...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
Hey guys,
I want to get a couple of opinions on what you guys think an ama should be doing when you’re sailing a pacific proa. Flying the ama is better performance-wise, since with one larger hull you always get more displacement per wetted area than with two hulls in the water, and thus generally lower drag and a higher top speed (C-Class cats always fly a hull for this reason). I’m personally not that fond of the idea of permanently trying to fly one hull just like that. You can probably only manage it when going to windward (too bad for reaching and going downwind…), and it’s sort of a case of balancing the boat on a knife’s edge. Even with a lee pod to break your fall (a luxury which cats and tris don’t have), it seems a bit precarious to me…
The other option would be to foil the ama. That’s bound to have more drag than just flying it by sheeting in a bit, but it’s also less precarious, and could work for a variety of courses.
What I have in mind is to angle the daggerboard case from the vertical, so the board projects into the water and to windward, at say a 45 degree angle. This would produce windward and upward lift. That’s pretty much the opposite strategy as what is being done with the board on the Jester class proa, which produces windward and downward lift to increase maximum righting moment. One could argue that by foiling the ama you’re decreasing righting moment, but as soon as the board is out of the water, the RM is pretty much the same as it was without the board.
The problem with the entire scheme is that depending on what course we are on, we want very different things from that board. When we are going to windward, we want lots of windward lift and not so much vertical lift (the boat wants to heel anyway), and when reaching or going downwind we want enough vertical lift to still fly the ama, but we don’t need a lot of windward lift. So the question is: can this be solved in a satisfactory manner by playing with the amount of water ballast in the ama?
—I don’t think so. When you are reaching, you are going your fastest, the foil is producing lots of lift per area, you have the ballast tanks empty, and the ama is flying high, with the board in the water only a little bit.—Looks great, but then you decide to go to windward. Heeling moment increases, you need a lot more windward lift to maintain a sensible leeway angle, and the boat slows down a lot too (= less lift per area). Now you need the board down deep in the water. The problem is that the board wants to go up, the heeling of the boat wants it to go up too, but you need the whole thing down, for the windward lift. In my opinion, adding more water ballast to counteract the excessive vertical lift of the board is just nonsense.
The solution perhaps is to use a banana-board (C-foil). You put it down partially when going to windward, giving you almost entirely windward lift, and just a little to lift the ama, though most of the lift for the ama comes from the heeling moment of the boat. The bit of vertical lift from the board makes the ‘knife’s edge’ on which you are balancing, a little bit wider.
On a broad reach you put the board down further, so the board has more horizontal area, giving you more of a vertical lift component from the board. It flies higher, thus a lot of the board leaves the water, and windward lift decreases, which is exactly what you want. If you want to reduce drag further on a broad reach, the crew can head over to the ama as well, to let the board, and not the vaka, lift their weight; that’s assuming the board produces less drag per weight lifted, than the vaka does though, which may or may not be the case. For a long time C-Class cats went without foils, because the performance without foils was actually better. You had less drag by using a hull to carry the weight, than a foil. Looking at C-Class cats now, the opposite seems to be true, I’m not sure what changed, but maybe the foils just got better?
Is all that effort worth it, or should the ama just stay where gravity wants it to be? How much faster is a proa going to go anyway, if you get the ama out of the water? I think it’s probalby the kind of thing that might give you a knot or three extra, but that for a heck of a lot of extra construction time (just build a bigger boat instead?). What do you guys think?
Cheers,
Marco
I forgot the really obvious way to fly an ama when on a reach, and that’s to just move some human ballast to leeward, which is exactly what the polynesians did, as you can see from the large leeward platform on the Tepukei for instance. That’s still balancing on the knife’s edge though…
Hi Manik,
thanks for your post; it gives a lot of inspiration to me. If i had read this earlier, I would have constructed my ama-foiler in your way. By the way, if you fix the foiler with 2 vertically laid out rubber mountings, you can either create lift or down lift by a lever, attached to the foiler. I will test this configuration during my summer vacations and hope to find a way to show the effects on youtube.
Regards
Dieter
Aloha Marco,
I like your foil development initiative ; I think your proa is in cat 1 . Size ? Right?
Category 1. Speedster. ( for afternoon speeding along, pumping lots of adrenaline)
Size 20 ft - 30 ft
30 ft - 40 ft
40 ft - 50 ft
50 ft - >
Category 2. Weekender ( as the name suggest, might have some elements of Cat1.)
