# Modification for a Dead-Weight (Grandson)

A project log for A Plywood Canoe

I built this canoe 10 years ago. It was fun but various design flaws has limited it use. This project is about a salvaging the canoe.

Took the grandson for a test paddle. The best place to put him was behind the main seat (circled):

The problem is that the cross-beam is in the way. Although he can wiggle in, it is not safe if the out-rigger capsizes as it will be hard for him to get out. I can move the seat back by cutting it out and gluing in a new one, or I can cut off the cross-beam, to make it safe (i.e. he will fall out if it capsizes)?

The Australian Standards (AS 4132.1 4.8(b)) says the cross-beam structure needs a moment capacity of M = 3*Disp*Bh where:

• M = required moment capacity (kNm)
• Disp = weight of boat with payload (t)
• Bh = distance between hull centre lines (m).
• The displacement is 165 kg (= 35 kg + 90 kg + 40 kg ) or 0.165 tonnes.
• The distance between centre lines of the hulls is 1.25 m.

The required moment capacity is therefore 0.619 kNm. This is somewhat less than I expected!

The working stress for Tasmanian Oak is 35 MPa so the required section modulus for the uncut beam is 18e-6 m^3. The designed section modulus is 58e-6 m^3. So easy, cut out the rear cross-beam to make room.

AlanX