Ama hull shapes

[Capn Mud]

Hi All,

I asked a sort of a Q under the Hydrofoil Trimarans in New Zealand : Attn Gary Baigent thread but got no reply so thought I would ask the forum in general....

What are good and bad ama hull section forms and why.

I see early tris (Piver etc) seem to have essentially triangular sections and often no (or very small) keels/ centreboards - presumably the sharp hull form provides sufficient sideways resistence in this case.

However in more modern tris the hull forms seem to get more rounded. Farrier tris seem to have a sort of rounded V...

The Weta has very much a rounded section with no V at all..... very similar to that shown by Gary Baigent at Hydrofoil Trimarans in New Zealand : Attn Gary Baigent post #3

What, physically in tri design drives this? Is this a development in knowledge about how tris sail, or different design principles or?

Cheers,
Andrew

[Capn Mud]

And is there a library of typical ama hull shapes somewhere that can be used? Like the aerofoil profiles for foils.....

Thanks is advance

[Gary Baigent]

My apologies Capt. Mud - but I thought the last image of the foiler on the hard would be informative. Anyway the amas on Harry are not a good shape IMO, they are too veed ...... but my intentions were that the boat would rely on the foils too lift the amas so there would be very little float in the water. If I had more courage I would build a foiler without any floats at all, just large foils. Whoops!!! On the other hand, in light conditions I can sit to leeward and sink the semi-veed floats enough to cant the mast so the main sail falls into a sweet shape. My larger boat has rounded below water shapes - but it also has far more dihedral in the beam, so the boat is always leaning over somewhat.
regards
Gary

[Capn Mud]

Gary,

No need to apologise. Yes the pics both interesting and informative.

Anyway the amas on Harry are not a good shape IMO, they are too veed ..

Too veed - do you mean as in the sketch shown below? Hardly seems to be any V at all. is it some elipsoidal type shape?

What would you recommend instead?

Thanks alot,
Andrew

PS. Love the name "Flash Harry"

[Gary Baigent]

I'm always apologizing it seems Capn. Mud because of my not labelling the jpegs clearly - that image of the float is not from Flash Harry but David Knaggs foiler, the white 5.5meter boat. Here is a jpeg of my larger boat Groucho Marx with more rounded floats- I think theyt are an improvement.
regards
Gary

[Chris Ostlind]

Gary,

Did you come up with that paint scheme shown above?

Chris

[tspeer]

Originally Posted by Capn Mud

...What are good and bad ama hull section forms and why....

Shuttleworth discusses ama shapes in general in his papers. Jim Antrim has a very different take on the ama design for the Antrim 40.

Basically you have to look at the requirements - and there are a lot of them for an ama. Piver's designs were intended for easy construction in plywood, and he believed that he could get away with no or reduced keel size by using lift from the hulls. Modern designs have a more integrated understanding of the factors affecting ama design.

The ama moves up and down a lot and even when completely out of the water is still subject to wave impact. A somewhat V'd bottom reduces the slamming loads on the ama. But you don't need the entire section V'd - that increases wetted area and provides a flat surface that can be slammed from the side. Farrier uses a fairly rounded section with deadrise that is a good compromise. Antrim used elliptical sections for minimum wetted area. Dick Newick has also gone to rounded sections with deadrise, as seen here:


The topsides need to be a compromise between windage, reserve buoyancy and practical considerations. Farrier's topsides have rounded gunwales and some flat area in the center. Antrim went with elliptical topsides. In Hoerner's "Fluid Dynamic Drag" you can find empirical data on the drag of blunt shapes that are relevant to the crossflow drag of an ama in the air. You get the most benefit by a radius that is 20% of the height of the section. Compared to a sharp edged cylinder, that cuts the drag by 80%. However, as you make the section more rounded from there, only a little additional drag reduction results. So I see little reason to go to a completely round deck on the ama.

A practical consideration in the deck shape is the crew getting access to the ends of the ama when underway or docking. You'll want secure footing to place fenders, fend off the dock, or place snatch blocks for spinnaker sheets and twings. My Chris White designed Hammerhead 34 has flat decks which could be improved from the point of windage, but I feel significantly more secure going to the ends than I do on an F-boat. I would hate to have completely round amas like Dick Newick's Traveler trimaran - beautiful, stiff, minimum area, but I'd feel like I was walking a tightrope every time we docked.

