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Old 12-15-2015, 11:51 PM   #1
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Trim Tab Thread (ttt)

So, Scot Free was slow to get up to full speed during sea trials and ran a little bow up for a while, ploughing throught the water before it gathers enough momentum to plane. So I thought that perhap I could use a pair of trim tabs to lift the ass end and help things along.

So, I came across Interceptor Trim Tabs by Hydro-Tab and sold by Globaltec Solutions. (GlobalTec Solutions for those who want to look them up) I have never seen anything like them. They have a vertical carbon fiber blade that is pushed down broadwise into the waterflow at the stern. I would have thought that this would actually cause a speed brake effect but manufacturers claim that they actually do the opposite.

Has anybody here heard of them? Do they work? I would love to hear of any real world experience with them before I commit some boat$$ to them
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Old 12-16-2015, 02:07 AM   #2
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Without straining the pic posting skills, have you got a pic of your 53ft Cheoy Lee on the plane? I`d like to see that.
My little boat has a 2" high full width wedge, PO added, at the trailing edge of the transom, tapering as it goes fwd. Consensus is it was intended to keep the bow down, planing with Lehman 120s is I fear, a step too far.
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Old 12-16-2015, 07:50 AM   #3
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Think I would talk to a naval architect before writing a check. Hull shape and tab size are going to be important in getting the sizing right. This isn't the same as trim tabs on a 25' center console.

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Old 12-16-2015, 09:36 AM   #4
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I have seen those tabs. But never used them.

I would speak to the folks at Bennett trim tabs.

Bennett Marine | The World Leader in Trim Tab Innovation, Quality, and Service

They have decades and decades of experience putting tabs on all kinds of boats. And are very helpful people with great customer service.
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Old 12-16-2015, 09:43 AM   #5
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Volvo was the first to the market with that concept.
http://www.volvopenta.com/SiteCollec...System_eng.pdf
I don't know anybody who has tried them.
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Old 12-16-2015, 11:26 AM   #6
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Quote:
Originally Posted by BruceK View Post
Without straining the pic posting skills, have you got a pic of your 53ft Cheoy Lee on the plane? I`d like to see that.
My little boat has a 2" high full width wedge, PO added, at the trailing edge of the transom, tapering as it goes fwd. Consensus is it was intended to keep the bow down, planing with Lehman 120s is I fear, a step too far.
That is a picture I do not have. I have not had anyone take a picture from outside the boat while I'm underway.

There is a pic in the brochure on board. I will see if I can find it and scan it in.
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Old 12-16-2015, 11:38 AM   #7
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Had a 30' Tolly that would not get up on a plane without trim tabs on "full." Always thought it was a design failure by Ed Monk Sr.
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Old 12-16-2015, 02:28 PM   #8
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Interceptors are tried and true and actually pretty old technology and work well within their more optimum operating range. These can commonly be found on ships, especially military types and has been around since WWII. There are at least a handful of manufacturers making these units, some have been around for decades.

It is counterintuitive to think that pushing down a vertical plane at the transom could someway induce lift, but it does.


The more optimal lift performance for interceptors comes at higher speeds, and trim tabs work better at slower speeds.

Act accordingly. The Bennett system has all but one moving part inside the boat. There are no electrical components outside and a vast majority of problems with this system can be fixed with vessel in the water. Not always true with most interceptor systems and some other brand trim tabs.

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Old 12-16-2015, 02:57 PM   #9
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Quote:
Originally Posted by BruceK View Post
Without straining the pic posting skills, have you got a pic of your 53ft Cheoy Lee on the plane? I`d like to see that.
My little boat has a 2" high full width wedge, PO added, at the trailing edge of the transom, tapering as it goes fwd. Consensus is it was intended to keep the bow down, planing with Lehman 120s is I fear, a step too far.
I have the same mod on my '78 Mainship done by the PO. I don't think it actually helps but I have no experience without it. My guess is there was a fad when this mod was done. I think the wedges were supposed to be placed amid the props - recall seeing some hydrodynamic reason.

I've also seen the sliding "blade" that comes down the back, and the claims about how it creates lift without a whole lot of friction. I believe they also hook them to a computer and manipulate them to reduce roll. IMO, roll reduction makes sense since the lift created is pretty clear. However I'm skeptical it would actually lift you up on plane.

