Flat bottom is okay for surf skis but not for kayaks?

Advice
#1

After a bit of kayaking I’m thinking about getting a fat newbie surf ski. One thing that’s surprised me is that reviews say these wide skis have flat bottoms and are great in the sea and surf!?

Two I’m looking at are the Carbonology Cruze X and the Stellar S18S. Both have videos on Youtube showing paddlers having a great time in all sorts of sea conditions. Unfortunately I can only find photos of the hulls in profile - the standard sales images.

I don’t know how many times I’ve read on this forum and elsewhere that a flat bottom on a kayak means low secondary stability and the first wave is going to upend you. What’s going on here? I’m genuinely mystified :face_with_spiral_eyes:

#2

I think @MCImes is one of the few dedicated open water surf skiers left in PCom. Pikeabike also has converted to surf skiing after many years of using sea kayaks but she has not been around lately.

In YouTube world, you might want to look up @@kayakintro. This guy has lots of videos of downwind runs with all sorts of surf ski models. He also occasionally throws in runs with sea kayaks into the mix, and thus has perspective on both.

For my interest in surfing, my Sea Kayak choices in recent years have all been towards high rocker, flat bottom and hard chined for performance and maneuverability on the wave face. I used to be fascinated by these discussionss of “primary” and “secondary” stability. But, increasingly in real world paddling and surfing, these concepts has little meaning for me. Either one has the skills to paddle upright, brace, roll or self rescue in rough conditions as needed, or not. Pick the conditions and venues accordingly while working to develop one’s skills level.

-sing

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#3

I look at surf skis as more of a rounded bottom, not flat bottom. Usually no hard chines.

Here is a review video of a Stellar 18S

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#4

If you go to Stellar’s website and to the S18S, for example, you will see a video that shows the top and bottom. As you will see, flat is a relative term. It’s less rounded than other surfskis might be.
Secondary stability is controlled by the volume beneath the waterline. If when you edge a kayak to the left, the result is more submerged volume on the left than there is on the right, you will feel a resistance to submerging the left side further. So the volume above the waterline at level becomes part of the volume below the waterline on edge. So secondary stability is really more about what you’re seeing above the waterline than below. A flat bottomed boat can still incorporate high secondary stability.
Think of a perfectly round hull. As you edge it, there is no change in submerged volume between left and right. There is no resistance to edging further. There is no secondary stability. But of course you can have a round bottom half with an above waterline design where edging to one side results in a changed displacement balance that provides secondary stability.
What you’re describing I think is something else. Imagine needing or wanting to traverse steep chop. If you’re in a wide, flat bottomed boat with high stability that doesn’t allow you to easily control your edging/leaning, it’s going to follow the surface of the waves. When they’re steep enough for you to start skidding, that lack of edge control can make it very difficult to prevent that leading edge from digging in. When the wave and gravity are pushing from one side, and your kayak digs in on the other, at some point, you will flip. But even before you flip, things can feel pretty out of control.
If a flat bottomed boat is designed so that you can still control the edges, it’s narrow enough and has appropriate volume distribution, you can take advantage of what a flat/flatter bottom has to offer.
I was out in a 3rd generation Epic V10 yesterday. I was told by an Epic rep that the 4th generation feels more stable. So they’re constantly evolving designs to strike new balances. Stability and sea manners contribute to a strong consistent stroke. How much stabilty is optimal and the importance of good sea manners is very dependant upon tbe individual and the conditions they find themselves in.
I think they’ve also discovered that if your main fight is against a bow wave and a little less round hull doesn’t effect that significantly, then why not design in a little more stability.

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#5

The simple answer is Stability and Surfing.

Stability -
The fastest (least resistance) hull shape is a semi-circle, but as capefear says, this would have zero primary or secondary stability.

Olympic K1’s are pretty darn close to a round hull with only minor flare above the waterline and an almost perfectly round bottom. Thus, they are like balancing on a knife’s edge - extreeeemly tippy.

At the opposite end are paddle boards. No roundness, very stable.

Surf Skis were historically unstable and fast craft. Round hulls and high seats gave old skis a reputation for extreme tippyness, rightfully so.

As the sport evolved, the genius boat designers learned that non-elite folks like to paddle too! This lead to a progressive widening and flattening of ski hulls, culminating in the beginner skis we have today like the V5, V7, S14S, Bluefin, Vega Arc, and others.

Even the fattest, widest surfski like the Epic V5 is 14’ x 24" with sleek lines. I see other beginner kayaks are often 28"+ with blunt lines. This means that the ski will still be much faster and responsive than a 28-30" barge, as a 10% beam change is very noticeable in stability and speed.

