Once again on righting moment versus heeling moment

You can also refer to page 62 of "The Design of Sailing Yachts" by Pierre Gutelle for a similar illustration, but of a keelboat instead of a dinghy.
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I haven't studied the details of what you've done so I'm not sure about lead. I do those calculations a bit differently most of the time: since most of my designs have been relatively high performance boats I use the estimated aero-dynamic center as opposed to the geometric center of the sails and the same with the board(generally about 25% aft of the leading edge) and use zero to a few percent lead depending on whether the boat is going to heel or not. I am NOT recommending that to you!
What I recommend is that you research "lead" thoroughly and then use the system that seems most accurate to you. Then keep a record of your designs and how they sail. That way you'll gradually gain the experience necessary to get exactly the helm "feel" that you want.

 

I am not sure if this has been cleared up but all I was trying to say was don't use the lateral area of the hull in the water, only use the CE of the daggerboard in the vertical distance between CEs in the righting calculation. It appears you did this in the picture. This is only about the vertical distance between the CE of the sail and the CE of the DB times the force on the sail, which must equal the weight of the crew times their distance from the center of buoyancy.

About lead -For a dingy my only major concern for lead is that you never have lee helm. When a gust hits and the boat is heeling over I want it to stay safely upright if you let the sheet and tiller go. Your concern about excessive weather helm is just not a problem -forces are too small relative to crew strength, you can adjust the relative areas of the foils and you can balance the rudder.

I think you have some misconceptions about weather helm from L&E, the sail center of effort does not move aft when the boat heels (or ever?). When the boat heels the forces from the sail move leeward along with the sail while the drag of the boat stays around the center of buoyancy. The horizontal distance between the force of the sail to leeward and the force of hull drag causes the moment that is weather helm. Big keel boats always sail heeled because they have zero righting at zero heel (upright) and maximum righting at 90deg heel. This constant heel makes weather helm which must be countered with lead. A dingy sails best upright and should always adjust sails to get back upright when it heels -there is no 'normal lead' needed. Dingy righting is maximum at 0 heel and drops to 0 at 90.

Dingy 'balance' would be considering the forces and their positions so that the foils are used efficiently -the DB handles say 80% of force at 3 degrees angle of attack and the remaining 20% on the rudder leaves it plenty of capability in reserve for control. Don't worry about lead, you can move the mast back to the same frame as the front of the DB trunk, we can figure the forces and make a bigger better rudder.

The calculation I suggested relating force and fluid speed is independent of units -the units must cancel out. Coef. of lift of a profile as a function of angle of attack and fluid speed can be estimated for the infinite case in a number of ways. I was thinking we could use thin foil theory for low angles of attack, and use panel methods for CL from 0 to stall. This 2D Cl is the max a real finite AR foil could achieve and the actual lift will depend on aspect ratio. The basic concept is that the foil profile can not get to theoretical 2D pressure when there is an alternate path of lower pressure from a nearby edge.

 

Thank you, Doug. Just one more question: I researched this aerodynamic center, and the calculations seem very complicated, they require 3d shape of the sail. Since I plan to make this sail out of Tyvek, broad-seaming is not likely to work, and edge curves won't work either because this is lug rig, so luff must be straight.

My hope is that since Tyvek tends to stretch, the camber will come naturally. If not, I can pre-stretch the sail with sand. I heard about this method.
So, for now, I plan my sail flat.

With that in mind, what is a good way to estimate aerodynamic center?

Very clear, Skyak. Thank you. I've done this in my model.

I see. I kinda messed up when I explained it, but I had the same idea. I found some info on this topic, but I haven't realized that dinghy, unlike a keel-balast yacht, is sailed best without heel. This is very useful information, I haven't thought about it. Thanks a lot, Skyak.

I adjusted my model so that CE and CLR have no horizontal distance anymore.

Well, as it is of now, I see no reason to move the mast... It would mess up the design and create a lot of troubles. You mentioned earlier that it's best if mast connects to the same frame as daggerboard. I agree, but there are a lot of strong connectors between the two frames (mast frame and daggerboard frame) right now, so I guess this won't be a problem. As I mentioned, the same arrangement is done on Klepper Passat, and it is known-to-work boat. But if in doubt, I can add some more joints.



I am still working on trying to understand your explanation on daggerboard forces. I'll look up some literature about this.
What concerns me is if: 1. It affects CLR or CE considerably; 2. How it relates to daggerboard area versus sail area.
Not sure yet how to calculate that.



