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TOPIC: Trim tab size

Trim tab size 19 May 2017 16:05 #1

  • alanmoor
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I have a Bayliner 2750 with 12"X8" trim tabs. According to this:

www.bennetttrimtabs.com/tab-sizing-guidelines/

and this:

www.lectrotab.com/order-guide/trim-tab-plate-sizing-chart-1.php

they should be 18" X 12". I don’t know what the implication of that is (slower getting up on plane?, not as responsive at slower speeds?), but I do know that a tiny adjustment at 25mph makes a big difference in the attitude of the boat (I've put in Bennett Auto-levelers so that solved that problem). I don’t think they do anything at no wake or just above that but they do make a difference in getting up on plane.

It's not worth it to me to upgrade to a whole new system ($500+) but I wondered if I could pop rivet or weld larger tabs on the existing plates. The pump is from "Boat Leveler Mfg Co. I can't get a serial number or model, but I attached a picture of the pump reservoir. The issues I see are whether the pump/pistons could handle the larger tabs, and if the pop rivets would introduce enough drag to be an issue. Also, the tabs would be lower by the amount of their thickness. I just don't know if it would be worth it. Better control at lower speeds wouldn't hurt and getting up on plane faster wouldn't hurt either.

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1985 Bayliner Ciera 2750

Trim tab size 19 May 2017 16:43 #2

  • Grantj
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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.
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Trim tab size 19 May 2017 17:22 #3

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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.

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Trim tab size 19 May 2017 22:59 #4

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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!

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Trim tab size 19 May 2017 23:18 #5

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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
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Last Edit: by Tabman.

Trim tab size 20 May 2017 11:15 #6

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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!


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Trim tab size 21 May 2017 00:12 #7

  • mister larry
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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
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Trim tab size 21 May 2017 17:01 #8

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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..

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Tom
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Last Edit: by Tabman.

Trim tab size 22 May 2017 13:49 #9

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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

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Trim tab size 22 May 2017 14:54 #10

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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.

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1985 Bayliner Ciera 2750

Trim tab size 22 May 2017 14:54 #11

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spaceman 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

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Last Edit: by Tabman.

Trim tab size 22 May 2017 16:10 #12

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Tabman 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.



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?

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"B on D C", is a 1989 2459 Trophy Offshore HT, OMC 5.7L, Cobra OD, Yamaha 15hp kicker. Lots of toys! I'm no mechanic, just a blue water sailer and woodworker who loves deep sea fishing.
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Trim tab size 22 May 2017 16:31 #13

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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.

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Trim tab size 22 May 2017 16:51 #14

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

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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 24″ x 9″, 36-foot boat = no less than 36″ x 9″.

Span = side to side measurement
Chord = fore to aft measurement

Whenever possible, choose a 9″ 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 12″ chord.

Recommended sizes here 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
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Last Edit: by Tabman.

Trim tab size 22 May 2017 17:18 #15

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Tabman wrote: 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

Recommended sizes here 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 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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"B on D C", is a 1989 2459 Trophy Offshore HT, OMC 5.7L, Cobra OD, Yamaha 15hp kicker. Lots of toys! I'm no mechanic, just a blue water sailer and woodworker who loves deep sea fishing.
MMSI: 367637220
HAM: KE7TTR
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BoD, North Olympic Peninsula Puget Sound Anglers, Sequim, WA
Kevin

Trim tab size 22 May 2017 17:50 #16

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We do make 18 x 12 with Drop Fins, but they are optimized to fit right in place on a Bennett system not on the Boat Leveler system on your boat. You would have have to do a lot of modifications and also move the actuators inboard so they were still in the center of your Tab.

Possibly the easiest way to make an improvement would be to fit some Boat Leveler 12 x 12s in place of your 12 x 9s and if you still need some additional lift attach a set of DF12s to the underside of the Tabs.

I just checked and they only make 11" chord tabs, so the DF12s will stick 1" past them, but maybe you could modify them.

I have not tried attaching a Boat Leveler Tab to a Bennett actuator so I can't be much help with that.

Tom
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Last Edit: by Tabman.