Also a Ski’s flat bottom is only a small portion of the hull, typically starting at the bucket and going back towards the rudder. On a rec kayak, essentially 100% of the hull is flat. A ski only ~40% or less may be flat.

Surfing -
A flat spot behind the bucket, as shown by experience, aids in wave surfing. Ignoring stability, imagine trying to surf on a barrel. It wouldnt work. a round hull does not ‘harvest’ wave energy. A Flat hull will harvest wave energy, which is the entire point of surfing a surfski downwind - linking waves energy. This is pretty simple to see in the design of surf boards.

Additionally, adding a flat hull to a skinny boat gives the perception of being more stable. I say perception because it only helps on flat water. As soon as you’re in proper waves (like a ski should be), your stability comes from your hips, stroke, and brace.

The prime example of an elite ski with a big flat spot is the Stellar SEA. It has one of the skinniest beams of any ocean boat at 41.4cm, yet you wont see it on any podiums (because stellar does not have good Ski designs - they are a sea kayak and rowing company that also makes skis).

The balance
The best hulls have a partially round hull with a moderate sized flat spot behind the bucket. A oblong / semi-round hull provides good secondary stability and good attitude control in rough seas. A properly sized flat spot in conjunction with a semi-round hull adds primary stability, reduces ‘twitchiness’, and increases wave energy harvesting ability / surfability.

Recommendations
My personal favorite boats that embody these charistics are the Kai Wa’a Vega / Vega Flex / Vega Arc, and the Epic V10 G4 (and ONLY Gen 4. Gen 1/2/3 are COMPLETELY different boats and share none of the magic of the G4).

Do not get the Stellar. Their buckets are huge, the hulls arent fun, and they are very uninspiring surfskis. All good skis come from cultures with big waves - South Africa, Australia, or Pacific Islands (Hawaii, etc).

Carbonology is the better pick if its down to these 2 boats. If you can handle a 21" beam, then the Kai Wa’a Vega Arc is the winner (Ozone has insane value with their sub $5k full prepreg carbon 20lb boats. The Vega Arc Pro is 16lbs!!!). If 21" is too skinny then I recommend the Epic V5. But if you have any natural ability or desire to learn 21" is generally approachable. The Epic V8 at 22" was the gold standard of beginner boats for decades until the 22"+ novice boats came around a few years ago.

Where are you paddling (or what type of water)? how much experience do you have? what’s the skinniest boat you normally paddle? do you know wing stroke? Do you know wing WELL? Are you willing to swim and learn or do you want instant-easy-no-challenge stability? how warm/cold is your typical water?

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#6

Amazing help, thanks. I’ve learned so much from this, and it’s helping me with confidence to give a comfy beginner surf ski a go.

In answer to your questions, MCImes, I’m really at the low end of the experience, skill spectrum, and also 65yo. Just done some up and down river paddling and a little on calm seas. One slightly rough sea that tipped my friend over in a 22" wide sea kayak, with the hassle of a flooded cockpit, was part of making me think that a wide ski would be so much easier to fall out of and of course get back into and paddle away. Just want something that will encourage me to get out on the ocean more, safely and reasonably stably and start to gently build skills.

I’m in Townsville, North Queensland, so right in the middle of what’s left of the Great Barrier Reef, with no surf but the Coral Sea does get considerable swell and chop. I want to be able to paddle out to Magnetic Island (with a group) and paddle round it for a few days. So the Cruze and S18S attracted me because I can put a bit of gear in them. Also because they’re not too long. The only place I can store boats is hoisted to the roof of my carport, which has only 586cm in available length. Anything 5.9m or longer is not possible.

I do keep hearing good things about Carbonology, and your warning about Stellar has been noted MCImes. Given I’m a bit remote and can’t sit in the boat before ordering, getting something that nobody complains about comfort-wise seems advisable too, and the Cruze seems to tick that box.

So all in all, the Cruze X is looking good at the moment. Thanks again, this has really helped.

#7

Go out to Youtube and (search) reach out to Kayakintro at YouTube. He is located in Aukland and surf skis similar conditions that you described. He is friendly, knowledgeable and pretty responsive about his sport. https://www.youtube.com/@kayakintro

-sing

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#8

Thanks for the link Sing. Indeed lots of videos learning skills on choppy sea. Thanks!

#9

Nice. With that in mind the Cruze X seems like an ok choice. On flatter water a 56cm boat should be very stable feeling. 56cm was the standard beginner boat up until just a few years ago, so I think you should be ok assuming average balance and fitness.

