John, it is narrow for a number of reasons. 1) The boat that will go on it is narrow! 2) We couldn't see the point of having a combi trailer for a boat that has to be lifted onto its side for rigging, as that implies that the boat is light enough to carry, which it is! So we didnt have to add the width of trolley wheels. 3) We bought a crossmember. If I was doing this again I would make one. The problem is that you need springs on the suspension units that match the weight of your trailer plus boat to get the best out of them. The smallest ones we could find were 350Kg rated, not ideal but will do the job. The only crossmember that towsure supply for the light weight springs is 915mm end to end, which with the addidtional width of the suspention units and wheel hangings gives a trailer a fraction wider than Flat Stanleys old launching trolley. Hopefully this will be fine for a 900mm waterline hull.
Phil, I am not sure I would like to bolt a road tariler together unless it was done with U-Bolts as per the Length adjustment feature of my new trailer. Even then, you will still need to do several welds for flanges and suspension mounts etc. And it would probably be best made in steel to make this easier. You are welcome to borrow my welding gear for Cherub related activities.
I will post a write up on the tech pages of the site in due course.