Backstay problem on Gunter rig

Lauke...
nice to see you are getting back to your boat plan.

I am wondering if everyone advising has grasped that this boat is built and taken apart for each use with no solid piece longer than 1.3M long. The boat will be stored indoors or in the trunk of a car. Hull and rig stiffness is very challenging. The mast is three pieces, every stringer in the hull is three pieces, and since the skin is fabric there is limited ability to transfer torque along the length of the boat. An un-stayed mast will lever on one frame forward, while the crew will sit several frames back and the buoyancy will be distributed further back. A stay running back to the crew would be the most direct support and it would take some serious diagonal frame to provide this support from the boat.

Given the shortage of boat structure it would be wise to plan a sail rig that does not rely on the boat structure -lug (or junk) and lanteen would be my suggestion and I would not rule out running stays or struts on a mast that would be un-stayed in a typical one piece solid boat. Remember, the stays and struts must end at a frame, preferably at the intersection with a stringer to give any stiffness.

By the way, have you found any decent plywood source? I was blogging with a guy making a three piece nesting scanoe that would be a great hull for you to consider.

http://www.nestawayboats.com/page4.htm This will give you the gist.

The attached picture is a klepper break-down sailboat frame for reference but not a suggestion.

 

Skyak! Thank you for writing. I'm glad you found this interesting. You saw possible problems that haunt me also. This boat is designed to fit into car trunk, so I really can't make any part longer. If I have to car-top some parts, it defeats the purpose of this idea.
And that's right, there is little that can be done about torque - one of the reasons I'm trying to make loads travel directly to the supports - hence the stays that wouldn't be necessary on similar non-SOF boat.

I heard of junk rig benefits, but it also doesn't work well with backstays, and generally doesn't perform as good as, for example, Bermuda... Upwind sailing is what this boat is meant for.

No, I have yet to search for material sources. I'll do this on later stage.

I'm modeling this boat in SolidWorks program, thought I could show some screenshots to give better idea of what kind of boat this is.

It is now skinless, for better view of structure. Many things are not modeled yet (rigging, bolts, some metal corners, and so on), so all this will be reinforced later. I use 2.5cm diameter aluminum stringers, and 2cm thick marine plywood (maybe I'll have to change these numbers if more stiffness will be required).

Back, bottom and front views of the hull:







Partially raised centerboard for better view. Mast step can also be seen (there is actually a fitting for mast step, but it doesn't show now):





Rudder (sandwitch method, raisable, these eye-bolts are for lines that raise and lower it, and the clamp on top to quickly tie it down):



And the bow. The bolts are obviously missing yet, but this is generally how it will look like. The hole in the plywood-part is for forestay attachment (there will be an eyelet in skin too).



So, yeah, basically this is the idea. Still working on it, stuck now because I need to decide on rigging, and start balance calculations. I finished reading Principles of yacht design by Larsson, Eliasson, but still can't figure it out how to translate weight and hiking positions into possible sail area for given windspeed. That book is for physicists


(Does anyone know how can I size the images I link to attachments? They are too big)

 

Solidworks is a great CAD program but not specific to boat design.

Nice model but I think it looks more like a keel boat than a dingy in hull shape. A keel boat has weight down low in the hull or below so round or deeper hulls get positive stability. Dingys don't have any weight down low. Most of their weight is in the crew, high up, so it is preferred that the hulls have lots of "form stability". The water displaced by a dingy sitting upright tends to have a wider waterline and less depth.

About your stringers -you are going to want to minimize the distance between stringers below the waterline. The wider the distance, the further the water pressure will push the fabric up between stringers, or the tighter you will have to pull the fabric to smooth the hull, but tight fabric starts to bend the stringers. More stringers evenly spaced low in the hull is better, The topsides above the water don't mater as much so you can space wider there (if they are in the water you don't care if the hull is smooth).

About toque stiffness, the simplest way to improve the hull is to add diagonal boards from say the frame edge ahead of the mast to the mast frame center to the opposite edge of the frame behind the mast. From above or below the brace would look like an x with the mast a circle at it's center and those three frames will resist torque.

 

Yeah, I know, but it's great for general design. When it's finished, I'll transfer the model (or just general hull shape) to DelftShip, adjust it for best hydrodynamic properties, and then transfer back to SolidWorks for finalizing.

