Foiler Design

Thanks, Alan-I am interested. The wand would not quite be at midships but at
the location of daggerboard. It could simplify things a lot like it does on the Rave foiler-particularly in light of the fact that the Rave foils(and mine) are retractable.There is also some video of a Swis guy (JPZ) using dual "midship" wands at the Moth Worlds. In that video he has zero crashes compared to numerous crashes for Moths with bow wands.

 

Doug Can you tell me where to find the video you refer to?
Just because moths with bow wands experience more crashes than a centre wand may or may not be the full story by a long shot. There are simply too many variables to draw a conclusion. In the math modelling I have done in the past the bow wand has come out superior. In most control system lead (rate of change) data improves the system and in many cases is essential to stability. Not withstanding it is possible to have too much of a good thing. It is important to realize that forcing functions (in this case waves) only effect control not stability. A simple explanation of YPZ's success is that he is making better use than others of the manual control avialable via the rudder elevator.

 

Alan, here is the midship wand moth at the worlds: http://www.youtube.com/watch?v=8C9u2m8cdvk
And here is video of other Moths taken at the same time and in the same place: http://www.youtube.com/watch?v=IPn6fqL2LNo&feature=related
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By no means do I think this is definitive-it is merely interesting. The fact is JPZ finished 31st out of 95 at the UK Worlds and boats with bow wands finished ahead of him. He also finished 14th out of 36 in the Velocitek Speed Challenge-with bow wands ahead again. These were his first two races with this boat.

 

Here is a comment by Julian Bethwaite on SA regarding the bi-foil system he is developing for the 49er:

"Yes we have gone the Canard route, because the Canard is inherently more stable than the tractor route. Firstly moths are quasi Canards anyway, secondly there is a huge difference in crashing a 30-40kgs moth at 18-20 knts compared to crashing a 130kg 49er at 25-30 knts. The 49er has another 50% of the power of a moth, it will take off at double to treble the speed of the moth. It should go 1/2 as fast again with the existing rig. If we go to a smaller rig, say 600mm shorter, it should go significantly faster again.

I also find it interesting that people are bemused about steering from the front when none of them drive cars with the steering wheels at the back. Stupid!

No we dont expect to fly first time perfectly, yes we expect to have some very trying days before we get it right. But we are very confident of what we are doing and we believe strongly that in 2-3 months we will be flying and operating in the mid 20 knts.

From the speed boat testing, we are now very confident of speeds up to 20knts, and have had a couple of successful runs at speeds mid and high 20 knts bracket. We are also confident we can do it without hurting people!"
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Question for Tom Speer, Alan Smith and Marc Drela: do you feel that this comment is correct for a two foil monofoiler? "..because the Canard is inherently more stable than the tractor route."

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Also see posts 154 & 155 from Julian Bethwaite and Mark Drela in "New High Performance Monofoilers".

 

I think he is referring to the rudder. It is more stable in front just as in bicycles. In the beginning of the thread it was tried in a moth but was left because of problems when tacking, I think. Have you seen the japanese waterjet two-foiler? I don't remember the designer's name but it is a clip on youtube. I think it uses front rudder (and also front elevator sensor and actuator).

 

Nonsense. A wheels in a rolling vehicle are stable due to gyroscopic stability - a spinning mass resists change in angular direction. There is no correlation between rudders and wheels - none of the same forces apply.

 

Yes and no.

On a byke the spinning front wheel will under the effect of precession automatically point into any lean which arises, making the apparatus self stable and the staying upright easy to master. In order to initiate a turn in this system one must actually steer the wrong way initially in order to make the the byke 'fall' into the desired turning direction. With the effect of precession one must apply force to the steering wheel in the opposite direction to desired turn. When this pressure i removed the byke returns upright going in a straight line.

In a bow rudder bifoiler the same principle apply (fall to the right, steer to the right to get back upright) but with no help whatever from precession so it would be more difficult , but not impossible as it would be something like the case of the byke as it approaches zero speed, or better yet a byke with tiny light weight wheels. Also turns need to be initiated by steering the wrong way first.

In fact the dominant method for maintaining balance in a bifoiler is through the same methods as normal dinghy sailing using combination of sail trim adjustments and body weight adjustment. With a stern rudder the lateral acceleration due to turning is offset by the roll torque of the rudder so is not a powerful steering for balance method.

I would have personally thought that a stern rudder is preferable for bi foiler handling as it allows one to steer fairly naturally using classic dinghy methods of roll control instead of having to also steer by bicycle methods which i would have thought would be an extra distraction that is not required for roll control as there are sails. (apart from straight downwind, which is a rare ase for a high speed sailboat, when sailtrim for roll control becomes tricky).

So in the case of motor propelled bifoiler i would say bow rudder is pretty much a requirement to obtain roll control through steering a la motorbyke style although without the guiding hand of precession.

For sail it is quite a different situation that is worthy of further investigation/experimentation i think.

 

Try to drive a forklift at high speed. It is difficult but fun. It might be easier if it had a longer wheel base.

 

While I love the metaphor, Sigurd, LOOK OUT!!!! as here comes the omnipotent wrath of Doug Lord, simply because you used an automotive reference to explain a salient point in the boating world.

