Leg trim advice

[MickJ]

Is there a right and wrong leg trim, over and above what just feels right?

My leg trims between -1 and +6 and the gauge has two backlit up/down arrows that wont go off until I trim the leg to +2, but below 20 knots the boat feels much happier at 0 or even -1. Using trim tabs to push the bow back down with the leg trimmed at +2 seems pointless. I'm not stressing anything unduly by ignoring the arrows am I?

Thanks,
Mike.

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[TwoStroke]

Dials and guages are not always accurate.

If it feels right - then you probably have it set up correctly.



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[sfh]

My boat will only work with the legs fully in, if i trim them out the props loose their grip on the water, the engines increase in revs and the boat slows down (in any conditions) im sure this is not correct

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[Col]

Find the boats happy medium (this may change with weight & sea conditions)
The bigger the boat, the less (useable) trim range available.
1st boat was Fletcher 21 Arrowbolt with 311hp V8 Volvo that ran best at between+3 deg in flat calm, and -4 in rougher conditions.
Current boat 38ft Sunseeker with Volvo diesels run best at between -7 & -5 depending on sea conditions, fuel & water load, passengers etc:

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[BrendanS]

There's only one really reliable way to trim. Forget the trim indicator. Try this first in flattish conditions. Trim all the way in, then set off. Set the throttle for your normal cruising speed. Then watch only the rev counter and speedo. GPS speedo is best for this as it's more accurate. Push the trim up control for one second, then run for at least a minute. watch revs and speed
Push it again for a second, run for a minute, watch revs and speed.
Repeat

You should find there is a maximum speed you attain for your revs. After a while, as you keep trimming up, you'll find that speed reduces, and you'll also find the props start to aerate (you can hear the difference)

After a bit of practice, you'll shorten this procedure considerably. I always trim by speed observations at constant revs, and can find that very quickly by trimming up until aeration starts, then trim back a bit, then fine tune, but that's my boat and set up.

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[duncan]

All good advice given and it should reassure you that you are in fact correct in your thought and actions to date.
Under normal conditions (more later) trim tabs fully retracted as soon as teh boat is 'up and planning' - visual check of the wake should have the wave motion perpendicular to your track (in centre of wake) pretty much gone - probably around 13-14 knots. Over around the next 10 -15 knots the boat will be reacting to different forces at different speeds with the hydrodynamic lift being affected by diffrent parts of the hull as she really picks up her skirts - some boats will have a constant trim throughout this range and others will benefit from a progressive increase in trim out of the drive at increased speeds through it. At this point you should be a BrendanS's correct trim point and it tends to remain correct for the balance of revs/speed.
Trim tabs can be usefully deployed whilst planning to either level the boat in a cross wind or if the beer is all stored on one side of the boat, and also to trim the bow down a little into a head sea (note you will also have to trim the leg in the same angle to maintain the right thrust angle too). It is probably a reasonable generalisation to say that not only should you have the tabs fully up with a following sea but you should probably have your drive trimed out to the extent that the warning lights are out in your case!
Sorry for the long post when all I was really saying was - you have got it right!

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[BrendanS]

Yep, should have made the additional comment to then try same technique through the speed range.

Only additional comment is that 'feels right' is usually somewhere in the +/- 10% range. Watching the (GPS) speed and trimming can gain a useful speed increase above and beyond that.Careful trimming can gain me 7knts at near full chat, but I can only do that by watching speed, there are no other useful indicators.

Also, conditions make a big difference, so the leg trim that works one minute, needs to be changed as the conditions change

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