The secrets of the SOT kayak’s underside

Have you ever seen a picture of the underside of a sit-on-top (SOT) kayak? –
It’s unique, and the bottom of no other vessel looks like it.

Below is a figure showing what a typical SOT kayak looks like when it’s turned over:

underside-of-typical-sot-fishing-kayak

Understanding the design of SOT kayaks’ underside

The ‘scupper’ holes

The most striking feature in a SOT kayak’s hull are the holes in it:
All SOT kayaks feature vertical holes connecting their deck to the water below. Kayak manufacturers call them ‘Scupper Holes’ and claim they were introduced into the SOT design as means to drain water from the kayak’s deck, similarly to what scuppers do in normal boats.
The truth is different –
To begin with, these vertical holes were introduced into the SOT kayak design not as means to drain their deck but for a totally different reason: They support the deck from crashing down as the user sits on top of it –
Kayaks’ hulls have thin plastic walls, and SOT kayaks’ hulls have a general form of an empty and flattish eggshell that’s not very strong, which is why its top side (let’s call it the ‘roof’) requires reinforcement.
The way to support the roof of a large structure is by means of vertical columns, and that’s essentially what scupper holes are: vertical, molded-in plastic tubes that act as supporting columns for the kayak’s deck.
This explains why scupper holes are rather dysfunctional as drainage holes – They were not designed as such in the first place.
So next time you paddle a SOT kayak and you notice water splashing through the scupper holes onto the deck as a result of the kayak moving in the water (that’s what kayaks are supposed to do), you’ll know why the manufacturer just had to put these holes there, and why the sensible solution of letting water drain from the kayak’s deck to its sides didn’t get adopted…

The hydrodynamics of scupper holes

Any detail in a vessel’s hull that can generate a noticeable change in the regular flow of the water is unwanted since it increases drag (resistance) and makes it harder to move the vessel. In other words, it slows the vessel down.
Therefore, a hull that features multiple drag generating elements such as scupper holes is very slow, and in kayaking terms it’s hard to paddle or pedal it forward at an acceptable speed without the kayaker making an unusual effort.
Call it a barge and you’d be spot on.

Channels / tunnels

The second striking element that SOT kayak feature on their underside are channels sometime called tunnels. These are the long and narrow grooves stretching along the hull’s middle section.

Why are they there? –

Water cannot be compressed, and it doesn’t like to be forced into narrow and long structures such as these. When it does, it generates friction (Frictional Resistance – FR) and turbulence, and thereby even more drag making the kayak even harder to move, I.E. much slower.
Kayak designers know these facts, or at least they’re supposed to be aware of them, so why do they add such channels to their kayaks’ underside? –
The typical answer you’d hear at a kayak dealership is that channels add to the kayak’s tracking capability. This should be a good thing because SOT kayaks or at least those designed to serve anglers track exceptionally poorly, which is why nearly all of them come equipped with a rudder (yet another undesirable element). But if a kayak features a rudder, it doesn’t need such channels molded into its underside…
So this common explanation is false, and it masks reality –
If indeed channels are there to improve tracking, why does only the middle section of SOT (and ‘hybrid’) kayaks feature them? It would make sense to make such channels longer, so they produce a more noticeable tracking effect, wouldn’t it?
Well, the reality is that such channels are essentially yet another means to reinforce the SOT kayak’s hull, which is why they coincide with scupper holes –
Knowing that the lower end of scupper tube generates considerable drag as it comes in contact with water, the kayak designer may attempt to position it higher, that is at the top side of a narrow tunnel, such as can be observed in the above image.
It doesn’t really work, simply because the kayak sinks lower in the water as soon as it is loaded with a passenger, and if that passenger happens to be an angler, the load is heavier since it includes fishing gear as well.

Other underside elements that slow down the SOT kayak

If you thought that kayak designers and manufacturers would stop at scuppers and channels, you were wrong. In fact, in the race between kayak manufacturers to overdo each other by introducing details and accessories in increasing numbers, many SOT kayaks today feature additional elements that generate extra drag, and further slow you down –
Those include fins, keels and skegs, and pedal driven kayaks feature flapping fins or propellers.
The additional effort required to propel SOT or hybrid kayaks that feature such elements is significantly greater than the effort required to propel simple sit-in kayaks of similar proportions, and it’s much greater than the effort required to propel a W kayak.

Clearly, as SOT kayaks become bigger, wider, heavier, over accessorized and dysfunctional through clutter, we are witnessing the end of a design cycle that began sometime in the end of the 1960s, when people started outfitting paddle boards with seats and footrests and called them sit-on-top kayaks…

15 Comments

  1. Pete

    I can see how the scupper holes can cause a lot of drag, but why would these narrow tunnels have a similar effect? Can’t the water just flow in the tunnel in a straigt line without much drag?

  2. fish kayak

    The water is pushed into the tunnel’s front end from its top side and from its left and right sides, that is from three different angles. These three currents or ‘mini wakes’ start interacting with each other and with that narrow tunnel’s walls right at the beginning of their journey through it, with no room the go elsewhere…
    The presence of vertical holes in the top of a tunnel amplifies this turbulence and consequently the drag it generates.

