Kahana Bay Beach Cruiser
I’ve also added a little stick built ring frame/mast step in the middle of the cockpit. I’m disappointed to have an obstruction in the cockpit, but I think it needs the rigidity here.
I’ve also modified the tiller so that they come readily to hand. They also get out of the way of somebody sitting on the cockpit sole while the boat is under Hobie Mirage drive power.
Looking very good, real capable looking boat. If you are having the ‘little stick and ring frame’ you could have a full bulkhead and split it two for storage and transport. Realy like the New tillers, I have visions of holding a realy cool laminated tiller.
Where is the Mirage drive going?
Tink
If you are having the ‘little stick and ring frame’ you could have a full bulkhead and split it two for storage and transport. Realy like the New tillers, I have visions of holding a realy cool laminated tiller.
Where is the Mirage drive going?
The reason for the ring frame is to provide enough room on the cockpit floor for mirage drive engine i.e. you. If you look at the first pic in post 43 of this thread, you’d be sitting with your back near but not against the white, angled bulkhead. You’d be sitting on an off the shelf sit on top kayak seat, which would provide back support and adjustability to accommodate folks of different heights. Your legs would be poking through the ring frame (hence the ring frame). The compound curved tillers put them closer to the helmsperson, and they make room for the person pedaling the mirage drive.
I like the idea of a two piece main hull, but not for this. My 16’ Wa’apa is two piece and I love that when I store it, but not as much when I use it. The smoot bottom makes it hard to put a bolt at the keel. And I love the idea of that big, continuous foam backbone to counter the tension of the rig.
aloha!
All very clear and well thought out, good point about the rig tension much better keeping it as a whole. I think you have far exceeded your original brief this is far more than ‘a simple, beachable, warm weather proa’. This would make a very versatile proa for daysailing, raids and expeditions in most latitudes.
You have however kept the most important design requirement, simple. The hardest job for any designer is to design in simplicity, and when you are done it just looks as if it was easy.
Great work
TINK
I’ve also added a little stick built ring frame/mast step in the middle of the cockpit. I’m disappointed to have an obstruction in the cockpit, but I think it needs the rigidity here.
You might be able to forego the ring frame with some ‘flanges’ stiffening the hull from just beyond the beams. Canting the frames in toward each other at their base would also help torsional rigidity as long as it didn’t compromise ergonomics. Of course the flanges would probably weigh more than the ring frame, so much for the free lunch.
It really is a good looking design.
Skip
You have however kept the most important design requirement, simple. The hardest job for any designer is to design in simplicity, and when you are done it just looks as if it was easy.
Thank you, Tink. That’s what’s getting me worked up about this design. Its very simple for a proa. The rudder daggers go up and down by hand. One, fairly conventional sail (per tack) with a wide sheeting base for lots of sail shape control. A fixed, well stayed mast. And its the size that makes all this work.
You might be able to forego the ring frame with some ‘flanges’ stiffening the hull from just beyond the beams. Canting the frames in toward each other at their base would also help torsional rigidity as long as it didn’t compromise ergonomics. Of course the flanges would probably weigh more than the ring frame, so much for the free lunch.
That’s a good idea, Skip. I’d be willing to add a little weight to keep that cockpit clear. I don’t want to have to step over anything when switching from one helm to the other. A flange could help. Or maybe there’s a timber truss, like the one under the mast step of madness. Or maybe that side of the cockpit gets a balsa or foam core for stiffness… or….
I quite like the jib only rig (that you have been playing with) as it’s the simplest and by far the cheapest of the proa rigs.
I think that if the lateral resistance could be moved forward enough and the jib tack was placed far enough aft, it would work.
How does this look from a CLR forward, jib tack back, perspective, Russell?
Hi Everyone,
I liked this design so much that I asked Chris if he would mind sending me the CAD drawing so that I could do a CFD model of it. He obliged and asked that I post my findings, so this entry is an introduction to what I hope to do in what spare time I can scrounge at work.
I do computational fluid dynamics (CFD) as part of my job at work, though it is mostly on pollution control equipment for power plants. Though I’ve sailed a fair bit, I haven’t done much design work with boats yet, so this is a great excuse to learn. In case you are wondering, the software this is being done in is called Flow3D, and is pretty good for free surface flows, but not so great for lift and drag of foils. There is a different code that I use that is good for lift and drag on foils (Fluent) but not so good for free surface flows.
