Bolger Advanced Sharpie Proa, now with Videos.
http://www.junkrigassociation.org/slieve
Thanks Robert!
Now i have even more to read and think about. This seems like a never-ending adventure of discovery and development. The split junk looks like a really good idea. It seems a little complicated to build and rig, but probably very easy to use.
With best regards.
Johannes.
I’m really enjoying this thread. One thing that seems to be preoccupying your thoughts is the crossflow concept. I think sharpies and the AS sharpies are going to be tolerant of variations, even significant ones to the “ideal” sea of peas theory. What isn’t quite so clear is what will happen to your full sized craft heading upwind if the boat is more of a piercer than a duck. I have been in kayaks, canoes, and multihulls that subscribed to the piercer theory and it does limit the pitch substantially. Lots of rocke OTOH generates more pitch. One video that underscored this was the EPic kayak video that compared their piercer design to a duck type of significant rocker (pretty much all the English style kayaks). If the boat does stay with the duck mentality of bobbing over the waves, it’s going to take lots of volume (which the pram will give you) out at the ends. The question I have is, will it be enough to change what is typically a piercer type design into one that will ride over the waves rather than slice through them?
Dan
One thing that seems to be preoccupying your thoughts is the crossflow concept. I think sharpies and the AS sharpies are going to be tolerant of variations, even significant ones to the “ideal” sea of peas theory.
Yes, i like the “Sea of Peas” theory. It makes a lot of sense.
I might have to try a deviation from that, just to compare side by side.
I know the proa is a natural wave-piercer and as such, should not ride the waves like a duck. I hope there is a good balance between the two different paths. I like it to ride like a duck, when it comes to large ocean waves, but punch right through the shorter and steeper waves along the coast or in the larger lakes in sweden.
Its a delicate balance, and there seems to be many different ways to go. I do like the performance of my model. So far it has proven to be a nice balance between the different aspects of bluewater cruising. It carries a lot of weight, its quite fast, and behaves like i expected it to do.
I believe an strict adherence to the “sea of peas” theory is a simple way to get good performance from a very simple (and simple to build) hull. Any deviance from the theory has to be tested with several models to optimize performance.
Wheight and Displacement: Don’t go with your superheavy structure and keep the boat lightly built.
My experience shows me that the AS-proa sails much better when loaded down a bit. It needs some weight to sail properly. I have filled the vaka with about one liter of water (equals 1000 kg in the full sized vaka), and it sailed better than empty. It points higher and pitch less. I think 4000 kg is not too heavy for a 46 foot proa.
I will have to test my new model before i can make any more conclusions.
I really appreciate all your comment and thoughts about this. I have learned a lot from this thread already - thanks to all your posts.Thanks!!!
Please keep posting any and all your thoughts and ideas on this subject.
With best regards!
Johannes.
@ Dstgean:
I fount the video i think you were refering to.
Epic Kayaks Rough Water Comparison
A very interesting video. I think a low rocker is a great way to reduce pitching, but i dont think its the only way.
I took a screen-dump from my videos of the proa-model.
One can clearly see some pitching motion. The forward bow is a couple of inches above the surface. I dont think there is many boats that would behave very differently under the same circumstances. I can see Jzerro pitch on youtube. I dont mean to criticize Jzerro, only point out that symmetrical hulls will pitch given large enough waves of the right frequency.
Regards.
Johannes.
Here are some pictures i found that i think explains my view on the hydrodynamic forces around a hull as it is moving through the water. Its a “normal” hull compared with a Fredrik Ljungström Arch Bow Hull.
I mentioned that there is only three discontinuities in the flow around the Arch-hull, but 5 or more in a “normal” hull.
This is why i want to use circular sections, an arch bow hull kind of sharpie. That explains my “obsession” with the Sea of Peas theory and the Advanced Sharpie. I will probably build a small model that doesnt adhere to this theories. I want to compare them side by side.
Your rockerline seems to be a part of a circle, thats the wrong line, because it does not support a stable fore and aft trim. Flat it out in the middle a little until you get a soft curve and add a part of a circle at both bows to bring the two forefoots near the waterline.
