I put a ridge athwartship on the spoon to see if it would break the coanda effect, and it did easily.  It might work on a similar shaped lee pod.  That said, the stealth version with different colored panels looks really cool, and probably does the same thing.

Should also be fairly obvious that I’m not in the leepod camp, having not ever built one or carried the concept beyond a very preliminary sketch. Part of this is I’ve tried diligently to avoid water that’s cold enough to kill me, balance is I lean towards the very conservative side of proadom preferring cruising to racing (quick cruising to be sure but I don’t have the competitive drive of the racer’s I’ve designed boats for).

Still I like Patsy’s pod in the sense that the faceted shape is really slick (old gearhead term was “Trick”). OTOH a couple of ‘steps’ or discontinuities in a more conventional pod might eliminate the Coanada effect.

I get the argument for pod-less-ness.  The part I like is not so much the ass over tea kettle prevention (though that sounds good, too), but the popping the boat back up on its feet, before the ama starts rapidly loosing RM, because the lever arm is shrinking.  According to Sven’s excellent account, that’s about 15 degrees.

So dynamic lift seems like a good idea.  Now I don’t know how much coanda effect is going on with a Russell Brown style pod.  But I know that the faceted thing is going to be MUCH draggier at low speeds.  But maybe, just maybe there’s some attainable speed where that sucker skips like a stone.  I hope that if/when it gets dipped, we’re moving pretty well.  For now I’m digging the novelty.  It also makes a flat, level triangular table in the pod for the folks in the seats.

It would be interesting to try a diffuser thingie like Ryan has attached to a spoon on a boat with a curved lee pod.  My gut is that its not going to be noticeable, but that’s why we try things.  Guts are often wrong.

There may very well be a Coanda effect on a flared hul or podl, but dynamic lift has to be much greater….otherwise there would be a lot of submarines rather than yachts out there….

I hadn’t thought about the idea that the coanda effect should be pulling everything with rocker (aka, everything more or less) should be sucking into the water.  Interesting!

The coanda effect is a boundary layer effect. While it may seem like a big deal on a spoon with a thin film of water flowing over it, it’s not such a big deal in the context of a hull in a deep body of water. The lee pod will not behave any differently to any other barge shaped hull, i.e. any lift will be mainly due to buoyancy, unless you can get good flow separation at a high enough speed. The ridges shown on the model above are unlikely to create enough separation for pure planing at the speeds likely to be encountered.

As speed increases, displacement hulls generally exhibit some some sinkage at around ‘hull speed’ and will then begin to rise as flow separates from the aft sections at higher speeds. Flow separation will occur even if there is no ridge or immersed transom. The ‘hull speed’ of the pod is fairly low compared to that of the vaka, so if you are doing hull speed for the vaka, the pod may provide a small amount of dynamic lift if some separation has begun to occur. Whatever the shape, a short pod will be very draggy, and I would advise against intentionally designing the boat to sail with the lee pod in the water.

Mal.

I would advise against intentionally designing the boat to sail with the lee pod in the water.

That part I got! 

But thanks for the coanda comments.

I have a question for the group about mast spacing.  My tendency would be to put the masts as close together as makes sense.  In this case that minimum is something like the sheeting base—about 9-10 feet.  That keeps the weight of the rigs out of the ends of the boat. 

Are close together masts an aerodynamic interference issue, or a potential to build a slot between the sails?

What about having the front rig far enough forward to bear away?  Is there a minimum for that?

Just sharing my latest internal design arguments.  Happy to hear your thoughts…

chris

daveculp - 24 June 2014 11:11 AM

Hi Skip,

You were at Weymouth in 1986-7 were you not? I have a bit of video of the boat—and you—at that event. If I can isolate it—it’s only 20 seconds or so, would you like a copy?  😊

Dave

Hi Dave
Did you ever find the video of ‘attempt’ at a proa, because I would love to see that

Tink

So I’ve been playing with pods.  This time the usual shape, with one difference.

My pods are drawn to touch the water at 15 degrees of heel.  In order to do that, without being TOO wide, they sit below the sheer line.  The lower they go, the narrower they are (in plan view).  The more freeboard, the wider they get.

Here are two responses.  One sits 6” below the gunwale, and is wide enough to stow the sliding hatch.  That gives 18” of freeboard below the lowest point of the pod, at a full load (1200lbs).  The other has 22” of freeboard under the pod, but it also sticks out REALLY far.  So I gave it a reverse transom.  As opposed to the water ski, it looks like an ama when it gets immersed.  I’m worried it might trip the boat instead of let it slip to the side.  But as an ama, its kind of a nice shape.  And there’s something neat and 70’s-licious about the shape.  Like those Swans with the crazy tumblehome aft.

with the origami pod, (which also has 18” of freeboard)

more pods

Another question is mast distance.  On the orange boat, the masts are 12’ apart and the cabin is 8’ long.  The shorter cabin looks better to my eye.

The purple boat hs the mast 9’ apart and the cabin is 9’ long. 

There was some discussion about schooner rigs, overlap and interference a while back on this forum.

I can think of reasons to like both solutions…

more of the same

...

the origami pod at 15 degrees heel

and the conventional, deep pod.  It has the same freeboard as the origami pod, but is a foot longer, because the cabin is a foot longer.

I like the greater volume of this pod better than the bet on skipping like a stone…

My 2 cents, I don think the shape of the pod is critical. Probably a flat bottom is easier,,, plus it allows for simpler stowing of items. Adding an angle to the front and back so that if it does dig in, it will push up smoothly seems the most logical way to go. The outside of the pod I think should be angled vertically or outwards,,, no massive difference either way.

I like a longer outrigger, less drag IMHO.

Of course being small, you need daggerboards that are vulnerable to grounding, especially with the schooner rig your using, Of course a tacking version eliminates all this downside…. but then it would not be a proa. Very nice design and nice renders by the way

My 2 cents worth here
http://www.tacking-outrigger.com/TO18b.html