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Old 09-25-2022, 08:16 PM   #1
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Trim Tabs on Trawler?

Trim tabs were installed on our CHB 42 by a previous owner. They currently do not work. I have 2 questions:

I see how they would be handy on a planing hull. What purpose do they serve on a 7-8 knot boat?

We have a hydraulic Dickson stern thruster that is not mounted in a tube. We also have an electric bow thruster. The stern thruster’s effectiveness is weak compared to the bow. Is it possible the trim tabs and associated hydraulic cylinders impede the water flow from the stern thruster?

Thanks.
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Old 09-25-2022, 08:46 PM   #2
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I tested the trim tabs on my boat when I first bought it. Haven't touched them since.


My theory is that they effectively add a little to the length of the hull, and therefore give me a tiny increase in hull speed and efficiency. Technically I have a semi-displacement hull, but almost never get much above displacement speed, and I don't think they'd help trim the pitch much at higher speeds anyway.


As to the stern thruster, I would think the same amount of thrust at the transom is going to produce less movement, just due to the hull shape. No doubt there's a theoretical reduction in thrust from the tabs and cylinders, but I can't see it making much difference.
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Old 09-25-2022, 08:49 PM   #3
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If the trim tabs are big enough they can indeed effect heel due to wind or loading, not so much fore to aft trim at 8 knots. If they don't work chances are very good they could be made operational easily if you have basic skills.

Thrust from a propeller is probably not effected by something like trim tabs. Thrust is an action/reaction law of motion, and the propwash hitting the tabs will decrease effectiveness, but the profile of the tabs on edge along with the actuator do not present much surface to impact thrust.

The bow is much easier to move sideways than the stern, there's just a bunch less boat at the bow and much less wetted surface.

$00.02
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Old 09-25-2022, 09:08 PM   #4
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Play with the tabs and see if they do anything useful. My boat has tabs as it planes. But even running at 6.5 kts the tabs are beneficial. Slight deployment reduces pitching a bit in a head sea. More deployment reduces and smooths roll noticeably in a beam or quartering sea. And at slow cruise they don't seem to noticeably impact speed. My tabs are fairly large though (48x12 on a 38 foot boat).
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Old 09-25-2022, 09:19 PM   #5
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I agree with rslifkin. I have an 7-8 knot boat with trim tabs installed by the original owner.

I first thought what a waste. I learned differently as I became familiar with their use.

THey can reduce and eliminate a list, they noticeably knock the stern wave down when I am pushing the boat a bit and I can hear the engine note change.

So don't just write them off untill you have seriously worked with them for a time.
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Old 09-25-2022, 09:54 PM   #6
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Our 34'planing hull Tollycraft has a wetted surface length that calcs to an equivalent 7.58 knot "displacement" hull speed. To get best economy, while still covering some otg miles, we often cruise at 7 knots. When going that speed the trim tabs will somewhat effect port or starboard list but do nothing for fore to aft trim. Going much slower than that it seems the tabs have no effect.

Planing at 16 to 17 knots the trim tabs can play a good part in list angle as well as fore to aft trim. By synchronizing the twins and holding steady on course... playing with the tabs can increase or decrease otg speed by 1/3 nmph.

Of course a lot of what the tabs can do has to do with how heavy you have the boat loaded as well as where you have the weight distributed.

In beam seas, while on plane, adjusted tabs can reduce the roll.
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Old 09-26-2022, 06:11 AM   #7
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Trim Tabs are like any other control surface, how effective they are is a product of how much water (or air) flows over them. Larger boats of course need larger tabs, and slower boats need larger tabs than faster boats.
A trim tab that needs to be deflected a little to have the desired affect will be more efficient than a smaller tab that needs to be deflected more for the same result.
My Mainship 34 is an SD hull with 42 x12 tabs, which I think are just right.



Quote:
Originally Posted by rslifkin View Post
My tabs are fairly large though (48x12 on a 38 foot boat).
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Old 09-26-2022, 08:37 AM   #8
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Quote:
Originally Posted by Keysdisease View Post
Trim Tabs are like any other control surface, how effective they are is a product of how much water (or air) flows over them. Larger boats of course need larger tabs, and slower boats need larger tabs than faster boats.
A trim tab that needs to be deflected a little to have the desired affect will be more efficient than a smaller tab that needs to be deflected more for the same result.
My Mainship 34 is an SD hull with 42 x12 tabs, which I think are just right.

I also had those on my old Mainship. At 7 1/2 knots that size was enough to raise the arse end to get it "on plane".
They also help quite a bit to reduce roll while at anchor.
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Old 09-26-2022, 09:58 AM   #9
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They make a difference in running angle at 8kts on our 40' O.A. They drop the bow ( or raise the stern) about a 1' at that speed. But the do collect a lot of growth and are a P.I.T.A. to keep clean.

