Hydrofoil surfcraft

[kcasey]

MTB,

Regrettably I was only able to get into your posts on Sunday evening.

I think the pathline treatment is interesting, but before I can really appreciate what you’re describing some further clarification would be helpful.

Is figure 1a and 1b correct given that,

theta is the angle between pathline and wave crest

tau is the angle the the tangent to the face of the wave at the position of the surfer

lambda is the projected angle of tau onto the pathline plane

psi is the angle of the transverse slope between the left and right side of the board

Some other points of clarification

Do you see envision the centerline (stringer) of the surfboard as lying along the projected tangent (defined by lambda)?

You’ve written to ‘a first approximation’ but if the above interpretation is correct, then at least lambda can be found exactly using the values of tau and theta.

As for psi, this is embarrassing but I actually don’t see how you’ve come to your approximation.

Also, when you’ve written slope do you mean tangent? I don’t see what else you could have meant, but terminology varies and clarification is always helpful.

My apologies if it turns out that I’ve simply missed the obvious. Anyway, I do wish to understand your approach.

Your discussion of momentum was appreciated.

Thanks,

Kevin

[maxmercy]

MTB,

Another question(s) for you. You are currently using a front surface-tracking foil for pitch stability on the HYPO, which makes for a very stable ride in pitch, except when chop is encountered.

How would you tackle pitch stability in a fully submerged hydrofoil scenario, say if your front foil on the HYPO were designed to be submerged, with the only control applied by the rider would be a change in CG (Center of Gravity) location (i.e. no control surfaces on the foils)?

Which brings this question forward: On the HYPO, is turning simply initiated by a movement in CG that causes a rolling moment? How does the front foil respond to bank angles? Does angle of bank determine turn radius, or can you ‘crank’ a turn more by increasing the AOA of the main foil by moving the CG of rider/board backward?

So, basically, not having any control surfaces places many more restrictions on design, considering the only control input is a movement of CG. I would just like to hear/read your thoughts on this subject, and how you would/have done designing with this limitation.

Thanks so much for your explanations.

JSS

[kcasey]

I’m not exactly why you’ve introduced speed at this point, but I do think it’s a big issue here, and in general.

In figure 1, is the standard quick and dirty take on curl speed*. It’s not meant to be exact, just rough, if only to illustrate the dependence on curl speed on angle of incidence of the wave crest to the reef or bar. But it’s probably not that far off. (At least for small sections of the breaking wave. The diagram is not meant to represent a breaking wave from start to finish. A lot more is actually going on.)

In the figure, a number of curl speed vs incident angle plots for various breaking depths have been plotted. Curl speed is indirectly related to wave height, that is a larger wave will break is deeper water, and therefore for a given angle of incidence will have a faster curl speed. Even so, it would seem that the opportunity to go pretty fast is possible on small waves - if you can figure out how to do it.

(By the way, the assumption here is that the wave has the opportunity to break sooner in deeper water. On a lot of reef breaks this isn’t the case and the speed of the shoaling wave isn’t given by the standard square-root of gravity times breaking depth formula, at least in the way its being used here. For example, those sucking ‘unbelievably fast’ monster lip reef breaks scattered around the Pacific basin. Though the potential for similar kinds of effects can also be found wherever the reef amounts to a submerged cliff, -i.e. a pronounced abrupt change in depth.)

It would be interesting to know your thoughts on speed requirements and how they are to be achieved. In particular, what mechanisms are in play and what factors need to be addressed? Start simple if you like, especially since I’ll be a member of your audience if you do decide to respond.

Thanks,

Kevin

• curl speed, here defined as how fast the leading breaking lip (edge) of the wave is traveling more or less along what MTB’s has called a pathline, see his post to me KCasey for a description.

