I've got a Mercury 25EL that is less than a year old. Today I was on the Potomac river, and the engine was running fine in the morning. On the return trip when I would advance the throttle to a point somewhat above 10 MPH (get on plane) the thrust would quit and the engine speed rpm's increased greatly. When I got the boat home, I noticed a greenish oily slime on the prop (some kind of lubricant--perhaps lower unit lube?) Coincidentally, it was really weedy where I was fishing. When the problem first arose we stopped the engine, pulled it up, and removed some weeds that had wrapped themselves around the lower unit. Is this as bad as it sounds? Any ideas would be appreciated.

Remove the prop (and thrust washer if it has one) and check the prop shaft to make sure there is still no weeds or fishing line wrapped out it. Sounds like it has gotten under the seal and the LU lube has gotten on the prop. Loaded up with weeds, the prop may have slipped and with lube on it, that's all she wrote, it spun the hub.

Change the LU lube and make sure no water is in it. If it is the seal, it will need to go to the shop to be pressure check and for repair.  If they find weeds/line wrapped pulled under it, they may not want to repair it under warrenty, depends on how your dealer want's to take care of you.

Ben,

I took the prop off and there were no weeds or line under it, but there was what looked to be traces of lubricant. I had some line wrapped around it several months ago and removed it, and it ran fine after that--but it may have done some damage to the seal that I was unaware of. I changed the LU lub this morning and there was very little that drained into the pan, and what there was had a cloudy appearance to it--so it looks like I'm off to the dealer to have it looked at. Thanks for the advice--at least I won't sound stupid when I get to the dealer.

Line will take one out in a heartbeat.  Probably has been leaking ever since and the weeds making it spin the hub may have saved you from needing a whole LU before much longer.  A word of advise. Play country dumb about anything ever being wrapped around the prop shaft. If some didn't get under the seal and stay there, they may just fix it.