Just read through this entire thread. I believe the fundamental topic as related to this whole "cone" discussion, is nothing other than the larger, after-market or non-stock 'cone' creating increased water pressure around our jet-ski or boat's impeller(s). Maybe I missed something and someone already pointed this out.
If you've ever spoken to any US Navy submarine folks, or read up on sub history & development, one related issue (to this thread) that comes up, is emitted noise from cavitation of the submarine propeller. Please bear with me.
Emitting more noise in a sub, makes you much easier to detect / puts you in danger. Scientists / engineers / naval architects, etc. who work on subs or sub designs, go to great pains to minimize noise in general, and noise from propellers in particular. Props that cavitate are VERY noisy. Cavitation in a sub prop must be avoided as much as possible.
So,
- Cavitation in submarine prop is NOISY => a big no-no
- A given sub prop can turn more RPM's (and the sub can go faster) WITHOUT hitting the threshold of prop cavitation when the sub is moving at DEEPER depths. Why ? DEEPER depths = INCREASED WATER PRESSURE. Increasing water pressure around the prop reduces the tendency for the prop to cavitate. Likewise, if the sub moves to shallower depth, with lower water pressure, the crew must be mindful of the fact that, the prop will begin to cavitate at LOWER RPM's than it otherwise would, when running at deeper depths with higher water pressure.
- The instant our boat impeller(s) spin up, the bigger or after-market cone(s) placed at jet pump output(s), 'RESTRICT', to some degree, the outflow of water from the pump, meaning the water pressure around the impeller will rise faster after impeller starts spinning up, & the 'steady-state' pressure around the impeller, once it has spun up, would be higher than it otherwise would, without the bigger cone.
This tends to reduce cavitation.
Of course, there is 'no free lunch': restrict water flow TOO MUCH or in the wrong way, and you get more 'losses' to overall pump efficiency. This explains why there is so much back-and-forth with tuning i.e. spending a lot of time and effort tuning and optimizing the spacers to make the cone occupy or displace a 'bigger or smaller' volume, changing impeller pitch, jet pump output diameter, etc. with trying to balance or straddle reduction of cavitation, while maintaining top speed.
- Even WITHOUT any 'cone' present other than the stock one, other people who have posted on these pages, as well as people with lots of experience with this stuff (i.e. company called "Group K" in Arizona), have commented about how different impellers affect cavitation. For example, Group K has a posting, wherein, from their work with some types of Yamaha 2-stroke jet boats, they found that the SOLAS "Concord" or Skat-Trak "Swirl" impellers generally cavitate less than the stock impeller does. So just the impeller choice alone - without changing the 'cone' setup - can have a big impact on cavitation. See this link, and scroll down to "About impeller choices".
http://www.groupk.com/yjetboat.htm
- The submarine propeller people go to extreme lengths, and spend a lot of money, to tweak and refine their prop designs to minimize noise, including minimizing cavitation noise. What exactly works best are closely guarded secrets. In the 1980's, the Soviets were able to surreptitiously buy very sophisticated metal-milling machines they did not have 'in house', from Norwegian firm Kongsberg / Japanese firm Toshiba - with these machines later used by the Soviets to more precisely shape sub propellers so as to minimize noise / cavitation. In short order, The West / NATO were not happy to find out that Soviet subs had suddenly become much quieter and harder to detect.
Analogy: "Stock" versus Solas versus Skat-Trak impellers - they are all slightly different with different blade shapes, different blade areas, different surface finishes, and they behave differently, including how and when they will begin to cavitate.
projectalpha.eu
" The consequences for international security and US-Japan relations must also be considered. In pursuing just $17 million and $10.4 million worth of business, respectively, it has been alleged that Toshiba Machine and Kongsberg caused somewhere between $1 billion and $100 billion (1980s prices) worth of damage to the US Navy. The sale was alleged to have made Soviet submarines twenty-fold quieter and much more difficult to track in a very short space of time. The actual cost of the damage to western interests is difficult to determine. In terms of US-Japan relations, the actions of these companies put the relationship of the two allies under serious strain. "
Anyway, in relative terms, we with our jet boats or skis are just barely scratching the surface of all of these topics. You have a lot of Physics / Fluid Dynamics stuff going on here, people. ( OK, Fluid Dynamics in a 'non-compressible fluid' ).
