2000 340 trim tabs too small?

Discussion in 'Bennett Trim Tabs Vendor Forum' started by Rob1961, Aug 30, 2012.

  1. Rob1961

    Rob1961 New Member

    7
    Aug 6, 2012
    Sandusky, OH
    340 Sundancer 2000
    7.4 MPI 310HP
    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.
     
  2. Tabman

    Tabman Member

    700
    Jan 11, 2007
    The Tab Cave
    boatless
    boatless
    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
     
  3. MasterFab

    MasterFab Active Member TECHNICAL Contributor GOLD Sponsor

    Aug 17, 2010
    Wallkill, NY
    2002 340 Sundancer, Rescued, Restored, Modernized, and now Sold
    8.1L Horizons, ZF 63 V Drives
    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
     
  4. Tabman

    Tabman Member

    700
    Jan 11, 2007
    The Tab Cave
    boatless
    boatless
    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
     
  5. MasterFab

    MasterFab Active Member TECHNICAL Contributor GOLD Sponsor

    Aug 17, 2010
    Wallkill, NY
    2002 340 Sundancer, Rescued, Restored, Modernized, and now Sold
    8.1L Horizons, ZF 63 V Drives
    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
     
  6. docofthebay

    docofthebay New Member

    461
    Sep 10, 2009
    Fort Morgan, Mobile Bay Alabama
    1985 390 EC
    twin 3208TA Cats
    Twin disc transmissions
    I added 50% more area to mine and dropped the bow about a foot and a half with better ride and better mileage
     
  7. Loneranger

    Loneranger Active Member

    May 2, 2008
    Knoxville TN
    2000 SeaRay Sundancer 270
    7.4l Mercruiser w/ Bravo III
    Larger tabs is on my to do list probably this winter.

    John
     
  8. Rob1961

    Rob1961 New Member

    7
    Aug 6, 2012
    Sandusky, OH
    340 Sundancer 2000
    7.4 MPI 310HP
    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!
     
  9. Mittens

    Mittens Active Member

    520
    Mar 5, 2017
    Huntsville, AL
    2002 340 Sundancer, All Cherry Cabin, Black Canvas.
    8.1 Mercs, V Drives, 4.5kw Westerbeke.
    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
     

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