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Old 03-03-2016, 12:23 AM   #1
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Trim tabs...?

I did search, and didn't find much information germane to my question, sorry.

I am looking at a boat on Saturday. It is a 43' SD hull with a max speed of 12 knots. It has Bennett 48x12 trim tabs.

I don't know anything about trim tabs other than the basics that they could help planing hulls get up on a plane more quickly or help with a ballast or wind induced list.

What possible use could trim tabs be on a slow SD hull? I am sure there are very good reasons, I just am too ignorant to figure it out.
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Old 03-03-2016, 12:37 AM   #2
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Apply your flight knowledge Dave. Pretty simple stuff.

It's not just about planing. You can trim nose up, down and port or starboard. In some seas they are just a poor man's stabilizers. They are nice even on a planing boat when going slow with a shifting load of people.
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Old 03-03-2016, 12:47 AM   #3
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I can figure out that much, but when flying the little planes I flew, the elevator trim simply helped neutralize the control input required on the elevator. No vertical control inputs on a boat (unless I am missing something!).

Do they do anything to improve efficiency at a given speed?
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Old 03-03-2016, 12:54 AM   #4
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The boat trim changes with load. Therefore so does efficiency. Think about the difference in the stern with 50 vs 500 gallons of fuel.
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Old 03-03-2016, 01:10 AM   #5
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The boat trim changes with load. Therefore so does efficiency. Think about the difference in the stern with 50 vs 500 gallons of fuel.
hm.... yeah that makes sense. lol, I am so used to a 35 gal tank. I imagine I need to get to thinking in terms of hundreds of gallons, not tens of gallons.

I imagine that at a given load and flat water, carefully looking at speed at a given rpm would give me an idea of the most efficient position.
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Old 03-03-2016, 01:14 AM   #6
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I imagine that at a given load and flat water, carefully looking at speed at a given rpm would give me an idea of the most efficient position.
There you go. Conditions dictate and you will understand quickly. And don't beat yourself up so much.
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Old 03-03-2016, 01:22 AM   #7
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Doesn't rudder trim on a plane do the same thing?
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Old 03-03-2016, 01:45 AM   #8
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Doesn't rudder trim on a plane do the same thing?
Just affects the yaw axis. I rarely flew planes that had rudder trim, they were too old. Helpful to counter engine torque on climb out, but the pedal pressures to compensate were pretty light and once at altitude, the trim really wasn't needed.

But dang, that was 30 years ago. I am sure I have forgotten more than I learned by this time.

Thanks for the help understanding the trim tabs on a boat.
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Old 03-03-2016, 01:56 AM   #9
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But dang, that was 30 years ago. I am sure I have forgotten more than I learned by this time.
Dave, if you can adapt to the pace, you will have a blast on a trawler and here's why;

Sailboat: tack, jibe, luff, telltale, furl, mainsheet, clew, leech genoa, in irons, spinnaker....

Trawler; reverse, neutral, forward, sandwich...
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Old 03-03-2016, 09:13 AM   #10
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On the boat you describe they will only be effective at full speed of 12 knots. Those are large tabs and could help push the bow down at that speed

At trawler speeds tabs will have little to no measurable effect.

Not enough water moving by them to produce much force. No bow to push down.
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Old 03-03-2016, 09:30 AM   #11
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I appreciate my trim tabs when my sister-in-law is aboard.
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Old 03-03-2016, 09:35 AM   #12
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I appreciate my trim tabs when my sister-in-law is aboard.
Now That's funny........
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Old 03-03-2016, 09:48 AM   #13
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On the boat you describe they will only be effective at full speed of 12 knots. Those are large tabs and could help push the bow down at that speed

At trawler speeds tabs will have little to no measurable effect.

Not enough water moving by them to produce much force. No bow to push down.
I used to think the same thing... but on my Ocean Alexander at 8kts the trim tabs do make a significant difference on angle and view over the bow from the helm.. about a foot of bow drop.

The sweet spot seems to be at about 75% of extension, the boat speed actually comes up as the bow goes down, the boat responds better to helm input and the boat "feels" better.

who would of thought..

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Old 03-03-2016, 10:11 AM   #14
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I used to think the same thing... but on my Ocean Alexander at 8kts the trim tabs do make a significant difference on angle and view over the bow from the helm.. about a foot of bow drop.

The sweet spot seems to be at about 75% of extension, the boat speed actually comes up as the bow goes down, the boat responds better to helm input and the boat "feels" better.

who would of thought..
Certainly not me.

Thanks for the input. My thoughts were similar to Kevin's, that they wouldn't be effective at low speed. However, obviously the PO had them installed for a reason.
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Old 03-03-2016, 10:25 AM   #15
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Even at lower speeds tabs that size will help correct port/starboard list due to load in-balance or wind conditions.

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Old 03-03-2016, 10:28 AM   #16
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I will ask the owner when I look at the boat this Saturday. I just figured I was too ignorant to either ask the right questions, or understand the answer. You guys have helped. Thanks.
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Old 03-03-2016, 10:49 AM   #17
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They may have an effect, especially at higher speeds and for free, they would be nice to have. Personally, I would not pay the cost to add them to my boat.


I pretty much agree with ksanders' post.
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Old 03-03-2016, 10:54 AM   #18
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Dave, if you can adapt to the pace, you will have a blast on a trawler and here's why;

Sailboat: tack, jibe, luff, telltale, furl, mainsheet, clew, leech genoa, in irons, spinnaker....

Trawler; reverse, neutral, forward, sandwich...

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Old 03-03-2016, 10:59 AM   #19
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Dhays. On my 42NP, I have tabs about the same size. So far I have not been able to see that they do anything. In certain conditions I will adjust them to where they should be helping, but have not seen any noticeable change. On my previous boat, they were very helpful. Current boat weighs 35k. They give me a few more square feet to paint and a place to hang zincs. I had thoughts of removing them, but NP recommended I leave them on.
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Old 03-03-2016, 11:59 AM   #20
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I will ask the owner when I look at the boat this Saturday.
They might have been original. I don't agree with a one size fits all answer. Speed, sea, load and hull design can all be factors.

Tolly 43 has soft chines, Tolly 44, hard. I'm told 42" tabs work well on the 43 at almost all speeds. Not so much on the 44 until it is up on plane, as Kevin was talking about.
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