Planing hull kayaks??

Yep, people with more mass need larger boards. Ok so a short board is ~8'x2' so it has ~ 1700 in^2 planing area. Given a 170 lb person+board and a 10 degree AoA, you need a speed of ~9.1 ft/sec (~5.4 knots) to support that weight. Double the weight, you need to increase the necessary speed to ~7.6 knots or double the area.

 

I simply agree. without some sort of foil to raise the boat, the "power" just is not there. What is interesting is how we do indeed get there. The vid by waikikin on post #7 is the closest I've ever seen under human power (and he's working very hard).
Surfing is one thing, planing is another:?:

 

OK, thanks Jman. Here is a video that shows flatland skimboarding with shallow and deep water. It does appear you get more support and glide further in shallow than deep water. But that may be because you can run faster alongside in shallow than in deep, Duh, I should have seen that sooner...

http://www.youtube.com/watch?v=agoInUtRP5s

Porta

 

Can you paddle this fast.

 

A sailplane's best Lift/Drag speed is typically around 90-110 km/hr where a long narrow wing is more efficient.

At this speed induced drag is about 50% of total drag. Induced drag decreases as aspect ratio (wingspan/chord) increases. High aspect ratios reduce draggy spanwise airflow that ends up as wing tip vortices.

Dino

 

Induced drag decreases with increased aspect aspect ratio if the span increases at the same time. This occurs if the planform area remains constant.

However, induced drag (not total drag) remains essentially the same as aspect ratio is increased if the span remains the same.

Discussed in a previous thread: The Myth of Aspect Ratio

The relevance to planing kayaks of wing aspect ratio is minimal.

 

Very informative link.

Dino

 

Planes & birds take great advantage of the compression of air molecules becoming trapped as a wing or any object moves horizontially, very close to a flat surface.

The same effect is taking place with those shallow depth Skim Boards. It takes a finite amount of time for the water molecules to move from the center of the board to a outer edge.
If the beach is sloped. There is a endless supply of new water being ""run over "" as the "" old "" water escapes out the back of the board.

Simple ??

 

Thanks, Cy:

Yes, this is what I have been wondering about in thin water. Do we have extra lubrication compared to deep water? Is there something of a lubricating surface effect going on due to the compression of fluid with the bottom being so close and not being able to get out of the way fast enough? Or is it all due to reduced loading per JM comments?

Porta

 

I don't know if this question makes any sense, but here goes. You might be able to design a planning hull that gets on plane at low speed/power, but would you be able to design a more efficient planning hull that was harder to get up on plane? That is, it took more power/speed to achieve planing, but once on a plane, drag/resistance would be less relative to a boat that got on plane easily.

 

The compression of water under a boat, kayak or skimming board is minute and totally negligable.

There is not a lubricating surface effect due to shallow water unless are comparing to the zero depth, no water case.

 

Throw the Skimmer Board in water 12" or deeper. It sinks. NO WAY can you jump on it and ride for 60' & more.

Compression of water ON TOP of the NON compressible sand causes the Ground / Sand effect . No mystery.

 

Your first statement may be true. However water at the pressures involved is essentially incompressible. The fluid mechanics of water around boats and similar, even extremely high speed boats, does not involve compression of the water.

I assume by compression you mean that the volume the water occupies becomes smaller and the density goes up.

 

Air bubbles are used to cut friction in the fastest torpedoes. So air bubbles should make it easier to get up and stay on plane?

P.

 

It Is not about the (in/)compressibility of water as it takes about 40 Megapascal to compress a water down by a volume of 1.8%. That is 4 kilometer down from sea level!

A stationary surfboard will sink at the same rate whether in shallow or deep water.

In Physics, it is about Bernoulli’s principle. Streamline fluid flowing through a tube will accelerate at the point of restriction which causes a reduction in pressure along the (pinch) point.

In Naval architecture, it is the shallow water effect or the effect of partially submerged moving body in a restrictive depth. Water flowing past the body must increase in speed more than in deep water, with a consequent reduction in pressure and increased sinkage, trim, and resistance and is further aggravated by lateral restriction such as near shore, river, or canal.

In Aerodynamics, air passing through an airfoil will speed up at the top curve, causing a reduction of pressure, providing lift. Reverse the airfoil and it sinks.

Illustration from "Physics for Scientist and Engineers" by Fisbane, Gasiorowicz & Thornton