Another Trim Tab Question-With a Twist

[rangerdog]

I, of course, have 12x12 Lenco tabs and want to upgrade to 9x24 tabs. Here's my question: Can I go to a 12x18 tab and get the same result as 9x24? They are both 216 inches square. It is also a cheap Lenco retrofit as an upgrade for about 200 bucks. Thanks in advance for your thoughts.

 

[Capt. Ronnie]

Get the 24 wide tabs, I have the same boat and have them, made a huge difference in the ride.

 

[cbigma]

Can I go to a 12x18 tab and get the same result as 9x24? They are both 216 inches square. It is also a cheap Lenco retrofit as an upgrade for about 200 bucks.

John,

I am leaning in the same direction for an upgrade this spring. I have Boat Levelers, not Lencos, but the concept is identical. I can upgrade my 12X12 Boat Levelers to 12X18 Boat Levelers with a non-instrusive direct replacement. Just a few more screw holes to drill for the longer tabs. No modification of actuator mounts, or hydraulic hose lines. I will certainly replace the actuators, pump/reservoir and all hoses and switches. I can do this however, without adding a host of new transom penetrations for all these items.

Parker Nation seems to favor the 24X9 Bennetts, claiming that if you are going to have 216 sq_in of lift, it is best to have a 9 inch chord than a 12 inch one. However, no one can point to the data that supports this.

Bennett recommends a longer chord for more lift. Bennett's Sizing Guidelines state that "semi-displacement hulls, boats over 50 feet, outboards on brackets, or boats with any other feature that increases the need for lift aft benefit from the 12" chord. The 12" chord provides greater surface area, thereby utilizing more water flow and providing more lift."

I carry a lot of weight aft, and my eventual repower will certainly be heavier than my 2 stroke OX66, so more lift aft sounds like a good thing.

I'm with you John, unless someone can point to some data that says 216 sq_in needs to be 9X24 instead of 12X18, fewer holes in my transom is better.

..John

 

[DaleH]

Parker Nation seems to favor the 24X9 Bennetts, claiming that if you are going to have 216 sq_in of lift, it is best to have a 9 inch chord than a 12 inch one. However, no one can point to the data that supports this.

I'd need to do some hydronamic drag equations to put real numbers in place, but trim tabs work by providing lift with a byproduct of 'drag' being produced.

All tab mfgr's state in their literature that 'given the same surface area', WIDER tabs across the hull (say 24x9) produce less drag as compared to tabs that aren't as wide, but extend FURTHER out from the hull (say 18x12s).

The 'Parker Nation' recommendation for at least 24" wide (span) by 9" projection (chord) tab planes are for a mod-V hull. If I had a deep-V Parker, I would have gone with 30" span by 9" chord tabs. They'd outperform 24x12s in a heartbeat! Meaning, better MPG for the same surface running angle/effect.

 

[esfishdoc]

The only surprise in the equation is why a 2120 has the same tabs as a 2520......

What size does the 2820 have?... don't tell me they are the 12 X 12's.

To support the American way of bigger is better I'm installing the 30 X 9.

 

[Outta Here]

www.bennetttrimtabs.com/size1.htm

 

[rangerdog]

Great information. I am still studying.....

 

[Megabyte]

John,

I took the liberty of posing your question directly to "the source"... at Bennett Marine.
Here is Toms response. I hope it helps.


>Hi Kevin!
>
>Good question! Both the 18" x12" and the 24" x 9" would be good
>choices on those boats. But the 24" x 9" is a bit more efficient.
>This is primarily due to the fact that the 9" chord lets less water
>escape out the sides in relation to the water flowing over the Trim
>Plane then the 18" x 12" Tab. In much the same fashion as an aircraft
>wing having flaps that extend a long distance along the trailing edge
>of the wing, the "longer skinny" Tabs make the most use of the water
moving across it.
>
>I hope this helps.
>
>Let me know what you think.
>
>
>Tom McGow
>Director of Client Services
>Bennett Marine

 

[cbigma]

Thanks Kevin!

Hmm.. an interesting explanation. Not at all what I thought was happening. He describes the reason that the smaller chord is more efficient relates not to drag, but to having a long continuous unbroken surface not allowing water to "escape out the sides" presumably breaking (reducing) the "lift" effect.

Both the 18" x12" and the 24" x 9" would be good
>choices on those boats. But the 24" x 9" is a bit more efficient.
>This is primarily due to the fact that the 9" chord lets less water
>escape out the sides in relation to the water flowing over the Trim
>Plane then the 18" x 12" Tab. In much the same fashion as an aircraft
>wing having flaps that extend a long distance along the trailing edge
>of the wing, the "longer skinny" Tabs make the most use of the water
moving across it.

So in this explanation, the 9 vs 12 chord length is not as important as the fact that the 9 chord has a span of 24 long,,, 6 inches more continuous than the 12X18. Hmm... So span is more important than chord? Dale, Ronnie, Richard and other proponents of 30X9 are right on track then! The longer the better for continuous "unbroken" lift.

I'm still a little unclear about using an airplane wing analogy however, because airplanes use BOTH upper and lower surfaces of the wing, not just the lower like our tabs. There's a lot more going on with an airplane wing than with boat tabs.

Thanks again Kevin, I was struggling with "drag" as the primary issue. It looked like the explanation was that for a rectangle of any given surface area, it was more efficient to tow it "wide side" first for less drag. That would make it more efficient to tow barges sideways! :shock:

I was having a tough time coming to grips with that one. :roll:

 

[DaleH]

The only surprise in the equation is why does a 2120 has the same tabs as a 2520 ... ?

