...in the water, carrying its own weight!
...in the air! They’re called flying proas for a reason!
...somewhere between the two.
Flying and foiling amas
It seems then that a lot of the difficulty of a foiling ama is caused by trying to have the foil provide vertical lift as well as leeway resistance. Why not separate the two functions?
To me the argument which speaks against separating the two completely is that the fewer boards you have in the water the better, the less induced drag you have. At present that would be two boards; the rudderboard and the daggerboard, and perhaps there’s a partial extension of the front rudderboard ontop of that (seems to be the case with Russell’s proas). If foiling the ama is what you want, then using the daggerboard in the ama to do it is the natural choice over adding another foil of some sort to the setup.
When it comes to foiling even when going to windward though, I think you are absolutely right. You could have say a daggerboard in the vaka to generate the sideforce, and use some other kind of foil in the ama, which has the sole purpose of keeping the ama out of the water. Then you could foil on all points of sail. I have no idea what a type of foil would be sensible for the ama then though, it should be something retractable, only needs to generate lift in vertical direction, has to control ride height, and it should be able to carry quite a bit of weight; the ama could get pretty heavy if you are going to windward with lots of water in the ballast tank…
I get that as the a canted ama foil lifts out of the water, leeway resistance reduces which supposedly lets the boat slide sideways and reduces overturning moment. But is that a real or theoretical advantage?
What do you mean exactly? I think what Skip was referring to was the opposite situation, that you are sailing downwind, say on a broad reach, in heavy seas. A wave passes underneath you, and the proa slides down the back side of the wave backwards / sideways. The ama foil, if canted, would be pointing pretty much directly in the direction in which the boat is sliding, which would mean that the ama foil would bury itself (and possibly the entire ama) very quickly, it would just stab right into the wave.
If you are infront of the wave on the other hand, you are right, in that the boat could sideslip better. I would say the risk of burying the ama on the backside of the wave is far greater than the small benefit of the extra sideslip infront of the wave though, so I would definitely have the board up, or at the very least set to vertical, if confronted with rough seas. In my opinion foiling the ama is something for good to moderate conditions only, when the shit hits the fan, safety definitely comes first.
A number of things here….....
1) Skip and Michael are right : If you only want horizontal (anti-leeway) thrust, the only way to go is with a vertical foil…..However if you want to do both, vertical lift and horizontal, it seems to me that a canted foil is best, because it has less foil length (and hence resistance) than any combination of vertical and horizontal foils (hypotenuse always being shorter than the sum of the other 2 sides of a triangle). Plus you lose lift area at the junction of vertical and horizontal foils plus T foils have 2 tip vortex losses instead of one..Plus a canted foil is simpler to build and is more practical in many ways.
2) Any foil system providing additional righting moment can be “popped” unless it is counter balanced somehow like “Hydroptere” so I aim to use it as emergency / gust response rather than basic RM…..
3) Marco is right, I currently have an 2 way asymmetric ogive foil and 0 degrees AoA….which gives a pretty miserable lift coefficient…....I don’t have to commit just yet, so I still am looking at ways of being able to vary the AoA and have a one way asymmetrical foil which flops over during a shunt..It can be done but at the moment my current solutions are too complicated for a singlehanded 9.5 metre proa and my engineering abilities.
4) If a foil looks like being “dangerous” downwind in big seas, don’t use it. It is still there ready to use for the appropriate conditions. Just like a spinnaker.
5) Marco, upwind is where you would use a canting foil most, and with or without a foil, you can fly an ama any time you want just by carrying sufficient (too much) sail for the conditions, just like any other multihull.
Rob.
Hey Rob,
I really like how you’ve broken it down to fundamental principles here! I do have to object with this one though:
2) Any foil system providing additional righting moment can be “popped” unless it is counter balanced somehow like “Hydroptere” so I aim to use it as emergency / gust response rather than basic RM…..
A foil which is to leeward of the center of flotation can’t really be popped. An example would be a foil mounted at the outermost point on the lee pod. If it leaves the water, due to wave action, the boat heels a little until it’s back in the water. I’m not saying that’s necessarily a good (or a bad) idea, but it’s possible.
