Short Amas and Single Beam ?

[Doug Lord]

MCM, I tried to answer your questions in red below:

i meant the main hull having the main lifting foil.
which seems to be your 'Fire Arrow' set-up.

Wind pressure ? So, the main hull foil is unloaded because it hasn't started to pull down, or because the increasing wind pressure forces down the lee ama causing its foil to generate enough lift to hold the whole boat up ? Think of an ama with the buoyancy(about 200%) to fly the main hull as a pivot point-as the wind gets stronger the whole boat will pivot on the ama.On my boat the mainfoil lifts the main hull and the ama foil lifts the ama in light air until windforce unloads the mainfoil and loads up the ama foil so the ama is carrying most of the load. That leaves the mainfoil AND rudder T-foil working to control pitch which is a lot more pitch control than a "normal" tri has. The system makes it practical to tap into the advantages of an oversquare beam. The three greatest advantages of a lifting foil on the main hull(in addition to the rudder t-foil) are:
1) the ability to fly the main hull in very light air,
2) The fact that ,as the boat goes faster and the wind is stronger, the main foil is unloaded reducing induced drag substantially.
3) the ability to allow the main foil to develop down force adding to the RM substantially. Keep in mind that the wand system is set for a particular angle of heel, so if the boat exceeds that, the wand causes the main foil to generate downforce to return the boat to the designed angle of heel.You can reef the boat if the downforce stays constant-or not. The main foil acts like the ultimate in gust response-automatically adding RM with every gust. The system is quite versatile allowing the designed angle of heel to be changed by simply adjusting the length of the wand.

A single up-tip foil on the lee ama can do that ? yes, absolutely-look at the pictures of my test model under way-click on the picture.But what makes it possible is that the mainfoil ,working with the rudder T-foil, controls the ride angle of the UptiP ama foil-this is real important because it prevents the ama foil from having pitch excursions on its own. The main and rudder t-foil would also control the ride angle of a planing ama working with or without ama foil assist.

Do you think such a system is applicable to off-shore racing ? Yes I do and it would have the same advantages as on the small boat

And About the 'Fire Arrow' amas; will that stage one planning half hit the water mainly at sub-5knts ? The lee ama is in the water below 4-5mph(not knots) of boat speed. But the ama hull(first stage) is designed to plane in case the ama is sailing in conditions where the ama makes incidental contact with the water at speed.



I see that, but this ? Even without foils a 200% ama will fly the main hull sooner or later as the speed/wind increases(the boat pivots on the ama). On a square or oversquare tri this can take a lot of wind and also be unsafe. The real wide boat can carry loads of sail upwind but w/o foils the pitch resistance is more or less the same as a normal tri during a bearaway or gybe and that can be dangerous.


i understood the ama buoyancy generates RM by lifting itself - how can it also lift the main hull way over there ?See "pivot" comment above. The 200% ama can support the weight of the tri as the wind causes the boat to pivot on the ama causing the main hull to fly.

By the way, i do appreciate you taking the time to help me grok this.
Don't hesitate to ask any questions-I and others will try to help. The physics of what happens on my boat is sometimes difficult for even an experienced trimaran designer to understand-probably why this configuration has never been seen before. And a damn good reason to build a large RC model to test it.

 

[mcm]

This all leads me to questions about the geometry of the set-up.

The flight lift-off angle of heel should be less than 15deg. right ? So the two factors here are the desired wind pressure for lift-off, the lifting power of the two foils (vaka and ama), and distance between the two foils.

Is that what determines the best span distance for the beam ?

Then there's the question of foil alignment between main and float foils along the fore and aft line.
And their relationship to center of effort and center of lateral resistance.

Up-tips in the amas and t-foils for main and rudder, but only the main foil needs wand control ?

The t-rudder can be transom mounted and kick-up, but i can't help but wish the other foils could be side mounted and thus kick-up instead of in-hull and crash-boxed.

 

[P Flados]

The t-rudder can be transom mounted and kick-up, but i can't help but wish the other foils could be side mounted and thus kick-up instead of in-hull and crash-boxed.

Ama foils can be kick-up.

For a longer displacement ama, they can be mounted just aft of the beam on the inside surface similar to a leeboard.

