Backstay problem on Gunter rig

Earlier, I thought that the builder wanted a very light sailboat. I didn't get why an SOF boat would make good use of stays. It seemed the reinforcement of the mast to gunwale structure would probably add some undesirable weight but also complication. A web bulkhead would, of 1/4" or 3/8" plywood, I assumed, be the best way to handle the requirements.
It does make sense that a boat that could go into a trunk would make good use of SOF design. Whether the use of stays would make sense in this case is in question. Well, this is a high tech design (reminds me of something NASA would come up with to drive around Mars) in that you're pushing technology to the limit at least if the boats going to actually have value to anyone because it could easily either be too flimsy or too complicated to be worth using.
So we're not talking about an easy build. I picture a boat that weighs 100 lbs at 12 ft that takes no more than 15 minutes to set up that goes in the trunk of the car that actually performs as well as a racing dinghy. Cost of labor doesn't seem to be an issue.
Not impossible, but no small feat to design and build.
Frankly, this is asking a lot. If only you concentrated on perfecting the take-apart feature. Really, this one feature dominates all the others. The fact that you also want to complicate the rig by staying the mast and in addition, that you want a fresh hull design might become overwhelming for a single project.
Why not find a satisfactory hull design (there are hundreds), rig it simply (a pair of free-standing carbon fiber take-apart windsurfer masts) for a cat schooner rig? Then concentrate of the take-apart aspect. No jib, just two small identical sails that might each reef. Boomless, just the masts, each of which can be purchased off the shelf and maybe 7 ft long sections that fit inside any car.
To get enough sail are without tall masts the sails could be full batten and about 40 sq ft. each making 80 sq ft total. This should make for a pretty exciting ride if the whole boat is kept relatively light. Depowering with no stays is easy even when running due to the sails being able to swing around forward when needed. They could use a mast sleeve and maybe an option for a single central mast location when it gets breezy. Very easy to scandalize as well. Light, efficient, and stowable. I imagine a hull based on a plywood design, easy enough to copy in SOF.

 

Petros, thank you very much. This was probably the most interesting and useful reply to the issue. It cleared things a lot.

So you're saying that I can use jib even with unstayed mast? I had this kind of idea - rig a line to the top of the mast that acts both as halyard and forestay for the jib, and keep jib lowered on deck, along with the line - and then raise that forestay only when jib is needed. This is similar to what AC45's used in America's Cup - they rolled genoas, and then lowered them along with the forestay to minimize wind resistance on upwind legs. So this would be something similar, except no rolling the jib.

One more question: if I design boat to run without a jib, and then add jib later if needed, how am I to balance it? I'm now making CAD-assisted calculations of sail center of effort and lateral resistance center balancing, aiming to bring both of these centers together as close as possible. Easy in one configuration - but with 2 additional reef points, it's almost impossible for me to make boat balanced in all conditions.
What I meant to say, is if I designed boat to be balanced with mainsail only, and then add jib, I'm worried it might bring CoE forward so much that I won't be able to point high enough, especially in low speeds - and in higher speeds it will be a gym-session with the rudder. I might be wrong, but if you remember, this summer I made this improvised sailboat from a fishing boat - and at first, the small 3m^2 sail was very unbalanced. I moved it just 20-30cm towards the stern, and problems went away. If such slight adjustment made such a significant difference, what will happen if I add a few square-meter jib to a balanced 4m boat with 4-6m^2 mainsail?
Are my fears about boat balance well-founded?

 

Balance is greatly exaggerated. The modern fashion in racing dinghies is for a good sized rudder and both the rudder and the centreboard to contribute to the lateral force. That way the boat will track quite happily with a considerable range of centres of effort.

Its important not to confuse side load on the rudder (generally good) with weather helm (generally bad).

If the rudder is on the centreline when the boat is bolt upright and holding a straight course then a pull on the tiller is from side load and is not an issue for efficiency, and if its excessive can be dealt with by tweaking the rudder blade angle.

If on the other hand the rudder is having to be significantly offset from the centreline with the boat bolt upright in order to maintain a straight course then you are greatly increasing drag, and this is a bad thing!

People often get confused by weather helm induced by not sailing the boat bolt upright. That's another problem!

BTW if you are using mast grade heat treated aluminium alloy then 75mm is a very generous mast indeed, a Laser mast is only 63mm for the bottom section and 51mm for the top one, and as much as 8m^2 square sails are rigged on them. If you are using ordinary general purpose aluminium tube its another matter, because that stuff is far weaker (but cheaper!).

 

put two (or more) mast steps that allow you to rake the mast forward or aft, and use a small jib (it does not take a very large jib to improve pointing ablity), and than adjust rake in mast to compensate for the jib. Easy with an unstayed mast.

you could also sweep dagger board forward and aft a little to assist in trim. Or you can make different shaped dagger board, one tapers aft (trailing edge straight), the other tapers forward (leading edge straight).

