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My IOM won't go straight

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Old 03-31-2011, 03:56 AM
  #1
SimonsDad
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Default My IOM won't go straight

Hi All,

I recently built myself a NImbus 3 IOM and after lots of advice on setting up the rig it is sailing pretty well now. I just have one real problem remaining:

It won't go straight!

By that I mean: after every course correction or change of tack I need to fiddle with my rudder trim to keep it on a straight course and stop it wandering off. Once trimmed it will follow a straight line OK.

There doesn't seem to be any logic in which way I need to trim it but Im sure there is a pattern and I just havn't seen it yet. Can anybody suggest what the pattern could be and if I can prove it then what do I do about it?

Cheers,
Andy
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Old 03-31-2011, 04:17 PM
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ron ward
 
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Default RE: My IOM won't go straight

 it sounds very much like a helm balance problem. 
     when sailing it on either tack, as it heels over, does want to wander  the same direction as the wind or does it want to wander into the wind ?
        if with the wind, reduce the size of the jib,or increase the size of the main, or move the rig ahead.   if into the wind, increase the size of the jib, decrease the size of the main, or move the rig aft.
       most times, with a high aspect rig... as found on an IOM...., just moving the rig will balance it. you don't want any tendancy to to wander with the wind at all.  a bit of windward helm is ok and a small trim on the steerage to correct it is fine, actually prefered. some trimming is naturally expected with a tall rig as the center of effort is high in the sail and has a greater lever as the boat heels, so different winds( and angles of heel) will need a different trim. the idea is to find the best rig position that has the least helm change in varying winds.
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Old 04-01-2011, 06:07 AM
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hew565
 
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Default RE: My IOM won't go straight

Also check your rudder servo to see that it returns to the same centered position each time. Unless it is a digital servo, it may not be centering properly each time, causing the random wander.

If it were always the same correction needed, it would be a helm balance thing, but random suggests a non centered servo, or a rig that is not centered (starboard to port) in the boat. If the rig is not centered, the boat would want to head up on one tack, causing you to trim the rudder down, and after a tack, the rig would be different (centered or not, but different) causing you to adjust the rudder again.

Always leave the rudder centered (assuming it stays there!) and do your trim adjustments to the rig.
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Old 04-02-2011, 11:14 AM
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Default RE: My IOM won't go straight

All good advices but have you also checked the fin keel and the rudder to see if they are absolutely aligned and with no twist. A fiend of mine who bought his IOM from a profesisonal boat builder had a serious twist in his rudder - a bit like a prop. And though it was built by a pro and in carbon after few months sailing the twist appeared. no reason found as the builder built plenty in the past with no issues... it was probably a monday rudder !!!
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Old 04-02-2011, 11:18 AM
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Default RE: My IOM won't go straight

Digital servos are neither more accurate nor more stable than analogue types, since they use the exact same sensing device.  They do drive to their new required position more rapidly, since they dont need the more gentle stopping that analogue ones need to prevent overshooting.
Go with Ron's advice, Analyze the problem and sort the trim of the rig.
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Old 04-02-2011, 07:12 PM
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ron ward
 
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Default RE: My IOM won't go straight

SimonsDad,
i just re-read my post and i realized i mixed up some advice to you.

where i say to....." move the rig ahead", should read...." move the rig aft",......and where i say to...."move the rig aft"...should read..."move the rig ahead"..... for whatever reason, it just popped into my head that i mixed that up!...... sorry about that!.... that's what happens when you get old and brain gas starts leaking from your head!!........
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Old 04-05-2011, 10:08 AM
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Default RE: My IOM won't go straight

Check the real easy stuff first.
I had the same problem recently and blamed everything but what was right.
Must be the servo not centering..change the servo...same problem
Rudder crooked on shaft,found the shaft was sticking a bit in the rudder tube...sanded it a bit to free it up...helped a little.
Finally,
Checked the slop in the rudder arm connection between servo and tiller arm.It had just enough play for the rudder to be off centre when loaded up ,different angle on each tack due to water flow.
Changed the servo arm so that the hole was a tight fit with the steering rod....damn the problem went away.
Look for the small least expensive answer first.
Sure was frustrating .
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Old 04-05-2011, 10:18 AM
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Default RE: My IOM won't go straight

A friend had a similar problem. Turned out to be the rudder shaft was actually turning a bit inside the rudder itself! It would slip under load, but not the same every time. He fixed it with a drop of CA where the shaft entered the rudder.
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Old 04-06-2011, 12:12 AM
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Default RE: My IOM won't go straight

Thanks to everybody for all the great advice.

Andy
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Old 06-08-2011, 06:16 PM
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Default RE: My IOM won't go straight

Put a motor on it
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Old 06-08-2011, 07:27 PM
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Default RE: My IOM won't go straight

Why put a motor on a SAILBOAT!! The wind is all you need
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Old 06-08-2011, 11:42 PM
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SimonsDad
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Default RE: My IOM won't go straight

Thanks to the last two silly posts for reminding me to let you know what the problem was.

Unbalanced Rig

Had to make a new jib and main boom to get the geometry just right and now I am no longer the last in the fleet!

Thanks again for all your suggestions.

Andy
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