For a planing ama, they can be easily mounted on the outside like a leeboard (as I did).

It may be possible to mount ama foils on the rear of a short displacement ama similar to a transom hung rudder. The ama to beam connection would need to be near the rear of the ama and the beam would need to be designed for lots of torsion loading.

It may also be possible to mount an uptip ama foil on the outside surface like a leeboard if it is close enough to the back.

For kick up lifting foils and Tee rudders you really need an adjustable solid stop at full forward and you need to make sure it does not kick up unless you hit something solid. The location of the pivot point is part of this puzzle. I put the pivot point near the back edge on my Tee rudder so that it would "lock in place" when lifting. However, on a downwind run it produced enough negative lift to overcome friction and kicked up. I have added a bungee loop that will keep it locked in place better (I hope).

 

[Doug Lord]

Since the boat takes off in around 5mph(4.4knots) of wind it is nearly level-maybe a 5 degree angle of heel. But the angle of heel doesn't matter that much. She is set up at present for about a 10 degree angle of heel when fully foiling.
--
Since this is a scale model of a 19.5' boat what determines maximum beam is the strength of the beams. I wanted beams with no waterstays(a diagonal stay between just above the waterline of the main hull out to the beam). Using water stays could allow more beam but this boat doesn't need more beam. A contributing factor in deciding on beam is the designed flying angle if a full flying foiler OR the "just flying angle of heel" -that is the angle of heel when the ama or ama+foil first takes the whole load.
--
The quarter chord point of the ama foil is slightly ahead of the daggerboard quarter chord point.
On multies,I generally use the quarter chord point of the rig as a whole with more weight given to the jib than if only its area was considered. Over many years that has worked very well in producing a boat that is well balanced with slight weather helm.
The main foil and ama foils will be retractable on the full size boat and there is a high likelihood that the main foil will be shortened about 2.5' to 3.5' from the present 6'. I didn't want to have any trouble with the main foil popping loose so I made the main and rudder foil deeper than I hoped they would need. And they can definitely be made shorter. The ama foils on the full size boat will be retractable and may be able to be lowered about a foot from where they are now(3.375" if it was done on the model)-that I haven't decided on for sure. All foil controls that need adjustment while sailing will have their controls led to the cockpit.
One of the few disadvantages of most foilers is that retractable foils can't kick up-they have to move vertically.If the rudder was to kick up while the boat was moving something serious would break! Some rudders on A Class cats kick up but they have a very small rudder hydrofoil. Still, I imagine if one of those kicked up at serious speed there might be trouble. If my ama foils kicked up at speed there would be catastrophic failure in the near vicinity of the foil and its mount.......Moths don't even have retractable foils and have to be carried out to deep water on their side! Dr. Sam Bradfield was a pioneer in making his hydrofoils retractable-but you have to retract them before you hit something.
--
The main foil is controlled by how the wand is set(length of the wand), and in turn, working with the trailing rudder foil, those two control the pitch angle of the boat and the running angle of the UptiP ama foil.The cool thing about UptiP foils is that they control their own altitude-you can see clear examples of that in the pictures marked "A" and "B" below. "A" shows the main hull in the water but the ama is flying. "B" shows the whole boat flying yet the ama still has the same altitude it had before the main hull flew. The ama foil lift increases due to leeway and speed so as the boat sped up the lift on the ama foil increased as the force on the sail did yet the ama foil maintained its altitude even though its speed and load changed! No other single foil has ever been able to control it's own altitude with no moving parts throughout the speed range of the boat. It's really a breakthru type of foil design whose principle was invented by TNZ in AC34. It is the perfect ama foil for the Fire Arrow configuration.

 

[Doug Lord]

MCM, here are some shots of Dr. Bradfields fairly recent 18'(by 22' wide) foiling trimaran,Osprey, using wands on each ama foil with all foils being retractable:

 

[mcm]

Ama foils can be kick-up.

For a longer displacement ama, they can be mounted just aft of the beam on the inside surface similar to a leeboard.

For a planing ama, they can be easily mounted on the outside like a leeboard (as I did).

So side mount is possible, that's good news, and your t-rudder kicks up.

That's great as well, but do your side mounted ama foils have any horizontal bend to them.

That could change the picture, and change it even more if there are sailing speeds that would prevent a kick-up without damage.

