High Performance MPX Foil/Self-righting Trimaran-The Test Model

The "full sized version of the MPX"? Does that mean you have some detailed design work to show us rather than just free hand sketches?

Now I'm confused by your counting of foils - are you saying that you expect the weather Ama to fly completely out of the water and the boat to fly on the leeward ama foil and the rudder? Because I could have sworn in an earlier post you said the ama foils were not intended to fly the ama...

I'm so confused. can you please clarify? Maybe some completed force couple analysis drawings would help. That's what I'm used to looking at for understanding the forces involved.

 

Actualy, I think I do.

There is no full size version yet, so you can't make claims about what it does, because so far it doesn't do anything.

Also, if your model is intended to use "3.25 foils" on takeoff, then that would be a number of foils greater than 3. The quantity of foils comes in integer values. The next integer after 3 is 4. So, my claim that you have 4 foils is looking rather good.

The reason I made that claim is because I've seen your photos of the model, which funnily enough show it equipped with 4 foils. You made it that way. Don't blame me. Aint my fault, cobber.

Ok, but that unloaded foil (if it does unload) is still going to have all its wetted surface, and form drag. The only thing you'd be saving is the induced drag. To save the induced drag on the main foil, you have to generate it somewhere else. So, offhand it doesn't seem like there would necessarily be any net benefit. Might be. Might not. Hard to tell at this stage.

Well, you have four foils. Maybe only one has a wand, but you still have four instead of three. And you still have to drag the extra foils through the water regardless of which one is carrying most of the load.

But really, what makes you think I didn't understand any of this before? It's obvious that the foil in the lee ama is intended to take a fair whack of load.

And Williwaw use the old style ladder foils, but they're not usually all counted as a bunch of separate foils. I'd forgotten about the one under the bow though, so you're right about it using four instead of three. So, in terms of general arrangement it's much the same as your model then: one middle foil at the front, one middle foil at the back, and one foil out each side.

 

Its interesting that a lot of the commercial foilers in service continue to use ladder foils because of their simplicity and functionality. Not that long ago I rode on a foiling ferry in st Petersburg. It too effectively used 4 foils (4 sets of ladder foils).

4 foils is a solid stable configuration with balanced forces. Things get much more complicated when you go to 3 foils or 2 foils.

 

the simple way to do this would have been to make the whole control box demountable. Fairly simple to do with screws and deadend nuts.

Why can't you just calculate this from the geometry?

unless you are using a curved track the way a StarBoat does this will not work. Because as the car moves to leeward or windward of its current sheeted position, it will change the amount of sheet tension you have between the car and the boom. That's why dinghies have largely abandoned travelers because unless you use a curved track, they dramatically change leech tension.

Now on an RC boat you could run a servo for the sheet and a servo for the car, and create a mixing profile that lets you use one control to adjust both: the amount of sheet change you need is given by

Sheet change = car distance travelled/tan(angle boom change)

of course that has to be multiplied by the purchase you might have built in.


and that's why most dinghies these days vang sheet. And that's also why the Star uses a circular track for its vang. And why the Nacira Mini 650 which also uses a "vangeller" but simply calls it a Main sheet traveler - runs a curved track from gun'l to gun'l.

but you may well know more than the naval architect who designed the Nacira... I'm probably not in a position to judge that

 

MPX Test Model

Heres the final radio box minus most of the radio equipment. Shots of the seal are included as well. It's bonded on to the carbon fiber flange with an acrylic glue which is part of the material. Worked very well except where I botched the corners. They need to be fixed with black silicone.

click:

 

Y'know, I've always thought the term "vang sheet" was rather daft. I know it refers to using a powerful vang to control leech tension, but controlling leech tension is what vangs were invented for in the first place. So, really, "vang sheeting" is just plain old "vanging".

I have no idea why someone thought up the term "vang sheeting", or why anyone else ever thought it was a great idea for a new term.

 

I agree its backwards (though I'm not sure it gets used correctly cuz the opposite of Vang Sheeting is "traveler sheeting. And to me that speaks of the main sheet being used as a traveler in one case and a vang control in the other.

But based on usage it seems to be the other way. Meh....

So a vang on a track is still a vang. And a mainsheet on a track with a traveler is still just a main with a traveler.

