Proas are pretty cool vessels, if only because they’re different. However, the sum total of their differences—lighter for length, cheaper for length, power for length—all these converge on length. We all know that the longest boat wins, but hey, we’re not millionaires, right?

What if you could have a longer boat, for free? I’m not talking about dollars here, I’m talking about having a longer proa at the same weight, the same sailing rig, the same transport and storage space—as a much “smaller” boat.

Say you had a 14’ proa, maybe 8-10’ wide, with sort of 120; maybe 150 sq ft of sail. Would it be too cool if that hull could be “stretched” to maybe 22’ overall, only while sailing and without adding but a few pounds weight, a few dozen square inches wetted surface and a few dollars added cost? Keep your rig, keep your trailer/cartop setup, keep the wife’s peonies safe from “boat bloat,” but sail FASTER than you ever thought your budget allowed?

Am I selling snake oil here? Certainly sounds like it, yet I think the above can be done. Mr Maguire whispered “One word; plastics” into Dustin Hoffman’s ear in The Graduate. I’m gonna whisper “inflated bows” into yours.

Imagine this:  We have a main hull drawn at 22’ overall. It has vertical bows, perhaps 14” of beam on the waterline, and for simplicity’s sake, has a simple rectangular midships section (I’d prefer a nice oval, but let’s not quibble). Call it 22” high by 14” wide, amidships.

Now let’s take this graceful and slippery hull and brutally whack 4 feet off both ends. We end up with a 14’ long hull, 22 x 14” midships section. The cross section at the cuts might be closer to 6” wide, by most of that 22” vertical height—and still be rectangular (or oval) in section. Let’s go ahead and seal off the “stumps” on the hull and set it aside for a bit. Note though that it weighs pretty much the same as any 14’ proa vaca, and still contains the cross beam mounts, daggerboard trunk, mast step, rudders/steering oars, etc; everything that the proa needs to survive. She’s just… shorter.

Now let’s look at those whacked off pieces. Each is 4’ long, with a root cross section of 6” X 22” (give or take) Because they aren’t running very deep at this point their at-rest wetted surface is on the order of 1.2 sq ft, each. The cutoff part of the main hull, for comparison, has about 17 sq ft wetted surface, so the two “extensions” add in total, something like 12-13% to the hull’s total wetted surface, over what we might have had in the 14’ hull (at the same mass).

But what about weight? A 4’ long inflated hull, built of 4 Oz Dacron sailcloth with inflatable PVC bladders inside (Let’s really control costs here, and use a couple of Walmart air mattresses for bladders), including a rigid “coupler” to attach it to the ends of the main hull, might need on the order of 2 lbs of cloth, a pair of 1.5 lb bladders (4 total, for redundancy) and half a pound of plywood for the coupler, for a total of 5.5 lbs apiece, 11 total lbs added to the boat’s mass.

I’m leaving he ama’s construction and mass out of this to simplify the value calculation, but of course the ama can be partially—or totally—inflatable as well, thus could become longer, again adding only the smallest of additional mass and cost. This might yield an even more 14 foot-like boat with 22 foot-like performance.

What about the rig? How much horsepower do we need here? Well, if the 14’ proa has a hull that’s 14” wide amidships and the 22’er has the same, we’ve moved from a L/D of 12:1 to one of 19:1. We’ve pretty much eliminated wave-making drag altogether. We’ve added 12% to the wetted surface mind you, but can we maybe call these a wash? Reduced wave-making drag equals increased wetted surface drag? This is a trick question because it’s very much speed dependent, but let’s be conservative and say that drag increases 5-10% more or less on average.

Did we just say total drag increases 5-10% when moving form a 14’ hull to a 22’er?? Heck, sign me up!

If we’re going to keep things strictly comparable, we’re gonna need to grow the rig by 10% in area to make up for that added drag. This is an area consideration, so we’re saying that the mast and boom each needs to be what, 110^.5; call it 5% longer?

If the mast needs to be 5% longer, than the crossbeam perhaps wants to be 5% longer to take the additional heeling moment (which means we go from say 8’ beam for the 14’er, to 8’ 6” beam for the 22’er. Not a lot, izzat?) My point is, you could almost take the little boat’s rig, crossbeams, trampoline and its ama wholesale, without a bit of alteration or added cost, and put it on the big 22’ hull, and Bob’s your uncle. Or put another way, we can consider a significantly larger rig or sail in higher wind, without increasing either total mass or drag very significantly, and have a much faster boat under us.

