It helps if you can remove your stanchions and rails, but it is not 100% needed.

The method is simple, create a series of hoops, using alkethene pipe the sort of black or blue heavy walled water pipe used on farms or building sites for semi permanent supply.  If you can match the outside diameter to the diameter of the sockets left on your deck, or the inside diameter to short lengths of copper pipe in those sockets all the better.  I used 1 & 1/8th inch OD pipe.

You can simply lash the pipe to the guard rail stanchions and also to the pulpit and stern rail. 

Trick is to make the hoops high enough to get under to work on the deck and cabin top, without making it ridiculously high and likely to catch every gust of wind!  Obviously the hoops will all have to be different lengths to ensure the ridge pole is level.

Along the centre line, for the ridge pole, I used a length of guttering pipe, you could use timber or whatever.  Best not to use the mast if you can avoid it, too many sharp bits to chafe through the cover. 

Lash the hoops to the ridgepole with thin line, being on the farm I used bailer twine, lasted for years under a cover. In addition I took long lengths of bailer twine and starting from the bow, joined all the hoops together, clove hitching each hoop in turn.  I used several of these lines each side, to make more support for the cover between the hoops and to stiffen the structure up a little.

Cleverly Nigel has used electrical ties to simplify fixing the hoops to stanchions, what did we do before them!

Then I covered the lot with poly tarps, I used light green to make it blend in with the greenery nearby, but it was a little dark underneath, clear covers allow the light in better.  I had several overlapping covers, and over the course of several years whilst  out side, I placed new covers on top of old, the older ones acting as chafe guards.

If you can leave a gap either end to allow ventilation, without the risk of rain penetrating, all the better.

I related this to Nigel last Autumn and he has sent in a couple of pics of 'Otteau' under such a cover.  She was left for several months and when the cover was opened up, there was no wet aboard!   Below are a couple of pics of 'Otteau' under this sort of cover.

As you can see there is plenty of room to work on deck and the cover can extend right down to the keel to protect all the woodwork etc.  Nigel used two layers of poly tarp.  I suspect all can be rolled away and stored for the next year too!

John Williams, pics by Nigel Seary