Minimal proa
Minimal Proa
Description, specification
Minimal Proa with balanced lug rig and moving leeboard to align CE with CLR. Single central AKA, nod to Skip Johnson’s AC/DC and Bionic Broomstick.
This is a concept for a possible next build after testing my current proa TP03 and will help clarify tests and developments to be done with TP03.
I am also working on another concept which is a slightly bigger design with more freeboard and a small central open concept. This will allow two crew and storage of camping gear. All other aspects and hence development tests for this concept as per here.
Main hull
LOA 12ft hull, (3.7m) beam and draft 230mm, depth of hull 360mm, freeboard 120mm
L/B 15.25.
Target weight whole craft 50kg, tough target, but research and a few calculation have given me confidence.
Main hull displacement 150 kg total hull buoyancy 250kg, little reserve but think about Hobie 16, trimaran floats, swath, Tepuke etc.
Deck section at bows, addition of foam and glass to create wave piercing bow. This should counter low freeboard and crew fixed in the middle.
Outrigger AMA
Outrigger buoyancy 50kg allows for crew at 50% beam, LOA 2.6m (8.5ft) beam 160mm, L/B 16.25
Overall beam
tbc approx 1.5 to 1.8m
Construction,
ply, simple box, plywood and polyurethane foam bulkheads, all hull panels 3mm Okoume, bulkheads and other structure 9mm
AKA
3mm egg box constructed ‘box’ based on International Canoe sliding seat. Hydrodynamic box shape lifts AKA 250mm above deck, as a result underside of AKA 370mm above static waterline. This reinforced box also contains a deep ‘bury’ for the mast and links righting forces with heeling forces without loading the hull. Leeboard loads are relatively close. The only other load being the buoyancy which is obviously spread throughout the hull. This all helps to produce a light structure.
AKA to AMA connection
Two flexible beams allow movement of AMA relative to main hull
Rig
balanced lug, unstayed mast, area 8m sq
Steering and lateral resistance,
leeboard mounted on track moves fore and aft to steer by achieving CE / CLR balance, subject to testing with TP03
Why’s and downsides
Proa (Pacific)
> Max performance for given materials, performance means many different things, in this case it is reasonable speed in a minimal package
Downside, control issues, additional complexity?
Minimal
> Low freeboard, very light weight,
> > Less materials
> > > Cost
> > > Weight
> Less room to build
> Easy to transport, fastest way to get upwind is behind / on / in a car
> Easy to handle on land
> Downside low freeboard drag,
> Downside low buoyancy limits use; single handed daysail
Lug rig
> Good performance for simple set up and small spars
> Lower hull stress
> Less set up time
> Low CE
> > Heeling
> > Pitching
> Downside ultimate performance
Free standing mast
> Lower stress
> Stress all in one place
> Less set up time
One central AKA, also the seat
> Simple
> Stress in one place as above
> Away from bow so possibly less drag from wave action
> Less set up time
> One item two uses, only one aka no trampoline
> Downside can’t trim fore and aft
> Downside limits to one user
Steering with moving leeboard
> One foil only
> Low drag, one foil
> Downside unproven, test with TP03
Sharpie (box construction)
> Very simple build
> Flat plate, ply, construction
> Max buoyancy for box size
> Helps leeboard system
> Bow sections promote lift
> Latest AC, A Class appear to have flatter sections
> Downside higher weight and wetted area for buoyancy
Pictures, rendering of proposal and quick CAD file to check bouyancy etc
TINK
Proasail
A 12 foot proa might be a good test for general layout and ideas, but don’t expect any real speed from it.
For such a small proa i would definitly go with a single sail. I think my models sail quite good with only one sloppy lugsail.
I like simple box-sections because they are easy to build out of plywood or foam+glass. With some asymmetry and a sharp leeward chine you probably don’t need any other foils than the rudder. That will simplify handling and sailing a lot.
Cheers
Johannes
Dear Johannes
Thanks for the positive comments, the design is coming along well, simplifying and lightening it up a bit.
I am a bit anti just using the hull for lateral resistance, on my first proa TP01 I found the CLR moved very far forward. I plan to test steering with a leeboard, on a track, moving for and aft on TP03 which is in the garage waiting to go. I broke my shoulder bone a week before launch. TP03 is a schooner so can sail steer.
On the minimal proa I plan to use a single lug sail, with the leeboard and a quarter paddle for slow speed steering.
TINK
Hi Tink,
FWIW in your quest for a 50 kg boat I weighed most of the bits for the Bionic Broomstck yesterday.
32# - Main hull 2# foam, 9 oz fiberglass 5.2 mm deck, graphite/epoxy coated bottom
10# - Float same basic construction as hull no coating yet
14# - Beam with sockets to connect to float and hull
6# - Rudder pods
7# - Padded seat
69# (31.4kg) at the moment.
Doesn’t include finish, mast, sail, rigging, rudder blades plus whatever I forgot but I think I’ll end up at less than 50kg (just barely).
Best wishes on your build.
Skip
Thanks Skip
That gives me confidence in where I am going with this. Working out a few more details before creating an accurate bill of material with weights and detail CAD model. My rough calculations seem to indicate 50kg with ply and foam bulkhead achievable. The method I used worked well on TP02/03.