Trim tab size 22 May 2017 18:26 #17

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Here is what an 18x12 with drop fins looks like:

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Trim tab size 22 May 2017 20:38 #18

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Tabman wrote: We do make 18 x 12 with Drop Fins, but they are optimized to fit right in place on a Bennett system not on the Boat Leveler system on your boat. You would have have to do a lot of modifications and also move the actuators inboard so they were still in the center of your Tab.

Possibly the easiest way to make an improvement would be to fit some Boat Leveler 12 x 12s in place of your 12 x 9s and if you still need some additional lift attach a set of DF12s to the underside of the Tabs.

I just checked and they only make 11" chord tabs, so the DF12s will stick 1" past them, but maybe you could modify them.

I have not tried attaching a Boat Leveler Tab to a Bennett actuator so I can't be much help with that.

Tom
Bennett Marine


Oh! I thought I had Bennetts. Okay. Thank you.

Hinges versus a hinge plate. Hmmmmm...... It looks like it is a SS hinge spot welded onto some 12ga 304 SS. I could just make some up with the drop fins? My local body shop has the spot welder, and have a friend who has a break. Hmmmmmm.....

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"B on D C", is a 1989 2459 Trophy Offshore HT, OMC 5.7L, Cobra OD, Yamaha 15hp kicker. Lots of toys! I'm no mechanic, just a blue water sailer and woodworker who loves deep sea fishing.
MMSI: 367637220
HAM: KE7TTR
TDI tech diver
BoD, North Olympic Peninsula Puget Sound Anglers, Sequim, WA
Kevin
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Trim tab size 22 May 2017 20:53 #19

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How do you know you should get trim tabs? or is it just an good idea to have regardless?

I have 21ft capri 2152 cuddy.

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Trim tab size 22 May 2017 21:01 #20

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CptCrunchie wrote: [ Hinges versus a hinge plate. Hmmmmm...... It looks like it is a SS hinge spot welded onto some 12ga 304 SS. I could just make some up with the drop fins? My local body shop has the spot welder, and have a friend who has a break. Hmmmmmm.....

That's what I'd do, probably cost ya less than 100 bucks. bolt em on with button head stainless bolts (smooth side down) and nylocks.
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Trim tab size 22 May 2017 21:04 #21

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vonstallin wrote: How do you know you should get trim tabs? or is it just an good idea to have regardless?

I have 21ft capri 2152 cuddy.

Tabs are used to correct list while running and can be used to assist getting the boat to plane quicker.

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Trim tab size 22 May 2017 22:18 #22

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vonstallin wrote: How do you know you should get trim tabs? or is it just an good idea to have regardless?

I have 21ft capri 2152 cuddy.


I have trim tabs only 2459, and use them anytime I am on plane. Yesterday, I caught some slightly bigger seas coming back., Rather than hammer my way home, I prefer to slow down and lower my trim tabs to stay on plane. However, the tabs are too small to really assist, where bigger ones would let me slow down even more, and it would hold the boat to a nicer profile on plane. I'm stoked about the drop fin idea.

Interestingly, if my steering ever went out, as long as I am on plane, I can actually steer using just the trim tabs.

I just remembered I have a set of Bennett pistons from the Chris Craft I wrecked. Will they work with the Load Leveler system?

Hmmmmm..... Since they will be bigger, I shouldn't need as much pitch. Rethinking, I may as well stay with what's on the boat.

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"B on D C", is a 1989 2459 Trophy Offshore HT, OMC 5.7L, Cobra OD, Yamaha 15hp kicker. Lots of toys! I'm no mechanic, just a blue water sailer and woodworker who loves deep sea fishing.
MMSI: 367637220
HAM: KE7TTR
TDI tech diver
BoD, North Olympic Peninsula Puget Sound Anglers, Sequim, WA
Kevin
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Trim tab lesson 23 May 2017 06:14 #23

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OK guys. There's lots of input here, some confusion, and possibly more than a little misunderstanding. Maybe I can help clarify some of this.

Possibly we should start by telling the boaters that do NOT have a trim system already to get educated by Tabman and learn about all things Bennett. They are the "big dog" in the tb universe and have survived other competitors. Depending on the size of your boat, weight, type of cruising, etc. they can sell you exactly what you need. The rest of you can read along. I'm new to this forum and my attempt at attaching visual aids might not be successful but bear with me.