Sizes. As above
Category 3. Passagemaker ( capable of quick transocean travel, extended live-aboard)
Sizes As above
Category 4. Live-aboard & charter purpose ( as the name suggest)
Sizes As above
IMO. , one should always define first; in what segment the intended write-up belongs, as such would make any discussion more to the point.
For a speedster I think foiling a ama or vaka would be realistic, and I would surrely want to develop it.
For a passagemaker is a definite no. Its steady high average tracking you seek. this without sudden surprises.
My 5 cents cheers Sven
Have you considered “S” boards?
I have seen some articles on super fancy sports catamaran, and right, I cannot find them right now… where the profile of the board is not straight as usual, or even C shaped like on the ORMA 60 ft trimaran and newer trimarans, but S shaped… So depending on how much board you put in the water, the whole board can be put at a different angle.
Of course, it means super complicated construction and high costs…
Laurent
Of course, it means super complicated construction and high costs…
—No thanks… :D
While I do have a taste of category 1 in my design, I’d put my ‘Firstborne’ into category two, maybe even a 2+ (small boats in general are a lot less seaworthy than larger ones, which bodes ill for ‘Firstborne’, but time will tell just how seaworthy it’ill actually be). I think your categorization is still sensible though; the best solution is very different depending on what your use-case is. In general, trying to keep the ama light-weight, but with a large ballast tank, will make any kind of flying or skimming easier.
Here’s a more complete list of the options available to get the ama out of the water (if anyone has even more suggestions, fire away!):
1.) Heeling moment and/or human ballast to leeward
2.) A short ama designed for planing, with a vertical daggerboard. I’m thinking something like a dinghy, but lighter, a bit slimmer, and with a lot less freeboard (sort of like the ama on the tepukei). Slamming of the ama could become a major problem in rough water though.
3.) Outward board, producing vertical and horizontal lift. If the vertical lift is too much, just add more water ballast.
4.) Have two daggerboard cases per board, one for reaching, angled outwards like #3, and one vertical trunk for windward work. Straight boards.
5.) Canting daggerboard cases, again with straight boards. Could be internal or side hung. Gives full control over the ratio of vertical to horizontal lift, but may be difficult to build.
6.) C-foils
7.) S-foils
Sadly, none of these options help you when going to windward so long as your daggerboard is in the ama, since the daggerboard has to be in the water when going to windward. Skimming the ama is the best you can do in that situation. Is stable skimming difficult to maintain, or does that happen more or less by itself when going to windward?
If you had a ‘speedster’ the easiest thing to do by far, is to just fly the ama using a combination of the heeling moment of the boat and human ballast to leeward. What I intend to do with my boat, is install a stowable trampoline on the leeward side; since about 1/3rd of the displacement of the boat is the crew, getting the ama out of the water that way, even on a reach, shouldn’t be much of a problem. That is balancing on the knife’s edge, and requires constant vigilance and trimming, but when I just want to have some fun and want a bit of extra performance to give the production ‘racer-cruiser’ cats twice my size or the passenger ferry from Cuxhaven to Heligoland a run for their money, then that’s by far the easiest solution.
Where it becomes interesting, is what if you are firmly in category 2 or higher (so weekend cruiser or above), and are searching for extra performance. My hope is that you can use a foil to fly the ama, so the boat can be cruising along nicely under autopilot or windvane self-steering, with the ama out of the water, without anyone having to worry about a thing. To me the C-foils, inclined daggerboards with a 2nd trunk for each board, or canting daggerboards, all seem like options that would allow precisely that. Building an extra set of trunks seems like the easiest one to me; the inclined trunks for foiling the ama when on a reach wouldn’t even have to be internal trunks, or full daggerbaord trunks for that matter, they would just have the be some sort of ‘side-hung’ fixture to get the board into the water at a certain angle.
@Sven: What do you mean when you say “sudden surprises?” I intially considered giving my vaka C-foils, but decided against it, at least for the time being due to the extra construction time which that would involve, and more significantly due to the pitch instability which would accompany C-foils in the vaka—that can make for very sudden and very unpleasant surprises (pitchpole), but in the ama I’d imagine that any sort of foils would be pretty harmless, or am I mistaken…?
Cheers,
Marco
Marco said;
#1.”Where it becomes interesting, is what if you are firmly in category 2 or higher (so weekend cruiser or above), and are searching for extra performance. My hope is that you can use a foil to fly the ama, so the boat can be cruising along nicely under autopilot or windvane self-steering, with the ama out of the water, without anyone having to worry about a thing.
#2.@Sven: What do you mean when you say “sudden surprises?
