Bulbous bow on kayaks?
Wednesday, November 7, 2012 (building), 13 comments
Occasionally I get questions about fitting a bulbous bow on kayaks. Many large ships have such a bow to decrease fuel consumtion at speed. Wouldn't it allow for greater speeds in a kayak?
Well, I wouldn't bother.
A bulb protruding under water from the bow, creates a wave ahead of the wave created by the ships bow. The two waves are out of phace and partially cancel out, reducing the ships wave resistance.
A ships cruising speed is normally far below the hull speed, which means that the bow wave is fairly short and the needed bulb can be short – three or four meter for a 100 meter hull. A kayak in speeds where a bulb might have meant a difference is moving close to hull speed, which means that the wave-lenght is close the kayaks loa. To produce a cancelling wave the bulb therefore need to protrude more than a meter ahead of the bow! Guess how that would affect the wetted surface and friction.
Furthermore the buld is effective only if it is positioned just below the surface. In rough seas it is more of a disadvantage than an advantage. A kayak moves most of the time in rough seas. Four meter waves for a 100 meter ship is like 0,2 meter waves for a five meter kayak.
For a bulb to be effective the ship must meet a few:
it must be more than 15 meter long,
it must operate in a narrow speed range,
and it must be loaded to the DWL at most times.
Outside these conditions the bulb increase wave resistance.
A bulbous bow kayak would be slow and sluggish (increased friction and wave resistance), hard to turn (super long waterline), expensive and heavy (more material and complicated construction) and and a monster to handle in water and on land...
Tempted, anyone?
(The image is Photoshop-manipulated, of course ;-)
