Performance / Cavitation / Impeller Slippage in Twin Engine Jet Boats - help me understand

[swatski]

[EDIT: If you use your boat to just cruise around, your twin Yamaha is perfect (leave it alone)!

But, if you plan on doing activities that involve pulling heavy loads at very low speeds, such as wake surfing, you WILL experience cavitation due to the fact that pump loading is less efficient (with boat NOT moving on plane). If that is the case, you may consider
an approach of tuning your anti-cavitation mods one engine at the time
to simply improve your testing efficiency, no need to prep the load etc. every time you test a new component. ]


The twin Yamaha jet boats, such as my AR240, afford sufficient thrust for excellent acceleration, stock. It would be hard to complain about the factory setup. Still, there are practical considerations to try to better it. For example, when running with a heavy load (say, 12 people), or hauling lots of wake surfing ballast -- one can hit a limit of a boat's planning capacity rather quickly w/factory (stock) setup. Or other scenarios, like running heavy-loaded in rough water, climbing waves in 5-6+ft seas, where big thrust can be vital.

In this context, I'm having a hard time wrapping my head around the issue of what would be the best approach to tuning anti-cavitation mods in a twin.

With the single engine/pump, it is pretty simple. You either have slippage or you don't. For example, hit the throttle hard (as in all the way to WOT, fast) and you will see what happens. In the case of my 190, I had a lot of cavitation/impeller slippage especially after I reflashed the ECU for more torque. It would present as a "wha-wha-wha-wha" sounding process of impeller slipping and engine bouncing off the electronic rev limiter for few seconds (the first 3-4 sec of the run in the video clip below, where there is a lot of slippage); after that the boat would pick up enough speed to help load the pump, the impeller slippage would stop, and the boat would pull off with no more impeller slipping:

Kind of the same issue as loosing wheel(s) traction in a car. I hated that, when my impeller was slipping, and experimented with various "solutions" before I settled on one that worked best -- it was the Lucky 13 anti-cavitation cone. It was a bit of work to dial it in but after that the boat would pull a lot stronger (and sound better):

If you compare those two clips (the first few seconds of each) I think it is quite evident the cavitation at the start of the former. Which is (mostly) eliminated in the latter. With no slippage comes much stronger pull out of the hole, ability to haul a big load, etc.

With the twins, I am a bit lost. The reason being that I don't hear or feel a lot of impeller slippage running with the two engines. But, is there really no cavitation? Or is it just masked by the excess thrust from two pumps?

I think it is the latter. The simplest test is running with one engine. I played with that a bit last weekend, and the results are obvious: there is TONS of cavitation when running on one engine, LOL. It is actually worse than in the single engine boat, but it is the same process I think. I don't have a video clip, not needed -- it is very obvious, running with one engine at high idle (to prevent flooding it) and pushing the other one hard creates conditions with just awful/excessive amount of impeller slippage. We all know it.

So, to me this presents an obvious question: should the cavitation issue be approached (and resolved) at the single engine level? And, if successfully tackled, would the boat gain epic amount of thrust running with two (individually tuned) engines?

Am I missing something?

And just to be clear - this is NOT about the impeller pitch angles and rev limits. I understand the correlation between the impeller pitch, revs, top speed. Of course I can eliminate impeller slippage by increasing the pitch (trailing edge), but that will decrease the top speed, etc, etc, an entirely different discussion. What I do not know, and wonder about, is if anyone tried to dial in their pumps individually in a twin boat, and what was the lesson/experience. Is there a benefit in tuning the pumps individually for maximum efficiency?

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[buckbuck]

Wow that first video was extraordinary. I have never heard my boat do that. But like you say, maybe it was being masked.
I will defer to the smart guys here to answer your questions.

 

[MrMoose]

No.
It's a twin engine boat that kicks a$$ out of the hole and carves heavy G's. It's a non-issue unless you plan on driving around on one engine. On a related note, I ran my starboard engine at idle because I had a plastic bottle cap stuck on one of the impeller blades, and the port engine pushed the boat just fine while returning to Harbour Towne marina in the ICW. No cavitation. Bimini 2016.

 

[MrMoose]

I think that you are just looking for a reason to spend money.

