Firstborne Revisited: a 7.5m Light Cruising Proa
@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?
Nomads Cp is .672 according to Freeship, I don’t have numbers for entry angle since designs are arrived from a different “angle” (sorry).
[Edit] turns out Freeship does calculate entry angle, in Nomad’s case Freeship says 14.53 degrees.
I do have more to say about prismatic coefficient though. This from years of designing racing canoes searching for that elusive 0.1%. http://www.duckworksmagazine.com/12/columns/guest/skip/index.html .
The following is based on doing enumerable simulations in Mitchlet and Godzilla, goes into the millions if you count each iteration in Godzilla. Bottom line.
The ideal Pc for any long lean hull is 0.50 period…...pause…...for a speed/length ratio of 1.0
Most of our boats are going to operate at a little higher speed/length ratio; what to do?
Shape a hull the length needed to match the speed intended (long sucker) then chop off the ends and fair the new ends into reasonable bow(s).
That’s maybe a little simplification but it is pretty much what happens in Godzilla optimizations. As has been said in a number of other recent posts, light and long are where it’s at, everything else is secondary.
Cheers,
Skip
It really does make you think, doesn’t it? I’ll speak to ama foils in another post, but a couple of comments on crew weight. Once upon a time I built and raced speedsailing boats. Crew weight is a very significant issue here; for instance, I eliminated the second person when similar boats had two. I also aggressively dieted before each campaign, aiming to reduce displacement 20-25 lbs. Both significantly increased my boat speed. The dieting concept also served to focus my thinking on taking weight out of the boat at every opportunity. Pounds and even ounces are worth considering.
Firstborne Revisited: a 7.5m Light Cruising Proa
In my view structural strength (resistance to chafe, crashes puncturing the hull or crushing the hull-bottom/side) is more important for a cruising proas then hard diet, thin fragile plywood and 2 knots extra speed. I fail to understand how a 500 kg 7,5 meter proa could be slow compared to a 7,5 meter long, 2,5 ton monohull with a deep keel and 2,5 meter waterline-width? Yes i know speed is addictive and fun, but having to be extremly careful all the time while cruising is not very fun.
A 7,5 meter deep V proa (just an example) would have somewhere around 7,1 m2 plywood. 7,1 m2 6 mm thick plywood will weight about 32 kg, and the 12 mm plywood will have a weight of about 64 kg. I fail to see why those 32 kg extra would make such a big difference, when the proa will have a total weight of about 500 kg anyway. Say 532 kg instead of 500 kg with the same gear, food and stuff. Will the 532 kg cruising-proa only do 8 knots while the 500 kg cruising proa with the same length, sail area etc, will do 15 knots??? How big difference in speed will it be between the two?
Some of the added weight from thicker plywwod will be mitigated by fewer stringers, less epoxy, fewer bulkheads etc, so the difference will probably be somewhere around 15 - 20 kg for a comparable strength.
2 mm Duplex stainless steel comes in at about 120 kg. A deep V proa built in steel would have a total cruising weight of about 620 kg with the same gear and stuff as the other two examples. Would this proa only do 6 knots?! How much difference does this added weight really do in real world usage? (beer, fishing gear and everything else that make a cruising boat heavier then the latest breed of AC-72s).
It is important to define what exactly is failure. Just using plastic versus elastic deformation as a criteria, steel deforms easily and permanently (plastic deformation) where plywood may deform elastically (bouncing back to original state) in the same type of impact.
I prefer a dent over a hole in the bottom of my cruising proa.
When exposing plywood for “the same type of impact” as steel, the steel will dent and the plywood shatter to pieces. The plywood proa would partly fill with water and be a dog to sail. The steel proa would dent a couple of mm, but otherwise nothing much will happen. This can be corrected by a hammer and anvil, or in worst case an angle grinder and a welder.
This is my experience from working with both materials. Steel will be twice the weight or so (it is hard to weld thinner steel then 2 mm without distortion), but the plywood will shatter pretty easy from a single blow with a hammer (or hitting something while sailing). It is easy to use much thicker plywood to a similar weight as the steel, but nobody seems to do it. I guess it is too expensive and/or too much work to build a 7,5 meter proa in 21 mm plywood.
