I'm trying to determine how long of spinnaker sheet I need. Help is appreciated. (c.f. attached drawing).

Distances measured:
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a) active clew to turning block = 8'
b) turning block to ratchet = 10'
c) ratchet to aft windward corner = 18' (14' + extra 4')
d) windward corner to lazy ratchet = 12'
e) lazy ratchet to lazy turn = 10'
f) lazy turn to forestay = 17'
g) forestay to clew (lazy) = 15'

spinnaker foot = 19'
pole length = 16'

sections c & d are the loop sections between the two ratchet blocks [purple].
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My question is, with a single sheet system like the one that I have drawn, do I need extra line in the loop section (c & d) besides what I have measured? If I used the line lengths measured and shown here, will there be enough sheet in the system to adequately ease the sail. As drawn, this sheet would measure 90'.

I can hypothesize a situation where the 19' foot of the sail extends straight perpendicular from the spinnaker pole tip to leeward. This would lengthen the span of section "a)" from 8' to 30' and would lengthen section "g)" from 15' to 36'. In this situation the continuous sheet would have to be 133' long.

If the spinnaker blew straight forward section "a)" would have to be 45' long and section "f) + g)" would also have to be 45 feet long. This would make a total spinnaker sheet length of 142' long.

In the last two cases the amount of unused sheet in the "loop" [sections c+d] would be pretty large: 30' in the shortest scenario, 73' in the second scenario, and 82' in the last scenario.

What length do I really need???

Whew, I thought I overexamined things! ;-)

You're talking about some 23' foot monster there - but for the I20 / F18 here is how I go about it. Of note, I have an idler block in the center/rear of the trampoline to help take up slack when the spinnaker is snuffed. I typically mount this on a buji going through a grommet in the trampoline so if, for some reason, we need more sheet in a hurry, the bunji will give way and give us a little more.

That said, with the spinnaker hoisted I shoot for enough spinnaker sheet for the crew to be trapezed at about the rear beam and able to sheet the spinnaker as it just barely starts to pull on the aforementioned idler bunji. We usually start with the spin sheet a little on the long side and slowly snip away a foot or two after test sailing until it is right. That has worked out really well for us.

I haven't looked thru your calcs carefully. A couple of things you may want to consider:

A friend of mine buys the cheapest line he can find and rigs up on the beach to determine the line length he needs, then goes and buys the right line.

You might want to consider that an ARC22/Supercat22 is about the same dimensions you have and could check your calcs based on what they use.

I would start with about what you calculated, add the idler block that Jake suggests, and see how it fits. If it too short, it is not as big a deal as it used to be with low-tech line, and a little effort. Hi-tech line can actually be made longer by cutting the core and pulling it out the ends. For the N6.0 I bought 60’ for the halyard, cut the core (Not necessarily in the middle), pulled the core out both ends, effectively creating a taper and about 83’ of halyard.

With the size of the spin you will be running, you will want the ability to double the active sheet, creating a 2:1 purchase. It is very easy and not that expensive to setup. This 2:1 part of the line and any part that is in blocks during active sheeting could be just core or tapered line.

take it from me, 2 spin sheets a little short from cutting too much off at once. Cut no more than a foot at a time until it gets to the right length. Have the spin hoisted and try the lenght from the rear beam trapped out. Try it with the spin snuffed to make sure it will completely snuff. And if you cut it too short, you can always add a tail to the clew of the spin and tie your sheet onto that.
Your graph looks great, but I would only trust the on the boat model.
Someone on here has a sayng at the bottom of their posts that sums it all up for me.
"I cut it twice and its still too short"

Just what I heard at regatta a while back though, Dave, was that it has always been too short for you.......

Do you HAVE to go via the back beam (A->B->C) ?



You can save a lot of sheet by just going A->C and you sill still get a similar amount of turn on all the block. And you will save a lot of clutter on the tramp.

Unfortunately, yes, I have to go back to B. The spinnaker clew comes back to the shrouds (A)at which point it is a couple feet/foot and 1/2 above the trampoline. The only place I can attach the turning block is on the rear beam.

How about moving C to the windward sidestay as in this picture of Tilted. I can see where the pull from the sheet is more forward than perhaps you would like.
Tilted

Also noteable is that if this is a refit of an existing boat - sheeting the spinnaker to the rear beam puts a good bit of direct stress to whatever attaches the beam to the hull. I know of two Nacra 6.0's that broke beam bolts due to the additional loads induced by sheeting the spinnaker to the rear beam. You are much better off if you can figure out how to attach the hardware to the deck - somewhere around the shroud is a great place. The sailmaker can also tell you at what angles the spin was designed to be sheeted (you probably already know all of this - if so, I appologize).