Firstborne Revisited: a 7.5m Light Cruising Proa
Hey Proa enthusiasts!
I kind of went below the radar a few months back; my entire boatbuilding project came to a screeching halt when the workshop I had been promised access to was pulled from me at the last minute. The workshop problem is now solved, I’ve been working on the design since then, and now I’m (hopefully) only a couple of weeks away from starting construction of the ama.
The boat is 7.5m long, intended for cruising in the North Sea (especially the Wadden Sea) and the Baltic Sea, singlehanded or with a crew of two. Overall I guess I could say that John Harris’ Mbuli served as the starting point for the design, so the two have some very pronounced similarities. Compared to Mbuli, Firstborne is longer, slimmer, has a greater overall beam, has a strip-planked rounded bottom at the centerline and a sharp entry at the bow (as opposed to the Mbuli’s flat bottom), sleeps two instead of one, has a bit more headroom (vaka height is 1.3m), has sliding-seat rowing for two as auxiliary propulsion, a lot of water ballast in the ama, a lee pod, and carries more sail area (albeit at a lower aspect ratio).
I’m going to build and store this boat in a garage and that means that no single component (except the rig) is allowed to be longer than 5.4m. The vaka will consist of a central 5m long section (cabin, rig, iakos, rudderboards), with removable 1.25m bow sections (crash compartment, empty space) connected at either end. I have a concept for a pretty solid interconnection which hopefully won’t weigh too much, but that’s still a work in progress.
The structure between the iakos and the ama will be a tetrahedron, the height depends on a design decision I still have to make: does the ama generally belong in the water, or in the air? At present the ama is positioned at the height it would be at for the flying option, otherwise I’d place it ~0.1 - 0.2m further down. I’m not a fan of free-flying hulls, because that’s a kind of precarious balance, but I could just incline the daggerboard case in the ama by say 45 degrees outward, have two daggerboard cases instead of one, use asymmetric foils (one for each tack), and then I’d have a foil producing lift to windward AND upwards, so that when sailing at top speed (and only then), the ama would be foiling fully. —Good idea, bad idea? See the discussion topic here.
What do you guys think? Is the foiling ama a good idea or bad idea? Is the large boom overhang going to cause problems with weather helm? Is 8mm plywood and the stringers / frames a good call, or could I get away with less if I fiberglass both the inside and the outside of the panels? What do you think of the design in general?
I’d appreciate any feedback you guys can give me; I’m hoping to have a construction thread here really soon! 😉
Cheers!
Marco
Stats:
Overall:
LWL = 7.5m
LOA = 8.7m (overhanging boom)
B = 4.6m
Displacement (loaded) = ~500kg
Sail area = 21.2m^2 (2x 10.6m^2 Finn-Dinghy mainsails, freestanding, freely-rotating)
Auxiliary propulsion: a double scull rowing setup, sliding seats (bidirectional), NO ENGINE
Vaka:
L/B = 15.2
H (draft, boards up) = 0.3m
B/H = 1.6
f_bow (freeboard) = 0.75m
f_midplane = 1.0m
DLR = 33 (ama skimming)
C_p = 0.57
Ama:
LWL = 6.4m
B = 0.325
L/B = 19.7
Waterballst tank: 150 - 200L
I like it a lot, Marco. 😊
I, too, have liked Mbuli and thought it would benefit from a little more length and volume and a lee pod. Though I understand John Harris’ goal of minimising construction time and materials.
You’ve done a good job of integrating it all and retaining Mbuli’s attractive styling.
Detachable ends have quite a few benefits in construction, storage and transport it seems to me and allow for a bigger boat in some circumstances where space available is limited.
Hi Manik, welcome back.
I like your general form factor very much. The rig is very interesting, are no battens required because of mast bend?
One aspect of Mbuli that I felt was not so successful was the “beach cat” trampoline cockpit - giving no protection from the elements whatsoever. Are you planning anything different for Firstborne?
Flying the ama: in theory a big YES, but in practice, not so simple. The leeward inclined foil must be larger than a vertical foil to achieve same lateral resistance, which increases surface area except when foiling and the ama lifts. Otherwise, it is a net performance decrease. I am curious to hear what others think of this! Seems like something a spreadsheet could illustrate rather well.
I like the detachable ends, they certainly help ameliorate the proa’s main advantage (waterline length) vs the disadvantage of storage.
A lot to like in the design.
Here’s my thoughts based on some limited experience.
I think the boat would be happier with a little higher prismatic coefficient but that’s just my gut feeling, no hard data to back it up.
I’d vote for the scimming float, precarious balance is precarious by definition and not all that suitable for overnight adventuring. I’m ambivalent about angled foils, I’ve read about problems when you get into a ‘crossflow’ situation like sliding down the back of a large wave and burying the float to the point of capsize. Almost by definition you are only 71% effective with a 45 degree cant. OTOH there might be some goodness in going with a tee or “L” shaped daggerboard properly proportioned so that the float flies and the tip helps keep the foil in the water at speed.
