Trim Tab Drag
 

What is the best set up position for trim tabs to insure that there will be no drag at WOT?

Re: Trim Tab Drag
 

It will always depend on water condition, but all the way up is fastest.

Re: Trim Tab Drag
 

HighPriority,

I should have phrased my question better. I ment to ask when you first install a trim tab system, where should the full up position of the trim tabs be to insure no drag at WOT with the trim position switch at 'full up'?

Re: Trim Tab Drag
 

Are you asking how to mount them on the transom? or adjust the indicator to read correctly? If mounting then its usually about 1/2 to 3/4" above the bottom of the boat.
If you asking about the indicator......well i usually take a straight edge and place it on the bottom of the boat and extending out the rear under the tab....then adjust the tab till its parallel with the straight edge. At that point i either mark my indicator where its at or adjust it so its in the middle on the line between 4&5

hope that helps....

Re: Trim Tab Drag
 

Wally,

How to mount them on the transom was what I was after but the indicator info was helpful as well.

What determines 1/2" or 3/4" up from the bottom of the boat? Tab length from the transom to tip? My tab length will be 16" or 19" depending on brand.

Is the full up tab position to be parallel to the boat bottom or some angle above parallel to insure no drag at WOT? Once I drill the holes for the power cylinder transom mounting I do not want to have to move them.

Thanks!

Re: Trim Tab Drag
 

Rage,
I would position your tabs so they are + 2/4 degrees at full up position. Here's why. Most of our boats run at 2/4 degrees + at WOT, relative to the water surface. In that scenario, your tabs are parallel to the water at full up. Most tabs still have plenty of ram range to give you all the negative trim you need, even when mounted + 2/4 degrees, full up.
BTW, regarding performance, less tab usually equals better performance. Tabs are very useful when you have a lateral imbalance, rough water, or heavy loads during plane off.
Regards,
Steve

Re: Trim Tab Drag
 

Here's a few pics of how mine are mounted. In the first pic you can see that they are about 3/4-1" above the hull. In the second pic you can see that they are angled up when in the full up position. I don't think they are touching the water when all the way up.

Re: Trim Tab Drag
 

the bottom of the boat to the bottom of the plate that mounts on the transom of the boat.......or should i say the lowest part of the assembly thats closest to the transom.....nothing much to do with the tab that extends out from the plate.

Re: Trim Tab Drag
 

Good information everyone, thanks!

Steve,

The "2/4 degrees +", does that mean 2 degrees to 4 degrees plus range? Just want to get it right.

Re: Trim Tab Drag
 

Rage,
If you look at Dave's tabs, that's what I am refering too. Fully retracted, or up, they should be several degrees above parallel to your hull. That way when your running with positive trim in smooth water your tabs will only be catching spray off the corners of the transom. If they were installed so that they were parallel to the hull when fully retracted, they would actually be slightly deployed when running with positive trim, which would increase drag and slow you down.
Tabs are a great tool to fine tune your ride, but you'll generally want to use as little tab as possible for the conditions you're running in.
Regards,
Steve

Re: Trim Tab Drag
 

OK, I am going to go through the the real answer to your question as I see it. You will have to fit this to YOUR particular hull/use/water conditions. You (I assume) have already installed your tabs on the transom at the recommended 1/2 to 3/4 inch above hull line. Now, determine from research or photos of your boat running just what the ideal angle of attack your hull runs at WOT in calm water and attains top speed. That would be that angle between the hull keel line and the horizontal water surface. Take that angle, (which would be extending that horizontal water line) and extend out from end of hull off keel line. Set your full up tab aft tip position just a bit (maybe 1/4" depending on tab length) above that line. Your tabs will never cause any unnecessary drag and will be close to the surface and react quickly when you need them. --- Jer

Re: Trim Tab Drag
 

Sounds like I have the info I need to do it right. Thanks once more!

Re: Trim Tab Drag
 

Don't forget that there is a significant amount of wave rise, depending on the trim angle of the boat, and the speed. I've generally seen K-planes mounted at 1" above the running surface, and they are capable of retracting about 10 degrees up (asssuming a 13 degree transom angle). Mounting them above the transom ensures that the water flow will not remain attached and be drawn up the tab. You must retract the tab quite a bit more than parallel with the hull to actually get it out of the water.

For instance, I know that my K-380 planes are parallel to the bottom of the boat at a indicated reading of about 4.5. At lower speeds, they will still produce lift as low as 3, and at top speed, I have to leave the port tab at about 4 to just drag the tip and overcome the propeller torque. I can really tell on my boat whether I have the tabs dragging or not. I have a SSMIII running a full cleaver, and the combination of the tabs being pulled up free of the water occurs right about where the prop finally hooks up at about 60 mph, and it takes off with little increase in power or engine RPM.

If anyone is interested in trying out a new set of Carbon Fiber tabs, I am developing a set based on my research. I have the molds made already, and am just about to lay up the first set. They are going to be a little more expensive than most, but will be incredibly strong and stiff. My design condition is a 20 degree deflection at 95 mph, which generates around 12,000 lb of lift per tab (this is the relief point on the hydraulic pumps). I still have a factor of safety of nearly 4 on the tab structure and around 3 on the pistons, according to both conventional analysis methods, as well as a Finite Element Analysys.

They will be supplied with a really cool piston made of solid 316 stainless with a 3/4" piston rod (compare to 1/2" for competitors...) I have the first two pistons done now, and they look really cool. The tabs will be 25" in length, which puts them squarely between K-280 and K-380. They will be the ultimate in looks and corrosion resistance!

My boat only runs 75 mph, so I have been looking for someone with a faster, heavier setup who would be willing to mount them and test them in exchange for a free set! The only catches are that they won't work with the same bolt pattern as existing tabs, and you'd have to report back your findings after testing.

Re: Trim Tab Drag
 

Here is one with the nuts actually installed

Re: Trim Tab Drag
 

Sorry to say that mine is in the ~mid 70's as well. You might want to start a fresh thread announcing your offer of free high tech tabs for trial installation. You will likely have more takers than you can count. You will not draw much attention in my thread.

Re: Trim Tab Drag
 

Icelano,

Re: "You must retract the tab quite a bit more than parallel with the hull to actually get it out of the water." how much more in degrees at ~70mph? I am installing 16" length tabs.

Re: Trim Tab Drag
 

I like them mounted this way

Re: Trim Tab Drag
 

My question would be, has anyone had tabs mounted flush or near flush and tested true/performance/drag by pulling them off or way up to be sure they were out of the way, and seen a good result?

Mine'sobviously not a performance hull, thus I need every advantage I can get. :D My tabs are factory installed at near flush and retract just enough that I have to look doe the running surface to tell if they are not flush.

I've though about (ok, bobl suggested :D) a way to test them would be to remove/unhook the ram from one side and tie them up. I have a feeling they are dragging quite a bit.