Recently, Silbs asks,
Meanwhile, I have been driving myself nuts while mulling over an issue around boat size and speed. For starters, we all agree that a longer boat (all else being equal) can go faster than a shorter one. So far so good. In addition, we know that that longer boat will need more horsepower (muscle) to hit those higher speeds. Correct? Here’s where the waters get muddy.
Say a paddler is capable of putting out, oh say, 5 units of power. If we put this paddler into a long boat that needs 6 units of power to hit top speed, he will fall short. Okay? Now, let’s put him in a slightly shorter boat that tops out with only 5 units of applied power. Hurray, the paddler hits hull speed in that boat.
The question is, in which boat can he go the fastest?
This question is pretty common in paddlesports and comes from the use of a few generalities used in marketing copy and sales floor speak. The generality is: In general, a longer canoe or kayak is faster than a shorter one. The generality developed from the use of a calculation discovered about 100 years ago by William Froude called Hull Speed. John Winters in The Shape of the Canoe writes about this calculation:
As the hull plows a furrow through the water, two wave patterns are formed. The first, the divergent waves, fan out from the bow and stern and their significance is minor. The second, the transverse waves, also form at the bow and stern but their crests lie at right angles to the direction of travel. These waves are the visible evidence of energy lost pushing water out of the way at the bow and suction at the stern pulling it back to its original level. The length of these waves (crest to crest) is equal to the natural length of a wave traveling at the same speed as the hull. About 100 years ago, William Froude determined that the speed of waves in knots was equal to 1.34 x L1/2 in feet. At low speeds there will be a large number of waves along the hull but as speed increases the number of waves decreases until the hull lies cradled between wave crests at the bow and stern. At this point, the so called “hull” speed has been reached. For heavy displacement craft, this marks the maximum practical speed attainable and higher speed is possible only with extraordinary power increases.
You calculate Hull Speed by entering the length of your kayak or canoe into a simple formula: Hull Speed = 1.34 * (LWL)1/2
So, for a 14′ boat, the Hull Speed equals 5.01 knots. For a 15′ boat, the Hull Speed equals 5.19 knots. For a 16′ boat, the Hull Speed equals 5.36 knots. For a 17′ boat, the Hull Speed equals 5.52 knots. For an 18′ boat, the Hull Speed equals 5.69 knots. Simply looking at the calculations, it’s easy to see why it looks like a longer boat is faster than a shorter boat. Simple subtraction tells us that the Hull Speed of an 18′ boat vs. a 15′ boat is 0.5 knots faster.
Example 1: Now imagine three displacement hulls all 15′ long: a bass boat, a canoe, and a kayak. All three of these boats have a Hull Speed of 5.19 knots. Stick your average all-else-being-equal paddler into each of these boats and answer this question: Which boat is faster? Or better yet, let’s mix up the boats. Our bass boat becomes 18′ with a Hull Speed of 5.69 knots. The canoe and kayaks stay at 15′ long with a slower Hull Speed. According to the Hull Speed calculation, the bass boat has the fastest Hull Speed, but if you’ve ever had to paddle a bass boat with engine problems back to shore, you know it isn’t as fast as paddling a canoe or kayak back to shore.
Example 2: Another displacement hull example to look at is surf skis. In places on the Internet, people report average speeds of 6.5 knots or greater during races on 18′ skis that should have Hull Speeds of 5.69 knots. (Tangent: Check out what Björn Thomasson has to say about speed and stability.) According to the calculation, these surf skis are traveling faster than their Hull Speed.
I hope that these two simple examples show that the Hull Speed calculation doesn’t necessarily predict which boat will be faster to paddle, and we see that in some cases paddlecraft are exceeding their Hull Speeds. I hope after thinking through the above examples, you’ll agree that we can put the Hull Speed generality to rest, but if not, then check out the resistance info below.
A second generality emerging in the sea kayaking world is that a shorter boat is probably faster for a paddler that can’t drive his kayak to the Hull Speed. I imagine that at some point we’ll see an new generality emerge, which will state something like: in general, a short boat will be faster for paddlers that can’t produce much power. Below, I’ll use some computer generated resistance numbers to show that this generality isn’t true.
How Fast Will This Kayak Go?
If we can’t use Hull Speed to determine if a boat is faster for a paddler, the question then becomes how do you determine if a boat will be faster for a paddler that can only output x amount of power? I posit, other than using controlled tests in each boat, the best way, assuming the paddler comfortably outputs the same amount of thrust in each boat, is by looking at resistance. (Comfortably outputs is making assumptions about boat stability, ergonomics, sea state, etc. being all-else-equal.) To show this, I’m going to show the resistance for three 40 pound boats paddled by a 160 pound paddler. I’m using Brian Schulz’s F1 (14’2″) design, my cedar strip Siskiwit Bay (17′) design, and Necky’s Chatham 16 (16′).
|Knots||F1||Siskiwit Bay||Chatham 16|
*smaller number is better.
Sea Kayaker Magazine claims that a fit paddler can maintain speed against 3 pounds of drag. So, if we assume that our paddler can cruise all day against 3 pounds of drag, we actually see that the longest kayak is faster for this paddler. He’ll be able to cruise along in the Siskiwit Bay a bit faster than 4 knots. We see that at speeds of 3 knots and lower all three boats are very similar in the pounds of resistance. At higher speeds, the shorter F1 is more efficient and thus faster at the same thrust than the longer Chatham 16. So, if our paddler can work against 4.65 pounds of resistance, then the F1 will cruise slightly faster than the Chatham 16, but the Siskiwit Bay beats them both. When comparing the boats and thinking about one of the generalities used by Silbs, “we know that that longer boat will need more horsepower (muscle) to hit those higher speeds. Correct?” we find an exception to the generality. In the case of these three boats, the longest boat actually requires less horsepower to hit the higher speeds. That doesn’t hold for all longer boats.
If we add more weight in cargo to these boats, the numbers may tell a different story. For example if we add cargo and run the numbers for 250 pounds of paddler, cargo, and boat, these are the numbers we get:
|Knots||F1||Siskiwit Bay||Chatham 16|
Now, the Chatham 16 becomes easier to paddle than the shorter F1 at 3.5 and 4 knots, but the Siskiwit Bay still beats both. And this will vary differently for each hull design. Looking at these two charts, we can cast doubt on another generality: in general, a kayak will paddle fastest when loaded at its design displacement. You can see that there is more resistance for all the kayaks at every speed with more weight in the boat. For a paddler only able to produce three pounds of thrust, these boats loaded heavier, all else being equal, will paddle slower, than if they were loaded lighter.
There really is no easy answer to Silbs’s question, “The question is, in which boat can he go the fastest?” The computer can give us suggestions. Sometimes it’s the shorter boat, sometimes it’s the longer boat. The only way to really know is design a set of on-the-water tests and put each kayak through the them.
I’m curious how you think about this. Any thoughts?