I’ve done a couple of trial simulations to get the hang of things, and here are the data inputs that I’ve used so far:
- Boat speed is fixed at 10-ft/s
- The displacement is 400-lbs
- The center of mass is fixed in the x- and y- directions (horizontal), but can float in the z- direction (up and down)
- The boat is free to pitch and roll (around the x- and y- axis) but cannot yaw (fixed in z-axis)
- I’ve done a flat water simulation, and an initial wave simulation with 2-ft waves.
Below are some pictures of what the model looks like so far, and I posted a short video here:
http://www.youtube.com/watch?v=WKIAl5EzCr4&feature=youtu.be
So far, the results are no surprise, there is a very small wake, and the boat sails nicely with a slightly bow up attitude in flat water. I haven’t really dug into any numbers yet, but here is a rough list of what I’m hoping to look at in the model as I have time to work through everything.
- Torque required to flip the boat up onto the safety ama, and then submerge it when stationary.
- Same as above, but moving forward (effect of dynamic lift)
- Drag as a function of speed in flat water for speeds up to 20-knots or so.
- Response to waves of various frequencies (upwind)
- Surfing down a wave (I haven’t figured out how to do this one yet)
- Various displacements. 400-lbs for boat plus skipper is on the lighter side, I’d like to look at it up to 800-lbs.
Please add any suggestions for what you’d like to see, if there is interest in a model run, I’ll probably get to that condition sooner!
Thanks again Chris for letting me play with this great design!
-Kimbal.
Hi Chris,
Have you ever looked at putting a reversed bow like this on any of your designs?
http://www.shaw4.org/blog.aspx
The aesthetics might be challenging with it on both ends.
-Kimbal.
Timothy’s lightning proa looks good with reverse bows!
I don’t like them on other boats, but on proa’s they look good and actually might work…..
On boats other than racing boats.
I’ve done a couple of trial simulations to get the hang of things, and here are the data inputs that I’ve used so far:
- Boat speed is fixed at 10-ft/s
- The displacement is 400-lbs
- The center of mass is fixed in the x- and y- directions (horizontal), but can float in the z- direction (up and down)
- The boat is free to pitch and roll (around the x- and y- axis) but cannot yaw (fixed in z-axis)
- I’ve done a flat water simulation, and an initial wave simulation with 2-ft waves.
How cool is that? Wow, Kimbal, thanks for your interest and industry!
So its going about 6 kt here. It looks sweet. I have a few questions…
I’m guessing 400lb might be a bit on the light side with a crew. The boat will probably come out about 400. I’m over 200 lb. I’d like to bring along some camping gear or another person (or both), so I’d like to think about displacements between 700 and 900 lbs. Maybe that’s wishful thinking for a 22’ canoe. My vaka is the same length as (though fatter than) Madness’ ama.
Yaw is very interesting to me. How much drag is there on the ama?
Where is the boat being “pulled” from? Can we include the height and ww/lw location (from the vaka centerline) of the CE, in order to figure out the effect of that on pitch, yaw and roll?
How do we take the findings of these simulations to make changes/improvements to the hull shape?
I’m really excited to learn about the dynamic lift of the safety ama, and the hydrostatics too. Next time I’m looking at the model I’ll take a look at the buoyancy of the safety ama.
So far, the results are no surprise, there is a very small wake, and the boat sails nicely with a slightly bow up attitude in flat water. I haven’t really dug into any numbers yet, but here is a rough list of what I’m hoping to look at in the model as I have time to work through everything.
- Torque required to flip the boat up onto the safety ama, and then submerge it when stationary.
- Same as above, but moving forward (effect of dynamic lift)
- Drag as a function of speed in flat water for speeds up to 20-knots or so.
- Response to waves of various frequencies (upwind)
- Surfing down a wave (I haven’t figured out how to do this one yet)
- Various displacements. 400-lbs for boat plus skipper is on the lighter side, I’d like to look at it up to 800-lbs.
All that sounds amazing. The yaw when surfing would be especially interesting. What about foils?
