GIS Yawl

**Boatmik** · 4th August 2012, 03:22 AM

Originally Posted by paulie

And I was trying so hard to be obnoxious! I guess I'll have to try harder next time.

As for "how much twist is too much", I think it depends on what you are looking to do. That was the point I was trying to make in my post.

Every time I look at those pictures, I'm struck by how far out Clint is hiking. Based on his height, I'm guessing he must weigh over 200lbs. And he's way out over the rail. Yet BLEAT is heeling, with the lee chine dug in.

Where is all that lateral force coming from? Don't say crew placement. Ellie's skinny cousin couldn't tip the boat like that no matter where she sat.

I'm convinced that lateral force is from having the lower part of the mainsail oversheeted.

With the twist in the sail, the upper part is set properly to drive the boat. The lower part is stalled, at least a bit, exerting more lateral force than driving force.

Is this "right"? It depends on what you want. I think you hit the nail on the head, that if you used more downhaul and sheeted out you'd be off like a rocket. But that lateral force is acting as a stabilizer, both directly and, by allowing you to distribute your mass outboard, thereby lowering the resonant frequency of the boat, indirectly. So you'd have a lot less stability. Clint and crew could probably distribute themselves so that BLEAT stayed upright. You, being alone and so light, maybe not.

Keeping the boat up on the chine also allowed your boats to cut through the chop more cleanly. Maybe not an issue on this tack, but it could make a difference going upwind. We saw that on the homeward leg the first day, when a little heel smoothed the ride.

I think the idea is to adjust the twist to get the proper balance of speed, stability, and heel for the conditions. And sometimes you have to sacrifice a little speed for safety.

Howdy.

Not sure about this reasoning. I don't think there is enough evidence to say the bottom of the sail is oversheeted and the top not. Generally oversheeting is very very bad for speed. You lose your lift coefficient if the sail is stalled in any section and produce significantly less force in any direction Additionally you don't get so much force from the front of the sail as the llow has separated at the luff and trails off in a downwind direction rather than following the sail curvature.

The force a sail develops is in part proportional to the curvature the wind has to make in each area. So when the wind is adapting from the true wind angle to match the angle of the sail is when you get the most force. and that is what happens at the sail front.

(in reality the circulation around the sail makes that angle even larger than you would think because of upwash in front of the sail. Then as the wind comes back along the sail the angles are much smaller ... really just following the sail surface)

So if the bottom of Clint's sail was stalled ... he couldn't keep up with you.

The thing to realise with the lug rig is that while the downhaul provides a reasonable amount of sail twist control ... it is by no means optimum. Our sails are always too twisted. But I assumed that when designing the goat. My thinking is that the 100 sq ft sail of the Australian NS14 is nice for two light to medium weight adults. The Goat's lug would be more twisted and provide less heeling force and so I would go up to 105sq ft ... also that the single sail would be more efficient than the two sails of the NS14 rig but would lose again because there were so many things optimised in the NS14 rig which is incredibly adjustable.

Other things that might make a difference are sail fullness (particularly at the top), Clint has some extra area with battens, Christophe's Duckworks sail has little roach on foot and none on the leach. The bend of Clint's boom making the bottom a bit fuller.

On a reach like this the boom angle is in line with the camera ... it is not on the transom corner there is another nice trick ... as so much of the force is forward.

The top can be trimmed properly which means the bottom has the tendency to be oversheeted. But on a reach the sail can have more depth ... so make the bottom very full which widens the angle of attack of the luff at the bottom of the sail but the greater sail curvature gives you more power.

A general number for foot depth in reaching conditions is that you can go to a 1 in 7 camber which is a monstrous bag.

Look at the photo again. The camera can see the lee side of the sail up near the yard but not much down near the boom. Turn the bottom of the sail into a big bag and you would see much more of the lee side of the mainsail down low. They say these types of camber give 20 to 40 percent more power.

Measure it out ... it is quite scary! So Clint could still get a lot more power.

From a rigging point of view ... I always set up the outhaul so when it is completely released the sail will go directly to 1 in 7.

MIK

**Boatmik** · 4th August 2012, 03:35 AM

With the "resonant frequency" argument ...

Upwind you can sail stupidly narrow boats quite easily. Like the 12" wide Moths before they started to be fitted with foils. Or BETH to some extent or canoes of various types.

