Auto-Flight
We have too little information to accurately predict the boat speed under each of the above conditions, but a (very) rough-cut estimation might be something a good bit less than the apparent wind speed in each situation, but surely more than half of it? Bruce number is not high at 1.2, but the boat is pretty clean, skinny, and after all will be flying its ama. This might lead to boat speed estimates of 10 kts, 17 kts and perhaps 5 kts, respectively.
Dave,
The lowest boat speed relative to wind speed scenario occurs upwind. Forget deep downwind in light weather.
If you look at the VPP tables for Jester 2 (Bruce Number ~ 1.76)
http://www.biekerboats.blogspot.com.au/2012/12/an-update-to-our-32-foot-proa-design-in.html
Assume that your proa is as fast, (it won’t be?) then your boat speeds will be:
Case 1: 12 knots AWS, 7.18 knots boat speed
Case 2: 20 knots AWS, 8.91 knots boat speed
Case 3: 8 knots AWS, 5.14 knots boat speed
This will mean that your auto foil needs to be significantly larger again???
If you already have a foil for lateral resistance, why not use it and avoid the cost and complexity of at least one other additional foil and the drag, however small, that goes with it???
I still love the idea and I reckon I can make it work for my proa using the lateral resistance foil ......
Rob
If you already have a foil for lateral resistance, why not use it and avoid the cost and complexity of at least one other additional foil and the drag, however small, that goes with it???
I still love the idea and I reckon I can make it work for my proa using the lateral resistance foil ......
You most certainly can do this, Rob; in fact it is my preferred method. For purposes of this demonstration, I wanted to provide a “bolt-on-and-go” gadget both easy to understand and (ridiculously) simple to build and trial, without permanent alteration of any kind to an existing boat.
Even for a new-build using the main foil for auto-flight sensor and actuator, I still to recommend trialing the dead simple version, even on a cobbled-together “proa-like” boat (anything from Coleman canoe with PVC aka to a borrowed Hobbiecat with C-clamped-on mountings for the gadget.) Actually flying with the gadget will offer you far more insight into how to build yours than any calculation or discussion.
Dave
The lowest boat speed relative to wind speed scenario occurs upwind. Forget deep downwind in light weather.
Neither is strictly true from the VPP you reference. The upwind case you quote is while pinching the boat up to 32 degrees of true wind; when the VPP clearly states best VMG will be at 44 degrees. Big difference in boatspeed and in rig force, although I fully agree with you that pinching the boat up, which increases rig force *and* slows the boat, will need a larger paddle. Off-wind the VPP indicates we might continue to fly the Jester’s ama as deep as 140 degrees true in strong wind with “default trim” (not supplied; crew to windward? Ballast tanks full or empty? Spinnaker or no spinnaker?), and likely as deep in light as well, by moving the crew to leeward—either until the ama flies or the max throw and force of the blade are demonstrated. This VPP also does not mention whether the ama flies or when. There should be a noticeable bump in boat speed as the ama comes out, but I can’t see it. I expect it may not be there there, putting the whole comparison in jeopardy, since the subject boat will have much less wetted surface than will the Jester.
Still and all, more powerful is always better than less powerful. If that means a larger paddle to cover all conditions, I see few negative repercussions. No one’s commented on the version where the entire (asym) ama pivots athwartship, thus acts as the sole and complete actuator/sensor with no linkage or fragility at all. Plus it gots lots of area… 😉
Dave
The lowest boat speed relative to wind speed scenario occurs upwind. Forget deep downwind in light weather.
The upwind case you quote is while pinching the boat up to 32 degrees of true wind.
I have sailed on dozens of boats that sail well at ~ 32 degrees AWA, which equates to ~ 45 degrees TWA. But I hope you are right, because my proa will be up to 3 knots faster upwind than I was expecting…..
But then 20 knots TWS equates to ~28 knots AWS, so does that mean that sail forces have increased by ~96 % and foil force much less so???
Clearly more foil area will be required any way you look at it.
Your work is based on the the foil lift arm below the pivot being equal to the mainsail control lever arm above the pivot to maximise sheet travel hence the rotational / dumping effect on the boom…..You can fine tune the system either by varying the length of the control lever arm or the position of the sheet control on the boom in order to minimise foil size and / or system forces, but it will affect boom rotation.
Attached are some numbers for my 9.5 metre proa FYGI.
Rob
Meaning our little ½ sq ft auto-flight blade, again working at a CL of about 1.0 (perhaps it’s a Speer reversible asymmetric section, set at zero AoA, relying on the boat’s leeway to give it ~5-7 degrees working AoA) creates a force of about 100 lbs at 17 kts, about 35 lbs at 10 kts and about 9 lbs at 5 kts boat speed.
