Trim Tab Mounting.

[chevy355mark]

Yep, I've used the search. there's not as much information around as i had thought there would be. So i gather mounting them about 3/4" higher then the hull bottom is about standard procedure but then there's the few i see mounted flat, or horizontal, assuming to get them out of the water all together at speed. My question isn't so much a matter of what's best, as what's best considering my limitations

So with the old bennet's out of the way, I've got a set of Eddie marine 24" k plane style tabs. The boat is about 7'6" at the transom and the chine flats are very wide, maybe 4". So with the twin outboard bracket in the way, there's really not much hull surface left. by angling them closer to flat I can get them a tad inboard where as mounting them even or following the deadrise angle, they would be right to the edge, if not slightly past the point where the hull bottom meets the chine flat. Would it be stupid to sort of cut the difference between even with the deadrise and flat horizontal? that seems to place them at the point where the outer edge lines up even with the outside limits of the hull bottom.

The boat is 23ft plus a 2ft outboard bracket, 24 degree bottom. Very stern heavy, needs all the tab it can get right up to about 55mph at which point I no longer need any trim tab, run the outboards completely trimmed out, and the boat feels as if its coming up on the pad. boat tops out at 64mph

Thoughts, Opinions, or who the heck cares because the boats too slow for any of this to matter lol.

[Jim Speros]

Opinions may vary. I believe tabs should be wide on transom. I giggle at narrow set ups. Narrow reminds me of training wheels. LOL.
I agree with mounting half way between horizontal and dead rise. Velocity has done just that since the 80's. I generally disagree with horizontal mounting unless boat runs 100 mph or more. Tabs always make a boat better. Gives more control in a wide range of situations.

[FUJIMO]

...12'' x 16'' bennett marine sport tabs...

[XstreamVking]

If you need that much tab, I would place them just above the bottom. 1\2'' -3\4'' up. Out as wide as you can. Might as well take full advantage of the max surface area. jm3cts

[chevy355mark]

So, with them closer to the hull bottom they will be obviously be more effective, but possibly too effective at top speed. does that mean that angling them towards horizontal makes them less effective at all speeds, or only less effective at top speed due to the water line dropping as the boat comes up on the pad? not that it matters in an under 100mph application as already mentioned, just curious on the theory.

the bennet's i actually had the rams mounted so that they could never tuck up past flat. I'm thinking the boat will gain some small amount of speed with the tabs getting out of the way. at 60 plus the boat feels well planted, runs level, could probably be a little looser. if that makes any sense.

I think these tabs I've got are so far overkill for a 23ft boat it really wont matter what i do, airing towards the less effective side will give me my greatest top speed and still give me plenty of tab cruising 25-30mph on a fishing day.

[Jim Speros]

3/4 inch up if possible . Needs to remain dry when up. Moving away from deadrise reduces the surface area on tab that actually sees water.
The tabs on cruisers and as noted on old switches tab use would push opposite bow down. On faster modern boats the tab fits the stern on the same side. I often explain as using tabs to lift stern to fly straight. Tabs on deadrise can have the diagonal reaction effect. EI You push port tab down it rocks stb bow over. Horizantal mounting does less of that. Horizontal mounting can have an undesirable effect in hard turns. To some extent can help boat to spin out. I have tried different angles over the years. I even moved tabs on a Velocity flatter and ended up returning to original position. I find half of deadrise to be a good overall option.
As far as when a tab is best used and how active they should be, I believe they should only be active when needed. If the boat is set up and designed well the tabs don't need to be active all the time to effect the ride. Tabs to some extent add drag. If the drag allows you to stay in the throttle and let it eat so to speak then overall advantage can be seen. When cruising I like use them to prevent to boat from being upset by the occasional bigger swell or wake.. In that case i like to add tab until you feel them drag and effect the boat then tap up a couple clicks so they only do something when needed without actually moving them. I never really lift them all the way up. Every time the bow gets thrown up you either have to slow down and/ or something bad happens.

Some boats have design short comings that tabs can help alleviate. There are examples of tabs that are installed or limited to not go up all the way. A 29 Fountain with twin big blocks left factory with limiting pucks so they could not be cleaned up by lifting clear. Some boats need a hook or tabs to keep them from porpoising endlessly. SOme are just dangerous at high speeds.

[99fxst99]

My favored use of tabs is to compensate for the tendency to lean into the wind when running cross-wind, particularly on a high deadrise hull at cruising speeds. Much smoother ride when you aren't slapping into a beam sea.

