Trim Tabs

[dbassauto]

Hey All, Replacing my trim tabs. any idea what size trim tabs works best for my 1975 scepter 5.7 litre inboard outboard set-up thanks Tom.

[McGillicuddy]

I think most would tell you bigger is better, so 18x12 or 24x9. 12x12 would probably suffice, but Bennett says bigger tabs are more efficient (I interpret that to mean more responsive and likely able to stay on plane at lower speeds).

[Bushwacker]

I'd interpret more efficient as less deflection required to generate a given amount of lift. Less deflection generally means less drag. In other words, more efficient means a better Lift/Drag ratio, or L/D, just like on aircraft wings!

[McGillicuddy]

Hi Denny,
I concur with your definition. By "responsive" I meant more effect with less tab movement or if I understand correctly, less deflection. That being said, would you suggest more tab area or more breadth of tab?
Gillie

[bigeasy1]

I have the same boat(74 23'Tsunami) with the 5.7 liter I/O.

I went with Bennetts drop fin tabs.I know this sounds crazy but I can't remember the exact size,but I'm pretty sure they are 12x12 drop fins.
Call Tom or one of the guys at Bennett,they are incredibly nice to deal with.They'll set you up with the right size tab.
They actually recommended using two sets of tabs for my Tsunami,but told me if not,then to go with the drop fin tabs.

They explained that the drop fin adds a lot more lift effectively acting as a much larger tab,and they also help make the boat track straighter.
They were right,as these tabs do a great job.
Don't confuse the drop fins with the sport tabs.The drop fins have a much larger size drop fin.(about a 3" drop)I don't see the drop fins on their website,but I know they have them.

I placed mine on the outside steps of the hull,and they do a perfect job of controlling the boats attitude.I don't really have to use them all that much to get on plane,as the boat hops right up pretty quick.I use them more in a quartering sea more than anything

Here's a pic of the tabs on my transom in the early stages of the restoration.I had just mounted them.

[cdavisdb]

Thanks for the drop idea. That one gets incorporated in my next set of tabs.

Race boats like narrow and long, more leverage, but I really liked the tabs on my old
Seabird, very wide, very stable and sensitive, seemed to reduce the roll a bit.

[Tabman]

The 12 x 12 Tabs with Drop Fins are a great Tim Tab to fit to the stepped hull Scepter.

The placement in Bigeasy's picture is perfect and they would be what I recommend. If you have any questions let me know, my direct e-mail is [email protected]

Tom McGow
Bennett Marine

[gofastsandman]

Originally Posted by McGillicuddy

Hi Denny,
I concur with your definition. By "responsive" I meant more effect with less tab movement or if I understand correctly, less deflection. That being said, would you suggest more tab area or more breadth of tab?
Gillie

Breadth and profile of the tab design? BH nailed it. Sales op there...

Bones has big and wide tabs. Many are smarter than I.

[Bushwacker]

Originally Posted by McGillicuddy

. . . would you suggest more tab area or more breadth of tab? . .

Hi Gillie,

Lift for a given amount of deflection would be directly proportional to area, so more area would be a plus, regardless of how you get it. The most efficient (highest L/D ratio) airplane wings are high aspect ratio (long with short chord) like a sailplane, and at low Mach numbers air does behave somewhat like water. We used water flow tables at Pratt for that reason to study complex flow patterns in stuff like combustors at Mach numbers of 0.3 and lower. The racing sailboat guys also seem to prefer high aspect ratio keels, so I suspect that they have better L/D ratio. That would indicate that high aspect ratio tabs (wide and short) are probably more efficient, but practical installation considerations are probably more important . . . for instance, if you used a wide tab that straddles the vertical step between panels on a SeaCraft hull, the area near the step is going to see a lot of foam and air, so it won't produce much lift! I'd vote for a narrow and long or square tab that's no wider than the outer panel, which is where it should be mounted.

I agree with Tabman that the large drop fins on the sides of the tabs would definitely improve effectiveness. When water flowing off the transom encounters a deflected tab, it will try to take the path of least resistance, i.e. off to the side and around the tab! The drop fins create a channel to stop that, so should be very effective and they'll also provide additional stiffness to the tabs in the fore and aft direction as well! Denny