3) Marco is right, I currently have an 2 way asymmetric ogive foil and 0 degrees AoA….which gives a pretty miserable lift coefficient…....I don’t have to commit just yet, so I still am looking at ways of being able to vary the AoA and have a one way asymmetrical foil which flops over during a shunt..It can be done but at the moment my current solutions are too complicated for a singlehanded 9.5 metre proa and my engineering abilities.
You’re right about the lift coefficient. I hadn’t really considered that. Changing the AoA of the board in the trunk, or of the entire trunk, seems like anything but trivial though, and one trunk for each direction of travel means more building time. Hmm…
5) Marco, upwind is where you would use a canting foil most, and with or without a foil, you can fly an ama any time you want just by carrying sufficient (too much) sail for the conditions, just like any other multihull.
I guess you’re right, you would have to carry a lot of the water ballast on the foil though. If you look at the RM, then that what you need to counter the heeling moment of the rig, you get lifted out of the water for free. Any reserve RM that you want (which is an absolute must), must be countered by the foil lifting the ama. You can choose how big the reserve of stability is though (adjust the amount of water ballast), and since the daggerboard has to produce a lot of lift to windward anyway, it’s not so tragic if it has to lift quite a bit of weight upwards on that course too (in case you want a large stability reserve).
Cheers,
Marco
.........I do have to object with this one though:
2) Any foil system providing additional righting moment can be “popped” unless it is counter balanced somehow like “Hydroptere” so I aim to use it as emergency / gust response rather than basic RM…..
A foil which is to leeward of the center of flotation can’t really be popped. An example would be a foil mounted at the outermost point on the lee pod. If it leaves the water, due to wave action, the boat heels a little until it’s back in the water.
Sorry, I thought we were talking about ama foils, not vaka foils…...
A canted (to windward) vaka lifting foil of course would not “pop”......the main effect would be to make the vaka lighter. Depending on foil length and cant angle, it might even be that you have reduced RM, but in any case the (dynamic) centre of buoyancy to shift to leeward and consequent RM gain would be peanuts in comparison to a canted ama foil…..A vaka canted to leeward lifting foil would shift CB further to leeward and further increase RM, (still peanuts relatively) but would increase leeway…..A vaka canted to leeward downward thrusting foil would be OK for leeway, but would add to heeling moment not RM.
Rob
Marco,
One last thing…....If you are serious about most of your sailing being in the Waddensee, for a proa of your size, I would strongly recommend that you have a good look at Garry Dierking’s side hung rudders…...complete with the accidental grounding flip back. I would actually use both Dierking rudders all the time….Better for steering, better for balance and leeway and you can sail in shallow water with both rudders half raised to cheat the tide.
Rob
Sorry, I thought we were talking about ama foils, not vaka foils…...
We were (and are) talking about ama foils, and I don’t think a foil on the leeward side of the vaka is a good idea either for the reasons you mentioned, plus the trouble and drag of another board.
—Have you had any ideas thus far how one could solve the AoA problem with the canting ama foil? You said you didn’t have anything practical, but any thoughts at all might be helpful for a place to start. So far my thinking has been limited to the fact that since it’s the AoA we want to control, it would be sensible if the mechanism which adjusts the AoA cants along with the whole setup, so you always control AoA directly, and never indirectly. The best I’ve come up with so far is that the entire daggerboard trunk could be hung on a big hinge of some kind, and then entire assembly, hinge and trunk together, is mounted in the canting mechanism. The canting mechanism itself could just be another hinge as well, mounted on the floor of the ama.
Marco
Marco,
One last thing…....If you are serious about most of your sailing being in the Waddensee, for a proa of your size, I would strongly recommend that you have a good look at Garry Dierking’s side hung rudders…...complete with the accidental grounding flip back. I would actually use both Dierking rudders all the time….Better for steering, better for balance and leeway and you can sail in shallow water with both rudders half raised to cheat the tide.
