Go fast kayaks, surf skis and sprint kayaks regularly are paddled above hull speed but that doesn’t mean that it’s easy to do so. You will only go faster if you have the engine to sustain the power output required. A shorter boat with a so-called lower hull speed might actually be faster for you, if it more closely matches your “engine” and how hard you can or want to paddle.
John Winter has some good information on hull speed. One article is at https://www.qcckayaks.com/SpeakingGoodBoat2.aspx .
@harry0244 said:
A long narrow kayak has more ‘glide’ than a row boat, therefore loses a little less speed between strokes.
This is exactly the principle I described in my above post, but I would modify what you said here and replace “row boat” with “a wider boat (of any kind)”. To illustrate, no typical kayak is as sleek or has as much ability to exceed hull speed as a rowing shell, or even a basic fitness hull that’s made for “casual fitness” rowing. That’s why in my post above I referred to my rowboats as “general purpose”. They may not be skinny but are far from pudgy and can carry a load of gear. In fact, waterline width with only a lightweight person such as myself on board would match that of some touring kayaks. It’s best to refer only to the shape of the hull, not to the method of propulsion when making these comparisons.
@Andy said:
In your instance, how is THS calculated? I have frequently see the calculation employed where Hull Speed = 1.34 times the square root of waterline length in feet.
People usually fail to indicate the units when showing how to calculate hull speed, and too many don’t even know that it matters. The formula you provide gives hull speed in knots, but most of us are more familiar with MPH, which also is the unit of measure used by the O.P. To determine hull speed in MPH, replace “1.34” in your formula with “1.54”.
Hull speed is not a useful concept for kayaks. Look at any of the drag vs speed curves, (In old Sea Kayaker magazines, for example.) Drag increases smoothly with the square or cube of speed. It would be impossible to pick out hull speed by looking at the drag-speed curves.
I think you’d be better off using the rating system used by kayak races to classify a boat’s speed capabilities. Basically an aspect ration of WATER LINE length divided by the beam. The higher the number the faster the boat. Note they use design water line length. String wants to know about a loaded boat that would likely be a different water line length. See the Sound Rowers Classification system. soundrowers.org/boat-classes/
The thing not rated and not readily quantified is the kayak engine strength, range and efficiency. “A strong paddler”… what ever that is. A strong paddler today might be a weak one tomorrow after thoroughly discussing these concepts around a camp fire partaking of adult beverages. Or in this case a fire place since most of y’all are up in cold country.
@string said:
I’m looking at a boat that has THS of just below 5. I typically paddle at 3-3.5 so it sounds like it would be OK.
THANKS.
Seems like you need to test paddle the boat! I think my 15 foot solo canoes have a THS around 6 mph and the faster one (Merlin II) cruises a bit over 4 with strong input (Swift Osprey a bit under 4) so it seems like you are climbing the resistance curve pretty seriously at around 2/3 of THS. Your example of a THS a bit under 5 represents a modest kayak right? Not some skinny sea kayak or surf ski bullet. At 3.5 it might be pushing back on you.
I fully agree that the shape factor (aspect ratio) is important…my Merlin II 15 footer is way faster than my 16’2" Swift Shearwater, at least with my engine.
If you feel like fooling around with changing designs & seeing what happens with drag & stability take a look at Ross Leidy’s Kayak foundry software: http://www.blueheronkayaks.com/kayak/index.html
Tom is suggesting that you might be encountering significant resistance at 3.5 mph, but I tend to think not. My way of thinking is to first see how long the boat actually is, and then apply experience. Applying the formula, we can see that the waterline length is just about 11.4 feet. That’s close enough to 12 feet that I’ll go with that. My 12-foot rowboat (an unusual boat in that the waterline length is virtually the same as the total length) cruises quite effortlessly at 3.5 mph. Not until it goes well over 4 mph does wave-making resistance start becoming all that noticeable. Aspect ratio may be a factor, but wave-making resistance is the main limiting factor with boats that are this short
I can’t imagine a problem here but we are sort of going about this backwards. Knowing the boat model and it’s specs would allow more people to think in terms of similar boats which they have actually paddled instead of making comparisons in the way that I just did here. Still, I can’t imagine a kayak that’s almost 12 feet long and surely better than a pool toy giving much trouble at a speed of just 3.5 mph.
