Reverse cavitation

[BakedZiti]

Hey guys, I have a 2019 sx240, came from a 2015 sx192. Compared to my 192 this thing is almost scary inadequate in reverse, I sometimes jump off the Rev limiter and or get major cavitation in reverse. I can approach most situations slowly enough so that I don't need to worry but there have been situations where I was coming in a little too fast and the boat just won't stop. I know my 192 is a smaller boat but it stopped in a dime. I have Cobra fins I stalled on my 240 btw

 

[Scottintexas]

are you sure your buckets are moving completely to the reverse position, maybe have somebody work the throttles out of the water and you can confirm their movement,
@swatski had both size boats, maybe he can give you some feedback in comparison of both,


.

 

[props2you]

Couple of things to keep in mind; The 192 weighs much less, thus requiring less thrust to slow/stop. You lose about 40%-50% of your thrust in reverse as it is simply a redirect from the static pump direction, versus the unrestricted forward. It takes a lot more throttle/torque/thrust to slow stop the 24's because of their sheer size/weight.

 

[BakedZiti]

are you sure your buckets are moving completely to the reverse position, maybe have somebody work the throttles out of the water and you can confirm their movement,
@swatski had both size boats, maybe he can give you some feedback in comparison of both,


.

Yes I have done this and they seem to be fully engaging

 

[BakedZiti]

Couple of things to keep in mind; The 192 weighs much less, thus requiring less thrust to slow/stop. You lose about 40%-50% of your thrust in reverse as it is simply a redirect from the static pump direction, versus the unrestricted forward. It takes a lot more throttle/torque/thrust to slow stop the 24's because of their sheer size/weight.

When this happens I'm usually coming out of my lowest no wake setting, to avoid cavitation I need to give myself enough time to throw it in "neutral" then slowly onto reverse.. are you saying the cavitation is a common thing?

 

[Cambo]

Did your 2015 have a rev limiter to 4K ? I think that's what the newer boats have

 

[Jameson Clark]

My 21' does nothing but cavitate in reverse. Obviously not close to the reverse power of my old I/O but it is about what I expected from a jet drive.

 

[props2you]

When this happens I'm usually coming out of my lowest no wake setting, to avoid cavitation I need to give myself enough time to throw it in "neutral" then slowly onto reverse.. are you saying the cavitation is a common thing?

I guess what I'm saying is nothing should (generally speaking) ever happen fast in boating. Back in the day before the rev limiters and redesigned hulls we could take these boats and do "sub moves" (I'll let you search here for hours of entertainment on that topic). The rev limiters are a factor as @Cambo mentioned. It's not cavitation, it's physics, because the jet never changes direction, it ALWAYS has forward propulsion, the buckets simply redirect the forward propulsion, thus losing a significant amount of thrust in reverse. I think you're misunderstanding the thrust redirection in reverse as cavitation. You will not have the same amount of thrust in reverse as you do in forward for previously mentioned reasons. You're also working against inertia moving you forward. The cobra's will help with your handling but no impact on your thrust.

 

[swatski]

When this happens I'm usually coming out of my lowest no wake setting, to avoid cavitation I need to give myself enough time to throw it in "neutral" then slowly onto reverse.. are you saying the cavitation is a common thing?

You won;t hurt a thing throwing it into full reverse, there is no gear box, but it does cavitate out of wack in reverse, and your new boat is limited to under 4k in reverse, the E_series are worse.
Yes, the new 240s are a slog to stop as compared to a 190/192 no doubt. Thank God for Cobras, the new large fins saved my butt several times I could avoid what I was about to hit while losing thrust.

--

 

[swatski]

I think you're misunderstanding the thrust redirection in reverse as cavitation. You will not have the same amount of thrust in reverse as you do in forward for previously mentioned reasons. You're also working against inertia moving you forward. The cobra's will help with your handling but no impact on your thrust.

