These pics are of a double Harbeck trailer. At the time it belonged to Lin, but now resides in New York with the Gorbolds. I was about to construct an aluminum version of the dolly (see elsewhere) and this was to give me a perspective on the balance points Harbeck worked out.
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The double accomodates the bunk or the strap dolly. The forward upper bunk is positioned to take the load approximately under the V bulkhead.
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The newer strap dolly did fit onto the trailer, however the pin holes anchoring the trolly did not line up , hence the strap (orange) as a quick fix. Notice how narrow the entire rig is.
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As much as they thought out the upper structure, the cross support bar was a half inch too close and would rub the lower boat. Not enough to scrape the boat, but the boat cover could be damaged if padding was not positioned properly.
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So these plugs are all the rage in Europe, but try to find an adapter for this in the US. (Hint, try Volvo)