Rob
Thanks a bunch for bringing that point up…again! 😉 I put a bit of thought into it, and for shallow water steering, using a steering oar has always appealed more to me than a sidehung rudder, for the greater simplicity. Now as it happens, Firstborne, which is to have a double scull sliding seat rowing setup on the deck, is lugging 4 oars around with it anyway. Adding additional attachment on the iakos for instance, to allow an oar to be used as a steering oar, hardly adds to build time or complexity, and whamo, suddenly I can sail the boat with a draft of 27 centimeters! 😊 I can’t imagine it’ll really go to windward in that state, with the daggerboard down 20cm, but whatever.
Now we just have to get this canting daggerboard business worked out somehow…
Thanks a bunch for bringing that point up…again! 😉 I put a bit of thought into it, and for shallow water steering, using a steering oar has always appealed more to me than a sidehung rudder, for the greater simplicity. Now as it happens, Firstborne, which is to have a double scull sliding seat rowing setup on the deck, is lugging 4 oars around with it anyway. Adding additional attachment on the iakos for instance, to allow an oar to be used as a steering oar, hardly adds to build time or complexity, and whamo, suddenly I can sail the boat with a draft of 27 centimeters! 😊 I can’t imagine it’ll really go to windward in that state, with the daggerboard down 20cm, but whatever.
Be prepared to switch to some form of rudder. I really like steering oars, have used them on three proas, great attributes of simplicity and versatility.
However.
Oar(s) worked very well on two 14’ proas, ACDC & ACDC2. Experience with larger P52 was mixed. Oar worked fine as long as there were warm gentle breezes (or cold for that matter probably, I’m a warm water wuss). When the wind picked up and forces got a little higher there was a runaway feedback mechanism where.
The boat had a little weather helm and oar adjusted accordingly.
Oar bent just a little bit so a little more adjustment was made which bent the oar just a little more….
(see where this is going?) Shortly you are screaming along at 12-14 mph; oar bent in a long arc with white knuckles and knotted biceps.
I couldn’t make an oar stiff enough to keep the deal from happening, uni carbon included.
It may be with a different geometry with less distance from the blade or a steeper angle to pivot might work.
FWIW P52 always steered two fingers light when I switched to quarter rudders.
I don’t want to completely disparage using oars, nothing else will swing your boat around like sweeping an oar but felt compelled to recount my experience.
Best of luck,
Skip
Thanks for sharing! Looks like I will have to find another solution, at least if I want to sail at the boat’s normal speeds in shallow water. I just had a look on wikipedia yesterday, even the vikings dropped the steering oar in favor of a ‘steerboard’ in the 10th century AD. The had a completely lashed setup, with two lashing points so they could set two different rudder depths. I’ll figure something out, but let’s get back on topic here!
Do you guys think it’d be practically feasible to control the AoA of a canting dagger foil? Or should you just have a separate trunk and board for each direction of travel, with a reasonable AoA fixed from the start? Adjusting the AoA of the canting trunk/board would face some of the same difficulties as the concepts for rudders that should go both ways: you’d get some pretty high loading and would have to have quite a stiff construction to keep the AoA setting where it is. Another alternative would be to just bite the bullet and accept the 0° AoA, but I think I’d rather build two trunks. What do you guys think?
Great but long video of the AC72 development for those that missed it
First half back ground but from about 45mins in lots of meaty details of what these guys actually did.