We have neither the expertise nor the resources that the US Navy has.
For me, running a 2006 Yamaha SX-210 at 6300 foot elevation? My boat is very under-powered (due to altitude, it's just fine at sea-level) and I would be happy with any reasonably-priced 'bolt-on' improvement I can get, to get better 'hole shot' and top speed. $600 or so for a pair of 'cones' for my twin-engine boat might be worth looking at, but given that price tag, not before I get totally squared-away on the impellers. (And after squaring away the impeller choices / pitches etc., if I added a cone, would certainly not want to have to go back and re-pitch the impeller(s) again after addition of the cone(s), nor would I want to grind out metal from the jet outlet pipes to get another millimeter or two of effective outlet pump cross-sectional area --- just too much work and hassle - would be better off just using fewer 'spacers' on the cone discussed here, to have less restriction of jet pump water flow, and less reduction of engine RPM......rather than grinding metal......)
Steps to take ?
A) Re-pitched stock impellers
B) after-market impellers ('standard' pitch as they come, or re-pitched)
C)"cones" like those discussed in this thread,
D) better air intakes to the engines, or other relatively simple engine mods to buy back a little horsepower ?
E) Cleanup of pump intakes, including proper sealing, 'smoothing' of non-ideal bumps, cracks or steps to water flow path?
Any other suggestions ? And what's lowest-cost way to get improvement(s)?
QUESTION: does anyone know, what the Power Output vs RPM curve looks like, for the 'budget' engines used on the SX-210? Factory says, these are 1052cc MR-1's, but mine (like all SX-210's) have choked-off air intakes, with only 110 hp rating (at sea level). Asking the question in a different way - as long as we stay below the RPM limit set by the engine's Rev Limiter (have no idea what the limiter is set to), is there any advantage in terms of increased engine output power by running up at 8500 RPM, instead of 7800 RPM? Or is there nothing to be gained by running the RPM's slightly higher?
So, for my first step, A), re-pitched 'stock' impellers, back in 2007, when the boat was pretty new and I was disappointed with the 6300 foot elevation performance of the boat, talked to Yamaha Customer Service (they were helpful):
1) Yamaha offered to pay the LABOR ONLY cost of having the 'stock' Yamaha impellers re-pitched by Group K in Arizona
2) Per Yamaha, I had to purchase (out of my own pocket) a NEW set of stock impellers and have them shipped to Group K (so I have two sets of 'stock' impellers, one original, the second 'new' set, now on the boat, were re-pitched prior to install by Group K)
3) the guy at Group K ( named 'Gerhard') nearly hit a 'bulls-eye". (By the way, I have no relationship to, or financial benefit from, Group K). He said, that he was targeting 7800 RPM for the re-pitch. Have a look at the attached photo of my tachometers:
a) ORIGINAL configuration, stock impellers, 6300 foot elevation, wide-open throttle: could only hit ~ 7000 RPM each engine, ~ 34 mph top speed
b) Photo: with Re-Pitched impellers, re-pitched by Group K: Port 7900 RPM Starboard 7700 RPM Speed 36 mph - so improvement to both 'hole shot' AND top speed
These re-pitched stock impellers are now > 12 years of age. Got a few nicks and pits on them. About time to replace them.
So, for Option B), after-market impeller(s):
I am going to try out a SOLAS Concord impeller on ONE engine to see what sort of improvements (if any) are to be had. I picked the YF-CD-11/14 Concord (lowest, least pitch I could find) for the STARBOARD engine, as the starboard engine has to 'work harder' than the port, due to combination of direction of rotation of impellers with how water flows across bottom of boat on starboard side.
If this gives an improvement, with 'improvement' being less cavitation at take-off, slightly higher full-throttle RPM, etc., , then will get another Solas Concord for Port side, but with Port side impeller pitched the next 'steeper' option I can find (YF-CD-13/20), as port engine "works less" than the starboard for reasons just described (impeller rotation, water flow direction on port side boat bottom).
If however the first Concord on Starboard side does not show any significant improvement over the re-pitched 'stock' unit, will then just get my 2nd set of 'stock' impellers re-pitched and put these back on. (Yes, I might need to also try a re-pitch on the Solas before giving up on it...)
Thanks for your patience in reading a long post.
I want a new Yamaha boat with more power but right now don't have the bucks for it - so will try to 'tweak' what I have.
