Kayak Sail on Tinnie?

I am currently undertaking a University project in which we are trying to make boating propulsion for islander fisherman using small aluminium tinnies 3-5 metres more affordable and one idea I have had is fitting something resembling a large Kayak Sail near the bow of the boat. Assisting a battery powered outboard motor.

I was wondering is this feasible?
How big can the sail get before a keel/ centre board must be used? (Would preferably not have to implement this into the idea)
Would additional aluminium bars have to be welded onto the boat to support the mast?

Any advice would be greatly appreciated.

Cheers

 

This being a university project, I risk telling you things you already know, so I apologise if I do. Whether you need a keel or centreboard depends less on the size of the sail and more on what courses to the apparent wind the boat is supposed to sail. To sail any course other than straight downwind, you need lateral resistance. Both the performance of all the bits underwater and of all the bits above the waterline can be described by lift to drag ratios or the corresponding drag angles. The closest a boat can sail to the wind is the sum of the two drag angles. A keel or centreboard reduces the hydrodynamic drag angle by providing lift more efficiently than the hull and rudder alone.

There is a link to the size of the sail because the hull and (in a small boat) crew just have drag, so when the sail is small, what little lift it provides just isn't much compared to that drag.

If you know the lift and drag of your sail as a function of angle of attack, and the drag of hull and crew, and the lift to drag of hull alone and hull with a keel or centreboard, you can work out how close to the apparent wind your boat can sail and at what point an additional appendage gives you enough to be worth the bother. Given the electric outboard, I guess sailing very close to the wind is not necessary.

The maximum static load depends only on the stability of the boat. A smaller sail just reaches that maximum load in stronger wind. Note that you should think not only about lateral stability. Sailing downwind, longitudinal stability may determine the maximum load.

I experimented last year with putting a sail on and aluminium canoe, a hull not designed to take the load: Hinged Bruce foiler - Amateur Yacht Research Society https://www.ayrs.org/phpbb/viewtopic.php?f=19&p=4429&sid=b5e54d5344ead4f09799fb426be47823#p4429 I designed the frame to take all loads except hull buoyancy and hull drag. It is far too elaborate for your purpose, so I offer it only as an example of keeping loads out of the hull as much as possible.

I am trying something else this year, with the same hull and the same need to keep sailing loads out of it as much as possible, but the build is taking far longer than intended, so I don't have anything to show yet. I can tell you that I am following up on an experiment from 25 years ago, when I stabilised a canoe using Paul Ashford's anchor dog design for a paravane:



You can just see the bridle between the mast and the paravane to port. Here is a close-up of the starboard paravane:



And showing the shape more clearly:



I steered the boat between courses on the wind to a broad reach by having the paravane bridle attached forward of the sail's centre of pressure, and attaching an additional line to pull it aft. The further off the wind I wanted to sail, the further aft I pulled the paravane. However, I never managed to tack or gybe the boat that way, and retrieving the leeward paravane was a nuisance. I now have a hull with a rudder, and an idea for better retrieval. That is what I hope to test this year. This is all fairly low tech (plywood and broomsticks), and the largest loads either act over short leaver arms (bending in the foil) or are purely in tension (the bridle). The loads on the hull should be less than anything else providing the same power, except perhaps a kite, though that is only my guess. If you want to say anything about that in a project report, either come up with a relevant calculation, or do a finite element analysis of a suitable model boat.

Other examples of keeping most sailing loads out of the hull, also too elaborate to be practical for your purposes, but you could use them as references in your project report, would be Bernard Smith's monomaran designs (Mr Smith's Amazing Sailboats - Designs https://www.oocities.org/aerohydro/designframeset.htm) and the revolving catamaran (Revolving Catamaran https://www.revolvingcatamaran.com/). I think the people who are closest to doing what you want are the Open Canoe Sailing Group - for canoe sailors across the UK https://www.ocsg.org.uk/

Consider using a kite as a sail. If this is supposed to be primarily a downwind sail, a single line kite would do. If the boats don't need to change course much under sail, it should be possible to set up a single line kite with three bridle settings, one for straight downwind, and one each to make the kite pull a little either side of straight downwind. I expect there is a trade-off between stability and how far from straight downwind you can have the kite pull. And the kite would have to be retrieved to change the bridle setting. But if your fishermen travel for an hour or more in a straightish line and you can get the kite to be stable enough that it needs no further attention after launch, this might be worth the trade-off. If manouvres are more frequent, look into what is the simplest kite that is steerable, but ideally doesn't need constant attention to stay in the air. David Culp used to have a site with a lot of useful information, but I can't find it now, so it may no longer be maintained.

 

Most interesting sail on the canoe. The zig zagged line that secures the sail to the mast has some possibilities. If it was arranged to run freely at the mast, adjusting the downhaul part could change the camber of the sail.

The picture shows some very stiff battens. The sail might work better with softer battens?

 

It is a luff parrel. Its function is to allow adjustment of the distance between mast and luff, both so that the sail hangs from the halyard without kinks, and to get better balance downwind.