See IPENZ web site for more info including a pdf of the references
Tink
Coming to this thread late, sorry. I was going to comment on half a dozen earlier comments, but most have been adequately covered, so…
In the great scheme of things, Pacific proas are woefully underpowered—they simply cannot generate anything like the RM both cats and tris can. Fortunately, they can also be much lighter, especially for their length and also far less stressed, allowing even lighter construction. So we have the opportunity to design light, cheap boats with power-to-weight ratios that are more than respectable—even superior if managed carefully. The mantra of light weight and long hulls must be followed, though. Building an overweight proa—even a “normal” weighted one, puts you straight into condo-maran territory. And if it’s a small one, you can’t even bring your stuff. http://www.youtube.com/watch?v=4x_QkGPCL18
But the important thing to remember is that our little engines (net rig thrust) just aren’t very powerful. (net rig thrust = net rig force component parallel to the water’s surface and directly aligned with the course)
So if we’re going to do things like fly amas, or fly whole boats, or hold down one or the other, we *must* do it in the most efficient way possible. For most conditions and courses, buoyant lift from a long skinny hull is the most efficient way to support the boat in the water. Playing around with hydrofoils on any sailing boat, let alone on our little VW-reminescent powerhouses, is a game of inches. The drag from the foil must be less than the hull drag it replaces—and if we’re not going to reduce hull drag (as for instance with “hook-in” or “hold down” foils) then we’d darn sure better increase the RM—a lot. There are no rules of thumb here, every different layout, at every different load, speed and course needs comparative analysis to determine where the “windows” of opportunity for foils will lie. Of course Oracle did exactly this, but they have a larger budget than we do.
Bottom line, we want to shepherd and husband our net thrust to keep it working on moving the boat forward, efficiently. OTOH, we have other ways to fly hulls which don’t steal net thrust (drag = negative thrust, right?) Moving the crew in and out to balance flight is a no-cost way to do it—no added drag, no reduction of thrust—but we’ve discussed how this is a walking-the-knife-edge approach, haven’t we? We can run a version of the same game with playing the mainsheet in and out, but that gets pretty darn tiresome, pretty darn quickly—and we’re still walking that knife edge. A combination of the two is useful, but no safer or easier to use. I’ll lay out another possible solution here, in a couple of days.
Lifting the ama with a foil—when it could have been done with weight shift, mainsheet, etc—is a generic loser. We’re stealing thrust to do it, when optimal solutions steal less—or none at all. An aside; inclined foils are an old-skool approach. Yes they’re self-limiting, and yes, they (sometimes) have less wetted surface, but in actual racing and cruising boats, it was found they ventilate quite easily—all that negative pressure, so close to the water surface. Adding fences to stop ventilation doubles up on wetted surface quickly—you have to count both sides of each and every fence. Last, it was found that, when sailing above design speed, on the foils’ “tippy-toes,” too little of the foil is in the water for reliable sideforce lift—so small vertical tiplets had to be added = more wetted surface, more induced drag, more form drag. The unlimited budget guys can find the clean efficient solutions for this problem—for a limited set of speeds, water conditions and crew talent—but it’s been found not competitive for the rest of us.
Another aside; many early hydrofoil boats used ogive sections—arc-of-circle top surface, dead-flat bottom—to good effect These were easy and cheap to machine, even of metal. Their lift coefficients weren’t stellar, but were certainly respectable—and Tom Speer’s improvements have made these better—and also eliminated the sharp leading edge—which are huge ventilation-inducers. Also don’t forget that leeway is giving you AoA. Depending on the size of the board, this can be 4-8 degrees with a zero setting on both tacks—plenty for an asym section.
Last aside; Pizzey’s leeward-leaning rig is indeed brilliant, but trades net thrust for stability and safety. The rig is never fully “on” in order to maintain the moment-chasing trick that keeps them from capsizing. Also, they were indeed safe in a sudden knockdown at zero speed—they didn’t require any way-on to achieve their non-capsizing. A similar yet more powerful approach was Newick’s 5 degree leeward lean with both Cheers and Godiva’ masts.The mast hits the water while there is still respectable righting moment so the boat has a good chance of re-righting—if the masts don’t break!. Pizzey’s is safer, Newick’s faster.
Never say die, though. There are lots of ways to skin cats.
Dave
Marco,
As I explained earlier and bearing in mind that minimum 70% of foil drag is there anyway limiting leeway, for my proa, to get a foil on the ama, it HAS to pivot from horizontal to vertical anyhow. At 45 degrees on the way down to vertical I get increased RM and I may as well go another 45 degrees past vertical the other side and get ama lift while I am at it.
