Thread: GIS spars

Originally Posted by Watermaat

Hi Clint,
I am considering to make a square hollow boom. I have ordered a loose-footed sail, so boom-stiffness is very critical for me. But also in general I like to have a boom as stiff as possible and use a flexible yard only.
I already made a square boom with rounded corners about 65x35 max in the center back to 50x35 in the front and 40x35 in the back end.......but I do not like the weight.
It's not a real issue ( will be around 4kg ) but it could be made better.

I have found a source of very nice 69x12 planks ( again cheap European Fir ) with 360 length. Two of these with 18mm infill and a spacers can make a very stiff and lighter boom. I am thinking to keep the top line straight and make a tapered bottum line with the max height somewere around 40% from the front of the boom. Sizes 42 x 69 max ( or slightly lower ) tapered towards 42x 40/50-ish in the ends.

Not the highest priority project yet, since I need to finish my hull first.

NB I have copied this into Joost interesting new Thread about GIS Spars !

Hi Joost, good initiative!
I have taken this answer from another thread to get these very interesting discussions together.

Not knowing the actual spec's of the GIS, I still see some fundamental logic flaws that should be addressed.

- A stiffer yard will give more shape to the sail (more depth)

This is incorrect, a stiffer spar of any type (mast yard, boom, sprit, club, etc.) will not offer as much "belly' to the sail as a lighter or more flexible piece.

- A stiffer boom will prevent the depth to be pulled out of the sail when applying downhaul tension which pulls a bend in the boom

This is a similar situation as the previous statement and though technically more difficult to understand, the same results can be assumed. In a free standing rig, you attempt to design a specific amount of "unloading" ability into the spars. This assumes the spars can accept the loads imposed from the boat's displacement against the wind pressure.

GIS spars seems to be very different from other designers like Viviers or Oughtred whose rigs have far fatter spars (but then again, their hulls are a lot heavier as well).

This is the classic novice designer mistake of attempting to compare similar size craft. There are so many variables that direct comparisons are usually imposable. Displacement is key in most the formulas used to calculate sectional stiffness, required modulus of elasticity, etc., change any of the variables, especially the displacement and your calculations get tossed right out the window, with the resulting spars (calculated to the new set of spec's) being considerably different from the previous set.

From my observations of GIS underway in various wind strengths, I think she's spared fairly well, with a reasonable reserve. The spars could be lighter, possibly by a substantial amount, but there are risks with this decision, typically involving splinters.

I suppose my point is, making guesses at the values used to determine the spar construction dimensions, employed on the GIS seem self defeating, without considerable understanding of the loads imposed. The alternative is a very costly trial and error sort of thing, with a fair number of busted up rigs to prove you needed just a wee bit more stave thickness on your latest version of a GIS rig.

Hello PAR,

Please let me know what kind of specs you need (if I am able to deliver them at least).

What happens to my current yard and boom is that they flex a lot when downhaul tension is applied and that seems to flatten the sail.

There seems to be some broadseaming in my sail, but most of the belly is to come from rounded edge curves along the head and the foot of the sail. As stated before, the spars seem to take the same shape as the cut curves of the head and foot, thus taking away the depth in the sail making it flattish which, as far as I have always understood, shouldn't be happening as much when the spars are stiffer as then the spars are not able take the same bend as the curved edges of the sail forcing the belly into the sail.

But I could be completely in the wrong here and by the sounds of it, you are of the opinion that it would be better to keep the current spars, but have a sail made with more depth in it.

Anyway, this lug sail business is completely new to me (very much used to bermudan sails and gaff sails), so all advise is appreciated to make the sail I have work as properly as possible (although I believe I did make reference to displacement, albeit perhaps a bit unclear by having written "weight" ).

Attached pic was made during the maiden voyage and might explain it better.



Best regards,

Joost

It can be costly ... but it can be fun too! The cool thing is there are three boats in the same geographic area - so the costs are "thirded".

The other way of course is to get the sails cut differently too - this is a further variable.

One thing that I was talking about with Duckworks was offering a second sail design which was fuller in the head even with the standard spar. But I would need an actual depth measurement using the Duckworks sail to know how much to suggest changing the depth.

Same problem with the spars really - the sailmaker does the best job on the available data, but may end up with something a bit different from expected.

If someone has the Duckworks sail and the chance to do it they could lie the rigged GIS on its side, set up the downhaul and mainsheet so the loads on the sail were close to moderate wind upwind and put a straight edge across the sail to find the depth compared to the chord.

