Skin Friction - Question for CW

we’re straying toward “discussion” here
I think this half of the thread needs to be moved to he discussion forum.

A few more thoughts
Re: the discussion of ‘effective weight’ or ‘added weight’, this is an alternate way of thinking about the boundary layer. If you are actually taking the boundary layer into account, with its velocity gradients and momentum transport away from the boat (at the expense of your paddling energy), then there is no need to invoke the concept of added weight. In particular, if you say that a thinner boundary layer implies less added weight, that would lead to the conclusion that going faster incurs less drag which we is clearly incorrect.



Angstrom mentions the past vogue for forming grooves in the streamwise direction by directional sanding, later by the application of riblets. The purpose of this was to modify the near-wall boundary layer structure for drag reduction. I believe it proved to be so sensitive to direction and speed range that the benefit was negligible compared to the effort involved. I know the Olympic swimmer outfits have grooves, but I doubt they function in this way.



Some of the coatings explored in the past were long-chain polymers that reduce drag by diffusing through the boundary layer, and actually modify the eddy structure in the middle layers. As Bill says, they were basically soap that had to be replenished. They do the job, but it’s a creepy idea - who wants their submarine or kayak leaving a snail-trail through the water? It looks like some of the current go-fast coatings are teflon or silicone based - but I don’t believe they would help any better than the very-fine-grit sanding jobs already mentioned - smooth is smooth.



The OP quoted (someone) claiming that scratches can raise drag by 50% which I doubt strongly. That sounds like an estimate generated from an assumption that the entire hull is deeply scratched which seems unreasonable. Offhand (subject to verification) I’d be surprised if scratches raised drag by more than 10-20%.



Much boundary layer talk centers on the laminar to turbulent transition. I’ve worked in wind tunnels and water channels, and I have to stress just how difficult it is to maintain laminar flow. In the field, I don’t think any of us are paddling through truly placid water, ever. Even a breeze or convective motion will set up currents in calm water that will tend to trigger the transition to turbulence early. Also, paddling creates a continuous surging motion, with an acceleration and deceleration with each stroke. This provides an unsteady forcing at the outer edge of the boundary layer, which is quite disruptive. Taking this into account, I think laminar flow is rarely present.




Got My Merlin II…!!!
I was quoting CEWilson himself. Sorry I didn’t cite that properly. It was from a thread “Got My Merlin II…!!!”, but I didn’t want to hijack that one.

Who was quoting
John Winters himself, “A moderately scratched hull has half again the drag of a new hull”, which is a 33% increase. John can generally come up with the research paper, but Carl Delo has a PhD in aeronautical engineering with study in fluid dynamics so he has special information too.



If we say drag increases somewhere between Johns 33% and Carl’s 20%, that’s close enough, because drag is only one component of canoe/kayak drag that increases arithmetically with speed. Wave making, or residual, resistance increases geometrically with speed, and so is over half the resistance slowing our hulls as we approach a two wave wash; theoretical hull speed.

Well
If Winters is the source, then I tend to think it’s correct - I’ve certainly been wrong about many, many things fluids-wise in the past. Something to check out experimentally someday…