Trim tab fitting advice

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Jonny2018

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Hopefully the pics will be viewable below.

We have ordered some trim tabs (Bennet SLT10's) for fitting to a fletcher 18gts. Hydraulics/manual no electrics
They are not arriving unill Wednesday so I'm sure there will be fitting instructions.
However I've started researching the fitting positions and there appear to be some conflicting instructions. Some say the hinge should be flush with the bottom but there will be gaps due to the ridges others say a few inches up and in from the corner (I forget the correct terminology)
Can anyone give me some advice or tips on fitting them?
https://imgshare.io/image/5UFGn
https://imgshare.io/image/5UDPY

Thank you!
 
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The Bennett trim tabs fitted to my 20ft Finnmaster are flush with the bottom of the hull, and as far outboard on the transom as possible.
 
There, at that angle

BBWuSbi.jpg
 
AllanG said:
The Bennett trim tabs fitted to my 20ft Finnmaster are flush with the bottom of the hull, and as far outboard on the transom as possible.
Click to expand...
Can I ask why? I've read some say 4-5" inch in from the corner and others say the same as you I'm just wondering your reasons behind it as you seem in the know.
 
BruceK said:
There, at that angle

BBWuSbi.jpg
Click to expand...

I've not measured that section to see if its 10" but thank you for the edited pic!!
So are you suggesting to place them as others have said, as close to the corner as possible? I'm just confused as to why some say to have it 4-5" in from the corner.
 
Jonny2018 said:
I'm just confused as to why some say to have it 4-5" in from the corner.
Click to expand...
Two possible reasons for that:
either who say so doesn't have a clue but wants to say something regardless (a pretty common situation, in the cyberspace),
or they were talking of an installation with some particular restrictions.

Ref. the height, if the position that BruceK suggested wouldn't be large enough for the width of your flaps, personally I would rather fit them slightly raised in the inner side. Not perfect, but better than having the flap internal side exposed under the hull surface.
The ideal setup would be with tab plates exactly as large as the red line...
...but I appreciate that if you already bought the kit this isn't a very useful suggestion! :rolleyes:
 
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PS: the following pic is just to show one example of an installation where the most outward position was restricted.
Rather self-explanatory, I think.
Mind, this boat still performs beautifully, also because those K-planes are longer than any Bennet tabs.
Nonetheless, a more outward position would have been even better, if feasible.
Hull.jpg
 
Jonny2018 said:
That's great, thank you Allan! I never thought to search for some online instructions. Can I ask how you decided which of the 3 mounting settings to put it in?
Click to expand...

I bought my boat secondhand, and the trim tabs were already fitted, so I don’t know who decided the position to install the tabs.
 
Jonny2018 said:
I've not measured that section to see if its 10" but thank you for the edited pic!!
So are you suggesting to place them as others have said, as close to the corner as possible? I'm just confused as to why some say to have it 4-5" in from the corner.
Click to expand...

You want them as far outboard as possible but not crossing any chine. It's ok to cross a lifting strake. 4 to 5 inches on your size boat is too far imo. ~2 inches from the chine should be about right (Maybe others were thinking in cm and repeated it in inches). I have made the assumption that the line is a lot longer than 10 inches. It's there to show where to place them. The further outboard they are the more sensitive / effective they are w.r.t. lateral trim, i.e. do not need as much deflection. This should then pay dividends in a hard turn at speed. I'd have the bottom of the tabs 1/2 to 3/4 inch above . The lower they are the more effective once again but as you are a trailer boat and liable to also beach you want to place your tabs in such a manner that at neutral deflection you have some room to back out in shallow water without having the tabs dig in to the sea bed at reasonable angles of entry if that makes any sense.
BTW. As Mapism says, this is opinion and observation and does not have the backing of scientific research although a small amount of trial and error on my part ;)
 
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