This needs to be taken at two places across the sail parallel to the waterline (very approximately).
At the "throat" of the sail - ie forward bottom corner of the sprit
Halfway along the sprit.

This might also allow me to make a rough calc about how much stiffer the spar needs to be.

The other way to play with the stiffness is to put some unidirectional glass along the top and bottom "faces" of the yard.

MIK

I just added an explanation in the first post.

I don't know what information is provided the sail maker for the construction of these sails. Some designer's like to control most element of the sails, others let the sail maker us their judgment as to what will work best. I've never seen a sail maker get it right, unless they took their work, bent it on and sailed in a variety of wind strengths. The later sails are always better then the first few as a result. Unfortunately, most sail makers don't get a chance to try out their efforts, particularly on this type of craft.

I can tell you this, a good sail maker, if given the opportunity, can dramatically improve the sail set, once they have a chance to "live with" a set of sails. By this I mean they need to see and feel what's happening with them on the boat. I have a racer I designed several years ago, that did fairly well, but came to life after the sail maker spent several hours across several days aboard the boat. He was able to make corrections, take notes and truly improve the set and performance of the boat. You come to expect this with custom sails, but not so much from "manufactured" sails.

Hi Guys,

I've been thinking of working up a diagram for my website and for teaching so now might be a good time to provide some visuals. This may help.

The diagram is purely conceptual. It shows what happens when forces are applied in a rig from the standing and running rigging and from the wind. Feel free to comment to make it better.

What I get from it is that the spar bend -- which is what we want! -- needs to be such that the draft in the sail is not reduced too much in light to moderate winds which would depower the sail sooner than we want. In moderate to high wind, we want the spar to bend enough that the draft aloft is really 'eaten up', flattening the sail, and therefore depowering it. As the highest tip of the yard bends downward, the leech opens as well, which spills some air, depowering the sail. We want this to happen as wind increases or gusts. This is why MIK's boats can be pushed into higher wind speeds whereas others may be reefing sooner.

It might be good to revisit this video and about 45 seconds in the camera shows the yard working. It looks like a solid 15-20kt wind here with two guys hiking out. You can see the yard bend, and the sail flattened along it, and the leech opened up so as to spill the air. Pretty damn cool.

[ame="http://www.youtube.com/watch?v=fseMm6Ddbho"]YouTube- Broadcast Yourself.[/ame]

The boom bend should be negligible, is that correct, or less than yard? If the boat is sailed hard enough or the wind is strong enough, the boom bend would add draft into the sail, hurting upwind performance and not helping to depower the sail, so we heel more and swear more. The sail is laced to the boom, and the mainsheet force is spread out more. (Please check my logic with boom bend...)

So the sailmaker rounds the head of the sail and the foot of the sail according to spar bend. The PDR sail has 67mm of round max, 2 3/4". If the spar bends that much, the draft is reduced by that amount or close to it (depending on how much broadseaming was done).

Hello All,

Thank you for your input. I guess I have to drive to the garage once it stops raining here to pull out the rig to make some measurements. Results are based on standard mast, standard boom and yard and the Duckworks sail. Will probably rig the sail on a couple of sawhorses, but that should not make much difference for the measurements.

Hi Clint,

I agree with everything in your post save for the bit concerning the boom where you state the following;

"the boom bend would add draft into the sail"

Currently I am still of the opinion that any bend in the boom will have the same effect as bend in the yard: both will flatten the sail. I am willing to invest in a second set of spars for testing. With 2 booms and 2 yards available and with Ralph's stiffer spars for comparison as well, we should be able to find a good combination and get more insight in the rig.

Dear Par,

Reading all feedback and based my own thinking, could you please explain the technical background regarding my fundamental logic flaws:

"This is incorrect, a stiffer spar of any type (mast yard, boom, sprit, club, etc.) will not offer as much "belly' to the sail as a lighter or more flexible piece."

Thank you all. This is an interesting discussion providing me (and perhaps others) more insight in the fundamentals of the balanced lug rig.

Best wishes,

Joost

Hi Clint,

Your diagram seems accurate, only thing is that you say a bending boom should increase draft?. I think a bending boom decreases depth of the sail in the bottum part.

