# Design/ speed question

Although exaggerated pushing and arrow through water is easier than a banana.

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The “design speed” of a displacement boat (does not plane) is said to be the square root of the length at the water line times 1.55 mph (1.34 knots), though in reality, it is slightly less for boats with a lot of taper at the ends. A boat can go faster than the design speed, but the effort required increases more than for additional speeds below “design speed”. This is based on the difference in distance between crests of the bow wave, the distance increases as the speed increases. After “design speed”, you are paddling up the side of the bow wave.

That is the best explanation I can do. If you need more you may be able to get it by asking a boat manufacturer. They usually have engineers who have studied this.

Edit: Due to differences in hull shape, my 16 1/2 foot sea kayak is slightly faster than my 17 foot sea kayak, Measured on flat water with no wind by a Garmin GPS.

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Best explanation I’ve seen yet.

So speed isn’t just determined by the length of the boat. Consider that I paddle 12’ long ww boats (vanguard and 12r). Both boats have a lot of bow rocker to facilitate turning. Because of the rocker, the boat doesn’t really get faster until you get it up to speed. Then I can feel and experience the difference between it and a shorter ww boat. In a shorter ww boat a pillow of water starts to build up in front of the bow. In the longer ww boats this pillow builds the same way for a bit but then instead of continuing to build the water dissipates a bit due to the increased lengthening of the boat as the water now meets the rockered portion of the boat.

All of this is trying explain that my boats have to be paddled above 3 mph to even begin use the length advantage over a shorter ww boat. My boats are only faster when I put more initial effort into paddling faster. I rarely care about speed but I do care about efficiency. They are more efficient than the shorter ww boats but only if actively paddled. When I’m lilly dipping and floating they seem just as slow as my shorter ww boats.

My xp10 (ten feet in length) doesn’t have the rocker and features a rounded bow (less tapered) and it plows pretty much like a shorter ww kayak. It takes a lot of effort to move it on flatwater. It has some other good qualities but efficiency in flatwater is not one of them.

I have a buddy Bob who is an engineer who built a very fast boat. Unfortunately, it was also a very unstable boat. He ended up buying a commercially sold waveski. As an engineer he doesn’t view it as a failure but rather a learning experience. My kind of guy!

Length is just one factor in determining how fast a boat is for the same effort.

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If the designs of both are the same, the shorter one will have less volume. That means that the same paddler in both will cause the shorter boat to sit lower in the water.

That will make more wetted surface to create friction.

I once raced a lady named Anita who had the same boat as I did. That boat is a racing boat, 19 feet long with a max beam of 18 inches.

Because I outweighed her by a little over thirty pounds, she had less wetted surface to deal with. That advantage almost counteracted that I was a stronger paddler than her.

Long, thin, hulls are faster than wide hulls. To get an 800+ foot battleship to 33kts, one has to have 210,000hp+ and the most optimized hull. The power to speed curve goes up exponentially, so one needs everything in their favor.Aug 25, 2023

I know very little about this subject. Read the post to learn. I used to sail a bit when I was a kid.

What I found interesting is no comment about doing anything to the hull. For example, polishing or waxing. Is that not something that is done to kayaks? Would that make any difference?

I’ve never seen anyone try to defend each as beneficial for speed and you’ll find many articles that flat out state that there is no benefit. The boundary layer of water has zero velocity on the skin of the boat and doesn’t care about the surface finish.

For smoothness the best analysis I’ve seen says there may be small benefits to keeping the surface as smooth as 320 sandpaper but smoother than that has no benefit. Plus the surface finish may only matter on the first 3 feet of the boat because in principle you’re trying to maintain laminar flow and avoid turbulent flow but in practice the bow of the boat is moving up and down (pitch) and back and forth (yaw) all the time so it may be almost impossible to maintain laminar flow even on just the bow.

You can find articles talking about “riblets” which are just shallow grooves running straight from front to back to try to keep the water flow straight. I think they are outlawed in the Olympics even though they only improve efficiency by something like 1%.

Hopefully others with more knowledge will chime in.

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America Cup boats sand the hull between each race. I forgot the grit could be 320 or 600.

Does sanding a boat hull make it faster.

For a kayak, moving at relatively slow speeds, a gel coat finish is plenty smooth enough. Sanding or polishing it further won’t help any. Certainly there is no need for wax or anything like that.

The faster you go through water, the thinner the boundary layer becomes on the hull. If you’re going slowly, your hull may be ‘hydrodynamically smooth’ even if it has perceptible roughness (bumps), since the boundary layer is relatively thick. At low speed, the bumps do not extend beyond the lower part of the boundary layer, so do not encounter higher speed flow which would generate additional drag.

