255 FSH Sport E acceleration

[FSH 210 Sport]

Came across this one tonight, you can hear it cavitate for about three seconds before it hooks up. Looks like it tops out at 7800 rpm.

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[TeenGee]

Sucking air through the clean-out plugs until the dynamic pressure overcomes the suction of the pump.
I'll bet the clean-out ports are full of water after running at speed.
Clean-out plugs need bee's wax.

 

[singl77]

Hummmm....I thought water in the clean out ports was unavoidable. Is there maintenance recommended that keeps a better seal and prevents water from entering?

 

[FSH 210 Sport]

Hummmm....I thought water in the clean out ports was unavoidable. Is there maintenance recommended that keeps a better seal and prevents water from entering?

Some members have reported using a wax toilet ring / bees ? wax on the sealing surface of the clean out plugs will keep the water out of the tubes.

 

[FSH 210 Sport]

Sucking air through the clean-out plugs until the dynamic pressure overcomes the suction of the pump.
I'll bet the clean-out ports are full of water after running at speed.
Clean-out plugs need bee's wax.

I’ve seen a couple of acceleration tests of the 255 FSH Sport E and both times when full throttle was given from a stand still there was some cavitation for a few seconds. Personally I think this happens with a single screw impeller and that much power is applied until the boat gets moving, of the throttles are rolled on rather quickly the cavitation probably wouldn‘t happen. I think this is what @Wicked92 was trying to overcome with the twin screw impellers on his 255 XD his post #54 in this thread https://jetboaters.net/threads/who-has-experiance-with-twin-screw-impellers.37689/page-3 talks about all but eliminating cavitation.

If the clean out tubes are full of water then no air can get sucked past them ?

While I don’t have one of those 255 FSH Sport E I’ve tested quite a few sets of impellers with different pitches in my search for the best pitch for high altitude impellers, and the impellers with not enough pitch were pretty easy to make cavitate from a stand still and full throttle is applied. The set of impellers I ended up with will still cavitate just like in the attached video if I go to WOT at a standstill but will not if I just slow the movement of the throttles a little bit.

Either way, that 255 FSH accelerates reeely fast.

 

[TeenGee]

Go to post #19 in this thread:

https://jetboaters.net/threads/another-clean-out-plug-thread.43036/

I saw something similar on my boat. It is min wake out to the river. First time accelerating one pump cavitated because there was no water in the clean out port. For the rest of the day I had no cavitation problems. When I got home both ports were full of water. I applied bee's wax to the sealing surface. No cavitation during the first acceleration and no water in the ports. I carry a can of bee's wax and some disposable gloves at the first sign of water in the ports, I reapply the bee's wax.

 

[FSH 210 Sport]

Go to post #19 in this thread:

https://jetboaters.net/threads/another-clean-out-plug-thread.43036/

I saw something similar on my boat. It is min wake out to the river. First time accelerating one pump cavitated because there was no water in the clean out port. For the rest of the day I had no cavitation problems. When I got home both ports were full of water. I applied bee's wax to the sealing surface. No cavitation during the first acceleration and no water in the ports. I carry a can of bee's wax and some disposable gloves at the first sign of water in the ports, I reapply the bee's wax.

Thanks for linking that thread!

So the water in the tubes did stop the cavitation, in my situation testing impellers that was not the case, but I am going to be trying the bees wax in a couple weeks. It’ll be nice to have dry clean out tubes!

 

[gthorson]

Have you guys already had it out over the difference between cavitation and lack of/losing prime? This talk about cavitation is killing me.

 

[TeenGee]

The outcome of loss of prime and cavitation are the same, loss of thrust.
The remedy for each is different.
Cavitation requires a gentle hand on the throttle.
If loss of prime is the cause the bee's wax can solve the problem as it did in my situation.
The point I am trying to make is that if air is getting sucked in past the clean-out plugs, bee's wax solves 2 problems. Loss of prime induced cavitation and water filled clean-out ports. As an aside water filled clean-out ports can also reduce thrust because not all the water forced into the pump by dynamic pressure is going through the pump. This may only equate to 1 or 2 mph of top speed, but who ever said "I want to go slower"?

 

[FSH 210 Sport]

Have you guys already had it out over the difference between cavitation and lack of/losing prime? This talk about cavitation is killing me.

Are you having a cavitation issue?

 

[FSH 210 Sport]

I was talking to my friend this morning about cavitation and he sent me the following response.

Cavitation in a captured system (below the waterline) is caused by the water boiling because the suction pressure gets low enough to create gas at low temps. Water boils at 212* at sea level and about 202* at 1 mile. At a pressure equivalent of 60k feet elevation that drops to 98 degrees. Water in a vacuum can boil at -70*C. Basically the suction of the pump created a low enough pressure behind the leading edge of the impeller blade that the water boils. Hence why the long leading edge of the Skat Trak impellers resists that cavitation better than the relatively sharp entry of the solas units. The decreased pitch allows the engine RPM to build at a rate where the low pressure induced boiling of the water is exaggerated.

So mass air leaks aside, this is the source of cavitation in the 255 FSH Sport E.

And I give you the case of the Turbinia, which was the first steam turbine powered ship, in its original design with a single shaft and screw it experienced massive cavitation, there is even a picture of a heavily pitted impeller in the video that looks like it could have come out of our boats.