Dinghy on a stick v.2
Here are some more recent musings on the dinghy as lee pod concept. With a less *aesthetically challenged* pod, it looks a lot better.
From a looks perspective, having the pod level looks great. But I wonder if canting it, so that the bottom hits the water more or less flat upon immersion, might have better dynamic characteristics.
Anyway, this is an all new, more carefully constructed model—this one is fully parametric. It doesn’t explode as much as the green one did. But still 24’ LOA, 12’ center to center beam.
The more I think about it, the more having a little dinghy makes sense for the Bay Area. It makes it possible to leave it anchored out, rather than being tied to a marina. I have a novel outboard idea brewing as well.
Best,
Chris
two more shots from head on (with a little perspective).
Again, flat looks better, but tilted might have some real dynamic lifting benefits when you hit that 15 degree pod interface angle. Then again, maybe immersing the chine is more gentle.
Russell, Sven, any thoughts on that?
Aloha Chris, yes, I like te idea ,
Beam, small note;
I see you place the beams into the ama like Pacific Bee (Cimba) previously had, I would run them straight towards the ama , and connect them well clear of the water, basically like traditional connections, In the heavy (Bay) short/ steep waves , against the tide outward bound for instance, you may bury the ama now and again, the exposed beamend and bend, will than hit the wave, consequently rounding you up all of a sudden. happened to me few times with Cimba, its quit violent if you are doing 17knots or so.
Another problem is that the beams starts to rotate in their sockets, caused by the frequent stops of the ama, and because it is at some 45 degrees, besides connecting it outside is easier and quicker.
interesting pod ideas, it would be nice to test that with a model, to fine tune the shape ,are you planning that ?
a view video’s would explain a lot.
Cheers Sven
Aloha Chris, yes, I like te idea ,
Beam, small note;
I see you place the beams into the ama like Pacific Bee (Cimba) previously had, I would run them straight towards the ama , and connect them well clear of the water, basically like traditional connections, In the heavy (Bay) short/ steep waves , against the tide outward bound for instance, you may bury the ama now and again, the exposed beamend and bend, will than hit the wave, consequently rounding you up all of a sudden. happened to me few times with Cimba, its quit violent if you are doing 17knots or so.
Another problem is that the beams starts to rotate in their sockets, caused by the frequent stops of the ama, and because it is at some 45 degrees, besides connecting it outside is easier and quicker.interesting pod ideas, it would be nice to test that with a model, to fine tune the shape ,are you planning that ?
a view video’s would explain a lot.Cheers Sven
Not such a small note! Those real life experiences are golden. .....Skip
Aloha Sven,
The torque on curved beams is a SUPER valuable insight. I’ve redrawn the dinghy on a stick with more triangulated streamlined connections. These would mount to brackets and pins under the aka. Maybe by having the pins perpendicular to the centerline of the vaka it will introduce some compliance as well. I’ve drawn them as CNCed plywood. But a sticks-and-string type of connection could be a cool option—in composites of course.
I’m definitely thinking models, both to try rigs (junk v sloop) and pod configurations.
Thanks for the advice about the connections. Here are some renders with that revision.
You can really see how much less exposure these ama connections have to oncoming waves.
But I couldn’t resist *a little* curvature. Even 2” makes a difference aesthetically.
Hey Chris , some ideas ....... Cheers Sven
[edit; 10thDec11 ; brace wires to run from bow to end of aka.]
Aloha Sven,
Excellent thoughts! I’m glad you like the ama shape. I’m especially proud of how the bows came out almost native-american-canoe shaped. I’m going to trim them down from 20’ to 18’ in the next version,
Should the guide wires attach to the ends of the akas or the ama attachment points?
I do not like the look of tubular akas. Plus, the bending loads exerted on them are more up and down than side to side (especially with the brace wires in place) so a round shape seems misaligned with the loads. But If the ama connection is going to pivot (as I’ve drawn it) the attachments might as well wrap around a round aka for maximum contact area/low point loads. I’ll draw that next.
I’ll put a dagger board in the ama too…
Thanks for your continuing advice!
best,
Chris
Chris,
Very cool boat you have comming together there. I love the images, what software are you using to generate them?
Tom
Hey Tom,
I’m a product designer so I used the tool I use every day to design—Solidworks.
The plug in for the renderings is called photoview 360.
Its a grea software program, but there’s no hydrostatics plug in that I can find. Bummer.
All the best,
Chris
Chris,
Ahh, ok. I’ve just recently downloaded and started using carlson’s free hull program, and I know that can be imported into autocad and some other programs as a DXF. Haven’t exactly figured that out yet though.
I’ve also downloaded the free version of freeship, but that baby is gonna take some serious study and playing to get going on.
