At a rough guess, I'd think the rigged boat and motor is 750 ish kg, without the trailer. But as your in Poole, why not run it down to the weighbridge and get a true overall weight.
We've used the one in the Port before, as well as the one at the metal recyclers on the Nuffield. There's also one in Dawkins road in Hamworthy, at the sand and ballast yard.
Chaos for Moderator.