Sea Kayak Waterline Length and Surfing??

Here is a question I have pondered for sometime and would like to start a discussion on the topic.

I do a good bit of surfing in sea kayaks…in the surf zone, in wind waves, and on standing waves in both rivers and tidal races.

I have found that longer boats with a longer waterline tend to be able to catch waves a bit better in surf and wind waves given their greater speed. Is it a big difference…no…but I find thre is a difference. Example: Romany vs. Explorer.

So…what about in current and standing waves?

Of course in standing waves the length of the hull will have an effect in spanning from the crest of one wave to the crest of another in the case where waves are close together and affect whether the bow digs in and therefore slows the boat / pushed it off the wave etc.

But how about just plain old top theoritcal hull speed of the boat as a function of its waterline.

Obviously the boat is not moving in relation to the land, but it is moving in relation to the water, or more accurately the water is moving fast under it. So in a way the boat is moving forward across the surface of the water at the same speed that the current is flowing (since you could view the water moving under the boat or the boat moving over the water).

Can a boat no longer surf once the speed of the current exceeds the boat’s theoretical max hull speed?

I as this based upon not only my own thoughts on the matter but what I have observed.

I was surfing at Sullivans Race in ME. I saw two paddlers in the race (which was flowing really fast at like 7 knots or more I would guess). One was paddling a Romany Surf (16 foot). The other was paddling a Impex Force 4 (18 foot). They were both in a section of water that required a lot of ooomf to move forward through in order to surf the front wave.

Teh paddler in the IMpex could reach the wave without too much trouble. The paddler in the Romany could but only with extreme effort and was only able to work his way up to the wave once or twice through the current, yet the paddler in the Romany was much more skilled as a paddler and much, much more physically fit than the paddler in the IMpex.

In this case I guess it was more a function of working against the flow of the current than surfing, but of course part of working against the current is surfing the small wavelets in the current that allows you to work forward.

So my observation would be that hull speed has an affect in working against current. I also feel it must have some affect on surfing on standing waves as well, and probably on the ocean too as their is a point when your boat in the ocean is just flying along at high speed and you eventually come off the wave as a function of the wave just overtaking you and moving under / or over you if in the event you have not yet broached. To me you see this more when you are surfing those big huge steep peaking waves you run into out on big shoals. You know…the kind that peak up very steeply and give you a hugely fast ride that you can stay on for quite sometime before the waves kind of goes down as a function of the water getting deeper or gets sloppy and breaks a little before continuing to shore.

Just curious what your thoughts are.



speed is speed
The boat doesn’t know if the water is moving relative to the land or not. All that matters for hull speed for a DISPLACEMENT hull is how fast the water is moving along the bottom of the boat. So catching wind waves or surf is just the same as catching that front wave at Sullivan Falls. You have to push that boat over the speed of the wave to get in front of it.

Often you’re pulling out of an eddy in a tide race, and you can sort of catapult your boat up to an upstream wave by paddling fast across the eddy line, and your momentum will carry you upstream a bit before the current starts slowing your boat down. So that means you don’t have to paddle as long sometimes to catch a 7 knot wave in a tide race as you would to catch a 7 knot wind wave.

As far as surfing - theoretical hull speed doesn’t apply when a boat is surfing because it no longer behaves as a displacement hull. Once a boat is surfing, it is, by definition, a planing hull. That exhilarating feeling of surfing any wave is caused by the boat getting pushed up over it’s bow, wave, and rising over the water coming at it, rather than pushing the water to the sides (displacement).

So I’d say that theoretical hull speed makes a difference for catching up with a wave (surf, wind, or standing). But it doesn’t have much effect on whether you can surf a wave once on it. Boat shape probably plays a much bigger difference with that.

My personal experience is that the Pintail surfs any wave about as well as any boat I’ve compared. It has a shorter waterline length than most sea kayaks, but it can catch wind waves that simply pass under my paddling companions in many conditions. I certainly can’t sprint as fast as some other boats, but once that wave picks up the stern, the Pintail seems much better at lifting up and accelerating, so it can stay on top of the wave. Point being that waterline length doesn’t count for everything. Shape of the bottom makes a difference.