There are lots of questions, like how much air pressure, what happens if we spring a leak, who’s going to do that sewing, how will we connect the bows, would this work on a tacking outrigger? A kayak? A catamaran/trimaran? I’ve given these some thought, but this has gotten long enough. What do you think?

Dave Culp

I like the idea a lot.  This has a lot of appeal for creating a one person car topper for the beach.  Michael and James’ The Pookie got this ball rolling a long time ago.

http://www.proafile.com/archive/article/teh_pookie

I don’t know about the wall mart air mattresses.  But simple welded PVC structures make a a lot of sense to me.  Why not make the ama inflatable too?  The rig, akas, roll up tramp/tubing and vaka go on the roof.  The bows, ama and sail are in bags in the car. 

Might it need a stiffener to keep the bow/stern from deflecting?

Dave,

I have always liked the idea of inflatables, they bounce off things, you can drag them over rocks sand and shells without too much damage to the paintwork so to speak and you can deflate them then roll them up to make a much smaller package for carrying carting and storing.

Michael and James’ Teh Pookie I thought was a lovely piece of design and very eloquent in many ways.

Your idea of a rigid central box would work well in the case of the unlikely event of a catastophic punctures, at least you have a floating box to get you home…Not sure about the rectangular underwater midships section though, the transition from the end corners to a round / oval shape towards the bow which is what is usually needed for inflatables would be a weak point, so why not stick with your favourite oval (or at least oval under water) shape all the way through?

I have often wondered about carrying an inflatable lee pod or a streamlined deflated tube along a proa’s leeward gunnel, which auto inflates if you get to say 60 degrees heel to greatly improve capsize resistance? It doesn’t have to be instead of a solid lee pod, but as well as?

Rob

Rob Zabukovec - 17 March 2014 03:32 PM

Dave,
Michael and James’ Teh Pookie I thought was a lovely piece of design and very eloquent in many ways.

Teh Pookie is da bomb. I’d have her in a second if I could.

Your idea of a rigid central box would work well in the case of the unlikely event of a catastophic punctures, at least you have a floating box to get you home…Not sure about the rectangular underwater midships section though, the transition from the end corners to a round / oval shape towards the bow which is what is usually needed for inflatables would be a weak point, so why not stick with your favourite oval (or at least oval under water) shape all the way through?

I specified a rectangle only as a visual example; didn’t want to get sidetracked by a hull shape discussion.  😉  I’d definitely use something more rounded, or multi-chined, or, or.

I have often wondered about carrying an inflatable lee pod or a streamlined deflated tube along a proa’s leeward gunnel, which auto inflates if you get to say 60 degrees heel to greatly improve capsize resistance? It doesn’t have to be instead of a solid lee pod, but as well as?

I have built and sailed with inflated lee pods/safety hulls. In this case it was about 14’ long by ~12” diameter (turned out bigger than it needed to be—shoulda gone 10”) The whole hull weighed around 15 lbs including some wood framing and the stub crossbeams added another 4-5 lbs. Truly painless. I used bladders from an AIRE inflatable expedition kayak. $120, but bulletproof and with an industrial valve on it. I still have the hull.

To Chris: I would try hard not to use any framework, and chose 4’ for the length of the inflated bits for just that reason—I want this to be blow-up and go sailing, no assembly. This much of the hull sis lightly loaded, so even if soft due to leaks, there’s not much performance lost (btw, I’ve never had a leak, either with my hull nor wth ~50 total hours in an AIRE, bouncing off rocks and sharp stumps). Also with a longer, thinner boat, you tend to run into more things—and people. Nice to be soft there.

The only and sole issue I have with Teh Pookie is that she still has a frame to schlep around, bump into things, etc. I’d look real hard at a tensairity (not tensegrity) structure if I were building a complete boat. A tensairity structure still has a single compression member, but this can be small enough to coil—or even remove if cleverly made. Another alternative for Teh Pookie, I believe,  is to go with high pressure air in the main hull, but this does better with kevlar for the shell and careful construction, enough to hold ~20-25 lbs pressure. The boat would be marvelously lightweight, making her wicked fast! Be no big deal to use 2, even 3 bladders/hull for multiple redundancy, even without additional inflation.

Dave

If you’re going to inflatable ends (and maybe ama) then you could use them as soft rollers (a la Rollogon) to beach the boat over all sorts of ground…