Thread: GIS spars

Originally Posted by Boatmik

Found the slide out menu! Good work.

I don't know how possible it is to do but under each spar - if that column could be subdivided into two with one as MaxDiam and the other column with Deflection.

Should have that number of diameter as it is the most important variable.
Also a weight would give some indication of density.

MIK

I'll have a go. I'm crap with spreadsheets though

Thanks to BobWes for all his help with the mods.

Here is the new link that should directly open the spar data file.

By the way, this site can be used to store all sorts of other documents, so if you think of anything let us know. I think it's handy to have a document repository like this on the forum...

Link: http://sheet.zoho.com/open.do?docid=703747000000007001

Fellow spar gurus:
I'm building the yard using cypress because I stumbled on a nice piece of it. Cypress is a bit lighter and weaker than douglas fir. I increased all cross section dimensions of the yard by 2 mm to compensate. With the corners aggressively rounded the yard weighs 2.5kg and deflects 43mm with a 10kg load at midpoint. The question is do I leave it as is or continue rounding to get a lighter more flexible fully round spar? I'm a bit concerned it will get too flexible once fully round.

Originally Posted by SimonLew

Fellow spar gurus:
I'm building the yard using cypress because I stumbled on a nice piece of it. Cypress is a bit lighter and weaker than douglas fir. I increased all cross section dimensions of the yard by 2 mm to compensate. With the corners aggressively rounded the yard weighs 2.5kg and deflects 43mm with a 10kg load at midpoint. The question is do I leave it as is or continue rounding to get a lighter more flexible fully round spar? I'm a bit concerned it will get too flexible once fully round.

Hi Simon, your spar seems to be in the same range as mine at the moment. Rounding it will of course only make it flex more. I think the general advice would be to wait until you have your sail and can go sailing before making further adjustments. Maybe just a single coat of spar varnish to seal it in the meantime.

By the way, please feel free to add your data to the site. If it asks for a login, the username is gisspardata and the password is goat.

Thanks Bruce, I will definitely add to the spar database. Since the spar bend characteristics determine the sail cut I'm trying to incorporate the experience based wisdom of this forum into the best spars I can make.

BTW, Stuart at Dabbler sails will most probably be the sail maker. He uses 3 measurements along the spar's length to define the bend characteristics. Makes sense since a tapered spar does not bend the same amount on either end. Sounds like he gets pretty sophisticated with the sail cut to match the spar. Dabbler's procedure for measuring spar bend is here.

Stuart is big on a luff and head rope and vertically cut panels, different than others who have a taped, reinforced luff and head and cross cut sail, like Michael Mcnamera in the UK. I think it'd be nice to have two sails, a more traditional sail like Stuart likes, and a sporty sail like Mcnamera's.

Stuart has built several GIS sails and I believe (will ask to verify) that they are modern cross cut construction. That is certainly what I want. Is there any functional difference between a rope and tape luff?

Below is a photo of Middle Ages Man's Dabbler GIS sail (hope you don't mind me linking it MAM) It's a good looking unit.

THAT one is cross cut...I think MAM urged him to do it that way.

The other sporty way is with leech battens. THis all starts making the sail more expensive.

The one place I'm not economizing is the sail. I love nice sails

Originally Posted by SimonLew

Stuart has built several GIS sails and I believe (will ask to verify) that they are modern cross cut construction. That is certainly what I want. Is there any functional difference between a rope and tape luff?

Below is a photo of Middle Ages Man's Dabbler GIS sail (hope you don't mind me linking it MAM) It's a good looking unit.

I would follow the sailmaker's preferences. The sailmaker here uses luff groove tape designed for grooved headsail foils.

But I won't be betting that it is any better than other methods.

MIK

There may have been a discussion about vertical vs. horizontal panels (my memory is foggy on the point) but Stuart didn't try to talk me out of the horizontal arrangement. I recall telling him I wanted a sail just like MIK drew. He preferred the roped edges and a loose foot. He got his way with the former and MIK's laced foot won the day for the latter.

Interestingly, Dabbler's sailmaker says that the boom does not need to be beefed up for a loose footed sail.

"Yard bend is accounted for in the design of the sail, once the offsets per my instructions are known. Booms for loosefooted sails don't need to be beefed up -- they function only in compression. It is booms to which the foot is attached that need to be stiff."

Seems to be a different take.

If sheeted at or near the end a heavier boom would not accomplish anything but if you're going to sheet it off per the plans I think it would definitely need to be stronger. With a loose foot the sail will not be there resisting boom bend.

LOL, the old: is the boom in pure compression or compression and bending. I am in the camp that says for a boom with a typical goose neck and the main sheet attached at the very end, the boom is purely in compression. Loose footed or not it does not make much difference. What convinced me of this is the fact that reef grommets, other than tack and clew, have very little reinforcing so the sail must be exerting a tiny force on the boom along the foot or the grommets would rip out. All of the loads are resolved into compression through the outhaul. As such the boom need only be strong enough to prevent buckling.

Now, I'm theorizing that all other setups with vangs or mid boom sheeting or balanced lug with down haul like the Goat will impart significant bending loads on the boom. Once again I don't think loose foot or laced makes much difference. The outhaul transfers sail loads into the boom putting it in compression. The other control lines impart the bending.

If the above theorizing is true a stiff boom is advantageous on the Goat. Mik care to enlighten or blast my theory to smitherines?