You mean that I should increase beam? I didn't exactly aim to make keel boat-like shape, it's just that PVC fabric (which will go for skin) is produced at 1.5m wide rolls, so perimeter of cross-section (not including deck) cannot exceed that number. It comes down to about 1.2m beam, or I'll have to join two rolls of PVC, which will complicate things. Another option is to scale down the whole boat, which doesn't sound appealing.
There are several other limitations. There can be no inward curves in cross-sections, and the bottom of the hull has to be completely straight, with no belly, or the PVC will wrinkle.


So, what do you think, for this type of construction, what rig would best deliver 10m^2 of sail for best upwind sailing? Stayed or unstayed? With or without jib?

 

10 sq.m sounds like an awful lot! An International Canoe has roughly 10sqM of rag and has a 2m long sliding seat to hold it up. Probably the best empirical way to size your sails is to look at dinghy classes and see what seems reasonable to you. For example the different varieties of Laser have 7, 5.7 and 4.7m^2 of sail, and unless your local wind conditions are very light indeed or your desire is for a very high performance boat I would have thought that a radial sized sail ought to be the very most you should be aiming at, especially as you don't have a planing hull shape, so there's little point in attempting to drive the boat beyond high displacement speeds. Bearing in mind your major engineering challenges I might go much smaller just to be sure the whole thing doesn't fold up on her first outing: 4sqm perhaps...

On the hull, I think I'd be tempted to trade freeboard aft for more beam and flatter sections. Also if you have to have no rocker in the bottom of the hull, which is what I think you mean by belly, then the boat will go down the mine with great enthusiasm at any speed, which is another good argument for small rags.

Building a whole rig with no component greater than 1.5m might be best described as brave... What do you have in mind for materials? I suppose it could be done with carbon fibre tubes, each one of narrower diameter than the one before, which would give you a tapering mast, quite handy for an unstayed rig. A round pseudo tapered mast and a sleeve luff sail might be reasonable. Maybe in this case there would be some mileage in emulating the 16*30 Canoe we've seen in another thread, and have two masts one forward and one aft, each with an unstayed three section mast (say about 3 m luff) and a 1.5m boom, plus fully battened sails with a generous roach. That ought to get you around 3m2 per sail without either mast being too long.

I think its all about the engineering challenge of transferring the loads from the mast to the daggerboard to the gunwhales... I dunno, I think your problem is not so much "how fast can I go upwind" as "how do I get upwind at moderate speed without the boat collapsing around me".

 

This is likely as accurate an observation as has been made and one I've eluded towards, in previous posts.

It's not difficult to scale a sail plan and there are several approaches to take, which will offer a good guide, but ultimately the real goal is if the boat, can stand against the proposed area, requiring a clue about it's initial stability and roll period. You can employ a wind pressure coefficient, use it's Dellenbaug angle, estimate or observe the boat's roll period, plus a few other techniques or you can guess. Which do you suppose will work out for you. You might be interested in reading (> http://sponbergyachtdesign.com/THE DESIGN RATIOS.pdf <) by one of our notable forum members.

 

Well, my idea now is to make a reasonable guess, then make a boat plan, and when everything is in place, start to tune it down with precise calculations.

As I mentioned, cross-sections will be made out of marine plywood, and stringers - out of aluminum tubes. There will be gunwales of plywood, centerboard box (also plywood), which connects to two cross-sections. Also, 3 most-stern cross-sections will be connected with several wooden planks for floor. All can be seen in these images I posted earlier, except for floor.

I understand. Well, I have no problem adding additional materials to make this boat stronger, even if it contributes to heavier overall weight. You see, I'm building this boat SOF not because I need it lightweight, but because I want to fit it in car trunk. It can be heavy as similar sized plywood boat, if necessary, I just need it sturdy and capable of decent performance. So I can add more supports to this boat, if needed.

Problem is, I can't find similar sized SOF sailboat for reference. There was one Russian design, but it used same aluminum tubes for cross-sections, and there was little info on it. So I really don't know how to determine what kind of hull strength is needed and what maximum sail area can be utilized without collapsing the boat.