[mtb]

Quote:

<1> Is figure 1a and 1b correct given that,

<2> theta is the angle between pathline and wave crest

<3> tau is the angle the the tangent to the face of the wave at the position of the surfer

<4> lambda is the projected angle of tau onto the pathline plane

<5> psi is the angle of the transverse slope between the left and right side of the board

……

<6> Do you see envision the centerline (stringer) of the surfboard as lying along the projected tangent (defined by lambda)?

<7> You’ve written to ‘a first approximation’ but if the above interpretation is correct, then at least lambda can be found exactly using the values of tau and theta.

<8> As for psi, this is embarrassing but I actually don’t see how you’ve come to your approximation.

<9> Also, when you’ve written slope do you mean tangent? I don’t see what else you could have meant, but terminology varies and clarification is always helpful.

<1> Not quite. <2> Correct <3> Correct <4> No. Or at least not as shown in Figure 1a. I tried to derive the equations for the pathline slope in terms of the wave face slope and the path angle in this reply–but the formatting came out virtually unreadable. I also doubt if one can use Microsoft tags in anything posted to this discussion board so I can’t just paste a in response in html. Hence I’ll check your profile and if you have an e-mail address listed, a bit later I’ll compose the derivation of the pathline slope equation in a MS *.DOC file and send it to you via e-mail (and leave it to you to use the same approach, or some other approach, to derive the “transverse slope” equation). <5> Yes. <6> I see it lying along the line defined by the intersection of a vertical plane passing through the pathline with the sloping wave face. <7> Lots of assumptions in the equations I gave: planar sea surface (vs a compound curved real surface, or, alternatively, a flat board of length and width. No gradient in wave face slope as a function of distance along an axis paralleling the wave crest, etc. Hence the equations are only an approximation (but representative of the effects) <8> I may not always be careful in my terminology as I know what I mean. But by “slope” I mean the change in vertical distance per unit change in horizontal distance. The corresponding angle is the slope angle (slope angle = arctan (slope)) <9> Dang I hate it when the formatter removes all the paragraph breaks!!!

[mtb]

Quote:

You are currently using a front surface-tracking foil for pitch stability on the HYPO, which makes for a very stable ride in pitch, except when chop is encountered.

Technically speaking, the canard foil on the HYPO board is not a " surface-tracking" foil but rather a “surface-piercing” foil (due to the transverse slope present from one side of the craft to the other when traversing across the face of a wave). In essence, the transverse slope of the wave face acts like the dihedral angle on a surface-piercing foil operated on flat water. As a surface-piercing foil, it does not follow the surface as precisely as would a surface tracking foil. Typically only a small area at the tip of the foil on the wave side of the craft penetrates the sea surface (unless going straight off) and the “rigidity” of the flight elevation depends on the magnitude of the slope of the wave and the path angle of the craft (relative to the alignment of the wave crest).

Quote:

How would you tackle pitch stability in a fully submerged hydrofoil scenario, say if your front foil on the HYPO were designed to be submerged, with the only control applied by the rider would be a change in CG (Center of Gravity) location (i.e. no control surfaces on the foils)?

A fully submerged forward foil introduces a number of design problems that would need to be solved–especially in a surf environment. A number of flat water craft depend on the change in lift as a function of depth below the sea surface to regulate the depth of the canard foil. The Flykayak(tm?) is one such craft. But since the forward and aft pair of foils would likely be significantly closer together on a wave-riding craft, I would expect the pitch stability to be weaker. In addition, that effect is limited to the forward foil being submerged to one or two chord depths or less. With the sea surface curvatures and wave face slopes that one can encounter when riding and maneuvering on waves, I would consider that approach problematic.

If one depends on a mechanical sensing and control system to regulate pitch and flight height the sloping sea surface, the curvature that can be present in the sea surface, and the normally desired characteristic of rolling into a turn can present some significant problems for the designer (as well as complicating the system). I have designed a small number of such systems (and constructed parts for the same), but decided that was going to take more development time than I wanted to spend given the odds against success.

