Shaft Log Angle

Discussion in 'Inboards' started by jimhales, Feb 4, 2007.

  1. jimhales
    Joined: Mar 2006
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    Location: canada

    jimhales Junior Member

    I have an older fibreglass 16 foot boat that I am (hopefully) transforming from outboard to inboard direct drive. I will use a small 4 cylinder gas engine and 2 spd. trans.
    My question is how does one determin what angle is the best for the strut or shaft log? 8, 10, 12, 15 degrees? (what if any are the formulas to be used)?

    Also location and positioning this properly, does it have to be through the bottom of the hull of the boat or, can it go through the transom?
    I know, these are very amateur questions from an ameteur but I haven't been able to locate any information on what problems I may run into or even how to make such an installation work. Any direction would be appreciated.
  2. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    The shaft angle is determined by the size of the propeller, the size of the engine, the type of gear box, and the total arrangement of everything. First, you need to know the diameter of the propeller and approximately where it will go underneath the boat. You would like to have a propeller tip clearance of 15-20% of the diameter--that is, the distance from the bottom of the hull to the top tip of the propeller. Then you have to know where the engine is going to go, and the type of gear box that it has (in-line, offset, offset with drop angle, V-drive???). Line up the engine/gear box with the shaft line coming out of the gear box to meet the propeller at its preferred location.

    You always want the shaft angle to be as small as possible, so if you have to choose between 8 deg. and 10 deg., go with 8 deg. Smaller is always better--less hydrodynamic shaft drag and turbulence flowing into the prop.

    Be careful on the placement of the engine--it is a heavy weight, and putting it too far forward may make the boat perform badly and be unsafe (bad trim and steering control because the longitudinal center of gravity of the boat is too far forward).

    You want the rudder to be behind the prop because the rudder works by deflecting the prop wash. So you do not want the shaft to go out the transom--that's a no-no except for surface piercing propeller designs, which your design likely is not. The prop goes under the boat, and the rudder goes behind the prop.

    The key starting place, then, is to determine the size of the prop. You can start with Dave Gerr's "Propeller Handbook" to see how that is done.

  3. jimhales
    Joined: Mar 2006
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    jimhales Junior Member

    Eric, thanks so much for the advice. I am learning their is much to consider when making such big changes. Your knowledge is very much appreciated!!
  4. Gilbert
    Joined: Aug 2004
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    Location: Cathlamet, WA

    Gilbert Senior Member

    Eric's post is excellent; except giving the impression that a small shaft angle is necessary. I know he didn't say necessary he said better. Lay out the geometry for the all the required equipment as he said and there you are. The shaft angle is what it is.
  5. Jango
    Joined: Aug 2005
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    Location: Mid Atlantic

    Jango Senior Enthusiast

    Since you are using a small 4 cyl, and not talking about higher speeds,(< 50mph) a 16 deg angle will be appropriate. This higher angle will enable placing the engine further back. Your application will most likely use a 12" dia prop, back of Strut (@ shaft centerline) 12 - 14 in. from transom. A 6 3/4 drop Strut will work out just fine.

    It's All a tradeoff. The disadvantages of higher shaft angle, usually is offset by the ability to place the Engine further aft.- especially in smaller Boats.


  6. jimhales
    Joined: Mar 2006
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    Location: canada

    jimhales Junior Member

    Thanks for your opinions/knowledge again!!
    Can any of you recommend a good book to purchase that will help me with understanding technical terms, determining formulas to calculate stuff like " draft, waterline etc. I am really wanting to klnow how to do this so that I can do things properly.
    Thanks again
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