Outhaul Settings and Terms

I have been reading the Hobie U tuning guide for my Hobie 18. For example, a setting reads for light to moderate air "bottom batten 5 to 10 inches draft". Does that mean set the outhaul traveler car to cause/allow the bottom batten to be pushed 5 to 10 inches off center of the boom?

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1984 Hobie 18

yes. there are two items to adjust for with battens. draft depth and position of maximum draft.
depth is measured from the centerline. draft position is a percentage of distance on the sail fore and aft.

ex.- 45% draft would be 4'-6" back from the leading edge if the sail was 10' wide at that point.

draft position is just as important as depth, probably more so as poor draft position hurts more than incorrect depth.
the great thing about full length battens is depth and position are easily seen.

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Chris
'88 H18SE Arís

Thanks for what I am sure is a great answer but man you just flew past me on that one.

In your example,



Is that 4 to 6 feet or inches from the leading edge? I'm thinking feet. When you say "if the sail was 10' wide" - does this mean the length of the base of the sail caused by the position of the outhaul?

So the Maximum Draft point is the point along the base of the sail that is furthest from the boom - the peak of the curve. This peak position is adjusted to be x% of the total of the sail base length measuring back from the luff of the sail.

And the draft depth is distance from the centerline of the boom (not the fore and aft center "point") to the peak of the sail curve?

Whew. Is this right or do I need to go the back of the class!!

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1984 Hobie 18

Draft position is the distance along the chord (measured from the leading edge of the mast to the leech) of the maximum depth of the sail. General rule of thumb would be approximately 45% in the lower sections of the sail gradually lessening to 30% in the upper portion of the sail. So to go off the previous example, if you want the draft position to be at 45% at your bottom batten and you measure the chord length to be 10 feet, then the position of the maximum depth (measured from the leading edge of the mast) should be 45% of 10 feet or 4 feet 6 inches.

Similarly if you want the draft position at your top batten to be 30% and you measure the chord length of you top batten to be 3 feet, then the position of your max draft for the top batten should be 1 foot back.

Draft depth is also measured as a percentage of the chord length. So if you want a 10% draft depth and the chord length of your bottom batten is 10 feet, then you want a draft depth of 1 foot. Draft depth is measured by stretching a line from the leading edge of the mast to the leech, then taking a perpendicular measurement from the straight line to the point of maximum draft depth.

To respond to the initial question of whether the outhaul should be set so the foot of the mainsail is 5 to 10 inches off the boom, I can tell you that for upwind sailng I would never have the sail 10" off the boom in any conditions, it's way too much draft. We typically set the outhaul at about 4" off the boom for all conditions and never touch it. The problem with adjusting the outhaul for downwind sailing (during a race) is that if you ever forget to re-tighten it, it will kill you upwind. In my experience, adjusting the outhaul during a race just isn't worth it.

sm

sorry if I confused you. the mainsail has a fairly uniform width (leading edge to trailing edge) until about 2/3 the way up, then obviously the width diminishes. as noted, the relative draft POSITION changes as a percentage of the entire width because the sail itself gets narrower.

as noted, the best initial practice is to set sail curve to a standard depth and position, sail with it and assess your performance. The H18 pages of the Hobie U tuning guide will give you basic settings for draft. if you have or can read the Berman H18 performance guide, there is a bit more detail.

think of an airplane wing, it has a uniform curve from the root (next to the fuselage) to the tip, where maximum depth of draft is the same distance from the leading edge along it's length. that is what you're looking to achieve.

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Chris
'88 H18SE Arís

Thanks guys. This is helpful. I going to print this out and chew on it. I have been reading the Hobie U tuning guide, which is what drove my initial question - I wish I had the Berman manual, but it seems as rare as hen's teeth.

Now if I ever get any wind here in this Memphis July I'll test these settings out.

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1984 Hobie 18