The pressure on the sail is very good at damping roll. And the only time you get a surprising roll is if the wind suddenly changes direction so that it is more head on (headed) and you suddenly lose power from the sail. If the wind "frees" ... comes from a wider angle ... the boat heels more and you ease sail or hike harder to get the dynamic balance.

Dynamic balance! Good term

So working and reaching you have dynamic balance because the crew can move to one side as the sail wants to fall in the opposite one.

But running .... there is no damping from the sail because the boat is unstable on its lateral axis but the sail is only providing force on the longitudinal axis.

That's part of the reason the "sailing by the lee" trick works .. it gives you some lateral forces back to play with.
http://www.storerboatplans.com/wp/de...-strong-winds/

But when you don't have enough wind to do this or too much wind to risk it ... there is no chance for the dynamic balance. So we have to rely on the stability of the hull. With the narrowness of the GIS bottom there is a possibility that if it rolls quickly there is not enough time for the increase in stability that will push it back to level to do anything and you go straight over.

But if you sit further apart the period of roll is slower which gives the hull much more time to exert the modest righting moment.

MIK

**Boatmik** · 4th August 2012, 03:44 AM

Oh ... I misunderstood the whole thing then. I feel very uncomfortable with the rope going down from the tack to the mast base... if it has tension in it then it is going to be taking tension away from the downhaul.

And if it does that it increases twist.

What I thought was happening was that it was a line from the tack going parallel to the boom and lashing around the mast. That has no downward component.

Or what about doing a similar trick to the halyard. The rope ties to the tack goes on the mast on the opposite side to the boom then has a loop around the boom. The more the yard tries to move forward the more securely it is held against the mast.

Or am I repeating the ideas above?

THEN I was thinking if that held the boom to the mast then you could move the downhaul further back along the boom to increase the vanging effect.

MIK

**IanHowick** · 4th August 2012, 09:32 AM

Originally Posted by Boatmik

Oh ... I misunderstood the whole thing then. I feel very uncomfortable with the rope going down from the tack to the mast base... if it has tension in it then it is going to be taking tension away from the downhaul.

And if it does that it increases twist.

What I thought was happening was that it was a line from the tack going parallel to the boom and lashing around the mast. That has no downward component.

Or what about doing a similar trick to the halyard. The rope ties to the tack goes on the mast on the opposite side to the boom then has a loop around the boom. The more the yard tries to move forward the more securely it is held against the mast.

Or am I repeating the ideas above?

THEN I was thinking if that held the boom to the mast then you could move the downhaul further back along the boom to increase the vanging effect.

MIK

Mik I think your understanding of what has been tried and works is correct, perhaps woodeneye has mis-understood what people are trying, or he wants to try something different. It would be interesting to see if he tries it, but I'm with you above, I'm doubtful of the sense of a line from tack to mast base.

My understanding of the variation from the square lashing described in the plans for keeping the boom in the correct place is that it involves a line permanantly attached to the tack end of the boom, it goes the other side of the mast and bowlines or similar to the boom just behind the mast like this:
Attachment 218195

This is how I have mine, I think I saw it described by Ross Lillistone on his website. I find it works well, does up quickly and restrains the boom exactly as required, keeping it close to the mast and stopping it sliding too far forward. It's also easy to change the amount of sail forward of the mast if you want, and would allow you to move the downhaul back a bit to provide a bit more vanging effect if desired.

I'm not sure of the sense of the 'boom limiter' from the boom tack to the deck in front of the mast, I think it creates too many interacting forces that I struggle to visualise, though I'd be keen to hear what Bruce thinks after he has tried it.

Close hauled, with my set-up I have complete control of sail twist - I have upper, middle and lower leech tell tails, and a tightly stretched spectra traveler just in front of the transom corner knees. I have no vang, just a downhaul, but I can control twist with sheet tension, more tension and the upper leach pulls in. I can pull the upper leach in to the point where it is stalling without pulling the boom in beyond where it should be, so on the wind, I'm watching all three leech tell tails, and adjusting sheet tension to keep all three flying as much as possible. I haven't paid quite as much attention to what is going on on a reach, and whether I have enough control of twist as I ease the sheet on a reach. If I couldn't get enough control of twist on a reach, I'd be thinking about a vang, or possibly moving the downhaul a bit behind the mast as woodeneye suggests. It is on my list to make it so adjustment of the ownhaul is easy on the fly.