Most boats with reasonable performance will sail with a leeway angle of less than 4 degrees, With a fixed angle of attack the foil will operate with a lift coefficient of around 0.4 or less (which for a some foil sections is conveniently within the low drag bucket, so not such a bad thing). You could achieve a lift coefficient of 1 if you used a variable pitch paddle, but that increases the complexity of the system.
Mal.
Most boats with reasonable performance will sail with a leeway angle of less than 4 degrees, With a fixed angle of attack the foil will operate with a lift coefficient of around 0.4 or less (which for a some foil sections is conveniently within the low drag bucket, so not such a bad thing). You could achieve a lift coefficient of 1 if you used a variable pitch paddle, but that increases the complexity of the system.
I’m sure you’re right Mal, though I have a dozen published sources, from Skene’s to Marchaj, agreeing that “very efficient” yachts may have as little as 5 degrees leeway, while “typical” yachts will have from 5-15 degrees.
Have you tried measuring this? It’s not difficult, with models or full size. Trail a long light line and either sight it with a compass (full size) or photograph it from directly above (model sailing under a bridge, or past a tall stepladder set in shallow water). Most will be surprised just how rare 5 degrees of leeway is and how hard it is to get under 10 degrees, when hard on the wind. 😉
I’ve attached a lift coefficient v AoA graphic for an ogive section. ~0.68 @ 4 degrees and ~1.0 at 7-8 degrees leeway. YMMV.
Dave
I was referring to reasonable performance by modern multihull standards as opposed to the pre 1980’s ballasted monohull standards referred to in the quoted texts. 😊
An asymmetrical section will help.
The chart above shows raw section lift coefficients. You have to allow for 3 dimensional effects (upwash due to finite aspect ratio) which reduce the effective angle of incidence.
Mal
Your work is based on the the foil lift arm below the pivot being equal to the mainsail control lever arm above the pivot to maximise sheet travel hence the rotational / dumping effect on the boom…..You can fine tune the system either by varying the length of the control lever arm or the position of the sheet control on the boom in order to minimise foil size and / or system forces, but it will affect boom rotation.
You are entirely accurate here, but my work isn’t meant to be “based” or restricted, either one, to any given leverage ratio (“gear ratio” if you like). The illustrated case is a cartoon example rendered to teach the concept, not as a working or scalable model. A wide range of gear ratios are possible, between lever arms, block and tackles, etc. Similarly, any amount of horsepower is available, depending on the size of the paddle—even a paddle wheel can be designed and attached below the akas—resulting in a very long “throw” indeed!
Every boat will need to play with gear ratios, blade size and gadget placement before they are optimized. I’ve made such adjustments as simple and inexpensive as possible, and am confident they will only need doing once; that the gadget’s range of power and motion will suffice for the great majority of flight-capable conditions for any boat. It will only be after half a dozen boats actually mount the thing and run with it that I might even hazard to guess specific lengths, ratios, forces and performance. (You’ll realize that the farther the gadget is from the ama, the less sensitive it will be in sensing the ama lcation; the closer the more sensitive. Does it matter? Maybe, maybe not.)
I strongly recommend starting too big and whittling down—most of us are light air sailors and a gadget requiring high winds might be a disappointment. I also strongly recommend starting with an oar, steel sleeve and hose clamps—even to the point of whittling your own oar from a 2 X 4 (Tim Anderson’s my favorite. Makes it look so easy—it is!—I always want to go outto the garage after viewing his vids): http://www.instructables.com/id/twobyfoars/
Dave
I was referring to reasonable performance by modern multihull standards as opposed to the pre 1980’s ballasted monohull standards referred to in the quoted texts. 😊
You won’t want to read Newton then, or god forbid, Pythagoras. Those guys are completely out of date. 😊
Dave
You won’t want to read Newton then, or god forbid, Pythagoras. Those guys are completely out of date. 😊
Dave
Well, in some respects, they are! 😊
Tom Speer points out:
A related approach would be to use the articulated fin as the primary side force generator. In this case, the moments on the fin may not act the way you anticipate. Because the side force on the fin has to equal the side force from the sail rig, as the boat heels the side force will not drop off. Instead, the leeway will increase to maintain the same side force. As a result the moment about the hinge will increase with heel, not decrease due to the lesser wetted area. The increase is because you have the same force concentrated closer to the tip. Until the tip stalls, at which time you have a big reduction in the loading on the foil. So for the early part of flying the ama, rigging such as you show would sheet in the sail as the ama rose, and then would release it just as the foil was about to emerge completely. Then it would dig in as the ama descended and sheet back in. It could make for some interesting behavior.
And he’s right! The Auto-Flight blade cannot be the only leeway generator, there must be daggerboard(s) or rudderboards elsewhere.
