Message-Id: <n1376149319.80301@566_QM>
Date: 28 Jun 1996 14:45:09 U
From: "Chris Young" <nosc.mil!young@imagelan.com>
Subject: Re: dimples, hull drag, etc
To: baidarka@imagelan.com
RE>>dimples, hull drag, etc. 6/28/96
WARNING . . . boring discussion of drag reduction ahead.
_____________________________________________________
The drag reduction due to the dimples on a golf ball is a result of energy
being injected into the turbulent boundary layer, moving the flow separation
point aft. This reduces the size of the wake and the drag. This phenomena
is highly dependent upon the Reynolds number (the ratio of inertial forces to
viscous forces) of the system and works only within a very narrow range.
The Reynolds number of a kayak at 4 mph is approximately 2 orders of
magnitude higher than that of a golf ball at 80 mph. It is unlikely that
roughess would have a similar effect.
Drag reducing film was applied to the hull of one of the Americas Cup
defenders some time back. This film contained 'micro-rivulets" which is a
fancy way of saying roughness. The roughness was very slight and the size
was precisely calculated for the anticipated flow conditions speed, location
on the hull, boundary layer thickness . . . ). The film had to be oriented
in specific directions with respect to the flow field. The drag reduction
results in the ocean were inconclusive but were very small if present.
In conclusion my advice is don't bother trying to produce roughness on the
bottom of your Baidarka.