Size Matters, but How Much?

Recently, one of my paddling buddies shared a discussion thread with me, which presented the idea that many people paddle boats that are too long for them. The idea being that if you can’t attain and sustain max hull speed in the boat, then the boat is too long for you.

I think it is a fair criticism, as often in sea kayaking, the push as you skill up and subsequently start upgrading boats(at least personally,) seems to be for a longer/narrower boat.

This is an interesting concept and led me to do some research on hull speed and subsequently a different formula for calculating hull efficiency called the Froude number.

What is Max Hull Speed?

As a boat moves through the water, it creates a wave formed by the bow of the boat, which helps to form the wake(wave formed downstream of the boat.)

Max Hull Speed is basically the speed at which the wave created by the bow interacts constructively with the hull of the boat as it moves through the water. When you exceed max hull speed, the boat becomes less efficient. As this relates to kayaking, as you exceed hull speed, it takes more energy to paddle and maintain a consistent speed.

It is important to note that max hull speed is not a limit on the speed of the boat and it can easily be exceeded, but as the speed is exceeded, it takes more effort to maintain the speed.

Max Hull Speed can be calculated by taking the square root of the water line* of the boat and multiplying it by 1.34. While the hull design is important, to a certain extent, the geometry/design isn’t as big of a factor and efficiency basically falls in line with this equation(see note[1] below table for thoughts on human powered boats.)

*Note that this is the waterline of the boat and will not always be the total length. For instance, my Pilgrim has a lot of rocker, so even though the boat is around 15′ 9″, the water line is probably closer to 14-15 feet.

The below is a quick chart that shows hull speed for a few of the common water lines found in sea kayaks[1]:

Length of Waterline Max Hull Speed in Knots
12′ 4.64
14′ 5.01
16′ 5.36
18′ 5.69
20′ 5.99

[1] I don’t think you could use the above chart to compare flat hulled boats, such as a recreational, sit-on-top kayak, or most white-water boats, as when being powered by a human, the inefficiencies in the hull would be come much more apparent and difficult to work with when compared to boats powered by an engine. So, while the max hull speed should be more or less correct, the amount of human energy required to achieve that would not be the same when compared to a sea kayak.

How long is too long?

As you can see from the above chart, if you aren’t going to be paddling more than 5 knots, an argument can definitely be made that at least in terms of efficiency, there isn’t a huge advantage to having a boat with a water line longer than 14′. And even as you move up in 2′ increments, the difference in efficiency isn’t that significant between boats.

From a personal level, when paddling on the lake with little to no help from current, it isn’t uncommon for me to hit 5MPH+ and when doing a fitness paddle where I am paddling at about 80% of my maximum, I can maintain 5MPH+ for several miles at a time without too much difficulty. So, having that longer boat makes sense for me(at least when doing my fitness paddles.)

However, I think the overall point is a good one, boat length is important, but once you get over 14′ in a sea kayak, the overall water line length is probably less important than choosing the right boat for the job.

So, aside from racing or other times when you need high-performance is needed, the stability(secondary and primary,) tracking, the ability of the boat to handle conditions(wind/surf/swell,) and of course the paddlers comfort in(and paddling) the boat should likely be considered first, rather than always just going for the longest boat you can get. When considering conditions, the amount of help you are going to be getting(or working against) is also likely an important factor, so if you are going downriver with a fast current or getting a nice push from the tide, you are going to get to this speed and likely exceed it much easier than on flat water.

Another consideration is that when paddling a kayak, you are using a human engine(ideally mostly your core and not shoulders/arms) to power the boat, so even just a marginal improvement in efficiency can make paddling much easier…it isn’t like you can just get a bigger engine from the store to overcome inefficiencies. So definitely for anyone doing non-casual distance paddling, the length of the boat is still a very important consideration.

Froude Number vs Hull Speed

While researching hull speed, I found that in modern navel engineering, the Froude number is more likely to be used than hull speed when determining how efficient a boat is.

In terms of boating, the Froude number is a formula that lets you calculate the efficiency of a given hull at a speed, so you can get an idea of the amount of resistance to expect when moving at that speed. This helps you find the sweet spot in terms of speed for a given hull and also get an idea of the resistance you could expect at larger speeds, as opposed to max hull speed, which just gives you the speed at which the boat will be come more(or less) efficient.

While the formula is more complicated, I found an online calculator and plugged in some numbers. Since we are dealing with relatively low speeds and lengths, as compared to engine powered boats, the numbers pretty much corresponded with the above chart and you could draw similar conclusions about speed vs kayak length. However, I think it more clearly illustrated the inefficiency of a 14′ boat at speeds over 5 knots and how when going for distance/speed a longer boat(16’+) is definitely more efficient.

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