Who knows jet boats?

Dude_Himself

Looks familiar:

The first one I flipped in college 2003: fuel pickup fell off in the Gulf of Mexico - that's the north side of an abandoned Key (forget which one) in the distance. We filed a float plan, were within site of the Navy base, under a numbered electrical tower, and called with our exact position but no one ever came and the cell phone eventually died. Waiting for dark to set off the distress flares, but considering the very real possibility we'd die of exposure after our gallon of water ran out. We were rescued shortly after I walked us 4 miles to land.



Numero Dos: the day I sold it.



Number 3: cause it had been left open, with the drain plug in, all winter. $2k I should have never spent.



Number 4: rafted up on the Potomac near DC:



Number 5 - the last I have photos of:

Liberty44140

glad to know that you know this boat so well Only two of the pics came in but it does look familiar. 

Liberty44140

Dude thanks for all of the info. I talked to the PO about your comments so that I know what I have to do:
- He said that he did grease the shaft zerk last fall when he did the impeller lube change so I am good for this season.
- He did remove and rebed the pump and shaft when he did the engine in 2015.
- He did not replace the fuel lines because he said the ones that were gray and flakes were only on the Jet Ski's, he said the USCG made them use something better and different on Jet Boats so it was less of a concern. I'll probably want to change them over the winter but doesnt sound like it is a must do today.
Thanks! 

Dude_Himself

That 787 should have a carbon seal on it from the factory. There's a grease fitting for the shaft where it meets the engine, and the pump is pressurized with lube to prevent water intrusion. That's likely what he meant.

The carbon seal is a stainless disk and a carbon ring - pressure is maintained by a rubber accordion hose, so water shouldn't be able to get in easily.



The problem is a jet pump creates a vacuum - not pressure. The accordian hose, while under vacuum, pulls in a little and lets air from inside the boat into the jet, creating cavitation. It wasn't as big a deal on the smaller jet craft because they weight a lot less - so they generated less vacuum and the cavitation impacted performance less. On a jet boat, with 1000# more weight, cavitation in a drive chain that's already 20% less efficient than a propeller is your enemy.

The older jet boats used a grease fitting to overcome this - but it was challenging design as new boaters (Seadoo's largest market share) didn't grease the fitting regularly and it would seize. When it seized it would rip the hose off the inlet shaft, and you'd have a 3" diameter hole 1' below the waterline - the boat would sink.