Sorry to go all the way back to the beginning, but that’s sometimes the best place to start… 😊

Manik - 27 May 2014 03:54 PM

I went for an L/B of 35:1, which is probably way too much), with low freeboard to save weight. It has a vertical sides and a semicircular midsection which is carried through most of the boat unchanged, which should make construction dead simple.

I approve (as you might expect!) although I think 35:1 is a little beyond the pale, but that’s what thought experiments are for—exploring boundaries. If this is a hypothetical racer, I’ll make some hypothetical comments and suggestions.

First, at 12 meters by .34 meters, with bottom a semi-circle and prismatic coefficient at .6, you’re already seriously overweight/under volume. The volume of either hull at the design waterline is 330 kg. If the boat weighs 500, it’s gonna run 3 inches or so below your DWL right from the start. (Not with both hulls equally weighted, then you have plenty. But as soon as you start to load one or the other, it’s overweight.

Not a deal killer; you just build your freeboard higher and gain a little bit of wetted surface, over optimal.

And about that 500 kg target… I don’t think you’re going to hit it with 2, 12-meter hulls, tubular akas and the big pod. Your hull surface area alone for both hulls is around 340 sq ft. Lightweight construction might get you below 2 lbs/ft but interior framing (hoop frames??), bulkheads and stringers might add half again that. Means each hull, if identical, will mass abut ¼ tonne; your half-ton is all used up just in the hulls. The pod is likely to go another ¼ tonne, akas and rig perhaps 75 kg each and we have a weight estimate close to 900 kg very lightship and also very lightly built. Go to PVC pipes for hulls and you will more than double, maybe triple that.
Again no deal-killer. Adjust the L/B ratio to maybe 25 and it will fall into place. It’ll be a stronger hull, as well.

I really like the sliding pod, but I think it might be problematic. The akas in any proa take a lot of wracking stress, and we want them to deform—so we don’t have to build them as massive as cat or tri akas. Unfortunately, this also means they’re going to bind in the holes under sail and the pod won’t be moveable. So, change the through-holes to 4 captive wheel arrangements (like the wheels on a roller coaster) and leave lots of room for them to flex.

What it doesn’t fix is that the boat keeps getting heavier. These dollies will be more massive than simple holes through the pod.  Divorcing the pod from the vaka means you need a lot of additional skin to “close” each one off—if they were mated together as a hull/pod they’d have considerably less surface area—thus weight—then when they’re separate.

Dave also pointed out that in general proas can’t produce anywhere near the amount of righting moment that a catamaran or trimaran can (for a given boat displacement). Maybe he’s right… snip… the crew and accommodations would probably make up half the displacement or more anyway.

No “maybe” about it, the physics on RM are easy to understand and not really refutable. If you like PM me and I’ll walk you through some examples. I’m hoping your accommodations aren’t half the mass of the boat; the weight estimate above doesn’t include any at all.  😉

The big red thing floating around in the air to leeward, without a connecting structure (like I said, thrown together) is an inflatable hull from a catamaran.

If this is a racing boat, how about making the emergency hull deployable rather than fixed, out there causing wind resistance all the time? It deploys automatically if the boat heels to maybe 60 degrees and stays out of the wind and weather all the rest of the time?

Accommodations can be pretty much camping cot and 1-burner stove for a racer, which might also mean the pod can be smaller—suppose it were just 4’ wide and 10’ long Just enough for 2 to sleep side-by-side and enough left over for a micro-galley at ne end? Could you live with that? Or take a page from someone else’s book (can’t remember who) and have two pods, one really minimal for racing and the other larger and a bit more opulent, for when the wife and kids are with you.

In terms of boards I was thinking of opting for performance and simplicity too. I’m thinking side hung daggerboards and rudders for each direction fo travel. You get to use good asymmetric unidirectional foil sections, you can get the daggerboard forward of the centerline slightly, and that at the time of construction you wouldn’t need to know where the boards belong, if they are side-hung you can try them out all over the place until you find the right spot.

You’re considering 4 foils total then; 2 daggerboards and 2 rudders? Might save some complication and potential damage if you could remove the forward rudder and bard when not needed—doesn’t need to fold, never breaks when big waves hit it, there’s less mass in the ends of the boat. Probably impractical, but it’s a thought

If you can hold it to 900 kg all-up (~2000 lbs) your 28 sq meter 300 sf) rig will yield a Bruce number of about 1.4. Not startling, the boat’s going to suffer a bit in light wind due to a lot of wetted surface with no hull-flying—hey wait a minute, you could use my auto-flight gizmo!  Lol.

What do you guys think? Totally crazy? Absolute nonsense? Will the performance be utterly miserable—if so why, and what can you do to make this boat give everything else a run for its money?

Not crazy at all, IMO

Dave

Hi Marco, Dave,
Like Dave, I thought the sliding pod, though a attractive idea, would prove problematical in practice. I saw trianguolar beams with three largish nylon rollers at four points on two beams. I’ve seen this in industrial applications and ot works very well. Though those applications did not have to deal with torque forces along the beams. I think triangular beams would minimize any twist and the rollers would handle any torque that is present anyway.

But, ironically, the problem that I see is related to how well it would work. This pod would rocket along the beams with this roller arrangement and holding it in situ in heavy seas might prove difficult or heavy at least. It is not the sort of thing you want to malfunction when you might be relying on it most. So a friction brake would be prone to failure and some sort of positive locating device (a pin or key) might prove awkward and difficult to use in practice.

Like Marco, I don’t see it being useful in many situations anyway. So I figured it wasn’t worth the weight, cost and trouble especially on a simple, lightweight boat.

I think Marco’s design would be a perfect application for Dave’s auto-flight gizmo (not that i understand it . . . yet)

Changeable pods is another good idea. It would work best if the boat is trailable and can be manoevered under a chain block or two.

Marco,

Having encouraged you to look at multiples of 5 metres, you seem have gone to the extreme….

Virtually all of what Dave says I totally agree with….so my suggestions and comments are:

1) Do an 8 or 10 metre long vaka in 2 pieces of a more practical L/B ratio. Bolting them together will be simpler lighter and faster.
But design them in such a way that you can add a third central 4 or 5 metre “cruising"section sometime in the future.
You can have a sporty camper with removable pods now, and something a bit more comfortable with the necessary displacement to go with it later. Ditto the ama (proportionately).

2) Forget semi-circular bottoms, they might give you a slight theoretical speed advantage for a given vaka length, but they are more expensive to build and aren’t as practical as a flat bottomed vaka, especially as your main intended cruising area is shallow and sandy / muddy. You want minimum draft.

3) I can’t see how your rig stands unless it is a balestrom rig. Any form of cantilever mast is heavier than one held up with wires and usually are more expensive as well. Saving weight aloft is the best thing you can do for a lightweight yacht. Adjust your vaka / aka support structure to get the mast out a bit further out so that there is enough leeward angle to put on forestay / shrouds.

4) I know it is early days and you have been concentrating on other things, but your vaka / aka / ama connections seem OTT. They need to be minimised and streamlined with wave impact and air drag in mind.

5) Side hung rudders, Use both all the time.  Better balance flexibility and better steering in tight situations or when both are partly raised in shallow water and Jzerro style ones would be already scraping the bottom. Watch out for wave wash on low slung assemblies which happens when they are mounted on low freeboard hulls (look at some of the Harryproa / Blind Date videos).

Otherwise, go for it….

Rob