Im afraid that will make the proa slower. I dont like the discontinuities in the flow, as explained above and by the pictures. Maybe its more important with less rocker and a flatter run amidships, than the more even distribution of the hydrodynamic forces along the hull. Im afraid this will turn into a very extensive test of many different aspects of multihulls. I could probably spend the next ten years or so, just testing and learning. I would probably be hired by some Volvo Ocean Race team to design their boats somewhere along that timeline, but i only want to go sailing. As fast as possible, as cheap as possible and as soon as possible.
In the last weeks I had a private chat about cruising proas and I made some rough design studies for a (2 - 4 people) offshore cruising proa. I calculated a needed displacement of 4000 kg. 1000 to 1500 kg of that displacement is payload, so the boat should not be heavier than 2500 - 3000 kg. I managed it to put everything together in a 12,8m long vaka ( 42’ ). If you are interested I can spoil your thread and post a basic linesplan of the vaka here.
Yes please do! I would love to see those plans!
I recommend arranging to sail on a Wharram, and to try reefing. It would be ideal if you could arrange that both when mast and sail are dry and when they are wet.
I think that is a very good idea, but Sweden is a very harsh and cold country. Very hostile towards multihulls. I have only seen one Wharram catamaran in Sweden during the last 10 years. I know there are probably more of them, but they are not very common around here. As usual. I will have to test with a model first.
Johannes.
The original Freedom 40 had a wrap around sail, I hear it did have problems lowering when wet. (they are now on a track) Wharram soft wing should not suffer so much due to the short gaff. When the gaff halyard is realeased the tension is also released over the whole sail. I saw a good video of a Tiki 26 on a braod reach going, say 12kts, dropping the sail with ease.
I suggest still look further into junk derivatives. One advantage on a proa is a stay, fitted to windward does not foul the yard . The disadvantage is even more string, though it all swings away from the cockpit (rather than over the top, as with a mono, probably the worse aspect of a junk). Robert made a very interesting junk wing sail for the 16 ft proa, (later passed to me without this sail), It is a concept I would like to develop further as it gives a wing without any moving parts.
Regards
Mark
I mentioned that there is only three discontinuities in the flow around the Arch-hull, but 5 or more in a “normal” hull.
This is why i want to use circular sections, an arch bow hull kind of sharpie. I will probably build a small model that doesnt adhere to this theories. I want to compare them side by side.Im afraid that will make the proa slower. I dont like the discontinuities in the flow, as explained above and by the pictures. Maybe its more important with less rocker and a flatter run amidships, than the more even distribution of the hydrodynamic forces along the hull. Im afraid this will turn into a very extensive test of many different aspects of multihulls. I could probably spend the next ten years or so, just testing and learning.
, ...but i only want to go sailing. As fast as possible, as cheap as possible and as soon as possible.
Offshore cruising proa. I calculated a needed displacement of 4000 kg. 1000 to 1500 kg of that displacement is payload, so the boat should not be heavier than 2500 - 3000 kg. I managed it to put everything together in a 12,8m long vaka ( 42’ ). If you are interested I can spoil your thread and post a basic linesplan of the vaka here.
Yes please do! I would love to see those plans!
Hello Johannes,
I agree more with your arguments than you think. I like the arch hull aspect especially in a V-hulled vaka, but rockerline on a flat bottom is a different thing. But take a look at my vaka and you will see that it is not so dramatically as you think now.
By the way, I mentioned “Mbuli” as a an example for the “perfect” flat bottom vaka, I measured 21cm of rocker on a bottom 610cm long. Mbuli has a BWL/LWL Ratio of around 10:1 and her bows are 8cm below surface at (theoretically) full displacement with the ama flying.
OK here ist my Vaka. As said before its 12,8m long, 1,8m wide and has standing headroom (1,8 - 2,0m) in the 4.6m long middle section. BWL is 1,2m , Ratio is nearly 11:1. The Vaka is shown with 3850 kg of displacement
Accomodation:
Fore and aft compartments are 3,1m long and have bunks 1,3m wide at the shoulders, a seating / reading location and 1.7 headroom, 0.7m over the bunks.
