degree
the depth and number of gouges on plastic are greater than on gelcoat
Laminar vs. turbulent flow
While the statements about laminar-turbulent transition are accurate, it is still most likely a minor effect on kayaks for the following reasons:
First, a much more forceful effect on the transition process is the left-right yawing of the hull due to the asymmetry of one’s paddle strokes. Transition on a streamlined body is strongly influenced by the angle of incidence of the oncoming fluid. The few degrees (perhaps 3-5 degrees) that a hull wanders from a straight line course will cause transition and perhaps boundary layer separation well before scratches play a role. Scratches might be more important during the glide after a stroke, while coasting straight ahead, if the boundary layer relaminarizes. This probably won’t happen if there’s any cross current, however.
Second, the water that is being paddled through is generally in a turbulent state before the boat ever gets to it. Wave action usually indicates that there are turbulent eddies in the upper layer of the water, created by the transfer of energy from atmospheric boundary layer, either locally or transmitted via wave propagation from elsewhere. These eddies will create a turbulent boundary layer on the hull with no laminar zone. In fact, in a water tunnel, significant design effort has to be expended to achieve smooth laminar flow. In general, the normal state of fluids in the world is turbulent. My gut feeling is that there is a very small (or zero) region of laminar flow on any given kayak hull in real-world situations.
Scratches should influence transition to turbulence in dead-flat water, if you ever find any. But the real drag increase mechanism is from little plastic whiskers extending up into the turbulent boundary layer that is nearly always present on the hull. The depth of the scratch will only present a significant drag increase at high speeds (i.e. generally never) which is why scratches aren’t much of a problem on composite boats (gel coat scratches get no whiskers).
As mentioned by others, any deformation of the hull from it’s ideal state (oilcanning, etc.) will mess up the smooth flow over the hull, causing flow separation and high drag. Plastic hulls deform with age, so this is definitely a problem, probably more important than fuzzy scratches, even.
OK, I’m off the soapbox, and am looking forward to finding a copy of the book mentioned on the fluid mechanics of sailing.
if you look at a scratch on a composite
hull you will just see a scratch…jsut a negative space…
the same scratch on a plastic hull will have a tail of the removed plastic MOST LIKELY (unless you were crusing and ripped that tail off) hanging at the aft end of the scratch…
that tail is what is going to create more drag…
me too
and even more glad it’s not made of kevlar because it would have sunk like a stone…
…which is why…
I shave off any noticeable tailings. They’re not doing any good and they look crappier than just plain scratches.
scratches and speed
Well, I figure my plastic yak is more like a golf ball with all the dimples. Speed might be slower, but it sure does track well!
badjer