A fully-submerged foil also has a forward strut that can introduce yaw instabilities. Moreover, if one uses that as a forward rudder, application of right rudder to turn right introduces a rolling moment to the left (potentially introducing a motion sort of like that in the old movies of naval destroyers in the process of turning). To gain a foothold in the market, that characteristic would probably have to be dealt with. I have modified one of my early prototype boards to have a forward rudder. The hope was that rider manipulation of this rudder might help with roll control. As it turned out, the combination of the design and the reflex times of the typical rider made the “flights” (using the term generously) even shorter than they had been without the rudder.

Quote:

Which brings this question forward: On the HYPO, is turning simply initiated by a movement in CG that causes a rolling moment? How does the front foil respond to bank angles? Does angle of bank determine turn radius, or can you ‘crank’ a turn more by increasing the AOA of the main foil by moving the CG of rider/board backward?

1. Yes…but to initiate a turn it takes so little shift in CG that as one of the surfers who tried it commented afterwards: “You don’t turn this thing, you just “think” turn.”

2. Not sure what you’re asking. Banking the forward foil certainly introduces a transverse component to the pressure force produced by the foil. The details of the magnitude of that force as a function of bank angle and the resulting motion and accelerations, depend on a number of design and operational factors.

3. Yes (but not necessarily totally)…and yes–although in some situations moving the CG of the rider aft can result in a decrease in the AOA of the forward foil (even to the point where it may even become negative although the AOA of the main foil is positive).

Quote:

So, basically, not having any control surfaces places many more restrictions on design, …

Yes (although as one adds control surfaces one is also placing restrictions related to them as well).

Quote:

…considering the only control input is a movement of CG.

Generally very small movements of CG

The control problem is not very significant in pitch once the rider becomes used to the fact that the maximum design fore-and-aft weight shift range totals only about 3 inches. Most of the time a beginner tends to shift his weight too far back due to the fear of pearling (actually the craft is remarkably resistant to this happening on take-offs). Too far back and the rider and board “get air”; too far forward and generally the only effect is that the drag increases–although at very low speeds, the foil may “stall” and submerge easily during abrupt maneuvering. When the latter happens, the foil will often resurface (due to the drastic increase in the wetted area of the foil as it pearls) and the rider can continue on.

The primary problem is in roll. The effort required to initiate turns is very small and over-control is an extremely common occurance when one first rides the craft. The situation quickly becomes more unstable to deviations from a coordinated turn as the bank angle increases. In this latter case, if one doesn’t coordinate the roll/turn very well large (transverse) weight shifts may be required to recover. Alternative, or additionally, he may chose to deploy a pair of retractable, and controllable emergency foils (rider’s legs and swim fins) as a way of saving the situation (albeit generally not gracefully). Just as with a bicycle, it is generally easier to ride the craft at higher speeds than at low speeds. In the latter situation one tends to wobble around a bit in both cases–and in the case of the HYPO board the low speeds do not provide a large reserve of lift available from the front during these excursions from the desired path.

Quote:

I would just like to hear/read your thoughts on this subject, and how you would/have done designing with this limitation.

By designing for both possibilities.

[maxmercy]

MTB,

Thanks so much for your detailed responses, and let us all know when you get the new model done this winter, and especially tell us how it goes. Best of luck to you! That thing looks like a ton of fun.

JSS

[kcasey]

Thanks. It would be nice to have more than basic text and graphics, but I won’t complain as Swaylock’s is free – and free is free?

I though I might provide you with my approach. Hopefully, if you’re so inclined, you will resolve your vectors in terms of those in figure 2, and then again, perhaps not.

My approach may not be as immediately as practical as your approach, but I’m sure, as you’ve indicated, in the end one will be able to move from one to the other with the correct transform.

In figure 1, the plane of the bottom of the surfboard is represented by a normal (a unit vector in the direction perpendicular to the plane of the bottom of the surfboard) and the motion of the fluid, by a vector in the direction of the motion of the fluid. The angle of attack is found by using the dot product of the two vectors.

