trig42:
The boat isn't 'that' heavy I don't think (4.2m Tinny with a 40hp motor) but its axle weight is obviously more than a corner of the 1.9t Kia.
I thought about AA, but this happened north of Coromandel on Labour Day - would have been a very long wait. I would have done it if the bloke didn't pop out of his driveway with a better scissor jack than I had - he had no issues winding it up.
I think I just need to find a 1.5t jack - that will be fine (and also hope, now that I've replaced both tyres on the trailer, that I don't need to do that again for some time).
You're right - that is likely not heavy at all. My mate has a 4 metre tinnie and without the outboard attached, the two of us can lift it onto his trailer with a bit of effort. Your mileage may vary if you have remote steering, seats, plus the weight of the outboard, fuel, oars, fishing equipment etc, and I know that modern welded aluminium tinnies tend to be quite a bit heavier than older riveted ones like my mate's. With the weight of your 40 horse factored in, I'd say maybe 500kg for the boat as a starting point. I think you'd find your trailer wouldn't be more than 250kg. - but again it depends how solid it all is. I'd be interested to see a photo of your setup to get an idea.
So yea, maybe 750kg as a total guess without seeing your particular rig. That's pretty light so I don't think you need to go too hard out looking for a heavy duty jack. If you think a small trolley jack just isn't for you, then I'd suggest getting any decent quality scissor or bottle jack to replace the broken one for the car, and as someone else suggested, carry a sturdy piece of timber to set it up on - either a piece of 15-19mm thick flooring ply, or an oversized piece of framing timber such as 240x45mm pine. Best to avoid jacking up on a main highway if you can avoid it - passing trucks etc have a nasty habit of blowing your trailer off the jack. This happened to our caravan and would have been a nightmare if a Unimog full of NZDF soldiers hadn't seen our plight and stopped to lift the caravan up for us using nothing but their arms - nice buggers they were.
My suggestion is an old Toyota Hilux/Landcruiser jack. People swear by them as they use old school mechanical screw gears, they're light and compact, they never fail, and they can lift quite heavy loads. I have one in my old Hilux Surf from the '80's and it's a beast. Below is a selection of videos of people singing their praises: