I believe that there is no better investment that you can make in the preservation of your boat than a good winter cover.
Even the most enthusiastic of us here in the northeast only have a six month boating season. That means that protecting your boat from the elements for the other six months is critical. Even if you back-off the 'shoulder' months for decommissioning and spring preparation, there are still four months of harsh weather with radical temperature changes for which you must provide protection for your beloved vessel. I refer here to boats that are 'dry-stored' or hauled-out of the water. Much of what follows, though, is applicable to the covering of boats that are left in the water, or 'wet-stored', too.
So what are the options? If your boat is small enough and/or your cheque book large enough, indoor, heat and humidity controlled storage is undoubtedly the best option. If the boat is winterized, the temperature doesn't matter; but what ever type of winter storage option you choose, the proper humidity level is critical. A humid, damp, poorly ventilated building is much harder on a boat than outside storage with a proper, well ventilated, winter cover. The goal is to prevent mildew and to keep decks, cockpits and bilges dry and free of ice and snow. Conversely a building that is too warm and dry can be damaging to a wooden boat and the woodwork on a fibreglass boat. So care must be taken in the selection of the right storage facility; but proper indoor storage may also afford better security and allow for maintenance to be carried-out in any weather.
For most of us though, outdoor storage will have to do. In this instance, a good winter cover is essential. I can't tell you how many times I have inspected boats that were left uncovered only to discover that they have bilges full of ice and deck-heads covered with mildew. Nothing turns prospective buyers off more than that combination. As well, once a boat is full of ice it is virtually impossible to do anything about it until spring, or at least until a major thaw.
I do acknowledge that a poor winter cover is perhaps worse than no cover at all. An often seen example is one where an u-stepped mast is used as a ridge-pole, and then a cover, usually a cheap poly tarp, is spread over the mast, over the lifelines then tied down around the topsides. The first late-fall rain storm causes large pockets of water to collect on the cover between the mast and the lifelines placing untold strain on each only to turn into enormous chunks of ice with the first freeze-up. If the next survey shows the deck around the stanchion bases to be stress-cracked with high moisture content there would be little wonder. Besides that, the mast and stanchions may not withstand the weight and survive the winter anyway.
I am not in favour of using masts as ridge-poles for winter covers. Masts are not designed to take the strain of the snow-load on a boat; and this is not to mention the difficulty that the mast and rigging present in allowing one to move around on deck. A proper ridge-pole is, however, a good basis for a winter cover frame. One can be fabricated from metal tubing (of adequate size relative to the span), laminated of wood or even, as mine is, simply two (4" tapering to 2") spruce trees, scarffed at the butts with the slender ends fore and aft allowing for considerable strength as they are tensioned from two vertical points. No matter what material you select, adequate vertical support obviously necessary. Some boat owners fabricate elaborate truss systems to support their winter covers. Others purchase or make for themselves tubular cover frame systems. Any of these can be fine if made adequately strong and secured such that high winter winds do not cause them to shift and damage the boat in the process. Make sure that the ridge-pole is high enough to afford adequate slope of the cover to encourage the snow to slide off of its own accord and lessen the snow-load.
By the way, one of the best things that I have bought for my boat is a 'Roof Rake', intended to help remove snow from the roofs of houses from the ground. They are perfect for dislodging wet snow from a boat's winter cover too.
Once you have your frame constructed there is the question of the best material for the winter cover. I strongly favour heavy cotton canvas duct. I do so because of its strength, its durability (both in terms of chafe-resistance and long-life), its disinclination to move in winter winds, due to its weight, and its ability to 'breath'; and therefore discourage condensation and improve ventilation. The cover I had made for my 34' ketch is made of 18 oz. cotton canvas duct. It is admittedly heavy but it is also very strong and secure such that I can leave all of my ports open including the vertical part of my companionway; and hatches are propped open to further enhance ventilation. Winter winds just blow through the boat and she is always fresh and dry. I also admit that a fitted canvas cover is not inexpensive; but the last one I had lasted for 25 years. Amortized over that period, it is a cost-effective option compared to most other alternatives. It is important to make sure that it is dry when folded for summer storage and that care is taken to prevent chafe when it is on the boat to maximize its' life-span, however.
There are a number of manufacturers of winter covers that can also provide tubular frames as well. If you have a production boat, these makers can often supply both cover and frame tailored to your boat at short notice as they already have the measurements. Some offer a variety of fabric options for the covers too. The choice of fabric and its weight will depend on your location, wind exposure, expected snow load and so on.
For many people, poly tarps available in various sizes and weights will be their choice. This is a mediocre option. Some of the problems with them can be mitigated by the use of netting to hold the light poly tarps down and the arrangement of adequate ventilation, however. All you have to do is go to a boat yard on a windy day in winter and hear the trashing of poly tarps to discover their short-comings. I cringe when I hear the racket that they make and think of the damage that they can cause.
My least favourite means of covering boats is shrink- wrapping. In the absence of recycling facilities, shrink-wrap is an environmental disaster in my view. Besides that, it's costly, if convenient, and the material does not breathe; although vents can be added. If you choose this alternative, be sure to specify plenty of them. I acknowledge that if they are properly installed, shrink-wrap covers shed snow and ice well and due to their 'shrink-fit' move not at all in winter winds. Boats covered in this way have to be more carefully monitored for condensation and mildew, though. In the heat of spring and summer, make sure that the shrink- wrap does not closely cover hull and deck gel-coat. I have seen heat- blisters of the gel-coat occur due to a heat build-up between the shrink-wrap and gel-coat (particularly if one or both are dark colours). Poly tarps can cause this problem too. Both should be removed in the spring before the weather warms-up.
No matter what cover material you choose, make sure that there is provision for access to the boat and its interior: a "door". There is nothing quite as disheartening as finding, in the midst of wind, snow and ice, if you absolutely must get aboard, that you practically have to uncover the boat to do so. Easy access will encourage you to check your boat more often to see how she is wintering. It might also encourage you to do some of those needed jobs aboard in the warm weather of late winter. If your boat is for sale, this is even more important. Easy access will make for easy viewing by potential buyers and will result in your broker being more motivated to show her.