For those of you that might have forgotten, the attitude of your boat ON the water is dictated by speed and dead rise. The boat has a hull designed to plane based on those factors.
On our conversation here, forget speed for a minute. Just assume we can get enough. So we are left with dead rise as the big factor. (illustration attempt #1) OK that didn't work - I need to rewrite the file - no problem. We have some good examples in this thread. Look at Tabman's illustration in post #14. See how close to level the bottom of the boat is? That is close to a zero dead rise - as opposed to the Tabman photo on post #8. That obvious slant in the hull shape is obviously near a 20 degree dead rise. The dead rise is the "V" shape of the hull at the transom.

Can you see how important dead rise in helping the boat get on plane. How much effort does it take to pull a piece of plywood with your nephew kneeling on it? Knowing that the steeper the dead rise the harder it is to get on plane leads us to assume that the steeper the dead rise, the less efficient our trim tabs are since they are mounted in line with the hull: not in line with the waterline. Make sense?

Let's make another assumption. As we introduce dead rise getting away from a flat bottom, we introduce a new exit path for the water coming out from under the boat. Our flat piece of plywood will have an equal flow of water along the entire trailing edge. But our boat with dead rise starts to experience diagonal flow. The water being pushed aside by the keel is coming back to the surface more towards the back corners. The faster we go, the more it straightens out but we're not talking speed - remember. Again, we have flat trim tabs mounted in line with the hull, so water is passing sort of sideways along the plates, towards the back, uphill corner.

Put a small dam on the outside, uphill edge and a significant increase in water pressure is created. The water is forced to change direction a little and the pressure created by that captured water creates lift. That's what the "winglets" on airplanes do. Capture the lift that was falling off the ends of the wings. We have a problem with the boat, though. The tab is freestanding and if wide enough, that extra lifting force on the uphill side can tend to bend the plate on the other side of the hydraulic ram. To get lift on the inside end we need to be more creative, but it can be done. (attachment attempt #2) I'll need to learn how to do that.

Anyway - if you ares till reading this you are visualizing the water pressure creating a high pressure zone in the pocket created by the outside edge drop down attachment. Now visualize a companion piece being bolted on the inside end but extending the tab surface about three inches inboard. The attachment with an illustration of this did come through, but I can't tell how large it is. If you can't read it - visit the fuelishpleasureboats.com site and look at the modification drawing on the "order the book" page.

In my experience, you can bolt these attachments on either the top or underneath most trim set-ups. IF the tabs are narrow, like the 12" variety, one could get away with only fabrication a set to mount on the outside. Much longer than that and the boater would need to have four pieces made. One comment here on Tabmans caution that they found drop downs to be ineffective on narrow tabs: if the tested hulls had much of a dead rise we can see why. The water doesn't have much opportunity to fill the uphill high pressure zone and the current bumping up against the downhill drop down would be counter forces.

All right. That's the trim tab lesson for now. Time for my attachment lesson.

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Trim tab lesson 23 May 2017 14:47 #24

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mister larry wrote: Time for my attachment lesson.


Interesting post, and I can certainly see your point.



If I understand correctly, you are saying this is the fin modification I should be doing to my boat with my dead rise. My original idea was to bolt a U shape plate to my existing tabs. However, this would create torque to the outermost point. But by putting two, this creates double the lift and spreads that lift out over the tab.

Makes sense to me. Thank you.

ADDITION: Now I'm wondering how long (deep) the fins should be. 2"? 3"? 4"?

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"B on D C", is a 1989 2459 Trophy Offshore HT, OMC 5.7L, Cobra OD, Yamaha 15hp kicker. Lots of toys! I'm no mechanic, just a blue water sailer and woodworker who loves deep sea fishing.
MMSI: 367637220
HAM: KE7TTR
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BoD, North Olympic Peninsula Puget Sound Anglers, Sequim, WA
Kevin
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Trim tab lesson 23 May 2017 16:06 #25

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I normally use no more than three inches. You want enough high pressure pocket to be meaningful, but be careful that the leading corner does not come even close to the hull if you are forced to put the tabs all the way down.

Even when you trim that leading corner to 45 degrees take care that nothing touches anything when the tabs are all the way down. One easy way to check is just clamp them in place prior to bolting and put the trim system through its' paces.
The following user(s) said Thank You: CptCrunchie, alanmoor

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