    It’s a very ‘messy’, ‘noisy’, inefficient and not elegant design – depending on what metaphors you prefer 🙂

    Yoav

  3. Pete

    But those multiple tunnels improve directional stability, don’t they?

  4. fish kayak

    Yes, they reduce zigzagging that plagues wide kayaks, but not as much as a single tunnel with high walls would, or a large size keel, if you prefer.

  5. Norman A

    I don’t think those extra large sots and hybrid yaks are made for long trips anyhow. They’re just too hard for a normal person to move, and not comfortable for sitting for long hours.

  6. fish kayak

    Some of them are reasonably priced between $300 – $500, and others that offer no substantial advantage over them except a hyped brand are offered for $1,000 and even over $1,500, which is ridiculous.

  7. Benny Suttendorf

    This solution involving hollow tubes molded into the kayak’s hull is dysfunctional, and the manufacturers must be aware of the problem. Isn’t there another way to strengthen the hull?
    –Benny

  8. fish kayak

    There are two different solutions that I’m aware of –
    The first is implemented in a small number of kayaks, and it offers a way for water to drain from the deck through ‘channels’ on its sides. Some of these channels are integrated with existing elements such as footrests. The bottom of the hull is reinforced by means of a high walled tunnel, and the manufacturer offers these models as ‘catamaran’ or ‘pontoon’ kayaks, which they aren’t.
    I guess the problem this solution creates is both aesthetic and functional, since the kayak doesn’t appear to have a deck, or a cockpit, or a place for the angler to work in.

    The second type of solution is more popular, and it can be seen in ‘hybrid’ kayaks that feature no ‘scupper’ holes at all. Their floor (I.E. ‘deck’) is lowered, and it is supported from below by molded-in tunnels.
    This ‘hybrid’ between canoe and kayak offers too little free board, which makes it suitable for fishing only in perfectly flat water, with no waves or eddies present.

  9. Pete

    I saw hybrid kayaks that come with scupper holes, but I guess that if they’re overloaded they could get flooded with water gushing onto their deck sfrom below, wouldn’t they?

  10. fish kayak

    True. In fact such hybrid kayaks risk getting their decks being flooded from below even if they’re loaded unevenly – There’s one particular hybrid canoe-kayak design that I saw, which features scupper holes in its rear, behind the cockpit. If it’s user loaded that end with heavy gear, and/or if they sat aft of the middle of that boat, the boat’s rear end would sink dangerously, and that could let water onto the deck.
    BTW, scupper hole plugs are available from several vendors, and some kayak anglers make such plugs themselves.

    What I find most striking is the fact that most SOT and hybrid kayak manufacturers exaggerate the load capacity of their kayaks, as if they completely ignored the simple physical fact that all vessels including kayaks sink lower as a function of their load, and a boat or kayak cannot have their deck get too close to the waterline because this is both inconvenient, unstable and potentially unsafe to the passengers.

  11. carpinator

    Fish, you mention that these sot yaks shouldn’t cost more than 500 dollars, and I agree in principle but how come they’re offered for more and people still buy them?.. some pay three times as much!
    H.C.

  12. fish kayak

    H.C.,
    Some of the upscale SOT fishing kayaks feature a lot of accessories that cost money to produce and add to the kayaks. For example, rudder systems, leaning bars, adjustable seats, fishing pole tip protectors and so forth.
    The fact that some of these accessories are ridiculously redundant doesn’t change the fact that there are people who are willing to pay for what they perceive as ‘more’.
    Every dollar spent by the manufacturer adds much more to the retail price, since distributors need to make a profit too, including dealers, regional stocking distributors and regional sales reps.
    There is also the marketing factor: Manufacturers of fishing kayaks who want to charge more for what’s basically very similar to a low-end model must spend a lot to push their product into the market. They have to enhance the brand’s image and spend on advertising, PR, trade shows etc., They also have to finance their ‘pro staff’ which are people that get paid to use kayaks from a certain brand and ‘endorse’ it both passively (e.g. by showing up in it in fishing tournaments), and actively, by posting their pictures, trip reports and ‘reviews’ on online discussion forums.
    Just for the record – Wavewalk sells directly to its dealers, with neither regional distributors nor sales reps involved. We also don’t have ‘pro staff’ or any other sponsor-endorsed relationships with anyone.
    We sometime donate a W kayak to serve as a prize in a fishing tournament, such as the Hudson River Striper Derby

  13. HookSetR

    I tried renting one of the SOT yaks before buying a yak and after 30 minutes my lower back was killing me and my legs kept cramping up. I knew from that rental that a SOT would not work for me. This is what led me on my internet search for a better alternative. Upon finding Wavewalk and watching the videos and reading the stories I decided to purchase one and have not regretted it since. Keep up the good work Yoav and the W team.

  14. fish kayak

    Thanks George,
    It’s nice to hear from you.
    I bet the weather in California allows you to fish already.
    Here we had 16F this morning, and it’s spring, at least officially 🙁

    Yoav

  15. Pete

    The most important fact about SOT yaks is that they’re not unsinkable. It’s important because it has to do with safety, and people who believe in the false notion that SOT kayaks are unsinkable could get into some serious trouble, as this eye opening article explains – http://wavewalk.com/blog/2008/09/22/are-sit-on-top-sot-fishing-kayaks-safe-for-offshore-fishing/
    –Pete

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