Thread: GIS spars

Hello,

It is all about finding the optimum sail - spars combination.

I have the standard Duckworks sail, and although it sets okay on the standard boom and yard it makes for a very flat cut (especially low down in the sail where you want some power) due to the boom bending much when you apply any downhaul tension.

I glued up a 50 mm square blank from 2 pieces or decent quality Oregon pine measuring 50mm x 25 mm last Saturday to make a new boom. It will be a round one (I just like the looks of my exisiting standard round spars), not sure yet about whether to follow MIK's plans for the standard boom + 10 mm on every measurement or to adjust the taper on the front end of the boom by only tapering the front 400 mm or so and to leave it parallel sided until the taper starts on the back end of the boom as on MIK's drawings (this would possibly avoid the downhaul tension bending the boom and flattening the sail).

After trying out this new boom with the old existing yard, and if the new boom works, I might try the old boom rigged as the yard (it has the same 40 mm maximum diameter, but not tapered as much. Otherwise I might consider making a new one with a 45 mm diameter (50 mm is overkill I think, at least for my sail).

I hope the above makes sense. Ralph (Watermaat) has a different sail (McNamara sails) and different spars, so over the Dutch summer we will probably find some good combinations.

Best regards,

Joost

Originally Posted by BradLH

I've read this thread with great interest and would like to incorporate your ideas into the PDR I'm building.

What I've gathered so far:
Outhaul - Robust and cockpit adjustable, 4:1 or 6:1?

Thanks,
Brad

Howdy,

If the outhaul only controls the in and out and the clew cannot rise relative to the boom as the outhaul is eased you will get away with probably a 3 or 4 to 1. However a bit more can make it easier to adjust when you just want to do it quick.

I always set up the outhaul so that if it is completely released the outhaull comes to stop with the sail at the maximum reaching depth position.

About a 1 in 7 draft in the bottom of the sail. Normal upwind is 1 in 10 for smooth water or 1 in 8.5 if the water is choppy.

This will work very well when reaching with all the sail tufts flying or close to (ie sail not stalled. Should be flattened out when reaching and the tufts stall - ie broad reaching and running.

Best wishes
MIK

About a 1 in 7 draft in the bottom of the sail. Normal upwind is 1 in 10 for smooth water or 1 in 8.5 if the water is choppy.

Mik,

You lost me here. Does 1 in 10 mean the depth of the draft should be 10% of the foot length?

Also does more tension on the cunningham move the draft forward on a balanced lug sail like it does on a conventional sail or does it just flatten the sail.

Thanks,
Brad

Hi Brad, yes that's correct 10% of the foot length.

The downhaul which pulls down on the boom tensions all the luff and pulls hard down on the yard and hence the halyard, bending the yard the more the downhaul is pulled.

So the harder the downhaul is pulled the more the yard bends and the more the head flattens to depower the rig. You need a fair amount of downhaul even in light winds to set the sail, Little downhaul will allow the yard to twist off very easily and the boat will not point well.

Hope that makes some sense.

Also, for you guys looking into spar bending, wooden and alloy, there two free free softwares you can put the numbers into to calculate the strakes for round masts and another to calculate mast size and strength for stayed and unstayed rigs based on sail area and wood types.

It's at the bottom of this page, under the title "Shareware for Boat Design"

Carlson Design Plotter/Cutters

Brian

Brian,

Thanks for the reply. I'm starting to get an idea of how the sail works. Yard too stiff, sail won't depower. Yard not stiff enough, sail flattens before getting sufficient luff and leech tension. Sail, yard, boom, outhaul, and downhaul all must work well together for maximum performance.

I'm going to take a look at the boat design shareware you mentioned.

Next question if you don't mind, interpreting tell tales.

First where do you put them? I'm guessing two on the luff and one about 3/4 of the distance up on the leech.

I assume you stream the leech tell tale while reaching but do you let it wrap around the lee side when close hauled?

Also on the luff tell tales, do you stream them always or do you let the windward tuft stall a bit when close hauled?

Thanks,
Brad

Brad, yes that's the general idea. A sail working with the spars across various wind speeds and angles as a unit.

Here's a recent picture of my Scow with tell tails on the sail.



One tell tail half way up the luff. Yes, I agree that sailing upwind as high a spossible the windward telltale should be lifting, leeward streaming. To me, both streaming is sailing just too low. I had a sailing instructor, a pal crewing for me, complaining I was sailing my Laser Stratos too high when sailing like this. We were sailing about 10 gedrees higher than the Wayfarers we were racing. Yes, I was going a bit slower, but sailing so much higher meant we easily beat them to the mark. My pal just nodded.

Another tell tale half way up the yard.

A third tell tale high up the yard. Adjusting my kicker/vang makes it fly or stall.

Three streamers on the leach.

Making all that lot fly is almost impossible and will drive you crazy trying!

However, I did wonder for some time why the leach streamers always seemed to be stalled. I learnt I had too much leach tension and the sail was stalling. Nice to see them flying some times now.

Interestingly when I was sailing much better some years back, I never looked at the sail even, never mind the telltales. All I had to do was adjust the sail and my weight position till the helm was neutral and the boat was fast and would sail in a groove away from other Scows.

I only find this groove now and again in my current boat. Something to do with the helm always carrying some load so the neutral position is hard to find.

Brian

Well, I hung 34 pounds off a 50x50mm round Goat yard (N. White Spruce) and got 2" of deflection from a straight line down to top of yard (so 3" to center of yard). The boom is much stiffer at 55mmx49mm ovalized section for a loose footed sail.

Stuart's recommendation was to hang a weight 1/3 the sail area from the halyard attachment point, which is what I did.

I'll need to measure for the standard weight and measure on the spreadsheet. I also have a second yard/boom set to measure.

--Clint

I'm building a GIS birdsmouth mast. I need to add length to the component pieces, an additional 570mm to each one. I thought I'd put 4x 8:1 scarphs on alternating pieces, low and high. Do you think this would work??
Thanks

It will be absolutely fine.

Generally you can assume the strength of such scarfs is the same as the original timber. But even I would reverse them as you suggest. It certainly is harmless to do so.

Read the plan appendix about gluing end grain too.

Best wishes
Michael

Brian, you don't find that the mast disturbs the behavior of your lower two forward telltales?

Grant

Originally Posted by keyhavenpotter

Here's a recent picture of my Scow with tell tails on the sail.