 

[Mainah]

@swatski - I owned two twin engine jet boats before my Yamaha. They were two strokes of that other brand that no longer makes boat hulls. In both of those boats I could easily tell when I was cavating like when pulling surface spin moves and trying to climb back out or when from a dead stop and slamming the throttles fowards with a heavy load.

I have yet to hear that with my 242 except when experimenting with hard reverse or right after backing down the throttles in a hard turn puttting the wheel back straight and hitting the throttles again in churned up water. So there you go slam it in reverse to see what it sounds like. I don't think it is much of an issue with these boats unless wear or a foreign object causes it.

 

[swatski]

@swatski - I owned two twin engine jet boats before my Yamaha. They were two strokes of that other brand that no longer makes boat hulls. In both of those boats I could easily tell when I was cavating like when pulling surface spin moves and trying to climb back out or when from a dead stop and slamming the throttles fowards with a heavy load.

I have yet to hear that with my 242 except when experimenting with hard reverse or right after backing down the throttles in a hard turn puttting the wheel back straight and hitting the throttles again in churned up water. So there you go slam it in reverse to see what it sounds like. I don't think it is much of an issue with these boats unless wear or a foreign object causes it.

Yes, I definitely get the reverse cavitation, and the rev limiter seems to cut out at about 4500RPM in reverse. But that's different.

With a single engine on, don't need the video demonstration, the cavitation is rampant (when one engine is left idling, and the other slammed WOT). Even with both together, I'm pretty sure that my port engine bounces off the rev limiter (at 7800+) when I slam both throttles down. My only mod so far is ribbon delete. I will take a video next time I go out.

The thing is, if I can reduce the cavitation on the pumps tuning them as singles, it gives me a very clear readout as opposed to dealing with both at the same time.

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[swatski]

On a related note, I ran my starboard engine at idle because I had a plastic bottle cap stuck on one of the impeller blades, and the port engine pushed the boat just fine while returning to Harbour Towne marina in the ICW. No cavitation. Bimini 2016.

Yes, once you pick up some speed running with one engine maybe fine, as the pump loads better at speed. But trying to rapidly accelerate from zero with just one engine is a different story. That's where I see some room for improvement, in terms of pump loading/efficiency. Lots of room, maybe.

I can't complain about the stock acceleration of the 240, it is very nice. But then, I think of what it would be like if I had the individual pumps dialed in as I had them in my single 190, no cavitation/slippage. I think the 240 would just jump out of the water, LOL.

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[Mainah]

Keep us posted!

 

[bronze_10]

Could the cavitation be due to one impeller trying to move a lot more mass with out the other engine running. when they both are floored there is enough thrust to get the boat moving quickly so the impellers load better at speed... but with one engine there thrust juat simply may not be enough to get the boat moving quickly causing the cavitation until the speed some up. I have played with this some with my 2008 sx230... if I dump one throttle it just recession and the boat is very sluggish. if it increase incrementaly the boat picks up speed daily well.. well enough to get somewhere. I have not tried it while on pane and pulling one motor back to see how well it stay in plane. seems like a good experiment for next year.

 

[swatski]

Could the cavitation be due to one impeller trying to move a lot more mass with out the other engine running. when they both are floored there is enough thrust to get the boat moving quickly so the impellers load better at speed... but with one engine there thrust juat simply may not be enough to get the boat moving quickly causing the cavitation until the speed some up. I have played with this some with my 2008 sx230... if I dump one throttle it just recession and the boat is very sluggish. if it increase incrementaly the boat picks up speed daily well.. well enough to get somewhere. I have not tried it while on pane and pulling one motor back to see how well it stay in plane. seems like a good experiment for next year.

@bronze_10 That is exactly what I think.

I find stock 240 acceleration is totally satisfying/acceptable, but that is not to say it can not be improved. And no, there is NO reason to ever run on a single engine other than in emergencies.

The reason I think a tuning approach of "one engine at the time" would work and may be warranted is that it would provide a clear readout of impeller slippage that is otherwise masked by (great amount of) thrust from twins running in parallel, and it can not be encountered in-real-life-when-running-on-two-engines. The end goal being to reduce slippage without sacrificing top speed. Basically, better "traction control" during the hole shot.