I have been 'told' that Parker is but one of many boat companies that buy their non-factory parts through a 'group buying' purchase. With that comes one item at a great price and committed quantity schedule. they still work ... they just kill your MPG efficiency, that's all .

 

[rangerdog]

Thanks, Kevin. John. I am scratching my head as well. I can see/understand the chord effect but it seems like the rake or drag would "dig in" faster.

On the other hand, digging in surely affects MPG.

I just know that my 2320DVSC is ass heavy. F225, two batteries, petrol shifting back, cooler, and big-ol' boys for a crew, I gotta do something. Depending on conditions, I sometimes have to have the port blade fully deployed and the starboard one fully retracted. One day last summer it was a flat as ice bluebird day. I was by myself and decided to make a WOT run to CCNPP. Hit 42 mph (GPS) with no tabs, but had a slight list to port. (Because of the batteries, I assume. It has always done that.) To get her to run true, I had to deploy the port tab, I dont know, halfway. Knocked down my speed and pushed my fuel consumption from a pretty good 9 something to almost 12.

What I want is to just be able to deploy tabs just an RCH to take care of any given situation and to stay on plane around 12 to 15. Looks like for me, that would be the 24 x 9. Would you all agree?

 

[Outta Here]

24x9 is OK (IMO). I have 24x12 and if I had it to do over, would have 30x9, you have room for these also.

I know of no advantage to small tabs. You mentioned wanting to plane off at what? 15 mph? The larger the tab, the more planing surface you have.

 

[Megabyte]

What I want is to just be able to deploy tabs just an RCH to take care of any given situation and to stay on plane around 12 to 15. Looks like for me, that would be the 24 x 9. Would you all agree?

Agree.

FWIW... I've been out (more than once) when it's kicked up to 3' and I've had to come home in head seas...
I can tell you that my boat (with 24x9's) will remain on plane at 9 knots if need-be, in that slop.
Thank goodness for pilothouses (and windshield wipers). Warm and dry... :wink:

 

[Megabyte]

Agree also with FF.
You can certainly go too small, but I don't think you can go too big.

 

[DaleH]

I am scratching my head as well. I can see/understand the chord effect but it seems like the rake or drag would "dig in" faster.

On the other hand, digging in surely affects MPG.

Exactly! That's most probably why there's no impetus on behalf of the fine folks at Parker to put on larger tabs - they work. Of course they do! I'm just a stingey (?) guy of part Scottish descent and I want my tab performance AND fuel efficiency too. So that's why Iput 24x9s on my mod-V.

If I were you, I'd put on, in order of choice 1-3, - 30x9s, 30x12s, or 24x12s. You want/need WIDER tabs on a deep-V boat with any power, nevermind a heavy 4-S.

 

[cbigma]

OK Tab Gurus. :lol:

Here's something that I still am struggling with. When you deploy a tab, you get lift and create a pressure wave in front of it, much like a bow wave. That pressure wave front has a certain resistance or "drag" associated with it. The wider the tab (span), the wider the pressure wave, and the greater the resistance.

If we agree that the bigger the tab, the greater the lift, why shouldn't you install your rectangular tabs with the shorter edge creating the pressure wave? You get the same lift with less pressure wave front, and therefore less drag? :?:

 

[Outta Here]

OK Tab Gurus. :lol:

Here's something that I still am struggling with. When you deploy a tab, you get lift and create a pressure wave in front of it, much like a bow wave. That pressure wave front has a certain resistance or "drag" associated with it. The wider the tab (span), the wider the pressure wave, and the greater the resistance.

If we agree that the bigger the tab, the greater the lift, why shouldn't you install your rectangular tabs with the shorter edge creating the pressure wave? You get the same lift with less pressure wave front, and therefore less drag? :?:

You are describing exactly the HiPerf "K-Planes" built by Mercury Racing are designed. That is narrow side hinged to boat, long side trailing.

 

[Megabyte]

K-Planes are a whole different animal.
They are generally found on narrow beam, high-performance 'go-fast' boats with twin outdrives, and precious little room on the transom for wide planes.

Combine that with that fact that said tabs must incur the wrath of re-entry to the water when the boats become airborne...
Essentially, the tabs hit the water with the weight of the boat on top of them. :shock:
They are certainly special purpose tabs, heavy-duty and well made, but well beyond the scope of what I need on my boat.

This is what a set of K-Planes look like.

FWIW... Bennett makes tabs for go-fasts too.
http://www.donzi.net/forums/showthread. ... ge=1&pp=15

 

[cbigma]

You are describing exactly the HiPerf "K-Planes" built by Mercury Racing are designed. That is narrow side hinged to boat, long side trailing.

It would seem logical to get as much lift as you can for as little penalty as possible.

This appears to be in conflict with the analysis presented yesterday by "Tabman" Tom. He stated that "the "longer skinny" Tabs make the most use of the water moving across it". He doesn't address the wall of water the longer (greater span) skinny tabs create in front of them.

I'll ask him. :wink:

 

[Megabyte]

I'll ask him. :wink:

Tom McGow <[email protected]>

BTW - Tom was getting ready for the Miami boat show when I asked the question, so he might be delayed in getting back to you.
Direct him to this thread, and I'm sure he'll chime in if gets a moment. :wink: