Once again on righting moment versus heeling moment

Bob Perry, thank you for your insight. According to my calculations of balance, the sail (without reefing) can withstand 7m/s winds before capsizing. That is, with hiking as far out as possible. 2 reefs enable the boat to sail in 10m/s and 13m/s respectively.
These reefs seem to be very easy to make, so I'm thinking, why not have that option?

I will look into your website.

The link you provided seems fine, however, we do not have anything like that in my country. We don't have any skin-on-frame builders, and there is no market for such supplies.

However, I found 2 part polyurethane paint

What do you think of this one?

There is very, VERY limited supply of such specialized paints here. I just hope that something works.

Yes, I remember. I believe we talked about PVC back then. Just thought that its different, while it's not.


Things are frustrating at my end. I cannot find ANY source of either polyester or nylon fabric in my country. All they sell are those bloody bedsheets. No one tells density in denier or oz or kg.

I hate to ask, but you mentioned that I could try fabrics from China - could you please give a link or something? I googled for hours, and found nothing. No one tells denier density of materials.
I am not even sure if they would ship to Lithuania.


In the meantime, I will visit several fabric shops in my town in person, and see what they tell me in the eye.

 

We have time don't we? If it turns out stuff is only available in the US maybe I can send you a care package. Any idea what shipping is from the US?

 

Thank you, Skyak. Still, I hope that there are options beside that. I don't know exactly, but I know it isn't cheap.

I found something on Amazon - they actually ship to my country. Check this out.

At the bottom there is a description, and it says 3,5 ounces. If this is per foot, then I guess it is too thin. The links you provided say that the minimum for boat skin is 8oz, but 12+ is better.

Can you confirm?

P.S. do you know how to convert from denier to ounces and vice versa? I found something on google, but it doesn't make a lot of sense.

P.P.S I found "ballistic nylon" on Amazon, here, but again, it is only 1050 denier, and already coated, which is bad.

Actually, I can find NOTHING when searching for like "2000 denier nylon fabric" on google.

P.P.P.S I found 1000 denier AND uncoated, but out of stock - here

 

I don't think the rip-stop nylon will be heavy enough. When weight is given it is typically per square yard or meter. When I looked, this is what I found

http://www.amazon.com/Silver-000-De...038&sr=8-13&keywords=nylon fabric 1000 denier

http://www.amazon.com/Nassimi-Imper...1402239282&sr=8-1&keywords=fabric 1800 denier

http://www.amazon.com/Red-Denier-Te...1402240806&sr=8-16&keywords=fabric 000 denier

coated like I use

http://www.amazon.com/Rochford-Supp...1402237430&sr=8-3&keywords=fabric 1300 denier

http://www.alibaba.com/product-detail/Flame-Retardant-PVC-Coated-Polyester-1000D_617977180.html

Like you said, most if not all the strong heavy denier fabrics tend to have some coating that tends to lock the fibers in place and provide some moisture resistance. I think the ones toward the top of this list are designed for luggage so they have bare fabric outside (for looks) and a urethane coating inside. I don't know what to say about if these fabrics are usable. I think heat would make them conform to shape but I don't know what the manufacturers coating will do to your coating. I think we should ask a skin-on-frame builder forum. If I was building your boat I would try to design it for the heavy PVC coated fabric -with a little adjustment of rocker and topsides I think it could be done without darts. Coating the fabric after it is sewn still needs to prove it can fold reliably.

FYI I found this guy torture testing post coated fabric with urethane (varathane appears to be a trade name of rustoleum).

http://www.qajaqusa.org/newsletter/Masik_Summer2003_07034.pdf

 

Skyak, thanks for reply. I was afraid that everybody had given up on this

I looked at your links, and yeah, these materials are already coated. The only reason I would buy fabric and sealant separately is because I could sew and heat-shrink it. If it's coated, I doubt that is possible. If it is coated on one side, as you suggested, I have no way of knowing whenever my paint will hold together with old one.

If I can't find totally uncoated heavy fabric, I think I should go back to PVC. Much simpler, and less risks. There is plenty readily available in Lithuania, mostly for truck covers. The heaviest I found (link is in English):
http://ramchos.lt/en/568-standartin-tentin-medziaga.html

It says it is PES, not PVC, but it looks pretty much the same to me. 1100 Denier, 900g/m2 (the only heavier fabric I've seen is 1100gm/2, but not for sale)

Comes at 3.9$ per foot. Do you think it is tough enough?

P.S. As for rocker, my current design has none. It would complicate design process a lot, and since I can't make the rocker deeper without resorting to dart-cutting in skin, I think I'll go without it.

 

That 1100 denier coated material looks just like the stuff I am using. There is no way it is PES (expensive high strength and temp material) -far more likely PVC over PBT like mine. From all I have found it is equivalent to what Klepper used. I will do a little test to see if I can stretch it with temperature.

As for the search for uncoated material, Petro has a contact with lots of it. He is talking about taking a load to mexico in another thread.

http://www.boatdesign.net/forums/boat-design/looking-cat-plans-suggestions-50595-2.html

 

That's great!! Then I'll definitely use this fabric.

I already wrote Petros a while ago. The only source he knew was Dyson company in USA. I contacted them, and they put up such a price that is totally out of question...

By the way, coming back to the main topic - I'm kind of worried about transom on my boat. Since my design has no rocker, even with crew in proper positions, the transom seems to be submerged a bit. As far as hydrodynamics are concerned, that may cause water drag, and it may significantly reduce boat speed.

