i550class.org

Chad,

I have some class rules questions .....

2.9.3 and 2.10.3 don't make much sense to me. We are measuring 'frame angle' with a linear unit. What are we measuring and how?

2.12.1 references rule 4.5: there is no rule 4.5 - but otherwise the rule is clear. Perhaps reference should be 2.3?

5.7.2 "The head of the mainsail shall not exceed an included angle of 100 degrees, measured to the last yard (meter) of luff." What are we measuring and how? Is this some sort of control on the 'squareness' of the head of the sail?

5.8.2 and 5.9.1: between them these two rules appear to do two things. 5.8.2 appears to prohibit overlapping headsails [genoa jibs] on a beat (or effectively at all, given 5.10.3), and 5.9.2, by adopting the reverse of the racing rules of sailing definition of the distinction between a spinnaker and a jib, prohibits the use of an old-fashioned, wire-luffed 'flat' spinnaker which could circumvent 5.8. Am I correct?

Hi Warren,
Glad to see you interested in our rules, and I'll try to convey understandable answers!

Warren Nethercote wrote:Chad,

I have some class rules questions .....

2.9.3 and 2.10.3 don't make much sense to me. We are measuring 'frame angle' with a linear unit. What are we measuring and how?

The plan describes a diagonal line, by measuring over and up for each frame. Draw that line, then take the chine as the datum, measure along the diagonal line 33" from the chine, and your frame's actual line needs to pass within 1/2" of this point. Carpenter frequently measure angles in "rise over run"; this is a similar concept in that it describes 1/2" of allowable deviation in 33" of radial "run".

2.12.1 references rule 4.5: there is no rule 4.5 - but otherwise the rule is clear. Perhaps reference should be 2.3?

Yep, 2.3 is the intended target there.

5.7.2 "The head of the mainsail shall not exceed an included angle of 100 degrees, measured to the last yard (meter) of luff." What are we measuring and how? Is this some sort of control on the 'squareness' of the head of the sail?

Yes, exactly. 18's, and some other classes, have way over square heads. How to measure? -lay the sail on the ground, measure 36" or a meter down the luff, get some kind of angle measuring gizmo (protractor, a calculator if you're trig-savvy, etc.), and make sure the head of the sail doesn't project above 100 degrees. The "yard or meter" just indicates that we're only looking at the top of the sail, not the entire luff. It's so nobody has to get out strings to measure the angle when there is a lot of luff-round.

5.8.2 and 5.9.1: between them these two rules appear to do two things. 5.8.2 appears to prohibit overlapping headsails [genoa jibs] on a beat (or effectively at all, given 5.10.3), and 5.9.2, by adopting the reverse of the racing rules of sailing definition of the distinction between a spinnaker and a jib, prohibits the use of an old-fashioned, wire-luffed 'flat' spinnaker which could circumvent 5.8. Am I correct?

The intention of 5.8.2 is twofold: it prevents overlapping headsails as you mention, as well as eliminating the incentive to produce a narrow-shroudbase boat. Keeping the shrouds wide reduces loads, and eliminates the need for some beefy structure to accommodate a high-tension rig.

The RRS on headsails changed from 2012 to 2013: when these rules were written a headsail mid girth could be no more than 50% of its foot, and almost all rating bodies defined a spinnaker as having a mid girth greater than 75%. If anything, the rules have changed "toward us". We didn't see the need to define a headsail then since the RRS took care of it; now the RRS define what isn't a headsail as a spinnaker in a way that is consistent with our rules.

Under our rules, you can build a taught-luffed spinnaker, but will face the challenge of how to deal with the excess, flappy leach if you try to go upwind with it. And you get to engineer your prod for the loads...

Chad,

OK, the mainsail stuff is clear (see sketch to confirm) but I am still struggling with 2.9.3 and 2.10.3. See second post.

Chad, I have sketched a transom in my attachment and drawn a diagonal. Is this the right one? If it is, I still don't understand what the measurement is.

Warren Nethercote wrote:Chad,

OK, the mainsail stuff is clear (see sketch to confirm) but I am still struggling with 2.9.3 and 2.10.3. See second post.

(To clarify, the mast band is unrelated to the head angle, as might be construed from the sketch)

Warren Nethercote wrote:Chad, I have sketched a transom in my attachment and drawn a diagonal. Is this the right one? If it is, I still don't understand what the measurement is.

The CHINE is the datum for the frame angle. I'll add a sketch later today.

Thanks Chad. I added the band (5.33) to the mainsail sketch to observe that no part of the head may be above the black band as 5.3.3 is written, so if the included angle of the head is over 90 degrees the sailor breaks 5.3.3 if the main luff is hoisted right to the black band. As worded, 5.3.3 is not easily policed. Was this the intended effect I wonder. Since there is a limit on head angle, having the black band control the height of the luff of the sail would be easier to control by on-water observation.

That applies to trilateral sails, but our rules doesn't restrict mains that way. For a flattop main, the "head point" is what would be called a "throat point" for a quadrilateral sail. Bands measure the edge of the sail that is related to that spar. If we were measuring sail luffs, a technical reading might suggest that the head be projected 90 degrees, but that's not the case.

This thread doesn't address all your questions, but maybe it helps with some:
viewtopic.php?f=5&t=287

Thanks Chad, your diagram clarifies 2.9.3 and 2.10.3. I see your stair-tread analogy now, although it was not the first thing that came to my mind (being a naval architect, albeit for ships not boats). Measuring these items might not be straightforward: presumably the origin of the arc is the virtual chine intersection point if the chine is radiused. My own inclination would be to build a go-no-go gauge running along the plus 1/2 inch radial line, and then measure in to the hull surface at the specified radius. The gauge might be fixed in space with a long straight-edge running under the hull and parallel to the cockpit sole or the gunwales, to establish a 'local horizontal'. The gauge would need to be adjustable on the straight edge to cope with bottom camber variations.

The measuring guidance in your link, together with the i550_ANGLE pdf you just posted would benefit from being combined as a single pdf and posted to the website along with the constitution and class rules, where the measurement guidance would carry the weight of an official publication rather than a contribution to the class blog. The two documents (rules and measurement guidance) would help late-comers to the party .... This applies not only to new builders but to race officials. Another of my hats is as an ISAF International Judge. This weekend, for example, I will be measurement liaison for the jury for the Canadian PanAm trials and will need to be able to appreciate a half dozen or so rule sets. The i550 won't circulate in those circles any time soon, but the odd PHRF handicap committee might want to peek at the rules when first trying to establish an i550 rating. It would be helpful in those circumstances if rules, together with measurement guidance, left no doubt in the committee's mind as to what an i550 was. I think the rules plus the measurement guidance satisfy that goal, but the rules on their own do not in all cases.

But now I think I appreciate what the rules mean, which increases my confidence as a builder.

Like this? -from 2011

https://youtu.be/n6pG-M9Ps9Y

Chad, you crafty fellow! Indeed, like that - well, yours is farther along than my first thoughts by displacing the whole thing an inch (? I think that's what it was). As I said, this knowledge needs to be captured on the main web page, particularly as you have developed some elegant measurement techniques.

At the time I built that jig, there was pushback against anything to do with angles. The "class" started with rules on a single sheet of paper, but there were lots of holes in them- big holes. There have been some different approaches to defining what should and shouldn't be open in an i550, from some advocating basically "if it looks like an i550 it's good enough", to those that wanted a clear set of rules so that the boundaries were nice and clear. What we have is the result of those discussions- mostly clear, but still some resistance to measurement procedures that are too cumbersome. When groups start sailing with each other, we'll have to look again at these and see what the stakeholders want.