Ric ochet:

[Quoting me]

3) Why "pivot set *well* in front of the center of pressure"? I could understand a *little bit* in front, to stabilize things (and I was actually assuming that), but... Setting it "*well* in front" would only make it harder to get the damn thing to turn, no?

Depends on definitions. I was trying to combine your desire for handlebar steering with Ash's wish for lean steering.

Ah, OK.

If the front ski is triangular, then as the thing gets up on step the center of pressure will move back, because the part that isn't in the water will only have airflow over it.

What do you mean, "on step", in this context? I was envisaging pretty much flat skis, not "stepped" like some mid-twentieth-century racing boat hull.

If you mean, "up and aquaplaning", then it'd actually be the other way around, since you obviously want the skis spring-loaded to have the forward tip up and possibly out of the water when you're not planing -- so when the weight goes onto the skis, this spring-load is eliminated, the front tip (well, the front *part*, if not the very *tip*, which is probably curved up) settles down onto the surface and moves the center of pressure *forward* compared to where it was with the tip out of the water.

If you mean, "leaning into a turn", then AFAICS, the answer is: "Probably not -- rather, quite the opposite". In fact, what would happen if you *didn't* taper off most of the front end of the ski, and widen the rear of it... is that the rear end of it would rise out of he water as you lean into the turn, the front end would cut down under the surface, and you'd get an almighty fuck-up because the center of pressure moved entirely to much *forward*.

Exaggerated example -- say we're turning left: Front end of ski goes left and rear goes right, we lean left into the turn, left becomes (to some degree) down and right up -- so rear end of ski is poking up outta the water and front end pointing sea-bottom-wards. To keep the direction of the edge of the ski we're now riding on -- the left half of it, more or less -- parallell to the surface, we *have to* slice off most of the left-front part, and add material to the left of the left rear. That will (in the ideal case, at the correct combinations of turn-in and lean) keep the center of pressure -- exactly in place.

In pure-geometry terms, the proportion of a triangle that is to one or the other side of a point at X percent of the *height* of the triangle stays the same if we slice the triangle vertically *or slantwise a little off-vertical* if we slice through the *top point* of the triangle. Draw some (upward-pointing, approximately horizontal-base and vertically-symmetrical) triangles for yourself and see.

An extension of this concept would be, as I think I alluded to elsewhere, be to have raised edges to the ski, which aren't in contact with the suface. As an extreme, say we have a 90-degree surface standing vertically, straight up, on each side: |______| ... (At the thinner end, the front, it's |_|) ... Going straight forward, not leaning, only the flat ______ surface is in contact with the water. Now say, still extreme-example, we lean 45 degrees in a turn to the left. The "L-shape" of the left edge *now becomes a V-shape*, as the upright left edge comes into play.

Generalize that, and we see that a couple more "kinks" (the "step" you were talking about? Naah, hardly...), of something less than 90 degrees, would probably be better. Continue generalizing, and the bottom of the skis becomes curved cross-wise; go all the way, and our whole skis probably become conical segments, or something...

You're going to want the pivot to be forward of the center of pressure in all circumstances.

Yeah... For the front ski. How does this work for the rear one -- do we want the same thing there, or just the opposite? Anyway, as I've hopefully (but not probably) shown above, a tapered (or in extremis triangular) shape would accomplish that "automagically".

Then, when you lean, the CP provides a side force that turns the handlebars. OTOH, if you turn the handlebars, the CP moves to the outside of the turn, and applies a force that tends to make it lean.

Yup, think so...

As I said, I haven't either the MIPS or the smarts to model it, which it needs before anybody starts laying fiberglass --

Fiberglass??? What's wrong with steel, or if you want to be fancy, aluminium? Remember, we are *_N_O_T_* building a boat hull here! (Or maybe *you* are, but *I* sure wasn't... :-)

And I really think the outboard motor version is a nonstarter.

Why???

Remember, I wasn't going to hang the engine off the rear ski, but the other way around... The prop wouldn't be at the end of the ski, but somewhere under the middle of it. What's wrong with that?

It needs a jet pump, with the intake in the rear ski

How is that different from a propeller, in principle?

My very first idea was based on a water-jet pump, yes... But I was thinking, hang it under the *middle* of the contraption, poking down into the water (powered from above) on its own, quite separately from *both* skis. Can't remember why -- probably just the first thing that came to mind, perhaps because of some naive desire for symmetry.

(which will carry most of the weight).

Possibly, but not necessarily. Depending on how far forward you sit on the contraption, and the distance between the skis, you could get a weight distribution pretty close to fifty-fifty. Would that be a bad thing? If so, why? In the inboard-motor scenarios (like the mid-mounted water-jet, above), we could probably even get most of the weight on the *front* ski, if we wanted to -- not that I can see why we would want to...

Christian, would you be willing to compromise on a hydrofoil? The rear "ski" would instead be a pair of fins sticking out of a vertical strut. It would solve some problems [while introducing new ones, of course].

Yes and no: Yeah, sure, that would be fun! But no, that would be *another* thing, that I'd like to *also* try -- a complement, not a replacement.

Among other things, differential trimming could provide forces in pretty much whatever direction you wanted, and I've been searching (and failing) for a righting force once the bike is leaned over, other than centrifugal;

Why -- isn't that enough? What more do you have on a motorcycle?

far as I can see, the best you can do with a two-ski gadget is metastability.

Uhh... What's "metastability"?

Sounds kind of like what you have on a bicycle or motorbike, or for that matter, on a single waterski.

And if that's what you mean, then it's *exactly*... What I *want* to achieve!