I think the only way to get the bow down "correctly" is to get enough speed to get over the hump. The only way I've been able to calculate how this can be done without changing the motor is to:
reduce the weight of the hull
increase the flat planing area
reduce water friction (fair aft keel, clean bottom)
optimize propeller

I've heard very large trim tabs can help. One vendor has a whole lateral hydrofoil under the swimstep, but it's more for mega boats.
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Old 12-16-2015, 03:32 PM   #10
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Quote:
Originally Posted by R_p_ryan View Post
I've also seen the sliding "blade" that comes down the back, and the claims about how it creates lift without a whole lot of friction. I believe they also hook them to a computer and manipulate them to reduce roll. IMO, roll reduction makes sense since the lift created is pretty clear. However I'm skeptical it would actually lift you up on plane.
What you describe above is an interceptor. Bennett and some competitors offer a similar option, Auto Tab Control, works.



[/QUOTE] I've heard very large trim tabs can help. [/QUOTE]

They can. The slower a boat operates the more tab they need, the opposite holds true for faster boats, that's why race boats have small trim tabs. Think fighter jet vs glider, compare wing size.

My '78 Mainship has 42 x 1 trim tabs which allow me to plane at 13.5knots and maybe hit 15 going downhill. Very little chance of planing without them.

Fixed wedges will work to force the bow down, but they are not adjustable, and boats with fine entry forward like Mainships are susceptible to bow steering which can be dangerous.

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Old 12-16-2015, 04:42 PM   #11
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[/QUOTE] I've heard very large trim tabs can help. [/QUOTE]

They can. The slower a boat operates the more tab they need, the opposite holds true for faster boats, that's why race boats have small trim tabs. Think fighter jet vs glider, compare wing size.

My '78 Mainship has 42 x 1 trim tabs which allow me to plane at 13.5knots and maybe hit 15 going downhill. Very little chance of planing without them.

[/QUOTE]

Do you have your prop specs handy, and happen to know/guess the RPMs at those ranges? Also, is this with the Perkins T6.354?

I'm jealous seeing those numbers!
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Old 12-17-2015, 05:59 AM   #12
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[/QUOTE] My '78 Mainship has 42 x 1 trim tabs which allow me to plane at 13.5knots and maybe hit 15 going downhill. Very little chance of planing without them. [/QUOTE]

Do you have your prop specs handy, and happen to know/guess the RPMs at those ranges? Also, is this with the Perkins T6.354?

I'm jealous seeing those numbers![/QUOTE]


The Tab size is 42 x 12, not 42 x 1, missed a key. There are a few 78' models out there with 200HP Chrysler Mitsubishi engines. These were factory installed and the story goes that was the HP upgrade that year or the other story was Perkins was having problems keeping up with demand.

Even with 200HP this boat wallows onto plane and a turn or even a wake will often bring her down, it's not something we do very often but I do like to open her up once in a while and sometimes to make a bridge opening we'll "run."
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Old 12-17-2015, 12:09 PM   #13
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Keysdisease,

Do you have the specs on your prop? While I have the lesser powered motor I keep my hull as light as possible and am trying to get to a sustainable 12 knots. Sometimes I can get there, others not so much. And when I do, the wake flattens out and steering tightens up a bit.

Currently I am under-propped, as I can achieve over 2500 RPMs at WOT with full tanks and a load of people. I believe that boat should be ideally propped so WOT = 2450 RPMs.

I would like to:
  • Add a fairing to the aft of the keel
  • Remove the wedges
  • Add 42 X 18" trim tabs

Then re-prop so I'm limited to 2450 RPMs, and ideally obtain 12 knots @ 2300 RPMs. When the boat "gets on top", everything feels much better and it feels like a different boat.
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Old 12-17-2015, 01:47 PM   #14
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I'll see if I can dig up the prop size when I get home tonight. It's a 4 blade, I think around 22 square. I find for my boat that 13.5knts is the minimum planning speed, but she's probably heavier than yours.

I did the keel fairing and think it helped some, I would do it again, every little bit helps.

