Assuming you have taken steps to protect your engine from the elements (see: How to change engine oil on a 4-stroke and how to winterise a 4-stroke: video) and the whole boat is snug as a bug in a rug on its trailer... take one last look at the top tips listed below before the frost bites, or your outboard could land you with a big, preventable repair bill.
Don't mummify the engine
I spotted this outboard engine last year (see photo) wrapped in an old shower curtain. At least this owner didn’t go all the way and seal the plastic with duct tape.
When you wrap an outboard in plastic you ensure that any moisture that’s under the cowl, or gets under the cowl, cannot escape through the vents in the cowl provided just for that purpose. This moisture, even if it’s just condensation, can start corrosion on electrical connections and moving parts like the throttle linkage. During the course of the winter there will be cold days and warmer days, the sun may shine on your motor, and the air under the cowl will need to breathe.
• Bonus tip: Before laying up the engine spray down the powerhead with a water-displacer like WD-40. Then secure the cowl but don’t cover it.
• Bonus tip: If you have your boat shrink-wrapped either don’t cover the outboard, or rig the wrap so that air can still circulate around the motor. And make sure the shrink wrap is not covering the vent of an inboard fuel tank.
Don't tilt the outboard up
One of the great features of an outboard motor is that it’s self-draining, but it’s only self-draining when it’s in a vertical position. If you tow the boat home on its trailer with the engine tilted up and then forget to lower it before storage, any water left in the engine will freeze and could cause damage. Another problematic possibility is that snow or rain water can collect in the barrel of the prop and migrate down into the exhaust passages of the gear case. Eventually it gets really cold one night and… POP! Your gearcase is cracked. Of course, this couldn't happen since you wrapped your motor in plastic and sealed it with duct tape… never mind.
See Lenny Rudow's video on this topic: Boating tips: outboard laying up blunder
Remove the propeller
If the boat is stored outdoors, you might as well put a sign next to it that reads: “Steal My Prop.” Besides, you’ll want to pull that prop so you can check the shaft for wrapped-up fishing line and remove it (see boats.com USA: how to remove fishing line from a prop-shaft) before it cuts through the prop-shaft seal and lets water into your gearcase.
Maybe the prop-shaft seal is already damaged. Well, now you know. Get it fixed over the winter when business is slow at the repair shop. The last step: slather that exposed shaft with some grease so it doesn’t corrode over the off-season.
• Bonus tip: If your back-up engine is small enough to carry, then unclamp it and put it somewhere secure. I keep mine in my office, so I can enjoy that outboardy smell all winter.
Don't dry-start the engine
I know your two-stroke sounds really cool when you start it up on the trailer, but there's no reason to start the engine without first attaching a supply of water – as per the instructions in your owner’s manual. Remember: your outboard is self-draining. Starting the engine to “blow out the water” even for just a few seconds can seriously degrade or destroy your water pump impeller, which relies on water for lubrication. If you really can't go without hearing that 2-stroke noise, get yourself a ride-on lawn mower. They're usually air-cooled!
For more about outboards, see How to clean your outboard engine and How to winterise an outboard engine.