A jib on an unstayed mast works fine, the forestay would actually counter the loads the main sheet puts on the mast.

I also highly recomend and second what alen white suggests about hull design. find a good performing dingy design that already exists, and than copy the lines of the hull. You can still make minor changes to the hull shape, but if you start with a known good performing boat, you eliminate a lot of second guessing when you are working out the bugs. It will also give you a starting place for the rig size, rudder and dagger board size. all of these can be tweaked a bit, but again, you have a known starting point.

 

But there are some limits, right? Could you give at least approximate estimation what is the limit of CoE and CLR distance difference, like, in percentage of waterline length?

Well, I guess it is ordinary aluminum in my case. By the way, what is best method of securing that mast insert used for joints? If it is welding, than welding area will be very small, since all the contact area is hard to reach. I was thinking... Does epoxy stick to aluminum?

Very interesting idea! Maybe you could name some good designs that utilize adjustable rake for reference?

Well, the obvious one is 420 class... It is racing sailboat, but it is a famous design, probably for a reason. Is it a good choice to copy lines from?

 

The 420 is a very full shaped 2 man dingy. The beam is 1.63M which is wider than your max storage. The laser is a great single hand dingy of 1.39M beam but that is with 4 or five inches of splash lip -the hull is right on your beam target. I would suggest the laser hull shape with the deeper side profile of the 420. I have a laser and can tell you it sails great to windward. If you flatten the bottom of your current model and maybe widen the transom a bit I think you would be pretty close.

 

Hmm, before you pick a model, do make sure you are selecting the right one for the job in hand.
Where are you going to be sailing this boat?
What are the wind conditions like?
What are wave/water conditions like?
What sort of loads, one person, two persons, extra equipment?
What sort of sailing are you going to be doing?
- day sailing exploring creeks and inlets
- long distance cruising with camping gear on board
- short sharp zooming round the local area going as fast as possible?
- racing?

 

Oh, right, I forgot the beam. Thank you. Okay, I'll try to find Laser blueprints, and adjust my model. Will post results once it's done.

Lakes. I have a few in my vicinity. I don't expect blue waters, if that's what you're asking.

Unpredictable shifts, vortexes, strength varying from dead calm to 15m/s in few seconds, occasional gusts up to 20m/s from any direction. Wind shear, microbursts... Yeah, that's about it.

As one could expect in lake - not too big...

I want to be able to handle the boat alone, but also have it comfortable for two persons (easier to handle larger sail area in stronger winds). It would be nice if it wouldn't sink with 3 persons, but that's optional.
Extra equipment - nothing too fancy. Extra lines, sail repair kit, PVC patches and glue, PVC cover for deck opening (in case of rain/longer boat storage without disassembly), screwdriver, pliers, some extra screws... And, if weight permits, a small anchor. I expect all that (except anchor) to fit under 5kg.
Depending on route, there could be some extra clothing, food, and other miscellaneous things taken. I don't see why even a foldable tent wouldn't fit for few days cruise.

Mostly - day sailing, although other categories you mentioned would also happen from time to time (except racing... I don't believe SOF would stand a chance against solid hulls - but if proves to be fast, hey, why not?)

I'll begin working on transferring Laser hull lines to my model.

 

epoxy is perfectly good to hold the sleeves in the tubes. do not weld it, it will distort the tube dia and weaken it since it will remove the heat treatment.

I built a 14' x 4'8" skin on frame dingy from the K18 hull lines, I scaled it down to 14 ft. Nice flat bottom planing hull. the only thing I would do differently is make the transom a bit wider. You can also get the Australian Javalin lines for free, also a good design, but you will likely have to scale it down some too.

 

Okay, after some hard work, I transfered Laser hull into Solidworks, and tuned it for SOF boat limitations. I made a preliminary hull model which I'll use for defining parts (stringers, cross-sections, etc.)

I'm attaching pictures of this hull transfer. Tagged with "1" is original Laser hull, and "2" is adjusted for SOF limitations hull.

Apart from being non-smooth, probably the most notable difference is side (left) view - bottom has no curve, it is straight, but if I make a belly, there will be wrinkles in the skin... At least that's what Yost says at his website of making SOF kayaks.

What actually worries me is that without this bottom belly, the waterline will be further up, and even with small amount of heel, boat will take on water. Also, with this low profile, there will be little distance in hull for holding the mast - I remember reading that 5 times the diameter of the mast should fitted be in mast step. If we take 7.5cm diameter mast, 5 times is 37.5cm, while now actual height between bottom of the hull and top of the deck is 27.5cm (practical height will be few centimeters lower).
I'm not sure what to do here. I could simply raise gunwales, but maybe there is a better solution?