I realize it won't be feasible to reduce cavitation/slippage completely while running on one engine in those big boats. But I think it can be helped, based on the fact that I see direct parallels/similarity to a (torqued up) single engine boat, the 190. "Wha-wha-wha" sound, or "tires slipping in mud" process that you see in video clip 1 above has been reduced (for the most part) as in video clip 2.

Basically, if the approach worked, the twins might gain an epic hole shot. The idea here is to do it by tuning the pump for maximum efficiency while preserving the top end (so -- no dual impellers etc. fly-surfers use with stationary jet boats -- that kill top end).

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[dan144k]

Would intake grates help with your problem?
@swatski

 

[swatski]

Would intake grates help with your problem?
@swatski

Most likely not. I don't think I want to touch the intakes or ride plates.

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[swatski]

Here is a clip demonstrating CAVITATION in a perfectly good 2016 AR240.
(running on one engine only, this clip was shot for demonstrative purposes only, I don't normally run that way, LOL).

The set-up is: from a complete stop (both engines at idle) I push the Port engine WOT; Starboard engine stays at idle. If you watch the first 3-4 sec, clearly the engine hits rev limiter and the prop slips like crazy, before it stabilizes after 10 sec or so as the boat's speed picks up. You can do this with either engine w/similar results, although the Port side is more pronounced. That is called cavitation. I guarantee, every twin boat will show something similar.

Is this normal? You won't see it to nearly the extent during a regular hole shot operation with both engines (unless you have some issues). But the point is, cavitation exists even in a brand new factory tuned pump. You don't see it much during normal operation, but it can be "unmasked" by pushing the limits such as running WOT on one engine.

To me, this is reminiscent of a situation in a single engine boat. If you look at the first clip in the first post in this thread (which is a stock single) - it is quite similar to what you see below (stock twin running on one engine). When you look at the second clip in there, cavitation is basically gone due to a couple of mods and that is what I am hoping to achieve in the twin as well. It can be done in several ways, the trick is to get rid of cavitation without adversely affecting anything else.

 

[blacksapphirez]

@swatski i knew it wouldn't be long before you were messing with your new boat

 

[MrMoose]

A glimpse into the future... @swatski is going to do the next crossing to Bimini on one engine without cavitation, just to prove his point.

 

[haknslash]

Keep in mind that when you're only using one engine on the big boats that's like adding two tubers to a 19' single and expecting it not to cavitate lol. The boat is having to work twice as hard to pull the boat up on plane. With two engines running your efficiency should double and would be a better performance/weight ratio thus less cavitation, if any at all. I don't think running one engine is an ideal test to see if you're cavitating under 'normal' twin engine use. I would imagine that if your boat was cavitating with twin engines you would still be able to feel it. You could always keep a log and inspect the impellers for cavitation wear.

 

[blacksapphirez]

I would think the design of the intake being on an angle would add to the cavitation vs the single engine would be on the bottom center.

I really like the ribbon delete and thats like most hp/mph gain out of any reasonable cost mods

 

[Mainah]

I like the idea of improving the efficiency of the pumps if it can be done. It is like putting better tires on your car. I have to agree that tuning one pump at wot with the other at idle may not be the best way. The available water to pump out will be much greater once the second pump is at wot completly changing the output pressure and volume dynamic as a result of the change in input. In my opinion making changes to only one pump at a time with both pumps running equal rpms may be the best way. The tough part becomes accurately measuring the change in an enviroment that is always changing. Keep in mind that I have zero experience in pump tuning so my opinion may be worth less than .02.

As long as @swatski is putting his time and effort into it while doing a great job documenting it for us I am happy and anxious to read his results.

 

[dan144k]

I always wondered what the twin impellers or dual inpellers would do. theoretically the best of both worlds for idle and top speed

 

[swatski]

Guys, please don't burst my bubble. LOL.
You are right, of course, about the limitations of running on one engine. Water flow is just one issue. And yes, it is similar to dealing with traction control --- and I'm tuning one wheel at the time... but it is not entirely crazy (I think).

What I am thinking is very simplistic, and the whole thing is a bit silly, but...
what if I could tune the pumps such that each alone would run with minimal or no cavitation...? That boat would want to jump out of the water. The low end would be just crazy awesome.

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