How would you define where the overlap of a headsail starts?

How do you define the overlap of a headsail?

With straight & raked masts, swept & in line spreaders how would you define where the overlap of a headsail starts? Do your PHRF/IRC rules define at exactly what point the overlap starts?

- Foremost part of the front of the mast

- Average midpoint of the mast

- Foremost part of the trailing edge of the mast

- Foremost part of the luff of the main

- Average of the mainsail luff

- Furthest aft point of the mainsail luff

- Base of the spreaders

- Tip of the spreaders

- Something else, such as the percentage beyond the J measurement?

Tried to run a poll on this but the SA polling format only allowed 4 possible repsonses and this question is a bit more refined than that.

 

jcc

Member
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How about a percentage of the J measurment.

J is the horizontal distance from the front of the mast to the intersection of the forestay and the deck.

You shouldn't try to make things more complicated than they are.

 

Innocent Bystander

Super Anarchist
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Lower Southern MD
J dimension on the Luff Perpendicular (LP). The LP is a measurement from the clew to the point where it intersects the luff at 90 degrees. Easiest way to find the location is to fold the tack up the luff until the fold is at the clew. Measure from that point on the luff to the clew and divide by J. Result is % overlap.

 

my nuts

Super Anarchist
I agree with the LP as a percentage of J for rating purposes. however, aerodynamic principles and fluid dynamic modeling may give you a different answer, but those usually aren't involved in a single number rule, to my knowledge. I could be mistaken, but I believe that the VPP stuff used mostly hull characteristics in the model. we do know, however that the cut of a headsail is very important to its performance, but I can't see a practical way to rate that other than with some measure of sail area (~0.5*LP*luff)

 
Okay, my thought was also that overlap consists of the excess beyond j and had nothing to do with the mast, mast rake or spreader position etc but I had heard some different opinions. (note how I asked about everything else before bringing the J measurement in) Do different areas use different standards?

 

my nuts

Super Anarchist
Okay, my thought was also that overlap consists of the excess beyond j and had nothing to do with the mast, mast rake or spreader position etc but I had heard some different opinions. (note how I asked about everything else before bringing the J measurement in) Do different areas use different standards?
but if you change forestay length, rake, bend, etc, you're going to alter the geometry of the foretriangle, making it difficult to consistently define said triangle for rating/measurement purposes. from a practical perspective, I don't really see how this could be made to work. from a theoretical perspective, it may make some difference, but I'm not an aerodynamics guy.

 

my nuts

Super Anarchist
maybe you could snap a chalk line from the head to a place J back from the tack along the line from tack to clue, accounting for foot curve. this doesn't really account for mast tuning characteristics, though.

 

Steam Flyer

Sophisticated Yet Humble
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J dimension on the Luff Perpendicular (LP). The LP is a measurement from the clew to the point where it intersects the luff at 90 degrees. Easiest way to find the location is to fold the tack up the luff until the fold is at the clew. Measure from that point on the luff to the clew and divide by J. Result is % overlap.
Almost but no cigar; you dont want to compare the J which is the bottom side of the triangle to the perpendicluar of the long side of the triangle.

Folding the tack along the luff is a good way to illustrate the sail's LP measurement, but the foretriangle also has to be measured from the front edge of the base of the mast to the forestay and perpendicular to the forestay... this is a little more difficult.

It's easier to measure the foot of the sail & compare to J. Since the foot is not perfectly parallel to the line of the J measurement, there is a little bit of error but it's tiny.

FB- Doug

 
Okay, my thought was also that overlap consists of the excess beyond j and had nothing to do with the mast, mast rake or spreader position etc but I had heard some different opinions. (note how I asked about everything else before bringing the J measurement in) Do different areas use different standards?
but if you change forestay length, rake, bend, etc, you're going to alter the geometry of the foretriangle, making it difficult to consistently define said triangle for rating/measurement purposes. from a practical perspective, I don't really see how this could be made to work. from a theoretical perspective, it may make some difference, but I'm not an aerodynamics guy.
I think the issue came as a result of discussing headsail size as a percentage of overlap, so the question arose "overlap of what?" but we are really not talking about an overlap at all but rather the difference, expressed as a percentage, betwen the perpendicular line from luff to clew vs the "J" measurement.

Just another misnomer, like "topsides" are not the top of the boat.

