Effect of scratches on canoe efficiency

[gumpus]

Many seem curious about this, including me.

Please go to www.scienceofpaddling.net and click on Part 3... that's the best practical summary that I have been able to find.

Key takeaways seems to be that the boundary layer is a fussy thing and that it is worth keeping the front of the boat as smooth as possible and that the BL will trip into turbulence no matter what you do and then there is still a laminar sub layer that would prefer that you keep the whole boat at least as smooth as 120 grit sandpaper.

And one might be able to feel the effect of scratches, especially at very low speeds...like 1 to 2 mph.

And if your boat can secrete oil like a fish it will go faster.

Comments are welcome!

 

[gumpus]

Sorry if the link does not work.

I also learned a lot about the impact of barnacles on fuel efficiency of oceangoing vessels if anyone is interested.

 

[gumpus]

https://thescienceofpaddling.net

 

[Guest]

So, if I don't care about looks, I can sand my scratches with 120 grit and stop? Going further is just for looks? Inquiring turtles want to know.

 

[gumpus]

So, if I don't care about looks, I can sand my scratches with 120 grit and stop? Going further is just for looks? Inquiring turtles want to know.

Nope...and for sure I don't have all the answers. You can find lots of info online saying racers go to at least 400 or 600 grit and then debate whether it is worth going to 1500. The article above says one key factor is trying to keep the laminar boundary layer at the bow attached as far back on the boat as possible, and that on race boats at 6-7 knots it may detach and go turbulent as early as something like 18 inches back from the bow stem...but for lower speeds maybe it stays attached longer...and then if your boat ever yaws that could matter, and and and. So you need to make your boats shiny from bow to stern to be safe.

 

[Odyssey]

So from now on whenever someone grimaces at my rock rash I can tell them "Those scratches make me go faster."

 

[BWCA66]

A scratched canoe is a happy canoe.

 

[yellowcanoe]

Let's deal with paddler efficiency. Shawn trains aerobically and anaerobicaly for canoe racing each day when he is not cooking or smoking meat etc.
Do we?

 

[deerfly]

well done gumpus.

I think YC is touching on my main take on this. There's no doubt the scratches are a factor that increases drag. Due to their randomness and variability along the whetted surface though I'm skeptical they can be measured and quantified in the same way surfaces are uniformly roughed up on purpose to effect laminar and Bernoulli effects. Regardless, the biggest performance problem with canoes is the people that paddle them, the paddles they use and how they use them, especially with course correction strokes that work against the forces in yaw on nearly every stroke of the paddle. I think those things account for the majority of calories burned. Skin friction is in there for sure, but its not whats killing you by the end of the day.

 

[frozentripper]

Thanks for providing that report, Gumpus... the author seems to be contradicting John Winter's statements on frictional drag dominating to as much as 80% of total resistance. I might have to read this thread's report again but my sense is that racing speeds are being what's covered here and not the slower cruising speeds that recreational paddlers operate in.

Also from your conversation with JW there is the statement that the resistance situation is complex with multiple variables in hull properties acting on total resistance. In comparing racing vs recreational hulls this could include hull shape affecting wave-making (residual) resistance where at near top hull speed, wave-making resistance dominates in total resistance. For racing hulls, paddling above maybe 5 mph could be where the hull's wave-making properties make going faster much more difficult, while is a less well-designed, inefficient hull such as an aluminum Grumman, this could happen at 2.5 mph. So in an efficient, sleek hull paddled more slowly at cruising speed, it's possible that frictional resistance dominates and residual, wave-making resistance is less.

So at these lower speeds where wave-making resistance isn't limiting speed, hull scratching contributes to frictional resistance and could be contributing quite a bit to total resistance as the extent of scratching increases. My unenginerd take anyway... tow tests would be the thing to verify and validate the effects. IIRC the paddling magazine quoted tested a beaten-up Grumman in tow tests which probably had a pretty inefficient hull to begin with where wave-making resistance was high from the outset and that could be masking the effects of frictional resistance.