 

Right, I got that, but it was the materials you propose to construct the spars from that were of relevance. If the stringers are all alloy tubing are you perhaps considering alloy tube for the spars? I think we can exclude marine ply from serious consideration for spar building

I suppose if someone forced me to do a rig for something like this I might start, as I said, with a 16*30 styles setup with two identical rigs fore and aft to keep down the height and thus twisting loads in the hull. Maybe something like the attached which scales to three 1.5 m mast pieces and one 1.5m boom. Full battens and pocket luff for simplicity. Not too much shape in the cloth, mostly in luff round. Then if it works out too large simply shorten the foot by moving the pocket luff. This would give you two reasonably efficient rigs of I suppose around 3.5-4m^2 each, which feels like lots to me - in fact I would think hard about shortening the booms to nearer 1m to keep the size down right from the start. If you put a 1.2m or whatever slab reef in you could shorten sail just by removing the top mast section! You could tune this capability by juggling mast section lengths of course - 2 * 1.5m, 1 * 1m, 1 * 0.75m (depending on the overlap) is the sort of thing that could give you some flexibility in rig height. More taper too come to think of it.

Space this apart as widely as feasible, depending on where your bulkheads are, then, as the man said, build as much triangulation as you can to deal with the torsional loads.

I can't help thinking though, interesting as the intellectual challenge of a craft with no component longer than 1.5m is, you may find that your assembly and disassembly time exceeds your sailing time each day. However as I have a very delicate 45 year old racing dinghy that seems to need more maintenance and repair time every year than she achieves time on the water, perhaps I should keep quiet on that score.

 

GGG,
nice work on a main made from 1.5M segments. I think any bermuda rig is going to gravitate toward this ~4M^2 main sail and it will help consider the forces from the rig.

About how to figure out sail area and righting moment -Dellenbaugh angle seems more suited to keel boats with their gradual rising stability curves. I think dingy designers need something else because almost all of them would be knocked down flat by 1lb/sf. What I was thinking is doing a moment diagram for the hull heeled to some angle and solving for the wind speed that would produce that heel angle tightly sheeted. This would give the wind speed at which the dingy goes from 'lift limited' to 'righting moment limited'. I think this would give a very sensible reading of how a dingy behaves. The downside I see is that since wind force rises as the velocity squared a low number would not be as alarming as it should be. Maybe there should be a similar calculation where the force of the sail luffing is applied to the same moment diagram -this would be the wind speed that would result in certain capsize and would carry the right cautionary message.

 

gggGuest, that's an interesting idea, but I don't believe I can make two masts with this kind of construction - there are too many reinforcements needed for mast step, and it interferes with the skin. But nevertheless, I will consider it. Thank you.

Your question on spars material - I thought the same aluminum tube, maybe 3cm in diameter. I already used 2cm diameter aluminum boom on 8m^2 gaff sail (boom was 3m long, if I recall correctly), and it holded even in strong winds. However, this time I don't want to risk, and long-life of the boat is a priority, so I'll go on the safe side, but I don't see why aluminum tube wouldn't work.

I don't know, but if I sacrifice some of sail area, won't simple Bermuda with jib work? I plotted it down. Attaching sketch I'm now working on.



This is still work-in-progress, but it could be the basis for my boat.
LOA is 4m, mast height is also 4m.
Mainsail is 5.15m^2 (including roach), jib is 2.35m^2. Total sail area is 7.5m^2.
Don't mind the reef profile. Still working on it.

So, how realistic this would be for a SOF boat with displacement of 180-200kg? Is it still in danger to collapse?

 

[actually written before previous was posted, but I went and had supper before pressing the go button]

I think that Bethwaite has done pretty much all that's required for sail area and righting moment calculation with dinghies - at least at the performance end of the spectrum where I operate. I guess the key thing is that with the quality of rig control available with modern gear and ideas there is an enormous capability to dump surplus heeling moment without compromising control too badly.

In the case of this boat though I think he'll well deserve self congratulation if he can power the boat up to the full potential of a reasonable size rig without structural issues.

Tell you one thing that occurs to me: I wonder if it would be possible to build in a Canoe style sliding seat, 1.5m long of course, and integrate a seat carriage, mast bulkhead and daggerboard all into one sort of slot together lump which would take all the primary loads, and thus separate that from the skin on frame construction? It would take some canny design to make it all work, and the number of largish pieces to carry around might be way beyond any kind of sanity...

