# Optimum Area & Overlap?

Discussion in 'Sailboats' started by RHough, Nov 11, 2005.

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### RHoughRetro Dude

Given: A clean sheet of paper, no rule to design to and known (calculated) RM of the hull.

Am I correct in thinking that the sail carrying ability is a function of RM and the length of the arm between CLR and CE?

RM = 3000 lb/ft
CE arm = 10 ft
Sail power = 300 lbs?

RM = 3000 lb/ft
CE arm = 20 ft
Sail power = 150 lbs?

From Bethwaite's book I understand that the ratio of Sail Power to displacement needs to be greater than 5-8% for displacement hulls upwind and over 11-15% for planing offwind. Is this reasonable?

That means the taller rig must have half the displacement to have the same power to weight ratio?

I've designed a few gliders and have a layman's working knowledge of the effect of span on induced drag. I've also sailed for a few decades and have decent knowledge of sails and rigs.

I'm using Bolger's Light Schooner as a base for the next boat to build. I'm looking at the sailplan to optimize it.

So far I have a projected hull weight of 890# and a ballasted daggerboard of 210# putting the VCG about 1 ft below the waterline. Total displacement of the boat is 1100#. With three crew it becomes 1525#.

I come up with 185# SCP @ 5 degrees heel for 12% power/weight and 294# SCP @ 15 degrees heel for 18% power/weight.

I'm playing with sail areas and distribution. I have to keep the sail area low to keep the SCP up. As area goes up the A:R goes down and inducted drag goes up. There has to be a happy medium.

If I remember correctly, a given force can be generated by a small area at high CL or a larger area at lower CL. Induced drag is reduced by low values of CL, would a low A:R sail plan with large area operating at a lower CL have a better L/D ratio than a smaller area working at a higher CL?

Since two foils (main and jib) combine to give a greater CL than either sail by itself, what is the best placement of the jib leech relative to the main luff? Is any overlap needed to take advantage of the interaction? How about the relationship between the foresail leech and the main luff? One option I'm considering is using an overlapping, boomless foresail (ala America) if getting the foresail leech closer to the main luff will improve the overall rig's L/D.

Is any of this going to matter? Beyond mental exercise, am I wasting my time even worrying about overlaps?

Things I'm committed to:
Unstayed (athwartsips) masts
Schooner rig

Mast placement and sail area distribution are relatively open. I can always loft new bulkhead positions and change the bowsprit length.

TIA for any insight!

-Randy

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### Van NostrumJunior Member

Jib leech should be slightly overlapping so that the jib helps to accelerate the airflow along the main.

Or have a look at a 49er sailing upwind.

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### RHoughRetro Dude

Thanks, the reason for my post is to see if anyone has a logical way to determine just how much the sails should overlap.

We know that 150% of J is not needed. At some point the leech of the leading sail will to far forward of the luff of the trailing sail for the higher pressure in the slot to work to combine the flow of both sails.

The forward sail works in a apparent lift created by the aft sail even when the two sails are too far apart to create the "slot".

What I'm looking for is the optimum spacing for a multi-foil lifting surface.

Most single foils stall somewhere around CL = 1.0 - 1.2 Two foils can combine to create a CL of 2.0+

Somewhere between 6-10 chords forward and some overlap the highest combined lift occurs.

My gut tells me that the leech of the forward foil does not have to extend as far as the 1/4 chord point of the aft foil.

The width of the slot is probably important also. It depends on how far "above" the rear foil the negative pressure region extends.

Should the leech of the forward foil be centered in the low pressure bubble?

Could you have a non-overlapping foil with it's leech just forward and leeward of the trailing foil's luff work with the trailing foil to become a single aerodynamic unit?

On conventional rigs the top part of the leech of the foresail cannot overlap the main, further down it overlaps the mast and overlaps the main sail at the foot.

Does this mean that there is no "slot effect" in the top third of the jib?

Should the leech of the jib be a constant distance from the leeward surface of the main?

Should the jib leech be a constant % of mainsail camber away from the main?

I assume that the area of lower pressure at the front of the main is in a fixed position defined in terms of local chord, camber and AOA. But, where is it?

Should I just try different leech positions until I find out what works?

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### Van NostrumJunior Member

I think it's (jib leech) meant to be right in the low pressure zone.

I've heard it's the same theory that allows jumbo's to take off... sorry I've no idea what it's called.

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### chandlerSenior Member

It's called the harmonic differential balancing axis

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### RHoughRetro Dude

Three foil optimization ...

http://syr.stanford.edu/HISWA_Tyler_2002.pdf

I found this study of optimum camber for semicircular camber sails.

The study shows that non-overlapping sails are best with increasing AOA in the trailing sails and increased camber in the leading sails.

This study shows that overlap in not needed to increase the load/area ratio of the leading sail.

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