I put the pivot point near the back edge on my Tee rudder so that it would "lock in place" when lifting. However, on a downwind run it produced enough negative lift to overcome friction and kicked-up. I have added a bungee loop that will keep it locked in place better (I hope).

Most foils around aren't kick-ups, but i don't know if my reaction time is quick enough at those speeds with that wide a span of beam.

 

[Gary Baigent]

You blokes need to consider that loads on hydrofoils are very, very high, when sailing at speed - and if you have kick up versions, you had better make sure they don't kick up unintentionally ... because if that occurs on the main foils, you'll be in deep defecation; like lee float/foil twist and bury and definite breakage ... and if the rudder T foil kicks up, the complete assembly plus your transom will also definitely be spat out from the excessive drag like an obese persons' flatulence.

 

[mcm]

MCM, here are some shots of Dr. Bradfields fairly recent 18'(by 22' wide) foiling trimaran,Osprey, using wands on each ama foil with all foils being retractable:

Jim Brown's review of Dr. Bradfield's fully flying, foiling tri 'Osprey'. A glowing review that would inspire even the most doubtful.
http://smalltrimarans.com/blog/?p=8890

Thanks again for taking the time and trouble to share all the useful info - i'm saving every word of it.

 

[mcm]

and if you have kick up versions, you had better make sure they don't kick up unintentionally ... because if that occurs on the main foils, you'll be in deep defecation;

That implies that even intentionally you had better be going slow !

 

[P Flados]

So side mount is possible, that's good news, and your t-rudder kicks up.

That's great as well, but do your side mounted ama foils have any horizontal bend to them.

That could change the picture, and change it even more if there are sailing speeds that would prevent a kick-up without damage.



Most foils around aren't kick-ups, but i don't know if my reaction time is quick enough at those speeds with that wide a span of beam.

The rudder on my boat extends down much more than the ama foils and has hit bottom many times. Probably max speed at ground impact has been close to 8 - 10 kts. So far, kick up at speed has not been fun, but has not caused any real problems. I do worry that a really fast grounding may result in excessive loads on something.

The ama foil groundings have mostly been as I was driving the boat up on shore. However, I have had rotation due to slippage while going faster. The drag at the ama has gone way up, but again, no failures or crashes yet. Ama foil remote retraction and deployment control is still an area I consider "ripe for improvement" on my boat.

The attached photos are from late October and do not reflect several changes. However, they do give you some idea. Remember that this is not intended to be a "finished boat", it is only a constantly evolving "test platform".

 

[mcm]

a constantly evolving "test platform".

Those are serious foils !
Where are the amas ?
I didn't notice, did you have a main foil on the main hull ?
That test platform looks life size and it sounds like you've done some valuable real life testing at 10knts +.
Does it fly ?
I know some of us hope you'll let us know all the test results !!


To my mind, a kick-up allows the foil to give way on impact and should cause significantly less damage.
Especially if the kick-up is held in place by a break-away release.
Keep it tied to a lanyard and reel it back in after a break-away.

 

[P Flados]

Those are serious foils !
Where are the amas ?
I didn't notice, did you have a main foil on the main hull ?
That test platform looks life size and it sounds like you've done some valuable real life testing at 10knts +.
Does it fly ?
I know some of us hope you'll let us know all the test results !!


To my mind, a kick-up allows the foil to give way on impact and should cause significantly less damage.
Especially if the kick-up is held in place by a break-away release.
Keep it tied to a lanyard and reel it back in after a break-away.

The starboard ama is there, it is just so small that you did not recognize it. It is approx 1' wide, 2' long, with a 3" thick chunk of foam on top for flotation. I will include an early photo of it in the water at low speed.

No "main foil".

"Life size" is pretty close to "minimum usable size", approx 11' main hull. Around 80 lbs with small sail.

Inverted Tee rudders are getting widespread use. I am sure that some have kick up versions that are much better mine. Ideal would be to figure out how to have the tee portion pivot all the way out of the water.

No flying yet. Go read the boat specific thread (link below) for details of my testing. For general questions on my specific boat, posting in that thread would probably be better.

http://www.boatdesign.net/forums/multihulls/small-tri-test-platform-51551.html​

 

[rapscallion]