 

I used to race motorcycles in 400 and 750 production here in Canada, so I completely understand counter-steering. I also worked in a beer factory (great job!) driving forklifts through University, so I have four summers experience driving forklifts as well. Steering stability is heavily dependent on the axis of rotation of the steering column being some degree off vertical in line with the direction of travel - about 5 to 12 degrees on a bicycle, and usually vertical (for short, easy and fast turning radius) in a forklift.

If you had a bicycle with zero steering axis camber, it would feel unstable and dangerous like a forklift at speed. If you had a forklift with cambered steering, it would be far more stable at speed but harder to maneuver slowly in tight spaces.

I do not think so ... the same forces do not apply. Basically, there is not adequate control of roll-axis using only the rudder to affect things. The only real substantive control on roll axis is movement of crew body weight in or out. In an airplane the ailerons handle this roll axis control.

Counter steering ("wrong way") would not result in a turn in the opposite direction for a front rudder foiler. You would have to steer the front rudder normally, and also move in towards the centerline to heel the boat and take advantage of the foil lift (as done in Veal Heel). I suspect that body movement and control of roll axis movement has a greater effect on steering than a rudder does. (Just as a it does in normal dinghies, or I could not sail well without a rudder). I can completely control a sail boat without a rudder via weight placement and hull shape.

--
Bill

 

Brief time out here...

Another point made. I used to race 350 production and twice, 350GP. After that, it started to get waaaay too expensive for a student, so I switched over to Observed Trials, sold all the MX, Desert and Road Race machines and got serious about my studies.

Want a challenge, guys, one that is the ultimate in learning about weight placement, balance and braking?... go take a really "out there" run down a very steep, two lane canyon road with the engine off and the tranny in neutral. The lack of engine noise, alone, will force you to look at your shorts afterward in case you need to use some tissue.

Ohhhh, those were the days when youth reigned and the future was so very far away.

OK, time in!

 

<<<I do not think so ... the same forces do not apply. Basically, there is not adequate control of roll-axis using only the rudder to affect things. The only real substantive control on roll axis is movement of crew body weight in or out. In an airplane the ailerons handle this roll axis control.>>>

I might have tried to express myself too briefly above.
I was not referring to righting moments (maybe i'm misunderstanding your sentence) only roll stability outside of righting moments. Ie heeling and righting moments must be equal and opposite, however there are instabilities that must be dealt with and steering torques are something that need to be considered for roll stability and avoiding positive roll/yaw coupling that can rapidly lead to uncontrolable roll oscilliations.

<<< I suspect that body movement and control of roll axis movement has a greater effect on steering than a rudder does.>>>

Yes of course the moments involved indicate that sail trim and weight placement, at least at the range of speeds that are realistic here, will be greater than centripetal effects and roll torque due to rudder. This situation however eventually reverses as the sheets are freed at reater angles to the wind.

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Some more detail and some ommited points on my previous post;

I think it is helpful to imagine the case of zero heeling moment first and then apply the heeling moment and lateral pressure later.

In the case of a motorized bow rudder bifoiler yes you must initiate a turn by counter steering.

With sails you can overpower the inertia/torque effects of the bow rudder with correct sail trim/weight placement adjustments, provided you are sailing at small apparent wind angles and below a certain V/W.

The point i had ommitted previously is the very important distinction to be made between bykes etc and sailboats, which is the lateral forces. This does not invalidate the previous points but are additional effects that must also be taken into account.

Subsonic foil borne craft must have greater foil loading on the leading foils than on the trailing foils. When analyzing this one comes to the conclusion that sailboats should for yaw stability have lee helm. Now with a stern rudder this is not at all efficient, but it is with a bow rudder. Also lee helm, from the point of view of control/safety, is a undesirable charachteristic in the case of boats that sail at moderate V/W but is a good feature for boats that consistently sail at high V/W.

In the case of classsic monohulls they will be yaw stable despite the weather helm, provided the weather helm is moderate, due to the overwhelming effect of changes in angles of heel to weather helm. However this diminishes as the angles to the wind free, until it is plainly observed that the monohull with weather helm will refuse to keep a course no matter what trim is given even though it could easily be trimmed to sail a steady course upwind.

In the case of bifoilers where sail and weight trimming keeps the boat at fairly constant angle of heel it would be a benefit to have lee helm for improved control.

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What i'm trying to say is yaw stability & control, yaw/roll coupling, and roll stability (stability in the current context is not +ive righting moment curves), for bifoilers (which are roll unstable in the normal sense of the word) is a fairly complex and multifaceted affair and i do not think the question of the superiority of bow or stern rudders can be resolved with a couple of opinions or analogies.

 

A small correction,regarding terminology,the inclination of the steering axis in the fore and aft plane is usually described as caster.Camber being the angle of the wheels relative to the vertical centre-line.

This thread remains a source of thought provoking information and I look forward to its evolution.

 

Doh! Fingers typing faster than brain proofreading. Thanks for the gentle reminder.

--
Bill

 

Tc regarding subsonic foilborne sailboats.
I am not sure if I am confusing yaw with pitch in your argument, but I don't agree that the front hydrofoil has to be more loaded than the rear. For the rudder/dagger it may be different, and heeling will complicate analysis, but essentially the sensor provides a fly by wire system so other stability in pitch or heave is not necessary. My opinion is worth its weight in clear fog.