The SRN4's were great for a quick crossing if the sea was reasonably calm and I used them many times from Pegwell,one crossing was 'interesting' when in the middle 'third' of the crossing in open water we encountered fairly heavy seas,the 'Hovercraft' had to slow right down and became a 'Bobbercraft' - closely followed by mucho vomiting by many pax,of course unlike a ferry - there was nowhere to escape to for fresh air,my travelling companions were quite ill LOL.
Once we got back into sheltered water the Bobbercraft could again lift its skirts and accelerate.
Later in life I used the high speed ferry from Newhaven which was probably a better solution for a 'fast' crossing.