Another thing about frictional resistance dominating resistance and perception thereof... at slow speeds where frictional resistance dominates, there isn't much energy or effort being put out by paddlers so the increase in resistance with more scratching wouldn't be noticed very much. At slow speed - it's easy paddling with a smooth hull and it's easy paddling with a scratched hull, so not much difference in paddlers' perception since there's not much difference in effort. Anyway, good discussion, keep it going...

 

[sweeper]

So are we supposed to carry sandpaper with while tripping?

My trips are planned to have relaxing days. Scratches happen!

The only race I'm concerned about winning is the one with the Thunder Storm.

 

[deerfly]

Maybe I'm too math challenged to even have an opinion here, but I think all we can say with high confidence is that no scratches are better than too many scratches or that just the right amount of very, very small scratches purposely and uniformly applied below the waterline can help reduce skin friction. Otherwise, its more prudent to prepare oneself to encounter far greater forces acting on your canoe than skin friction with much of it being self inflicted as a result of paddling skill or a decision to paddle in conditions beyond your abilities. Beyond that it seems to me calculating effect drag with canoes is more of an academic exercise where terms like "all things being equal" or one of my favorites from my scientist buddies "sensitive dependence on initial conditions" are introduced to smooth over inconvenient factors introduced by real world application. My strategy is to try and work it out so there are a few beers left at the end of long day to ease the tired muscles from the rigors of paddling, real or imagined.

 

[gumpus]

My intention was to try to develop a better "feel" for the relative importance of scratches and I did a bunch of Internet surfing and traded some emails with John Winters because my own experience (with about 20 solo boats) did not seem consistent with some of the extreme statements I have seen including John's statement on the greenval site that a year's worth of scratches might offset all other design factors. I've also seen extreme statements on paddling forums like "the difference between two different boat designs may be more than offset by scratches" or "the glide will soon be gone after some scratches". I'm pretty sure that my scratched up Merlin II will still run away from my almost pristine Osprey.

I think I agree with everyone's comments. The overall impact of scratches seems to be small...just a few percent at race speeds and at low speeds (like 1 mph) where friction dominates it may still be imperceptible or barely perceptible since it may take just a couple pounds of total force to move the boat at 1 mph versus over 20 pounds at 3.5 mph or so...so the effect is most likely lost in the mud. And frozentripper I'm not sure tank testing would give the answer (just more clues) since in the real world there is yaw and speed variation from stroke to stroke and turbulence from the paddle messing up the flow over the hull and who knows what else is going on.

John Winters was not in any hurry to defend his statements as a real theoretical analysis and he did mention that he was trying to sensitize people (like Swift rental fleet customers) to not abuse their boats.

Even the 25% effect that frozentripper mentioned from another John Winters statement may be high, but it seems more logical for two boats starting from a dead stop when shape effects are least important. I've also seen statements that Swift boats with aluminum gunnels have less rocker than those with wood gunnels, and it has always felt to me like boats with more rocker feel more free on the water...and my current Shearwater with aluminum gunnels seems a bit less free and playful than the one I had with wood gunnels...but that could be my imagination.

I also think it's interesting that the current state of knowledge is such that the effect of scratches still isn't understood very well. There was one post from a guy that attended conferences on applied fluid dynamics that said "the jury is still out".

So I don't think anyone needs to carry sandpaper while tripping. I've never polished up any of my boats. But I'd like to see Turtle polish the front half of his boat to perfection and the rear only to 120 grit smoothness so when people ask him why he does that he can tell them that the smoothness in the rear does not to be any better than 0.002 inches to make sure it does not disturb the laminar sublayer in the turbulent region.

There's also a cool article on the epickayak site that says that it's better to work on the paddler than the scratches.

 

[gumpus]

Interesting article for those interested, again focused on race speeds (for kayaks).

http://www.epickayaks.com/article/article/to-wax-or-not-to-wax

Also a pic of my Merlin II after 18 years of faithful service. I'm still not sure if it's worth polishing it up.

