# How does theoretical hull speed

Relate to reality? How does it relate to total load?
If a strong paddler has a boat with a THS of 5 mph , what speed can he or she expect to sustain on flat water with no wind or current?

In my experience, checked with a GPS when rowing either of two general-purpose hulls (and I find it far easier to maximize speed when rowing than when paddling), theoretical hull speed relates very well to reality. I can exceed hull speed by a few tenths of a mph only during the strongest power stroke I can make, and it falls back to hull speed or a tenth of a mph below hull speed before there’s even time for the next stroke to start, but averaged out, my max output puts me really really close to theoretical hull speed.

I’ve always heard that extremely fast hull designs are more capable of exceeding hull speed under just human power, and in that case the “hull speed limit” is “softer”, instead of being extremely abrupt as in my example.

If you are paddling a boat with a hull speed of 5.0 mph, my guess is that you won’t even come close to being able to “sustain” a speed of 5.0 mph unless you are a racer and the boat is designed for racing too (of course no boat designed for racing would have a hull speed that low, but I understand you just picked a number out of the air). It so happens that my 12’ rowboat has a hull speed of 5.3 mph, and I can make it go that fast, and if I remember right, even 5.5 mph, but the effort needed is ridiculous. In fact, to maintain 5.0 mph, like an hour or so, takes everything I’ve got. For me, paddling instead of rowing would lower those speeds substantially.

By the way, I haven’t tried to row at hull speed or even close to it for quite a few years. You don’t have to watch that GPS for very long to realize that trying to go anywhere near that fast is a useless waste of energy. Again, in a racing hull, it would be more do-able (but I bet still plenty hard).

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.

Go to marinerkayaks.com
and click on FAQ where hull speed is discussed along with other relevant factors which affect kayak speed.

@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.

Then the last thing you should look at is probably hull speed.

The relation between hull speed and paddling effort below hull speed is very weak, perhaps even negative.

In other words: If you buy a kayak with a higher top speed than your current kayak, it may actually be slower than your current kayak at your normal cruising effort.

I can’t recall the URL, but I have seen a table of drag force vs. speed for around 100 different sea kayak models. In this table, it was very obvious that the fast kayaks actually had a higher drag at low speeds.

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. While I do not understand all the nuances of friction, waves, wind, water SG, etc., I do know you do not see 40 foot long kayaks as they theoretically should be about 2 times faster than a 10 foot hull. I also know that my “engine” can rarely if ever get my hull to speed.

@Allan Olesen said:

@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.

Then the last thing you should look at is probably hull speed.

The relation between hull speed and paddling effort below hull speed is very weak, perhaps even negative.

In other words: If you buy a kayak with a higher top speed than your current kayak, it may actually be slower than your current kayak at your normal cruising effort.

I can’t recall the URL, but I have seen a table of drag force vs. speed for around 100 different sea kayak models. In this table, it was very obvious that the fast kayaks actually had a higher drag at low speeds.

I probably downloaded that on my computer. Looking for it on net with phone.

Theoretical hull speed can be confusing. It is based on the distance between crests of the bow wave increasing with higher speed. When this equals the length of your boat, you have reached ths. Assuming your boat has good displacement through out its length, when the wave is longer than the boat you are going ‘uphill’ past ths. My sea kayak is slower than ths, because the bow and stern are very narrow and sharply "V"ed to reduce side slip when entering or leaving a wave at an angle. The stern
two feet has less displacement than any other two feet of the boat, meaning it sinks into the wave pointing the bow up sooner than ths. Because of this my 14 1/2 foot boat is easier to paddle at speeds over 5 mph. I have quit over thinking speed, and now just try with a GPS and see what happens. A long narrow kayak has more ‘glide’ than a row boat, therefore loses a little less speed between strokes.

@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. While I do not understand all the nuances of friction, waves, wind, water SG, etc., I do know you do not see 40 foot long kayaks as they theoretically should be about 2 times faster than a 10 foot hull. I also know that my “engine” can rarely if ever get my hull to speed.

The manufacturer gave me the THS.

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 .

Greg

@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.

10.54 for my extreme now I need water line length but I guess I need to be in it to see what that is.

@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.