@string said:
I’m looking at a boat that has THS of just below 5. I typically paddle at 3-3.5 so it sounds like it would be OK.
THANKS.
Seems like you need to test paddle the boat! I think my 15 foot solo canoes have a THS around 6 mph and the faster one (Merlin II) cruises a bit over 4 with strong input (Swift Osprey a bit under 4) so it seems like you are climbing the resistance curve pretty seriously at around 2/3 of THS. Your example of a THS a bit under 5 represents a modest kayak right? Not some skinny sea kayak or surf ski bullet. At 3.5 it might be pushing back on you.
I fully agree that the shape factor (aspect ratio) is important…my Merlin II 15 footer is way faster than my 16’2" Swift Shearwater, at least with my engine.
Yes, it is a light weight SOT, an Eddyline Caribbean.
Hey string, as you say I think you’ll be fine. I bet your normal stroke puts you somewhere between 3 and 3.5 even in the 12 foot Carribean. My point is that with a shorter/wider boat, by 3.5 you are getting into the zone where the shape of the boat affects the shape of the speed/resistance curve and in turn the “feel” of the boat. For me I like the feeling that the boat will still accelerate easily from cruising speed…something all 5 of my solos are good at. Very cool software link from rival51.
A lot of boats these days are so poorly streamlined that hull speed makes no difference - you will never get them going that fast anyway.
On my 14’ boat, hull speed 5mph, I can see when I reach hull speed, no speedometer needed. What happens is the bow rises and falls with every stroke. He is why.
Hulls create bow waves and resonance creates another wave behind it (which creates another behind it…) like so:
The next resonant wave appears to be coming from the hull somewhere behind the bow. As you speed up, the bow wave gets higher and the crest of that next wave moves back along the hull. When it reaches the stern you are at hull speed. At that speed your hull is sitting centered in the trough between the two waves and is level.
As you increase speed, the back wave moves behind the hull and you are no longer centered. In fact, as you continue to speed up more and more of your hull is on the backside of your bow wave and you are basically paddling uphill. More speed pushes more water so the wave that you are climbing gets bigger and bigger. This is why on a water skiing boat the bow rises up so high that you can’t even see where you are going until it find hops on plane.
So if your power stroke pushes the wave up and your bow rises with it, and between strokes it slides back down, that’s hull speed slowing you down.
The question is how do you know (calculate) the “hull speed” of your boat?
Because the S/L ratio of kayaks (in knots) can vary from 1.34 up to 2.2 for Sprint Kayaks and the like.
Based on 1.34 and waterline length. https://en.wikipedia.org/wiki/Hull_speed AFAIK the only way to change the Froude number from 1.34 is to change the force of gravity.
As I understand it, the hull speed depends on the S/L ratio,
which varies with the hull design (and weight).
This explains why Sprint Kayaks can go so much
faster than their “hull speed” calculated with S/L 1.34
Hull shape affects the height of the wave but not the wavelength and therefore not the hull speed, but with a good hull the hill that you are paddling up is not as steep so it takes less effort.
Exactly, Unless you have the hull shape and power to climb up and over that bow wave and start planing, trying to go any faster than hull speed will simply dig a deeper hole in the water that slows you down from excess drag.
In my experience and limited knowledge hull shape does affect the “hull speed”.
With my 14’5" kayak the hull speed with a S/L ratio of 1.34 would be 5 knots and with a S/L ratio of 1.51 would be 5.7 knots.
The 5.7 knots “hull speed” sounds more realistic to me in case of that kayak, as I can feel a kind of limit there and not at 5 knots (5.8 mph).