Not sure about weather helm with the long booms but can assure you that you will need to go wing and wing or over the float downwind, the other option will be uncontrollable.
I agree with your Editor that trampolines are better in theory than practice at this size.
6mm ply should be fine for most of your build.
Cheers,
Skip
I also thought the prismatic coefficient looked a bit low. I seem to recall that for narrow hulls, you want a higher prismatic for higher speed. (For a given length, that is). I can’t remember the reference sorry. So I could be wrong.
There are boats of a similar size built mostly of 4mm ply. Switching from 8 to 6 doesn’t automatically mean you need to glass it on both sides. It all depends on the rest of the construction.
Thanks for the kind words, and for all the feedback!
@Skip & tdem: Higher prismatic coefficients are indeed better for high speed performance, and 0.57 is relatively low. Proas will always have lower prismatics than cat or tri hulls though because of the double bow—particularily if you want a very fine bow with V-sections. Maybe I’ll play with it a little bit and see what I get if I make the transition from the almost semi-circle to the deep-V a little bit more abruptly and closer to the bows. Do you guys have C_p values for for any other proas?
@Editor: The rig is going to be two off the shelf Finn-Dinghy mainsails. Unfortunately the class rules specify that you are only allowed to have 5 comparatively small battens at the leech. I’m going to have to have a look at the Finn rig to be sure, but I think the prospects of getting a headsail (particularily a gennaker) on there also look pretty slim, since they are simply not designed for that. It is a very practical setup though, the masts and their supporting structure don’t get in the way of rowing or into the cabin, and you don’t have the mast and main in the way somewhere between the hatch and the ama. The compartively low aspect ratio and in general quite durable construction of Finn dinghies should make for a setup which is relatively well suited to somewhat rougher weather as well. Something like a tornado rig, while offering about the same amount of sail area, would be downright frightening in my eyes…
@tdem: Where the materials are concerned I think you’re right. Yesterday I was in my garage and spent a bit of time with one end of my flat bottomed canoe propped up on a low stool, me walking around in the hull and bouncing around a bit. That’s 6mm cheapo plywood, epoxied both sides, but only fiberglassed at the seams, with one stringer down the middle of the boat. It held up fine, so I think 6mm with fiberglass on both sides, and a moderate amount of stringers in there (at every joint between two panels) is probably overdimensioned. Jzerro is much larger and is built in 2x 4mm glassed on the outside, with a generous amount of stringers. Cheers was only 3x3mm strip planked, no stringers, glassed on the outside. 4mm sounds pretty iffy though; what kind of boats were those, and what were they used for? Did they go to sea, or were they just open daysailors with about the same LOA?
It’s too bad I have to drive a few hundred kilometers to buy the marine plywood for the boat, otherwise I’d make a few material samples and just destruction test those to see what I want to use… It’s a tricky question: just how light can I go before the north sea smashes my boat to bits? Every millimeter I take off the hull scantlings saves me at least 12kg…
Cheers,
Marco
Oh and Skip, thanks a lot for this heads up…
I’d vote for the scimming float, precarious balance is precarious by definition and not all that suitable for overnight adventuring. I’m ambivalent about angled foils, I’ve read about problems when you get into a ‘crossflow’ situation like sliding down the back of a large wave and burying the float to the point of capsize. Almost by definition you are only 71% effective with a 45 degree cant. OTOH there might be some goodness in going with a tee or “L” shaped daggerboard properly proportioned so that the float flies and the tip helps keep the foil in the water at speed.
I started a thread on flying and foiling amas in the tech section a few days back, you can have a look at it here, then we can keep that part of the discussion in one place. 😉
Cheers,
Marco
Thanks for the kind words, and for all the feedback!
@Skip & tdem: Higher prismatic coefficients are indeed better for high speed performance, and 0.57 is relatively low. Proas will always have lower prismatics than cat or tri hulls though because of the double bow—particularily if you want a very fine bow with V-sections. Maybe I’ll play with it a little bit and see what I get if I make the transition from the almost semi-circle to the deep-V a little bit more abruptly and closer to the bows. Do you guys have C_p values for for any other proas?
Marco
P52 had a designed Pc of 0.635, Nomad is a little higher but I’m not at the office at the moment. The Bionic Broomstick has a Pc of over .78 but not actually rigorously calculated. It is easy to increase Pc in narrow hulls, just fatten up the ends a little bit, your boat will thank you the first time the wind blows.
Skip
I might have misremembered on the same LOA boat having 4mm ply. But I believe it was one of the Woods catamarans which was built for racing.
Jarcats (5m) would probably have similar loads to your boat and are 4mm with 2 oz glass on the outside only. They have been around for a long time.
Having said that, I’ve seen a number of Hartley 16’s (6mm ply no glass I think) holed on the racecourse. There’s water pressure loads, and then there’s other boat’s transom impact loads.
