posted 11-15-2006 10:09 PM ET (US)        

I have twin 250 outboards, and when I needed to trim the boat I simply adjusted the trim on the outboards. not the trim tabs.

Am I missing something?

posted 11-16-2006 10:43 AM ET (US)            

The mostly useful feature of trim tabs is their ability to keep a boat on an even keel. Running in a cross wind will cause a strong list into the wind. Even twin outboards will not be able to overcome this tendency.

I have trim tabs on my 25 foot , twin engine Whaler. The twin outboards have no trouble getting that boat on plane so I rarely use the trim tabs, but when running across the wind I almost always lower the windward tab to bring the boat back on an even keel.

I always use the trim tabs in conjunction with motor trim.

Lowering the trim tabs adds a lot of drag so I try to avoid them as much as possible. I trim the motors to a point where they help the situation and if more correction is needed, *then* I lower a tab.

It is only under very extreme conditions that I would ever lower both tabs to bring the bow down. I would only do this to maintain the absolute lowest planing speed possible when punching into nasty short chop. Running in this condition is NOT fuel efficient at all. I try to avoid it.

posted 11-16-2006 11:14 AM ET (US)            

I think we do disagree regarding which strategy is the least efficient if considered on its own. My opinion, and I think it is commonly shared, is that trimming with tabs is generally more efficient than trimming with motors hydro-dynamically, and consequently is more efficient in fuel usage.

Running with the motors trimmed down as far as they can go at the lowest possible planing speed is certainly less efficient than doing the same thing with tabs. I think this can be proven by coming to a recorded planing speed with the tabs up and the motors at neutral trim, then trimming the motors all the way down and record that speed drop once stabilized. Once that speed is recorded, trim the motors back up to neutral trim, and the initial speed is reached again; then lower the tabs all the way without changing the motor trim and record *that* speed drop once stabilized. Compare the differences in trimmed speed drop.

My boat is out of the water for the season; maybe you'd be interested in running that test?

posted 11-17-2006 01:00 AM ET (US)            

The wind is going to exert an listing force force in the
downwind direction.

posted 11-17-2006 09:25 AM ET (US)            

The reason is very simple. The wind will exert force against the profile of a vessel. Most of that profile is well forward of the motor and the portion of the hull that is running in the water. Thus the tendency is for the wind to "weathervane" the hull in the downwind direction.

In order to maintain a straight course the helm is adjusted to steer into the wind a little bit. What happens when you turn your Whaler? That's right, it leans into the turn. But because the amount of steering input is only equal to the weathervaning tendency of the wind, the boat is left with no course change, just the list.

posted 11-17-2006 09:47 AM ET (US)            

I would not advise anybody to set their motors to a "neutral" trim position and then adjust running attitude with the trim tabs. An outboard motor's trim is too useful and beneficial to not take advantage of.

You seem to imply that a boat will be operating more efficiently if the motor's thrust is directed straight ahead and not with any up or down component. I disagree with that too.

My advice is to achieve some trim with the motors first. How much is something that comes with experience in operating a particular boat. In all the boats I have operated, there is *almost* never any time when the boat is operated with the motor(s) at the extreme end of its trim range. The vast majority of the time the motor(s) is trimmed just ahead or behind vertical.

Trimming a motors out can add bow lift (or stern depression, if you prefer) where a trim tab can only exert stern lift and drag. In the case of my twin outboard Revenge, I often run across Puget Sound where the wind is running either north or south and thus I frequently am running in a cross wind condition.

To keep the boat on an even keel, I usually trim the windward motor out more than the leeward motor. Typically, the leeward motor is close to vertical while the windward motor is trimmed out a ways but not so far as to ventilate at all in a chop.

This results in some torsion to the boat to correct for the wind induced list and is the limit of how much correction can easily be had from the motor's trim. While I could trim the leeward motors all the way in, I do not because it seems to operate with more drag and the RPMs will start to decline. There is also more spray coming up from the motor on my boat when trimmed that way.

I could also trim the windward motor out more but then it would start to ventilate and "growl" a bit going through chop. The RPMs also rise and then I have the problem of keeping the motor synchronized.

