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Old 12-21-2016, 10:44 PM   #1
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Trim tab Question

Bought a '84 34 Mainship recently and am very pleased with it. It has fairly large trim tabs which I have not had the opportunity to try out yet. The controls for the trim tabs on the flybridge are rocker switches with "BOW DOWN" when pushing the rocker forward, and nothing written when pushing the rockers to the stern. I ASSUME that "BOW DOWN" lowers the trim tabs to force the bow down into the water, correct? Next time I'm doing 10 knots or so I'm going to experiment with the "BOW DOWN" and see what happens. What SHOULD happen?
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Old 12-21-2016, 11:08 PM   #2
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Yes bow down means when you push the switches in that direction the tabs will extend forcing the bow down.

Play with them while watching your GPS to see if there is a sweet spot that gives you better speed.

You need to do it a step at a time. Move them a bit and then give the GPS time to catch up and give you an accurate speed reading.
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Old 12-22-2016, 01:09 AM   #3
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You can also use them to level the boat side to side while you're running.
Say all of your guests have moved to the starboard side and the boat is a little down on that side, use the starboard switch to pick that side up.

Go easy with the switches. Feed the tabs in a little at a time. If you push the bow down too much, the steering can get squirrelly.
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Old 12-22-2016, 05:21 AM   #4
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Tabs will have little to no effect until boat speed is up above hull speed on most boats. Experiment with them.
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Old 12-22-2016, 05:25 AM   #5
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"Fairly large" is relative. Boats like the Mainship need larger tabs because the operate at lower speeds. I have 42" x 12" and with 200HP I can get on plane, but without the tabs that's a maybe. Also other benefits as mentioned, the high profile might make a MS heel with wind on the beam, or being able to put one side down a little to knock down spray.

Enjoy


Donít Worry About Which Trim Tab is Moving
Bennett Trim Tabs operate the reverse of what you might think. The port trim tab lowers the starboard bow. Conversely, the starboard trim tab lowers the port bow. The control is wired so that all you have to do is press the control in the direction you want the bow to move. The proper use of Bennett Trim Tabs becomes second nature after a short time.

Probably more than you might ever want to know about trim tabs:

How Do Bennett Trim Tabs Work? - Bennett Marine

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Old 12-22-2016, 06:54 AM   #6
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I also had 42 x 12 tabs on my old Mainship 1. One tab down will keep the boat level in a beam wind also.
They also act as "rocker stoppers" when anchored.
And remember.....heavy following seas, tabs UP.(bow up)
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Old 12-22-2016, 12:12 PM   #7
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What really happens when you push the "bow down" switches is the stern is pushed up by the tabs. That has the effect of pushing the bow down or making it seem like it is down.


This only works when the boat is moving and only works well if the boat is moving relatively fast. At or below hull speed there will not be a large effect.


And yes, you can use one tab to trim the boat side to side but again, you have to be moving.
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Old 12-22-2016, 12:12 PM   #8
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Using them in following seas could be exciting.
Especially on the Main 34.
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