Benefit of trim tabs on semi-d boat?

Keith-i

Keith-i

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We have a Channel Islands 32 which naturally has a semi-displacement hull type and a typical cruising speed of around 14kts. Many years ago a previous owner fitted a pair of trim tabs which now no longer work properly. So, before going to the expense of replacing them, or at least the pump unit and controls I wondered whether they are really of any use. I can appreciate they might help control list when in a beam sea or wind, but can they add anything in practical terms to performance or comfort? Unfortunately I can't judge this for myself as only one side operates.
 
I have semi displacement Swift Trawler 34, so we’re no too far apart. I use the trim tabs to level the boat, port and starboard. When the water tank is full the boat lists to port, the tabs can level her up but it does make steering more of an effort. I also occasionally use them in a beam sea.
I also use them to lower the bow (probably by raising the stern) this makes the boat ride much better, more level and there’s not that hole in the water at the stern. But while it looks and feels better, there is no increase in top end speed, or decrease in engine revs at any speed.
In short, the ride feels better but the boat doesn’t go better.
 
Had a set of the now defunct Volvo Penta blade type trim tabs fitted to a Broom 37 hull.
Max speed about 15/16 knots.
Made virtually no difference to top speed (.5 knot)and only real benefit was did bring the bow down a bit which was an aid to forward visibility.
Considering the amount of money the original owner would have spend buying and installing the system bit pointless on this particular boat.
For fast :) long off shore passages some marginal visibility advantage, for displacement short trips waste of money.
 
Keith-i said:
Unfortunately I can't judge this for myself as only one side operates.
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If the boat listing is meaningfully affected when the working side is lowered, also the whole hull attitude (angle of attack) is bound to be affected when both are working.
I half disagree with oldgit btw, because a lower AoA doesn't mean just better visibility, but also a less bumpy ride, in principle.
And I'd be surprised if that wouldn't be true in any 32 footer cruising at 14kts.
 
omega2 said:
Ours definitely assist to lift or lower the bow, 15 knot Aquabell 33, but what If any speed/fuel gains unable to assist, just keeps the windows clear going into a head sea, re passengers you would need to nail them down.

https://youtu.be/KqfZ6ZXl5pc
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agree, we have similar boats, our Mitchell 31 has them, we cruise around 11.5 knots with tabs full down position, if I bring them fully up speed drops to 10.5knots so it gains me a knot, drops nose down a touch and makes for a more comfortable ride and I use them all the time. in a following see I don't use them. Interesting thing on the Mitchell they have no notable difference in trimming boat angle side to side. She always runs flat (side to side) almost regardless of where folk sit but she is heavy, doesn't move much when folk wonder around, main difference between old British and new French I guess. .
 
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Aquastar 33. Fitted from new allegedly. Makes a very slight difference at 15 knots as she has a tendency to bury her stern. Frankly though I don't bomb around at that speed much as its too damn expensive. If I wanted a speedboat I wouldn't have a semi-displacement hull..... At 8-10 knots it makes booger all difference so personally I wouldn't bother.
 
Thanks for the responses; a bit of a mixed bag then but very interesting. Seeing as the tabs are sound I might replace the pump unit at some point if I'm bored with some cash to spare, but I won't rush out to buy one.
 
Volvo tabs fitted to our 805, definite improvement to performance and trim. Delighted. Used to have CI 22 that could have done with them as well
 
gordmac said:
If one side works would that not mean the pump is ok?
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They are a very old QL system. The defective side will extend but not retract properly. I’ve had the whole lot apart and repaired what I can but the unit just isn’t worth saving. Connections were heavily corroded, the motor is on its last legs (the magnets fell out) and I suspect the valves are now at fault.
 
MapisM said:
If the boat listing is meaningfully affected when the working side is lowered, also the whole hull attitude (angle of attack) is bound to be affected when both are working.
I half disagree with oldgit btw, because a lower AoA doesn't mean just better visibility, but also a less bumpy ride, in principle.
And I'd be surprised if that wouldn't be true in any 32 footer cruising at 14kts.
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On a planing boat with flat forward chines it may help keep the slamming down, on the bath tub bow of the Broom it makes precious little difference.
The Broom hull rolls like a drunken pig in beam sea and all.... :0
 
The CI 32 like my Aquastar 48 , and previously my Halmatic 34 as well as my Mitchel 31mk 2 and mk1 and my 2 are all round bilge.
They all behaved differently to deep V or hard chine boats.
with round bilge the boats have tabs but don’t need them To level the boat or to gain speed.
If it slows you down it’s costing diesel.
If in doubt phone IMC in Guernsey builders of CI 32, pm me for no.
 
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Personally I don’t think the wonderful CI 22 needs them. It’s wouldn’t suffer the effects of windage and listing trim issues that larger boats would, just by its very nature, size and hull dynamics.
I’d think if the designer thought they did require them, they would be on them. Not as if this particular and well respected boat was ever compromised on build or finish when new.
Just my opinion.
 
The first 6 built of the Channel Island 22 did not have the extended trim tab, kinda fixed 'tab' from memory but afterwards all did
The original ones built buy 'Silver Yates Plastics' didn't have a wheelhouse either and later a wheelhouse was 'bolted' on until the Firm made the vessel with the open wheelhouse integral
Sorry to digress but I am a CI 22 'anorak'!
The fixed Tab on the CI22 gave the vessel 'lift' and especially those with the 140 turbo Mermaid could achieve 19kts The fixed 'trim tab' as we call it gave the hull extra length in effect to enable the boat to kinda 'plane' as it were, I aint no hull design Expert here peeps just my findings an honest opinion as it were
I have fitted trim tabs to a few semi displacement boats including an 805 as mentioned earlier, which suffered from a 'list', ie it was and some were, handicapped by the nature of the layout of the weight distribution of batteries fuel tank etc etc so trim tabs helped to 'trim' the boat to level aspect and therefore keep all aboard happy
The answer is Yes, Keith-i, fix the busted tab!
The CI 32 is a bonny vessel, which will serve you well, great boat.
With both Tabs working it will help your fuel economy, your boat will almost 'plane' if you want it to
Yes, it is a semi planer but She will benefit from both '
Tabs working
Trust Me!
:encouragement:
 
Bouba said:
I have semi displacement Swift Trawler 34, so we’re no too far apart. I use the trim tabs to level the boat, port and starboard. When the water tank is full the boat lists to port, the tabs can level her up but it does make steering more of an effort. I also occasionally use them in a beam sea.
I also use them to lower the bow (probably by raising the stern) this makes the boat ride much better, more level and there’s not that hole in the water at the stern. But while it looks and feels better, there is no increase in top end speed, or decrease in engine revs at any speed.
In short, the ride feels better but the boat doesn’t go better.
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This is exactly what I find on our Scand 32 Atlantic.

Great for levelling (for example when one fuel tank is empty or the holding tank is full, or when the boat just won't sit 'right')

Also useful for trimming the Bow down a bit to optimise the speed (very marginal though) and improve forward visibility when sitting down.
 
The
Ferris said:
Didn't most CI22's have a fixed GRP tab instead? Certainly the ones with the mermaid t4's.
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The fixed trim tab is referred to in the CI as a “Sugar Scoop”.

semi displacement hulls don’t seem to need to level side to side in the same way as planing boats.

I had a Sunseeker 31 many moons ago and you had to level it to compensate for a side wind as it leaned in to a side wind.
 
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