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.

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.

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?

The boat trim changes with load. Therefore so does efficiency. Think about the difference in the stern with 50 vs 500 gallons of fuel.

Hawgwash said:

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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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.

dhays said:

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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There you go. Conditions dictate and you will understand quickly. And don't beat yourself up so much.

Doesn't rudder trim on a plane do the same thing?

Hawgwash said:

Doesn't rudder trim on a plane do the same thing?

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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.

dhays; said:

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

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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...

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 appreciate my trim tabs when my sister-in-law is aboard.

obthomas said:

I appreciate my trim tabs when my sister-in-law is aboard.

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Now That's funny........

ksanders said:

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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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..

HOLLYWOOD

hollywood8118 said:

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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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.

Even at lower speeds tabs that size will help correct port/starboard list due to load in-balance or wind conditions.

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.

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.

Hawgwash said:

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...

Click to expand...

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.

dhays said:

I will ask the owner when I look at the boat this Saturday.

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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.

Rebel112r said:

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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Thanks Rebel, your experience is the most pertinent to this boat, as it is a NP43. I will see what the owner thinks.

Hawgwash said:

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...

Click to expand...

Sounds very nice Hawgwash. The most appealing aspect is being able to do it all out of the weather and be able to sip a cup of coffee with that sandwich.

dhays; said:

Thanks Rebel, your experience is the most pertinent to this boat, as it is a NP43. I will see what the owner thinks.

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Looking at YW NP43/42, 4 out of the six have tabs. So, either way, factory or owner installed, must have been a reason for it.

Dave,

Having had two boats where trim tabs were almost a necessity, (C-Dory 22 and 25), sometimes the effect of trim tabs are hard to see without a fuel flow meter or very accurate measurement of speed and RPM. Generally speaking in any boat, the most efficient way to push it is with the shaft and prop parallel to the direction the boat is moving. There are many more factors involved but looking at only the prop, anything you can do to make the thrust of the prop more parallel to the direction of travel, the more efficient. At hull speed or less, in an inboard boat, you probably are as close to parallel as you can get. beyond that speed, everything from the wave forming at the bow to the downward thrust of the prop is pushing the bow up (or stern down). At 10 knots if putting trim tabs down, gives you another .2 knots, then the question becomes are they worth the investment. The problem is accurately measuring the .2 knots.

In the C-Dory's I could tell the difference with trim tabs and motor trim at 18-24 knots where proper trim would give you 1 to 2 knots more speed at the same throttle setting. At 12 knots the difference was not as measurable other than once on step you could stay on step down to about 10 knots with everything properly trimmed. I have always wondered what trim tabs would do on the Nordic 42, but I am not at the point to fund a science experiment yet.

Tom

"The most appealing aspect is being able to do it all out of the weather and be able to sip a cup of coffee with that sandwich."

With your feet up listening to music and the AP doing the steering .

Tabs on a trawler are of little or no use.

First, rest assured they were used for a reason. At some mix of weight and speed, the boat could benefit from them. I've seen even FD boats that ran at a slightly different angle and speed depending on weight on the boat. For many boats, full water and full fuel can make a significant difference. The ability to adjust for conditions is something I find very worthwhile. You're not just changing the level of the bow either, but adjusting side to side. If you have a load that leaves the boat sitting lower on one side, just trim it. Also, a windy day adjusting the angle at which the waves hit you. You think initially you won't use them much but you find yourself tweaking things with them all the time.

Speaking of Thomas' fat sister-in-law... Our new motoryacht is ass-heavy. Tanks and batteries are in the stern. TBH, I think our trim tabs are a little small, but when I get up (or try to get up) on full plane, if I trim the bow down, I can easily pick up 5 or 6 MPH. I am assuming it is pushing her the last little bit over the bow wave to hit full stride.

I've noticed that at 6.5 to @8 mph I can give some down tab and pick up a few tenths mph. When going faster than that I got to have the tabs full down to get the bow down and gain a couple of mph. I think the gain at slower speeds would of been more before I added 250' of chain to the original 50' but I had no way to measure speed before adding it, the extra 275# help hold the bow down.


Sent from my iPad using Trawler Forum

hollywood8118 said:

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..

HOLLYWOOD

Click to expand...

+1. Tabs can be effective a slower speeds. Especially if the are sized correctly.