Can someone explain to me how cavitation happens? I know I've seen it on here before, but I can't seem to find it now...
Also, I know most of the physics involved in a jet, but I've got some questions...
How does the ring affect how the impellar works? I mean, it's always sitting in water, so why does the clearance of the ring effect the performance?
Why does a jet lack the torque of a prop?
Why does it take more horsepower to drive a jet than a prop?
Just some things I've been trying to figure out....
4 replies to this topic
#2
Bob86ZZ4
-
- Members
-
- 103 posts
Novice
- Year of DOO:1996
- Model of DOO:Speedster 160
- Engine in DOO:Twin 85hp Rotax (720)
Posted 09 June 2004 - 07:56 PM
I'll take a stab at that and then others can correct me if I'm wrong. Cavitation is when the jets are sucking air, or just not sucking enough water to make pressure and thrust. If you do a spin turn for instance and leave the throttles up, there will be a high revving of the engines until they draw in enough water to create pressure and thrust and the boat gets going again.
The wear ring. It's supposed to be rather close to the edge of the impeller to make a good seal and thereby good thrust. It's softer plastic verses the impellers stainless steel. Even tho most of the stuff pumped through is water there are weeds, sand, gravel, and rocks. As well as sticks and anything else nearby. As those get ground through they tend to wear out the wear ring. The spinning of the impeller pushes the foreign objects out (centrifugal force remember) so that's how they wear out. They are cheaper to replace than a impeller.
Jet less efficient than a prop? Weeellllll, I dunno about that one. I know prop guys claim that a jet is less efficient. Anybody got any input on this one?
The wear ring. It's supposed to be rather close to the edge of the impeller to make a good seal and thereby good thrust. It's softer plastic verses the impellers stainless steel. Even tho most of the stuff pumped through is water there are weeds, sand, gravel, and rocks. As well as sticks and anything else nearby. As those get ground through they tend to wear out the wear ring. The spinning of the impeller pushes the foreign objects out (centrifugal force remember) so that's how they wear out. They are cheaper to replace than a impeller.
Jet less efficient than a prop? Weeellllll, I dunno about that one. I know prop guys claim that a jet is less efficient. Anybody got any input on this one?
#3
tiggerman
-
- +SDSB Club
-
- 1,356 posts
SeaDooSportBoats.com Sport Boat Specialist
Posted 10 June 2004 - 05:56 AM
1997 Seadoo XP 800 (sorry, at least it is a DOO!!!)
#4
TexasA&MSpeedster
-
- Members
-
- 25 posts
Novice
Posted 10 June 2004 - 07:59 AM
That article answered a lot of questions....
So where do our pumps need to be sealed to prevent more cavitation?
So where do our pumps need to be sealed to prevent more cavitation?
#5
tiggerman
-
- +SDSB Club
-
- 1,356 posts
SeaDooSportBoats.com Sport Boat Specialist
Posted 10 June 2004 - 08:42 AM
The answer:
Wear rings, which you just did. Pump housing to pump support, which you would have done with the weatherstripping I told you about, or the OEM seal for a '96 model- which is neoprene as well. A new carbon ring, stainless disc, bellows hose, and o-rings for sealing the driveshaft, or the carrier retrofit I've talked about. The retro can be done with OEM parts for the '95 model boats, or the modified assembly that I have mentioned. The ride plate and pump support can leak as well due to poor application of silicone from the factory. I don't recommend these be removed unless you know exactly what you are doing. They are very difficult to get off the boat without possibly breaking something. I did it, but would not doo it again. You can, however, apply some ultra black to the intake tunnel in the seams where the ride plate / pump support meet the hull. Fill in the gaps and smooth it out, and it will be OK. Make sure you apply it to clean surfaces though, or it's not going to stick.
All these things can make a difference in whether you cavitate or not (not you, your boat). Chances are any cavitation you experience will come from the driveshaft seals or the wear ring.
Wear rings, which you just did. Pump housing to pump support, which you would have done with the weatherstripping I told you about, or the OEM seal for a '96 model- which is neoprene as well. A new carbon ring, stainless disc, bellows hose, and o-rings for sealing the driveshaft, or the carrier retrofit I've talked about. The retro can be done with OEM parts for the '95 model boats, or the modified assembly that I have mentioned. The ride plate and pump support can leak as well due to poor application of silicone from the factory. I don't recommend these be removed unless you know exactly what you are doing. They are very difficult to get off the boat without possibly breaking something. I did it, but would not doo it again. You can, however, apply some ultra black to the intake tunnel in the seams where the ride plate / pump support meet the hull. Fill in the gaps and smooth it out, and it will be OK. Make sure you apply it to clean surfaces though, or it's not going to stick.
All these things can make a difference in whether you cavitate or not (not you, your boat). Chances are any cavitation you experience will come from the driveshaft seals or the wear ring.
1997 Seadoo XP 800 (sorry, at least it is a DOO!!!)
1 user(s) are reading this topic
0 members, 1 guests, 0 anonymous users