Your question concerned sections, but I want to put in a plug for sheer. Straight or even negative sheer seems to be the style these days. But if you consider the worst case loading of an ama, it's providing diagonal stability. In that case, the bow is down 5 - 10 degrees and the boat is heeled to submerge the ama.

When the bow goes under, the center of flotation shifts rapidly aft, and you lose pitch stability. With straight sheer, the bow is the first thing to go under. If you build in positive sheer, you can design the ama so the deck is horizontal for this worst-case condition. That preserves waterplane forward so the ama still does its job of providing stability. So I think a reasonable design criteria is for the slope of the ama sheer at the bow to equal the pitch trim of the boat at it's diagonal capsize condition. What you get from this is much like the sheer of the classic Newick designs.

[Gary Baigent]

You don't like my anarchic colours, Chris? Goes with the boat mate?

[Chris Ostlind]

Originally Posted by Gary Baigent

You don't like my anarchic colours, Chris? Goes with the boat mate?

Actually, Gary, I do like the paint scheme quite a bit. It has a direct graphic quality that fits the mission of the boat it dresses. Long time ago, I used to compete in a rally prepped Mini Cooper. We found ourselves gravitating to a bold graphic paint scheme as it helped course observers to identify our car when the numbers were blotted-out by dirt, mud and snow. Also gave the sponsors a boost.

"Did you come up with that paint scheme shown above?", is just a simple question with no innuendo attached.

Fact is, I'd like to know more about the sailing habits of the craft shown. What have you learned from the design direction you took? What kinds of positive and/or negative characteristics did the boats demonstrate on different ponts of sail in different conditions? If you had to do all over, what would you do differently? What happened to the boat... do you still own her?

Chris

[Gary Baigent]

Hi Chris
In my acutely defensive and well trained paranoia (joke) I thought, for a second, you were taking the Mickey out of me. Actually my arty farty sailing friends are always doing so – so I have been well trained and can handle ALL criticism. But have you ever noticed that the sensitive arties actually prefer ultra conservative boats, preferably a century old at least. Not that I have anything against traditional designs. Back on subject:
Groucho Marx has been changed a lot from original design (when it was called Misguided Angel). First I had 40 degree angled inward floats and foils (the idea being that the foils would exit the floats at the same angle, no cavitation areas, less drag) but the floats were too deep so it sailed like an overweight Piver, all three hulls dragging. When the breeze was up a bit it would lift the windward float, which was an improvement. Also I had two 11 metre wing masts (una rigs) set halfway out on each beam and, because the boat is a single beam tri, there was nothing to sheet the main sheets to, just air and space - so I had a half wish bone set up, heavily reinforced with carbon, but they were still useless, once the mains loaded up, the wishbones would flex and leech fall off was disgusting to observe. The boat was not good on the wind because of this failing but reaching and running in fresh winds, was very fast because of the large sail area set low. On that point of sail it was really a rocket BUT a boat has to point high and go to windward otherwise you are just fooling yourself. So I changed over to a new single (conventional) wing mast 15.5 metres tall, something I should have done from the start. Plus new, more vertical floats and a new 11 meter beam (original was over 12) with greater dihedral (now I wanted the windward float and reversed L shaped foil (now angled outwards) to sail well clear of the water), new cabin, different cockpit and so on.
Don’t think I’m going to make many other changes, just refinements with the foils and better battens perhaps. The boat behaves well now, excellent to windward in light and heavy – one thing on Groucho, the wing mast has an 0.5 m chord, (which is large for an 11 metre boat) in strong winds with too much sail up, you can ease main traveler and sail very well on the mast - but I have found on such a light boat that it is better to reduce sail earlier than to lug it. Platform is steady in waves as you would expect with a foiler, no pitching like a light cat. I do have problems taking off into the sky when crossing the savage wakes of poorly designed, gross pig, gin palace, power boats, no trouble with container ships, fishing boats, ferries though, whose wakes are sweet and round by comparison.
If I was doing it again: I’d make the boat 40 x 40 foot with very narrow hulls that flared in the accommodation and beam attachment areas, dreadnought wave piercing bows but otherwise very much the same. But I’m not going to do that as I’m quite happy with what I’ve got. The boat is superb in following seas, never nose dived, (so far) the foils do great work and on that point of sail there is only the main hull (and not much of that) plus the leeward foil in the water. You can do this in light airs too, have the crew move out on the windward beam so only the leeward foil skims the water.
Hope this answers a few questions - but it reads like a lot of waffle to me.
regards
Gary

[tspeer]

Originally Posted by Gary Baigent

...Here is a jpeg of my larger boat Groucho Marx with more rounded floats...