Let me try and describe this, because I don't know I can sketch it in reasonable time...
You already have a centreline panel from the mast backwards which includes the daggerboard case. OK, lets have two more panels the same length. Let them meet the centreline panel at the keel, and each be angled outwards so that they meet the gunwhale at the sheerline. They are anchored onto the bulkheads at each end, just like the centreline panel, so this construction is going to be very rigid if you can get all the joins working well (big ask I admit).
At the rear of the daggerboard there's another transverse bulkhead (lets say leading edge of daggerboard is .5m behind the mast bulkhead, and daggerboard is .5 metre chord), and both this bulkhead and the end one have rails for a sliding seat (basically inverted L section. There's also, at the end of this intermediate bulkhead, optionally an anchor for a shroud. The sliding seat, also 1.5 m long, slides in the rails, and is limited to slide no farther than the centreline. Thus the effective beam to the end of the plank is equivalent to a 3m boat, and the whole plank/daggerboard/seat structure is integrated into a pretty rigid (if you can make the joints work well enough) box that takes all the torsional loads out of the skin on frame part of the boat. The optional shroud anchors can give the mast a bit more support. With this I reckon you could go back to a single sail, and probably get up to about the 6.5m^2 region, lets say with two more mast segments...
I might also give the, still basically free standing, mast a couple of shrouds from about two thirds up to the points mentioned, which would give extra support reaching square running where the load on the mast can be greatest. Maybe a forestay too, but not with any great tension in it.

Well its a nice mind game for me anyway, I think in practical terms the sheer complexity of the assembly would be daunting, not to mention that the 1.5m trunk of your vehicle is probably now piled 2m high with bits of disassembled boat which probably defeats the original point of the exercise...

[since previous post]
As for danger of collapse, well I have no experience at all with skin on frame boats and your structural techniques, so my uneducated guesses are probably no better than yours, and doubtless you will learn a lot more as you build your craft and take it for a paddle or two before actually putting any rig on! But jibs add structural complications and load paths that would concern me - more so than a second mast really I think. With a jib you need much tension, which is much load in the structure.

I guess my thinking on this interesting challenge is pushing me towards a craft that is more of a sailing canoe, albeit perhaps with a transom stern, whereas your drawings look a bit more like a baby yacht. I sail high performance sailing Canoes though as many will have guessed from the above, so maybe that's not surprising.

With your mast, I would definitely be keen on using nesting alloy sections so that as you go up the mast each one is smaller diameter than the one before. Not only does it make the joints easier, but I reckon it also gives a better result.

[later yet] attached sketch of my central box, maybe the colour helps make it understandable (or maybe not). Mast step is built into the pink bulkhead.

 

Have you considered an inflatable cat? Can be homemade from pvc, or you could buy one of the many commercially available ones. The setup time would probably be a lot quicker than slotting all those tubes together.
-Thomas

 

I have built a number of skin-on-frame sailing dingys, non were folding but the mast loads issues are the same. I have built a folding SOF kayak, and many non-folder too. Making the boat folding will make it a lot more work to build, and heavier, but it can be done certainly. I have drawings of a folding SOF sailing dingy that I want to build, but no time right now. Similar to your concept, one that could fit in the trunk of a car, be stored in a closet at home.

As far as rig goes; KEEP IT SIMPLE. It will save time and set up, and reduce the number of parts as much as possible.

that said, a cantilever mast is not a bad set up but as noted you will need more structure at the hull to transfer the mast loads to the hull. It also means a heavier and stronger mast, perhaps a heavy walled 75mm aluminum tube. I would use a deck piece between two frames that the mast goes through like a mast partner, down to the keel. This will spread the loads out over a longer length of the gunwales through two frames rather than one as you have.

About seams on the skin, do not be afraid of having them. the modern solvent types adhesive is very strong and will allow you more flexiblit in hull shapes. you lap glue them and than add a strip over the top to double glue all seams, no sewing required. Also consider that you will need a way to tension the skin both latterally and length wise in the hull. the skin can be laced on along the length of the frame to give you skin tension around the hull, but you will still want a way to put tension on it length wise. Some kind of bow peice in the frame that can be adjust outward to make the frame a few inches longer when you tighten it down should be all you need.

On the other hand a tri-stayed rig would allow a lighter mast and spread the loads out better on the frame, so the frame can be lighter as well. this means more parts and a longer set up time.

I would go with a fully battened bermuda sail, it will point the best and be easy to handle. A junk might also be a consideration, simple and light, but it does not point as well so you might not want to use a junk rig if where you sail requires a lot of beating into the wind.

most of your frame looks reasonable, adding a few diagonal members around the dagger board and mast area will stiffen things up nicely. For a dingy however you need to make the bottom flatter, the hull shape is too deep and too round.

You should probably find a dingy about the size you like and copy the hull lines and rig size, and than use that shape for your SOF hull. Any stitch and glue multi-chine plywood panel design hull can be adapted for skin-on-frame construction.

