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Old 15 August 2009, 20:46   #31
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is that an Aqua star 27? Lovely boats.
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Old 15 August 2009, 20:54   #32
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Atlantic 75's must be different to most other boats. I guess if the boat is light enough and a single overloaded engine still powerful enough then ok. I don't believe it will reach max revs unless it is under propped in the first instance. Or, possibly, scooting down a following wave but that's not a reasonable comparison.

The Aqua 27 I'm not familiar with but at the speeds you quote will not be fully planing, if planing at all. The figures you give for this boat do make sense because at 13knt the boat will be attempting to climb its bow wave so require considerable power to continue to run at that angle and speed.
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Old 15 August 2009, 21:51   #33
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however, although your single motor will be capable of half your power while both engines are running, it wont be able to produce that half power if one engine fails. This is because it will immediately become over propped. With a planing hull, it is quite likely that one engine of a twin setup will not be able to get your boat planing so you will be little better off, in terms of speed, than you would be with a decent sized auxiliary motor. It would be possible to carry a spare prop of a correct pitch for one engine but then you're back to faffing around at sea trying to swap props. Probably not what you want to be doing.
Interesting - I've always wondered about that side of it - I guess it depends on the boat then. If you couldn't get up on the plane then it wouldn't be much point if you only got another couple of knots.

I think my Destroyer is rated for a "minimum HP" of 40 which I assume is probably what is needed to get it up on the plane - but I suppose that would be if correctly propped.

Variable pitch props, that's whats needed
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Old 16 August 2009, 02:21   #34
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In my opinion the primary reason for fitting two engines to a rib is for redundancy. Consequently, it would be interesting to know if the rescue services have any statistics on the probability of a single engine failure.
Extra weight. loss of HP of around 25%, double servicing costs, extra drag, initial cost etc,etc are a large penalty to pay for, especially if modern single engine installations are proven to be almost totally reliable, which I believe to be the case.
Nevertheless, reliability on a modern engine is only achieved in conjunction with sensible use, quality servicing and the availability of water-free fuel!!!
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Old 16 August 2009, 02:36   #35
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Consequently, it would be interesting to know if the rescue services have any statistics on the probability of a single engine failure.
I believe that the vast majority of lifeboat shouts to "broken down" vessels ends up being fuel related, either a kink in the hose, water in the fuel or just plane run out.

When we are called out to an on-water outboard engine issue it is usually fixable with minimum effort. For this reason we have single motor on our RIB, there is a chance that it will conk out but the extra operating costs (even though we do our own servicing) would be prohibitive for twins.

Recently a local skipper lost one motor, the performance was appalling and to my amazement he did not call it into the local CG even though he was in some of the nastiest water around. The end effect was a feeling of over confidence, because he had "redundancy" he felt the boat was quite capable on half power.
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Old 16 August 2009, 04:20   #36
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Malthouse,

"The end effect was a feeling of over confidence, because he had "redundancy" he felt the boat was quite capable on half power."

And that is what a lot of boat people I know tend to feel. A pretty dangerous way of thinking. An underestimation of the situation, perhaps. Too much trust in the twin engines.
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Old 16 August 2009, 06:04   #37
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Recently a local skipper lost one motor, the performance was appalling and to my amazement he did not call it into the local CG even though he was in some of the nastiest water around. The end effect was a feeling of over confidence, because he had "redundancy" he felt the boat was quite capable on half power.
How would he have coped with a small auxilary though?
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Old 16 August 2009, 06:14   #38
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How would he have coped with a small auxilary though?
IMHO He would have done what he should have done with a single twin and motor into the nearest shelter and drop the hook. Let someone know what had happened and wait for the tide to settle.
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Old 16 August 2009, 06:24   #39
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flying around, but I don't yet see a solution to Willk's (implicit) question
Oi! Are you implying that I'm implying something? That's my wife's favourite pastime

... although my electrical contractor did look at me strangely once and say "it's not what you say, but what you don't say that I have to listen out for"

He's Irish too
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Old 16 August 2009, 06:42   #40
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And back on topic:

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Atlantic 75's must be different to most other boats. I guess if the boat is light enough and a single overloaded engine still powerful enough then ok. I don't believe it will reach max revs unless it is under propped in the first instance.
My Redbay will plane at about 20kts on one motor at WOT (with the dead stick in the water). It IS overpropped for single engine use (19 pitch) and has to be coaxed out of the water and she isn't making her normal revs either. I wouldn't fancy trying to maintain plane in short lumpy water.

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The Aqua 27 I'm not familiar with but at the speeds you quote will not be fully planing, if planing at all.
I thought these were SemiDi hulls?
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