Can you say a bit about how leeway affects the foils? It's not clear to me how you keep the foils from driving the leeward ama into the water.

Is the mast mass-balanced? Any problem with flutter when tacking in heavy winds or with the bare pole?

[Gary Baigent]

hi tspeer
Actually it is quite difficult to describe my angled out foils but here goes: they are set up exactly the same as conventionally angled in foils with an average angle of attack of three degrees, by average I mean they are similar to a helicopter blade in that at base (at exit from float) the angle of attack is five degrees which then twists to less than two at the tip. Why did I do this? Because I was aiming for higher lift at lower speeds to lift out the float and at higher speeds, with the float lifting clear, less lift (naturally) and importantly less drag, therefore higher speed. However I can't see or feel any difference to angled in foils like Flash Harry's - so probably a waste of time. The out foils probably counter some leeway but they are there primarily to lift the float - Groucho has a daggerboard in the main hull. Theoretically this is supposedly unnecessary but I find it helps going to windward and the helm feels easy too. Harry has no main hull daggerboard and she goes sweetly to windward. So there you are! Angled out at 40 degrees to leeward on Groucho the foils still lift the float but as the platform heels the angle steepens and lift is reduced, there obviously is a loss there, but not much. However I don't have the annoying cavitation or air sucking problems of Harry's because the foil is in deeper, less disturbed, non aerated water than if it exited on the windward side close to the surface. On Harry sometimes going through waves, a huge air bubble appears just below the case exit and the top section of the foil and the foil washes out almost instantly, then just as instantly, as the foil bites clear water, jumps up again. Spectacular sheets of water thrown but a jerky ride, and you lose that wonderful flying sensation and lose speed. Groucho doesn't do this. The reversed L tip was again a compromise solution, I wanted a bit more area and at the same time, as the boast heeled, the L section would be lying flatter, probably going to near 30 degrees, so more lift for the float and a compensation for the more vertical angle of the main foil section. Does that make sense? I should provide a drawing.
Actually inverted Y or T section foils would do a similar and probably better job - but they are fixed, and I wanted foils I could lift, for light airs, clearing weed, someone's anchor warp (joking, hasn't ever happened).
Your question about mast mass balance. I guess you are meaning the pivot point position for the mast base - it is set slightly forward of the mast I beam which is at its thickest point in the cross section. Tacking in strong winds I have the mast spanner lines tightened and cleated so the mast is set closer to the wind (in lighter or moderate stuff I have more mast rotation) - then as you go through the eye of the wind there is a moment when the mast can flutter but it is only momentary before you cross the eye and the mast slams across to the new tack. If you don't make it through the eye the mast will flap alright but only as far as the mast spanner lines will allow it. On an una rigged wing mast boat the mast can act like a headsail and you can use it if you hold it back against the wind, to more quickly get set up for the new board. One other point, as you go through the eye, I drop the main sheet off completely and pull the boom and sail over my head so that there is no chance for the wind filling the sail, if it does you will weather cock the boat and end up sailing rapidly in reverse.
regards Gary

[Gary Baigent]

Hi Tom, me again
You are concerned that the angled out foil is providing lift to LEEWARD - and it will be but it also has an upward lifting component too, plus there is the L bottom section that is also lifting to WINDWARD and upwards as well. But there is another interesting point: if the boat is making to leeward, what is that angled outward foil doing? - my thoughts it is also digging into the water and countering that movement too. I know things may be fighting one another and that is why I have a deep daggerboard in the main hull to counter it. The reason they angle out is primarily to reduce the length of the main beam, less material, less weight, less bending loads and more stability the faster you go, not less stability as in angled in foils.
Actually I'm thinking of changing to lifting, inverted T foils in the floats anyway (the rudder is already a T) with a symmetrical vertical blade sections and asymmetric lifting wings.
regards Gary