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Old 21 February 2008, 17:19   #11
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I think a lot of it is down to different operating conditions and requirements, though I'll be honest and admit it is not my area of expertise so I could be mistaken.

I think perhaps KNRM don't have to deal with the same distances and extreme conditions that the RNLI faces. The RNLI's all weather boats are 'only' capable of 25 knots, but can keep a significant percentage of this speed in all but the very worst conditions. The KNRM boats are faster in good conditions but have to back off sooner and further when the conditions get bad.

I know a lot of time is spent by the RNLI looking at what is already available before any decision to design a new boat / piece of kit is designed. Hence the E class on the Thames, and the Arancia SIB used by the Beach Lifeguards.

Cheers

Chris
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Old 21 February 2008, 21:45   #12
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To be fair - the debate isn't about the water jets - it's about hull performance in big seas. I think that the jets have been tried and tested well enough that they are happy with that method of propulsion.
You don't see many water jet tugs............

As to hull performance I am not so sure. I always used to believe in heavy displacement hulls being the best but some RIBs with planing hulls can cope with horrendous conditions. Bit like a cork bobbing on the sea!!!



Obviously when towing the bigger and heavier the boat the better.





The last one is a monster!!!
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Old 22 February 2008, 07:13   #13
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You don't see many water jet tugs............
True. But the RNLI have picked waterjets, and they seem happy about the decision.

Quote:
Originally Posted by codprawn
As to hull performance I am not so sure. I always used to believe in heavy displacement hulls being the best but some RIBs with planing hulls can cope with horrendous conditions. Bit like a cork bobbing on the sea!!!
That is the problem - finding a compromise. With the RNLI having a stated aim of a 25kt fleet - I can see how it would be difficult to get a small ALB up to that speed whilst keeping the sea keeping qualities of a displacement hull. As Chris points out, with the semi-displacement hull of the Mersey (FCB1) she shouldn't need to slow down in anything but the worst weather/sea conditions - but the fastest Mersey's only get to 17kts flat out.
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Old 22 February 2008, 07:55   #14
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What have the RNLI picked waterjets for???
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Old 22 February 2008, 14:07   #15
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What have the RNLI picked waterjets for???
The FCB2 - the 12m Mersey replacement boat.
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Old 24 February 2008, 05:23   #16
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What have the RNLI picked waterjets for???
As DGR says, the Mersey replacement, FCB2. The Mersey is beach launched / recovered, so the ability to drive up the beach during recovery makes the process much safer. Switching from props to water jets makes this possible.

From what I can gather, water jets in this application are different to props, rather than better or worse. The different handling characteristics of jets require different techniques to be learnt to get the best out of them.
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Old 24 February 2008, 05:47   #17
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As to hull performance I am not so sure. I always used to believe in heavy displacement hulls being the best but some RIBs with planing hulls can cope with horrendous conditions. Bit like a cork bobbing on the sea!!!
My broadband is down at the moment so I'm stuck on dial up and can't watch the vids, but horrendous conditions for the RNLI means waves up to 14 to 15 metres high, or perhaps more critically the sort of confused seas you get in areas like the Pentland Firth.

In big seas, any boat traveling at speed is going to tend to take off from the wave crests, particularly a planing hull, and when it lands the flatter the hull the more it is going to slam and the more it is going to hurt (just the same as shallow vee vs. deep vee on a RIB).

You can overcome this to an extent by driving on the throttles, but this is not always possible, particularly at night when you can't see what is coming up and anticipate it.

25 knots in a semi displacement hull is not a problem, but requires more power and therefore weight than a planing hull. Not quite such a problem for a slipway launched Tamar to weigh 35 tonnes, but FCB2 has to be towed on a tractor and trailer so weight is critical.

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Old 30 March 2008, 16:05   #18
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JC did a feature on the Dutch lifeboats he designed on Country file today.
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