Skin Friction - Question for CW

“A moderately scratched hull has half again the skin friction of a new hull.”

I didn’t know. So, what can I do to my gel-coated wenonah from 1985? I have lots of scratches on other canoes, but they don’t matter, as I race the Wenonah once a year at a fun event (so serious friction reduction is necessary!)

How about wax? Car wax? Boat wax?

Thanks, as always.

Sand paper
Fill the gouges with gel, sand out through 600 or 1000 wet.

It’s a nice thought, wrong, but nice anyway. Yes with lots of scratches you have increased surface area, but that doesn’t translate directly into drag. Any boat carries with it a thin layer of water that sorta sticks to the hull, that layer fills most scratches so the drag doesn’t change much. The effective weight of the hull does though and that’ll slow you some, but it’s not nearly double.

Bill H.

Skin friction:
I understand and agree with the boundary layer concept, but what is the weight factor you mentioned? The weight of the boundary layer water? This could develope into an interesting discussion, as some think a roughed up surface can actually be faster than a slick one.

Roughed up

– Last Updated: May-26-09 11:56 AM EST –

In my long-ago sailboat racing days, "roughed up" wetted surfaces meant 400 or 600 wet sandpaper used parallel to the direction of water flow. The idea was to have a smooth surface that water would form a sheet on and not bead up like a waxed surface. I don't know what the current thinking is.

For maintainance, you're probably better off using a good marine polish on gelcoat and accepting the tiny drag penalty.

I don’t think anyone mentioned scratches as significantly increasing surface area. Hull length is the major contributor in surface area followed by deadwood and goofy shapes as per rec kayaks or some pack canoes as additional factors in that order.

Frictional resistance increases arithmetically with hull speed. Again, major factors are hull length, deadwood, cross section and skin condition.

Water flows over water molecules attached to the hull, the friction being between those molecules as they flow across each other in the boundary layer. As the boundary layer increases in thickness due to surface condition, speed and hull length it’s drag increases as more layers of water are drawn along with the hull.

As hull length is set on the day of purchase, and speed is a function of paddler skill and power, that leaves surface condition as the factor we can, kinda, control. Roughness in mils is a handy numerical measurement of skin surface.

The I’ll spar you the math, but a new gel coated hull has a surface roughness of 1 mil. At a hull speed of 3mph, frictional resistance increases with roughness larger than ~3mils. A moderately scratched hull has roughness of ~20 mils and a badly scratched one ~ 40 mils. [This is why our new canoe always seems faster than our old one.]

The best discussion of Frictional Resistance for paddlers, and from which the above numbers are taken, is in John Winters “the Shape of the Canoe” chap 4 pgs 18-22, and John’s earlier article in Canoesport Journal, midseason 88, pgs 20-22. Also see Gilmer’s “Modern Ship design”, pgs 106- 112, Benford’s “Naval Architecture for Non Naval Architects”, pg 90, and Van Zorn’s “Oceanography and Seamanship”, pgs 266-268.

The key to reducing resistance is to minimize skin friction by maintaining optimal hull surface condition. minimal surface roughness maintains as much laminar, low drag, flow as possible. When laminar flow breaks into turbulent flow, drag increases.

With a gel coated hull, this is best done with wet-dry sandpaper. With a wood hull; paint. ABS and PE hulls seem resistant to maintenance. It is worthy of note that bumps, outies, like rivets, cause more drag than innies, like gouges, which kinda puts paid to riveted aluminum hulls.

How would the effective weight
of the hull really change the performance. From what I’ve read, it takes quite a bit of weight to significantly change the performance of a hull. How much weight do you think this thin layer is adding?

Just curious.


Charlie, do you remember
a spray-on product from teh mid-to-late 70s that was designed to increase hull speed? My memory isn’t so good…


Thank-you Mr. Wilson. One more question then, when I sand the bottom will it look awful? If so, is there any sort of polishing compound or fiberglass treatment or varnish or anything that will restore the shiny red colour. I suspect shiny red goes faster than chalky red, you see.

Thanks again.

After refinishing a fuzzy Kevlar kayak,
and sanding it very smooth, I tried a spray product, marketed for motorboaters, that was claimed to reduce hull friction and increase speed. Probably still have some in the can somewhere.

The kayak seemed real fast, but there was no control conditon. So I have no opinion as to whether it helped anything.

two options

– Last Updated: May-26-09 1:27 PM EST –

Option A is to mask off say 4" by truing the hull on a roadway and scribing your 4" line by holding a magic marker on top of a piece of 2x4. Then just sand the bottom. One might turn the hull over, mask the sides and paint the bottom with a top grade off white enamel. Very cool, because the mil rating for paint is 2 and touch ups become easy.

If you start with whatever grit it takes to remove scratches and sand up through 320, 600, 1000, 1200, 1500 and 2000 grit wet/dry, then buff with 3M rubbing compound followed by 3M machine glaze you will arrive at the hull's original finish and a very sore elbow.

Winters mentions three hull treatment options, two of which, pliant skin and structuring are expensive and heavy, so of no interest to paddlers. The third, dispensing long chain polymers along the hull to reduce water viscosity is, again, heavy due to the quantity required and probably not very green. Submarines, with unlimited cost and power use the pliant skin option.

I agree with CEWilson’s explanation with a couple of caveats. It is true that the boundary layer grows in thickness as it progresses along the hull at a given speed. However, as a boat speeds up, the boundary layer gets thinner at all locations. Drag is increased due to the resulting higher velocity gradient, as well as increased turbulent energy production in the turbulent portion of the boundary layer. Any increase in surface area due to roughess is almost certainly negligible.

The three mil roughness height he gives at 3 mph (.003inch = 0.075mm) is correct and is very small - about the roughness of a good paint job. As you go faster, the allowable roughness height decreases - i.e. at higher speeds the boat must be smoother. This is because at higher speeds the boundary layer becomes thinner, and the same roughness elements that might have been negligible at low speed now protrude farther into the high speed flow, hence increasing drag.

He mentions that increased roughness size will trigger the transition from laminar to turbulent flow earlier (i.e. closer to the bow). This may be true, however, I don’t think this is an important effect, as most (>90%) of the flow will be turbulent anyway. Roughness increases drag within any boundary layer, whether laminar or turbulent (although by different mechanisms). It is my gut feeling that in real conditions virtually none of the flow is laminar, so early transition is a moot point.

“Only racing once a year”.
Wax it before the race. Always works for me.


IF you are constantly pushing the boat to its limits, want the fastest hull you can get, sand the hull with wet/dry 600 grit until its baby but smooth.

It might make a .01mph difference. Frankly for 99.9999999999999999999999% of canoeing…its a canoe, wear the scratches proudly. It means the boat is being used.

Very nice
Simple, elegant yet scientifically accurate response. The paddling community needs more like this.

Excellent down and dirty (albeit necessarily brief) treatise on skin friction. It’s surprising how much you’ve picked up in a mere 40 years; give or take a decade or so.


The weight of the water in the boundry layer is effectively added to to the weight of the boat. It resists the forces of paddling. So the ideal is the thinnest boundry layer possible. Figuring this out would be tricky at best :slight_smile:

Bill H.

spray on coating
It’s basically soap, was banned for sailboat racing, though how long it stayed in place was always a question.

Bill H.

Thank you
Thanks Dressmeister, you just made my day.

the REAL stuff. None of this “I can’t believe it’s not” crap.