High hour 496 loosing power
 

So a friend of mine has a Nordic Heat with a 496 mag ho and 910 hours. 10 years ago he was hitting 67 mph with a labbed prop. Summer of 2011 he was at 64 mph and last fall he was hitting 62 mph but this summer his speed is down to 58 mph and he struggles to get on plane. In fact with 5+ people on board he needs full tabs down and it takes him over 30 seconds to finally see the nose dropping. Obviously, he is going to need to repower soon but will it last the summer? How does an engine die? Will he eventually just loose more compression and not be able to get on plane at all or will the engine eventually just not fire?

With that many hours it could easily be down on power from a compression loss. Only a comp test and leak down will tell for sure.
It could also have some hook in the bottom that has steadily gotten worse over over time because of the trailer or lift it sits on.

I think Griff is right about the Compression loss. Piston rings and/or cylinder walls are just worn out.
As far as engines dying goes, this is pretty graceful...

Isn't 750-1000 about time for fresh'n up on a 496??

I agree with Griif, Pull a compression test and if all of the cylinders are close then it might last the summer, but if one is way lower causing the loss of power then it will probably be junk when it finally blows, It is an easy rebuild right now, but he would be without the boat for probably the rest of the summer unless he just buys a long block and replaces it.

Loss of compression from worn rings would probably also cause excessive blowby and high crankcase pressure. A lot of times this will force the dipstick out of its tube. You will also see a lot more oil blown out of the breathers. Is he seeing any of this? If it is worn valves, he might not see as much of the blowby, but it would probably idle rough. If it seems to run fine at lower RPM's but runs out of steam on the top end much more quickly, the valve springs may be losing pressure. If this is the case, he could be in danger of a broken valvespring and possibly a dropped valve - which would be catastrophic. I would run a leakdown test on it to try to hone in on the power loss. Probably a good idea to play nice with it too, if he wants to limp through the remainder of the season.