In the ocean, it may be a while before you feel comfortable in chop. I would say just start small and build up in wave size and chaos factor.

If you are at all stability compromised, there is no chance for you to build a good Wing stroke, so make sure you spend as much time as needed on calm waters until you have half decent wing stroke form. The Ivan Lawler video below is a good reference, or ideally you can find a paddle coach and take a couple lessons early on.

Besides learning proper wing stroke, learn to remount and brace early on. Knowing you have a bombproof remount will quickly add confidence in tippy conditions. Start on calm water, then seek out choppy water that passes, such as timing a remount with a passing boat wake. This way if you fail to remount in waves they pass in just a moment, and you can try again with little consequence. After you can remount a boat wake, then start remounting in progressively bigger and messier waves.

Lack of confidence in your remount will significantly hinder your growth as a paddler, so build this skill.

Last, learn the nuances of bracing. There is “Stroke bracing” where you are adjusting the feather and paddle path slightly during the forward stroke to maintain and enhance balance. A Stroke Brace does not take away from forward propulsion much if at all, and is the preferred method of maintaining balance.

When a stroke brace fails, then you “Slap brace” where you push the back face of the paddle against the water. This results in substantially more righting moment, but kills your speed severely. If you do not correct quickly, a slap brace turn into a slow-motion swim as you lose momentum and the brace becomes ineffective, you slowly roll into the water.

Being able to execute both a Stroke Brace and Slap Brace almost instantly is key to avoiding swims and maintaining forward progress in large and sloppy conditions.

Last, learn and truly understand the Triangle of Stability - that is your Feet, Butt, and Paddle. Your butt is easy to understand, its the anchor.

Your feet only provide stability if they are in good contact with the footboard. You will see as soon as it becomes chaotic, one’s natural instinct is to remove pressure from your feet, which compromises stability and power through a lack of leg-drive. Leg drive and consistent pressure against the foot plate is key for stability and effective power generation. Understand this nuance as early as possible.

Last is the Stroke, which I covered above. Another natural instinct is to stop paddling, delay strokes, or take strokes with very poor form as a result of instability. Force yourself to paddle properly or swim. Focus on how much your lower arm bends (not much!), leg drive, and an early exit.

Early on, its about learning the basics of stability and proper form, and avoiding bad habits that must be unlearned later.

Let us know how your journey progresses!

Ivan Lawler Racing Kayak Technique Masterclass -

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#10

McImes you are an absolute treasure! Every word of this advice is exactly what I need right now. Thanks so much.

There has been a development. I had a look at the second hand market, and a guy down in Airlie Beach (3 hrs drive from Townsville) is selling a Fenn Bluefinn S Carbon Hybrid for a decent price - he actually has two of them which is good as it will take me a couple of weeks to get down there and check it out. I’ve worked out that if I store the 5.9m boat diagonally in my 5.86m rafter space it will fit.

Note that this is the Fenn Bluefinn S not Fennix, so it’s the previous model. It seems to have good reviews. A little narrower than the Cruze X at 53cm, but it seems to still have good stability and weighs only 12.5kg, the same as the Cruze X Hybrid. Some people have posted about scuppers leaking but that seems like a fixable potential problem.

If I buy the Fenn though, I need to drive it back at highway speeds - 100kph - which my roofrack at present would not support, as the bars are fixed at 80cm apart, with the front bar about in the middle of the car. So about 3m of ski would be hanging in air at the front, which seems unwise. The specialist rack extensions for skis - V rack and J rack - are extremely expensive. Even with my rudimentary handyman skills, I think I can build a simple rectangular frame with aluminium square tubing (good supplies of any size I want in Townsville) about 2.5-3m long (longer than the V/Y racks) and attach my Yakima Sweet Roll kayak cradles to it. This long explanation is because I have a question, not having any engineering knowledge. What size and thickness of aluminium square tubing should I use? I’m thinking 20mm square with 3mm wall thickness, which would fit well with the cradles. I could go 40mm square (30mm is awkward with the design of the cradles), but it adds weight and is perhaps overkill, given that the ski itself provides stiffness, and I’m using 2 tubes?

I’ve decided to let go of the ski with hatches for touring idea, as it adds a lot of expense ($500 for the Cruze X Touring upgrade), and I can use my Delta 14.5 or Bay Sports Expedition Zero kayaks for that. The ski is to get me out on the water easily and more often for fitness and skills building.