I'm attaching two screenshots from my design. These are underwater profiles for 1 person (128kg displacement), and 2 persons (225kg displacement). First one doesn't seem that bad (submerged 2.2cm underwater), but with 2 persons onboard, transom is kind of deep (11.5cm).





Now, the transom looks like this (skin is removed in these screenshots for proper visibility of structure):





I was hoping if you could shed any advice on how can I reduce the stern water drag. There are 2 seats in the boat, one in the middle, and one in the back, so either one of 2 sailors has to sit in the back.
Angling transom would be over-complicated in this project.

What can I do?

 

Crew weight is everything for trim in an unballasted boat, so I'd think very seriously about whether your seating arrangements are practical. desirably crew position will probably vary by feet between different sailing conditions, you must be able to trim fore and aft.

Basically you need lots more rocker though. There was a fashion for rocker well aft in some 80s racing dinghies. In racing boats its a bad idea because it kills top speed (boats tend to top out several knots short of what boats with better rocker distribution do) but it was a bit more slippery at lower speeds. It also tended to make the sterns squat which meant they were very safe sailing downwind in heavy weather with much reduced tendency to pitchpole.
For your purposes the top speed downside is utterly irrelevant, because you aren't thinking of the sort of size and shape rig to get to that sort of speed, so if you can get more rocker in, and especially aft of the centre of crew weight, it will probably help.

 

Thanks, but as I mentioned before in this topic, due to design limitation, I just can't make any rocker. I have to find other ways.

 

I raced a dinghy called a "US 1" which was based on the Windmill I raced for years. The problem with the US 1 was that the freeboard aft was too low and in the right conditions the boat could swamp during a turn. It didn't have a self bailing cockpit.....

 

My experience of sailing craft does include a 12' hull with no rocker. It was a total disaster on the sea and incredibly difficult to manoevre. Admittedly it was an experimental hull but lots of other normal features of naval architecture were properly taken into account. The intent was to sit forward in light stuff and keep the transom out of the water, move back as the wind came up.
It performed far worse than the original 45 year old first design, it was a development class. It did have some virtues though. A brief description of this craft can be read by using the link below. Sorry I could not find a picture.

http://national12.org/private/Newsletter/Annual Newsletter/1978/winter/the_wedge.shtml

 

Rocker is how displacement boats keep their transoms out of the water to avoid eddy drag. The other ways are canoe stern (sharp, no transom) or make a ledge at the transom to make the eddy efficient (I don't see this done, might not be so good).

I didn't want to get into the rocker/draping issue, but since you asked I think that a little rocker helps the fabric fit. If you take a piece of paper and fold it in half longways when you hold it at the ends -if the bottom is straight (no rocker) there is no volume, if you angle the ends the middle bows out to a canoe shape. The same would be true of fabric on your boat. The transom makes the topside curves smaller relative to length. A little rocker would make those make the fabric follow those curves. The troubles I see are that optimally the rocker would be aft of center but the effect I just described naturally puts rocker where the beam curvature is -more forward. The other difficulty fitting is that the rocker just gets you to the bottom shape with flared topsides. To get the topsides to vertical likely takes darts, but the good news is they are above the waterline -below is smooth.

I don't know how to quantify the amount of deviation from natural draping mathematically -I will give it more thought. For now I am thinking that the best tool is a scale model. If you cut a board at intervals and printed out your bulkheads to fit in the cuts you could see how fabric of various stiffness fits on it.

It would be difficult right now but I have my laser hull and a length of similar material to do a full scale fit test. I am sure I could get to it before you graduate.

 

Sorry for my late reply. Got caught up in finals and such.

Thank you all for your opinions on the rocker thing. I guess it is inevitable: I have to make it. I really didn't want to, as it complicates design process a lot (parts join at 0 or 90 degree angle without rocker, but with it, every part will have to be cut precisely at more complex angles), and because I have no idea how much fabric can stretch before it wrinkles and requires darts.

Skyak, I will surely make a model, maybe around 20-30cm long. However, I have no way to recreate fabric on such scale. Paper will most likely have very different properties than the PVC fabric. That being so, I'm afraid that the model won't tell me much if I'll need darting the skin.

Thank you for your attempt on the laser hull. I really appreciate that.

I will work on my virtual model now to add rocker. It will take some time. I will experiment with displacement models to set it's position as is best according to hydrostatics, but for now, could you give me a ballpark figure of how DEEP this rocker should be at the deepest point? LOA of my boat is 4.2m excluding rudder.

P.S. Off topic question, but I keep wondering: there will be some high-precision requiring cutting for plywood parts. Such precision that transferring blueprints from smaller-scale prints by hand is out of the question. I managed to fit all these parts onto 2 15x1525x1525 plywood sheets. Here is how it looks:



As you can see, it is very tight fit. In ideal world, I would just make full-sized blueprints, print them, glue on those plywood sheets, and cut them out. It would be perfect. The only problem is that no one prints on anything larger than A0, and A0 is 594x1189, not 1525x1525.

As I said, if I would transfer it by hand, there would definitely be some mistakes, such as parts not fitting on these sheets, or being out of proportions. Precision is absolutely crucial here.

What should I do? How can I transfer the image I shown above onto these plywood sheets with dead-on accuracy?

 

Spirit Line Kayak Skin Boat Store

 

Tag for later study. Interesting stuff to be certain.