Toll Free Information Hotline 1-800-309-5190. Sales hours: Monday - Friday 8:30a.m. to 4:30p.m. Central Standard Time. They are suitable for STORAGE, MOORING, AS WELL AS TRAILERING. The cover is made from marine grade polyester canvass. The cover is made from 230ml/600d/600 denier solution dyed polyester. The fabric is then coated twice with polyurethane on the bottom or downside, UV protected, and mildew protected. What this all adds up to is the best cover available. It is nearly twice as thick as what you see readily available commercially. It is designed to water out, as it is WATERPROOF. Yet, water vapour can permeate it, so it will moisture out, but will not moisture in! 600d is an indirect measurement of density and weight. The higher the number, the heavier, thicker, and heavier duty. This is the thickest cover you will find. You will see a lot of 300d, even 150d covers, which are proportionally half as thick and 1/4 as thick as our covers. These are very waterproof. These will collect water. If you are going to use this for storage or in any condition on your boat where rain could collect on it, we would highly recommend some type of support system. Support systems are readily available online. It does not have to be anything incredibly complex..just enough to create and arch so it can shed rather than collect water. A very common and economical way to do this is to build a support system or 'skeleton' with pvc pipe. Grey electrical conduit works very well and is even less expensive. SPECIFICS FOR THIS COVER: This particular unit will fit a pontoon or deck boat with deck or playpen area between 18"1" ft and 6.1m with a beam of up to 102". You can trailer with it, but you have to eliminate the movement of the cover. That is nearly impossible, and it is generally better not to trailer with this or any cover.