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Old 08 December 2005, 16:23   #1
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Cavitation

I haven't taken delivery of my RIB yet but I have a question I'd like answered that I have been mulling over.

Q: Is it necessary to throttle back if the prop comes out of the water for a brief moment? e.g. jumping off a wave
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Old 08 December 2005, 21:10   #2
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It all depends really,

The official way of going over waves is: -
1. slow down before getting to the wave
2. Power up the wave,
3. Power back as you get towards the top (to bring the bow down)
4. Power up as your going down the wave so you dont dig the bow into the bottom of the wave.


If your engine does come out of the water for only a short time, you mite be lucky enough for your engine not to cavitate.
If it does cavitate you will need to power down till the prop regains it's grip in the water.

Hope this helped you


Sammo
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Old 09 December 2005, 02:54   #3
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It does make a nice sound though


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Old 09 December 2005, 03:15   #4
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No don't touch the throttle, unless you are a pro throttle man you'll nie-on throw yourself over the console on re-entry The prop will only be out of the water for a fraction of a second and the rev limiter will prevent any over reving. You'll get a great noise as the exhaust comes out. I used to get my 21ft Sports Cuddy out of the water with a 5.7ltr V8 in it. The roar was awsome
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Old 09 December 2005, 03:55   #5
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Yes do touch the throttle. In practise it is often only necessary to dip the throttle as Sammo says, as you get to the top of the wave and then power back on as you start down the other side. Its all about timing but even with practise you will get a few wrong. If the prop breaks the surface there is a good chance of it ventilating and loosing grip which normally requires the throttle to be closed before it will grip again.

Btw, cavitation is lots of little bubbles imploding (think of vacuum) around you prop which takes the nice paint job off and damages the surface. Ventilation is a pocket of air or exhaust gases enveloping your prop so there is no drive. There is a better description here which is worth a read.

Click

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Old 09 December 2005, 05:20   #6
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If you don't touch the throttle imagine the shock load on the gearbox, saddle and the transom as the prop re-enters the water doing twice the speed of the boat!


Not to mention eventual damage to the engine ............ and if you have a modern DFI engine they get the hump when being over revved repeatedly!
A 5.7ltr V8 coming out of the water is a bit different to a modern 2 stroke revving its bits off ..............
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Old 09 December 2005, 07:23   #7
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It's a hell of a lot easier with a foot throttle though - unless you have some nice Gaffrigs or similar!!!
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Old 09 December 2005, 07:31   #8
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Quote:
Originally Posted by codprawn
It's a hell of a lot easier with a foot throttle though - unless you have some nice Gaffrigs or similar!!!

Do you have much experience with foot throttles? I'd be very interested to learn about this. What boat was it fitted on? Do you prefer this to hand throttles for every day use? What are Gaffrigs?
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Old 09 December 2005, 07:41   #9
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Quote:
Originally Posted by Alice
Do you have much experience with foot throttles? I'd be very interested to learn about this. What boat was it fitted on? Do you prefer this to hand throttles for every day use? What are Gaffrigs?
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Old 09 December 2005, 07:43   #10
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Quote:
Originally Posted by Alice
Do you have much experience with foot throttles? I'd be very interested to learn about this. What boat was it fitted on? Do you prefer this to hand throttles for every day use? What are Gaffrigs?
Have driven a couple of boats with them - in particular an old Fletcher 199 Bravo - the foot throttle made control so much easier. The problem is in a normal RIB you tend to be standing up most of the time so a foot throttle would be a no no.

Never used Gaffrigs but loads of racers use them - they must give you better control than the standard sloppy throttles you get as standard.
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