Conceivably you could then go on to define different angle-of-attacks as in figure 2. I realize this may not be standard a standard approach, nor useful to parse up the respective angle-of-attacks in this manner. (I somehow doubt it makes much engineering sense to do so. It’s not really something I’ve done.)

Nevertheless, given my approach, which tends to view a surfboard ‘when in motion’ as planing on multiple planes –i.e. not just the standard horizontal plane used by naval architects, it’s the best approach I’ve been able to muster so far. The need to do so comes from my interpretation of where the thrust is coming from. In particular, that planing in one direction provides the thrust in another, if the bottom is oriented correctly. This however is my problem, and I’ve offered it not so much as a counter argument or to start a controversy, but just as an explanation as to why I’ve tended to take the approach I have. (In my prior Dynamic threads, I referred to thrust as propulsion, though there is no assumption, nor expectation that you have read those.) You need not comment on my interpretation of the mechanisms of thrust, it’s the resolution technique that will hopefully allow you to answer my questions.

Anyway, thanks,

Kevin

(Edit, 09/25/06. Added diagram and additional notes. KC)

Figure 3 may be of some help.

Though here, tau and lambda look wrong, the geometry would show they are the same angles as referred to in the prior post. Both these angles are resolvable using the ‘cosine angle’ treatment as indicated in figure 2. Similarly for theta.

(Edit, 09/25/06. Additional notes. KC)

Also above, I actually have used the dot product approach to resolve flows, it just that at this point I haven’t attempted to use the full Lift/Drag treatment for each. I have no idea at this point how useful such an approach would be - I’m not an engineer and have little experience with these kind of lumped parameters

[iam2sam]

Impressive, but I’m pretty sure I don’t want to be trapped in the washing machine with that, or some of the other contraptions…

-Samiam

[iam2sam]

Quote:

on the “hydrofoil” subject, I don’t know if oyu guys have seen this, but it definitely is interesting.

http://www.metacafe.com/watch/28123/sea_bike/

I remember seing other videos too, but I can’t find them.

if refined, I can see a cool surf-BMX thing coming out of this?

d

Interesting, yes. The same marketer has a prop-powered surfboard hydrofoil. the “HydroGlider”. However, they don’t show it on any waves, although they do show some very low level surfing with the “AquaSkipper”. My biggest concern with the AS is: how do you get back up after stopping? It is a planing device that appears to have very marginal positive buoyancy by itself, and I’m having difficulty visualizing generating enough force from a submerged dead stop to get it back on plane (although it does look to be very efficient). I note that all the videos seem to show “land” starts…

http://tinyurl.com/fgho7

-Samiam

[iam2sam]

Quote:

Dale,

For a paddle board alternative take…check out

www.foilkayak.com

The thing already looks almost like a paddle board and gets the mind racing about an open ocean unlimited class board catching swells and the hydrofoils kicking in.

Could go like a banshee…

Pete

Site appears to have been taken down (hopefully temporarily). Some of the content is available from the Wayback Machine (unfortunately, no pics that I could find):

http://tinyurl.com/jogz2

-Samiam

[DaleSolomonson]

“kind of like a paddling Formula 1” The Flyak has already been charted at 28 kph!

Imagine adapting it for surfing open ocean swells… standing or sitting…

http://en.wikipedia.org/wiki/Flyak

http://www.aftenposten.no/english/sports/article1161296.ece

[ambrose]

Nuff to make a simple man resolve his vectors.

…ambrose…

standing to keep an eye out

for debilitating debris…

[TomBloke]

Quote:

The induced drag diminishes with increasing speed and varies approximately as 1/(V^2);

Sorry MTB, but that is correct in only one of many possible cases.

If we take a fixed angle of attack as the primary case, lift induced drag increases as speed increases.