Well there is A LOT of cavitation in reverse as you massively disrupt laminar flow/pump loading through the intakes.
The Cobras help you stop if you have the FANGS, those redirect and channel the reverse thrust better/more efficiently.
Cobras also permit avoidance maneuvers while moving forward with diminished or no thrust, that's the part than can save the day, saved my butt several times. Otherwise you can find yourself bolting towards and obstacle kind of... hapless.

--

 

[props2you]

Well there is A LOT of cavitation in reverse as you massively disrupt laminar flow/pump loading through the intakes.
The Cobras help you stop if you have the FANGS, those redirect and channel the reverse thrust better/more efficiently.
--

Correct, but not in the sense that there is something wrong with the boat.

Now...where's that mod to delete the rev limiter in reverse??

 

[BakedZiti]

You won;t hurt a thing throwing it into full reverse, there is no gear box, but it does cavitate out of wack in reverse, and your new boat is limited to under 4k in reverse, the E_series are worse.
Yes, the new 240s are a slog to stop as compared to a 190/192 no doubt. Thank God for Cobras, the new large fins saved my butt several times I could avoid what I was about to hit while losing thrust.

--

I agree having the Cobras is a Godsend

 

[BakedZiti]

Did your 2015 have a rev limiter to 4K ? I think that's what the newer boats have

No it did not have a rev limiter that I'm aware of, stopped on dime

 

[BakedZiti]

I guess what I'm saying is nothing should (generally speaking) ever happen fast in boating. Back in the day before the rev limiters and redesigned hulls we could take these boats and do "sub moves" (I'll let you search here for hours of entertainment on that topic). The rev limiters are a factor as @Cambo mentioned. It's not cavitation, it's physics, because the jet never changes direction, it ALWAYS has forward propulsion, the buckets simply redirect the forward propulsion, thus losing a significant amount of thrust in reverse. I think you're misunderstanding the thrust redirection in reverse as cavitation. You will not have the same amount of thrust in reverse as you do in forward for previously mentioned reasons. You're also working against inertia moving you forward. The cobra's will help with your handling but no impact on your thrust.

Thanks for the info, makes me feel better, I do understand how the reverse works, just so different then my 192.

 

[Gary Tait]

The dealer explained to me that you can exceed the reverse RPM limit in no-wake mode - of course I haven't confirmed, but I'm curious if it will work for you. No wake mode, in and out of neutral seems to work well for me, but I consider myself to still be a novice. I do experience the cavitation and it can be unnerving, even with the Cobra enhancement that swatski mentioned. He's right, it does help, you can feel the difference.

 

[Volffas]

your new boat is limited to under 4k in reverse, the E_series are worse.
Yes, the new 240s are a slog to stop as compared to a 190/192 no doubt. --

This is good info for those of us with older boats looking to upgrade. I love how fast our 2007 AR230 can stop, I didn't realize they changed that on the newer boats.

 

[mwalker4]

Does the sx240 have the same new reverse buckets as the 190s? As I posted today, I feel that my new super ultimates are causing problems with the new buckets. The new reverse sends thrust sideways which the ultimates might block.

 

[swatski]

Does the sx240 have the same new reverse buckets as the 190s? As I posted today, I feel that my new super ultimates are causing problems with the new buckets. The new reverse sends thrust sideways which the ultimates might block.

Fins or fangs? Fangs protrude forward of the pivot point of steering/reverse nozzle, fins don’t.

—

 

[mwalker4]

Fins It appears visually that the fins block the new buckets sideways thrust. On the old buckets the result would be to focus the flow. Here I wonder if it causes turbulence at the worst or blocking the intended function at the best. My reverse was sluggish today unlike before I had fins. I have raised them and will test later in week.

The boat is new to me so I do need more testing before I say something conclusive.

 

[mwalker4]

This is the bucket and fins from below with the fins on their highest setting. It doesn't appear to block much sideways flow. When they were on the lower setting, I think the may have blocked the flow.