# Trim tab theory

Discussion in 'Boat Design' started by 640 blazer, Nov 2, 2008.

1. Joined: Oct 2008
Posts: 9
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Location: New Zealand

### 640 blazerJunior Member

I have a 2 tonne 21 ft deep V pwrd by a 200 opti , I bought the boat new and put a set of trim tabs on myself , they were 300 wide 150 deep , after using them for a while I decided that sure they worked but in some occasions believed they were to small as the drag curve was out weighing the effect so I made a set same width but twice as long with a definite improvement . I am led to believe that the most efficient lift versus angle to relative water flow is 3 to 4 degrees same as an aerofoil to relative air flow on an aircraft . SO lets take this as an example , if the desired effect was reached with the tail of the tab down 5 cm the tab would have a 20 deg angle to the relative water flow , with a tab 300 long and the tail down 5 cm it would have 10 deg angle , same amount of lift for less angle ie drag ie efficiency . I like all this stuff as my job requires some knowledge along these lines but its air not water so I would like someone to let me know if I would be wasting my time going even bigger with the tabs until I get as much effect as I require without the tab having a greater angle that 4 degrees . Building time and so on is not an issue as its all stuff I enjoy doing . Scallie season my favorite .

2. Joined: Jul 2008
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Location: italy

### brunelloJunior Member

Most conventional tabs tend to be undersized. Increasing tab area is rarely detrimental and normally helpful. It is not only a question of optimum angle of attack (which of course matters), but also of plane loading. Increasing the area will lower your plane loading and increase efficiency. Optimum aspect ratio is yet another issue, mostly connected to the vessel's relative speed (and practical factors, as transom layout); normally, wide and short tabs for lower speeds, narrow and long ones for faster boats. The point is that the conventional rigid tabs, connected by a mechanical hinge at the transom, are not the most efficient lifting devices from an hydrodynamical view point. The flow passing along the normally flat aftbody of the planing hull, suddenly "bangs" into the tab's leading edge, which forces the flow downwards, with a peak of pressure located in the forward part of the tab. Once the flow direction has been changed, it continues its travel along the tab, generating little more lift - but frictional drag. This is why, over a certain extent, lengthening the blade even more, may result in quite a bit more frictional drag (most important on planing hulls) and little more lift at a given angle. A nicer way of doing the same lifting job, is to employ Flexitab's composite trim tabs, which flex up and down in a fair curve, hydraulically operated, to generate both transom lift (bow down) and - when lifted up - 'rocker', which makes the bows lift. In the more conventional tab-down position, Flexitab's gently curved profile will be much more efficient that traditional flat trim planes, cutting down drag and producing a very smooth and 'full' pressure curve, distributed all along the composite blade chord. Visit www.flexitab.com

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