No, because the battens are as rigid as weight and money allow.

The perspective doesn't show the most characteristic sign of a junk rig, the sheeting. East Asian sailors developed an ingenious sheeting arrangement that connects a single sheet to most of the battens. It helps with reefing, and it reduces loads on the sail fabric. But that does mean that if the battens bent enough to affect camber, then camber would increase with wind pressure. That is why I implemented one of the ways in which to give a junk sail camber, which is sewing it into each individual panel. Further explanations, ranging from short to very detailed, can be found at The Junk Rig Association - HOME https://junkrigassociation.org/

But that does bring me back to the original topic in that Liam will also need to think about what sail to put on the tinnies. I think prime candidates would be, in order from the simplest to the most complicated build, the crab claw sail, the lug sail, the junk rig, and kites. The junk rig would only be worth the bother if the sail is large enough to be worth reefing.

 

Cheers Robert,

This is all extremely helpful
I have only just started researching and haven't come from any maritime back ground so I am learning on the fly.

Your design is amazing but i think it may be a little too complex but I will definetely keep it in mind. The kite is an option I hadn't considered but it may be the simple design I could be looking for.

I know that each boat is different but if I was looking into installing a centreboard on the tinnie would I have to worry about the extra lift making the boat prone to tipping and therefor have to make some sort of ballast? Or if the centreboard itself was made of solid steel could it perform both jobs?

 

Then I recommend looking into traditional boats. Millenia of informal R&D have gone into creating economical craft with materials that have only modest mechanical properties. Rigs like those of modern windsurfers get their shape and adaptation to wind strength from the mechanical properties of expensive materials and sophisticated production methods, which translates into not economical by the standards you probably must apply. Also, if you think of the whole life cycle, fibre-reinforced plastics are a pain in the backside when it comes to recycling. Untreated wood can decay or be burned, aluminium can be recycled, so I would focus on those. Ropes and sail fabric will likely be some variety of plastic. That is difficult to avoid. But if you want easy fabrication with local methods and materials, you need to look at rigs with modest mechanical stresses. Traditional rigs had to limit those stresses, because the available materials could not take much. Therefore those would be a good starting point. And there have been studies of such rigs. One secondary report is in Practical Boat Owner #264, December 1988, pages 52 - 55. I know there was another report, possibly UN-funded, on giving sail-powered artisanal fishing boats more stability. The best method they found was a double Bruce foiler. I can't find the reference for that now, I might have seen a secondary report in Practical Boat Owner. The boat resembled this one, only still lower tech, with the foils just being flat plywood sheets.



See also Ngalawa - Wikipedia https://en.wikipedia.org/wiki/Ngalawa for a very low tech version from East Africa.

It may well be too complex. Then again, if you put a conventional rig on a tinnie hull, you will need a rudder, because the outboard leg will not do the job. The paravane can steer the boat. An easily lowered and started electric outboard could power and steer a rudderless paravane-stabilised boat through a tack. And there may be a way to gybe without a rudder. If I get on the water before the sailing season is over, I will try. So the rig could have exactly the same complexity as the auxiliary you were proposing, the paravanes would replace the rudder and possibly a keel, and the load on the hull structure might be less. So maybe the complexity of the paravane-stabilised boat is more visible, but not necessarily greater.

Hobbyists have made their own ram-air traction kites on sewing machines. Not having tried myself, I don't know how much knowledge and skill is needed. Certainly more than making a crab claw sail, which can get a good shape from a flat sheet of fabric, as you can see here:





If a kite with an inflatable spar is needed, then the airtight bladder/inner tube may add more complexity to manufacturing, and I have no idea how much. How important is it to your project that things can be made locally? How long do the bladders last, especially when exposed to possibly tropical sun?

You need lateral resistance for courses other than straight downwind. I guess that a tinnie hull with a rudder and a crab claw rig in the forward half of the hull should be good enough for about 30 degrees either side of downwind. If you place the rig well aft, so that most of the lateral load is on the rudder, and you make the rudder large enough and give it good balance, then you should be able to sail a lot closer to the wind. See the interview with Matt Layden in Small Craft Advisor #57, pages 17-22.

Most arrangements of foils in air and water create a heeling moment, which you need to oppose with a righting moment. Whether you need ballast for that depends partly on the size of the boat and the skill of the crew. Alternative ways of getting righting moment would be the Bruce foiler shown in the first picture, the paravanes in my last post, or not generating a heeling moment in the first place, as is possible with kites. I forgot the most interesting configuration, the Windfly Rig. It's inventor linked to a now defunct web site on this short thread on The WindFly Rig - Fast and stable kite boats - no heel, no leeway - Amateur Yacht Research Society https://www.ayrs.org/phpbb/viewtopic.php?t=2284, but the pdf to which he linked is still accessible. The part most relevant to you is that the whole rig clamps to the transom in place of an outboard, so there would be absolutely no modification to the hull of the tinnie. The drawbacks are that the rig seems to needs a traction kite with an inflatable spar so that water launch is possible, and probably it needs low friction blocks. And the web site being defunct may indicate that the inventor failed to find investors, so the rig would not be readily available. Is the availability of specific parts, rather than generic types of parts, a criterion in your project?

