the high prices listed there. Plenty of used book sellers out there have this book available in good condition for $10 or so. It might be worth having it in your library.
    Last edited: Jul 15, 2019
    crankshaft likes this.
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