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Author Topic: Lee helm in a DN?  (Read 8616 times)
Geoff Sobering
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« on: April 29, 2009, 04:24:00 PM »

My "new" ('05-'06 season) hull has always had a "lee helm" (i.e. it consistently wants to bear away if I don't hold pressure against the tiller).  This effect is evident in all kinds of wind speeds, with the mast-step in every position (from 35 1/4" max. forward to nearly max. aft.), and with mast-rake all over the place.  I'm also using steering runners from my previous hull where I didn't have this problem.

It's made it hard to hold a consistent groove when I'm sailing without paying way too much attention to my heading.

Ken Smith (and others) noted that it might be the result of the steering runner "rocking back" onto the aft end of the runner due to contact between the steering-chock and the stiffener.  There was some evidence this might be happening (wear marks on the top of the steering runner stiffener), so last season I cut away enough of the stiffener on one of my runners so there was no noticeable contact.  That seemed to reduce the problem, but it never went away, and late last season (ex. the Westerns) it seemed to have returned in full-force (and if anything I have my rig further back and more upright than ever).

In soft-water boats this is almost always due to a the center-of-effort (CE) being ahead of the center-of-lateral-resistance (CR).  On a bow-steered boat, the CE is always ahead of the main runners, so the only effect of moving the CE has is the ratio of side-force on the bow vs. rear runners.  In general, the ratio is about 1:3, so changes in rig tune wouldn't appear to alter the ratio substantially.

At the end of this season I assumed the problem was related to the rake I put in my steering post, and planned to reinstall it perfectly upright.  Careful measurement showed the rake was a modest 2 degrees, so that wouldn't appear to be the problem.  I plan to replace my steering chock with a short Euro-style item, so any issue with the steering chock and runner would appear to be eliminated.

I'm looking for suggestions on how to diagnose (and ultimately correct) this problem.
Any thoughts?  Huh

Cheers,

Geoff S.
US-5156
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Man Why You Even Got to Do a Thing
JOHN BUSHEY
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« Reply #1 on: April 29, 2009, 10:32:53 PM »

Geoff,

Have you tried a different steering runner?  The location of your crown flat relative to the steering post would also effect the helm.

John


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Paul Goodwin - US 46
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« Reply #2 on: May 02, 2009, 11:51:43 AM »

Runner crown "flat" relative to the steering pivot.  I assume this means the flat is too far forward.  Think about this - if you put a runner with crown on a surface plate, it will touch the plate directly under the CG.  It can't do anything else without a little help.

When measuring crown, we (I) lift the front of the runner so it touches the surface plate directly under the pivot hole, then slide the 0.008" shims in.  I have never been convinced the location of "flat" (my runners don't have any actual flat spots if I can help it) relative to the pivot bolt has any great influence on lee helm.

Has anyone tried taking a front runner from a boat with lee helm and putting it on a boat that didn't have lee helm?  Or the opposite might be even better - try several different steering runners from a boat without lee helm to see if it makes a difference on a boat with lee helm .  It seems like this would be the easiest way to isolate the influence of the steering runner on the lee helm problem.
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Paul Goodwin
DN US-46
Geoff Sobering
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« Reply #3 on: May 03, 2009, 01:58:10 PM »

Runner crown "flat" relative to the steering pivot.  I assume this means the flat is too far forward.

Aside: I think a pivot point forward would give weather helm, and aft would give lee helm. (?)

Think about this - if you put a runner with crown on a surface plate, it will touch the plate directly under the CG.  It can't do anything else without a little help.

I agree.  Of course, the there is a "little help" from the weight/force on the front of the boat pressing down on the pivot bolt.  Even with my relatively heavy plate steering runners, I've always believed that there's more than enough force on the steering chock while sailing to rock the steering runner so the contact point is directly under the pivot bolt (absent a mechanical interference like the chock/stiffener contact).  I've seen it first-hand when checking runner profiles on a stone, the contact point is essentially directly underneath the point where one presses down (assuming one applies enough force to overcome the runner's weight distribution).

OTOH, I'm adhere to the maxim from Sherlock Holmes, "Eliminate all other factors, and the one which remains must be the truth", so at this point I'm unwilling to categorically state it's impossible for the shape of the steering runner profile to affect the helm feel.  I'm pretty sure I don't have a uniform radius curve along the length of my runner, so even through there isn't any "true flat" area, I think there is a region near the center with less curvature than the lead-in and exit sections.  Could this be significant?  I don't know.  The experiment proposed below would seem to be the answer:

Has anyone tried taking a front runner from a boat with lee helm and putting it on a boat that didn't have lee helm?  Or the opposite might be even better - try several different steering runners from a boat without lee helm to see if it makes a difference on a boat with lee helm .  It seems like this would be the easiest way to isolate the influence of the steering runner on the lee helm problem.

I've not tried that in an immediate sense of swapping runners on the ice, but I'll give it a shot when we have ice next year. 

What I have done is use the same steering runner that I had on my older hull (neutral helm) with my new hull (lee helm).  I don't remember any re-profiling between the two seasons (I doubt I did anything because I was busy building the new hull almost the entire off-season).

Cheers,

Geoff S.
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Man Why You Even Got to Do a Thing
dn4379
ADMIN

Posts: 55


« Reply #4 on: July 30, 2009, 08:04:06 PM »

     When lee helm is a problem I move the plank one position forward.  I carry the plank in the second or third hole from the front on the standard Sarns hull/plank plates; all the way back in the 4-6 knt wind conditions like the race I won at this year's Worlds. Opps!, did I just give away a secret?!?  Body weight too far back could also contribute.  Both of these techniques move the center of resistance forward.  Sometimes I accept lee helm upwind to make control easier in super fast slippery downwind conditions.  Dull back runners will also "fix" the problem; not that I recommend that very often!
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