design a beach / lagoon proa

What does your design brief look like? What are your requirements for the boat? What do you want to use it for? What does your budget look like?
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jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

design a beach / lagoon proa

Post by jpn » 29 Dec 2015, 05:16

This is my first post. I am kinda nervous to stick my ignorant head into this , but....
I want to build a pacific proa, as small and cheap as is practical, mainly for fun and also to test a few old ideas.
It will be used on a lagoon which usually has strong winds in the summer, so I wont be pushing for maximum sail area.
I would like to build in the 16' - 18' range. I am light , 68 kg , but imagine the boat will often be crewed by two. 20' seems a better length, for speed , for payload, etc, but to keep costs and rig stress loading down I'll probably settle for the 18ft. Ply/epoxy.
The basic design will copy - as near as possible- Jzerro ... but will be simplified into multi-chine shapes, and won't have a cabin. I may add a rubber (Newick- Godiva) air float in place of the leepod.
I will follow the recent trend of the R2K and Jester 32 proas in making the ama slimmer in proportion than Jzerro's, although may hold back from going as far as these boats have gone. The Waka will be fatter in the ends, and follow Jzerro , rather than Mbuli /Jzero. I have an armchair sailor's hunch that Jzerro hits a sweet spot, and I am anyway very drawn to the idea that these boats are stabilised canoes (like a narrow dinghy with weight out on the trapeze).
I will also use the main and jib rig of Jzerro, and mount the mast more or less as Russell Brown has, on a beam between the akas, about a 1/4 of the overall width to windward. I plan to use a second hand Hobie Cat 14 main & jib, and mast. This should be enough power for the winds that we get.
BRUCE FOILS: The other changes from the Jzerro type will be in the addition, for experimental purposes, of two extra dagger board cases inboard of the rudder cases, just under - or slightly towards the bows of - the akas, but at 45 degrees off the vertical.
In the early 1970s I built a series of bruce foil atlantic proa models. In some conditions the foils actually slowed the boat, but in others they worked really well. I want to play with trimming them in fresh breeze to off-set some of the proas tendency to run nose down. This is sheltered water and I don't have to worry about extreme pitching. I hope to use symmetrical sections and just lift and lower, not flip, when shunting.
Last edited by jpn on 29 Dec 2015, 14:30, edited 5 times in total.
jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

Re: design a beach / lagoon proa

Post by jpn » 29 Dec 2015, 05:17

Rudders will be Newick/ Brown cassette type.
I see that there seems to be a trend (in the boats that Russell Brown is involved with) towards less buoyancy in the ama. I imagine that this may be to counter the effect of swells/waves/uprisings of water smashing into the topsides of the ama , and also of the buoyant ama kicking skyward as these waves/swells pass under. It makes sense to go through some of this water, as many of the island proas amas seem to do. But , to go back to my idea that these boats are stabilised canoes - the lurch back to windward , either in a lull or when the wave passes, may see the ama partly buried, and the akas dragging , at their ends , through water.
I have been looking at pictures of traditional proas with their leeward platforms, and wondering if they weren't balancing against the over-submersion of those narrow, low amas, presumably made of a heavy log. This would exert maximum righting moment as it left the water, when its full weight would suddenly come into play.
I want to build the ama with a hexagonal cross section, which may not be hydrodynamically perfect, but will allow me to have a section in the middle - the two foot or so that holds the dagger board case - that is built separately from the bow and stern and bolted in place. This will be to allow it to be rotated through 90 degrees for test purposes. The fail safe position would be with the dagger board vertical. The obvious test to perform will be with it rotated to form a Bruce Foil at 45 degrees, adding to the righting effort of the ama. However , I am most keen to use it turned outwards, as a small canted Foil lifting the ama slightly, aiding skimming, shock-absorbing as the ama drops back during lulls. Because I am only sailing up and down a lagoon for pleasure i can off-set the loss of righting moment by adding a bit of water ballast or sitting further to windward.
The other ideas I want to test are setting the jib permanently on a flying tack-pole, a bit like a spinnaker pole, that will be pulled around to windward on shunting and then hauled down and out to the new bow. This obviously would only work on a small boat.
Last edited by jpn on 30 Dec 2015, 15:49, edited 3 times in total.
jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

Re: design a beach / lagoon proa

Post by jpn » 29 Dec 2015, 05:50

It is when thinking about the amount of detailed work involved in a project like this that I realise that it is probably a pipe dream, that I should just build a metre long test model. The cassette rudders while simple in concept look very daunting to make. Without some carbon fibre engineering can they be strong enough to withstand side strain, and how will they be surfaced to enable easy sliding up and down?
I have drawn a kick up or rotating rudder, but it seems to offer even more work to build and iron out. This would revolve around a forward pivot, more or less - in side view - like a Finn dinghy centre board, but with shaped thickness and be divided to give a substantial trailing edge flap, the rudder. I am a very slow carpenter, and have little experience of carbon and epoxy and none of vacuum bagging. It is instructive to read Jak Mang's Proa 32 blog (http://proa32.blogspot.com) to understand the man hours involved in DIY of the many components .
Last edited by jpn on 29 Dec 2015, 07:11, edited 1 time in total.
jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