Cheers,
Johannes
Well I know that running resistance, for slender multihull hulls at multihull speeds, the running resistance at a fixed speed is roughly linearly related to the displacement of the boat (within realistic limits). 1/3rd less weight = ~1/3rd less hull drag, that’s roughly the picture which emerges from Norwood’s “High Speed Sailing Design Factors” anyway. Drag climbs very rapidly with increasing speed however, so something like a decrease of displacement from 530kg to 500kg will probably give you something like half a knot of extra speed, on that 12-15 knot proa. In part the point is though that if you choose to build lightly, it’s not just going to be the hull skin which is a bit lighter, but you are probably going to try to save weight on other components (and payload!) too. Provided you started with normal / somewhat conservative scantlings like 6mm plywood glassed both sides (‘Madness’), and decided to build light, but stick with plywood/fiberglass, then in all you’ll probably make your boat ~1-2 knots faster in all, and if you work on getting a good rig and boards (if you had decent ones to begin with), then you might get as much as 5 knots improvement in total. I don’t think it’s going to be more than that. Double the LWL and stick with ‘normal’ scantlings for the now larger size, and you’ll get the same, or probably a larger improvement in boatspeed.
A non-foiling C-Class Catamaran, which is about the same length as Firstborne, but built far lighter than Firstborne could ever be, and has a highly optimized wingsail, reaches speeds of 20-25 knots at a few select course angles.—I definitely won’t reach those speeds. Mbuli (6m) with its wingmasts and large sail area reaches something like 15kn at best, and I’ve seen a report of a Dierking T2 scaled up to about 7.5m (which I would say was rather conservatively canvased, just like the original T2) making a steady 12-13 knots with a moderate to fresh breeze. My hope is that a boat like Firstborne, built lightly, with a lower DLR than Mbuli and greater overall LWL, slightly more sail area, and for instance featuring Dave’s hull flying mechanism, could perhaps make it up to a steady top speed of ~17 knots if the breeze is really up.
It’s not a world of difference we are talking about here, it’s a couple of knots, provided the reference from which you are starting from was decent / good to begin with, but I’d rather take ‘em than leave ‘em, so long as construction time doesn’t jump through the roof. 😉
Cheers,
Marco
Thanks for a good explanation Marco!!
” the running resistance at a fixed speed is roughly linearly related to the displacement of the boat (within realistic limits). 1/3rd less weight = ~1/3rd less hull drag”
Seems like a likely relationship between speed and displacement for a fixed length (within realistic limits) as you write.
Double the LWL and stick with ‘normal’ scantlings for the now larger size, and you’ll get the same, or probably a larger improvement in boatspeed.
This sounds much more like my tune. I rather add length then hunt kilos for getting higher speed.
A really slender and long hull will not only be faster, but it will have a much nicer motion in any given sea-state, pitch less, keep a better air-flow around the sail(s).
Cheers,
Johannes
Yeah, it’s not an option for me though. I have a 5.5m garage to build my boat in. I’m not entirely happy about having to make the bow sections detachable, but with the system I have in mind I feel quite confident that it will hold, even in very rough seas. The connections will probably cost me about 10-15kgs in total though, and I would feel very uneasy about making the bow sections significantly longer than the currently planned 1.25m. The shorter the bow sections are, the lower the loads on the connection.
For me, light weight has some other added advantages too: it makes the boat easier to transport and haul out of the water, which is a big deal for me since I basically have NOTHING. I need to rent a car to transport the thing to the sea (which is 300km from here btw), and I’m going to have to build the trailer for the boat myself as well. The easier it is to haul the thing around, the better.
Less weight also means that the boat will perform much better under oars. It may have gotten lost in all the other discussions, but I intend this proa to be a good and proper rowing boat as well, for those days when there is no wind. Less weight helps a lot when it’s down to your own arms and legs to produce the driving force for the boat! 😊
If you live directly by the sea, and/or have a nice big car, and have plenty of space to build and store the boat, then just go larger. Had it been an option for me, I probably would have designed and built something based off of Madness instead of Mbuli, and maybe I wouldn’t have cared quite as much about light weight. In addition to the extra performance, the ability to propel the boat well under oars definitely has a strong appeal for me though, so perhaps I’d have been trying to build as light as possible anyway. 😉
Marco
Nomads Cp is .672 according to Freeship
...entry angle, in Nomad’s case Freeship says 14.53 degrees.
That surprises me… I don’t have all that much rocker left in the hull, and yet I have a larger entry angle, and lower Cp… I’m going to have to play around a bit more to see if I can get a better configuration, but I don’t really want to sacrifice more rocker for fear of making the boat more vulnerable to broaching.—I’ll post some new pics this weekend.
