Trim Tab Orientation Discussion

Discussion in 'Hydrodynamics and Aerodynamics' started by Barry, Mar 5, 2017.

  1. Barry
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    Barry Senior Member

    Pretty much all of the hulls that I have seen up on blocks or on trailers have the trim tabs oriented parallel to the deadrise angle, with the mounting maybe an inch or so above the bottom of the hull.

    One of the purposes of the trim tab is of course to lower the bow for better fuel efficiency, efficiency defined for this case, more miles per gallon, or km per litre. ( or alternatively hull efficiency, the lowest drag for the thrust value)

    Say the deadrise is 20 degrees and the tab is parallel to this angle. When the tab is in a down position, there will be additional drag from the tab which is not producing an upward lift.

    Ie there will be a vertical component to the force on the tab for the lift but as the deadrise angle and trim tab angle when viewed from the rear increases so will the drag due to the tabs orientation.

    So perhaps, trim tabs should always be mounted horizontal to maximize lift and minimize drag
     
  2. PAR
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    PAR Yacht Designer/Builder

    I suspect you'd have more drag associated with a horizontal mount. You'd need vertical end plates, which will pierce the flow, you'd have a triangular opening between the outboard edge of the tab and the hull shell, which will make significant turbulence, etc. Yeah, they'd be more effective in the vertical pressure aspect, but enough to overcome the additional drag?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Have you tried it that way Barry ? I'd expect a spectacular jet of aerated water sideways from the open side of it, when any decent angle of attack was applied. You could put a fence on that side I suppose, but you would have a lot of wetted area without the whole of the area generating lift. It may actually be a way of steering a boat when the steering packs up ! Maybe I have misunderstood Barry's idea though.
     
  4. Barry
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    Barry Senior Member

    Assume that two boats with the same chine width, and weight.One a flat bottom and the other a deep V at the same speed

    The flat bottom boat will give the best lift and lowest drag A deep V would have a higher drag and lower lift. Hence a trim tab on the flat boat will give a higher lift and and lower drag than a trim tab parallel to the hull with the deadrise angle.

    Certainly there will some issues wrt to the tab not coming into the flow off the bottom of the deep V with a horizontal tab, but perhaps these issues can be solved

    I could not develop the proper 3 dimensional i,j,k (or xyz) vector to illustrate this but was hoping that one of the contributors might be able to do it.



    It is easy to develop in two dimensions but things got fuzzy with three dimensions
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    So long as the objective is achieved - by using the trim tabs - does it matter?
     
  6. Barry
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    Barry Senior Member

    If you can decrease the drag for the same amount of lift, this should be a good thing
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    The objective is not to increase/decrease lift/drag from the tabs, but to alter the attitude of the vessel to one that allows the vessel to achieve its goal; where a different hull form and/or LCG distribution would not.
     
  8. Barry
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    Barry Senior Member

    I was not clear, I was only referring to the trim tab drag and lift.

    The object is to lift the rear of the boat with the tab without adding a lot of drag from the drag.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Tabs when applied are likely to increase the overall wetted area of the hull anyway.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed, I understood that.

    It is all relative! If the boat is going slowly and the tabs are fully deployed, it creates a lot of unwanted drag and thus negative contribution. If the boat is travelling max rpm, and then the tabs are deployed, it is a positive contribution.

    One must not be fixated on absolutes. Trends and objectives are all that maters...
     
  11. 640 blazer
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    640 blazer Junior Member

    Hi there just my 50 cents on trim tabs. I have a 21ft fibreglass 21deg deadrise 200hp opti weight 2 ton. Bought new 13 years ago and bought a set of Lenco tabs for her. They worked but the drag curve versus effect was huge so I cut the 30cm x 15cm plates off mounted 30cm x 30cm plates on and mounted the ram further aft so the end of the tab still only went down 7cm. The tab now being twice as long and going down the same distance meant the angle to the relative water flow was halved. The difference was massive, more effect and way less drag. Couple of years latter I cut the whole lot off and made my 3rd set of tabs 38cm w x 46cm L, I have never looked back. The most efficient lift to drag angle is 3 to 4 deg.
     

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    Last edited: May 11, 2017
  12. tom28571
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    tom28571 Senior Member

    Hi Blazer,

    I think you have rediscovered something that has been known for quite a while. A longer trim tab will have a lower trim angle for less drag and greater efficiency. If this "trim tab" is incorporated into the aft hull bottom, it can be very long and have a very low trim angle relative to the aft buttock lines. Just what the optimum length and angle is depends on the rest of the design and its intended use.
     
    640 blazer likes this.

  13. 640 blazer
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    640 blazer Junior Member

    I see so many boats that I believe are under tabed ie tabs are way too small. I think companies who sell tabs dont tell the truth. My friends look at my trim tabs and laugh, say oh my lord look at the size of them. To me they look perfect because of how they work. A tiny tab will work fine at high speed but it was low speed effect that I was after. My boat now planes at 11 kts, I can set a small amount of power where the bow points at the sky and it sits well in the hole then put both tabs full down, the bow comes down and shes up on the plane, lift the tabs and she sits back in the hole. Most of the time I sit on 25 kts when traveling, the effect versus drag is near perfect.
     
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