Trim tabs improvement

[forbes210]

Hi All,

Currently I have trim tabs 12"x8" which came with the 25' boat maxum 2500 single mercruiser 5.7 about 6000lbs. I'm on the east coast mostly cruising in open waters which often are pretty rough. I did some researches online and according to some manufactures 12x8 wouldn't be efficient and they recommend 1" of tab per each foot of the boat length. I was wondering if it will make sense to change them to larger tabs maybe 18x8 so I could stay on the plane at lower speeds.

[mmwjr]

Can you even fit that wide of a tab on your boat?

While tabs help planning there primary function is to get the bow down while climbing onto plane and compensate for minor side to side weight offset.

Maintaining plane is a function of HP/RPMs.

I am sure TabMan will be able to provide a much better response.

[forbes210]

18" should fit with no issues. From my small experience keeping the bow down helps to handle rough water for which I'm exposed a lot. That will be the main reason for me to change the tabs to larger. Is there any downside of having large tabs? The bottom line I won't have to drop them down that much to achieve proper balance.

[mmwjr]

The larger the tab area th more drag but with all the other effects from current and wind it really isn't noticable.

TabMan (Tom from Bennet trim tabs) has started posting on this site an I am sure he will respond with the best answer for you.

[forbes210]

Thank you Mike! Very helpful, as usually

[mmwjr]

Your welcome.

Just being myself, boat nut :-)

[Tabman]

Actually the larger the Trim Tab the less drag it makes in relation to the lift it generates. This is because it has to be deflected less to make the same or more lift than a smaller one. The larger the Trim Tabs the lower the speed you will be able to stay on plane.

Do you know if you have Bennett or Boat Leveler brand Trim Tabs?

Also do you have a photo of the Tabs that you can post?

Let me know and I will be delighted to help!

Tom McGow
Bennett Marine

[forbes210]

Tom,

Thank for valuable info.

Attached are photos of existing tabs. It looks like its a boat leveler system. What will be preferably the size of proper tabs? It looks like I can fit 18" wide tabs but if it make sense I could move transducer to accommodate even larger tabs.

[forbes210]

[mmwjr]

Tom/forbes regarding the 1 inch of tab per foot, is this for both tabs combined? 2: 12" tabs = 24 which is thesize of this boat.

forbes how slow of a speed are you trying to maintain plane? Yes the larger tabs will help but at some point lck of power rules and the boat will not plane

[forbes210]

I wasn't sure about that but for me it looked like everybody was talking about 1" for 1 foot per tab.

When cruising in rough waters I can stay on the plane at around 3200 rpm and 19knots but then I'm on the edge and when slightly pulled back by wave boat might come of the plane and then I might have 2800 rpm and be moving only 9 knots. Like I said very often I'm in a fairly rough water and simply can't keep the speed because the boat is being hit hard by waves. I wish to benefit from larger tabs by having more stable plane or ideally be able to plane at lower speed.

[mmwjr]

I find it hard to believe that it would be 1" per foot for each tab, that would mean that a 40' boat would require a pair of 40" tabs.

Yes larger tabs will allow planning at lower rpms but in this case I doubt it will help much.
At 2800 rpm and being slowed down to 9 knots after a wave hit does the boat then pick up speed and get back on plane?

Is your 5.7 a 2 bbl, 4 bbl, or EFI?

[forbes210]

Yes you're right about that it seems to be way oversized in some cases but I could fit with no issues 18"x8" versus current 12"x8".

When the boat is out of the plane I'll need to apply more throttle to get back on the plane and then reduce it to desired rpms and speed. I'll have to go through this process often while covering distance on open ocean.

I'm not sure what bbl means. Its a 5.7 mercruiser with 250 hp (serial #om322432) and it seams to be efficient for the boat.

[mmwjr]

I have had to do the same on the Chesapeake Bay when running in > 2ft waves. In 3 ft or more I pull the throttle back when cresting and apply it once over. It's part of boating. The bigger the boat the better it handles this. Hitting waves to hard just puts stress on the boat and you

bbl = barrel. 5.7 250 Hp = 5.7L which has a 2 bbl carb.

A 4 bbl handles this a little better as it opens the secondaries when load on the engine has from hitting waves. EFI (electronic fuel injection) is the champ.

[Tabman]

The old rule of thumb is 1" of Trim Tab per foot. That is each Trim Tab and at a 9" chord, fore to aft measurement. If you look at our sizing chart we would recommend 24" x 9" or 18" x 12" for that boat. If you could fit the 18 x 12s that is what I would recommend.

Tom
Bennett Marine

[mmwjr]

And there we have it.

Thanks Tom

[forbes210]

All clear

Do you know how close I can get to the outdrive or as long it won't interfere with it its fine? The other thing is the transducer which with current set up will not let me put 24" tab. I could move transducer but was wondering if it will work correctly if close to the tab or outdrive.

[mmwjr]

The transducer probably should not be moved any closer to the O/D because of turblance. Having the tab too close to the transducer may also cause turblance affecting it's reading, I would say 6 inches.

[dbriceno]

Tom, I'm looking to replace my 48x9 trim planes with 48x12 planes. (I need full tabs down just to cruise confortably). After I disconnect the cylinders on each tab, would I be able to slide the trim plane out without removing the hinge? Or do I need to remove the top hinge plate?

Dan

[Tabman]

Dan.

You really need to remove the top hinge plate to do the job correctly.

Tom
Bennett Marine