Trim tabs advice

[kashurst]

I find that 100% trim tabs down gives the best speed In most conditions. Easily get an extra 1 to 1.5 knot for a given rpm. However the trim tabs don't seem to actually move down very far.
If I arranged for the tab to go down further into the water would I still get a speed improvement or is there a point where the drag created by the tabs exceeds the stern lift effect?
Or is it a case of wider tab being more effective than a narrow tab ( mine are a bit tiddly)
Some sealines have a hull extension between the rudders that I believe increases the effective hull length. Would wider tabs create a similar effect?

 

[julians]

I find that 100% trim tabs down gives the best speed In most conditions. Easily get an extra 1 to 1.5 knot for a given rpm. However the trim tabs don't seem to actually move down very far.
If I arranged for the tab to go down further into the water would I still get a speed improvement or is there a point where the drag created by the tabs exceeds the stern lift effect?
Or is it a case of wider tab being more effective than a narrow tab ( mine are a bit tiddly)
Some sealines have a hull extension between the rudders that I believe increases the effective hull length. Would wider tabs create a similar effect?

I cant help with many of your questions, but my boat (whilst being a totally different style of boat to yours, but still probably useful as an example) is the opposite of yours, its fastest with the tabs retracted all the way up. So I would imagine with yours that there is a point where they will start to add drag rather than speed.


I suspect it will depend very much on the shape of your hull and the weight distribution as to whether your boat would benefit from more lift at the stern. I cant think of a simple/cheap way for you to try beyond moving some heavy items into the front cabin and seeing whether that makes a difference.

 

[Portofino]

Hard to be specific without a pic of the set up and some speeds , 1.1/2 knots on what ?
The holy grail of hull design for a true planing boat is zero tab .
Tabs down in head seas to inc the V slice effect
Tabs actually - ve yup up more the hull line in downwind seas to raise the bow decrease WL to prevent broaching, stuffing it in .

How ever theses days they are often used to compensate for - let’s put it this way some sort of compromise in hull design , engines and weight too far back as an example but there are others like lack of lift at the stern .
Lack of lift is actually excess negative suction pulling the stern down so these hulls require a lot of + ve tab to compensate .

Fast planers have long relatively thinner tabs so you can control as mentioned the amount of bow you want in .

The AoA is paramount as you seem to realise .
Again ideally about 2 degrees only certainty no more than 4 .
Theses are tiny No theses days as you go faster the AoA should naturally decrease if true planing .

How many boats do you see with the bow pointing to the moon and the stern digging in ? Sure doing 20 odd knots - just moving water about .

Thought about heavier anchor + chain 1 st and dumping excess kg on the bathing platform?

If you want to fanny about with the tabs go longer not wider .

 

[Portofino]

I cant help with many of your questions, but my boat (whilst being a totally different style of boat to yours, but still probably useful as an example) is the opposite of yours, its fastest with the tabs retracted all the way up. So I would imagine that there is a point where they will start to add drag rather than speed.


I suspect it will depend very much on the shape of your hull and the weight distribution as to whether your boat would benefit from more lift at the stern. I cant think of a simple/cheap way for you to try beyond moving some heavy items into the front cabin and seeing whether that makes a difference.

Windy ?

 

[julians]

Windy ?

yes

 

[Hoblands]

Where is your water tank located? Mine is right at the bow under the V berth and have found it actually performs much better when it's full despite being around 100kg heavier!

 

[J12345T]

I find that 100% trim tabs down gives the best speed In most conditions. Easily get an extra 1 to 1.5 knot for a given rpm. However the trim tabs don't seem to actually move down very far.
If I arranged for the tab to go down further into the water would I still get a speed improvement or is there a point where the drag created by the tabs exceeds the stern lift effect?
Or is it a case of wider tab being more effective than a narrow tab ( mine are a bit tiddly)
Some sealines have a hull extension between the rudders that I believe increases the effective hull length. Would wider tabs create a similar effect?

Hi Kashurst,

I had exactly the same issue and exactly the same thoughts with my boat. One year I had a wedge added to the mounting plates of the trim tabs effectively giving them another few degrees of 'down' deflection. The result was absolutely no change. My conclusion was that I was creating just about the max lift that the tabs were capable of creating and deflecting them more was likely only going to create more drag..

FWIW.

Cheers.

James.

 

[kashurst]

thanks for the feedback etc
I have been rummaging about on internet. Bennet and Lenco both have a sizing guide. Both advise to fit as wide a trim tab as possible.
I already have a pair of bennets and they seem to use the same ram etc up to the size I could physically fit so an upgrade not too expensive. Just watched a Youtube video where someone swaps to much wider tabs and the effect is quite pronounced so as its a relatively cheap mod, I will fit new wider ones in the spring when the boat is next out and see what happens. Worst case no change, best case bit more lift.

 

[Offshore57]

I know props are a bit of a black art, but when I raced the prop specialist was able to build in a certain amount of sternlift or bow lift depending on what the individual team needed.....just a thought.

 

[snowbird30ds]

This is something I'm mulling over at the moment, my princess 30ds seems under tabbed to me as they are only 20" wide and 8" long and it always seems a bit bum down, admittedly I do carry a fair bit of weight in the lazarette as ther's very little spare space up in the pointy bit.
I always need full tabs to get any speed.