By going with a fully radiused main traveler, you don't need the vang or leach tension control but there's no need to invent a name for that, at least not if you want to maintain clarity for other readers, which I guess sometimes iisn't that important.

 

Well, breaking it down into functionality, if you use a radiused traveller the traveller control effectively becomes the "mainsheet", and the "mainsheet" effectively becomes the "vang". Not that you'd ever convnce anyone to call them that just because it makes sense.

But yeah, radiused travellers make a lot of sense for multis.

 

Well only if you can keep it flat in the Z axis - which is harder than most realize since you typically don't have any structure between the Akas to run the traveler on - unless you are one of these big piggy Lagoon 45s or such.

What you really need is a wide hull planform. which is something you can get more easily in a beamy mono. Like the Minis or Volvos (damn why am I talking about cars )

 

Vang sheeting makes sense in a boat where you have to do a fair amount of rapid sheeting control - like on a dinghy or a foiling boat.

Traveller sheeting makes more sense on a boat that is a lot less tender - because you essentially set the traveler and sheet optimally for the lull and you use traveler as your fine tune, your mainsheet fine tune as your medium tune and the actual main sheet for gross tune.

Another way to think about it is that in that sort of boat the traveler is for the transition from puff to lull cycles, the mainsheet fine tune is for gust response (ie above the mean puff variation) and the mainsheet is the Bordelle De Merde!! control

 

good thing something like a wing renders such silly debates moot

 

Really good point! I forgot that Doug went high tech on this boat and is using a wing!

BTW I saw the GroupAma C-class boat at the boat show today. And it was Cool (though access was restricted)

 

MPX Test Model

Big changeover today and maybe tomorrow: both big tables need to be cleared 100% then poster paper will cover them for laying out the rig.
Don't really have the room(!) but it will get done somehow(there goes the second bedroom).
Nicely cool today with spectacular whitecaps on the river-quite beautiful!
Before and after pictures of the great changeover when its done.....

Picture: The River of Distraction-cold front has just come thru-north wind 25+ knots-hard to concentrate with such natural beauty staring at you. Man, am I lucky to be able to work in a place like this!! Simply awesome........

click,expand and click again for best view--

 

MPX Intermission

As the original poster and author of this thread I hereby present one of the only things that can distract me from working on the boat. I mentioned in the previous post about the "River of Distraction" and its awesome beauty when it's blowing 25 knots. Upon reflection, it occurred to me that about 5 miles away there would be a nest of kites! I wasn't wrong-over 30 on the lee side of 520-a tremendous, inspirational scene:

click, expand and click again for the best view---Three of the pictures show the guys flying -one really got some air. Almost everybody jumped really high every chance they got---

 

Ok, I have a question about this thing. Something about it is bugging me.

You say only the main foil has a depth-sensing wand and trim tab. Ok, so that will deal with the height control of the main foil, and presumably the longitudinal trim of the whole boat.

However, the foil on the lee ama, which is intended to carry 80% of the load when foiling, doesn't have any depth control system. All that foil can do is generate a fixed amount of lift (predetermined by section and angle of attack) at any given speed.

So, if the amount of lift generated, at any particular speed, is even slightly greater than what is required, the ama foil will rise towards the surface until it starts losing lift due to proximity to the surface.

If the lift generated is even slightly less than what is required, the foil will sink until the ama hits the water and starts taking effect (either through bouyancy or planing forces).

To put it another way, the transverse stability of this model seems to be very critically balanced, and extremely unforgiving of real world variations in speed and loading. To get around this problem, you'll probably need wand control of both ama foils as well.

Incidentally, this is a good argument for either ladder foils or Bruce foils (or any other dihedral foils) in that they give automatic lift response to changes in depth, thereby making them fairly stable in terms of ride height without external input (which is a large factor in why some boats use them).

Also, the idea of using the wand on the main foil to "increase righting moment" seems infeasible, unless you are prepared to accept the boat not flying once you reverse load the main foil.

This is tied in to the situation already mentioned with the ama foil: if its lift is balanced to fly the boat nicely and you then effectively increase the weight by dragging the hull downwards with the main foil (which is what you are proposing), that will again mean the ama foil sinking in depth until it is stopped by immersion of the ama itself.

So to make that idea workable, you'd again need wand control of both ama foils as well as the main foil. Looks to me as though by the time you get this thing foiling stably in real world conditions, it'll have to be running three independent wand systems.