@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).
