: Get the book "The Aleutian Kayak" by Wolfgang Brink. It has
: everything you need.

: Grant

With modern skin coverings such as canvas, nylon and polyester and their modern coatings a three piece keelson can only slow you down though if it is tightly lashed the amount of speed lost is not measurable.

The Aleuts had sea mammal skins and animal product lashings all which loosen to varying degrees when moist or wet. Just why the 3 piece keelson was used is not precisely known perhaps a result of scarcity of materials where the longer wood pieces were saved for the gunnels.