The main middle compartment is 4,6m long and this is the living room. The closed shower / toilet cabin (0.8m) is located here, followed by the pantry (1,5m) and the nav station on the other side of the hull. On the “aft” end ist a dinette (2,0m) in L-shaped form which can be transformed into a kingsize double berth (2 x 1.5m). On the imaginary Leepod is a bunk for the “stand by watch” and room for a lot of light stores
I hope you can imagine the leepod and the cockpit. Unfortunatly I cannot show it due to to the limits of freeship, which is a programm for the devellopment and calculation of symetrical hulls. The Leepod will have the nice shape of the Russel Brown proas or “Madness” ... in style of Dick Newick.
Plywood will be Okume / Gabun Marine Ply in the following dimensions: Bottom 15mm, Sideplanks 12mm and Decks 10mm. All covered with 200gr/m² (6oz) glass on the outside.
I appreciate the ease of construction and hobby-horsing-dampening qualities of a 3-sided hull, but I have a concern: those hard corners may contribute to instability during shunts, potentially leading to back winding.
In my understanding, a proa is stable during shunts because there is relatively greater windage and relatively less hydrodynamic drag on the leeward hull. It helps that the rig is generally in the downwind hull, for example. (Was Cheers unstable during shunts???) But if the main hull has sharp edges as in an advanced sharpie, this may make the proa “unhappy” to have the main hull to leeward. This can be partially aided by a windward daggerboard as in Madness for example, but I’m wondering if it would be too little in this case.
Is this concern well-founded?
Thanks Luckystrike118!
That looks like a real nice cruising-proa! Lots of usable space and fairly simple construction. I hope it gets built.
but I have a concern: those hard corners may contribute to instability during shunts, potentially leading to back winding.
I dont think it should be any more problem than a deep-V Vaka or a Vaka with deep rudders/daggerboards down.
An advanced sharpie hull is very directionaly stable so it wont turn very easy. It actually quite hard to pull the boat sideways or turn it on a spot, when im sailing my smal model. It makes a LOT of lateral resistans. Im more worried that it will be too hard to turn it into or out of a shunt, even though i dont have to “fight” the wind as one does on a tacking mono. I believe i need very strong and powerful rudders to control it. All that rocker it has, is probably needed to make it a little more easy to steer. I probably need to round the chines a little, out toward the ends of the hull to make it turn more easily.
Johannes.
Thanks Luckystrike118!
That looks like a real nice cruising-proa! Lots of usable space and fairly simple construction. I hope it gets built.
Hi Johannes,
it won’t be built ... not by me. Its just a rough dash in freeship of my thoughts for a offshore cruising proa. I wanted to see how big a proa must be to have enough interior space and payload for that purpose. 40 feet is for my feelings the maximum of what a two man crew can handle at sea without problems and what can be done (and paid for) by a normal skilled and situated home builder. Iam happy that it can be done with just stretching this limit a little bit..
Possibly my backpackers cruising proa will be built after I have finished my trimaran. The tri will be berthed behind a lock and that needs to much time to bring the boat out für an afterwork evening sail. So for my sitation it would be nice to have a small boat lying direct at the sea where the beachcats have there place. The backpacker or a Formula 16 Cat are two possible options for that.
Don’t waste any thoughts at shunting behavior. Any pacific proa will do it without problems, no matter the rocker or bulkhead shape.
If you want to speed up your modelmaking phase, use styrofoam and a hot wire for cutting it. Together with a sharp knife and a piece of finishing paper you ill be able to build new hull in one day. Think that if you have a satisfying sharpie hull you have to do the design your vaka interio, the ama, the leepod, cockpit and the rig. This must all be done before you start the actual hull.
Best Regards, Michel
I have been on a short vacation, and we visited The Viking-ship museum at Roskilde in Denmark. Nice place. I like the long slender viking-ships. The remind me a lot of Wharram Catamarans even though they predates them by 1000 years or so. We sailed one smaller ship, that was around 33 feet long and weighted empty about 1400 pounds (700 kgs). We did not have any wind that day, but they told us they have been sailing at sustained 14 knots with it. All with a handwoven wool-sail.
