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Several plastic sit-on-top kayaks are being marketed as "unsinkable" and "sea-worthy". However, I read that few people had to beach when the did not close the front hatch properly.
My question is: how difficult it is to close the front hatch "properly" and how safe are those "unsinkable" kayaks anyway? Does it help if a kayak has water-sealed bulkhead separating the front hatch and the rest of inside of the boat?
The boat itself is effectively unsinkable, it's a sealed plastic box. The hatches normally have bulkheads separating them from the rest of the boat. If the hatch sections become swamped it's not going to sink the boat, just make it very difficult to handle.
It should be noted that most properly set up kayaks are "unsinkable". All kayaks should have adequate buoyancy* to prevent them sinking. That doesn't stop them getting swamped when things go wrong. Here they're using "unsinkable" to say that it behaves like solid block buoyancy i.e. like a surfboard. You might be swimming but the boat is still floating.
Sea-worthy is an entirely different game. If you're thinking of using a SOT kayak at sea you shouldn't be straying away from lifeguarded beaches or organised groups. You're much more exposed, more likely to come adrift from the boat and probably inexperienced just because you're asking the question.
How "safe" is it? as safe as the person on board.
I'm yet to see a proper SOT sea kayak. They're mostly recreational toys or fishing platforms. If you're splashing about off balmy beaches in the Bahamas they're fine. If you want to cross the Pentland Firth, then you'll need proper boats and an experienced group.
*"buoyancy" in this case means foam blocks, sealed bulkheads, airbags, etc, design features or addons specifically designed to stop the boat from sinking.
No they are not unsinkable.
There is a class of boats where the hull material itself is buoyant, like a surfboard. These sit on types are not of this caliber. They are plastic boats with a sealed (to varying degrees) hull which traps air creating buoyancy. Any break in the seal; either by way of an open drain plug, storage hatch ajar (if part of the buoyant hull), or puncture (disrepair, shark attack, slamming into rocky surf) will cause a loss of hull buoyancy. The severity of the mishap will depend on exposure and the size of the breach in the hull and what methods of evacuating water from the hull you have available.
In regards to seaworthiness of SOT vs Sit In kayaks, it depends...
Typically a SOT, because you are sitting on top of a air cavity of some depth, will have a higher center of gravity and in that sense be more tippy than Sit inside. Initially the SOT will feel more stable because its like a big surfboard compared to a sit inside sea kayak, which is narrower, but when a sit-inside goes, it goes suddenly. It has little reserve stability.
If you do swamp or flip a sit-inside sea kayak, (and are unable to roll it back, which takes considerable strength and skill, )attempting re-entering a sit-inside, often full of water, can be difficult. A SOT can just be climbed back on (by most fit people anyway) like a surfboard
Some specific hull shapes, available only in sit in sea kayaks ( e.g. Icelandic) , seem tippy at first, but stiffen up when leaning, and have good reserve stability. This type of sea kayak can offer more control in terms of maneuvering it begins to become unstable.
So given a unexpected wave, I think you are more likely to find yourself in the water when you paddle a SOT, because of the higher center of gravity, the lack of secondary stability, and the less optimal hull shape of SOT in regards to controlling the kayak.
But given that you suddenly find your self in the water, I think you are a lot less likely to be able to get safely back into the Sit Inside than the SOT, especially if you paddle solo.
