Bolger Advanced Sharpie Proa, now with Videos.
Hi Johannes,
I have played with freeship, added your data and made some calculations. Unfortunally freeship cannot make asymetric hulls, but a few variations showed, that there are nor significant changes in surface areas and volume if your hull is asymetric or symetric. So my symetric model is working, even if your hull is asymetric.
The good news is, that you can live with your overkill scantlings (for the case you ahve already bought the plywood). But it is really hard on the edge. Loaded down to 25cm draft your vaka has a displ. of ~605 kg. Stepping drafts down the displacements are: 22,5cm = 535kg, 20cm = 460kg, 17,5cm = 390kg, 15cm = 320kg. Displacements are for saltwater.
The bad news is: you cannot achieve your desired whieght of 125kg for the vaka. Alone your plywood for deck bottom and sides have a surface of 13,2m² and a wheight of 75kg ... without bulkheads, glue, epoxy sealing, reinforcements, stringers, paint etc. (I calculated 700kg/m³ for the heavy pine plywood which is a realistic wheight for it). See the details in the Data sheet attached and my calculations for your proa at the bottom.
bolgerproa72.fbm
Vorgegebene Länge : 7.200 m
Länge über alles : 7.200 m
Vorgegebene Breite : 0.600 m
Breite über alles : 0.598 m
Vorgegebener Tiefgang : 0.250 m
Schiffsmitte : 3.600 m
Wasserdichte : 1.025 t/m^3
Volumeneigenschaften:
Verdrängtes Volumen : 0.591 m^3
Verdrängung : 0.606 tonnes
Gesamtlänge des eingetauchten Körpers : 7.200 m
Gesmatbreite des eingetauchten Körpers : 0.598 m
Block Koeffizient : 0.5498
Prismatischer Koeffizient : 0.5498
Vertikaler Prismatischer Koeffizient : 0.8277
Benätzte Oberfläche : 5.398 m^2
Auftriebsschwerpunkt im Längsschnitt : 3.600 m
Auftriebsschwerpunkt im Längsschnitt : 0.000 %
Tranverse center of buoyancy : 0.000 m
Vertikaler Auftriebsschwerpunkt : 0.141 m
Eigenschaften des Mittschiffs:
Fläche der Mittschiffsebene : 0.149 m^2
Koeffizienten des Mittschiffs : 1.0000
Eigenschaften der benätzten Fläche:
Länge der Wasserlinie : 7.200 m
Breite an der Wasserlinien : 0.598 m
Benätzte Fläche : 2.858 m^2
Koeffizient des benätzten Fläche : 0.6642
Auftriebsschwerpunkt : 3.600 m
Eintrittswinkel : 8.689 degr
Transverser “Inertia”-Schwerpunkt : 0.058 m^4
Inertia-Schwerpunkt im Längsschnitt : 7.402 m^4
Anfangsstabilität:
Transverse Metazentrische Höhe : 0.239 m
Tranverse metacentric radius : 0.098 m
Metazentrische Höhe im Längsschnitt : 12.655 m
Longitudinal metacentric radius : 12.514 m
Lateraloberfläche:
Lateralfläche : 1.265 m^2
Gewichtsschwerpunkt im Längsschnitt : 3.600 m
Vertikaler Gewichtsschwerpunkt : 0.148 m
Hull characteristics above waterline:
Lateral wind area : 2.446 m^2
Z coordinate of wind area CoG : 0.420 m
X coordinate of wind area CoG : 3.600 m
Z coordinate of wind area CoG above DWL : 0.170 m
Distance from bow to wind area CoG : 3.600 m
Die folgenden Eigenschaften wurden für beide Rumpfhälften kalkuliert:
+————————————-+————-+—————-+—————+————-+————-+————-+
| Ebene | Fläche | Dicke | Gewicht (Masse) | COG X | COG Y | COG Z |
| | m^2 | mm | tonnes | m | m | m |
+————————————-+————-+—————-+—————+————-+————-+————-+
| sides | 7.459 | 7.000 | 0.037 | 3.600 | 0.000 | 0.328 |
| bottom | 2.862 | 12.000 | 0.024 | 3.600 | 0.000 | 0.043 |
| deck | 2.857 | 7.000 | 0.014 | 3.600 | 0.000 | 0.590 |
+————————————-+————-+—————-+—————+————-+————-+————-+
Gesamt 13.178 0.075 3.600 0.000 0.285
Spantenflächen:
| Stelle | Fläche |
| m | m^2 |
+—————-+—————+
| 0.000 | 0.000 |
| 0.800 | 0.027 |
| 1.600 | 0.078 |
| 2.400 | 0.119 |
| 3.200 | 0.145 |
| 4.000 | 0.145 |
| 4.800 | 0.119 |
| 5.600 | 0.078 |
| 6.400 | 0.027 |
| 7.200 | 0.000 |
+—————-+—————+
605 kg total displacement sounds a lot in the first thinking, but it is’nt. You calculated 280 kg crewweight/payload, but you have not counted that the crew needs personal gear like clothes or safety equipment, survival water and food, lifejackets, etc. Further your proa is to big to be paddled, so you need a small outboard motor and some fuel. These are additional 20kg min. My advise would be: Count around 320kg for a realistic payload.
That means your AS-Proa is allowed to wheigh in at max. 280kg and this is very hard to achieve with your planned scantlings. 125kg for your vaka is not realistic, expect to have 150 to 180kg. You Ama will be between 40 to 50 kg, together 190 to 230kg.
More to come in part two..
Part Two….
So, together your Vaka and Ama will have 190 to 230kg, that leaves 90 to 50 kg for all the Akas (Beams), rudders, tillers, the whole rig and sail, trampolin, sheets and all the bits and pieces not thinking of yet. Please be honest to yourself, is this realistic?
Furthermore design advises,
- your freebord is way to low, 35cm is really not enough for a 7,5m Proa. You should have at least 10cm more and even this normally is not enough. I know you want to cut the sides out of a half plywood sheet, cut lenghwise. Plan to buy one more sheet of ply and three scarfs per side, then you can make it.
Even with the raise of the freeboard, make your trampoline higher with struts so that you have at least 60 cm air under your beams and trampoline.
- a 4mm ply deck needs very tight stringers and deck beams to be stable enough to walk on. Use 6 or 7mm instead. Not much wheight penalty and a faster work. (I calculateted the Vaka-deck with your 7mm plywood.)
Areas where you can save a lot of wheight!
- Your covering 300gr/m² glass laminate is far to heavy, use 165gr/m² (or max 200gr) instead.
- the bottom can be made out of 7 to 10mm ply without beeing fragile. If you use 7mm make two layers of 165gr/m² glass (one inside and one outside for maximum stability).
-the sides can be made out of 6mm plywood and one layer of 165gr/m² glass outside without beeing fragile.
- cut lightening holes in every bulkhead (if not sealed for watertight appartments, bulkheads every ~1m, depending on stress points (beams, rudders etc.)
- make bulkheads out of 6 or 7mm ply.
-you will need one , max two pairs of stringers made from 15x25mm nordic fir for your vaka, glued with the small face to the sides. One stringer in the bottom if you plan no rub strips on the bottoms outside.
So, thats it for today. I wish you the best for your boat.
Best Regards, Michel
Attachment: linesplan
I have played with freeship…
The good news is, that you can live with your overkill scantlings (for the case you ahve already bought the plywood). But it is really hard on the edge. Loaded down to 25cm draft your vaka has a displ. of ~605 kg. Stepping drafts down the displacements are: 22,5cm = 535kg, 20cm = 460kg, 17,5cm = 390kg, 15cm = 320kg. Displacements are for saltwater.
Thank you for taking your time and helping me with FreeShip!!! Its very much appreciated!!
I don´t think my scantlings is that much overkill. I have tried to explain that i´m more interested in a rugged cruising Proa that can take the kind of abuse normal cruising will expose it to, than a fast racer. My previous sailboat was a Swedish Amigo 23. A heavy, fat, long-keeled 23 feet sailboat. 23 m2 sailarea, 5 meters waterline length and about 2000 kg weight. 800 kgs was lead. A Proa with 7,2 meter waterline length, 600 kgs weight, 60 cms (vaka) width will be much much faster. With 15 m2 sailarea i should be able to sail at 10 - 12 knots without any problems.
The bad news is: you cannot achieve your desired whieght of 125kg for the vaka. Alone your plywood for deck bottom and sides have a surface of 13,2m² and a wheight of 75kg ... without bulkheads, glue, epoxy sealing, reinforcements, stringers, paint etc. (I calculated 700kg/m³ for the heavy pine plywood which is a realistic wheight for it).
I don´t think 120 kgs is so far off. If the plywood alone weights about 75 kgs, i hope the rest will add no more than 50 - 60 kgs. With thicker plywood there is less need of stringers and bulkheads. I will have both stringers and bulkheads of course, but i dont need that many of them. 75 + 60 = 135 kgs, and that is ok with me. Its still much lighter than any 24 foot monohull i know of…
As i said in the very beginning of this thread, this Proa is the pacific multihull version of a Matt Layden Paradox.