The first picture is me and my kids in the Sea Stallion from Glendalough
http://www.vikingeskibsmuseet.dk/en/boatyard/the-boatyards-building-list/boat-description/boat/havhingsten-fra-glendalough/?tx_ttboats[backPid]=1301&cHash=39d780f63d514a46e59badace83ad8a6
The next picture is my Wharram Tiki like ama.
Im starting up my 2:1 test of the Advanced Sharpie Proa again. I will post pictures and updates as it proceeds.
Johannes.
All covered with 200gr/m² (6oz) glass on the outside.
I currently have a Matt Layden designed Paradox, and the builder covered the edges where two panels join with light fibreglass. If I correctly remember the weights of the stuff I have used, it probably is 200gr/m². This has a tendency to crack. The wood seems to move more than the glass and epoxy.
There are two solutions I have know of. The one I used was to redo the joints with heavy biaxial glass (600gr/m²) covered with 200gr/m² cloth.
The alternative is to use cloth and epoxy that stretch more before breaking. For cloth, Xynole is suitably stretchy. Sven Yrvind sourced a quite elastic epoxy from a Swedish supplier, I think it’s this one: http://www.batepoxi.se/.
Whenever joining two materials, think about how much each of them will move either under load or when temperature and humidity change.
Regards
Robert Biegler
Johannes,
To back up a bit to the junk rig, I just uploaded some new video taken from inside the boat after my sail modifications. it’s not the split rig, but it will give you some idea of the functionality of the rig.
http://www.youtube.com/watch?v=pzaFUShASAE&feature=plcp
Tom
Thanks Tom! Very nice sailing. The junk-sail looks very easy to handle.
I have been rethinking my ama. Trying to understand what is needed and wanted from an ama.
I remember Russell Brown writing something about waves slamming into the ama, which transfered the load into the akas witch would flex and vibrate in a very disturbing way. It sounds like there is a lot of energy being absorbed by the aka and the connections. I think that is something not good, as the proa is all about low stress, low impact, “spiraling down the path of least resistance” kind of sailing. Looking at how the Micronesians and Polynesians do it, i think the ama should be different.
# 1 Low resistance going through the water, partly submerged or fully submerged. (wave-piercing shape)
# 2 Low windage. (both ama and aka) Very important in a severe storm, with the ama flooded.
# 3 Very little flat area sideways, to elude what i cited Russell Brown about above. An oncoming wave should more or less only lift the ama slightly and pass with very little impact. The vaka should be the dominant hull in everything except righting moment.
# 4 Simple to build out of cheap materials (plywood). This is in a square-law relationship with the probability of it being built. Very important to me!
# 5 Strong and suitable connections between the ama and aka (and vaka). They should be easy to inspect, easy to maintain and repair anywhere in the world.
The result is a square section with pointy ends. Kind of a advanced sharpie hull standing on one of its corners.
Since i want a wave-piercing shape and a very low windage/low resistance shape, it will be sharp in the fore and aft. No blunt ends. Seen from the side it will follow an arc. It will be symmetric in every way.
It will have to be long to have sufficient buoyancy. I’m thinking something like 85 - 90 % length of the vaka.
Its going to have a very low PC to give very little resistance at low speeds. At higher speeds it will partly lift out of the water. I might have to build something like a ridge on top of the ends, to part the water more efficiently if its pushed down under the surface. I’m building a 16 feet long and 1 x 1 feet wide and high model to try it out. My living-room is filled with plywood and polyurethane-glue everywhere. I’m lucky my girlfriend will not be home for another couple of days. It looks like 1000 dollars worth of divorce…
Regards.
Johannes.
Sven Yrvind sourced a quite elastic epoxy from a Swedish supplier, I think it’s this one
Its from Nils Malmgren. Its the place i always buy epoxy from.
They are were knowledgeable and helpful.
Nils Malmgren