 

Um... I know this design, but seriously, on SOF? I'm not even sure I'll be able to sit on gunwale to hike without breaking something... I thought sliding seat is used when boat has surplus sail area, and needs extreme methods to balance it out - and in my case, other sailors here say that even the normal amount of sail for dinghy of such size would break SOF boat apart - so what's the point? And I can't imagine fitting such complex and heavy mechanism in delicate and fragile structure.
Thank you for your idea, but I don't see how and why I could implement this...

Well, you see, this is was the problem with my previous attempts to make something sailable. As I look back now, I see that I made many stupid mistakes, wasted time and money just because I didn't plan far enough, and later had to spend even more time and money by working in constrains of old errors, trying to fix them.
This is why this time I want to spend as much time as possible for preparations - getting educated enough to make a reasonably sound boat. I know it isn't a substitute for experience, but I want to make the best I can at the moment, so I'll have something I can practice sailing with, and take it from place to place without fuss.

Could you please elaborate more on that? How is my design like a baby yacht? Too deep and narrow hull?

Well, yeah. I even found some used ones to buy near me, at the similar price I'm dedicating for this SOF boat. I haven't dismissed the idea, but after some thinking,and talking with a friend of mine, I decided that catamaran isn't the best type of hull to start learning how to sail on. All the delicate things of keeping the boat at balance, tacking, recovering from capsize, are best learned on monohull, my friend said. It's like learning to drive car with manual gearbox before moving to auto - if you start in this order, you'll know both, but if you start with auto, you'll be lost with manual. Sounds reasonable to me.
There is another reason - I have yet to see a catamaran that could be folded and fitted in car trunk. See, I lost a car that had roof-topping capability, so only trunk now remains.

Petros, thank you for joining discussion. If it wouldn't be much to ask, could I please take a look at your drawings? It would be a valuable reference, since, as I mentioned, there is only one similar foldable SOF boat I could find.

That isn't a problem. I can get one, even larger diameter, if needed.

Exactly my thoughts! I was decided on lacing. Still looking for a simple method to tension lengthwise. All I found in kayak constructions is dreadfully complex to build.

So, what do you see as preference here? Stayed or not stayed? I'm kinda lost here - some people on this thread state that stays will collapse the boat, while others say that they'll help to distribute loads better, and, in fact, is preferable in SOF. So, what is the final word?

And, I presume, you suggest no jib, right?

 

here is the rig issues: stayed will have lighter frame and mast, but will have more parts to assemble and tension, and will have some limits on boom location and affecting sail shape going down wind. Unstayed rig (cantilevered mast) will have fewer parts to assemble but these parts will be heavier.

So if you want light, use stays and design for that load path. if you want faster set up and fewer parts, with weight be a secondary consideration, than design for a stayless cantilevered mast design. I have built both types, and I am now favoring simple unstayed mast only because set-up is faster and the boat is cleaner and less rigging in your way. Faster launch is one of the most desirable traits in any boat, and one you will have to assemble will already be more time consuming as it is. Keep assembly as simple as possible, keep the part count down and by any means possible try and keep loose small parts to a very minimum.

Only silly inexperienced people will claim stays will collapse a hull. You will collapse a hull if it is stayed or unstayed if it is not strong enough to carry the loads. It just means you have to design the hull for the loads on the stays, or for the cantilever loads if unstayed. It is all about designing a load path for each of the designs, and than designing each element in the design load path to carry the loads. If any component in the load path is too weak, like the weak link in a chain, the hull will fail. It does not matter that it is designed for stays or not.

No jib is again another issue you will have to decide. A jib is simple to rig and operate, but it does add parts and assembly time. No jib means you will get the boat assembled and be sailing sooner. Raise one sail and cleat it, and off you go, less rig, less lines on the boat, less things to get tangled and foul and less things to mess with while under way. OTOH, the jib greatly improves the boats ability to point into the wind. Since one of your stated goals is the ability to point high into the wind, than a jib is almost a necessity.

What I would do is deign and build it to perform without a jib, but be able to add one if you find the performance requires it. You could add one later, and than have the options of using it or not, as conditions require. It would not take up much room to keep it stowed if you do not need it.

I do not have any drawings or plans really, all of them were built off of sketches with overall dimensions only, and I calculated the size of the various members (based on the loads) and just made a materials list. Saved the time of making drawings, which I would need if I expected someone else to build it. I might have some pictures of the frames I can post. I will see what I have and post them later.