And as such i dont think some weight is much of a problem. The Paradox has a waterline-length of 4,1 meters, is 1,2 meters wide and weights about 600 kgs. Its easy to scull with a yuloh even though its shorter, wider and as heavy as my Proa will be fully loaded with 4 people aboard. The Paradox is a small heavy cruiser, and its known to be able to sail at 6 knots (there is a Youtube Video of that) with a good wind.
My reasoning is that a Paradox with (almost) double the length, half the width, same weight and much bigger righting force and bigger sail should be a quite fast sailboat. The slender hull will have much less wave-resistance and wave-making resistance. The 12:1 L/B is much better than the Paradox at 3,41:1 L/B.
Since we are going to sail 4 people with some food and water aboard i don´t think 50 kgs more or less in the hull is such a big deal.
You calculated 280 kg crewweight/payload, but you have not counted that the crew needs personal gear like clothes or safety equipment, survival water and food, lifejackets, etc.
I calculated with about 10 kgs extra for each person. This is lifejacket, some cloth, water and 3 candybars each…
We will probably never sail more than a slightly extended weekend all 4 of us. This is a test if its feasable to build something larger in the future along this line. 20 -30 kgs more of water and stuff does not make that big difference.
- your freebord is way to low, 35cm is really not enough for a 7,5m Proa. You should have at least 10cm more and even this normally is not enough. I know you want to cut the sides out of a half plywood sheet, cut lenghwise. Plan to buy one more sheet of ply and three scarfs per side, then you can make it.
I don´t think my freeboard is to low. A kayak has about 10 cms freeboard even if it weights 150 kgs fully loaded.
I dont want to have a centerboard or daggerboards so i have to work a lot at lowering the windage instead. Its once again a lot like the Paradox. I know it will be a wet ride in short steep waves, but a Hobie-cat or a International Canoe is a much wetter ride in the same conditions.
Everything is a tradeoff. This is a test, and i hope i will learn a lot from my mistakes. I expect a lot of mistakes as this is my first serious try at designing a Proa that will fit my needs.
I appreciate your concerns and all the thought you put into my project! I might not agree with everything you write, but in that i have to think everything through from differnt directions. Its very valuable to me, and i hope you will keep on commenting on my thoughts and ideas!!!
Cheers
Johannes.
Hi Johannes,
you cannot compare a paradox monohull with a proa and not with a kayak and in no way with a heavy displ. long keeled sailingyacht. That is as compairing an apple with a pear and an apricot ... all are fruits but beside this they have nothing in common because they come from different trees.
You have to treat a proa like a multihull and in designing you have to respekt the nature of the multihull. A multihull is very sensible to overwheight and I mean this not in the sence of speed ... I mean sailing motion/behavior and safety.
At least you now have some secured mathematical data for your vaka. I hope I helped you with that.
I don’t know if Iam willing to comment your posts and progresses in building and design. I feel I have too much contrary opinions to you. I don’t mean this negative and Iam not offended in any way. But beside being sharpie-proa fans we have total different ways of thinking. You are willing to go your own way, make your own experiences and your own mistakes. This your right and this is what makes you and your project so interesting.
May be Iam a little boring in this sence, but I want to avoid mistakes and be glad if i can learn from other people experiences. Iam a quite experienced sailor (yes, also on multihulls) and boatbuilder (having 6 boats on my back and helped building 42 footers) and I know how much work it is to build a (big) boat and how big the disappointment is if a new boat does not fulfill it’s expectations.
I wish you all the best for your project have fun building and experimenting ... and have good boats in the end.
Best Regards, Michel
Luckystrike118:
If you have seen the above video you would understand just how important your comments, thoughts and suggestions are to me! If nobody says anything or everybody just agree there would not be much progress in this field.
We might see things very different, but that is the exact reason i appreciate you sharing your veiws on my work. I often choose to go my own way, and i expect failures, and try to learn as much from them as possible. One failure is worth more than two successes, because they force me to learn more, understand it better and evolve my designs faster. Failure is very important in any kind of development.
Thank you for the FreeShip data! They help me a lot!! Very much appreciated!
I wish you Luckystrike118, and everyone else reading this thread, a very good new year, with lots of proa talk/sail/build/development and much fun!!!
Cheers
Johannes
ROCKER
I wonder why the rocker? It is not required in a proa. It will increase the resistance as water has to travel around a sharp edge. With no rocker the load carrying / freeboard will be improved.
