hydrofoils for ocean-racing sailboats

[ntunnacl]

hey all, !this is my first thread!

ok, my interest is building and designing a lifting hydrofoil for an 21ft ocean racing sailboat called the open 6.50m (also called a "mini-transat").

http://www.ssaa.com.au/SSAA_Mini/mini_index.htm
http://www.minitransat-blue-one.com/....asp~mainFrame

Few restrictions are imposed on the design of a Mini 6.50m. In fact almost anything goes and no doubt, has already been tried. Swing keels, water-ballast, asymmetric foils, canards the list is endless. Areas of particular interest on the development front are the rig and sail design. I have an aquantaince who has built 2 carbon fibre mini hulls already, and he and I are interested in the concept of building 2 or so lifting foils into the hull.

By Design Group has designed a catameran with hydrofoils that involves retractable hydrofoils,
http://marine.bdg.com.au/spitfire12.html
and with the hull width of the mini transat, i'm sure that with some struts (hiking racks) a monohull version could be done. With hydrofoils, hull size hardly matters, the boat need only be big enough to carry the crew and supplies for the duration of the race. If the foils could lift the hull high enough to clear the wavecrest-trough distance, then speed would not be restricted by the possibility of slamming into a wave face, so there would be a very high upper limit on the sail area that one could carry.

!fascinating!

so, tell me what you think!


http://www.int-moth.org.uk/

[jehardiman]

First, Minitransat 6.5M are required to be monohulls, you will need to study the rules very carefully to ensure you don't violate their definition of a monohull. If I was the measurer and you showed up with a boat like MONITOR or WILLIWAW I'd toss you because of the very precise requirements of the rules (i.e. single waterplane in static or sailing condition).

If you go ahead, check out the two boats above or NF^3(NF "cubed"). The problems associated with supporting sail loads does not allow for much ability to profile which greatly restricts sea state.


MONITOR and WILLIWAW

[ntunnacl]

so how am I to interpret this as meaning, then:

"A monohull is defined as a vessel having only one flotation plane, when stationary or with sails up in sailing conditions, in which the depth of the hull in all its sections must not decrease in the direction of the axis of symmetry."

http://www.xs4all.nl/~blvrd/html/designrules.html

What exactly IS the depth of the hull? Does this simply say that you can't build a Transformers-style hull that slpits into two under hydraulic power in the water or something like that?
When I think 'depth' I think of its vertical depth from waterline to the keel-hull intersection.

The hull length is measured independantly of waterline length, and "This dimension does not include rudders and their fittings, the bowsprit bobstay fittings (without devices intended to extend the waterline length) as well as the solar panels, pushpits, pulpits and windvane pilot."

So if a horisontal (or anything that was not vertical) foil that gave lift sufficient to clear the hull proper from the water's surface was considered a 'centreboard' type foil, then a hydrofoil could be possible. (After all, a static planing hull is certainly of a different depth in that sense then that hull on a plane)... so how are we the rules readers supposed to interpret that definition?
If it is the case that lifting foils are 'centreboards' (as they certainly appear to be in some int. moths)... then the measurement rules say that:
The max beam is 3m including the keel and masts, but not the rudders or centroboards! so in this case, a system similar to that used on Gordon Baker's MONITOR employing two retractable lifting foils deployed from the beam of the boat and one submerged foil on the rudder would be feasible. Gordon's boat used retractable ladder foils, but the system on By Design Group's Spitfire 12 and Brett Bruvill's Windrush (another innovation by By Design Group) uses a similar configuration of lifting appendages, but with angled, single foils each (and a t-foil on the rudder) instead. Incorporating this technology, including a sophisticated mechanical computer like MONITOR's that reads input data from loads on the shrouds and foils and a central balance detection system controlling trimtab settings and 2D/3D canting keel position would make for a nearly completely automated racing thoroughbred! All the single crewmember would have to do is change sails and eat!

With a very responsive trimtab adjusting system, i wonder if it would even be necessary to reduce sail area in a storm (providing the boat was built strong enough)? With no possibility of a pitchpole or other such mishandling fuckups, I wonder what the upper speed limit would be? Or what other factors I may have forgotten.....

http://www.foils.org/monitor.htm
http://marine.bdg.com.au/

[jehardiman]

so how am I to interpret this as meaning,

To be honest, I can't answer that. I, as a measurer would focus on "one flotation plane", i.e. the shape of the water plane is a single closed figure. And as I move in from the edge of this figure "the depth of the hull in all its sections must not decrease" until I hit the centerline, i.e. no tunnel hulls.