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Old 09-26-2022, 02:56 PM   #10
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What’s the recommended way to use them, to find the optimal setting?
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Old 09-26-2022, 03:02 PM   #11
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What’s the recommended way to use them, to find the optimal setting?
If using tabs for ride comfort, experiment until you find what feels best in the current conditions. If using to optimize performance at semi planing or planing speeds, leave the throttles set in place and adjust the tabs to find either highest speed or lowest engine load (if speed isn't changing meaningfully and you have access to load data).
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Old 09-26-2022, 03:09 PM   #12
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Like some others, Tabs were spec’d by my boats original owner. I have played with them some and frankly at the 7 knots that I normally run, they don’t make a bit of different. The PO liked to burn money so he would regularly run the boat at 10 knots. At that speed, the tabs are effective. Now, I wish they simply weren’t there as it is another system to deal with, anodes to change, something to clean etc…
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Old 09-26-2022, 03:19 PM   #13
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Tabs all the way up in a following sea. Play with them in calm seas to figure out what works best on your boat. Then adjust accordingly.
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Old 09-30-2022, 12:57 PM   #14
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Trim Tabs Removed

The tabs on our Mainship 390 were undersized in my opinion. The trim effect was minimal, and I don't run at that gas guzzling speed.



When installed, one of the screws was installed wrong and penetrated the hull. Several years ago we discovered the screw corroded, leaving a pencil lead sized hole flooding the swim platform.


We got the boat to the yard, had them remove and glass over the trim tab areas. One less low-value complication to worry about.
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Old 09-30-2022, 01:07 PM   #15
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Had tabs on a 38' Trawler and found them useful is leveling out the boat from a strong wind on the beam.
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Old 09-30-2022, 07:57 PM   #16
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Quote:
Originally Posted by rjwilliams11741 View Post
Had tabs on a 38' Trawler and found them useful is leveling out the boat from a strong wind on the beam.
Since my boating area is primarily in relatively protected waters with good forcasts, I just avoid winds strong enough to cause an uncomfortable list
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Old 09-30-2022, 10:31 PM   #17
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We had an older Fleming 50 with Cat 3208N's, 11 knots and change at max RPM's but we cruised at 8.5 to 9. It came with trim tabs. They made a slight difference at max RPM but at the speeds we actually travelled, all they accomplished was to increase the fuel burn. They ate zincs! We removed them and never looked back. Good luck!
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Old 10-01-2022, 12:52 AM   #18
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I always thought the tabs on my old GB42 were there to keep me occupied, on those very long 8kt passages, observing observe minute and meaningless differences in trim, speed or heel as I pushed up and down.
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Old 10-01-2022, 04:00 AM   #19
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I took a gamble and installed interceptors on my SD boat. Below 7 knots, they remain off. At 7 knots, which is my preferred cruising speed most of the time, they turn on, but still provide little benefit. Perhaps they reduce roll by ~10%, as they move fast in an attempt to offset roll. Nothing spectacular, however.


At "higher" speeds (~9-12 knots), which I occasionally need to run in certain bad weather and wave conditions, the interceptors are of great benefit, as they make a big difference bringing the bow down. I would not be able to see over the bow without them. They also reduce fuel consumption for me by up to 16% at higher speeds...but the savings is not that exciting, as I rarely run at higher speeds.


If I were in the OP's shoes, I would probably take a gamble and try to get the existing tabs working, if the repair cost was not prohibitive.


And about the stern thruster. The OP could be making a valid observation. I have once seen a ridiculous installation on a new Greenline 39 where the transom-mounted stern thruster in a tube with "hoods" shot water directly on to standard trim tabs (i.e. not interceptors). The installation totally defeated any benefit the thruster could provide. There were only two ways out of this that I could see: 1) Change the tabs to interceptors or 2) remove the transom mounted thruster and install a bottom-mounted model (ideally retractable...but these are expensive). I don't know what they ended up doing to solve the problem.
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Old 10-01-2022, 10:03 AM   #20
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Holy S - Scott... I just spent 15 mins reading and watching video on ZipWake Interceptor- trim tab improvement/replacements. Heck of a good product... it seems. I'd go 1/2 batty setting up the coordinates on its control station. Once coordinates are set up correctly for a particular boat, I imagine staying on the "Auto" selection would suffice. What do you feel may be the average cost range for a 36' to 45' boat to get professionally installed Interceptors?

Important "Trim Tab" Possibility - To the OP:

Per quote from ScottC, post #19... "If I were in the OP's shoes, I would probably take a gamble and try to get the existing tabs working, if the repair cost was not prohibitive."

When we purchased our 1977 great condition Tollycraft in 2008, its Bennett trim tabs did not work. Soon after getting our Tolly I found that the tabs' [obscurely placed] hydraulic fluid reservoir tank [less than a quart in volume] was empty. I figured there must be a leak somewhere; but, could not locate or notice any place for the leak. Sooo... I began carefully filling the hydraulic tank. Realizing as I filled it that either the fluid was immediately leaking out [no sign thereof] - or - the fluid was traveling through the lines to reach and also fill the trim tabs hydraulic pistons in the water. Looking over stern I could not see any fluid discoloration in the water from a leak at the pistons, I then kept filling the reservoir. Eventually the tank reached its full level. I went to helms [in salon and on the fly bridge]... began to use the trim tab switch in all its positions... while wife watched the tabs. Lo and behold... the tabs worked perfectly well. From that time in 2008 [for 14 years now] I have never had to add a drop of fluid and use the tabs whenever desired.

The only two things I can figure for what I describe above: #1. The original owner [he passed away, we are second owners] had tabs newly installed and never filled them with fluid?? OR - 2. Orig owner had new hydraulic pistons installed due to the 1st ones failing - and - never refilled the system with fluid???

Goooood Luck OP!!!
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