12YJ, Technically Speaking

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« on: August 26, 2012, 11:20:28 AM »
Hi all,

This may be my first post here, but I'm not oblivous; I know "YJ" is code for "duck and cover" in most performance arenas and to some is a 4-letter word in at least 2 ways.  But, I run one, and I'm interested in knowing more about its limitations (and strengths, if there are any).

I've seen a good amount of generalizing about the YJs, but little technical backup.  That's not to say I'm disbelieving, I'm not trying to defend the YJ as a performance pump or anything, I just want to know WHY the limitations exist. 

So I have some questions on some of the common verbage that pops up every time someone wants to know if a boat w/a YJ is worth looking at, can go fast, can be serviced, etc, etc... Let's allow things to be theoretical (just imagine there's a deranged fabricator in the mix); for example, say things like a smaller or larger nozzle, higher cut impeller, etc were possibilites, what would change?

So, statement 1:  "The YJ does not respond to horsepower over 350/400".  Why is this? To me the most likely reason would be A) the axial/open impeller design can only create a limited amount of head pressure and basically "slips" beyond this output.  Another would be B) impeller cuts only go so large, such that to absorb more HP you'd have to run at too high an RPM for either the engine or cavitation. 

It seems A could be solved with a larger nozzle, and B could be solved with a smaller nozzle?

Statement 2: "The YJ is good at doing what it was designed to do; getting heavy boats on plane with low HP".  I think I've mainly seen this statement made by Tom at JBP, so if he wants to chime in that would be awesome.  My main question here is in the relativity of the word "good".  Let's say you had a heavy boat with low hp. Keeping all other factors the same, would the YJ get it up and going faster than an energized YJ?  If so, why?  My understanding is that the mixed-flow pumps are more efficient--which to me means more thrust per HP--so why would the YJ be good at anything compared to a mixed flow when it all comes down to thrust?  Or, is it that the mixed flows are only more efficient at higher hull speeds where the axial jet can't impart enough discharge velocity? 

Any technical discussion or experience with direct comparison of the YJ to an energized YJ or different jet behind the same motor would be welcome.  I have little interest in an energizer kit, but I would be interested in modification (even extensive) to the YJ itself, if beneficial.  I'm sure many would say to just drop in a berk if you're going to go to such lengths, but I enjoy the engineering and fabrication challenge of creating something unique as much as I do the end result in performance.




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