I would just tread lightly, and see if there is something like the old 12m AC rules committee, who would rule on anonymous proposals from the teams.

[tspeer]

Originally Posted by ntunnacl

so how am I to interpret this as meaning, then:

"A monohull is defined as a vessel having only one flotation plane, when stationary or with sails up in sailing conditions, in which the depth of the hull in all its sections must not decrease in the direction of the axis of symmetry."

It rules out multihulls. It also rules out hulls like the I-20 scow, that humps up in the middle.

...With a very responsive trimtab adjusting system, i wonder if it would even be necessary to reduce sail area in a storm (providing the boat was built strong enough)? With no possibility of a pitchpole or other such mishandling fuckups, I wonder what the upper speed limit would be? Or what other factors I may have forgotten.....

I think the answer is "Yes, you will need to reduce sail."

Have you wondered how Hydroptere ended up looking like a pretzel? Twice? From www.hydroptere.com: "Heading for the Dover Straits, with speed hovering between 35 and 41 knots in a true wind of 20 to 25 knots, the experimental yacht suffered a failure of the windward crossbeam. The load measurement and control unit indicated a value of 47 tonnes on the brace that absorbs the thrust of the windward foil. " The problem is flying out of one wave and into the next - and entering at a different angle of attack. Or flying straight into a wave and experiencing a sudden increase in area of a surface piercing foil (which is apparently what happened to Hydroptere). In both cases, there's a sudden increase in the load, and the inertia of the boat won't let it react fast enough to avoid the increase. Plus, you probably wouldn't want to be on board if it could.

So there's a limit to how fast you want to push in a seaway, even if the yacht can stand to carry the sail area.

[ntunnacl]

i gathered that trimeran was a massive 18.3m in length and was all built out of struts going here and there and had no autoreactionary stabilising system. i'm, not surprised that that craft had a mechanical faliure. what the hydroptere accident sais is: build it strong and manageable. a 6.5m craft will have a fraction of the loads that would exist on a 18.3m (47 tones at the faliure point)...
anyway...

[tspeer]

You can't change physics with software. An automatic control system doesn't change the fact that the encounter frequency goes up as the speed increases, and the heave acceleration required to follow the wave surface goes up as the square of the encounter frequency.

Regardless of the size or configuration of the boat, at some point you have to reduce speed to maintain tolerable ride quality as the sea state builds. With a hydrofoil, you can get some relief by changing the strategy from contouring the waves to platforming, but there's a limit to the size of the wave you can platform. The limit can come from the depth of the foil system or from the authority of direct lift control.

So you're forced to contour the big waves. The bigger craft have an advantage here because of the greater depth of their foils. But they still have a limitiing speed.

I know of at least two designers working on this basic concept for an ocean racing monohull: the boat would have a canting keel with a daggerboard in front of the canting keel fin/strut. The daggerboard would carry the main hydrofoil and the rudder the other. The daggerboard mounted hydrofoil would be designed to retract into the hull in non -foiling conditions. A variation on this basic concept involves the use of a trim tab(flap) on the lower end of the canting keel strut designed to be used at near max cant angle to develop exta righting moment thereby allowing the total ballast to reduced.
This has GREAT potential in my personal opinion.Also no mono/multi questions! People opposed to this will also raise hell about the altitude control system violating Rule 52(I think-check the Moth forum-e-mail me for the url-very benefical discussion site) but that "tack" isn't expected to work for the anti-foilers in the Moth class. Two foil Moth monofoilers have shown what is possible and that it works-even though many including one of the worlds top hydrofoil experts said it couldn't be done. Theres a way to go between a Moth and a 21 or 40 footer BUT it's not as much of a jump as you might suppose and I think you'll be seeing this configuration flying before too long.

[ntunnacl]

the problems that the moth monofoilers have is balance.
i was thinking of a cantreboard monofoil, but the swing keel would have to be mother******* nimble and 3D judging by how those monofoil moths skippers need to balance in order to not crash n burn. With the trifoilers, the crafts are somewhat more stable, though balance is still a key issue.

WIll their keel be autocorrecting? I dont imajine that an open650 could support a lifting foil tall enough to platform in the ocean, and the weight distribution would need to be in constant check.

Athwhartship balance is not a serious issue especially with direct control of RM via a flap equipped canting lower fin.