Hi Graham
I opted for an electric winch from Warrior and having used it 3 times I can say its absolutely fantastic, entering into the Frome at right angles to the flow, albeit not very fast but still pulls the boat downstream I have threaded the dyna rope through the roller coaster trailer and then pull the boat towards the rear rollers, then small gentle tweeks on the remote control and the boat will pull itself inline with the trailer and then I simply put it into fast mode and the boat glides on, finally attaching the winch strip around 2 foot from the final position to remove the electric winch hook.
Extreme supplied the mechanical support with the trailer, the winch is from Winchmax and a 3000 but it only ever pulls a couple of hundred KG force according to Warrior..