Damn! I gotta get to one of the tide races this week! All this talk is killing me!

Whitewater makes it obvious

Much of what you are observing is more pronounced in ww boats. Park and play boats are very short and boxy. River-runners are longer and less boxy. I can get to a desired spot quicker and easier in my Diesel. I can more easily sit on a standing wave right down to tiny ones in my I3.

As is always said it is a trade off.

I can surf my Romany with more confidence than my Aquanaut which requires more attentiveness once on a wave. I found that at Sullivans Falls I could get where I wanted faster in my Aquanaut, but could more confidently park on a wave in my Romany.

I agree with you
that it seems easier taking advantage of smaller waves and swells in faster kayaks. I also think that faster hulls seem to make a good difference in currents, or really anytime you need to really push it. Getting in and out of inlets against the current is where I typically appreciate my faster boats the most.

Of course, as pointed out, while maintaining yourself in front of the wave, you will plane and travel the speed of the wave on average. So there it depends upon how long you can stay on the wave without it passing underneath for whatever reason. I imagine different conditions get different results between the same two boats. And I imagine paddlers with different skill levels and/or conditioning levels could also get different results.

I’d love more opportunities to put these things to the test, but open water paddlers always seem to be in short supply. So finding among those a few people who really want to see how fast they can cruise with a following sea would definitely be a rare treat.

Under your own power
you will be in displacement mode so a “faster” (let’s say that means longer and skinnier etc.) will do better.

Once on the wave, if the wave is fast (e.g long period wave) but not steep you need a fast boat to stay on it. If the wave is steeper you can use a shorter boat and still maintain same speed. Both cases - displacement mode.

Once you begin planing, then the shorter the better - less friction, I guess. But in a sea kayak you ain’t planimg at 7 knots is my guess. Mine feels like it begins to plane nicely above about 12-15mph or so (fast standing waves or steep shore surf or very large boat wakes is typically what it takes). You can’t paddle “over” these to catch them - can only drop on them from the side or be caught by them …

Length and shape
While a hull with higher potential speed can be paddled faster to “catch” a wave, or fight a strong current than a shorter one with the same shape, some shorter hulls with shapes conducive to catching rides can be easier to get on a wave than a longer hull with a less conducive hull to catching rides. But, the shorter hull with less potential speed would have no advantage in fighting a current for example.

Case in point: The 14 ft. surf tour proto I’m paddling catches everything easily! Far more easily than my sea kayaks, but I wouldn’t want to paddle against a 6 kn current with it, whereas I could fight that short term in a longer touring boat.

Again, it’s about compromises.

Shape maters, true
Yup. Especially for ease of catching waves the shape matters. If the rear lifts easy (as I presume is the case in your surf-tourer prototype), it will see a steeper wave so to speak than a narrow pointy ended boat that would stay more horizontal or farther back on the wave till it experiences the same effect (by which time it will have a significantly shorter waterline than what it started with).

The whole picture has to be taken into account.

My Caribou catches waves easier than my Anas Acuta, especially in tide races, and can stay on them longer, but the Anas is more fun while on the wave.

This is all interesting and makes sense and now does explain many of my observations and experience. One of the most interesting things I have read here that makes me think is the affect of length and shape in influencing the angle of the boat on the face of the wave or the “steepness” that the boat “sees” Very interesting point.

so…what hull shape characterisitics are more conducive to better surfing?

I would imagine a flatter bottom and more volume in the bow and stern to prevent pearling and aid in lifting of the stern on a wave…what else???

Displacement vs. surfing vs. planing. I will have to take a look at John Winters Shape of the Canoe which I have upstairs. I recall he makes a comment in there about the fact that there is a difference betweeen surfing and planing and that a sea kayak has to be going really fast to actually start to plane. I will have to take a look at thta again.