Planing hull kayaks??

Good observations Porta.

Skipboards operate in the " Sand Effect " area.

You are right in picking out the odd surface effect devices.

ALL " Skipboard Naval Architects " report to this post.

 

Propulsion by the combination of the wave shape and gravity.

Inertia.

 

I tell my Kids there's a ward down the hospital full of skim board victims................................ Just doin' the Dad job.

 

now there's a sport I havn't been reminded of in years!!!

I did a heck of a lot of "Skim-boarding" in my youth.
Never heard the term skip boards before

We would build a new board every year, mainly so we
could paint a new design on them. The design of course got better
every year. Basically the longer the bevel on the underside to the
the round centre, the better the board would skim. There was always the issue of building one too big or too small in diameter. I can't remember the exact diameter now, but I believe it was around 30" and the thickness was around 3/4" to 1 1/4". Great fun all summer long.
We would wait for the tide to go out and have long runways of water to skim on.
Waikikin is so right! People would always want to come up and try our boards.
we usually would say no, cuz you knew they would get hurt. I remember if we let them, we would always tell them "no mater what!!! jump on the board with both feet!" Of course they wouldn't for fear, and would do variations of the splits that were painful to even watch. Sometimes when they did jump on with both feet the boards would shoot out from underneath them and would almost become horizontal in the air before thier "rears" and tailbones would gravitate back to earth. Which was another painfull site to see. the trick is to jump on the front leading edge of the board with forward momentum.

thanks for the memory.

 

Dirt eater:

Skim boarding was my description, didn't know it was the official name. The boards are not just round anymore, to my surprise:

http://www.youtube.com/watch?v=WZmE491awWE

You are PLANING at LOW speed, so I don't think it is just an inertia effect? The skim board does not have enough displacement to float you. Unlike the skimming of flat rocks, skimming is LIMITED to a shallow flat water layer for execution, AFAIK.

Maybe this is a hydroplaning effect as with automobile tires, which is a modified lubrication effect? But hydroplaning occurs at high speeds with great mass and small surface area, where the water film can't get out of the way fast enough?

Porta

 

"Inertia" is where the energy comes from which propels the skim-board. The board is planing and occupant are planing; they are supported by hydrodynamic rather than hydrostatic pressure acting on the surface of the board. Or to put it another way "inertia" is overcoming the horizontal drag and planing is providing the vertical force.

 

oh such great fun it was.
what I find interesting as well after watching the vid provided by portacruise is the shape of the boards. They are more like surf boards than the round boards we used. The surf style boards were kind of the new upcoming approach when I was doing it *L*. The trick for us most of the time was distance, and hanging ten, and my big trick after hanging ten was to kneel down stick a couple fingers in the ground for a wonderful twirling finish! awww too funny. sorry, I dont' mean to get off track but it really does take me back.

I also think there were two inertia's. one when you threw the board and another when you jumped to the front edge of the board. Because you were running faster than the board was travelling you would add speed to the board.
when the tide was coming in you could also get a little surf time on the incoming waves if your timing was right.
again, I do thank you for the memory.
(now back to your regular station )

 

On Myth Busters they water skied behind an eight man rowing shell. Water skis are planing after you get up, but the boat didn't plane. That is a whole different thing. A surfboard is, again, using gravity and the force of waves to gennerate enough speed to get up on a plane but, no, they don't get up on a plane paddling out to catch the wave. As for the small boards that you see people ride in the surf, they throw them, then run and jump on. That isn't the same as gennerating the speed while on the board.
The question is, can you sit in a kayak and generate enough speed from just the power of the paddle alone, to get up on a plane? I don't think so and so far haven't heard anything to convince me otherwise. I think part of the problem is that the power isn't steady. You might get up on a plane for a brief moment, but as soon as the blade leaves the water, you'd come off the plane.

 

Why is this hydrodynamic vertical pressure not possible with canoes/kayaks, or is it possible with proper hull design?

P.

 

You could get the same effect as a skim board with a kayak by using a launching ramp to get up to speed. In both cases you only plane until friction overcomes the inertia from the run up.

The closest thing I'm aware of to a kayak planing under human power is a hydrofoil kayak. http://www.youtube.com/watch?v=U95UReP4mdo

 

Very possible. But the power required to propel the canoe/kayak with a planing hull at planing speeds is generally more than a paddler can produce, even for a short time. A mechanically propelled planning canoe/kayak should be possible. Note thought that the high length to beam ratio generally means there will be a more gradual transition to planing than most planing hulls, not an large "hump" to go through.

At lower speeds the planing hull canoe/kayak would have higher drag than a hull not designed for planing.

 

I was thinking about this on the drive to work. I think we need to define what planing is. If you built a 20 foot long surfboard, it could skim along the surface at very low speeds. But is that planing? Surface friction would probably be so high you wouldn't be able to generate much speed. When a power boat gets "up on plane", there is a sudden and dramatic reduction in resistance. My understanding of what is happening is that with enough power the boat climbs on top of it's bow wave. At low speed a planing hull acts like a poorly designed displacement hull, pushing up a big bow wave. Give it more power and it climbs up on top of that wave. Going back to the 20' surfboard, is that what is also happening here and the transition is just more subtle? Or is it not really planing at all?
I'm thinking now that with the right hull shape and a strong paddler, you might actually be able to climb up on top of your bow wave. But would it faster than a well designed displacement hull? Maybe for a short distance, but I think the power you would need to accomplish this would be too much to keep up for long.

 

RR:

Some comments below inserted into your message.

P.

 

No, there is no decrease in power when a boat transitions to plane. Nor does it "climb over it's bow wave". All vessels are to some extent supported in the water by both static displacement pressure and dynamic speed pressure. Together these two pressures generate what is called form pressure (or form drag) which, along with skin friction, makes up the drag of a hull.

What a planing vessel does is to use a hull optimized to make use of the dynamic pressure to lift the hull out of the water to significantly reduce wetted surface and form pressure at speed, just like an wig uses speed to generate lift in the wings or a hovercraft uses skirt pressure. A surfboard or skimboard works because the loading on the hull is very low and not much speed is needed to generate significant lift, enough to totaly lift the board out of the water. It is possible to generate significant lift to allow a "vessel" to be totaly supported by dynamic lift with human power, but it desn't look like a kayak.

http://www.youtube.com/watch?v=In8f0C_B9HA

 

Wonder if the same skimboard works for a skinny or fat individual? It does with a bannana peel when you slip..

P.