How much stress on an engine in a jet boat?

[Mandelon]

Well that's it then. Thanks!

[jim@pj]

Hey before all you folks insult each other any further let me tell you how I see this boat vs. auto thing. I have been doing jet boat appraisals for twenty years and one thing I always tell my client is this. In my opinion, after tearing down many stock jet boat motors, with varing numbers of engine hours, I have come to this conclusion. One hundred engine hours in a jet boat is equal to 50,000 miles on a car engine. So if a jet boat has 300 engine hours, it would be like buying a car with 150,000 miles on the engine. I have not seen a jet boat engine with 400 hrs that was not in need of a rebuild.
Jim www.performancejet.com

[TopCat]

the load on a pump increases the faster you go loading it to the point it becomces inefficent causing so much pressure it slows the impeller so if you could come up with a variable sizing intake you could make your boat really fast ......but to answer the question marine use is more than 5 times more stressful..when was the last time you saw a boat with 100k miles on it
[ February 23, 2003, 10:05 PM: Message edited by: TopCat ]

[wfodude]

jim@pj:
Hey before all you folks insult each other any further let me tell you how I see this boat vs. auto thing. I have been doing jet boat appraisals for twenty years and one thing I always tell my client is this. In my opinion, after tearing down many stock jet boat motors, with varing numbers of engine hours, I have come to this conclusion. One hundred engine hours in a jet boat is equal to 50,000 miles on a car engine. So if a jet boat has 300 engine hours, it would be like buying a car with 150,000 miles on the engine. I have not seen a jet boat engine with 400 hrs that was not in need of a rebuild.
Jim www.performancejet.com Can I get an amen brother
smile_sp

[LVjetboy]

Topcat,
For the run above, about 100-150 rpm loss from hookup to finish, which is approx. 5% increase. Considering full hookup near 50 mph and finish well over 100 mph, I'd say that's not significant compared to overall numbers. Which is why I chose the words for intake flow effect on engine loading vs. speed as "not significant" For your typical lake boat topping out in the mid 60's to low 70's, the term "total wash" maybe even more appropriate.
A variable inlet could recover some, and would be interesting to play with. But likely not worth the added complexity even for a racer, unless jets start running a lot faster?
As for the original question...yes, I think that's been answered quite well by now.
jer
[ February 24, 2003, 03:02 AM: Message edited by: LVjetboy ]

[Hal]

Jer, You got one fine looking boat...

[bp]

yeah, but he's gotta learn to keep it in the water, so it'll go faster.
that data sheet is gonna get worn out from all the cutting and pasting. no one has answered this question yet.. how can a pump NOT be loaded, and have bowl pressure climb at a significant rate?

[LVjetboy]

Thanks Hal.
Ok, I'll venture a guess. Let's see...the bowl pressure was the green thingy right? Well, how about if the impeller is partially loaded during the rev limiter bump with zero intake pressure.
Then as speed increases, that partial load goes to full load as the intake becomes more effective. In this case bowl pressure could still increase along with impeller loading to the point where available engine hp begins to limit rpm instead of the rev limiter. In other words, an impeller spinning at rev limit with 50% cavitation generates less bowl pressure than that same impeller spinning at rev limit with 10% cavitation. The point where the rpm drops to a steady 6750 may mean full loading. At that point there is very little change in loading to the end of the run.
Just a guess,
jer

[jim@pj]

Hey... before you guys insult each other too much over motor stress in a jet boat let me relay what I have found. I have torn down alot of jet boat engines over the years with various running hours on them. What I found was, the average jet boat engine takes a good beating depending on the owner. I have been appraising jet boats for many years and I always tell my clients that 100 hrs on a jet boat engine is at least equal to 50,000 auto miles as a rule of thumb. I very rarley see a stock jet boat engine with 400 hrs that does not need a rebuild. Jim

[jim@pj]

Boy ... I'm losing it... I couldn't find my post so I wrote a new one, only to find out that I was on the wrong page ! I need a beer ,
.. Jim