hull smoothness vs. glide efficiency

A recent canoe hull restoration I’ve done raises this question. The final hull finish is spar varnish that was brushed on. The spar varnish finish has a slight orange peel texture. How smooth a finish do I need to achieve to not introduce excessive friction? How does the surface finish affect glide?



Thanks.

Read this
http://www.epickayaks.com/news/news/to-wax-or-not-to-wax

There’s a related discussion on the
Paddlers Place Discussion Forum.



I seemed to have good results when I took the Kevlar fuzz off a kayak, covered it with a thin epoxy layer, sanded to a satin smoothness, and sprayed on a speedboat product that promised higher speed. I think the boat was faster, but probably it’s too small a difference for me to tell.



As long ago as 1962, MIT paddlers were wet sanding the varnish on their shells, but this oarsman doesn’t think we achieved anything except to psyche out a few competitors.



There’s agreement that a really scratched, hacked, fuzzy boat is a little slower. But will your orange peel slow your boat? I kind of doubt it. Almost certainly the difference, at typical canoe speeds, will be impossible for you to tell.



Eric Nyre, who shows up here occasionally, has done coasting tests on rough and smooth kayaks, and says his data shows a difference.

Very good article. But I’d still like
to see hard data for boats going in the speed range typical for all day canoeing, not that typical for rowing shells and flatwater racing kayaks. I suspect that the OP is not going to get enough out of smoothing orange peel to make it worth the effort, and I’m sceptical that there’s a speed difference for 600 grit versus 2000 grit sanding.

Little is really known
Every year at the American Physical Society Division of Fluid Dynamics meeting there are dozens of talks related to this topic. Many theories are proposed, but no definitive answer exists, to my knowledge.



In general, the lower the speed, the less it matters.



I think Greg’s article is right on. I also think he would say, “train more.”



To answer the last question, the farther the surface roughness protrudes into the still water, the more it slows you down. However, any boat, every boat, drags a layer of water along with it. The slower the boat, the thicker the layer. At three knots, at the trailing edge of your boat, my gut says this “boundary layer” is on the order of 30 cm thick. The millimeter or two of orange peel isn’t going to make a big difference.



FWIW,



D

Years ago windsurfers used to use
600 grit on their boards to capture just a very thin layer of water on the theory it would be a liquid lube to reduce friction and max their speed. Just for thought. R

Shark skin?
Don’t sharks have rough skin?

Water layer
As Dressmeister said, every surface drags a thin layer of water along with it, it doesn’t matter what the surface treatment is.



He is also correct that a surface irregularity can only cause significant drag if it protrudes above what is called the ‘viscous sublayer’ of a turbulent boundary layer. If the texture on the boat is contained within this layer, then the hull is ‘hydrodynamically smooth’, which means essentially that the water doesn’t notice the bumps - they’re too small.



The viscous sublayer is thinnest at the bow, and gets thicker (slowly) as you go to the stern. This means if you’re going to get crazy about polishing your boat, concentrate on the bow area where it will give the most benefit.



As a boat goes faster, the sublayer gets thinner all along the boat, so a boat that is meant to go fast should be smoother overall.



I made a very rough calculation and found that the roughness length is in the ballpark of 1/2 to 1 mm for the first couple of meters of a boat going 3 knots. This indicates that a fine grit is probably sufficient for most boats, and minor orange-peel probably doesn’t matter much. When I get a little more time, I’ll try to make a more accurate estimate.

link to John Winters Articles
Check these out:



http://www.greenval.com/shape_part1.html



http://www.greenval.com/shape_part2.html

Clean and smooth

– Last Updated: Apr-21-11 3:11 PM EST –

It's all a lot of hoooey for an extra 0.00001% of speed.

Just keep it smooth and clean and forget all the nonsense.
Greg Barton and Epic already played this song Ad Nauseam
It's a myth, an untouchable holy grail, a ghost for common kayak folk.

The sharkskin-like coating on Virginia Class subs
was for sonar-absorption, an anechoic coating
to absorb waves of active sonar so it does not bounce.
The submarine coatings also fail miserably, always coming off.

Yep
I went to a presentation by Nick Schade of Guillemot Kayaks last night on boat design, and he said the same thing - at the speeds canoes and kayaks normally travel at, the paddler is the biggest factor in determining speed. He said as long as the hull isn’t all marred up (Think plastic boat that’s been bashed off a bunch of rocks), any other difference in the surface is negligible when it comes to efficiency of a particular design.



So if the hull is just simply smooth, the motor is a far more important determinant of speed for everyday use.



Make it purty, and paddle it !!!


Engineer
I did stuff like this for 20 years and ten days.

Minor surface roughness and irregularities are going to be well into the negligible range. Very shortly after the leading portion of your boat, a turbulent boundary layer will form. Surface roughness is smaller than the boundary layer, so the effect of any roughness is puny. If you could maintain laminar flow (the flow, in the smallest of detail, following the shape of the hull, you would reduce your drag. You would spend, oh, say a half million dollars researching and developing a surface that might pull that off for part of the hull. Of course, the first time you set the hull on a beach, you would lose the effect…it’s really finicky stuff.

2000, 600, 150 grit…won’t matter much performance-wise.

In a very controlled experiment.
Bill Hosford and Stuart Cohen did a study of canoes at the Hydrodynamics Labrotory at the Univ. of Michigan. They sought to answer several questions, but on point , one was the effect of hull texture on speed. I’m oversimplifying and summarizing here so I hope they will forgive me. The hulls tested were a Mad River Explorer (16.5’), a new Grumman (17’) and a battered Grumman with over 2000 miles on it. All were loaded with 500 lbs. Sticking to the question at hand, there was negligible difference between the two Grummans and the conclusion was that texture was relatively unimportant as compared to hull shape.



Without going into a very long technical explanation, their study concluded that bow shape and length are most important in determining resistance of a loaded canoe and hull smoothness has almost no effect on efficiency.



Pag

Shark skin…
…was quite well covered here:



http://www.paddling.net/message/showThread.html?fid=advice&tid=1176222