42 x 12 tabs are single ram and a standard Bennett size. The transom is not straight so you will have to make a fairing block to attach the tabs. If you do tabs then taking the wedges off is a good idea, otherwise they will just disturb the waterflow to the tabs and degrade their performance.

I have heard in the deep dark past that the 160/165 HP Perkins powered boats could get on plane by plowing like hell creating a giant wake, then you stop suddenly and punch it at just the right time that the wake coming up behind you "surfs" you up onto plane. Wives tale? I don't know, maybe?

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Old 12-17-2015, 03:24 PM   #15
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This seems odd to me that trim tabs would actually show up on a forum for trawlers. I consider them about as useless as cupped propellers on a slow boat. A drag to lift issue. Most trawlers are very heavy and even if the device would lift the stern the boat probably still wouldn't plane.

Mugillicudy,
This must be something I've never seen before.
Pictureswould be very helpful ..., umless these "tabs" are nothing new. The best a tab could do is to adjust the hull for it's optimum angle of attack .. and of course it has to pay for it's own drag.
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Old 12-17-2015, 07:03 PM   #16
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Mainships are Downeast style hulls that plane with very little HP, so they are not full displacement hulls at all.

These are the Volvo brand interceptors, the blades go straight down vertically at the transom.



Quote:
Originally Posted by manyboats View Post
This seems odd to me that trim tabs would actually show up on a forum for trawlers. I consider them about as useless as cupped propellers on a slow boat. A drag to lift issue. Most trawlers are very heavy and even if the device would lift the stern the boat probably still wouldn't plane.

Mugillicudy,
This must be something I've never seen before.
Pictureswould be very helpful ..., umless these "tabs" are nothing new. The best a tab could do is to adjust the hull for it's optimum angle of attack .. and of course it has to pay for it's own drag.
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Old 12-17-2015, 07:34 PM   #17
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Keysdisease,
Yes the Mainship is more of a planing hull than even a SD hull. And I agree w you their hull form should plane easily but I suspect they are not much lighter than most similar boats. They slowly go from deep deadrise fwd to no deadrise at the transom. They do have a twisted bottom that should have low resistance at medium speeds. But I think the M34 does make 20 knots w the 6cyl Perkins most of them have if the owners haven't put too much weight aboard.

Now re the "tabs". I'm sorry I can't tell what that image represents. Below the WL the M34 has a fairly narrow hull by modern standards and that would make the boat less likely to positively respond to benefits of tabs. The gain in performance must make it worth the extra drag of the tabs themselves. So if your boat could gain from tabs it may gain more w/o the added tab drag by moving the CG to optimize trim angle. Care should be taken to insure she floats very close to "on her lines". You may need a conversation w Mainship to learn where her lines are. Design WL.
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Old 12-17-2015, 09:46 PM   #18
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I agree with your observations about the hull, it's flat as a pancake at the transom with hard chines aft of the transition about amidships, its a planning hull with a keel.

Trim tabs on a Mainship help the boat run at the optimum design angle, run on an even keel regardless of loading or sea conditions, plane at slower speed and get on plane faster and easier.

The early models had 160 and 165 Perkins engines, later models had the 200 Perkins. A few had something different like mine with the Chrysler Mitsubishi and a few with DD power.

The full keel made handling anything above about 17 knots kind of "unusual" according to some owners that repowered with extra HP (Jay, you there?)

Those higher speeds made bow steer with the fine entry bow a real possibility with the shaft angle driving the bow down.

15 -17 knots it seems by consensus was the sweet spot if on plane, and somewhere between 12 and 13 the minimum planning speed.

With full fuel (300 gal) and cruising load my MSI with 200HP would not plane without trim tabs, a little tab down and the bow drops, speed goes up, wake flattens. If the wind is on the beam the high freeboard and house will push her off her lines, a little tab adjustment and back on an even keel.

Regarding the extra drag, a large tab deployed a little will get the desired effect with minimal drag which is typically more than offset by the efficiency gained by running on her lines. Those owners with fuel flow meters installed bear out these observations regarding fuel burn vs speed with or without tabs.

My experience with my '78 MSI over 11 years was that tabs made the boat perform the way I wanted her to and I believe closer to the lines the designer (Cherubini) planned regardless of loading or sea / wind conditions.

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