 

[Glenn MacGrady]

This seems to be the summary conclusion of the OP article:

"So to ensure that your hull is “hydrodynamically smooth,” make sure it is no rougher than about 1 to 1.6 thousandths of an inch average roughness height, or about one fifth of the sublayer thickness. In more familiar terns, 1.4 thousandths of an inch is about the average grain size of 320 grit sandpaper. If your hull is as smooth as that, as far as the water is concerned it is perfectly smooth. Roughness greater than that will introduce more turbulence into the boundary layer, and a (small) increase in overall drag.

"So where should you focus your energy and resources to reduce friction drag, if anywhere? Certainly a scratched and rough hull will foul more easily, and deep scratches should be repaired if for nothing else than to prevent moisture penetration into the underlying fibers of a Kevlar or carbon lay-up. Those are reasons enough to keep a hull in good repair. And if you really want to do some polishing, and the polish fills or grinds down in any superficial scratches, then there will be a tiny decrease in frictional drag. This will most important in the leading couple of feet of the hull, since small surface imperfections there can help destabilize a laminar boundary layer and lead to an even earlier transition to turbulence. So by all means polish the bow and the front meter or so of the hull, and keep the rest smoother than around 0.002”."

Hull roughness can be caused by many things other than scratches -- such a dents, pores, pittings, lumps, bumps, wood imperfections, fabric wrinkles, unfilled cavities, nail heads, nail holes, uneven or blotchy paint and gel coat layers, and squashed-on bugs.

In order to paddle the most efficient canoes, other things being equal, we must devise a list of canoe hull materials in increasing order of "intrinsic turbulence potential", and only buy canoe hulls made from materials at or very near the top of the ITP list. I suggest the very top of the ITP list would be a hull encased a layer of flawless industrial diamond, polished by the University of Arizona's Steward Observatory Mirror Lab. That's definitely the hull I'll buy if I get a new one this year.

This site and it's predecessors always make me spend surreptitious money on apex equipment like diamond hulls. Why, just this morning, I bought another knife (Benchmade Bugout 535) and saw (Silky Ultra Accel 446-24) after hours of turbulent internet streaming.

 

[frozentripper]

Well, wax-or-not-to-wax advises that keeping the hull as smooth as possible will help win races, forget the waxing. When I was rowing those long, narrow racing shells, the coach made us wax the hull before a race, but even then I suspected that it was for looks mostly. There were girls watching, too.

This book states that at cruising speeds, it's frictional resistance that creates most of the drag (see skin friction)... using the formula given for calculating the hull speed which is the theoretical max speed for human powered canoes, ie. when the canoe start to climb the bow wave it's creating and wave-making resistance increases dramatically... hull speed for a 16-foot canoe works out to be about 6 mph or over 9 kmph. Holy speeding snails Batman, keeping that up for any length of time ain't gonna happen, even if trying to get to the best campsite before those party-hearty yahoos carrying a load of screaming kids.

So maybe those saying that frictional resistance normally forms 80% of total paddling resistance might be on to something.


https://books.google.ca/books?id=M3d...2tA4kQ6AEILDAB

 

[lowangle al]

I'm a believer in waxing although I haven't studied it much. I waxed one of my wc boats down by the water and when I flipped back right side up in it's regular resting spot it would have slid into the water if I didn't grab it. The slope angle was very low, maybe 4-5% and the ground was mossy grass with some fall leaves on it. It impressed the heck out of me and the paddle that followed did seem to have a nice glide.

 

[dougd]

Well after reading through this thread I've decided that all of my boats should be slower then hell, maybe going back upstream. My OT Disco, the Hogged Backed Saint, has more gouges, scratches and dents then one can count. She still be slow but not slower with all those battle scars. I guess in my case I just don't notice as my canoes are meant to be used and I go a lot of places so where the bottom meets the water and rocks or whatever is fair game.

 

[Guest]

I sand and then buff a little over the winter each year. I don't get carried away and it makes way for new scratches.

 

[yellowcanoe]

I sand. With 20 grit. I epoxied some chipped gel coat and not watching what I was doing epoxied about a tablespoon smear of sand into the chips too. At my pace it's not been a big deal and I have a ready sander