Small props 11 degrees down

Discussion in 'Boat Design' started by eyschulman, Nov 27, 2011.

  1. eyschulman
    Joined: Jul 2011
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    Location: seattle Wa USA

    eyschulman Senior Member

    I am having a fast downeast semidisplacement boat built. 45.8ft WL-13.4 ft beam 34,000Lb loaded 36" skeg. The twin motors are in cockpit boxes with 10 degree down ZF 301A tranys. 1.8:1 ratio-JD 6068AFM75 rated from 230-330 Hp depending on M class and pump chip. The prop shafts will be 11 degrees down from waterline and this allows for 22" props with good clearence. My question how much loss of efficency can I expect due to angle and size of props? Will there be a speed range where the loss is less or greater?
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    15 degrees is the usual maximum. You are well within reasonable limits.
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Any shaft that is greater than 8 degrees starts to lose efficiency owing to the loss of thrust and Va. It can range from 10-20% depending upon the arrangement and of course the actual props 'design' point too.
  4. viking north
    Joined: Dec 2010
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    viking north VINLAND

    You mention small props ?? Has the engineering been done on the props, I.E. shape,size, number of blades, pitch. These combinations give you so much choice I would be surprised if you would have a problem with prop selection. The other feature in your favour would be, with a twin drive setup the shafts would be mounted on struts thus giving you the best possible arrangement for clean waterflow to your props. As for the down angle on the shafts--with that wide flat bottom (little to no deadrise) typical of the downeaster hull(Novi-Lobster boat) I doubt you would lose much efficiency due to the downward suction created by the off vertical angle of the props. There will certainly be some but because of the boyancy volume of that stern it should be a minimum. The proof will be in the pudding as they say, but if it becomes a problem with high RPM suction pulling the stern down to the point where it creating unwanted excessive drag you still have an ace in the hole by adding a set of trim tabs. I took the time to look into these also and a set of electrically controlled tabs suitable for your craft (boats 34 to 65 ft.) listed in Stright Mckay catalogue sells for $670 plus tax. Unless there's something i'm not seeing here(I'm not an engineer just a semi-retired builder) I don't see where you are getting into an insurmountable situation--

    A yacht is not defined by the vessel but by the care and love of her owner--
  5. eyschulman
    Joined: Jul 2011
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    Location: seattle Wa USA

    eyschulman Senior Member

    Yes we plan to use trim tabs and are working with 22x25 four blade no cup as a calculated starting point on props useing 1.8:1 gear.

  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Compared to what? What are the trade-offs you are considering? At what point is a bigger shaft diameter required? (edit- delete). How fast? What sea conditions? Is this a known hull as in it has a known resistance curve? Is a different gear ratio in play or is the 1.8:1 a given? What is the shaft angle relative to the hull. What deadrise above the prop. Any splay to the shafts? Have you calculated the absolute slip in ft/sec at high cruise?

    ok, found your motor. Had a typo in my search the first time.
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