So what I do if the wind is very strong and the small amount of list correction from the motors is not enough is that I lower the leeward tab just to the point where the boat is back on an even keel. I know the tab adds drag because by the time the boat is level side-to-side again, the boat will also yaw downwind slightly as a result the trim tab's drag so I try to avoid that if at all possible.

posted 11-17-2006 11:23 AM ET (US)            

1.

quote:

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I would not advise anybody to set their motors to a "neutral" trim position and then adjust running attitude with the trim tabs.

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I would, I do, and that's how I've found it to work best. I guess we'll have to agree to disagree here.

2.

quote:

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You seem to imply that a boat will be operating more efficiently if the motor's thrust is directed straight ahead and not with any up or down component. I disagree with that too.

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I didn't intend to imply that; I intended to state it as a fact given my experience and the way I have come to understand hydro-dynamics. Agree to disagree. I'm simply unable to understand how a motor's thrust can be more efficient by trying to push the stern up or down *and* straight ahead rather than simply straight ahead. For sake of clarity, by "efficient" I mean the most speed through the water attained for the least given unit of fuel used. By "efficient" I *don't* mean the most effective way to raise or lower one side or both sides of the stern.

3.

quote:

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My advice is to achieve some trim with the motors first.

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As I stated above, this is a fine line and I think we are closest to agreeing here: My advice is to trim the motors to neutral ("vertical", or perpendicular with the surface of the water), then take it from there with the tabs. Some fine adjustment may be necessary to get the motors back to neutral trim after the tabs have been adjusted, but for the most part I'm talking *really* fine here.

I certainly agree that trim tabs can only act to lift the stern and not to depress it; if you have a high performance prop that will "bite" and you don't have a jack plate, the highest speed can only be attained by trimming the motor out, which acts to depress the stern (lift the bow) and to reduce the drag of the motor and prop.

I also agree (I *think* I'm agreeing) that the greatest and most frequent use of trim tabs is to level the ride laterally. I don't agree that to accomplish that with the motors is more efficient than to accomplish it with the tabs, for the same reasoning used under #2.

Under all conditions except depressing the stern which we know tabs can't do, the use of tabs and motor trim together will gain more effect than using either strategy separately, but not necessarily more efficiency.

John

posted 11-17-2006 11:46 AM ET (US)            

I believe you are discussing the same idea with different terminology.

The problem relates to John's use of the word "Neutral" trim.

Tom seems to think that this means "no" trim, while John implies it to mean "vertical" running position, so the actual "trim" setting on the motor will vary with speed and conditions that affect the running attitude of the boat, and hence, the amount of trim adjustment to get the motor to have a "vertical" orientation with the water.

My understanding and use of my tabs is to dynamically adjust the engine trim to the most efficient angle for the current use (speed, conditions, etc) so that the motor is vertical to the water and the propeller is delivering maximum forward thrust that is parallel to the water's surface. If conditions are such that the boat is listing due to wind or loading, or that the sea state is causing the boat to leap through the waves, I'll give it a little bit of trim tab adjustment that will appropriately address whatever I'm experiencing.

On calm conditions, tab use would definitely decrease efficiency, however in moderate seas where the boat is running at cruise, the tabs can actually INCREASE efficiency by keeping the bow down and thus, the motor's thrust applied parallel with the direction of the boat for most efficient passage.

Just my take..but I'm starting to worry that Trim Tabs may be as hotly debated a topic here as Outboard Motor Choice, in which case I'll have to sequester myself from posting further information/opinions here for fear of losing another boating buddy over stupid stuff.

:)

posted 11-17-2006 12:57 PM ET (US)            

As far as motor trim goes, I have had CW participants comment that my motor is trimmed up too far in a futile attempt to compensate for my over pitched prop. I disagree. When I am on plane, my motor is vertical but in this picture, it sure looks like I am trimmed high.

http://i34.photobucket.com/albums/d137/jmartin-/092506115.jpg

On my little Whaler, a 15 with a too heavy 70hp 4 stroke, I trim out the motor so there is no feedback to the steering. That seems to be the best trim, but it changes with speed.

I do not have trim tabs, but I could. I think once you get into the 24 foot range, trim tabs would be a nice thing to have. John

posted 11-17-2006 01:57 PM ET (US)            

Tabs are useful after getting the boat on plane and trimmed out to a nice purr. Then adjust the tabs to correct the list of the boat due to load and wind.