Thanks,
Tom
hi Chris, I’m really enjoying watching this design iteration taking place. Like Tom, I’m impressed with your renderings.
If you move the leeward ama brace as Sven suggests to the max position to leeward and if both braces extend down to the keel, you then have a tetrahedron shape, the strongest and most rigid 3D shape there is in the whole known universe (and maybe beyond!) for a given amount of material. All forces are taken in compression and tension by the members without any bending moments on them. The Pacific Islanders knew what they were doing!
If you were to add two more wire braces you could make two more additional tetrahedrons to further strengthen your design. Run them from the point where Sven’s braces attach near the ends of the akas directly under the beams to the vaka’s hull sides. These then describe two cross-over tetrahedrons and would account for all forces except downward force on the ama (in an unstayed rig situation) and forward horizontal force on one of the beams but this force would be taken up by the aft beam supported by its tetrahedron-making braces. This transforms any bending moments into compression forces along the beam which is ideal use of material and strength. The ‘almost’ straight beam is ‘almost’ ideal for this as well. 😊
Sven’s braces would reduce any ‘shuddering’ forces from running the ama into waves being transferred to the vaka to a minimum. The additional braces would contain any upwards forces generated at the same time. They would help in a caught aback situation as well as the occasions when the ama is flying and is suddenly ‘dumped’.
I like the 2” curvature you have put in for aesthetics. It works visually. Dead straight lines can often look like they are curving the wrong way i.e. away from the body of the object under design. Great job!
As a product designer you are probably well aware of all this. So please pardon the structures lecture but I thought it might be of interest to others
Aloha Tom,
Let me know how the naval architecture freeware works for you. I wonder if Google Sketch Up has ever been used in conjunction with that—to draw the rest of the features of the boat. A lot of people use that to do CAD—even in the business. Anybody have any luck designing boats in Sketch Up?
Hey James,
One cannot be reminded of the magic of tetrahedrons enough. That sounds sarcastic, but I mean it. Spheres know a lot! I incorporated that suggestion in this version!
I like the idea of stays near the waterline to turn the windward shroud forces into compression, but I worry about dragging wires through the water. Thoughts?
Having said that, I put weak-in-the-up-and-down-direction aluminum tubes on the latest iteration of the dinghy on a stick (which is getting a bit of apple crate back). I have been thinking a lot about the torque Sven talked about. Connecting the amas around that axisof a tubular aka does make sense once you start down that road. I’ve tried to capture a bit of the visual taper back with the spray guard. It looks big but its just a bit more than a foot tall. It might help keep it drier….might…
Thanks for all of your insights. Here are some new renderings.
3 more…
I like the idea of stays near the waterline to turn the windward shroud forces into compression, but I worry about dragging wires through the water. Thoughts?
hi Chris, I can’t speak from experience but I look to the late Lock Crowther who was very performance oriented and used them on many of his trimarans. Obviously there will be some drag at times but how much? Common sense says ‘not much in the scheme of things’ as the frontal area and total surface area are very small.
Unlike a trimaran, a proa has only the stays to windward of the vaka so they are lifted to some extent away from the water as speed increases. A trimaran has stays to leeward as well and has them pressed towards the water surface as speed increases and still Crowther used them and to very good effect.
I imagine that by the time the stays are encountering serious green water the drag from those stays would be the least of the increased drag the proa is suffering from. But these are just opinions from someone without any direct personal experience let alone from someone who has carried out detailed experiments 😊
Aloha James,
You make a good point about them being to windward…and the further down the aluminum tubing route I go, the more sense triangulation in opposition to the windward shroud makes sense.
Mike Leneman, down in Marina Del Rey has some cool ultra light rigging bits and pieces. His L-7 trimaran has all composite rigging and they saved a LOT of weight with that—even with an aluminum mast. With all this string holding the dinghy on a stick together, composite rigging might be a cool way to go.
I’ve added a few more details in these renders and tweaked some overall dimensions. The ama is now 18’ (down from 20’) and the beam over all is 12inches—not quite 20:1 but the V is quite pronounced, so BWL will probably be in the 20:1 range.
The Vaka is a couple inches narrower over all, with a bit of a deeper V. BWL should be about 14:1. I raised the bows a couple of inches. The stems would be faired with foam.
I extended the “cockpit” to leeward a bit so that it fits snugly against the dinghy, and added some necessary gussets. The idea now is that the major parts are the vaka, akas, ama, cockpit (including mast beam), dinghy/pod and the rig. The cockpit break water looks like it might interfere with tillers—the design of which I’m still considering.
Also turning up the perspective factor in the rendering software makes it look sexier.