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

[swatski]

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

Would be great for low end but the 1.8 would need a lot more power - like a turbo, or you would loose just too much speed. So, I do not plan on going that route.
I'll try to find what Jerry Gaddis had to say about it and post. He tried a lot of crazy things while modding his VXR.

 

[swatski]

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.

@haknslash I think the amount of cavitation in twin boats is almost none under normal operating conditions, or minimal even if you push very hard, certainly not enough to cause cavitation burns. But those are excellent points!

To illustrate, here are two short clips of hole shot with both engines at WOT:

A couple of pertinent points (I think):

1. Overall amount of cavitation is minimal, but not zero
2. Port side pump shows more slippage (and also revs up much faster )
(That is not entirely unexpected, Yamaha used to pitch port side impellers more aggressively from factory, but not any more)

Back to your other point, imagine pulling several tubes (not my thing!) or running with 2,000-3,000lbs of ballast etc. Instead of the tiny amount of cavitation you see above, you might get A LOT. And - if the load is big enough - it could be similar to the situation in the post #13.

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[swatski]

So, back to the original idea of tuning one pump at the time, it may just be a useful approach. It would mimic running the boat under slow speed/extremely heavy load conditions. Without actually having to load the boat with a ton of ballast (or tying her to a tree, or trying to climb a 30ft wave, LOL).

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[Bill D]

@swatski I'm not sure if it is coincidence or not, but I also notice more slip/cavitation out of my port engine than starboard if I go to WOT from a standstill. Did you perform the same test with the starboard motor while the port was at idle?

 

[swatski]

@swatski I'm not sure if it is coincidence or not, but I also notice more slip/cavitation out of my port engine than starboard if I go to WOT from a standstill. Did you perform the same test with the starboard motor while the port was at idle?

Of course, same location/conditions as in the clip in post #13:
(I just didn't want to confuse the issues too much, LOL, so I did not post it right away).

While the differences between Port and Starboard sides are quite pronounced (to me anyway) w/Port side showing a lot more cavitation, those differences are one thing, cavitation (in general) is another.

An interesting observation is that cavitation (either side) is leveling off as the boat picks up speed, again more so w/Starboard than with Port only.

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[swatski]

So, the first lesson is this:

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 NOT aided by water flow on plane. If that is the case, you may want to consider an approach of tuning your anti-cavitation mods one engine at the time. It may dramatically improve the efficiency of the process as you will not need to load the boat every time you are testing.

That's all there is to it. Now, lets get to the mods!


(EDIT: On re-reading that last sentence... Anyone knows a good divorce lawyer? LOL)

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[Bill D]

This is all a bit out of my league, but I remembered reading this article talking about how the engines are loaded on a twin engine boat and how the port and starboard load differently. Maybe it helps?

https://jetboaters.net/threads/informative-read-on-jetboat-cavitation-pump-load-and-prop-slip.5557/

"Twin Engine Pump Loading

The phenomenon described above affects the twin motor jet boats in an entirely different way. The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull. This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat. This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down). At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold). This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines."

 

[haknslash]

I bet @swatski will be the first to retrofit twin 160mm jet housings complete with duo impellers, Lucky 13's and RIVA turbos!! Nothing says fast like 800 hp fast

 

[swatski]

This is all a bit out of my league, but I remembered reading this article talking about how the engines are loaded on a twin engine boat and how the port and starboard load differently. Maybe it helps?

https://jetboaters.net/threads/informative-read-on-jetboat-cavitation-pump-load-and-prop-slip.5557/

"Twin Engine Pump Loading

The phenomenon described above affects the twin motor jet boats in an entirely different way. The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull. This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat. This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down). At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold). This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines."

Absolutely, and that is a very good read. Exactly as Group K and others here pointed out over the years, Yamaha uses pump drives that rotate in the same direction, rather than counter rotating engines that would load more evenly. With the 2015-current boats equipped w/ “articulating keel” they stopped using different pitches in the impellers from one side to the other that was meant to compensate for the difference, so one engine (Starboard) will load easier while the other will starve for water thus causing some cavitation (primarily on the Port side). The keel in between the intake grates was somehow supposed to alleviate that, but it does not. (It could all be just about cost cutting)

Beyond that, however, there is intrinsic tendency of those pumps to cavitate when pushed hard beyond certain point, such as when you run with one engine only in a twin. And I think (or - am convinced frankly) that can be reduced.

Importantly, these issues are not necessarily caused by any missing sealant (around intake grates etc.), worn our wear rings, or issues w/clearance between the impeller and the liner, etc. So - simply "blue printing" your pumps alone will NOT take care of it.


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[Noko]

I thought the impellers were pitched differently on the older twins to compensate for torque that was pushing the boats to the left? I didn't think it had to do with cavitation.