You get much the same effect automatically by simply using a C or J foil attached to the ama. As the ama rises, the vertical part of the foil emerges from the water and more load is concentrated on the horizontal part. This increases the downward component of the force, providing more righting moment. There will be a stable flying height as long as the sail trimmer doesn’t push it so hard as to overcome the righting moment from the foil or the foil breaches.
This is not only the way the AC catamarans do it, it’s the geometry on the Brown Bieker ama foil as well. I first read this foiling proposal on J G Hagadoorn’s “Ultimate Sailing” in 1975. He goes a step farther and uses the curved foil with no ama at all.
Most boats with reasonable performance will sail with a leeway angle of less than 4 degrees, With a fixed angle of attack the foil will operate with a lift coefficient of around 0.4 or less (which for a some foil sections is conveniently within the low drag bucket, so not such a bad thing). You could achieve a lift coefficient of 1 if you used a variable pitch paddle, but that increases the complexity of the system. Mal.
There’s a funny supposition in “modern” times that lower leeway angle and high performance are somehow related. They aren’t. Leeway angle is a simple function of board size versus loading. A big board lightly loaded will have a small leeway angle, a smaller one or one running at higher load will have a larger leeway angle. What one designs for is L/D, what one gets is leeway.
Virtually all symmetric foil sections, including those used for daggerboards and rudders on perhaps 95%+ of all “modern” sailing craft use foil sections who’s L/D max is found between 7 and 8.5 degrees AoA (NACA 00 series, 63, 64; also so-called “bucket drag” laminar flow sections, etc, etc). Those with L/D max under 7-7.5 degrees are twitchy or designed for specialized applications.
It’s not advisable to run any foil at a lower AoA than its design optimum as you only reduce performance by doing so. Changing AoA as you suggest may have merit, essentially running the hull at a different AoA than the board, but again as you suggest, that adds complexity not wanted for this application.
Cheers,
Dave
The Auto-Flight blade cannot be the only leeway generator, there must be daggerboard(s) or rudderboards elsewhere.
Tom Speer makes an excellent point.
But if the ama lifts, the leeway resistance foil lifts reducing area and increasing sideforce per unit area on the auto-flight foil because the boat will be sliding off to leeward more than it did before and due to increased AoA???
But I wonder…....If you have a dual purpose foil with 45 deg windward cant as your starting point, if the ama foil “pops”, in Sidecar’s case, you gain ~400kgm RM (by losing ~100kg uplift) and a dumping sheet which allows the foil to swing down to vertical, you have lost uplift AND gained leeway resistance as the ama comes down plus a half dumped sheet and if it swings all the way to 45 leeward cant, you eventually end up with the same leeway resistance as you started with, but with an extra ~400kg RM making ~800kgm in all which is about a 25% increase in the overall righting moment plus a fully dumped sheet.
My foil has to pivot anyhow and there is still dynamic RM potential to explore, so I have nothing to lose…“Sidecar” has 2 full time working rudders in any case, and I am fairly confident that they will be sufficient for leeway resistance by themselves. I made them over-sized and it was only going to be a matter of how much I eventually reduce them once I got to know the boat.
It could be that rudders only are better, in which case, I can reduce the foil down to “wand” size.The rudders also work semi lifted, so it should be fairly easy to establish what works best before I cut anything down.
Complicated stuff…...only one way to find out!!!
Rob
With all that talk of increasing RM in the pacific/atlantic hybrid proa thread I’ve been thinking about the downward pulling foil which the Jester-Class proa features... It increases RM, doesn’t weigh much, and the load can be supported by the shrouds, but it results in increased drag and is dynamically instable (foil leaves the water due to waves and you’re lying on your pod in no time).
Using the downward pulling foil only makes sense performance wise if for a given amount of downward force on the ama, the drag increase is less, than it would have been had you added water ballast (which with auto-flight would come down to the ‘L/D ratio’ of the vaka). Assuming that’s the case though, then we’d only have to keep the downward lifting foil from plopping out of the water completely and we’d be all good to go.
How about trying to use the auto-flight mechanism to keep the ama flying AND the downward pulling ama foil submerged in the water? If the ama foil wants to plop out of the water, then autoflight will be dumping the sheet anyway… The downward puling foil would have to have a pretty long strut to get it down deep enough, but with that setup the ama foil could just as well be side mounted and wouldn’t have to be retractable through the hull, since it’d be surface piercing anyway.
It’s perhaps an akward solution, but it wouldn’t require straying significantly from the pacific proa setup and all the weight benefits that brings with it. Do you guys think this could work?
Marco
Along similar lines, I’ve been thinking that it might be a good idea to combine Fritz Roth’s ‘hinged vector fin’ with the auto flight mechanism. The hinged vector fin can be used as the surface follower and since it is already a hinged structure it should be easy to adapt the sail control mechanism to it.
Mal.