Why a tandem canoe isn't twice as fast as a solo

This has always bugged me.  Solo I can paddle in the 6-7km/h range, tandem we're between 8-9km/h.

Why aren't we 12-14km/h?  We have twice as much power!

I figured it was due to the weight of two paddlers displacing more water, which was a good thought - but it's not the whole story.  In fact it only accounts for a small part of the story.

Then I found this article on the Science of Paddling.

The real answer is, in my opinion, far more disheartening.  In fact it's a bit of a drag.

From some kayaking site but basic principle is the same... 

Drag And the Law of Diminishing Returns

The main force on a canoe is water - you are punching a hole and sliding through it.  This creates drag.

Unfortunately the faster you go, the more the drag increases... but it's non-linear!  In other words, you need a bigger increase in force to go from 8km/h to 9km/h than you do to go from 2km/h to 3km/h.

Now if we were a motorboat we would eventually get to a sufficient speed to lift most of the boat out of the water and reduce the drag to near zero...

Weeeeeeee!!!

  ... but sadly that isn't going to happen at 9km/h in our canoe.  We are stuck forcing our way through the water, and every tenth of a kilometer per hour of speed is harder to obtain than the previous tenth.

Weight

The other factor I alluded to is weight.  More weight means more of the boat under the waterline, and more drag.  Two paddlers weigh more than one, thus more boat is under the waterline causing drag.

The article I referenced above does some serious math and comes up with a 5% gain in weight resulting in a 1% speed difference, 10% gain results in 2%.

So it's significant... to a point.  If you're in a C2/tandem and you have a combined weight of, say, 350lbs, you'd have to gain a combined 15lbs or so to cost yourself 1%...

What This All Means

The most important take-away is that because the power/speed relationship is non-linear, a steady pace is the most efficient strategy (all things being equal).

And weight is important - if you can drop it, either from your butt or your gear, then do it!