Aloha Marco , about #1 . been out there done it , and believe me this is NOT what you want unless you are a fan of “‘sudden surprises” , and depend on regular outside help.
about #2 I took a bit of time to read through your previous posts, I think I now understand that you are trying to concept a weekender proa of some 20 foot, For all the foil stuff you describe , best look at A-class foil development, perhaps even C-class. If you really going to build soon, my best advice is keep it as simple as possible , Like Harmen Hielkema’s Toroa , a great proa or Gary Dierking’s book building outrigger sailing canoes.
[url=https://www.youtube.com/watch?v=a4FADWXmuHk[/url]
in all honesty I think smaller proa’s are a handfull to sail in bigger wind, let alone playing with foiling amas, ie. “sudden surprises ” for sure.
The bigger the proa the more predictable behaviour you want, its not funny to whip-out on a 30 ft+ Pacific Proa
cheers Sven
Where my design is concerned I have something quite different in my mind, you can take a look here. That said, I’m well aware of the fact that flying the ama freely is essentially asking for trouble, and that’s why I’m searching for solution that would keep the ama up in the air even while I’m down below making a cup of tea, a solution which is safe, and applicable to a cruising proa of any size. Maybe such a solution doesn’t exist, I don’t know, but I’m still curious what you mean when you speak of sudden surprises. 😉 Are we talking about something like sudden knockdowns here? If so why? In your opinion, what are the dangers / problems of trying to foil an ama on a cruiser?
Cheers,
Marco
Have a look at the articles from John Pizzey.
He advocates a canted out board, which lifts the hull.
He says you can go below, make tea, whilst flying a hull!!
I think he has it:
At lower speed, the ama is immersed, a canted in or out board not helping, but not too much drag? A good sized board is worthwhile.
As speed increases, the need for a large size board decreases, with a slim hull & efficient rig, the vaka hull + rudder-board sufficient?
With increased speed the ama can start to skim, so down force not helpful.
The canted out giving lift will give skimming mode earlier. It will also damp down the rise and fall through waves.
At highest speeds the canted in board would give useful downforce.
The most benefit is from the canted out board?
The canted out giving lift will give skimming mode earlier. It will also damp down the rise and fall through waves.
At highest speeds the canted in board would give useful downforce.
The most benefit is from the canted out board?
Marco / Mark
Of course the real answer is to have a foil set up which can cant both ways. Ideally adjustable on the fly as well. It also means that you can have vertical as well on the way through when all you want is windward lift.
I am currently working on a torsion / tension spring loaded central board set up which will pivot from 45 degrees out to 45 degrees in automatically in “dump” mode all controllable (and retractable) from the cockpit. On my proa design it gives approximately +/- 115 kg theoretical lift at 10 knots.
Or it could be a long spade rudder(s) in a cassette(s) in the ama canted out 45 degrees and controlled by a wand, just like current foiling boats.
Complicated, but if it can be made to work sufficiently well, you have the best of all worlds.
Rob
I might be having a stupid moment, but how does canting in or out change the lift or downforce? Isn’t the effect identical?
Ok I see, this is assuming the ama foil provides all the sideforce right?
But in reality, won’t the rudders and main hull also contribute to quite a bit of side force?
Doesn’t it make more sense to just set the angle of attack to provide lift in whatever direction? Regardless of cant angle? And having something else provide the leeway resistance?
Finally, how about a lifting foil like the AC cats have?
And finally finally, how about a lifting foil like the AC cats have, but with another vertical bit attached onto the bottom. Your ama will lift until the V’d area is breaking the surface, then the rest of the foil below it is still providing plenty of sideforce.
I’m avoiding real work. Oops. Thinking out loud. How embarrassing.
Have a look at the articles from John Pizzey.
He advocates a canted out board, which lifts the hull.
He says you can go below, make tea, whilst flying a hull!!
Pizzey’s proa succeeds because of the leeward canted rig, which creates a balance between rig and ama that makes a large “safety zone” of ama flying. As the boat heels, the rig overturning moment is falling at a faster rate than the ama righting moment. It’s sort of an inverse keelboat. Brilliant, really.
Safe ama flying is not a result of the canted hydrofoil, but of the canted rig.
But in reality, won’t the rudders and main hull also contribute to quite a bit of side force?
Doesn’t it make more sense to just set the angle of attack to provide lift in whatever direction? Regardless of cant angle? And having something else provide the leeway resistance?
Finally, how about a lifting foil like the AC cats have?
tdem,
Of course the main hull and the rudder(s) provide a lot of leeway resistance, but there aren’t too many yachts of any type which rely solely upon it…....
