Here is a problem when the front end of the trailer is too heavy for an extension. In this case, a set of robust dollies under the main hitch would be required.
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For a trailer that needs an exceptionally long extension, there is often some form of third wheel mounted to the front of the trailer. We have several boats with fixed fin keels with 4.5' of draft (Impulse 21, Santana 20, J/24) in our club, and their trailers have 25' long extensions. The flimsiest wheel on the front is a trailer jack, although a beefier model. Others have a jockey wheel which pivots down from the trailer, and has a pneumatic tire. That's a pretty damn nice trailer, too. Triad Trailers seems to be a pretty good brand for building this kind of stuff into a trailer for a deep keel boat. The worst trailer, unrelated to the extension or jockey wheel issue, has a keel tray (where the bottom of the keel rests while on the trailer) that is not too much longer than the actual bottom of the keel. Every year at haulout, we have to do all kinds of crazy cranking on the bow of the boat to pull it forward and up on the bow stand, because as the boat pulls out of the water, and begins to level not with water's surface, but with the actual trailer, the dang keel tends to slide back on the tray. If it's maybe 1/2 way off, we can get it pulled forward, but I really just wish the owner would get a longer damn tray welded on during the summer when the boat is off the trailer. I've been through this with him 4 or 5 years, and each year there are no trailer modifications. I suppose I should count my blessings, as 2 years ago I shamed him into buying a new carburetor for his outboard, so now he can successfully start the motor and run it to the ramp, as opposed to the previous 3 years when we had to tow him to the ramp every time. Ahhh, he's a good guy though, and I wouldn't trade him!