I have changed from having an IC style seat to twin alloy extrusions, the IC seat was much stiffer but weighed more. I could go for a smaller wooden aka but the connection was getting fiddly. I want to transfer the stresses of sitting on the aka to be transmitted to the mast with out transferring too much the the vaka. I have been influenced by the P5 design (on German proa site) with a few additions to support the mast.
Anyway below latest CAD, wave piercing bows not yet added.
Sounds as if bionic broomstick is coming together well, have you posted any new pictures and when are you expecting to launch
I would love to hear how you get on with the rig, IMHO it could be a perfect rig for a small proa like this,
Thanks again
TINK
Sounds as if bionic broomstick is coming together well, have you posted any new pictures and when are you expecting to launch
I would love to hear how you get on with the rig, IMHO it could be a perfect rig for a small proa like this,
TINK
Actually build is going slow at the moment, trying to get epoxied pieces together early so it can harden more, the relatively inexpensive epoxy I normally use takes a long time to reach an ultimate cure.
I’ll start a new thread about the build when the pieces come out from under the house in a couple of weeks, probably be May before it gets wet.
Still conflicted a bit about the rig, a lot of time and effort has gone into the biaxial rig, but the offset clubbed jib like Chris has shown on Snelson’s canoe sure resonates if the cambered panel rig can go upwind without really high stresses in the rig. Really look forward to finding out, the winner will probably be the rig for a schooner version of Nomad.
Skip
New image of minimal proa below
Have added track for the moving leeboard and wave piercing bows. The nearest on is a tortured ply affair and the furthest glass over foam. Not really a true representation just learning a new CAD package
TINK
Why not just give the deck some “rocker”. If the beam of main hull is 230, then you can have the stem 100 above the mid height of the boat if that makes sense. On the ply layout each side panel would be nested around the deck or bottom.
Dear Tdem
The funny thing is I had envisaged some good sheer to the deck when I first imagined this craft. Then when I drew it I was swayed by the current fashion for wave piercing and reverse sheer. Quick calculation says the extra ply is 300g however that is easy offset by the alternative complexity of creating a wave piercing bow.
The minimal also refers to the building process. The deck simply overlaps the sides of the hull and filleted with epoxy externally, see below. Apart from simplifying construction this should be a bit of a spray rail keeping the deck drier. Having deck sheer helps this construction considerably.
If I had issues with the bow burying, my biggest fear for the project, I could add some foam on glass wave piercing on the deck
Anyway thanks for the input and helping me see the error of following fashion
Next development is to think about pvc tube AMAs.
The CAD file is quick and dirty to thrash out ideas, in reality the whole deck wold have a full length sweeter sheer, I hope
TINK
Hi Tink,
I designed a 10 feet proa for the very special and popular “10-feet-class” in Italy (http://www.diecipiedi.it/ns/main/index.html). May you can get some ideas from it: http://www.multihull.de/proa/p5/p3.htm
Cheers
Othmar
Hi Othmar
Thanks for the links I’ve seen the German 10 ft proa but not the diecipiedi class. What a fantastic and diverse class, we are obsessed with one design dinghies here in the UK. Other than the odd class, mainly very expensive performance classes, we have very few development dinghies more is the pity.
I will have a good look at the diecipiedi website
TINK
I have been researching making inflatable tubes for a biomimicry inflatable luff (BIL) and it seamed that tube manufacture was doable, there have also been some good fresh posts in Nuts and Bolts of an Inflatable Proa.
This got me thinking about the minimal proa again. One of the main advantages of the minimal proa (especially when combined with the Lug rig adjustable CE) is all the stresses are concentrated in one central position. This structure would be ideal for an inflatable proa, so below is
1) minimal proa
2) inflatable vaka and ama
3) structural frame combines, aka, seat, mast support, board support and vaka, ama mounting
4) Lug rig adjustable CE - not fully shown see additional picture.
This is just a rough CAD model to get a feel for the craft, I need to do some more research.
TINK
Work in progress, I am looking at material sizes, specs etc before finalising dimensions
Tink
Hi Tink,
Looking good!
you can save yourself a strut by moving the feet of the f&a mast support struts to the outboard structural glass fiber tube. These then brace the mast not only f&a but also amidships. You then don’t need the amidships strut that has it’s foot towards the seat.
See below for new frame for inflatable minimal proa
Orange is the inflatable vaka, now optimised LOA 4.6m, diameter 360mm total buoyancy 400ltr. Estimate of weight based on surface are 4 to 9 kg for 600g and 1300g material respectively.
Blue, GRP partial hoops, laminated over male mould as complete set and then cut to size, weight 2kg
Silver, 32mm alloy tube, weight 2kg
Red, support: for mast, aka (note integrated lashing hooks) and board. 12mm ply, egg box construction CNC cut. This all stress localised here. Weight excluding glue and finishing 4kg.
Total weight so far 12 kg to 17kg therefore final estimate 14 to 19 kg.
Ply and foam target weight 15 kg. Therefore will:
1) new CAD model of ply minimal proa and create more accurate weight estimate
2) look for weight reduction of inflatable minimal proa
3) SWOT the two options
TINK