WEIGHT
Half the payload is crew, which is movable, which must influence how she will sail. Where do they sit? I do wonder if weight in multihulls is the big no-no. Many older and some newer cruising cats are not that light and whilst they are not fast, they give a comfortable ride. I have a copy of the book about the first multihull to cross the altlantic - it was made of steel, with a full bridge deck, and it worked ok.
FREESHIP
I must look into this sometime, how easy to use? any problems & limitations?
All the best for 2013. (it will be one year closer to starting to build my dream)
Mark
ROCKER
I wonder why the rocker? It is not required in a proa. It will increase the resistance as water has to travel around a sharp edge. With no rocker the load carrying / freeboard will be improved.
Rocker has a few benefits. A hull with rocker has less wetted surface, hence less frictional drag than one without it. Also the rocker will provide some hydrodynamic forces at speed, which are useful for resisting nosediving, so even though the wave drag may theoretically be slightly higher, the more level trim may actually reduce the drag verses an un-rockered hull sailing with bow down trim. I don’t know why you say that the resistance will be increased due to the chine. Sure, if a chine exists, some separation may occur which may increase (or maybe decrease) the resistance of the hull, but the local flow direction will vary with speed so it’s a bit indeterminate as to what shape is best. In general, chines don’t add a huge amount of resistance.
For the same amount of hull material, the rockered hull could have more freeboard than an unrockered hull, but it would also have have slightly more draft.
I wonder why the rocker? It is not required in a Proa. It will increase the resistance as water has to travel around a sharp edge.
The rocker is part of the lifting body that resist leeway and enable this Proa to sail to windward without any additional foils.
The water does not travel across the sharp edge sins the rocker in the bottom and the sides are balanced. With the same rocker on both the bottom and the sides the pressure will be the same and hence no crossflow over the chines. See Bolger “Sea of peas” theory earlier in this thread or via google.com.
Many older and some newer cruising cats are not that light and whilst they are not fast, they give a comfortable ride.
I have seen many examples of “not so light” multihulls that still sail fast once the windspeed pics up, and often with a smooth ride even though the waves start building. Wharrams are not very light, and often quite heavy loaded with (to) much cruising gear, food, water etc, and still manage to sail quite well.
Every boat ever built is somewhere on the scale between a solid cube of lead and a ultralight and ultra-slim all nano-carbon tube laminate - super high tech speed monster.The solid cube of lead will float in mercury and actually move slowly if pushed by a steady stream of air. I don´t want to call it a sailboat, because most sailors will not recognize it as such.
I have tested a heavy barge Proa and it sails way better than i ever thought it could. I´m not afraid of some weight. I believe one just have to design the boat, multi and mono, to suit ones need. I believe i could build a heavy 5:1 (L/B) Vaka - Proa that could sail much better than most comparable normal production monohulls. It would not be Jzerro fast or elegant, but it would probably sail at 1,5 X theoretical hull-speed.
Thank you everyone for thoughtful comments and ideas!!
Cheers
Johannes
The water does not travel across the sharp edge sins the rocker in the bottom and the sides are balanced. With the same rocker on both the bottom and the sides the pressure will be the same and hence no crossflow over the chines. See Bolger “Sea of peas” theory earlier in this thread or via google.com.
I believe Bolger when he wrote that the matched curvature leads to better behaviour, but I am still sceptical about the explanation. Sailing boats usually have some leeway. That will give you flow across the chines. Then add some pitching and a surface wave system and I really don’t see how the flow lines could remain aligned with the chine, equal curvature or not. The average flow across the chines may be less than with unequal curvatures, but I can’t believe no flow until I see an underwater video of a sea of peas hull with tufts to visualise flow while it goes upwind at some speed, pitching in a seaway.
Regards
Robert Biegler
I believe Bolger when he wrote that the matched curvature leads to better behaviour, but I am still sceptical about the explanation. Sailing boats usually have some leeway. That will give you flow across the chines.
Yes that is true. The “no cross-flow” should be “less cross-flow” instead.
I have some asymmetry that i hope will help minimize the crossflow. There is 35 cm rocker at the wind-side, 30 cm rocker at the bottom and 25 cm rocker at the leeside. I hope this is a reasonable amount of asymmetry to counteract the pressure from the sails and reduce the crossflow over the chines.
I have noticed that the sharp chines is very pitch-damping. I hope these things add up more and faster than the crossflow, greater wet area and other negative properties do.
Cheers
Johannes
FREESHIP
I must look into this sometime, how easy to use? any problems & limitations?