It warms my proa-nerd heart to see this kind of collaboration on this forum. Thank you, Kimbal, for bringing your unique skills into play for the good of proa kind.
Have you ever looked at putting a reversed bow like this on any of your designs?
http://www.shaw4.org/blog.aspx
The aesthetics might be challenging with it on both ends.
I have mixed feelings about reverse bows. Having seen the AC45s race in anger up close and personal, you can see the good and the bad. The good happens when the nose stays up, the bad happens just as the rudders are leaving the water because they sheeted the screecher too fast. At least that’s what it looked like to me. I get the idea of maximum waterline, buoyancy down low and the reduced windage. But if you don’t have racing rules telling you what the maximum LOA is, and you don’t need reduced windage to get through a tack, it may be less relevant to a proa. I feel the same way about tall, full, bows that I do about safety amass/pods. You might be faster in perfect conditions, but when you need them, it sure seems like it might be nice to have them.
Having said that, I really dig the bows on Nigel Irens’ recent round the world trimarans IDEC 2 and B&Q. The crisp chine extending some fullness above the waterline, but with a gentle reduction of the windage (and buoyancy) right at the bow. But I often think “what would Nigel do.” He is a master.
Thanks everyone for your interest.
My dream for this boat would be to offer the CAD files to cut the plywood and foam under a Creative Commons license, and let people do whatever they want for the rig. That would produce something between a one design and a development class. I bet we’d learn a lot.
Cheers,
Chris
Hi Chris, mmburrito can obviously do some interesting analysis. I would be very interested to see what difference his studies would show if the same hull was analysed in a “no rocker” configuration.
Regards
John
The plywood dory, without the foam bottom, has no rocker. The idea is that it can be built upright on a flat table.
The foam bottom is only 6 or 7” thick in the middle and about 2” thick at the stem. So there’s only 5” of rocker end to end.
Would you like to see a comparison to the same boat without the foam?
I believe that the foam creates a smoother transition from the stem to the bottom—I’m sure Kimbal knows a better term for this. Seen from the bow, the foam below the stem is tangent to the sides, and it transitions into a circular section by the time we reach the middle. So there is effectively no chine at the stem, transitioning to a chined but low wetted surface midsection.
So here’s a comparison I’d like to see;
The current design
The current design with no foam
A similar dory shape without foam, but a similar rocker (4-5”), and a flat bottom.
Tink believes that the flat bottomed dory is an optimum, based on VPLP tris. I’m not sure that hard chined dowries and the soft chined—transitioning to pure curvature—VPLP shapes are analogs.
I’d love to be wrong! There’d never be a need for any more than 3 sides for a skinny boat—as espoused by Skip and Gary Dierking. And for home builders there may not be!
But a closer look at the fastest oceangoing multis shows a lot of shape—especially in the bows.
...so said the armchair designer ; )
Chris
But a closer look at the fastest oceangoing multis shows a lot of shape—especially in the bows.
Even if the simple 3 sided hull was the quickest, best or whatever (and it is probably not) the designers have to do something to justify their fees 😉
That being said I suspect you would be surprised at the minimal difference between most of the scenarios, what one gains in certain conditions is usually lost in another. ad infinitum.
cheers,
Skip
I’ve been MIA for a while - I finished building my paddleboard and have generally been enjoying summertime. I’ve still been thinking about proas though, and this sort of proa (the KBBC) in particular. There has been some discussion about hull shape here, and I came across an interesting discussion of multihull shapes with respect to seaworthiness:
http://www.john-shuttleworth.com/Articles/NESTalk.html
What I take from this is that a rounded vaka with a daggerboard in the ama is my preferred configuration, which makes the CNC cut foam hull very attractive.
Cheers,
-Kimbal.
Welcome Back!
What rig do you think you would run on KBBK? I agree that that’s probably the best foil configuration. Part of the idea of KBBK is to eliminate stuff like raising and lowering lines for the dagger rudders—you can just grab them. But you might want a raising/lowering set up on the ama dagger board. More string.
Furl jib, shunt along track, unfurl jib. Done.
This, but without a track. A loop of Dyneema with both ends attached to the furling drum and a pair of cam cleats on either side of the cockpit. Furl jib, uncleat Dyneema, haul it to other end of boat, cleat the Dyneema tight, and unfurl the jib.