By the way, I looked up a nice lug design where a halyard also works for holding gaff at the mast:



This made me wonder: won't different halyard tensions affect the horizontal gaff position? Also, gaff-mast connection being so wide, won't gaff slide (due to aerodynamic forces) horizontally either way, moving CE to an undesirable position, unbalancing the boat?

Due to my lack of experience with lug sails, I really don't know if sail tends to slide into balanced (in terms of CE-CLR distance) or unbalanced position.

Can you please comment?

 

Lead(leed)

I don't think you should use the estimated aerodynamic center until you have substantial experience doing lead calculations the traditional way.

 

I will do some calculations about the DB lift and include it later. Your DB is plenty big. High performance boats use smaller area higher aspect ratio DBs to the extent that they risk stall at low boat speed. If we do the calculations you could do the same but I think you just want good performance that does not stall or have excessive leeway. To do this I would produce a leeway vs boat speed graph assuming full righting force. Your rudder is too small but we will deal with that after the DB size, profile and position are set.

I don't have any experience with lug sails but I do know physics and based on that;
1) you need camber -aerodynamically all that is good comes from camber and all that is bad comes from the loss of camber. At a minimum you should curve the sleeves that the yards fit into so the center of the sail can develop camber.
2) the positions of the halyard on the gaff and the downhaul on the boom are important to the twist of the sail. People with experience designing and sailing lug rigs have very detailed opinions of how to do it right. If I was doing a lug I would find the greatest authority I could, match their proportions, then make the halyard attachment to the gaff adjustable. The picture with the halyard around the mast does nothing but make a strap around the mast that is tighter raised than while raising -could be of value on a highly tapered mast.

Unfortunately all the decent low aspect foil calculations don't apply to the lug for various reasons. Petro did a few lugs and junk sails on boats similar to yours -we should get his comments. I think he has some ideas for a flat junk sail with a roughly elliptical profile. When you talk about a lug sail in tyvek I worry about the sail flogging when you ease the sheet in a strong gust -the highest stress in the fabric (impact stress) happens in flogging. Junk sails (or any sails with lots of full length battens) resist flogging more effectively.

 

Yes, exactly. I noticed that similar designs have smaller and higher aspect daggerboards. And yes, my aim is to have a boat that is not a racer, but versatile, especially in tricky winds I described earlier. Small lakes is where I'll sail most, and these tend to unstable winds, narrow passages to sail through, so leeway is bad.

Yeah, I'd like to make that kind of graph, but I lack the knowledge for that yet... As for rudder, I haven't started to work on it yet, so it'll improve. Problem is that due to skin-on-frame design, it is very difficult to attach rudder to the transom frame. Klepper Passat uses one joint at the top of the transom, and it looks like custom cast steel.

Photos of it:





I really can't make something like this. And I'm not sure what can I do. My experience shows that door hinges found in hardware stores work for a rudder, but if only one is used, forces on the rudder will bend it.

A while ago, Petros suggested lashing for rudder attachment:



Maybe that would work if rudder was lashed to the reinforced skin at the transom. However, I fear that the forces will stretch or tear the fabric over time. Currently, I plan to use 8oz. polyester fabric with Rustoleum paint as sealant.

This is a bit off-topic, but maybe you have some suggestions on how to attach rudder to SOF design? Maybe some strong magnets? I don't know...

Since I have no ideas at the time, I don't want to make the rudder high aspect (deep), because it will twist more, and will require stronger reinforcements.

Well, I can do that. I was thinking of loose-footed sail in regard to boom, but maybe I can make the curves you describe.

Since Tyvek stretches, how about the sand method I mentioned? I red they used this for cotton sails. Stretch the sail above ground, pour sand onto the desired locations, pour water for additional weight, leave overnight.

I will look for these authorities, then. This, if I remember correctly, is Goat Island Skiff, highly-regarded lugger. But I'll find some alternatives.

Well, I've done Tyvek sails twice, and I always go big on reinforcing them. Edges are overlapped, with reinforcing rope inside, all corners have huge patches. Not sure if that'll be enough in lug rig due to free luff... But I can always add one more reinforcement!

 

The thread on your previous post had a formula for calculating 3D CL from 2D CL so maybe I won't have to make up my own. I was thinking we could just exchange spreadsheet files -easier than typing formulas. I have MS office 2002 -did we establish compatibility?

Your concern about the rudder mount is valid but not a big problem. You want a solid lower mount but you don't want a leak path.
1) you could just put a reinforced hole in the skin, bolt through to the transom and seal the hole with a rubber gasket and pressure from the bolt.
2) you could have a triangle rudder mounting board with two bolts spaced wide at the top of the transom. These bolts take the strong moment from the rudder. The bottom of this rectangular board is pulled forward (only forward, not side to side) against the transom by a strap along the keel -it only pulls one direction so it won't stress or rip the skin.

