Trim Tabs force on transom

Discussion in 'Boat Design' started by sean-nós, Jan 25, 2014.

  1. sean-nós
    Joined: May 2010
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    sean-nós Senior Member

    I guy's I hope this is the right place for this, I'm just about finished my Glen-L crackerbox and as it was porpoising a bit on the first test run I made up some trim tabs now I'm a bit worried about the force that will be put on the transom when it's on the plane, where I have attached the turnbuckle to the transom is just a small two screw plate will this be enough or should I go bigger. I understand that they work more like a skimming stone than actually having to hold up the weight of the boat but would like to know if at any point would there be a lot of force put on them.
    Thanks for any help or advice.

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  2. Mike Graham
    Joined: Feb 2013
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    Mike Graham Junior Member

    I don't see why they would be any less susceptible to slamming loads than the bottom shell they butt up to. (The dynamic, local slamming load is usually big compared to the force just to hold up the weight of the boat.)
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    It depends on the speed and how much incidence you have to crank into your tabs, to make them effective. I think you porpoising issue would be easier to fix with trim adjustment, assuming you're not running your chines dry.

    On another note, take a grinder to the trailing edge of that rudder and make it really a crisp, 90 degree edge. She'll love you for it at high speed.
     
  4. sean-nós
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    sean-nós Senior Member

    Thanks for your help guy's.
    PAR when you say 90 degree edge do you mean to a point or to give it a flat trailing edge.
    Cheers.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yep, you want a dead flat trailing edge, with really crisp corners (do NOT sharpen it to a point). That photo looks like the rudder trailing edges are rounded off quite a bit. These crisp edges are hard to maintain, but is the way to go to offer good rudder response at high speed. Sounds crazy I know, but trust me, it's the go fast ticket.
     
  6. messabout
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    messabout Senior Member

    The trim tabs act so as to modify the angle of attack of the hull or to keep the hull at some favorable attitude. That function will require some force. So yes the trim tabs are loaded to some degree. The incidence angle of the tabs will vary the load, obviously.

    I would have wanted the turnbuckle ends at the transom a little higher. The turnbuclkes are the items that carry the column load to the transom. By having the forward end as low as the picture indicates, you are putting an increased load on the fittings (and the transom). The small parts that accept the transom end of the turnbuckles are concentrating the load. A bigger pad will distribute the load more favorably. The ones in the picture look like a bimini top fitting.

    There may or may not be large loads on the tabs when running in smooth water. Consider the instances when you may be jumping a wake or sizable wave. In that case the nose of the boat comes up and transfers a signifigant loading to the trim tabs and their fittings.

    With all that said, your picture indicates that you have done a very nice job on the cracker box. Kudos for your good workmanship.
     
  7. sean-nós
    Joined: May 2010
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    sean-nós Senior Member

    Good point messabout I didn't think about the landing after hitting a wake or wave I'm sure the force would be quite a bit if I was ever airborne and came down on them.
    You are right they are bimni top fittings :) I try to use things I have around me to save on cost as this is a no budget build but I don't want to cause any damage or make her unsafe, I will cut out some stainless steel plate and put it behind them and try and raise them up a bit or reinforce the inside of the transom.
    Thanks for the help guy's.



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  8. tom28571
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    tom28571 Senior Member

    Time will tell if the fittings are strong enough. Slamming loads can get pretty high and are not very predictable. One thing wrong is the geometry of the transom fitting as it is not in column. The bottom screw will surely want to pull out. A wedge block to make it align with the turnbuckle will make it much stronger as well as spread the load on the transom.
     

  9. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    I have tabs the same size as those on a 16' runabout, but with the Bennett actuators instead of fixed links. I've broken one of the actuator mounts from the force of a "hard landing" after jumping a wave. I dare say that actuator mount was probably stronger than those top mount fittings...or at least about the same.

    The typical mounting angle and location is not as you have done it; the support rod( or actuator) typically lands more forward on the tab, not all the way back on the trailing edge, and higher up on the transom. The relatively flat angle of your adjustment/support rod is not favorable from a load-reacting standpoint.

    Oh..And beautiful boat! I'm partial to the style...and still miss my '53 Chris Craft.
     
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