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**Grantj** · 05-19-2017, 09:43 AM

You might want to put a call into Bennett tech support. The fellow there (M.J. Thomas) knows about everything there is to know about trim tabs. They (Bennett) actually recommend 1" of tab span (width) for every foot of boat length (assuming a 9" chord). So that's more like a 24-28" tab for you, if you can fit that much width on your transom.

Check out their website, it has a lot of good info. Your observation that a small adjustment at cruising speed has a significant effect is to be expected. The biggest advantage to larger tabs in a planning hull like yours, is the ability to stay on plane at a lower speed, by deploying the tabs. This is helpful when you can't cruise at normal speed due to rough water, but you want to stay on plane in the 15-20 knot range. That's difficult to do on a lot of boats that come (typically) factory-equipped with tabs that are far too small...

As far as riveting extensions - sounds a little mickey-mouse to me. I wouldn't bother. One of the things you'll learn from the Bennett website is that span is more effective than chord, which is why they recommend trying to stay with the 9" chord if possible. Save your money and put on some larger tabs during your next haul-out. It's not that hard, I've done a few boats about your size. Bennett will be probably be able to advise you on re-using your existing rams, even if they're from the competition.

**builderdude** · 05-19-2017, 10:22 AM

FWIW I have 12 x 12 drop fin tabs on my 2556 and they seem to work great. Like you say just a touch of the button and the attitude of the boat changes. I know there are several members that have increasesed their existing tab size by attaching extra stainless sheet metal with good results.

**Dakota** · 05-19-2017, 03:59 PM

Bennett has bolt on drop tab parts. Drill a few holes and you are in business. No need to replace the entire trim tab. Ask about this option when you call Bennett.

Good luck!

**Tabman** · 05-19-2017, 04:18 PM

The Bennett Drop Fin add on DF12 may not work well with Boat Leveler (Insta Trim) Tab because if I recall correctly Boat Leveler Tabs are bent up on the edges at an angle. Plus they only fit on 12" chord (fore to aft) Tabs.

Larger Trim Tabs will be worth the effort, faster on plane and lower planing speeds. I am not a fan of welding larger Tabs on to the existing ones as it invites corrosion.

If it were my boat and they are Boat Leveler Tabs I would contact them and see if they have 12 x 12 Tabs that could be put right in place of your existing ones without having to move the actuators. Or larger ones if you have room on both sides and won't have to move the actuators.

Tom McGow

Bennett Marine

**2850Bounty** · 05-20-2017, 04:15 AM

As per the image in post #1, the OP has the old "Boat Leveler" system.

The Boat Leveler actuator pistons are smaller than the Bennet acuator pistons, which typically means that they will not provide the same force as will the Bennet system. In other words, use caution if going to a larger TT plane while staying with the Boat Leveler actuators.

Personally, I would switch to a Bennet system!

.

**mister_larry** · 05-20-2017, 05:12 PM

Grantj is right: the science says the more width the better, not chord (depth). This is partially because as you drop the tabs deeper into the water you create more drag. So go for wider tabs if you want a bit better performance. Naturally, the Bennet folks want to sell you as much tab as possible but I'd say your existing system is a good place to start. Your hydraulic rams may be a bit small, but the real work comes from the pump, and most companies use the same pump for a variety of widths. It's a safe guess that your 12" wide plates are about the smallest Boat Leveler used, so your desire of getting a wider planing system may be accomplished by just getting wider tabs.

In my years of working with trim systems I was forced to come up with a universal kit that consisted of drop down devices, two per tab. This is because on the wider tabs I wanted to balance the pressures on each side of the ram so as to not tweak the plates. Your tabs are not wide enough to worry about this, and I would suggest you add a drop down device only on the outboard edge. Even if you have a slight downward bend at the edges of your boat Levelers you can bolt on an attachment outboard of that. I have yet to encounter a trim tab installation that doesn't leave some room on the outboard edge. Most of my installations call for adding three inches to the width of the tab, on each side. Here, we are talking about only adding width to the outboard side.

I always taper the leading corner of the drop down attachment. Any debris encountered will be deflected easier that way. We have identified that the trailing edge width is important. However, equally as beneficial is the pressure created by the drop down. Have you noticed that almost all commercial aircraft have vertical tips on the ends of their wings now? (some call them winglets) This is what you want to duplicate under water. Once (years ago) at the Seattle Boat Show I was taking with a Boeing engineer. This was before winglets were popular and he confided that they were doing some research on them. So far the results indicated a 7% improvement in the efficiency of the wing. And that is with air which is a lot more compressible than water.

If you want to see a visual of my attachments, visit fuelishpleasureboats.com and click on the "buy the book" drop down. Naturally, you don't want to buy the kit but the illustration of the kit shows you what the modifications look like. Work with a sheet metal shop, bend and trim some stainless, and bolt the parts together with some stainless, pan head bolts. You will notice an improvement, and only be into the project less than $100 I'd guess.

Or you can buy the book, get a free set of plans and get answers to a lot of performance and maintenance questions.

Happy boating.

Larry

**Tabman** · 05-21-2017, 10:01 AM

Bennett DF12 add on Drop Fins. Google "Bennett DF12" and dealers sell them for about $30.00 for a set of 4. They are designed to work on 12" chord Tabs, we have found that adding Drop Fins on 9" chord Tabs does not add significant lift..