You have it easy. I am trying to do a firm rudder on a folding kayak -no bolt access, no transom -much harder.

About Tyvek shaping - Tyvek is "spunbond olfin" -this is the companies way of answering the question in the most useless manor possible. All I know for sure is that it is an olfin plastic -my guess is polypropylene or polyethylene but it could be something they made up. At any rate the fibers will stretch permanently when heated beyond their glass transition temp likely between 100 to 150C. The thing I don't know is how the spinbond will hold up to temperature and how uniform it will stretch. I think a little experimentation with a common clothing iron will tell.

You have experience with Tyvek sails, I do not, so I trust you like them and don't care about any negative concerns I might have. My thought is you prove the design with tyvek and replace them with polyester when you ware them out in a season or two.

So you plan to seal polyester rather than use pre-preg. That makes shape easier. Rustoleum is a brand -It tells me nothing about the material. The SOF crowd has decided that urethane is best. My inclination is to follow their lead since maximizing skin life is key. Whatever you use be sure you can make strong repairs to it.

 

Sure, we could. Just as of now, all my formulas are in Solidworks. Just name what you need and I'll send you an Excel spreadsheet.

Yes, exactly, the biggest concern is leak. This rubber-bolt combination sounds perfect. I don't plan this hole to be below waterline, but during sailing operations, it might receive substantial amount of water. Do you believe this solution will be waterproof enough?

Also a nice idea, but the first one seems easier to make.

True! I always wondered how they do rudders on kayaks strong enough, having only one attachment.

I care! I want to know as much as I can before I start building, negative info as well. During my experience with Tyvek, I haven't noticed any stretching - then again,I haven't sailed long enough for these sails to stretch substantially.
However, I red what other sailors wrote in various forums. They say Tyvek stretches over time. General consensus is that Tyvek sails are not suitable for performance sailing (racing), as it's shape is too unpredictable. However, they also say that Tyvek sails are perfect for a newly designed boat, to try out the balance, reefing points, and other things, so that adjustments can be made before switching to Dacron.

Thing is, that these Dacron sails may cost as much as my boat. Material isn't available in my country, I can only ship it from USA. Few sailmakers that we have also order Dacron in tons. Since sewing Dacron is such a delicate matter (I tried!), this is probably best to leave in sailmakers hands. And as I said, they charge so much that it would double the budget of my boat.
Used sails are also expensive here, and no one sells lug sails.

So for better or for worse, I have to stick with Tyvek. Maybe someday, if my design proves to be sail-worthy, I will save up and make an order for sailmaker.

True. My initial idea was to buy PVC fabric, and glue it with HH-66 Vinyl Cement, as suggested by Yost in Yostwerks.com, however, this cement is unavailable in my country, it requires a heat gun, and I also red that if PVC wrinkles due to hull shape or amateur gluing, there is nothing a mortal man can do to make wrinkles go away.
Polyester, on the other hands, requires sewing (to me, it's preferable to heat guns) and painting. The paint I had in mind was Rustoleum Pro. I took this idea from Kudzu Craft. Rustoleum Pro is also unavailable in my country, so I was hoping you would suggest an alternative. Thank you. I'll search for urethane.

And, if you had experience with urethane - how waterproof is it? How many coats of it does polyester require? And, since my boat is folding, and I'll have to fold the skin, won't the paint break once skin changes it's shape?

Speaking of polyester, all I find in my country right now are... Bedsheets I guess these are too fragile, too thin for this job.
What kind of properties do I have to search when looking up polyester fabric? What is strong enough for this application?



I'm sorry for so many questions, Skyak. I hope you don't mind it, and I really appreciate you helping me out. Without this information, my project is flawed.




P.S. Just thought you might be interested - I nested all the plywood parts, just to see how many 1.5x1525x1525mm sheets I'll need. It turns out that everything will fit in 2!

Screenshot (borders around these parts are dimensions of plywood sheets).

There will be one additional .4x1525x1525 sheet for other parts.

I did a bill of materials, at least on what I know now - timber. Currently, total cost for timber and plywood is only 120$. For the whole boat. Sure, there will be hardware, polyester, Tyvek, oil, paint, ropes, not to mention tools, but I expected the timber to take up the largest portion of budget...

It looks like this project might turn out even cheaper that I initially thought!