As you speed up, the boundary layer becomes thinner, and bumps that were not a problem are now big enough to project into the middle part of the boundary layer, impeding flow and causing added drag. So your boat which was smooth at slow speed can become ‘hydrodynamically rough’ at higher speeds. So roughness is not an absolute measure, a surface may be smooth or rough depending on the local flow speed and corresponding boundary layer thickness. It also means that smoothness is more important near the bow, since the boundary layer is thinnest there, with thickness increasing down the length of the hull.

Since racing sailboats move fast, fine grit sandpaper is worth taking the effort to use. Wax may be beneficial - not for smoothness, but to keep crap from attaching to the hull and causing drag. Kayaks speed is an order of magnitude slower, so the smoothness is not critical. A composite or plastic hull with normal wear scratches is really nothing to worry about, smoothing it more will have marginal or zero benefit. Even deep scratches, unless there’s a whole lot of them, won’t add to the drag perceptibly.

Way back when, I did some experimental work with riblets - their benefit only kicks in if the flow is steady and almost perfectly aligned with the riblets, i.e., within a few degrees. In the real world, on a hull moving through water paddled by a person, this will happen approximately never. Re the competitive swimsuits with riblets that were banned - as I recall, it turned out the benefit was attributed mostly to compression of the swimmers leg muscles, and had nothing to do with the boundary layer flow.

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I hadn’t heard that. Very interesting.

I think I’ll stick to once every 20 years

Thanks for the education carldelo!

I have a couple of follow-up questions…

1. Can you comment about yaw losses on paddlecraft? I think I saw one piece of info that said yaw losses might be in the range of 11-14% (I think they were discussing sprint kayaks). Makes me wonder if I should switch sides more often or jshorten my strokes or just try harder to keep the boat pointed straight. I also wonder about yaw losses on a zero rocker boat vs a rockered boat. With a rockered boat there is so much less lateral resistance of the bow that one can drive the bow towards a point destination on every stroke and the bow just skids and obeys. I just wonder if there are some kind of sneaky yaw losses on zero rocker boats since they can suck up a lot of energy without the bow changing direction much. Not that I’m biased.

2. Can you comment on the biomechanics/power/ efficiency of a sitting versus kneeling position in a canoe? I feel like kneeling is significantly more powerful and maybe even more efficient over a long day. My experience is in solo sport (not race) canoes. Sean Burke felt that sitting was more powerful due to the leg push. I told Sean I have much more time kneeling so that may be part of my perception. I’m very curious if you have any insight on kneeling vs sitting.

Sorry to bug ya. I’m curious.

Yaw is problematic, since it means you’re not going straight forward, and are expending energy off-axis (unavoidable when paddling) and not necessarily in the direction of travel. Unsteady flow is notoriously difficult to analyze well, or even generalize about. You’re right that the hull shape will make a big difference, as will paddler position, cadence, left-right frequency, etc. I don’t think I can even make a reliable ballpark estimate. The 11-14% loss estimate (loss of what exactly?) may be reasonable, although I think it has too many significant figures…

Biomechanics is a mess, from the POV of mechanical engineering. Both humans and hulls are endlessly variable, as is paddling technique. I think an experienced paddler who has varied his/her technique and come to a preferred approach is likely a more reliable indicator of good practice (for him/herself) than any hand-waving analysis could provide. Or even a detailed computer simulation, for that matter, since those always require multiple simplifying assumptions that can’t come close to mimicking reality.

OK, those are my excuses, and I’m sticking to them. Additional feedback may require collateral beverage consumption in a social setting.

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There was a thread that went into great detail on that topic. Maybe the contributor will read your comment and direct you to the thread.

Postscript: I’ve gotten over thoughts of another boat until I get in better shape. Recovering from 2 surgeries, one minor, is not the time for another boat. I paddled 7 miles up a river a few days ago and I was wasted afterwards. Thankfully, I had a good backup driver.

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Speedy recovery prayers for you.

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I like some of the simplified rules of thumb in The Science of Paddling since they help ground my thinking and it’s just as helpful to know that some topics can’t be distilled to simple rules of thumb.

Cheers!

Tom

John Winters mentions a figure of 5% in his article about “Glide”

Yes, it is all about practicing good technique:
as a matter of fact I can go straighter with less yaw when paddling on my right side
than when paddling on my left side.

Sitting is more powerful for paddling forward.
If it wasn’t, Sprint kayakers would kneel too

Dirk

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