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Marine (nautical not marine corps) - forces acting on an interceptor plate

  1. Jul 20, 2012 #1
    Despite the name interceptor, this is marine as in water, not marine as in marine corps.

    Interceptor plate on a boat is like a horizontal flap (airbrake, alieron, elevator) on an aircraft but is mounted below the water. Usually it's a flat rectangle (not airfoil) that drops down from the bottom of the boat. It's not hinged. It comes down like a sliding door, however vertically downwards. (Think guillotine blade).

    Purpose of the plates are two-fold. 1) To trim the boat - adjusting the angle at which it rides, and 2) to somehow make the boat faster by reducing friction underneath (makes an airgap behind the interceptor or so they say. Never looked under it while it's running fast!

    I was thinking of doing some kind of geeky experiment regarding this and an old boat. It never trimmed right anyway. Roughly how do I work out what sort of forces this thing must be designed to endure? The boat is 6.5meter, 1500kg, 200hp. Supposed to do 45+ knots but the salesman lied :-) I am sure I can rig up something that won't break but it's going to massively over strong and thus the weight will be more than it needs to be. How to determine more accurately?

    Any other any good ideas on how to adjust the trim?
  2. jcsd
  3. Jul 20, 2012 #2


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    The usual way is to add weight fore or aft, as appropriate.
    You could just load a few hundred pounds of rocks in burlap bags and shift them to retrim the boat. Maybe just get a few friends to come with you, they can shift as requested.
    For an interceptor, the obvious mounting place is the transom. There may not be another place on the hull that is strong enough, especially if the boat is supposedly good for 45 kt. You might also check if your motor is mounted right, because that could be hurting your performance.
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