Note for all: It is easy to adjust the tabs to the fully deployed position and leave them there. This caused tremendous drag. I always adjust all the way up (10 seconds) before using.

posted 11-17-2006 03:46 PM ET (US)            

quote:

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Kingfish, Efficiency is misused here. What you are missing is wetted surface.

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You are misreading or misunderstanding my post; I stand by my use of "efficiency". See the following quote from my last entry:
1.

quote:

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I certainly agree that trim tabs can only act to lift the stern and not to depress it; if you have a high performance prop that will "bite" and you don't have a jack plate, the highest speed can only be attained by trimming the motor out, which acts to depress the stern (lift the bow) and to reduce the drag of the motor and prop.

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Everyone understands that trim tabs can't have an effect to lower the stern (raise the bow or reduce wetted surface); that's not what I'm talking about so let's just leave that behind us. I'm saying that in the exercise of lifting one or both sides of the stern, the use of trim tabs allows greater operating efficiency than the use of motor trim.

John

posted 11-18-2006 08:36 AM ET (US)            

Normally I do not fool with my engine trim at all and use the trim tabs to adjust the boat's attitude. My engine trim is usually set at the 2nd bar of my Yamaha trim gauge and left there. The only time I adjust the motor trim is when entering shallow water or trying to outrun Jimh!

Go Bucks!

posted 11-18-2006 12:19 PM ET (US)            

But now I find some new theories being introduced.

John,

I'm not sure I am following you. In one post you say:

by "efficient" I mean the most speed through the water attained for the least given unit of fuel used.

OK, I like that definition.

Then you say:

I'm saying that in the exercise of lifting one or both sides of the stern, the use of trim tabs allows greater operating efficiency than the use of motor trim.

Well OK, maybe, but that is only partially relevant. You initial advice is to set the motors to a neutral position and use the trim tabs after that. I do not read your initial advice as only pertaining to side to side trim.

Now I have to actually disagree with the assertion that is all you are trying to do is level the boat side to side, that trim tabs are more efficient at it.

There are two possible condition under which your theory is wrong:

1) Trimming one motor in may be more efficient than lowering the tab on that side. I find no compelling argument to convince me that having the propeller thrust purely horizontal is necessary more efficient than any other state.

2) Depressing the opposite side of the boat by trimming that side's motor out may result in more efficiency than use of a tab by means of the aforementioned (and acknowledged by you) increase in speed from a raised bow and thus reduced wetted surface.

Oh I love a good intellectual debate among friends. But talk is cheap and today we have very nice weather here in Seattle, an open schedule for the afternoon and two new propellers I have been wanting to try out on my boat. I think I will squeeze some trim tab testing into the agenda, whadya think?

I will take your suggestion about bringing the bow down by use of motors then by use of tabs. We will see what speeds results.

I will also see if I can find some good cross wind conditions to test list correction by means of tabs vs. motors.

posted 11-18-2006 03:28 PM ET (US)            

The thing I like best about your argument is the phrase, "...among friends." That is the operant condition here as far as I am concerned too.

Now then, for the sake of the argument, and since my boat is *not* in the water, so I can't use *any* excuse to go out and play, here are a couple of responses to hold up my end of the conversation.

Oops - game's starting. Back atcha later-

posted 11-19-2006 01:08 PM ET (US)            

First of all, yesterday was a fabulous day here on Puget Sound. Mild temperatures in the mid 50s, partial sunshine and zero wind. The pleasure boat traffic, or at least the pleasure powerboat traffic, in November is only a tiny fraction of what we see in the summer.

No yahoos in 50 foot tubs plowing around oblivious to the rules of the road or the potential damage from their wakes, no nit-wits on jet skis buzzing about, so the water was baby butt smooth. Nice! Gotta love off-season boating here in Seattle.

Ultimately, the differences in performance are very small. This is a fairly academic discussion and the results of managing our boat's trim is not going to vary a great deal no matter how we run our boats.

I also suspect that the use of trim tabs will depend on the particular model of boat in question. Henry started this thread asking about a 305 Conquest. John and I have classic Whaler hulls of a very different design. Steve has yet another hull shape. Furthermore, John has a single, I have twins so I have options John does not.

The trim tabs themselves and how they are installed will influence the efficacy of them

Now to the evidence, and John, It's again' ya.