Hi Mark,
I started a thread about freeship in the Proa Tech Forums. There are discribed the the first steps for designing the hulls. Unfortunally Iam not trained in CAD and cannot look deep into the programme. Iam sure there are a lot more possibilities that I have not diccovered and learned yet. I learned designing the old way and do a lot of things by hand on paper.
Free!ship is freeware and is for designing hulls. Making fair lines and checking all relevant hydrostatics. further you can set stations and print out their measures ready for lofting. you can add rudders, keels, boards, masts and sails. Its easy to use but needs some training and the devellopment of a feeling for it.
Chined hulls:
For sitch and glue hulls you can check the devellop-ability of the panels and devellop the flat panel offsets. calculation of wheights and surfaces
Round hulls: of course you can make round hulls and print table of offsets for the stations.
You can feed cad cam routers with the dxf files, but don’t ask me how this works.
Limitations: the renderings are poor and asymetric hulls are not possible which is bad for designing asy - proas, leepods and so on. I don’t know how much details you can add. The biggest limitation is that freeship cannot design the boat for you, it can just speed up the calculations and drawing work, but you have to know what you are doing. You can explore a lot of alternatives in a short time.
Best Regards, Michel
ROCKER AND FLOW AROUND THE CHINE
With rocker the water hitting the bottom has to travel somewhere, that is around the chine, which adds to the water displaced by the sides froming the bow wave.
With no rocker, no water hits the bottom, so no water travels around the chine. (Think about cutting a plane level with the bottom, or moving through water the same depth as the hull draft) The bow wave is formed by the water displaced by the sides only.
Of course in a real world the water is not flat and the wind strength constant, so there will be some chine flow.
There is some lift at the bow, as the bow wave increases the pressure in this area which will act on the bottom (pressure but no flow, water being not compresible).
Well thats my theory. Interestingly, as an (ex) windsurfer, I first though rocker was important. But noticing the likes of Harry-proa have none, I gave some thought to this. They work because the hull is comparitvely long. Also rocker tends to promote hobby-horsing, which is never good news.
Mark
A hull without rocker will get a strong bow down tendency, and is much more likely to pitchpoole in heavy weather. That fact alone is enough for me to avoid rockerless hulls. All the water still needs to be pushed aside and if all the rocker is in the sides, the wake will get much higher, hence more resistance. Its better to have all the rocker in the bottom than in the sides of the hull. The rocker in the bottom will fit the hull better in its own wave-system. The rockerless hull will try to ride on its ends, with a deep through in between.
Please buildand sail some simple models. Its very clear and obvious that a sharpie needs some rocker. When i had to little rocker my sharpie-proa was much slower, even though it was more slender and slightly longer.
Cheer
Johannes
ROCKER AND FLOW AROUND THE CHINE
With rocker the water hitting the bottom has to travel somewhere, that is around the chine, which adds to the water displaced by the sides froming the bow wave.
With no rocker, no water hits the bottom, so no water travels around the chine. (Think about cutting a plane level with the bottom, or moving through water the same depth as the hull draft) The bow wave is formed by the water displaced by the sides only.Mark
As Robert mentioned, the least cross flow occurs when the change in slope of the hull panels, moving away from the stem point in the direction of water flow, is roughly equal. So for a box hull with vertical sides, for least cross flow, the rocker slope at the forefoot should be equal to the half angle of entry in plan view (ignoring free surface effects).
If you have no rocker, you have a large cross flow because the chine is shearing the water flow, causing vortexes.
Not sure this has much to do with proas but I’m compelled to make a rocker/rockerless confession.
Almost thirty years ago when I was first designing stripper canoes I made a nice 14’ solo that had an absolute straight keel line, influenced by the dogma of canoe racers that a flat keel is fastest. It was a good boat but I looked out my office window innumerable times at the boat strapped at the ready on my little pickup and meditated on the nature of things. I came to believe that Mother Nature didn’t really like straight lines and swore a mighty oath to never do another boat with a straight keel line. Subsequent playing with Mitchlet when it became available gave me some insight into the matter ONLY as it relates to simple(!?) drag of long slender hulls. The reality of proas have a lot of other factors to consider so there’s a lot of room to experiment.
Confession time; while doodling out the Bionic Broomstick it began as an extruded shape with a flat bottom, natural quick construction w/ cheap ply. When it morphed into an easier to build hotwired foam structure it was natural to think about rounding the bottom to cut down on wetted surface. That little voice whispered “why not?” so once again I’m building a boat with a straight keel line. For any serious boat I’m still of the opinion that at least some rocker is appropriate, but for a 14’ boat I’ll make an exception.
cheers,
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