2000 340 trim tabs too small?

Rob1961 · Aug 30, 2012

Tom I have read alot of your posts and find them very informative. I purchased a 2000 340da with 7.4 MPI's in June and love our dancer! The smooth ride and handling are awesome but when we get into 3 to 4 footers running heavy(6 or 8 people on board, full of fuel and water) it rides very high in the bow even with tabs full down. It also seems to need nearly full trim even when running light in 2 footers or less. It gets on plane quickly with tabs full down but the bow never comes down if I decrease tabs even a little. I have read a lot of threads about increasing the tabs anywhere from 40 to 60 percent. Bennett's sizing chart calls for 42" x 9" for this length boat which is nearly double the size of the tabs from the factory. What size tabs does this boat need for optimum handling and performance? Is the obvious reason of saving a few dollars why Sea Ray made them so undersized? I plan on "fixing" them this winter as soon as the boat is out of the water after I figure out what size they need to be. Will Bennett tabs bolt right on without modifications? Thanks in advance for your help.

Tabman · Aug 31, 2012

Rob,

Simply put the Trim Tabs need to be as big as possible for the best results. The bigger they are the more lift they will generate and allow you to keep the bow down at lower speeds. Also larger Trim Tabs actually make less drag for the amount of lift they make since they have to be deflected less than a smaller one.

I am not sure why many boat builders fit what we consider undersized Trim Tabs. I am sure cost is one reason, but also suspect that they sea trial new hulls with the biggest power plant option and without a bunch of people on board. Also boats get heavier in the stern over the years as swim platforms, generators and other equipment is added.

What I do know is that adding larger Trim Tabs can really help.

Without seeing the transom configuration and Trim Tabs that are currently on the boat I can't tell you if larger ones will bolt right in place. We have mad many different size Trim Tabs for SeaRay and they did not tell us which size was used on a particular model.

If you take some pictures and measurements when the boat comes out of the water I will be delighted to help find the best size to upgrade to.

Tom
Bennett Marine

MasterFab · Aug 31, 2012

Rob,

I performed this modification on my 340, with fantastic results. I have a fairly detailed post in my blog - look at the May 24, 2011 posting. Our boats are slightly different, as mine is powered by 8.1 engines at 370 hp, but the hulls are the same. Tom, if you look at that same posting, you can see some pics of the hull and mounted tabs, and that may help. I believe the original tabs are 12" x 18". The rare diesel versions of the 340 (most went to Europe) use larger 12" x 24" tabs, but they are a tight squeeze laterally, and don't quite fit. As such, I made mine about 20 inches wide, and extended them a few inches. I would not go any larger than that without adding a second cylinder, but I would defer to Tom's judgement as to the maximum safe loading for a single cylinder.

Bottom line performance - I can hold plane down to about 15 or 16 knots depending on loading (17 knots with full fuel, full water, and 4-5 people on board). This is at about 2700 - 2800 RPM, with the tabs fully deployed. This is NOT an efficient speed for the boat, but works for rough seas. At roughly 3000 RPM, 22 knots or so, they are only about 50% down, and at cruise speed of 3300 - 3350 RPM, my speed is about 27 knots with the tabs fully retracted, other than slight deployment to level the boat.

Dale

Tabman · Aug 31, 2012

Dale,

I remember seeing you fabrication and it's pretty cool. I agree about not going with any more chord (fore to aft) without another actuator. Actually we would recommend another actuator for the size you fabricated just to be safe. But since you have run the boat with a single one, the proof is in the pudding!

If at all possible extending the span (side to side) is a bit more efficient than a longer chord. This is because more water pressure is lost out the sides on a longer chord Tab. The long narrow racing Tabs you refer to in your blog are not really made to increase lift on those boats, Rather they are for very small corrections in trim. They are mounted a bit higher up on the transom so that the smaller tapered trailing edge contacts the water first and with very little drag. They actually also work as "wheelie bars" holding the bow down when the boat leaves the water.

Your results speak volumes about the benefits of installing larger Trim Tabs!

Tom
Bennett Marine

MasterFab · Aug 31, 2012

Tom,

I agree, the K-Planes are for a different application in high speed planing powerboats. I used them as an example of how to gain lift with minimal drag. They are also considerably more rigid than a bent steel tab - the applied dynamic loads in true high speed vessels, as you know, can be tremendous, especially when re-entering the water.

I bent some drop fins into my tabs, to mitigate the lost water pressure out the sides, as you mentioned, and to improve directional stability. If I recall correctly, you recommend drop fins that extend 2 to 3 inches down, while mine are only about one inch. This may be something Rob should keep in mind as he proceeds.

I am not sure if it is clear in my pictures, but I also raised the leading, forward edge of my tabs, as you mentioned. When in the "Up" position, there is no water flow over the tab surface.

Dale

docofthebay · Aug 31, 2012

I added 50% more area to mine and dropped the bow about a foot and a half with better ride and better mileage

Loneranger · Aug 31, 2012

Larger tabs is on my to do list probably this winter.

John

Rob1961 · Sep 5, 2012

Thanks Tom and Dale, both of you have been very helpful. Since Dale's design works so well I believe I wil copy it with a slight modification. I'll try to get 22 x 14 to fit so I can get similar surface area and try to go a bit wider as Tom suggests. I'll also drop the edges 2" versus his 1" as Tom suggests for more stability in Lake Erie's constant chop and the nasty mixing bowl we call the Mosley Channel coming out of Sandusky Bay into the Lake. Dale your blog is quite impressive and I'm certain I'll be referring to it often as I make the 340 my own this winter. The notes on the stereo are awesome and very informative and that was the 2nd thing on my priority list. I've been a truck mechanic for too many years but a boater for only 4 years. As I bring my boat skills up to speed help from guys like you and Tom is priceless! I wish I had the access to your machining tools. That would be a huge help but I've been fabricating parts for trucks for years with stone axes so I'll manage. Thanks again!