I was reading Dave Gerr’s “Boat Strength” a bit yesterday. He specifically states that his scantling rules are not suitable for multihulls, but I imagine that has more to do with the beams and beam attachments than the other loads on the hull. So if I take his equations and just plug the values in anyway, then I get 6.5mm for the plywood skin thickness (in the book only the joints are glassed), or if I were to strip plank (which applies to the keel) you’d need 7.5mm strip planks sheathed from the outside with 150g/m^2 fiberglass, or 6.5mm planks if you sheath inside and out.
6mm plywood + one layer of say 200g Fiberglass for the sides should thus be a bit on the safe side, but going down to 4mm with glass would seem a bit iffy to me for the portions of the vaka where the cabin is. What I am thinking though, is that for the removeable bow sections, and for the entire ama, 4mm sheated with fiberglass from the outside could be good, since they are almost compeltely closed bodies with a couple of full bulkheads and some stringers to boot. Due to their smaller size you can also pretty much laminate around the entire thing in one go to give you one nice monocoque part, so if I do my best to avoid stress concentrations (make as few holes in them as possible and make everything rounded) then those parts should be fine with the reduced scantlings.
With the cabin on the other hand, the size of the free surfaces is a lot larger, so 6mm with glass outside plus a bit of framing and some stringers as suggested in the book would seem prudent, to make sure nothing buckles under the loads there. Looking at the strip planked underwater portions of the hull it would probably be far easier in terms of construction, to not have to glass the strip planked portions of the hull (keel) from the inside, so going with sheating from the outside only and using a greater plank thickness of 7.5mm to compensate (with a smooth transition between the 6mm ply and 7.5mm planks) seems like the better idea, but then I woul dhave to make a smooht transition between the two somehow, which complicates the making of the bulkheads and frames slightly. Having a bit more stiffness in the bottom of the boat seems like a good idea where beaching and transporting of the boat is concerned though, so maybe it’s worth the extra effort…
Marco
I know i am considered extreme in my affinity for strong durable structures, but i like to think that if i can make a crack or a hole in the hull side with one single blow with a hammer, i don´t trust the hull for cruising. Racing around the cans 200 meters from shore maybe, but not when my life depends on it. I have hit rocks, logs and other stuff enough times to really appreciate the confidence and the mental calmness gained from a strong durable boat hull.
Cheers,
Johannes
Hey Johannes,
sure it’s good to have a hull which is strong, but I want a boat which is:
(1) strong enough to withstand the forces which are normally imparted on it, and not stronger
(2) has gentle modes of failure—essentially that a damaged boat doesn’t directly equal danger to the crew (plenty of sealed flotation, crash compartments in the bows, high inverted waterline)
My thinking is that if I ever crash into a rock or a log at really high speed, it will could damage the bow section of the boat heavily, but it will neither compromise the flotation of the boat significantly, nor take away the ability to at least carefully sail, row, or have the boat towed back to safety on the other tack. I don’t expect the boat to come away unscathed from serious human error / collisions, only that the boat can still get me back to port safely in most of those cases.
Marco
P.S. - Something like smashing away at the boat with a hammer gives an unfair advantage to the larger boats over the small ones, since they need thicker skins just to transmit the greater structural loads. That said though, I doubt you could smash a hole through 6mm plywood with a layer of fiberglass over it in a single blow. Cracking of the epoxy, some local crushing of the plywood, and local delamination maybe, but a hole, no way. 😉
@Skip: Could you have a look what the C_p of Nomad is? Also, do you have the values for the bow entry angles on hand?
The original hull I posted had an entry angle of 14.3°, which made for the relative low C_p value.
Edit: If I make the bows fuller to give them an entry angle of 19.2°, and decrease rocker from 175mm to 100mm, then the wetted surface goes up only very slightly (~0.5%), the bow sections maintain their nice deep v shape, the draft (boards up) decreases from 0.30 to 0.27m, and the Cp goes from the original 0.570 up to 0.647, which seems like a really good improvement to me. According to a figure in “High Speed Sailing: Design Factors”, for a boat of my size and DLR, running at (only) 10 knots, that should equate to a hull drag reduction of around 10%, which would be awesome. 😊
Does anyone have typical values the hull entry angles for proas or high performance cats or tris on hand?
In the study plans for John Harris’ ‘Madness’ it says that the vaka and ama both have a Cp of 0.60. In the Mbuli plans, no Cp is given, but it has an entry angle of 18.8°.
Build a boat with 6mm plywood from experience, I say that this is not enough!
6mm plywood poor resistance to shocks.
beyond 18 feet is not enough!
watching a boat hull laminated wood rubbing on rocks: It is immediately wire 4mm you remove!
10 or 12 mm is the right thickness. If you used the marine grade plywood (okoume moabi), the use of tissues is not necessary. Only enough protection 250g is for outside the immersed portion.
That is my opinion! After ...