Cavitation

[Andy Moore]

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

[Sammo]

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

[The Jackeens]

It does make a nice sound though


missus

[Mollers]

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

[Pete7]

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

Pete

[Cookee]

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 ..............

[codprawn]

It's a hell of a lot easier with a foot throttle though - unless you have some nice Gaffrigs or similar!!!

[Alice]

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?

[roycruse]

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?

[codprawn]

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.

[Richard B]

Originally Posted by codprawn

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

A top mounted throttle gives a lot more control than a side mounted one.

[codprawn]

Originally Posted by Richard B

A top mounted throttle gives a lot more control than a side mounted one.

Hope so - that's why I asked for one. Are the standard Suzuki ones any good???

[Richard B]

Don't know about the Suzuki ones.

However, I reckon that you're better off with the "stick" bit mounted on the right hand side of the mount box. I find that as the stick on ours is to the right of the box, I have somewhere to rest my hand, so small and quick throttle adjustments can be made easily.

[Alice]

Hi there. Thanks for your reply. I'm thinking of a foot throttle for the Scorpion but I'm now a bit wary after your comment about standing up. It makes sense as I'm only 5'2" tall and have to stand up for almost everything. You certainly meet some very knowledgeable and experienced people here on this forum.

I'm still a bit puzzled about Gaffrigs though. The only Gaffrigs I know about are large wood pole thingy's with a crutch and with red sails on because my uncle has an old wooden boat like this near Ipswich, and I'm sure you're not talking about those are you?

[Mollers]

[QUOTE=Cookee] 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!



I don't agree, the oposite is true. In a big sea constant working of the throttle is required, but if travelling at WOT or there abouts and crossing wake or whatever, causing the boat to leave the water, the boat doesn't half it's speed on re-entering the water so why half the engine speed? The faster water passing over the prop on re-entry would then apply reverse force on the gearbox which would strain the bearing carrier and the other componants above mentioned.
In a race situation where the boat is leaving the water on a regular basis and over-reving could become an issue, perhaps. But, in a recreational boating situation I reckon that our friend is better off keeping both hands on the wheel and choosing his line rather than grabbing at the throttle to prevent a momentary rise in revs. In my opinion



As for Alice and her Gaffers! We've got a whole fleet of 'em in Falmouth which race in the summer. If you wanna get all 5'2" of ya' down here we'll fix you up with a ride. It's the best fun you'll have without an engine, if you can cope with the post race "P" ups.

[Andy Moore]

Thanks for all you replies. Interesting reading.

[Richard B]

Originally Posted by Mollulnan

But, in a recreational boating situation I reckon that our friend is better off keeping both hands on the wheel...

I think that recommended "best practise" from most sources is to keep one hand on the throttle at all times.

Alice - If your Scorpion has a standard console and jockey seats, then considerable modification would be needed to convert to a foot throttle.

[Mollers]

Originally Posted by Richard B

I think that recommended "best practise" from most sources is to keep one hand on the throttle at all times.

Alice - If your Scorpion has a standard console and jockey seats, then considerable modification would be needed to convert to a foot throttle.

Also Alice it would be bloody awkward performing "best practise" and keeping one hand on it!! Come on Dickie, you need bucket seats and your arse on the floor for a foot throttle!

[Pete7]

Originally Posted by Mollulnan

I don't agree, the oposite is true. In a big sea constant working of the throttle is required, but if travelling at WOT or there abouts and crossing wake or whatever, causing the boat to leave the water, the boat doesn't half it's speed on re-entering the water so why half the engine speed?

In practise though the prop looses grip and just spins, the boat slows down and your going nowhere. Quickly dipping the revs brings the prop speed down enabling it to grip again and your off.

Pete

[Richard B]

Originally Posted by Mollulnan

Come on Dickie, you need bucket seats and your arse on the floor for a foot throttle!

Exactly