I seem to have in my mind that a Shetland 498 has a Dory type hull, dunno why and if so, unless custom made for the hull, a roller coaster will not be suitable due to the hull shape.
I am not saying this with any authority or experience though, just tarnished memories and opinion.
I have a reputation for saying the wrong thing
Maybe a Shetland expert will crawl out of the woodwork with direct experience later.
It not the type of trailer, its how the rollers are set up on it (as far as my experience is concerned).
We had a 498 in the 90's. Our trailer was a break back with twin axle. Rollers were fitted and a hand winch. I could put it in and out anywhere, off the beach in shallow water, off ramps etc.
We always had a 50' long strong rope, in case we had to fully immerse the trailer and then pull it out with the rope, as my vehicle was only a people carrier.