Also, regarding the transom - since there is no bottom "belly" on this adapted hull, I'm worried due to heightened waterline, stern might drag water, causing increased resistance, severely reducing speed. Is my assumption correct?

P.S. Buttock lines on my adapted hull are the stringer path lines. I lowered them, so there is less unsupported fabric between stringers, but maybe not enough, what do you think?

P.P.S. Out of curiosity, I checked with Delftship if original Laser hull is developable (if it can be layed flat). Surprisingly, it shows very low error values. It doesn't seem possible to me, but does this mean that original Laser hull can be made out of fabric? Attaching development table dialog.

 

A bit off-topic, but I was looking for SOF designs, and here's what I found:

http://www.youtube.com/watch?v=EAs7yDBaydo&feature=player_embedded

Ingenious mechanism... Too bad it costs astronomical 2500 euros. And doesn't fit in car trunk

 

the Laser is a good design, fast and simple lines, however it gives a rather wet ride and was designed for one sailor. I suggest starting with something larger, with more freeboard.

If you want to use the laser lines, I would make it deeper (just extend the gunwales up perhaps 10 cm or more), keeping the below water shape similar.

 

Nice work on the laser hull! The space between stringers looks like it would be fine if the keel had some width (made of wood) which I have been thinking of suggesting anyway. Developable means the hull surface can be broken down into a number of conic surfaces -so it can be made from a bunch of bent planes. At all points there is a plane that will intersect the surface in a straight line. It doesn't mean much about fabric unless you are willing to cut and join the fabric at every chine. Developable means that the hull shape could be reproduced in plywood or SOF without much change in hydro-static properties.

"belly" is referred to as "rocker", and it is important. I still don't know where Yost said there should be none, but on a double end hull like the kayaks Yost builds there is no penalty for dragging the stern below waterline. Yost also makes long skinny boats out of skinny tubes -he gets rocker just sitting in them, he doesn't have to build any in. Look at the top of the boats when he is floating in them vs sitting on shore -some bend a lot. If you take a piece of paper, fold it in half at one end, and hold the other end tight against a piece of cardboard cut to the shape of the transom. Looking down from the top you have a triangular hull (cone) with zero rocker. If you try to bow the hull out like the shape of a real boat with max beam near the middle, it will cause an increase in rocker. To make the rocker the right amount in the right places you will likely have to cut darts in the topsides aft of the max beam and or at the keel. If you want an idea of how hard it will be to wrap the skin on your boat, print the frames to scale , cut them out of cardboard and mount them in position. Now try to wrap paper over the frames. You will need to fold the paper to make it fit tight -darts.

The thing I think you do need to change is to increase the depth of the hull to make it better suited for a crew of 2 and you need to have some deck around the cockpit for a place to sit and to keep water out to a large heel angle. The laser is a bit more like a surfboard than a boat -fine for 1 but 2 would prefer a deeper hull. If the cockpit extends to the mast, you can make the frame at the mast taller to support the mast above the deck. The lower half of the laser mast is very thick and heavy and the mast steps do break.

Your description of local winds sounds terrible. I will be interested to hear what more experienced sailor/builders recommend. My feelings about the laser in those conditions is that I want a flat, low center of effort sail rig. I have also thought about a sliding seat to make it fast and easy to shift my weight.

Boats that fold flat are interesting. My mother bought a "fold-a-boat" which is plastic and folds to 3" thick and about 16" wide. The 350 days a year it wasn't being sailed it was better than any other boat and the few days a year it was used it was tolerable. I am still interested in building an efficient hull for paddle and sail that stores flat in less than 6 foot.

 

All right, I think I'll go with extending the gunwales. Will post images once it's done.

Well, I'm not sure how I can make a keel that is foldable. Most Klepper kayaks use double tubes for keel, but they are joint with some little metal sheets, and welded. I couldn't find these joints to buy, so I went with single tube...

Sorry for my loose terminology. I don't remember exactly where he said it, but it seems reasonable. Just like your example with paper (if I understood it correctly) - if you fold a sheet on one axis, you cannot smoothly fold it on another axis without extra material creating wrinkles.

That's not difficult to do, I'll get right on it - but still, without the rocker, won't stern drag water? How do I solve that problem?

Yeah, it's a hell out there. There are hills and forests surrounding the lakes, wind gets lost in them, and comes out totally unpredictable, therefore the extreme conditions...

 

P.S.

I just remodeled Laser hull in DelftShip without rocker, set both hulls displacement to 170kg, and ran resistance calculations.

At 6 knots (hull speed according to Delft Series '98 calculator), the original Laser hull has 0.1278 kN total resistance, while my adapted hull has 0.1313 kN - that is an increase of resistance by only 2.66%. Doesn't seem significant... Maybe stern isn't dragging so much. I though that removing rocker will ruin hydrodynamics much worse.

Assuming these calculators are accurate.

I'll now update my SW model on this adapted hull.