@Rex said:
My experience: Round hatch covers are good. Oval hatch covers leak badly.
I have Sealect covers. My stern oval hatch is 10" x 17. The cover has two alignment arrows as does the rim of the hatch. Embedded in the rubber of the inside cover are the following inch-tall words: “For waterproof seal, align the arrows”
I have a hatch cover for dummies… 
I really should clarify. The oval cover on my Tempest leaks a little. The oval cover on my Alchemy leaks like a screen door; like a spaghetti strainer; like there’s really just a spider web there.
I have a friend that swears by using di-electric grease (the stuff they use on electrical connections on car trailer plug ins) on his covers. In my opinion it makes them come off too easily, and I have witnessed them implode in heavy surf. On the other hand he has been using the same covers on his Valley Avocet for at least 7 years. I had to replace them all after 3.
Interesting. I was thinking this would be a bad idea due to having a petroleum base, but Wikipedia has set me straight with this paragraph:
“Silicone grease is commonly used for lubricating and preserving rubber parts, such as O-rings. Additionally, silicone grease does not swell or soften the rubber, which can be a problem with hydrocarbon based greases.”
From: https://en.wikipedia.org/wiki/Silicone_grease
I’m not sure it would have a better preserving effect than 303 and others, but I’d certainly try a small amount to lubricate the sealing surfaces.
How do these hatches “implode”? I’d think it would damage the hatch before that happened.
@BoozTalkin said:
Other than falling on the boat, I can’t imagine a scenario where my knee is on the hatch cover,
Raft up a group of kayaks on the water. The two persons in the outermost kayaks leave their kayaks, crawl over the other kayaks in the raft and sit down in the empty kayak at the end.
That is a pretty common exercise where I come from. The kayakers will learn how to form a solid raft (or someone will learn to swim), and their trust in the stability of a raft will increase. But I can assure you that there will be hands, knees and feet everywhere while doing this exercise. Everyone will be so focused on the task that they will not think about where they place their knees.
@trvlrerik said:
I have a friend that swears by using di-electric grease (the stuff they use on electrical connections on car trailer plug ins) on his covers. In my opinion it makes them come off too easily, and I have witnessed them implode in heavy surf. On the other hand he has been using the same covers on his Valley Avocet for at least 7 years. I had to replace them all after 3.
Silicone grease works great on rubber gaskets on you car like door seals. Very little and spread it thin and far. They sell tiny tubes like 2" or less. They also sell tubes like tooth paste which makes it much cheaper.
My CD Sirocco is about 12 years old and the Kajak hatch covers are just like new as is the boat. All I’ve ever done is remove the covers while the boat is stored and I use Armorall on them. They are still just as tight as ever. They are not particularly easy to get on and off, but with the right technique they work just fine.
@Sparky961 said:
How do these hatches “implode”? I’d think it would damage the hatch before that happened.
Usually happens during wet re-entry or heavy surf breaking on deck of boat. I have only seen this happen with oval covers.