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Trim tab dilemna

Discussion in 'Technical Discussion' started by Pascal, Nov 23, 2017.

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  1. Pascal

    Pascal Senior Member

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    trying to decide what trim tab size would be best for my 53 hatt MY which is being repowered

    Benett recommends about 1" or tab per foot of LOA along with a 12" cord. According to their information, more cord creates more drag than more span

    Hatteras used 40" ish trim tabs with 9" cord recessed in the bottom of the hull. At some point, someone added a 6" extension although im not sure why since with 325 DD natural the boat had no chance to get on plane

    Now that i am repowering with lighter and more powerful cummins, i want to optimize the tabs. Problem is that switching to the commended 54" wide tabs means extra glass work to close the existing recess. Not a huge deal i m wondering if it s really worth it...
  2. SeaLion

    SeaLion Member

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    What about the "interceptor" types? There seem to be some real advantages over tabs. I would assume that cost would be higher, of course. I saw the Zipwake product at FLIBS and I think it's much more reasonable than Humphree.
  3. YachtForums

    YachtForums Publisher/Admin

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    Interceptors aren't very effective at displacement speeds. Only a tab would generate enough pressure to induce trim.
  4. SeaLion

    SeaLion Member

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    Oh, right. I recall the interceptor guy said (something like) they kick in at about 12kts and would be effective down to about 10kts.
    So on a displacement only boat that can't run faster than that, maybe a fixed wedge on the hull? At a few knots it doesn't do anything and then adds some lift at higher speeds. No moving parts, zincs, etc.
  5. Capt Ralph

    Capt Ralph Senior Member

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    On our Bert, I added to them (fore - aft) another 6 inches. At 10kts it helps lower the bow in the skinny/shallow areas of the ditch. It helped a bit more keeping level from beam wind around 12 kts.

    At 14 to 16 kts, we really start consuming fuel to break free to plane so they are all down. Once up on a half plane (best it gets), I can ease up on the tabs and move along at 20kts and fuel consumption becomes realistic (still not good for the faint of heart).

    Before the mod, other than up at speed, they were worthless.

    I'd just ad to them some more. Speed boats have those long (fore-aft) skinny tabs.

    I guess the only time to large of tabs would be bad would be in a slow boat, faster following sea action. Have a safety wire installed to keep your rams intact.
  6. PacBlue

    PacBlue Senior Member

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    Chord length is your tabs best friend, much more effective at 15 - 18" . The 9" Bennett came from the small production boat needs.

    Just make sure in the all the way up position they are no lower than your transom edge if you were to run a straight edge aft' otherwise you would be running "hook" from the get go. Twin rams would be preferred for the longer chord length.
  7. Capt J

    Capt J Senior Member

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    This, and I'm not so sure the extra chord length increases drag, it also increases planning surface and lift (if not recessed into the hull). The raceboats all run extra long tabs and it doesn't slow them down. The EURO motoryachts which tend to be faster than the US ones always run tabs with long chords and much less width. I'd be inclined to run what you have and see what it does......

    I've also found running hook with 1" of tab doesn't do a darn thing to effect the trim of "most" yachts. I have had 1" spacers installed in half a dozen yachts where the tabs weren't effective enough and the tunnels didn't permit a wider tab.

    Some of the 70's 53's did get up on plane when they were newer with Naturals, my buddy ran one,back in the 70's. had to do WOT for a few minutes tab it, then start backing down on the throttles to cruise...and they'd do 16-17 knots........granted they probably weighed less than the 80's ones with all of the stuff added......enclosed aft deck etc.

    Quite honestly, I'd repower the boat with nibral props and run it, see how it acts, see how it behaves, then start making other changes......sometimes when you change EVERYTHING you end up with an issue but not sure where it's origionating from...... You're not gaining anything with surface area of the tabs....53"x12= 636 sq inches and 40"x15"= 600 sq inches and your existing ones with the longer cord will have more effect IMO.

    I've experimented with 5 blades versus 4 blades on a Hatteras MY and the 5 blades made docking less easy (boat didn't twist as well with 1 fwd and 1 reverse) and the boat slowed down 1.5 knots. With modern 4 blade props, you should get more stern lift and more HP you shouldn't have any issues. I'd spend the money on nibral props, and keep your existing tabs as long as they're in workable condition.
    Last edited: Nov 23, 2017
  8. Pascal

    Pascal Senior Member

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    Update... ended up filling up the holes and installed the massive 54" x 12 recommened by bennett. The old girl got on plane on the way back from the yard... probably for the first time ever!
  9. PacBlue

    PacBlue Senior Member

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    Sweet, getting closer to enjoying the benefits of your comprehensive re-power work!
  10. Boomer

    Boomer Member

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    Glad to hear the right call was made

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