It is only when the angle of attack is decreased at higher speeds that lift induced drag reduces with speed.

Furthermore, a reduction of drag as approximately 1/(V^2) will only occur when the value for the amount of lift produced remains constant.

Thus the relationship of velocity to lift induced drag 1/( V^2) is a ‘rule of thumb’ which applies only in a specific set of circumstances. . . and is not a direct relationship between velocity and induced drag as you have implied.

With a fixed angle of attack, lift induced drag increases with velocity !

Regards, Roy

[mtb]

Quote:

Quote:

The induced drag diminishes with increasing speed and varies approximately as 1/(V^2);

Sorry MTB, but that is correct in only one of many possible cases.

Quote:

If we take a fixed angle of attack as the primary case, lift induced drag increases as speed increases.

Correct (for a fully-submerged foil, an airplane, or a planing hull with a constant wetted area).

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It is only when the angle of attack is decreased at higher speeds that lift induced drag reduces with speed.

Correct (again, for a constant wetted area).

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Furthermore, a reduction of drag as approximately 1/(V^2) will only occur when the value for the amount of lift produced remains constant.

Correct. I don’t know about you, but at the end of one of my surfing sessions, the combination of the board and I tend to weigh just about the same as just before I went out. If a foil system is able to support me out of the water at 6-7 mph, and I maintain the same angle of attack and accelerate to say 20mph (increasing the lift to about 9 times the weight of the board and I), I’m literally going to be soaring above the water (at least briefly).

Quote:

Thus the relationship of velocity to lift induced drag 1/( V^2) is a ‘rule of thumb’ which applies only in a specific set of circumstances. . . and is not a direct relationship between velocity and induced drag as you have implied.

As noted earlier, the “specific set of circumstances” is that the wetted surface area of the lift generating device remains constant (e.g. a fully-submerged foil, an airplane wing, some planing hulls, etc.).

Quote:

With a fixed angle of attack, lift induced drag increases with velocity !

Yes. So why would one want a fixed angle-of-attack–i.e. a design in which the induced drag increases instead of decreasing as speed increases?

The only reason would be if an associated reduction in parasitic and form drag with increasing speed more than compensates for the increase in induced drag as the speed increases. There is also the problem with controlling the lift generated as the speed increases above that necessary to support the board an rider at the fixed angle of attack (esp. as the lift increases as the square of the speed for a constant wetted area and angle of attack).

One possible approach is obviously a design that results in a reduction in wetted surface with increasing speed so as to reduce the parasitic drag–as with a surface-piercing foil or (within limits) trimming a planing hull. In the case of a surface-piercing foil, there is an accompanying reduction in efficiency with ventilation of the foil. In the extreme case ventilation, the lift-generating efficiency of the foil can be reduced by more than 50 percent through the loss of flow over the entire upper side of the foil. There will also be an additional decrease in efficiency associated by the (often) decreasing aspect ratio (as more and more of the foil emerges from the water as the speed increases). And then there’s also the loss of efficiency of a foil operating close (within a chord depth or two) to the sea surface, and the loss of efficiency associated with wave generation.

[smallwaves]

i thought about adding hydrofoil wings to my waveski! Never got much further than thinking about it though

Kayaks that long surf unbroken swell anyway probably, but imagine the speed you get with foils: flying over unbroken ocean swell without paddling!

Like the idea a lot!

[daklaw]

The flyak in action

Cheers,

Rio

[TomBloke]

Took my new 10’9" tunnelfish out today in smooth offshore peaks, just over head high, and went 35.3 mph ! stoked as it wasn’t even cranking, just a little takeoff and zippy section sort of wave, starting to think that 40mph is going to be a breeze !

( that crunching sound is the sound of a few swaylockians eating their 2004 words)

[NormDePlume]

Roy- congratulations! Video and GPS documentation?

[janklow]

One question

Is that the one with the big keels…?