I looked up tinnies, and found one with a plate thickness of 1.6mm, about the same as my aluminium canoe. That is about half the thickness that I have seen described as the minimum for welding, though a video of manufacturing an aluminium canoe does show some welding. So you will need to check under what circumstances and with which techniques it is possible to weld aluminium as thin as that in a tinnie. Riveting may be safer. But if you can't weld, then a centreboard or daggerboard is out, you will need to fit leeboards. Look up Open Canoe Sailing Group - for canoe sailors across the UK https://www.ocsg.org.uk/ for examples.

Yes, if you can provide a structure that holds it in place, if that fixed keel doesn't interfere too much with pulling the tinnies out of the water at the end of the working day or if leaving them in the water is a viable alternative (needs more infrastructure), and if you can find a way to weld steel to aluminium, and if you can avoid galvanic corrosion between steel and aluminium.

 

Hi Liam, I think the easiest, cheapest low tech, easily repairable system for a tinny would be a single wood leeboard on a tether, swapped side to side as needed when tacking, and a balanced lug sail rig. The leeboard can double as a cutting board or seat when at anchor fishing, and a balanced lug mast can simply be dropped into a hole in a forward seat, with the mast foot located in a wood or metal block or cup.
The mast, yard, and boom doesn't need to be big, and so could be stowed in or on the hull when not needed, and the sail used for shade while fishing. Also, most tinnys I know have holes in the gunnels for rowlocks, and rowing to assist any electric outboard motor would be great and simple assistance. Rowing while sailing in light winds also makes a big difference to speed. I recently made a balanced lug sail from bed sheet material, which was then painted with cheap watered down acrylic paint, and it works very well. A rudder will be needed, unless an oar can double as a steering oar at the stern. All of the above can be sourced or fabricated easily almost anywhere, and being low tech, can be repaired and/or modified by anyone of moderate skills. The aluminium tinny and electric OB would be the most technical part of the system.

 

Comments below based on a "tinny" "tinnie" made with thin sheeting and riveted.

Some quick comments on welding aluminum
1) While welding thin aluminum is possible, you can expect a lot of distortion in the weld area. 1.6 mm is extremely thin.
2) Welding in a stiffener or bracket to support a mast base will more than likely introduce a ridgid (bracket) to sheet (hull) area that may create high stress concentrated areas which with flexing can crack over time
3) Many "tinnies" that people brought for us to repair was for leaks around the rivets. On our first, out of lack of knowledge, we tried to weld (tig) the interior stringer to the sheet (hull) and
discovered that the manufacturer had installed a mastic ( tape) between the stringer flange and the hull. This created quite a mess as the heat melted the mastic making a weld area
that could not be welded.

If you decide to attach mast supports, there may be, depending on the shape of the stringers, a chance that you could, with an extremely careful welding process, weld to the top of the stringer
in short welds, followed by cooling the area immediately so as to mitigate the mastic from melting and losing the watertightness.
And of course I expect that the gunwale will be riveted down the length of the boat which could provide support for mast stays etc

I adapted a front mounted mast base to an 18.5 foot composite Sea Clipper canoe that we used on long paddle trips. The base was right at the intersection of the two gunwale extrusions
at the bow. I riveted part of the bracket to the extrusions, built some exterior " backing washers" and added in some SS bolts to the bottom of the bracket.
Into this bracket which mainly was a round receptable for the base of the mast, we would slip the extendable mast (similar to a tent expanding pole) . The sail was an inverted triangle
with a pocket sewn in which accepted the top of the mast. The inverted V lowered the center of thrust to minimize roll.From each side of the lower triangle we had two ropes attached. The effect was similar then to a spinnaker though not bulbous
My wife would just hold the two ropes at gunwale height, and would be able to adjust the shape of the sail by varying the length of the ropes and this allowed her to release the ropes if
we caught a gust of wind. The canoe was tippy so an immediate release of sail thrust was imperative.
It worked well with a following wind.

 

Cheers everyone for the wealth of knowledge!

Got a lot of thinking to do but I am extremely grateful for all this help. Thank you again!
I'll let you know what I eventually decide on.

 

There are a few similar projects going on, and some sort of collaboration would be excellent for all involved. @rob denney and associates are helping Fijians shake off the petroleum yoke by combining traditional vessel styles with modern knowledge and materials. Tinnies really need petrol engines to be useful as they have a lot of drag, as anyone who has rowed one can attest to, and a very shoddy planing hull form that keeps crew alert with regular salt showers. I feel that they will only be viable as a stop gap solution due to Islanders having them already, so are they worthy of a lot of attention? This is Rob Denny's website. Cargo Proa - Fiji — Harryproa https://www.harryproa.com/blog/cargo-proa-building-blog