To add on this, from my point of view:
A bending yard, similar to the mast-top in a bermuda rig, flattens the top part of the sail and opens the leach. Both resulting in reduced heeling in a gust.
A bending boom flattens the bottum part of the sail ( more specific: the depth of the sail ) resulting in less drive-power . This makes sense if you have too much sail for certain conditions.
You need to realize that the boom and yard work together, if they have similar flexibility they will have a similar effect on the sail. If one is more rigid, in a given situation the other one will have to bend more.
In increasing wind I would like to have the effect of the yard ( flattening the top and opening the leach ) before the effect of the boom ( reducing depth and power ).
So it is better to have a relatively stiff boom and flexible yard.
My choice is even more extreme, I ordered a loose-footed sail to be sailed with an almost rigid boom. Than every flex will be in the yard ( flattening the top, opening the leach ) and no effect on the depth of the sail.
NB this depth can still be controlled by more or less tension on the leach. If I have too much overall power, I probably have to put a reef earlier than with the original flexible boom and laced sail. It will be interesting to see what will have better performance; an almost flat sail with flexible boom or a reduced sail with sufficient depth on a rigid boom.

Hi all,

some more data from me.


Suspended at the ends and weighted in the middle, my yard (Thickness to MIK's specs, but eightsided, not rounded completly to compensate the slightly lower specific weight of the used hemlock fir) bends almost linear 4 cm per 6,25 kg, so 16 cm with 25 kg.

The boom is a bit stiffer and bends ca. 3,5 cm per 6,25 kg and 13,5 cm in total with 25 kg, and the bending curve is far more asymetric than the curve of the yard.

I've done pics for the sailmaker (Michael McNamara) and would like to submit them here, but have still no idea how to insert them. Cut 'n' paste don't work.


Greetings - Jörn

Hi Joern

there is a little "attachement" symbol right next to the "smilies" symbol on top of the message window. Click on that and another window opens where you can upload your pictures.

Nice to hear you also will order from Michael McNamara. I have received his sail,did not had a cance to try it yet, but it looks very good overall.

Ralph

I was a little uncertain about the boom, so I am glad to have that questioned. In the Goat, with the sail's foot laced to the boom, and with the mainsheet pulling down from three points on the aft part of the boom and the downhaul forward, wouldn't that make the middle of the sail flex upward, pushing sail material up, making the sail baggier? Because of the three mainsheet points, I would think any boom flex would be minor. So which way is it flexing? I think the ends are coming downward slightly when the mainsheet is tightened up. It'd be great to have McNamara's thoughts on all this as the sailmaker is pretty key once the bend is determined.

Howdy,

The question about a spar increasing or reducing the depth of the sail is a good one.

If a sail is loose footed bend will only bring the tack and clew (the two corners) closer together making the sail fuller. (Are the words tack and clew used in Dutch too? The Dutch were the masters of windward sailing in the region for centuries).

So for a loose footed sail the boom needs to be quite stiff. I have been misled a bit by a couple of sailmaker friends. They thought that if the boom is laced or loose footed, it will make no difference to the spar bend. But it does make quite a substantial difference.

however if the sail is laced to the boom - Initially when the spar bends it will get to the point where it matches the round in the edge of the sail. Then the sail is probably close to its flattest. If it goes further than that the sail will start getting fuller again as the tack and clew get closer, particularly in the lower third of the sail - even though the edge of the sail will look flat.

Best wishes
Michael

Originally Posted by Compass Project

I was a little uncertain about the boom, so I am glad to have that questioned. In the Goat, with the sail's foot laced to the boom, and with the mainsheet pulling down from three points on the aft part of the boom and the downhaul forward, wouldn't that make the middle of the sail flex upward, pushing sail material up, making the sail baggier? Because of the three mainsheet points, I would think any boom flex would be minor. So which way is it flexing? I think the ends are coming downward slightly when the mainsheet is tightened up. It'd be great to have McNamara's thoughts on all this as the sailmaker is pretty key once the bend is determined.

Hi Cint

You need to realize that the main force on the boom is the pully that tightens the front leach and pulls the boom down where it crosses the mast. I have seen this on Joost's goat; if you have sufficient tension on this leach there is already a curve in both boom and yard without wind and without sheet tension. So the initial bend of the boom is downward ( reducing depth of the sail ). As the wind increases the leach tension increases and therefore this bend will become stronger having this pully as a fix point. The nice spread of the mainsheet reduces this effect, but certainly does not eliminate this.
NB that's why you basically need to have the max diameter of the boom closer to the front part; the main force down is close to one end.

Joost, am I right in this perspective?