My yard is more than twice as stiff as the wooden ones, probably that is helping.

I agree that Clint needs more draft in the lower part of his sail. Even if you're getting overpowered, you're better off with the lower part of the sail working well, then you can let the upper part twist off if necessary. Oversheeted and stalled at the bottom, and all the force is just making it heel, it's no longer helping you go forward.

You guys need to get at least three leech tell tails on your sails so you can see what is going on properly.

Shouldn't we move this discussion to the sail tuning thread and keep this more for yawl specific issues?

Ian

**woodeneye** · 4th August 2012, 10:20 AM

Originally Posted by Boatmik

The rope ties to the tack goes on the mast on the opposite side to the boom then has a loop around the boom. The more the yard tries to move forward the more securely it is held against the mast.
MIK

Well, I like the idea of the boom restrainer from the tack going around the mast and looping around the boom as shown in Ian's drawing. If you move the downhaul aft, you need some sort of restraint to prevent the boom going forward. If it can do two functions, then that is efficient.

With the line from the tack attached to the mast base, I did not notice any problem with unwanted forces at all. It is just a restraint fixing the fore/aft movement of the boom. Nothing more complicated than that is happening. However, the idea of it being used to hold the boom to mast is a better one. Perhaps I have misunderstood what you guys were talking about and the tack line with loop around the mast to the boom is what you already have?

So, in summary, you have two parts, each performing dual functions:

1. A line from the tack, around the mast restrains the forward movement of the boom AND holds the boom to the mast
2. Moving the downhaul aft provides luff tension AND increases the vang effect to control twist. (well, improve twist control at least)

**Boatmik** · 4th August 2012, 11:23 AM

Great stuff Bruce and Ian.

I am wondering a little about the phrase.... Move the downhaul back to get more vanging effect.

I always want to know the result of the extreme case immediately!

)

Make the downhaul look like a conventional vang!

You lose some downward component so would have to increase tension.

Also the angle defines the split between downhaul and detwisting function so might need to be moved for different winds.

If experimenting double the vanghaul by making a cascade system. A single block on the boom. Spectra line tied to downhaul fitting on deck, going through the block on the boom then attaching to the existing downhaul.

Gosh! Want to know how it works. Either it will or it will bind up.

MIK

**woodeneye** · 4th August 2012, 02:34 PM

Originally Posted by Boatmik

Great stuff Bruce and Ian.

I am wondering a little about the phrase.... Move the downhaul back to get more vanging effect.

I always want to know the result of the extreme case immediately!

)

Make the downhaul look like a conventional vang!

You lose some downward component so would have to increase tension.

Also the angle defines the split between downhaul and detwisting function so might need to be moved for different winds.

If experimenting double the vanghaul by making a cascade system. A single block on the boom. Spectra line tied to downhaul fitting on deck, going through the block on the boom then attaching to the existing downhaul.

Gosh! Want to know how it works. Either it will or it will bind up.

MIK

Don't tempt me too much MIK, I have a 15:1 Laser vang on hand

**woodeneye** · 4th August 2012, 04:38 PM

Well, I was tempted!

However, I moved my post to the GIS sail tweaking thread, here:

**CCBB** · 7th August 2012, 01:26 PM

Thanks for rigging thoughts. I am distracted for now with some hull tuning left to do.

Yesterday I found my self in 30kt conditions with big seas. I nearly capsized, but got the reefed sail down into the boat (I was not out with the mizzen) just in time. Only had maybe 4-5" of water in the boat. I got a tow from someone. It was really crazy conditions. I anticipate some gusts were in the 30's, a 25kt constant breeze against an ebbing tide = big waves breaking in the whole bay. Even the schooners were hove to and taking down all sails!

The tow in was nice, but we tried a hip toe and I banged up my stbd gunwale pretty good, mostly dents. The inwale tried to break at the very end where it slots into the knee. All fixable and can wait until winter.

