London Great River Race - cushion or not?

[Noseve]

I am rowing a fixed-seat gig in this year's race, on September 14th. Twenty-one miles up the Thames, about three-and-a-quarter hours, although there will be some rotation, with four oars between six crew (tradition dictates that a passenger must be carried).
Although I have never rowed with a cushion, once whilst rowing a near half-marathon on my c2 I did suffer from real backside pain. It wasn't a case of being able to 'man up', I simply couldn't function as a rower until I'd walked around a bit, and even after that the pain soon returned.
The thought of letting down my entire crew on the big day is not a good one.
So, to any of you with fixed-seat rowing experience - should I use a cushion?
Any personal anecdotes would be welcome. Thanks.

[Cyclist2]

The only "fixed seat" rowing I've done is on my erg, but as far as cushions go, the shape is more important than material or thickness. If the seats in your boat are flat, or nearly so, you'll need something to take the pressure off your sit bones and tailbone. Look at rowing shell seats.

Since you won't be bending your knees, the sit bone relief isn't as important as the tail bone, but would still ease some of the pressure. So make a pad that looks similar to a rowing shell seat and try it out, adjusting before the big race. You won't need much thickness (which actually can make things worse).

Good luck, that sounds pretty challenging!

[Noseve]

Since you won't be bending your knees, the sit bone relief isn't as important as the tail bone

That could explain why I don't feel any pain after over an hour on the water - as yet, at least. Useful to know.

[Noseve]

UPDATE FYI: Cyclist2 was right on the mark - I completed the row, without any cushioning, without the slightest hint of pain. Interesting experience, but not sure whether I enjoyed it enough to do it again, though, as it's not the cheapest day out.