Decreasing maneuverability is another not so nice downside of decreasing rocker, though the extra course stability (= lower maneuverability) can be of value once you’re out at sea…
Is this the full entry angle or half angle? One of those areas confusion could arise.
Thanks for pointing that out tdem!—I am referring to the full angle, if Skip is talking about the half-angle, then that would definitely explain the discrepancy!
Thanks for pointing that out tdem!—I am referring to the full angle, if Skip is talking about the half-angle, then that would definitely explain the discrepancy!
Sorry for the delay, been away from computer a bit.
Tdem is correct, I should have said “Freeship says halfangle is 14.53 degrees.
Skip
Marco,
I did have one other totally mad thought…....buy 2 identical second hand foiling moths, cut them both in two at Bmax use the noses and make your own central 5 metre long section . Lots of foils to play with, carbon to canibalise and rigs to use schooner wise.?????
Rob
So after that little foray into a different direction with the ‘Delirium’ concept, I’m back on the horse I rode in on. I’m eager to wrap up the design of the ama really soon because it’s the first thing I’m going to build. As soon as the ama and ama-iako-connection design is done, I can start construction , and I’m sick and tired of designing and speculating.
Since I intend to fly the ama with Dave’s auto-flight mechanism, the ama will not feature a daggerboard. It will however feature a ~100l water ballast tank which will be self-bailing when inverted (when capsized).
At the moment I’m still a bit conflicted as to how long I should make the ama. 5.4m is the longest I can make it without having to make the ama in two pieces, and it’s the same length the ama has on Mbuli, but it does seem a bit short to me, especially since I intend to include water ballast, which Mbuli didn’t have. At a 20:1 L/B with a relatively sharp cross-section my current 5.4m ama design comes in at only 330l total, which I feel is not enough if there are potentially two people sitting on it and water ballast in the tank… Lengthening it to 6.4m would push the fully submerged dispalcement to around 500l, but the extra hassle of the interconnections (because only 5.4m will fit in my garage) would be a real pain in backside…
What shape to give the ama is causing me a bit of a headache too… I want a bit more of a wave-piercing ama than the one on Mbuli (vertical bows and not so much freeboard). That means the deck has to be able to shed water in a hurry though. Avoiding large flat surfaces on the sides will make the ama a bit more resilient to waves slamming into the side, which would allow me to build it a bit lighter (the plan is 4mm strip planked, glassed from the outside with a single layer of ~150g cloth, a few extra layers at the keel-line for beaching).
I’ve got a design with a semicircular deck which I think looks absolutely gorgeous, but it’d be rather tricky to build, even with strip planking, because a lot of planks will need a lot of cutting, to get all that compound curvature in there… I will definitely strip plank the ama, as practice for the bottom portions of the vaka, and because I can probably get the timber for strip planking locally, whereas for the marine ply I have to drive 300km in each direction, so I’ll do that once I know the exact scantlings for the vaka.
What do you guys think? Anyone have an easier to build design which won’t slam when it crashes, will shed water very quickly, and looks reasonably good?
Cheers,
Marco
Foam and glass… Might be the way to go here, and I can get those materials locally too, so long as I don’t insist on using marine ply for the interior skeleton of the ama.
The strip planking on my 1:5 model of the vaka is going so well that I’m starting to think that getting extra practice by planking the full sized ama, is really not going to be necessary. When I get to planking the bottom of the vaka, then I can always just figure it out / learn as I go along then. A foam ama is lighter, and probably faster to build as well…
Hi Marco,
the 5.4 Ama should be long enough and it is absolutly not neccesary to hold the 20:1 Ratio. A 16:1 is more than enough for a very low resistance hull.
Foam and Glass is a possibility, but be aware that if you shape it from a full block of styrofoam (styrodur, jackodur) foam (30kg/m³) it will weigh a lot. Better to build it like a moth dinghy with 3cm thick walls and foam bulkheads. Glass over with around 500gr/m² glass and epoxy. Wheight will come out similar to strip planking.
A good possible wavepiercer design could be the “optimum hull” I found in a old book and posted some time ago. You just have to make ist equal fore and aft without the 90° rotation of the shape at the middle bulkhead.
Minimum surface, good volume, good hydrodramatics, low resistance.
http://proafile.com/forums/viewthread/197/
Best Regards, Michel