The experience made me realize I still have issues to resolve with my tanks. 1) rainwater is getting into the second mast step, the one that goes through the tank. I knew it would be a problem so I thought I did an excellent watertight construction job, but I was wrong. It is freshwater and mostly because the limber hole is not effective enough at draining the step. But clearly water is entering the tank around the step, too. I intend to do a pressure test and confirm that. I will then need to find a solution. Another Bomar hatch will go into the bulkhead so I can go in and mop up any water on both sides of step. Also, I will make a cap for the partner so when the mast is in the aft position, water will not get in too much. I also intend to make a canvas bootie to go around mast and a flange on the tank top.

2) The aft tank, the mizzen pierces the tank and is open to the tank. My plan is to have a canvas bootie that ties around a flange on the tank top and around the leather on the mast to seal it. Oughtred does this on his double enders, which is why I decided not to close in the tank.

If I were building the boat again, I'd likely do some things differently.

My main point it so give a heads up to all builders, esp. the yawl builders: it is surprising how hard it is to make an airtight tank truly air/water tight. If you do the yawl thing, I would likely use the same construction for the fwd tank but use polyurethane, flexible bedding compounds (e.g., 5200) to install the mast tube. The epoxy may have failed in the bond with the Aluminum tube and/or from stresses in way of the partner and step.

For the aft tank, I would consider closing it in and drain it through the transom. I think the mast boot will work fine, though, but will update everyone on that idea.

Any thoughts on any of this is welcome. Making the rainwater stay out of the fwd tank may be the trickiest part. We will see. Check your tanks and be sure to air them out...if they are bone dry, good job! If moist, air them out. There may be some Goat surgery in my shop this winter. Nothing stops the season though, but I do want to get the tanks tightened up before I go out again. Then I can get back to my rig.

Cheers

**paulie** · 7th August 2012, 11:11 PM

My aft tank is truly airtight. I know this because pressure changes can make it difficult to open the hatch covers and then air rushes in or out once they are open.

The forward tank is a different story. Even without the extra mast step, I've got a leak in there somewhere. Not so bad that I'm worried about not having floatation in a capsize, but enough to require regular mopping/airing out. The fact that I don't know where it is worries me. I intend to find and fix it at some point.

**davlafont** · 8th August 2012, 12:41 AM

Originally Posted by CCBB

For the aft tank, I would consider closing it in and drain it through the transom

I went this route myself. It was a little tricky laying in a step with a (somewhat) level sole and drilling a limber hole that slopes away yet still exits out the transom face, not the hull bottom. Maybe that wasn't really necessary. I did a "wet run" of water-tightness of the mast tube/box before installing the tank top. I poured a bucket of water into the tube and promptly watched it spill out of two corners. I got back in there with filleting mix and the second attempt drained out the limber hole only. Time will tell if that was sufficient.

(Pics tonight, promise...)
(liar, liar, pants on fire. I don't seem to have pics of my tube. I swear I remember taking an up-close photo of the step with the wedge of plywood to make it level. I suspect there was a technical error on my part.)

**CCBB** · 8th August 2012, 12:28 PM

My pride is hurting a little right now. The terrible sail on Sunday and discovery of the leaky fwd tank is a royal pain! I have a compressor and some fittings now to do a pressure test on the tank. Hopefully, it is fixable with out cutting into the fwd tank. I'll get through the season with some sort of cap for the partner.

I am engaging my canvas guy to see if you has some bright ideas for boots to go around the masts and partner.

Paulie, it is really hard to get an airtight tank truly tight!

**callsign222** · 8th August 2012, 12:46 PM

*ahem*

(seat top hatches/ports as opposed to bulkhead hatches)

**CCBB** · 8th August 2012, 01:00 PM

huh? You mean I shoulda done hatches in the top. Yep, well I'll likely be cutting one to see what the heck I did!

I like to look at these things as learning experiences. It does bring up the issue of "how tight should a tank be" and how to test it during construction to make sure we're good to go.

Dave's simple test is brilliant. Why didn't I do that?!

**callsign222** · 8th August 2012, 01:14 PM

Sorry Clint, I couldn't resist. Maybe I shouldn't have. I just don't understand bulkhead hatches-- anyone's bulkhead hatch. It just doesn't make sense-- if we have any gear we need in an airtight tank I don't know why we'd compromise the ship with a hatch on the BH as opposed to the seat top. The only point of water entry that I can see is the hatch in the bulkhead, especially when there's water in the boat.

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