Technically, according to the manual, the 155hp non supercharged engines are self draining which means the water in the exhaust manifold is supposed to gravity drain out when the boat is on the trailer. the S/C engines with inter-coolers are the only ones that need special attention and be flushed with antifreeze. But, to be on the safe side I always flush mine with RV antifreeze (155hp sportster).
What I do get an old bilge pump which is hooked up to a clear hose and a fitting on the end that screws into the flush port near the jet pump. I then pour 1-2 gallons of RV antifreeze (pink stuff since it is non-toxic) in a bucket, submerge the bilge pump. Next I start the engine, turn on the bilge pump and circulate the antifreeze until I see pink coming out the jet pump and exhaust port, takes less than a minute. Then shut the bilge pump off, a couple quick revs on the engine (too blow out the exhaust box as much as possible), then shut the engine off. I'll be winterizing our boats next weekend and maybe I'll try to post up a video of this procedure which is really pretty simple and can be home made with stuff you probably have in the garage.
In my opinion it is better to be safe than sorry next spring. Hope this helps!
--Deven
