Best Trim Tab Rest or Fully-Retracted Angle

S

stwagstaff

New member
I'm installing a set of Bennet 120 trim tabs on an '83 classic. Since the transom is rounded in back, I've fashioned 10-degree wedges that both straighten and sqare the trim tab brackets relative to the "keel" chines on the bottom of the hull. The hinged-end of my tabs are now mounted to the boat 1/2-inch above the bottom.

I had to raise the bottom of the brackets 1/2-inch (rather than the recommended 1/4-inch) so the through-bolts would clear the floor. I wanted through-bolts to secure the wedges since at the outside end the wedges are almost two inches thick and I did not trust wood screws getting torqued by that extra leverage. The down side of this is that I've got through bolts that I have had to seal very carefully with 5200, since the nuts/washers are right in the wbottom corner where bilge water gathers at the back of the boat.

Now I'm trying to decide on where to mount the actuators and my question is, does anyone have experience with a best "rest" angle for the tabs when they're fully retracted? Bennet's instructions say to put the trailing edge 5/8-inch below the hinged edge, which would put the trailing edge 1/8-inch below the bottom edge of the boat at rest.

I've read some posts that said the tabs can make the boat squirrely in following seas and I'm considering raising the back edge of the tabs an inch or two above the leading edge when fully retracted to give me more clearance in choppy conditions? Anybody have experience with this?

Thanks,
Sean
 
I have always had my tabs equal to the bottom of the boat. The tab itself is not the main factor in the handling--it is the bow down attitude which allows "bow steering" of the hull.

Hopefully you sealed all of the bolt holes in the transom with epoxy, before you put tabs on.h
 
RE-Check the instructions. The back of the tab should be 5/8" ABOVE the hinged end at the rest position. This should allow the tab not to effect the boat at all if desired at cruise. It would also help them from getting bent if your trailer bunks extend beyond the tabs. Or when the stern is against a sandy shore. You don't want the tabs to drag below the boats bottom all the time.

Since you had to mount the hinged edge so high I would opt for a much bigger (longer tab) to help compensate for the higher mount point. If a kicker or some other item does not pose a clearance problem. You will lose allot of the tab effect with the higher mount. And increased drag will result with the higher tab angles required to get the effect of a normal installation.

I have the M120's on a 22 and would have gone bigger if not for the kicker mount. I am even using a permatrim. (heavy boat/stuff)


I know you are committed at this point. I have read where Bennett is more than happy to exchange for bigger tabs if the need arises during installation to keep you happy. If you find yourself in following seas where broach is a possibility about the only quick fix is to move weight aft to hold the stern/prop in the water.

I just re-read your post. Maybe 1/4" higher than normal is not that big of a difference. Kinda hard to tell without seeing your set-up.

Good luck with the install.
 
Chris is absolutely correct. From the installation instructions:

" Step 3 - Secure the lower hinge of each actuator to the tab with 1/4-20 x 3/4" Phillips head machine screws. Using a straightedge under the hull bottom, set the "negative angle" of the tab’s trailing edge as shown in figure 7. 9" chord tabs should have a negative angle of 1/2", and 12" chord tabs should have a negative angle of 5/8"."

The negative angle means that the aft end of the plate is 5/8" above the plane of the bottom of the hull.
 
So, I decided to follow the community advice and mount the tabs according to directions, with the exception that the whole assembly is about 1/3-inch higher than Bennett recommends, because of the way I mounted the wedges. Also, I ended up, inadvertently, with the tabs about 1/8-inch higher than recommended at the trailing edge because of the way the mounts pulled up when I screwed them on, but I figure this can't hurt, except for reducing my downward travel by that much.

I'll post my experiences after I've had a chance to test the boat for awhile.

If they don't work as hoped, I'll have to make a new set of wedges with counter-sunk screw holes, that would allow me to lower the leading edge of the tabs, but this would mean re-drilling and re-seasling everything...I'd rather be fishing...

Thanks again,
Sean
 
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