For your proa, I would keep it simple and have one board and one trunk. Make it oversized so that you can experiment using different blocking or fillers / wedges. Get some inspiration from Dave’s “Simplest Proa Rudders” thread…...You could even turn it into a third rudder!!! This has to be way better than having 2 trunks and swapping a board between them.
I know this should be on your other thread, but Dave is right….long and narrow is fast, so if you are limited to say a 5 metre long garage for your build, budget allowing, you should be looking at either a 10 metre long vaka or even a 15 metre vaka not a 7 metre one. Put the expense and effort of extra trunks and canting / pivoting foils into raw boat. All-round faster and safer.
Rob
First off, thanks for the George Carlin video, Dave. As usual, George is/was spot on.
I know this should be on your other thread, but Dave is right….long and narrow is fast, so if you are limited to say a 5 metre long garage for your build, budget allowing, you should be looking at either a 10 metre long vaka or even a 15 metre vaka not a 7 metre one. Put the expense and effort of extra trunks and canting / pivoting foils into raw boat. All-round faster and safer.
So is this a fair summary of the important design parameters? -
1. Speed is principally determined by the cross section area of the widest point in the hull i.e. Length/beam ratio (L/B) is king.
2. Weight is crucial i.e. displacement/sail area (D/SA) is also of prime importance and
3. Wetted surface area (WSA) is important but crucial at lower speeds. So WSA/SA is very important but less so.
4. Points 1-3 are listed in order of importance.
5. The proa format takes best advantage of these governing principles for any given pile of money and/or materials if one is pursuing speed and perhaps seaworthyness
6. The ratios L/B, D/SA and WSA/SA should be optimized first before such things as foils and daggerboards/leeboards are considered?
Thanks for the really great posts, there is really tons to think about in there!
@James: I would give a slightly different list for the parameters that are important for speed:
1. Displacement to length ratio
2. Sail area to displacement ratio (limited by maximum RM, so you want a high value for very little weight)
3. Wetted surface & rough water drag (the latter can be 50% of the total if you are underway in a good chop)
4. Lift to drag ratios of boards and rig (for windward work)
Marco
Thanks for the really great posts, there is really tons to think about in there!
@James: I would give a slightly different list for the parameters that are important for speed:
1. Displacement to length ratio
2. Sail area to displacement ratio (limited by maximum RM, so you want a high value for very little weight)
3. Wetted surface & rough water drag (the latter can be 50% of the total if you are underway in a good chop)
4. Lift to drag ratios of boards and rig (for windward work)Marco
I’ve noticed over the years that people like to take a reductive approach understanding things. We would all like a minimal set of simple, solve-able equations to reduce the complex to something that can be handled by a spreadsheet.
The attraction to using these equations is that you can “predict” and “prove” success without the burden of actually trying, collecting data and empirically proving things. If you are interested, here’s a video worth watching. Russell Brown is by general consensus a pioneer in adapting and modernizing the Pacific Proa. His work wasn’t based on equations or simulations - he reasoned out his choices from Dick Newick’s efforts and those of Pacific islanders long past. By all accounts, he got just about everything right. His boat’s performance has been used as a baseline from which to gauge others - and his results are what a lot of proa enthusiasts have used to derive equations to be used for future predictions of performance.
Duct taping a new, radically different feature (lifting foils) on top of an existing design breaks the paradigm upon which the equations are based. My personal opinion would be that this is a situation where trying, collecting data and developing results is necessary if you wish to explore the concept.
I balk at the idea of using anecdotal evidence (foils worked on AC boats, therefore foils will work on all boats) as the basis of design planning. The systems and design premise of proas is deceptively simple in appearance, but fiendishly complex in execution. The Fulleresque combination of long, narrow length, light weight, tensegrity structure and efficient power transmission is kind of magical - and it falls apart if you mess with the formula too much.
LIfting foils depend on angle of attack as a primary design element - proas are constantly twitching on the pitch plain due to accelerations (both up and down) which may be a problem not considered in the ratios and equations. Rather that speculate, build!
—
Bill S. in Ottawa