IMG_0363

426009_368108973207226_126783417339784_1376524_762 55403_n

Tom

Bennett Marine

**Guest** · 05-22-2017, 06:49 AM

I had same problem with my 2452,so I got some 3/16" thick galvanized metal and extended them out the rear about 5",made a big difference in getting to plane and handling

**alanmoor** · 05-22-2017, 07:54 AM

Great information in these posts, especially with regard tot he drop downs ("wings"). I think I might try the extended tab with the drop down bolt ons similar to what fuelishpleasure outlines. I'd essentially make it a 16-18" span with "wings". On one side the depth sounder is a restriction and on the other side is the speedometer pickup. Having written that I wonder what effect this will have on the speedometer accuracy. I have a friend with a metal brake and I may or may not have the stainless I need to do it (I suspect the stuff I have is too thin) so the project could be pretty cheap. I wouldn't have thought of pan-head bolts as opposed to rivets, I like that. I guess if I screw it all up (burn out the pump or whatever) I'll have to get a new Bennett system!

I checked on larger plates, but 2 plates big enough would be a major portion of a new Bennett system and I' can't see the sense in spending that much money on what might be a 30 year old system.

I found Bennett's DF12 for sale, but I didn't see it on their web site. I think I'll still call them to make sure they don't have some other magical cure for me.

**Tabman** · 05-22-2017, 07:54 AM

"spaceman" post=810134 wrote:
I had same problem with my 2452,so I got some 3/16" thick galvanized metal and extended them out the rear about 5",made a big difference in getting to plane and handling

If the boat is kept or used in saltwater combining two dissimilar metals like this is a recipe for corrosion and I would advise against it.

Tom

Bennett Marine

**CptCrunchie** · 05-22-2017, 09:10 AM

"Tabman" post=810041 wrote:
Bennett DF12 add on Drop Fins. Google "Bennett DF12" and dealers sell them for about $30.00 for a set of 4. They are designed to work on 12" chord Tabs, we have found that adding Drop Fins on 9" chord Tabs does not add significant lift.

Interesting thread. I have the very same issue on my 2459 as the OP. Tom, could you please explain:

....how something basically vertical like the dropdown tabs increase lift?

....how speed is an element of the size needed? I'm confused. Because bigger would need less angle, speed should only come into it if they are really small. Then why use such a small trim tab?

....Is there a more common size that supports better performance over a variety of speeds?

Here are my tabs.

[attachment]36691 wrote:
BonDCTrimTabs.jpg[/attachment]

I generally cruise around 20-23kts, though yesterday it was closer to 17. After reading this thread, like the OP, I'm thinking about increasing the size. However, 2 - 24" x 12" is out of the question. Can I use the same rams, and if not, what is the biggest and best size I can get to use with the current system?

**SKUA_28_Contessa** · 05-22-2017, 09:31 AM

I put much larger dropfin tabs on my Contessa. With the bigger surface I didn't need to put them done much at all, so they stopped acting like speed brakes when deployed. Picked up about 2 MPH at cruise . Also converted to 2 rams per plate. 1 centered looked way too spindly.

**Tabman** · 05-22-2017, 09:51 AM

Drop Fins capture water pressure that is normally lost out the sides of the Tab and channels it aft to generate more lift.

305739_2

Boat builders almost always install smaller Tabs than we would recommend. I suppose cost is one factor, plus the usually do there testing on a boat with the max horsepower option and lightly loaded.

Speed, size of the Tab and angle of deflection are the three contributing factors in the lift a tab can generate. But the angle of deflection is limited, too much and instead of a lifting surface it becomes a speed brake.

Larger Tabs create less drag since they need to be deflected less.

Sizing recommendations are based on average performance. Your choice may vary based on power, engine configuration, weight distribution, type of boat, and use. The greater the surface area, angle of deflection and/or speed of the water flowing under the trim tab, the greater the lift.

When making a choice between trim tab sizes, remember that the largest trim tabs that will comfortably fit on the transom will be the most efficient.

As a rule, choose at least one inch of trim tab span (per side) for every foot of boat length. (Examples: 22-foot boat = no less than 24ÔÇ│ x 9ÔÇ│, 36-foot boat = no less than 36ÔÇ│ x 9ÔÇ│.

Span = side to side measurement

Chord = fore to aft measurement

Whenever possible, choose a 9ÔÇ│ chord trim tab, and gain lift through a longer span. The span of the trim tab has more of an effect on the amount of lift. However, a longer chord can be used effectively, and there are situations where you may need to use a 12ÔÇ│ chord.

Recommended sizes here https://bennetttrimtabs.com/tab-sizing-guidelines/

The largest Tab we make with a single actuator is 42 x 12, Boat Leveler may make comparable size or maybe a little smaller. So your actuator should handle any size that is suitable for your boat.

Tom McGow

Bennett Marine

**CptCrunchie** · 05-22-2017, 10:18 AM

"Tabman" post=810187 wrote:
Drop Fins capture water pressure that is normally lost out the sides of the Tab and channels it aft to generate more lift.

https://flic.kr/p/URGCqB wrote: <br ....jpg[/img]https://flic.kr/p/URGCqB wrote:
305739_2

Recommended sizes here https://bennetttrimtabs.com/tab-sizing-guidelines/

The largest Tab we make with a single actuator is 42 x 12, Boat Leveler may make comparable size or maybe a little smaller. So your actuator should handle any size that is suitable for your boat.

Excellent image and description. It appears my best bet is the https://www.amazon.com/Bennett-Marin...br /> TPA1812. Does Bennett make these with the dropdown tabs? With the angled trailing edge, how does this one attach to my current base? Or is there another one I should be looking at?

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