Aerodynamic question

[CORMERAN]

Yes Raggi_Thor,

a fine entry - followed by a rounded afterbody - is found in many
hull forms.
However - a balance needs to be found.
Too fine an entry with excessive volume aft could be a disaster.

Re: Canoe speed.
Maybe 5 knots - with an athletic paddler. This low speed is mostly
due to the puny human - powering the vessel.
A 100 ft. catamaran - is after all - basicaly two long canoes.
And they definatly go fast. With sufficient speed potential - that aerodynamics ARE a concern.
Given that: - at 37 knots, hydrodynamic lift starts to be taken over
by aerodynamic lift.
If you've ever operated a boat, well past this speed barrier, you will have
experienced this first hand.

[CORMERAN]

On cars and canoes,
to be clear. What I was referring to, was; that cars have different
aerodynamic concerns than boats.
However: well away from Surface Effect - high in the sky - or deep
in the sea, there are valid similarities.
I.E. Water tanks are used for flow tests - with similer data to what
can be seen in a wind tunnel.
Therefore: it is not suprising to see that a long tailed race car on the
- long Le Man's straight - will have a gentle taper trailing away from the
windsheild. Similer, in geometry to a canoe's hull. Aft of mid - ships.

However, as discussed; everything starts to get confused when we approach
the place where air and water meet.
So it's best, to be careful, using data useful in one medium and trying
to transpose it to another.

For instance - check out the exact density ratio - between water and air.
It's huge.
Anyone one know the exact proportions ?

[Raggi_Thor]

Water is approx 800 times the density of air, 1025kg/m3 for salt water (more in the Mediterranean, less in the Baltic), 1.2(?)kg/m3 for air depending on humidity and pressure of course.

If you are paddling straight against a strong wind, then MAYBE it's worth while to round off the stem above the waterline :-)

[tspeer]

Originally Posted by Shaunk

...Why is a canoe pointed at the front and the back if the ideal streamlined shape is rounded at the leading edge?...

Because the optimum area distribution for minimum wave drag at low speeds is spindle-shaped and symmetric fore-and-aft.

There are several sources of hydrodynamic drag. Two are due to the boundary layer at the surface. These include skin friction due to shear stress in the boundary layer, and form drag due to the apparent change in hull shape resulting from the flow displaced by the boundary layer. These drag sources are proportional to the wetted area and velocity squared.

Wave drag is due to the radiation of energy in the form of surface waves. Wave drag depends primarily on the cross sectional area distribution and the constructive or destructive interference of the waves at different speeds. Wave drag can increase or decrease with speed, but generally increases at the speeds you're talking about with a canoe.

See these papers by Leo Lazauskas for more information on the relative importance of different drag contributions and hull shapes:
UNCONSTRAINED SHIPS OF MINIMUM TOTAL DRAG
Low Drag Racing Kayaks