trim tab position

cosworth mirage · 15-04-2009, 08:56 PM

im looking for some help on positioning of some trim tabs ,
the tabs are quite long 450mm they are 200mm wide at transom end and narrow to 100mm at tip, im thinking of mounting them up from the bottom edge of the hull so only the end of the tabs touch the water.
will this give the tabs more leverage and less drag or neither ?
the boat is a 24 foot rib with 4cyl yb cosworth engine on a volvo e drive
hull starts at 50deg v at bow flatening out to 25 at transom.it has a pad 2500mm long that widens to 500mm at stern its only 20mm deep at stern and thins to 0 at front.
the boats has strugled to plane correctly in the past depending on what drive has been fitted (running to deep) hopefully this drive will help as is 6" shorter than last leg. propshaft centre line is still 7" bellow pad

larby · 15-04-2009, 09:09 PM

Wow, that's something a bit different!
There's people on here that would know much better than me but i'd imagine trim tabs would be best about 1/4" or so up from the bottom of the boat so they don't drag when lifted but are still effective when in use.

James

Jon Fuller · 15-04-2009, 09:22 PM

As a general rule is with a straight edge along the bottom, the tab should be about 1/2"-3/4" above the running surface. And when fully retracted (up) it should run up at at least 20 degrees from the straight edge. However, those numbers will be different if it's a stepped hull (depending on the dimensions / angles of the steps.

That's my take on it anyway.

Also, are you intending to put the tab running parralell to the deadrise, or horizontal? there are differing trains of thought on how it should be.

Oh well, onwards & sideways.

cosworth mirage · 15-04-2009, 10:18 PM

Quote:

Originally Posted by Jon Fuller

As a general rule is with a straight edge along the bottom, the tab should be about 1/2"-3/4" above the running surface. And when fully retracted (up) it should run up at at least 20 degrees from the straight edge. However, those numbers will be different if it's a stepped hull (depending on the dimensions / angles of the steps.

That's my take on it anyway.

Also, are you intending to put the tab running parralell to the deadrise, or horizontal? there are differing trains of thought on how it should be.

Oh well, onwards & sideways.

Not sure whats the pros and cons

Burty · 16-04-2009, 01:00 AM

parrellellell to dead rise will have more effect but will also steer the boat if only one tab is down, horizontal will have less effect so you will have to put the tab down further but wont steer the boat

cosworth mirage · 16-04-2009, 09:12 AM

Ah i see

do you think running them higher up transom is any advantage/dissadvantage

Jon Fuller · 16-04-2009, 09:36 AM

As said, there are many 'theories' on positioning and fitment of trim tabs.

My own opinion is that parallel to deadrise looks best, and if your correcting a list when running in a beam wind (using only one tab), this works just fine. But, if you have cause to use both tabs at the same time to drop the bow, the tabs are swinging down & apart with that geometry, and drag is increased considerably, so not very efficient. Probably only an issue if racing, where every tiny bit of efficiency and speed matters.
Set horrizontally, looks a bit nobish, but when both are deployed, they work together much more efficiently to lift the stern / drop the bows as they're both lifting vertically / in the direction.

If you can picture a more exaggerated situation, and the deadrise were much, much deeper, you can see that the two tabs would swing outwards, almost like a pair of brakes. Some passenger airliners have a pair of air brakes at the tail end of the fuselage that work like that, they open up like a chute.

As I say, it's all just personal theories, mine included. Nothing is black & white.

Here's two examples, My Phantom 28 pleasure boat with tabs parallel to deadrise and the SwipeWipes 32 raceboat with them horizontal.

cosworth mirage · 16-04-2009, 12:44 PM

Thanks for all the info
gonna try horizontal

Matt · 16-04-2009, 01:05 PM

Do you know of anyone that's actually done some proper tests?

Quote:

Originally Posted by Jon Fuller

As said, there are many 'theories' on positioning and fitment of trim tabs.

My own opinion is that parallel to deadrise looks best, and if your correcting a list when running in a beam wind (using only one tab), this works just fine. But, if you have cause to use both tabs at the same time to drop the bow, the tabs are swinging down & apart with that geometry, and drag is increased considerably, so not very efficient. Probably only an issue if racing, where every tiny bit of efficiency and speed matters.
Set horrizontally, looks a bit nobish, but when both are deployed, they work together much more efficiently to lift the stern / drop the bows as they're both lifting vertically / in the direction.

If you can picture a more exaggerated situation, and the deadrise were much, much deeper, you can see that the two tabs would swing outwards, almost like a pair of brakes. Some passenger airliners have a pair of air brakes at the tail end of the fuselage that work like that, they open up like a chute.

As I say, it's all just personal theories, mine included. Nothing is black & white.

Here's two examples, My Phantom 28 pleasure boat with tabs parallel to deadrise and the SwipeWipes 32 raceboat with them horizontal.

Jon Fuller · 16-04-2009, 01:08 PM

Quote:

Originally Posted by Matt

Do you know of anyone that's actually done some proper tests?

Quote:

Originally Posted by Jon Fuller

As I say, it's all just personal theories, mine included. Nothing is black & white.

No. And there have been race boats with both setups that have been hugely succesful, but one has to believe in something, one way or the other.

For me, if performance is crucial, I'd go down the gay route, if it's just a fast pleasure boat, I'd go down the strictly hetro-route. I voted with my feet, as the pics in my previous post show. Swinging from Hetro to Gay whenever it suits me!

Matt · 16-04-2009, 01:11 PM

Yeh, I know you haven't, but someone must have. Adam was developing a new type of trim tab a few years back, I wonder if he did some tank tests.

Does believing in nothing amount to believing in something?

Quote:

Originally Posted by Jon Fuller

No. And there have been race boats with both setups that have been hugely succesful, but one has to belive in something, one way or the other.

Jon Fuller · 16-04-2009, 01:17 PM

Tank test results with the two styles of fitment would be very interesting. And you could do it quite easily I guess, without a University.

Model boat, spring ballance and some fast flowing water.

Matt · 16-04-2009, 01:37 PM

Plymouth University tank wasn't much more complex than that.
The difficulty is in having flowing water without setting up a standing wave, that's why they usually have static water and moving boat.

PerformanceDodge · 08-09-2010, 02:14 AM

Quote:

Originally Posted by Jon Fuller

As a general rule is with a straight edge along the bottom, the tab should be about 1/2"-3/4" above the running surface. And when fully retracted (up) it should run up at at least 20 degrees from the straight edge. However, those numbers will be different if it's a stepped hull (depending on the dimensions / angles of the steps.

That's my take on it anyway.

Also, are you intending to put the tab running parralell to the deadrise, or horizontal? there are differing trains of thought on how it should be.

Oh well, onwards & sideways.

Hi Jon, just searching old posts for any info on fitting trim tabs or the design of. Just wonder if you have any ideas, theory's or advise for trim tabs on a stepped plain hull's

Have been advised size should be around 12in at he leading edge, 6in at the rear off set by 1in towards the centre of the boat and 18in long. Are they any more or less effective on a stepped plane hull ?

look forward to any helpful advise

Regards David