 

[swatski]

@dan144k is right that twin/dual props would totally help with loading/anti-cavitation, as those work great in fly boarding where the boat is stationary. However, based on my previous discussions with guys who know a whole lot about it, I think the sacrifice of top end acceleration and speed would be just too great for general use, as things stand right now.

It would be great if someone was willing to experiment with it. But I can tell you that, as of right now, those impellers are not completely developed/tuned for use in any watercraft, so the costs or developing a perfect dual impeller combo (2x for the twin) are prohibitive.

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[swatski]

I thought the impellers were pitched differently on the older twins to compensate for torque that was pushing the boats to the left? I didn't think it had to do with cavitation.

I believe you are correct, but that is ultimately related to (or caused by) the differences in loading efficiency for the two sides.
So, with the Port side pump loading less efficiently it becomes easier to cavitate (when pushed beyond a certain limit).

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[KXCam22]

I think the cavitation is somewhat inherent in the small diameter, high-rpm impeller that we use. At stopped or slow boat speeds the impeller moves more water out the back than water pressure/gravity can replace at the front, causing a vacuum and cavitation. Keep in mind that to reduce cavitation potential on the nuclear submarines they went to huge, slower rotating propellers. Without forward motion you are relying on water pressure/gravity to supply the inlet water. Cam.

 

[swatski]

I think the cavitation is somewhat inherent in the small diameter, high-rpm impeller that we use. At stopped or slow boat speeds the impeller moves more water out the back than water pressure/gravity can replace at the front, causing a vacuum and cavitation. Keep in mind that to reduce cavitation potential on the nuclear submarines they went to huge, slower rotating propellers. Without forward motion you are relying on water pressure/gravity to supply the inlet water. Cam.

I agree, but that is not to say it can not be helped!

For example, the first clip in the first post shows about 5-6 sec of cavitation. The second clip shows the same boat, same start, similar conditions -- with cavitation reduced to 1 sec or so (less than 2), with major improvement in pulling and handling.

The difference was a bunch of mods... Doable with a single, but with twins I'm worried about losing my a$$ in the process, that's the main challenge.

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[swatski]

I placed the order for two L13 cones today, from the greenhulk store.

I think this is the most logical route for me to take, with the twins. I want to keep the venturi nozzles completely stock and just play with the spacers, at least initially. If that looks promising, I may just need to bend/adjust the trailing edges of one (or both) impellers slightly and call it a day! If the weather holds, might start working on it next weekend!

Ideally, I will not need to change impellers or make any major changes w/repitching or boring the venturi. I definitely do not plan on messing with intake grate or ride plates, as that has not been proven successful in any of the boat mods that I know of.

The L13 is pretty simple. Basically, it increases the efficiency of the pump by increasing pressure and thrust. Since it makes it harder for the impeller to force water through, it can reduce RPM. You can control the RPM (to some extent) by adding/removing rings/spacers - so that the cone moves towards/away from the venturi nozzle opening, in effect reducing/increasing the path through which water can exit.

I have previously tested a Lucky 13 in a single engine 190, but it is a long thread.
https://jetboaters.net/threads/anyo...ce-adjustable-pump-cone-in-a-boat.7967/page-2

If I can make the individual pumps of the 240 respond to those mods similar to what happened with the single pump of the 190 - it would be a dream machine (or - close to it) .
But, I really have no idea.

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[blacksapphirez]

haha i knew it wouldn't be long!!! I know you will but please provide detailed feedback on them

I would be interested. but really though, the performance of the 240 is more than enough to satisfy my need for speed!

 

[blacksapphirez]

I was able to go out today for couple of hours... cavitation on single engine is horrendous...i guess it is a lot of weight to move with one pump. but with both engines, it was almost non existence.

Port


Starboard


Both

 

[swatski]

I was able to go out today for couple of hours... cavitation on single engine is horrendous...i guess it is a lot of weight to move with one pump. but with both engines, it was almost non existence.

Port


Starboard


Both

@blacksapphirez Man, she sounds awesome! I agree you do not have (almost) any cavitation running with both, great hookup, and so is the acceleration.

[BTW - and as an aside - are you running over 7,800 RPM? I can not quite see on your digital gauges. If you are -- you may be hitting the rev limiter and may have more RPMs to spare which are getting wasted right now!]

Anyhow, thank you for posting! I just got back from the river myself, might post a couple more clips, although the weather was a bit rough today, so don't know how much of today's footage will be useful.

You already know it, but obviously if you start rigging the boat for some serious wake surfing, at some point when you load her w/enough ballast she is going to start cavitating. May not be quite as bad as what you see running on single engines, but it will be close! (with enough ballast). That's why I think tuning anti-cavitation on single engines is the way to go.

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[blacksapphirez]

7800 was port side.
7650 on starboard. check my other thread regarding the rim variation. maybe you can chime in on the reason

 

[swatski]

7800 was port side.
7650 on starboard. check my other thread regarding the rim variation. maybe you can chime in on the reason

Got you! I didn't see it.