As one who paddles a long (17’ plus) boat in downriver races, I have been musing about whether the efficiency advantage of a longer hull (compared to a 14’9" wildwater hull) disappears when racing into a significant headwind.
If, for example, my 17’ boat has a cruising speed of 6.5 mph and the headwind kicks me down to 5.5 mph, a shorter but narrower (and also lighter and quicker turning) wildwater hull may be just as efficient, if not more so. Am I missing anything here?
Factor in the presence of current in sections of the river (lets say a 2 mph current) and now my hull speed is just 3.5 mph more than the current – and, it would seem, the advantages of a longer hull are reduced even further.
I am making these speculations based on the understanding that longer boats are “faster” (i.e. more efficient) only when paddled close to hull speed. I own a wildwater boat as well as a 17’3" x 19 inch Ruahine Swallow and am wondering if the longer boat is the best choice when paddling in a race in which a significant headwind is expected.
Size matters
There are normally two advantages to a longer boat. One is that these boats all run in displacement mode, and the upper limit of displacement speed is proportional to the square root of length at the waterline. The second advantage is that for the same load capacity, a boat will be either longer and skinnier or shorter and wider and the skinnier boat will have lower drag and present less resistance to waves.
Yes and no
"There are normally two advantages to a longer boat. One is that these boats all run in displacement mode, and the upper limit of displacement speed is proportional to the square root of length at the waterline."
Yes, it's true that a longer hull has a higher theoretical hull speed.
"The second advantage is that for the same load capacity, a boat will be either longer and skinnier or shorter and wider and the skinnier boat will have lower drag and present less resistance to waves."
No, this is an oversimplification, in that it doesn't take boat speed into account. And I would point out that resistance to waves is not the same thing as wave drag, which is due to wave-making by a displacement hull. Total drag on a hull is the sum of skin friction (due to fluid viscosity) and wave drag.
At low speeds, skin friction dominates the total drag on the hull. A hull with less wetted surface will therefore have lower total drag. Shorter boats generally have less wetted surface, so will have less total drag at low speeds.
At higher speeds, wave drag dominates the total drag, as more and more energy must be put into creating the large wave system around the boat (both bow wave and wake). Longer boats do in fact have more skin friction that shorter boats, but have less wave drag at higher speeds. At some point, there is a cross over when the total summed drag favors the long boat over the shorter. In the kayak world, considering boats we're all familiar with, that seems to occur in the range of 3 to 4 knots, but is heavily dependent on hull shape.
The OP is asking a pretty sophisticated question, and I think the answer is that he needs to know the crossover points for the boats he has, based on the relative velocity (between the hull and water) he expects to face while paddling. If the boats are from a while back, that info may be available in the performance curves that were published for years in Sea Kayaker magazine.
If it was me, and I had both boats,…
I would simply try them both on days when the winds current, etc are equal and time myself.
Jack L
But then what would I have . . .
. . . to think about while paddling?
You are right, Jack, I need to do more trials with the different boats I have available.
In a sense, the trials have already been done -- in that the guy who has been chasing me all spring (and paddling a wildwater boat) finished closest to me in two races in which there was a headwind. That's what got me thinking. Definitely need more data though.
(Unfortunately, Sea Kayaker hull speed data doesnt help me out any with these boats: Ruahine Swallow and Kyl Wildwater.)
You win.
Chances are that if you have a 17 footer that cruises at 6.5 mph, you’re likely going to win the race, no matter what the wind is doing.
thread-killer!
volume, hullspeed, and whatnot
I can paddle a 17’ low volume sea kayak upwind just as fast as I can paddle a 21’ racing surfski if not a tad faster.
The reason is that the headwind prevents me from pushing the surfski to (or slightly above) hullspeed and the lower volume kayak has less windage than the surski. In other words, the drag from the wind becomes the dominant form of drag in a stiff headwind and the lower windage boat does better in that case.
I find 5-6 mph into a stiff headwind to be easier in the a low volume skinny kayak than in a ski. But if there is any side or downwind component to the race then I would be faster overall in the ski.
Longer is Better (!)
As long as you are paddling at speeds relevant to the water that are faster than around 4mph, I think a longer boat will still be better. Shorter boats I think only have advantage if you are going really slow, like below 4mph. And as someone else mentioned, the moment the wind stops you will have an instant advantage in the longer boat.
I have not compared 15’ racing to 17’ racing, but 20’ racing has been always faster for me than a 15 foot touring, no matter the conditions. While at 3-4mph the 15’ touring might be easier to paddle, how often do I race at these speeds? Most races 'round here would be cancelled if there was that much wind that paddlers would be stuck at below 4mph speeds vs. the water…
shape not length
The head seas associated with head winds can do more to slow a boat than the wind itself, unless the wind is very strong. So how the hull shape affects pitching/plunging behavior in head seas can be more important than length. I opine that a relatively low rocker, plumb bow hull, is faster in head seas than high rocker with extended bow.
the shorter boat will always be faster
when you paddle both sideways into the wind
What if the longer boat is red?
hahhahhahhahehhe…
right ! Spring comes to Portland....
yessiree 21 foot hulls rip especially when powered by a 150hp Honda.
people ask really complex questions requiring either a Craig computer, an expert, or several years of trials in different hulls or ...as the OP sez a visit to the course.
Windage is everything going downriver. If 2 hulls exactly the same ceptin' freeboard, hull with less freeboard is faster.
When was the last time you saw a stylized upturned end hull travel across Lake Woebegone ?
https://goo.gl/fvy4mX
Across the river, a long hull with something of a keel will ferry faster (sail) but delay maneuvering required for down river: the hull 'hangs up' - hesitates.
Now, if you added a wee dropping centerboard to your short very low free board hull...is this cheating ?
YES ! try that...
drat!