[chevy355mark]

3/4 inch up if possible . Needs to remain dry when up. Moving away from deadrise reduces the surface area on tab that actually sees water.
The tabs on cruisers and as noted on old switches tab use would push opposite bow down. On faster modern boats the tab fits the stern on the same side. I often explain as using tabs to lift stern to fly straight. Tabs on deadrise can have the diagonal reaction effect. EI You push port tab down it rocks stb bow over. Horizantal mounting does less of that. Horizontal mounting can have an undesirable effect in hard turns. To some extent can help boat to spin out. I have tried different angles over the years. I even moved tabs on a Velocity flatter and ended up returning to original position. I find half of deadrise to be a good overall option.
As far as when a tab is best used and how active they should be, I believe they should only be active when needed. If the boat is set up and designed well the tabs don't need to be active all the time to effect the ride. Tabs to some extent add drag. If the drag allows you to stay in the throttle and let it eat so to speak then overall advantage can be seen. When cruising I like use them to prevent to boat from being upset by the occasional bigger swell or wake.. In that case i like to add tab until you feel them drag and effect the boat then tap up a couple clicks so they only do something when needed without actually moving them. I never really lift them all the way up. Every time the bow gets thrown up you either have to slow down and/ or something bad happens.

Some boats have design short comings that tabs can help alleviate. There are examples of tabs that are installed or limited to not go up all the way. A 29 Fountain with twin big blocks left factory with limiting pucks so they could not be cleaned up by lifting clear. Some boats need a hook or tabs to keep them from porpoising endlessly. SOme are just dangerous at high speeds.

"half of deadrise" meaning halfway between flat and following the angle of the hull bottom. Just to be clear, a lot of terms being thrown around in threads like these can get confusing.

Another question comes to mind, If the the tab was 3/4" off the bottom but even with the deadrise angle would it have the same result as a tab that was flush to the hull on the outside and 1.5" higher then the hull bottom on the inner edge. Question being, if the same exact surface area of distance is achieved between the bottom of the tab and the hull bottom, does the angle matter still? Is there any real distinct difference that the angle achieves or are the differences merely a result of a flat tab being much further from the hull bottom?...

I agree on the trim tabs use, the goal is as little drag as the boat needs to maintain safe and easy operation as boat wakes and moving passengers make your life difficult. right when you had everything dialed in somebody has to lean over the side... center consoles are awful about that, I've grown to appreciate a sit down boat where people stay put.

Ultimately, there's only a few ways my application can be mounted, id like to post some photos later and get a final opinion before I drill the holes.

[FUJIMO]

...if you go with the long 24'' tabs, with that transom heavy boat, just make sure you don't back up under a lot of power or in rough water with the tabs down...

[Jim Speros]

Half the deadrise. Half way between horizontal or flat and angle of deadrise on transom.

On your angle question. If tab matches deadrise the entire surface of tab is exposed to water and the more effective it is. However it will have the tendency to rock the boat across to pushing the opposite bow down. When tabs were first used on cabin cruisers the switch actually was labelled that way to push stb bow down the port tab was actuated. On high speed boats the tabs are used to lift the stern on the same side as the tab in order to get the boat to fly straight. So like just about anything a compromise is in the the order. If the tab is flat or horizontal, which ever way you call level across the back of the boat only the outside corner sees water. The faster the boat goes the more the tab effects the ride.

I learned to operate and offshore boat in early 80's in a 28 Cigarette with no indicators. Learned by feel and trial and error. I have rigged many offshore boats with tabs and remember re wiring tab switches so port switch moved port tab. After all these years the wiring has changed. I use the tab buttons more then the trim buttons on most v bottoms I operate. I have raced many different brands and sizes of boats. In short you trim the boat for speed and use the tabs so you can stay in the throttle and fly it straight ahead. Tabs make a good boat better. I can't enjoy a boat ride with out the tabs working properly.

Tabs are easier to use and learn to use with mechanical indicators. I recommend them. Tabs at high speeds used improperly can get you in trouble.

I included a pic of my 10 Meter. I felt so strongly about tabs I moved them to half the deadrise with a raised the x dimension.

[Jim Speros]

In order to relocate tab and keep steering geometry correct I had to notch corner of tab for steering mount.

[jpf091959]

I placed my tabs over the tunnels on 22' Daytona. They were Lenco's 12"x18" mounted 1/2" above bottom of the transom. I used them to help get on plane. Once on plane I raised them up. I did notice at high speed I could raise and lower them just a little to adjust the attitude of the boat, the boat would react to the adjustment even though the tabs were out of the water. The air moving through the tunnel at 100 mph really affected the aerodynamics.

[Jim Speros]

Tabs in a tunnel is an entirely different scenario. Yes the effects can be dramatic. You are changing the compression and ultimately the lift on the hull. On some boats you can cause it to blow over by dropping the tunnel tab at high speeds. Be careful.

[FUJIMO]

...how fast is that trs boat jim?...

[FUJIMO]