You could do longitudinally pivoting AC 72 type L/J or moth T foils to provide up / down force and even positive leeway thrust, it is a technically simpler solution, but how do you fully retract them, so that the tips don’t cause unnecessary resistance and drag when you don’t need them??....and there are other practical issues.
For several reasons, an ama foil retracting vertically won’t work on the proa I am building, hence the transversally pivoting approach. I don’t mean to imply that it is the best or only approach, just that there is significant potential in having an ama foil system which can do uplift, downlift, (positive) sideways, fully retract etc…...
Michael is quite right that the principal self righting feature of Pizzey’s proas is the canting / auto- dumping rig, but if you pop a foil and lose around 100 kg of uplift on a 4 metre long lever arm, that has to be significant and it happens sooner and faster than an auto-dumping rig….....
Rob
Hey guys,
I agree with Rob here in that the J or L foils are probably quite problematic for a proa. When beaching the boat for instance, I’d definitely want to be able to retract the boards fully. From what I’ve read, making the AC72-style Foils fly stably at all is extremely difficult both from a design and handling standpoint as well. The guys in the America’s Cup might make it look easy, but there’s only a very narrow range of angles of attack and foil depths in which those things will fly smoothly, so they require constant adjustment by the crew (or some other control system). Doing that with a bidirectional foil, and when the water gets a bit rougher… well good luck. It’s really difficult to get right even if it only has to go one way in flat water.
@tdem: I’ve attached a sketch of different foil types and the canting board. which might help clarify the situation a bit. I think Rob and I both automatically assumed we’d use an asymmetric foil, set at a (constant) 0° angle of attack, or at least I did. With an asymmetric board, if you cant it to windward, you get windward and upward lift, and canting it to leeward gives you windward and downward lift.
To me canting to leeward to increase maximum RM has the decisive disadvantage that you are creating a system where you have the potential for a very abrupt loss of righting moment. You’d better hope that board doesn’t ever leave the water due to wave action, pitching of the boat, or a sudden gust, because if that foil leaves the water even a little bit, then you have less RM, meaning more of the foil comes out of the water, means less RM, and so on, in the blink of an eye. I think you’re essentially setting yourself up for some very sudden and unexpected knockdowns—but maybe it won’t be such a problem practice? With the curved boards of the Jester class, the doward lift is being produced at a greater depth below the surface, which definitely seems safer to me: you need a greater heel angle until you set off that chain reaction to a knockdown.
Personally I think the easiest solution is problably to use a single asymmetric bidirectional daggerboard in a canting trunk, and make it freely retractable at that. I’d probably just design the trunk for canting from vertical to +60° to windward. If you can adjust that while sailing, and if you can also control how far down the daggerboard is, then you have really good control over how much vertical and horizontal lift you get, which should give you a foiling ama for just about all points of sail other than close-hauled.
@Rob: Since the foil is surface piercing (goes in at an angle), it controls its own flying height passively (more foil immersion = more lift), so it doesn’t require any sort of wand or control system like the T-Foils do, it’s a dynamically stable system. In general surface-piercing hydrofoils have lower performance than T-Foils, because the hydrofoiling powerboats for instance have V-Foils which produce a lot of sideway lift on both sides that just cancel each other out. Unnecessary lift = unnecessary drag, but since we actually NEED the sideforce and have to generate it anyway, the performance difference between surface-piercing and T-Foils would probably be quite small.
The only thing which is unfortunate with this setup (other than construction time) is that foiling the ama when close-hauled remains elusive! The racing cats and tris will all be flying their hulls, and we’re left with both hulls in the water…
Cheers,
Marco
I just wanted to throw this in here:
I’d vote for the scimming float, precarious balance is precarious by definition and not all that suitable for overnight adventuring. I’m ambivalent about angled foils, I’ve read about problems when you get into a ‘crossflow’ situation like sliding down the back of a large wave and burying the float to the point of capsize. Almost by definition you are only 71% effective with a 45 degree cant. OTOH there might be some goodness in going with a tee or “L” shaped daggerboard properly proportioned so that the float flies and the tip helps keep the foil in the water at speed.
I think that’s definitely a danger worth thinking about… The crossflow burying the ama sounds really dangerous to me, especially since you’ll probably have more water ballast in the ama in heavier weather. If you did have a freely canting trunk and the possibility to raise and lower the board at will though, then if the weather is bad, you could just set the trunk to vertical and/or the board up, and avoid that risk.
Cheers,
Marco
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? 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?