The biggest downfall to a cover, and believe me I've been through a bunch since I've owned the 170, is the frame around the windshield and anything else you have that has an edge (doesn't even have to be sharp) that a wet or dry rotted tarp can rub against. If the tarp is not supported correctly a sudden downpour (we get like 5"/hr here) will find the unsupported area and fill it with water which will weigh it down with like a bath tub full of water. This will stress against anything that has an edge(windshield, trolling motor, engine, even the cleat on the deck above the anchor locker, etc) and that starts the demise of the tarp/cover.
If you can afford it do yourself a favor and save some long term money by getting one custom made now in sunbrella or I think there is a new material out there (took me 22 years to finally break down and do this - after a cover every 2 years). The main thing is that they make it to fit one way - motor in the same position, trolling motor in the same position, etc and then they re-inforce where they need to, to make sure it doesn't "pond" with water and rip. My 230 has poles installed and reinforced, my 170 does not - but they used the custom brace/guard I finally figured out to take the edge off the windshield frame, so they didn't need poles. Great investments in the long run but a little pricey at the moment.
To give you a rough estimate it cost me about $800 for the cover for the 230 and $600 for the 170 - seems like a lot but I was buying a $125 cover from walmart every 2 years and I'd been through tarps and all kinds of covers for it in the 20+ years I've owned her. These should last 12-15 years and will be low maintenance versus me using a 2% mixture of bleach to keep the mold off the inside of the store bought covers every couple months (that's another demise of the cover but it was getting black from the mold).
My $.02