Re: design a beach / lagoon proa

Post by jpn » 29 Dec 2015, 05:56

The problem is that I am hooked. I have watched - over and over - every available clip of proa sailing. I can only imagine the intense engagement to be experienced in sailing fast to windward or close reaching on a design like Jzerro. It is not that I believe that these are the ultimate boats, but I am drawn to the way (I imagine) that they balance and respond to the natural forces. This has nothing to do with proving that one type is better or faster than another. My compromises have to be around issues of cost, ease of build, length of build, ease of launching and suitability for enjoyable sailing in fairly high winds on sheltered water.
Last edited by jpn on 29 Dec 2015, 07:14, edited 1 time in total.
jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

Re: design a beach / lagoon proa

Post by jpn » 29 Dec 2015, 06:23

There are many examples of Bruce foils , in trimaran and proa configuration. I am not trying to repeat the super-powered French flying foilers of Tabarly or the L'hydroptère. This is impossible, the forces involved are too extreme, the solutions too exotic, hi-tec and expensive.
I am trying to add a small component of lift to a fairly simple craft. The nearest example of canted foils to lee (although here used in an Atlantic rather than Pacific layout) in the youtube/proa file/proa forum range is the 1980 French Atlantic proa Epicemar (http://proafile.com/archive/article/bruce_was_right) . But compared to my old models Epicemar is underpowered, and narrow - it may be a practical and comfortable boat at sea, but the lack of length of the lee hull doesn't make sense to me. I am also not drawn to Atlantic layouts.
One mystery is the angle of attack of the canted (Bruce) dagger boards. Would leeway create enough attack to offset the proa's bow down attitude? Can they be set in practice to give a more or less self regulating ride? The hunch is to run with the front board right down and the back perhaps half up. On my models, when working properly the increased drive from the sail as the boat accelerated balanced the lift of the forward foil. When this wasn't set up right for wind speed, the boat did sometimes climb into an angle where the foil suddenly became less efficient and created lots of drag. But these were only my observations. I had no way of measuring what was happening, and the models were without radio control for steering or sheet adjustment . I have lost all my old photographs. It was interesting to me that in 1971 the great sailor Eric Tabarly spent quite some time watching my model and discussing it with me. He was in Cape Town preparing for the Cape to Rio race with the schooner Pen Duick 111. This was years before he developed his Bruce foiler trimaran. He spoke of seeing a hydrofoil tri in San Francisco that was probably the ladder foil Williwaw. He seemed to not be familiar with the foiler boats then fairly common in the pages of the AYRS journals. It was from these magazines , via a few Cape Town experimenters, John Goodwin ( http://www.oocities.org/aerohydro/image ... ohullB.jpg ) and David Buirski (http://www.ayrs.org/repository/AYRS074.all_A5.pdf -page 180) that I had access to these ideas. The great stimulus was of course Cheers and before that the many wonderful drawings and articles of J.S.Taylor.
Attachments
buirski 2.tiff
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Buirski outrigger.tiff
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buirski3.jpg
goodwin test.tiff
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Last edited by jpn on 02 Jan 2016, 18:45, edited 2 times in total.
jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

Re: design a beach / lagoon proa

Post by jpn » 29 Dec 2015, 07:31

Buirski's boats were outriggers, with a Bruce foil added to the single ama. He tested both dagger board and low aspect ratio foils, the latter with and without buoyancy. The most successful of the early AYRS foilers was probably that of George Dibbs, a trimaran with flotational low aspect ratio canted amas. It has recently resurfaced on the internet as '21 foot foiler' (
being rebuilt in the UK , although without the original AYRS/Bolger sail rig. Goodwin's starting point was Cheers /Atlantic Proa and the Aerohydrohull ideas of Smith (Mr Smith's Amazing Sailboats). His aim was to build a large cruising proa for a voyage to and in the West Indies. He later built a 20 ft approx. test model, and sailed it offshore here, even making a 100 mile coastal passage. This had two identical hulls ( the reference http://www.oocities.org/aerohydro/image ... ohullB.jpg shows the first built, windward hull) with the cabin and crew weight to windward and a canted foil under the ama and two jibs flown from the ends of a fore and aft yard atop a lee mounted mast. It suffered, I was told , from steering and tracking problems, and had two small rotating spade rudders in the windward hull.
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John
Posts: 142
Joined: 05 Aug 2015, 05:57

Re: design a beach / lagoon proa

Post by John » 29 Dec 2015, 19:41

jpn,

I think you will find this page by Russell Brown very interesting. http://www.ptwatercraft.com/Begining%20 ... %201-6.pdf

An interesting thing about chine boats is that each "plank" is a a project. The work is primarily in fitting the edges. You fit and remove each plank multiple times in the process. The more planks your boat has, the more work it is. This remains true until the planks become so narrow that you can bend the planks to aid fitting them, and then you are strip building...