The Minimum Planing Speed Test:

[Revenge 25 with twin 150 HP Mercury outboard motors, Mercury High Five 19" pitch props, light on fuel but with four adults and a large dog on board, full mills canvas.]

Minimum planing speed that I could maintain with motors alone and tucked in a far as possible and without fiddling with the throttles was 16.5 MPH on smooth water. With no hands on the throttles engine speed was 3000 RPM.

Without touching the throttles, I raised the motors to a neutral trim position (propeller thrust horizontal) and then lowered the trim tabs. Engine RPM rose sharply to 3400 RPM but the boat speed remained basically the same eventually dropping to 16.0 MPH.

The attitude of the boat was unchanged. Retracting the tabs at this point caused the bow to rise a little bit and the boat speed to slowly fall off to the point where it no longer maintained a plane. More throttle would be necessary to keep it on plane with this bow (slightly) high attitude.

Conclusion? For running at minimum planing speed, use whatever technique you prefer, or use both techniques in conjunction with each other, which is what I do when it gets really nasty out and you just have to punch into it.

For my boat, it was more pleasant to run with the motors tucked in and the lower RPMs because it sounded so much less strained than when the tabs were lowered and the motors raised to a neutral trim position.

The List Correction Test:

Well, with no wind I was unable to test with a strong cross wind, but as luck would have it, as Kate and I were headed down to the launch ramp with Whaler and dog in tow, we crossed paths with my neighbor and her boyfriend so they joined us for a joy ride. Little did they know they were just guinea pigs to be used as ballast in a test. Ha!

This added weight onboard allowed me to create a significant list by having everybody move to one side of the boat.

So while running at a nice cruising speed of 32 MPH and the motors turning a comfortable 4150 RPM while trimmed to be as near vertical as I could gauge, I had everybody move over to my side of the boat.

With no wind I was easily able to correct this list by trimming in the starboard motor and trimming out the port motor. The amount of trim was clearly visible by looking at the motors but was by no means anywhere near the extremes of the full trim range.

Boat speed was unchanged at 32 MPH.

Then I returned both motor's trim to an equal and vertical trim position and then corrected the list by lowering the starboard trim tab. The boat lost 2 MPH slowing to 30 MPH. I could also feel the boat yaw to starboard as the tab came down and added drag.

And this is the essence of what I have been trying to explain. Whatever slight loss of efficiency is due to some component of a motor's thrust vector being directed downward is obliterated by the added drag if one chooses to use a trim tab.

This is why I advise Whaler operators to use trim tabs only as a last resort and only after making best use of the motor's trim.

posted 11-19-2006 07:13 PM ET (US)            

So what do we know? I don't think it's *all* agi'n me - based on your evidence I do have to give up on some of the comparisons between motor and tab efficacy (read operational "efficiency" as described earlier) on a boat like ours (yours) with twins. Particularly with respect to lateral trim.

I'm standing pat for now on a boat like ours (mine) with a big single though.

I'm guessing that a big part of the greater efficiency shown by twins than by a single is from the greater surface area of the props from two motors compared to the single prop of a single motor.

It did sound to me from what you described, that on the first test, since the engine speed came up by 400 RPM when you raised your motors back to neutral trim and dropped the tabs, you could have throttled back to a lower planing speed than 16.0 MPH with the tabs compared to 16.5 MPH you were able to achieve with the motors? Not central to our discussion but interesting nonetheless.

I think I've been enlightened about the relative efficiencies to be gained from trimming motors and trimming with trim tabs when you have twins, especially with regards to lateral trim, and will add that knowledge to the way I think and talk about their relative merits. But nothing I have seen or heard yet has had any bearing on my conclusions about the efficacy of trim tabs on a boat like ours with a single engine.

So for my part I continue to advise Whaler operators, as well as other operators of boats with *single* motors about the advantages of trim tabs, and about the strategies in using them as I initially outlined above.

Good discussion Tom, and thanks again.

posted 11-25-2006 12:07 PM ET (US)            

I used be ambivalent about the usefulness of trim tabs on outboard powered boats, but recently I had an opportunity to ride on Dave's GAMBLER, an OUTRAGE 18 with twin engines and trim tabs. The use of the trim tabs to hold the bow down was quite effecting in improving the ride of the boat when running is some short, close-spaced, two-foot waves. Raising the tabs caused the ride to become less enjoyable.