Recutting sailboard sails

Nic, I actually asked the same / similar question in the thread.... www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=37622&SearchTerms=luff,curve.
It was taken from previous sails that have worked ok.

Now I know that that didn't answer your question completely so I will tell you how I did the luff curve on the "Sandpiper" yacht that I designed the sail for, many years ago.
The rig was set up on the yacht (no sails on) and a rope was tied to the top of the mast to the boom and sheeted in to about the fully sheeted in position.
A string line was then used between the top of the mast to the gooseneck point taking measurements at equal intervals along the string line.
This was with a 2 piece aluminium mast, now im not yet convinced that all fibreglass masts have the same flex, I have acess to 4 f/g masts from various craft and they all seem just a bit different.

Here is a link to "How to make a sailboard sail" worth having a look at.
http://www.sailrepair.co.uk/makingsail.htm

Here is some of my thoughts on landaycht sail design.....(now im happy to concede that i might be wrong but it makes sense to me)
The sail is the "engine and gearbox" for yachts and as landyachts are different from water yachts because of the minium and maxium wind speeds they need to sail in.... the rig needs to be different.
By using a rig with a fixed draft in the sail (ie. sailboard sail with wishbone boom) would be like using just one gear in your car.
My belief is that a sail that changes shape while sailing is the way to go (like a car auto transmission)
You need a sail with plenty of draft at slow speeds (low gear)and a sail with very little draft at high speed (top gear) and this needs to change while you are sailing!!!
The things that are easy to change while sailing are... sheeting tension, downhaul and mast bend.
If you have a sail cut to suit the bend of the mast when fully sheeted the sail will be very flat and when the pressure is released the mast will straighten and give draft to the sail.
An ultra flat sail will have some shape put in it by default due to the wind acting on it.
Hiko i hope this helps with your question?

The general principle with fore and aft sails AND square sails if employed that way, is that the wind energy is harnessed by changing the direction of the air flow rather than capturing the air (as in direct down wind sailing).

You guys know all this anyway but there may be some reading who are new to the game and might gain a better understanding of the principles involved.

I had the good fortune a few years ago to be part of the crew aboard the WA built "Endeavour Replica" and to helm her across the Whitsunday Passage. The sailing leg on that occassion is what I think is described as a "shy reach". If not that, it WAS a "beam reach".

The wind was from the south blowing up the passage and our destination that day was to the east not the north. The yards were hauled around as close to fore and aft as possible and the windward sheets were hauled down rather than back, creating a leading or cutting edge out of the windward edge of the sails.

This leading edge is equivalent to the mast on our land yachts.

My task on that day as helmsman was to steer the vessel as close into the wind as possible without the leading edge folding back on itself. That is called luffing and it is no luffing matter when you are trying to go somewhere.

I watched the GPS speed read while doing this and that big old boat was touching 9 knots. Certainly no slouch for something that was designed over 200 years ago.

So what was making her hoot along like that was changing the direction of the wind from striking the vessel at 90 deg to the direction of travel of the vessel, to as near as possible to the opposite direction of travel of the vessel.

I seem to remember something from high school Physics Classes about that. For every action there is an equal and opposite reaction.

How does this relate to mast curvature (luff), and sail draught (belly or lack of in the sail) with a land yacht??

We already know that hauling on the sheet and/or downhaul bends the mast and thereby pulls the belly out of the sail due to the curved cut of the sail.

In lighter airs, to extract maximum horse power from the wind, we need to change the direction of the air flow more drastically and to do this we have a more drastic curve in the belly of the sail. Vectors, resultants etc. As we gain speed the apparent wind speed increases as the apparent wind direction moves forward. Therefore we then need less curvature in the sail shape to maintain or increase speed.

There is another factor that comes into play as well. That is called "lift".

On the windward side of the sail we gain power by changing the direction of the air flow. As the sail moves through the air, on the leeward side of the sail, just behind the point of greatest curvature, low air pressure occurs which tends to suck the sail forward. This is "lift".

In summary:-

Notice how when you start sailing, you apply a moderate amount of sheeting and you experience a surge of accelleration, but if you leave the sheeting at that the yacht then slows. The sail has stalled. Increase the sheeting and speed builds. Why??

Then when you have been having a fat old time sailing around and you feel like a break for a drink or whatever, which you have, and then it seems like the wind has dropped because it takes so long to get back to the speeds you were doing just a couple of minutes ago. Why??

This is the exciting dynamic that sets land/ice sailing apart from any other sailing except for the fastest catarmarans and trimarans.

I am open to any corrections on what I have said here but this is my understanding of it. Cheers Cisco.

I guess you are right I dont have a lot of experience with unstayed masts
My first yacht weighed 20 tons and had a mast like a tree trunk fully stayed with
9mm rigging did a lot of miles with that one Since then they have got smaller but
my class 5 is the first unstayed rig I have used
I would guess Finn sailors would have some answers about unstayed mast tuning
Seems to me though that the stiffness of the mast would also effect sail twist
as we are not sheeted in hard all the time and we only have control in a fore and aft direction?

Good on you Paul I needed a reality check

Hey!!! That does look good

Thanks, that's a really old photo from the first time I took it out and the boom is lower now. I will move the front pulley forward though, thanks for the tip!

When you're landyachting its a good idea to smack yourself around the head regularly otherwise you might come to your senses and realise how rediculously dangerous it is to skid along on your bum at 100kph with no brakes!

Someone will probably give you some lame technical answer soon, but this is the real answer...

Aaarrrgh, look there is no sense taking any of this too seriously cause none of us is going to get out of this thing called LIFE alive anyway.

Thanks for the reply Nebbian. I stand corrected...The kites don't look that big.

However, I thought they'd decided it was all a bit of a myth about closing the slot etc. Just because some or even a lot of people do something, doesn't always mean it is true or best practice.

Landyachts post was (most likely) written with tongue firmly in cheek, however, people do a lot of things to look cool, which in reality don't improve performance enough to warrant the risk. Personally I'd rather the boom was that 200mm higher and cop a slightly higher centre of effort, rather than a whack in the head...not to mention feeling a little more comfortable about allowing someone inexperienced to have a go. The blockart sailors in the Videos/photos are fairly well "sat up" and the booms are reasonably high to clear their heads with what seems to be minimal disruption to their stability. Of course I haven't sailed one and am purely basing my opinion on some video and still photographs. As I see it from the replies so far it comes down to personal preference...which isn't a bad thing.
Nick