 

my nuts

Super Anarchist
Okay, my thought was also that overlap consists of the excess beyond j and had nothing to do with the mast, mast rake or spreader position etc but I had heard some different opinions. (note how I asked about everything else before bringing the J measurement in) Do different areas use different standards?
but if you change forestay length, rake, bend, etc, you're going to alter the geometry of the foretriangle, making it difficult to consistently define said triangle for rating/measurement purposes. from a practical perspective, I don't really see how this could be made to work. from a theoretical perspective, it may make some difference, but I'm not an aerodynamics guy.
I think the issue came as a result of discussing headsail size as a percentage of overlap, so the question arose "overlap of what?" but we are really not talking about an overlap at all but rather the difference, expressed as a percentage, betwen the perpendicular line from luff to clew vs the "J" measurement.

Just another misnomer, like "topsides" are not the top of the boat.
the other problem with "overlap" has to do with defining the sheeting angle, and designed shape of the sail when trimmed. you can't really pull a sail back along centerline and measure it, unless your mast doesn't have stays. if you have a 100% jib, defined by the LP, then the foot will be longer than J (Pythagoras and all that) if you're looking at a 150, you're really screwed. I would look at "overlap" as a practical definition from a sail handling/trimming/rigging perspective rather than a ratings perspective, where I would stick with some way of measuring sail area.

 

Innocent Bystander

Super Anarchist
11,749
757
Lower Southern MD
J dimension on the Luff Perpendicular (LP). The LP is a measurement from the clew to the point where it intersects the luff at 90 degrees. Easiest way to find the location is to fold the tack up the luff until the fold is at the clew. Measure from that point on the luff to the clew and divide by J. Result is % overlap.
Almost but no cigar; you dont want to compare the J which is the bottom side of the triangle to the perpendicluar of the long side of the triangle.

Folding the tack along the luff is a good way to illustrate the sail's LP measurement, but the foretriangle also has to be measured from the front edge of the base of the mast to the forestay and perpendicular to the forestay... this is a little more difficult.

It's easier to measure the foot of the sail & compare to J. Since the foot is not perfectly parallel to the line of the J measurement, there is a little bit of error but it's tiny.

FB- Doug
I guess it depends on whether you want to measure actual overlap or overlap for the purpose of a rating rule. Every rating scheme I've dealt with defines overlap as LP % of J. Clew height and foot measurement in relation to LP is an individual design. J is well defined by rating rules and is used to determine non penalized headsail size, spinnaker girth, spinnaker pole length, spinnaker luff, etc. See PHRF Chesapeake definitions here Linky

Rating rules don't care about clew height and there can be a significant difference in foot length based on clew height. Foot length will increase as clew height goes up. Imagine a blast reacher as opposed to a deck sweeping #3. Both can be (and will measure in as) the same if LP is the same (say 110% of J) and the sail area (but not performance) will be the same. Rating rules I'm familiar with presume the foretriangle is a right triangle. A 155 genoa is, by definition, a sail where the LP is 1.55 times J.

Actually the foretriangle is measured by J and I, both clearly defined.

 
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Nomenclature

Super Anarchist
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Cloud 9
It's easier to measure the foot of the sail & compare to J. Since the foot is not perfectly parallel to the line of the J measurement, there is a little bit of error but it's tiny.
You can only consider the LP relative to the 'J' length.

The foot length can vary with different clew heights, while

the LP stays the same.

 

WunHungLo

Super Anarchist
5,896
4
PNW
J dimension on the Luff Perpendicular (LP). The LP is a measurement from the clew to the point where it intersects the luff at 90 degrees. Easiest way to find the location is to fold the tack up the luff until the fold is at the clew. Measure from that point on the luff to the clew and divide by J. Result is % overlap.
Almost but no cigar; you dont want to compare the J which is the bottom side of the triangle to the perpendicluar of the long side of the triangle.

Folding the tack along the luff is a good way to illustrate the sail's LP measurement, but the foretriangle also has to be measured from the front edge of the base of the mast to the forestay and perpendicular to the forestay... this is a little more difficult.

It's easier to measure the foot of the sail & compare to J. Since the foot is not perfectly parallel to the line of the J measurement, there is a little bit of error but it's tiny.

FB- Doug
You're entitled to delete this post to preserve your image on SA :lol:

IB is right.

 




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