Mittens · Mar 12, 2019

So how did the 22x14 and 2 inch drop fins work? About to do this my self.

I know old thread.... hears hoping they still respond

Mittens · Apr 16, 2019

Well about to put 24x14 tabs with 2.5 inch drop fins on my 2002 340.

Plan to weld them onto the factorys. Grind smooth and paint with bottom paint.

Made out of 304 stainless and have a 1 inch rear up fold for lateral strength.

Mittens · May 13, 2019

Boats coming out tomorrow. currently with old bottom paint, and 8.1s and stock tabs all the way down.

26mph, 3600 rpms, and getting 0.7MPG...

Hopefully see 2-3mph and less tab, which should bring the MPG up some... results to follow

Stee6043 · May 13, 2019

Mittens said:
Boats coming out tomorrow. currently with old bottom paint, and 8.1s and stock tabs all the way down.

26mph, 3600 rpms, and getting 0.7MPG...

Hopefully see 2-3mph and less tab, which should bring the MPG up some... results to follow

Seems like something is amiss. You should be getting better mileage and better speed than that. I assume you're using statute miles in your math (you say mph and not knots)? On my 400 with 7.4's I'm running .71MPG (statute) at 3500/3600 rpms which is 27/28MPH, 24knots SOG. 37/38gph all day long...with full tab.

Mittens · May 13, 2019

Yea that's MPH GPS speed. SOG. When I got it and it was pressure washed bottom from the pull out it ran 28ish. Dirty bottom and not being pulled for 3 years and its 26mph... hopefully new paint, new tabs will get her closer to 30 mark and maybe .8 or .9mph.

Wide open it runs 41.8mph so its not slow there for what it is... haha Just mainly sucks at cruising... which is where I run 99% of the time. ha

Mittens · May 19, 2019

New tabs are on. One bottom painted will test! Ha

Mittens · May 22, 2019

Small update to my trim tab mods. Built super nice 24X14 tabs with 2.5 drop fins incorporated(pics above)

Boat came out of water, tabs came off, all looked good. Welded new tabs to underside of old tabs. Square as can be. Once I put the tabs back on the boat the drop fins did not look right. Each tabs fins are not facing forward, close but not.... they point inward little as the mounting of the tabs is not 90 to the boat. The stern of the boat is not flat. So take note other 99-02 340 people the round stern is not allowing the tabs to follow/deploy 100% square, and thus the drop fins are not right when square on tab.... Bennet said this will be extra drag (obviously) but also stress the mounts, in odd ways bending it, twisting rams, or also pulling strews holding mounting hinge into stern out/ popping heads off. Also the fins would be trying to stear the boat, granted canceling each other out, but a lot of drag/and stress for no reason.

Now others have done some drop fins and no issues. and I left them on the boat for 2-3 days while still on the hard, and after all my research it was not worth the risk.. so I removed and cut the drop fins off.

so now back on the boat is 24X14 tabs but with a flat bottom, so it does not matter about the round stern.

Bennet did say I could Mount drop fins at the angel needed to the bottom of the tab, but that's not in my mind right. And would change some with how much tab is down.. so big flat tabs that don’t care about round transom.

Mittens · May 22, 2019

Small update to my trim tab mods. Built super nice 24X14 tabs with 2.5 drop fins incorporated(pics above)

Boat came out of water, tabs came off, all looked good. Welded new tabs to underside of old tabs. Square as can be. Once I put the tabs back on the boat the drop fins did not look right. Each tabs fins are not facing forward, close but not.... they point inward little as the mounting of the tabs is not 90 to the boat. The stern of the boat is not flat. So take note other 99-02 340 people the round stern is not allowing the tabs to follow/deploy 100% square, and thus the drop fins are not right when square on tab.... Bennet said this will be extra drag (obviously) but also stress the mounts, in odd ways bending it, twisting rams, or also pulling strews holding mounting hinge into stern out/ popping heads off. Also the fins would be trying to stear the boat, granted canceling each other out, but a lot of drag/and stress for no reason.

Now others have done some drop fins and no issues. and I left them on the boat for 2-3 days while still on the hard, and after all my research it was not worth the risk.. so I removed and cut the drop fins off.

so now back on the boat is 24X14 tabs but with a flat bottom, so it does not matter about the round stern.

Bennet did say I could Mount drop fins at the angel needed to the bottom of the tab, but that's not in my mind right. And would change some with how much tab is down.. so big flat tabs that don’t care about round transom.

Mittens · May 28, 2019

So new tabs...After a long weekend.

Before if fully loaded 0.64mpg and 0.74 when light. 24-26mph

Now .92-1.01 mpg and 28-30mph.... loaded up with water and fuel and food and beer.

Hugh improvement. Using 1/2 tabs mostly for best numbers, the tabs all the way down can bury the nose if needed and way more reactive and responsive..

Jason T Insley · Nov 12, 2019

Mittens said:
So new tabs...After a long weekend.

Before if fully loaded 0.64mpg and 0.74 when light. 24-26mph

Now .92-1.01 mpg and 28-30mph.... loaded up with water and fuel and food and beer.

Hugh improvement. Using 1/2 tabs mostly for best numbers, the tabs all the way down can bury the nose if needed and way more reactive and responsive..

What is the thickness of the metal you used to fabricate your new tabs? I just pulled for the season and this is on my list for the winter on my 2001 340 w/8.1's.

2000 340 trim tabs too small?

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