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#11

Correction. The owner describes the Bluefin as a Fenn, but I just noticed the sticker in one of the photos that says “Fennix”, so it is in fact the latest model Fennix Bluefin S.

#12

The bluefin is an ideal boat for your specific waters. The bf is kinda slow on flat water in the grand scheme, but an excellent rough water boat that is ideal for big downwinds.

I would say it’s an excellent first boat and one you can keep as you progress for surfing on big days

#13

Thanks again MCImes. That’s encouraging. I am wondering, though, how can a 6m ski be slow on flat water? How slow? What slows it down?

#14

Look at the side profile of various skis here - http://surfskicomparison.com/

Compare the bluefin to something like the Carbonology Cruze. You’ll notice the BF has substyanitally more rocker (banana shape) than some other models.

High rocker improves performance in bigger waves at the expense of flat water speed.

High rocker means that on flat water, there is effectively a lower waterline length and a less hydrodynamic hull shape. Remember, the fastest flat water hull is a K1 (round hull, no rocker, knife edge entry and exit, nearly symmetrical).

But on bigger water, such as swell over 1m (and particularly over 2m), or steeper wind waves over 0.5m, high rocker really starts to help with maneuverability. Long, flat boats such as the Epic V10 g1/2/3 or Stellar’s have a tendency to get “Locked On” a wave.

A low rocker hull at medium-high speed becomes very difficult to turn, up to the point of impossible. This means you will miss waves because you cannot overcome the “lock” of the wave or overcome the direction the wave is pushing you. Locking In results in shooting off in the wrong direction, missing a hole forming in front of you, or being uncontrollably broached.

The best surfing boats like the Bluefin, Vega series, and Epic V10g4 all have substantial rocker which makes them very nimble on waves. They resist ‘locking in’ a course and remain maneuverable so that once you learn to read waves, you are physically able to be where you want to on the wave (once you learn where you should be)

In big waves, hull speed (beam) means almost nothing - all your speed comes from how well you read and surf waves, so there is only benefit to high rocker in a big wave boat.

And Fast/Slow is all relative. I would expect the bluefin to maintain ~8kph on flat water, where as a flatter rocker boat may be closer to 9 or 10kph. An elite ski could be closer to 12kph.

Not huge, but 1kph makes a big difference in races and thus the BF gets labeled “slow” on flat water. But conversely, it is regarded as one of the best big wave boats around that absolutely shreds 1.5+ meter downwinds.

#15

Don’t conflate the wide, flat width of a boat with the fore to aft “bend” or rocker. I wouldn’t really consider any modern surfski to have a wide, flat hull from side to side. Not like a lot of rec or fishing kayaks.

#16

Thanks MCImes & Dave. Haven’t seen that explanation of “locking in” on waves before. I’m remembering my friend who got upended in a sea kayak which had pretty much no rocker - my Bay Sports Expedition Zero.

I think the only speed concern I have is getting to around 9kph, which seems to be the agreed speed at which the scuppers drain. I watched one of those kayakintro videos with a Bluefin, and he was getting up to 16kph on brief sprints in an almost flat sea, I imagine downwind though it couldn’t have been a strong wind with the calm water. And in another kayakintro video the other boat is a Bluefin, and yes you can see the rocker effect in a calm sea, with the bow often out of the water.

So the Bluefin is looking pretty good. I have to admit though, the 56cm width of the Cruze gives me a sense of security for my initial adventures, with the Cruze still having some rocker and nobody complaining about it in choppy seas. It may come down to the price - the second hand Bluefin is a third of the price of a new boat (Cruze or Bluefin) so if it’s in good condition I’ll definitely go that way. Unfortunately I won’t be able to get down and look at it for a couple of weeks. I’m thinking of perhaps asking the guy if we can do a Zoom video inspection and if it looks okay putting a holding deposit on it.

#17

Here’s a site showing observations of the wave heights this week, which is I think fairly typical of what I’ll be paddling in. Given onshore winds tend to pick up in the afternoon, it’s generally calmer in the early morning, which is probably when I’ll schedule my first sea paddles. My very first outings might be in the river.

#18

Man, you live in surfski paradise!

Re the bailers, the Stellar Bullet bailers reduce the minimum bailing speed to maybe 7kph, very easy to maintain. I am a big fan of the Stellar Bullet Bailers specifically. The Fenn might have bullets already too.

I think both the Cruze or Bluefin would both be good boats. Beginner boats like this maintain value because the market is large, compared to skinny 43cm elite boats

Also my guesstimate speeds were for like a 1 hour race pace on flat water, like a 10k race. Sprints or wave riding you can go much faster.