What the best design to catch the current?

[alexlebrit]

The title says it all really, but to flesh it out a bit.

I'm building a small kayak-like boat for gentle cruising along the Nantes - Brest canalised river system. The current is hardly rapid, but there is a current, and for 2/3rd of the way I'll have it behind me. So I thought I may as well try and use it.

Originally I'd imagined a simple kayak, equally pointy at bow and stern, but I'm wondering, would a truncated version be better? or is there an even better way to make use of free power?

[alan white]

Drifting, you won't have steerage way, so you'll be cross-ferrying to stay pointed and to put the boat where you want it. Any boat will ferry at the relative speed you'll be going, so better to design or buy a boat for the sections where you'll be paddling, and don't worry about the downstream legs.
Shape matters not a whit going downstream.
Choose the boat for efficiency at your personal level of sustained paddling where you have to paddle, which if average would be about a fifteen-sixteen foot length. Longer will be less efficient at average cruising speeds (especially loaded). and depending on how loaded, an even shorter boat would be better.
Pointy ends are low prismatic shapes, meaning the envelope of the hull has more surface area than fuller ends would. The question of what coefficient of prismatic (CP) you want is dependant on your happy cruising speed--- the higher the prismatic (closest to a rounded section over the full length of the boat) the more efficient at low speeds. You need low surface/volume ratio for efficiency when loaded because your speed will be limited by all your gear.
So choose the shape based on sustained paddling, and forget about the downriver considerations.

A.