20MA Trim Tabs

[ste6168]

Hi all -

Have a customer that has purcahsed some trim tabs and wants me to do the install. I stopped by to go a quick assessment, but I am thinking the trim tabs are far too big for the boat.

On one hand, I think they would work, but there are a few issues. They are about 2/3" wider than the first chine. I would think they would still work, just portion of the tab is not going to be fully "in the water" underway.

Second issue, the scuppers on the transom slightly interfere with the mounting brackets. The only real workaround here I see, is either modify the scupper a bit, or modify the hinge portion of the tab taking a bit off the top corner.

My other option is to install them on the inner chine, but would prefer not to do that for several reasons, one big one being the top of the actuators will be mouting behind the splash-well, and the wires will be coming out there as well.




Thoughts?

[JohnC]

Them's some biggons! I imagine they will work but for the expense and work of installing the whole system, I would prefer tabs that are sized right for the hull.
I don't know if there is a downside to too big tabs, maybe someone here can offer an informed opinion.

[Capt Terry]

Added trim tabs to my 20’ Seafari a few months ago. Bennett recommends the tab outboard edge be 3”-4” from the chine. I mounted mine a 5” to avoid major mods to swim platform. They recommended 12”x12” for my use mostly watersports these days. The tab straddles the step between out outer and middle panels which means it is not parallel to either panel. Tried to balance out and is nearly tangent to the radius on the step. Never had tabs before, but seems to works as desired.

[Bushwacker]

STE, I think you have the tabs upside down in those pics! No benefit to having the ends turned up. That will let water escape out the sides and reduce lift! The old Bennett tabs I put on my Seafari had the ends turned down 90 degrees to prevent water from escaping out the sides. Result would be similar on your tabs if you flipped ‘em over!

As for mounting location, I’d put ‘em all the way at outboard edge and trim the mount flange as required. I used the tabs mostly for roll control. (Deep V’s tend to lean into the wind. In a short steep chop, rolling the boat AWAY from the wind and waves increases the deadrise relative to the waves, providing a significant improvement in ride!) Outboard tabs will give maximum rolling moment for a given amount of tab deflection. More deflection = more drag, so you want to minimize that!

I believe the mounting instructions said to mount the tabs about 1/8” above hull surface. If the tabs are wide enough that the inboard edge is inboard of the outer step, several inches of the tab may be out pf the water when on plane and not doing much. However if you invert them relative to your picture, the resulting concave shape will trap water and may eliminate that problem.

[Fr. Frank]

Originally Posted by Bushwacker

I think you have the tabs upside down in those pics! ... if you invert them relative to your picture, the resulting concave shape will trap water.

What he said.

[ste6168]

Originally Posted by Bushwacker

STE, I think you have the tabs upside down in those pics! No benefit to having the ends turned up. That will let water escape out the sides and reduce lift! The old Bennett tabs I put on my Seafari had the ends turned down 90 degrees to prevent water from escaping out the sides. Result would be similar on your tabs if you flipped ‘em over!

As for mounting location, I’d put ‘em all the way at outboard edge and trim the mount flange as required. I used the tabs mostly for roll control. (Deep V’s tend to lean into the wind. In a short steep chop, rolling the boat AWAY from the wind and waves increases the deadrise relative to the waves, providing a significant improvement in ride!) Outboard tabs will give maximum rolling moment for a given amount of tab deflection. More deflection = more drag, so you want to minimize that!

I believe the mounting instructions said to mount the tabs about 1/8” above hull surface. If the tabs are wide enough that the inboard edge is inboard of the outer step, several inches of the tab may be out pf the water when on plane and not doing much. However if you invert them relative to your picture, the resulting concave shape will trap water and may eliminate that problem.

I have never heard of them being inverted, seems it would look a bit funny to me? I have always thought the benefit to having the ends turned up was the way the water rolled past the tab, putting them down would increase drag a tony bit.

At any rate, I wouldn't install them the other way. Lencos installation instructions call for the 'fins' to face up. I can't say that I have ever seen a boat with them mounted in the opposite direction. If yours are, can you post pics?

[ste6168]

Originally Posted by Capt Terry

Added trim tabs to my 20’ Seafari a few months ago. Bennett recommends the tab outboard edge be 3”-4” from the chine. I mounted mine a 5” to avoid major mods to swim platform. They recommended 12”x12” for my use mostly watersports these days. The tab straddles the step between out outer and middle panels which means it is not parallel to either panel. Tried to balance out and is nearly tangent to the radius on the step. Never had tabs before, but seems to works as desired.

Can you post a picture of your setup? These are 12"x12" tabs, as well. I am not sure I am following your description without a picture.

[Capt Terry]

Originally Posted by ste6168

Can you post a picture of your setup? These are 12"x12" tabs, as well. I am not sure I am following your description without a picture.

These pics show my 12” x 12” trim tabs as I arranged them to avoid major changes to my swim platform and ladder. Like a lot of things there have been mods through the years since Bushwacker and I made our platforms together about 1977. I later added the ladder, but wanted it underneath so one could sit comfortably on the platform. The other pic shows No Bones 18” x 9” tab. I mounted mine at 5” from the chine (instead of the recommended 3” – 4” Bennett recommends) so the actuator would avoid my device which supports the ladder. I went with the 12” x 12” as one recommended by Bennett. As I shut the engine down frequently for watersports, but don’t want the tabs to retract every time, I wired the tabs to a separate switch instead of to the ignition. So far I have found the tabs and mounting effective for my needs.

Best wishes, hope this helps.

[fishstu]

My SC 20 MA has 12 x 9/10 trim tabs, the 12 is across the transom. That size trim tab can correct the pitch and roll of my boar more than I have ever required. So I assume bigger trim tabs are over kill, might trim too quickly and may make setting the trim adjustment more sensitive.