I think the tortured plywood method has a lot of promise to build a hull quickly. I would start building 1/4 scale model hulls to figure out the process.

One of the design ratios on Jzerro that I personally believe is very important, and largely overlooked, is that the length of the mast is equal to the length of the boat. The length of a Hobie 14 mast is 6.78 m (22' 3"). I think your boat should be about that long.

It seems that you are building a day-sailer. If that is correct, then you get to skip building all of the live-aboard systems. That is a huge savings in time and effort.

A though that occurs to me is to build a simple ama with a simple dagger board and get out on the water. Then build the ama with the more complicated Bruce foil set up later. On a boat the size that you are considering the ama is not a very big project. And if you do not intend on making open water passages with this boat I think you can afford to not design for broaching waves crashing down on the ama.

I would consider designing the ama so that the entire crew weight can be supported by the ama. In the worst possible situation the entire crew is hiked out there to hold the boat down in gusting conditions, but you don't want the ama to sink between gusts and turn the boat into the wind.

I think that this is a very doable project.

Just out of curiosity, where is your lagoon?

-John-
Last edited by John on 29 Dec 2015, 19:51, edited 1 time in total.
John
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Joined: 05 Aug 2015, 05:57

Re: design a beach / lagoon proa

Post by John » 29 Dec 2015, 19:50

jpn,

Another thought for you is to attempt a planing proa.

Look up Skip's Nomad project and Rob's Sidecar project form more information. They may be able to help you with your design if you choose to go in that direction. (It may be interesting to you to know that Skip is a canoe designer of some note and decided to build his proa with a flat bottom.)

And when it comes to ease of construction, it is hard to beat a boat with square cross-section.

-John-
jpn
Posts: 54
Joined: 28 Dec 2015, 15:28

Re: design a beach / lagoon proa

Post by jpn » 30 Dec 2015, 04:41

Thank you, John: perhaps it is possible to build a flat bottomed narrow hull with sides that curve as they approach the bottom? I will have to make a model. One begins to see a very different boat, with straight pole aluminium akas, flat sheer, slab sided, flat bottomed. It is important for me to lessen the workload that I take on, but suddenly the object - if too compromised - lacks all magic, and one is less compelled to do any work.
The only experience I had with flat bottomed narrow hulls was with the same model proa in the 70s. I worked for a while off and on building test tank models of fast power boats, tunnel hulls, surface piercing tris, etc, for a local designer, Bob van Niekerk (http://www.bobkatboats.com/files/TheGen ... iekerk.pdf), and he persuaded me that my proa's hulls were far from optimum. To test his ideas I converted it into a cat. The proa's hulls were different, the main hull being of standard asymmetric micronesian pattern, the lee ama more like a double ended Tornado with semi circular underwater mid frame. I cut this ama in half, added flat bottomed sections to complete the length. At the place where the two shapes merged were small steps supposedly to allow air to be sucked into the layer of water next to the hull. This boat , with the rig and foils from my proa, was noticeably less successful. It had a resistance hump, and while it could be towed most impressively, when sailing it never seemed to get through this hump. It seemed to me, watching, that the theory did not take into account the actual flows of water moving past a real sailing hull , which moves not just straight but up and down and sideways, even on the relatively sheltered farm dam where we tested it.
You ask where this lagoon is, it is on the west coast of South Africa, about 150 miles from Cape Town. The weather report for the past month had daily wind speeds- without respite - of between 17 (rarely) and 25 knots , gusting on some days to 35. It is usually lighter in the early morning.
John
Posts: 142
Joined: 05 Aug 2015, 05:57

Re: design a beach / lagoon proa

Post by John » 30 Dec 2015, 06:36

jpn,

I think it would be possible to build a tortured plywood hull with a flatish bottom. I have thought about Jzerro-shaped hull with just a bit of a flat bottom to take the wear and tear of beaching. I think it would be possible to build that shape from three panels.

I think the (boat length):(mast height)=1 relationship is important for proas. Traditional sailboats have much more displacement at the ends than proas. Even catamarans sort of have twice the displacement at the ends. Both traditional boats and catamarans have comparatively wide sterns that resist being lifted. I suspect that putting too tall a rig on a proa drives the bow down more than folks expect. Jzerro handles this with a low rig and bows that sweep up. Cheers was effectively speed-limited by how much wind it could take before the bow went under. Sidecar has a flat bottom with some rocker that Rob calculates will lift the bow up when the boat is at speed. Given the rig you are contemplating, I think you should be looking at a 22-24 foot long hull.

Since you are on an inland body of water with great wind I would be tempted to try to make a really fast boat, and that would probably tip me into attempting a planing proa.

Is your lagoon Langebaan Lagoon?

-John-
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