 

I just read this about tyvek;

TEMPERATURE RANGE
Toughness and flexibility are retained down to -100°F (-73°C). When exposed to heat, Spunbonded Olefin begins to shrink at approximately 270°F (132°C) and melts at 275°F (135°C). Under actual processing conditions, where tension is required to handle the web, the web temperature should not exceed 175°F (79°C).

This indicates to me it is made of polypropylene. The low stressed temperature suggests the bonding breaks down well before glass transition. Doesn't look good for shaping. Putting an arc in the sleeves should work.

About urethane sealant -I have never seen less than two coats. The answer is whatever it takes. I have been skeptical about sealed fabric for folding boats. My fear is that it will develop leaks at creases from storage or the fabric will stick to itself while stored folded. I have no basis for this fear. The factory PVC impregnated fabric is used in inflatable and folding boats commercially so no worries.

About fabric and part availability, I find that most parts and materials can be purchased and shipped from china for the same or cheaper price than I can find as long as it is not premium shipping. The reason I don't use this more is that there is no assurance that what I receive will be usable or of any value at all. There is no recourse for fraud. For fabric I think you are pretty safe buying from China because that is where most fabric is made. When I find something is only made in China I don't worry too much about buying domestic.

 

Okay, but what properties I should look for when searching for polyester? What is desired thickness or density? I want this skin to be as strong as possible.

About urethane: I am now searching for what urethane paints are available in my country. Found this. Very expensive for a 750ml.

It says it can cover 12-14m^2 with single coat (I don't know how different numbers would be for fabric, not wood). My current estimates is that the total area of the skin is around 12m^2. This means 3 cans of this paint for 3 layers, that is ~125$, or 1 2.5l can for 133$.

I need to be sure. Could you please take a look at the link, and tell me if you recognize the paint? Is it flexible enough not to crack when folding skin, and leak-proof?

Because there are alternatives, however, they are not meant for boats, so I'm not sure if they are just as good.



I am very, very unexperienced when it comes to paints. I've already been to several hardware shops, and asked for a "paint to waterproof polyester fabric while retaining flexibility". They looked at me like I just returned from asylum

 

Can you post a two dimensional sail plan?
Two dimensional lead calculations can be very misleading ( no pun intended).

 

Sure. Here it is:



It's not in English, sorry.

"Sverto SPC" is CLR of daggerboard;
"Svorio centras" is center of gravity (with two persons aboard)

Overlapped, there are two points - BC (CoE) and ^SPC (CLR of daggerboard raised to the height of CoE).
BC1 and BC2 are CoE for reef 1 and 2.

Lines on sail are reef points.

Everything else is hull and mast.

 

Laukejas:
You can go to my blog and see some of my design work:
http://perryboat.sail2live.com/

I am not scientist I design with all the math tools I can but in the end I rely upon my eye.

Your sail plan looks fine to me. No problem.
In a small boat like this there are so many dynamic variables that some weight shifting in various conditions may be required to keep the boat sailing optimally. But you rig looks like it is in the right place to me. I would not try to calculate this myself.

If I had one thing to change I think I would go with about 15% more sail area. Forget reefing. This is a small boat. You don't reef small boats you hike.

I can see that you are having a lot of fun with this. I congratulate you on your thorough approach. Do not forget that yacht design is an art/science.

 

Bob,
Thanks for the use of your 'calibrated eye'. I sure wish we could reproduce it but we can't and are left trying to develop mathematical tools that can be passed on.

I would also highly recommend visiting Bob's site. I am searching it for answers
-how can his latest 62ft 'sliver' design produce so little wake at speed? The absence of wake (energy lost to waves) is the only explanation I have for why the boat is so fast.

-talking about lead and helm balance, That 62ft boat requires only fingertip touch control -at a wide range of heel. This defies reason.

-windward sailing angle is another incredible capability of Sliver.

I tend to think I can at least get the science to explain the art but this is a tough one.

 

The boat urethane in your link is specifically for wood (one part with solvent is not good)-there is better for fabric (generally two part).

http://shop.skinboats.com/6-YD-65-1050-URETHANE-COATING-KIT-skin-boat-cover-kit-1050goopkit.htm

This is the kind of material I would look for. This is my first search. We have time to look for better. This is nylon -the SOF builders like it because they can shrink it to fit. We don't need to shrink (or do we?). For now let's just budget $150 for fabric and sealant.

We talked before about material 1000+ denier...i have seen up to 3000 used and 4000 existing.

some experienced opinions here;

http://www.kayakforum.com/cgi-bin/B...70/sbj/skin-on-frame-durability-of-materials/

Gotta go, more later.