Need the basic instructions for installing a pair of trim tab`s to the transom. How far from the botom "edge", how far or close to the center (drive) etc. 25 feet boat, 500hp engine, Bravo drive. Thanks.

logic would suggest that they be installed such that when nuetral that they be flush w/ the bottom. that would be the lowest drag ..,

logic also suggests that they be as far apart as possible to gain the greatest leverage for the least deployment ( less drag)

the mountings on my TG support these "theories" but i can not speak with authority

generally as far out as possible, but not to the edge, maybe 4" in, and 1/4" high, use a straight edge on the bottom of boat to get the right measurements.

Swedish manufactured boat that is simmuare to the Baja 23 Outlow. Don`t know the actual name. Does this help?

What size of tabs? What type?
Basic rule is mount the tabs 1/4" - 1/2" above hull surface, to be sure that they do not in any way touch the water when fully retracted.
Secondly, if the upper ram mount is not on a backingplate, make sure you mount them so that the trailing edge of the tabs are at least 1 1/2" above the leading edge in fully retracted position.

Next, the faster the settup, the nearer the tabs should be to centreline. Mild powered sportboats run the tabs outboard because it gives the maximum correction possibility at cruising speeds in crosswinds and with unbalanced loads. At speeds up to 60 mph, the whole width of the transom is in the water.

A performance boat (and that includes a 500hp 25 footer) will be running high out of the water at top speed (74-78 mph) and will respond better to the tab inputs if they are as near to the centreline as possible. The water is cleaner there and has a more constant pressure. You will need less tab angle to achieve the desired effect. Less angle means less speed loss.

As near to centreline means as far inboard as you can go without getting in the way of the drive in ANY trim or full steering lock position.

It is often said that high performance single engined hull work best with the tabs mounted horizontally (and not parallel to hull V). This is particularly true if the boat in question runs a prop (30" and above) and x dimension that makes getting on plane diffficult without the tabs. The horinzontal position is also supposed to help dampen chinewalk.
I can´t judge this ´caus I haven´t tried it.

But my (25", 525hp, 80 mph) boat run (24" EMI billet) tabs parallel to hull (1/2" above) mounted mid way between outer edge of transom (8`beam) and drive, and it runs really good in all conditions. I run the tabs level to hull in rough water and it makes a BIG difference. I can trim out the yaw in crosswinds, and I can run the boat down to 21mph keeping on plane using full tab.

A Hydrolift is a narrow beamed deep V hull. You may not have to go far from the outer edge of the transom in order to get the right effect.

Length 23", Height 10", Width at tip 3-3/4"
Looks like the EMI billet except for the narrower tip.
Wild Card 09
Thank`s for your reply.

logic does suggest that they be mounted flush with the running surface of the boat but for some reason i have never seen a set mounted like this. most factory instalations are about 1/4 to 1/2 inch from the bottom edge. i was always curious about this and hope someone can pop on here and shed some light on it.

i installed a set of 280's on a new 38 Donzi zx and the mounting template showed a 1/2 inch from the edge and if memory serves so did mounting instructions.

my 34 ScarabIII on the other hand had them flush but they were Bennets

my guess would be that , as mentioned above, it is to be certain that the are clear of the water at zero deployment...

when i look at mine closely, they are not precisely flush... they are about .090 above but my feeling is that this is " just making sure" as opposed to some critical dimension they were looking for. these were done by cigarette and i figure they know what they are doing.

having them above by some substatial amount leaves a void which when they are deployed would promote turbulant flow and as such , much greater drag and reduced effectiveness.

its the same ( in my opinion) w/ mounting them horizontal and closer to center... turbulant flow and vastly reduced leverage requiring much greater deployment for the same net effect....

after reading what was previously written, i walked around the marina and looked at what was there and i didn't see any done like that or even close on anything...

i would think you could just call the engineers at K plane or and see what they say... seems they would know the real answer and be happy to provide it.

http://www.offshoreonly.com/forums/s...=fountain+tabs

some more insight..

post #2 gives a good idea as to what's going on...

Thanks, always a plessure to get your input