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Old 25 July 2005, 13:52   #51
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Quote:
Originally Posted by Richard B
A BWM will need some careful driving as the bow doesn't ride particularly high.
Not all of them.
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Old 25 July 2005, 15:24   #52
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Quote:
Originally Posted by Jonny Fuller
Many of the 'pure race' type ribs have relatively small diameter tubes, these Buzzi 36's had quite large ones, and I know of 3 instances of them rolling in this way (whilst running straight, rather than 'in a turn', the latter is understandably more common)'

The later generation of that boat from Buzzi is 'tubeless'.
isnt buzzi one of the few high speed rib builders who leaves his tubes in the water?? theyre not upside down as much as honda 225s but theres been a few hasnt there!


i been looking for an article by tony lee-elliot that mentions tube height, now his own personal boat might look like a shoe but he seems a far more knowledgable guy than me and the way he explains things although sometimes long winded always seem to make sense due to his natural common sense. I remember him writing something along the lines of tubes in the water when underway = drag (less speed/less acceleration/increased fuel/decreased range). Tubes in the water at speed also = substantial collar wear and potential chaffing between grp and hypalon!!

Several builders use same moulds and tubes for both outboard and inboard installs, the extra weight of an inboard will be carried easier than on a conventional hard boat due to the tubes, but this does mean that the outboard models will sit higher in the water.. A well designed rib imho has tubes that just kiss the water or are just off aft when laden at rest but are lifted clear when on the plane.. The upstand should be sufficient that if the tubes were deflated the scateboard would still float!! but how far should the tubes protude behind the hull, thats almost as critical as tube height .. too much and the tubes will be under constant stress (just like dragging them in the water me thinks)
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Old 25 July 2005, 15:36   #53
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Quote:
Originally Posted by Richard B
BWM will need some careful driving as the bow doesn't ride particularly high.
Hmm, Have to disagree with you a tiny bit there Richard. My DS21 rode pretty well bow high although needed more positive trim from the engine than the Scorp. I also have managed to stuff a Scorp (both 7.5m and 9.5m) much more thoroughly than I ever did in the DS21 but mebbe I was trying harder! The Scorp has more bow sheer than the DS21, although the DS21 has a significant sheer which I think is a good thing over say a Prosport with its dead straight toobs.

Comparing the handling of the two though is like comparing apples to kumquats. Both are capable hulls but the Scorp is a much better ride but then it is a much bigger RIB and the old adage of size matters is very true of RIB hull perfomance!
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Old 25 July 2005, 15:45   #54
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Originally Posted by Alan
Hmm, Have to disagree with you a tiny bit there Richard.

yew carntt doo dat clooliss inn noofowndlannd orr dat leweeze wil powke yew inn de fkin iye wiv a blunnt stikk

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Old 25 July 2005, 17:37   #55
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mmmm... I dunno about a lot of the finesse written here - but it seems to me that the guy in the original photograph can choose to have only some hard hull in the water, or he can choose to have his sponsons in the water as well by reducing his speed...

unfortunately I dont have that choice - even at WOT the ends of my sponsons are in the water - this adds to the wear on the seam taprs (which I've just reglued!) - and I reckon it costs me some fuel as well....
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Old 25 July 2005, 18:11   #56
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Quote:
Originally Posted by Richard B
One of the most interesting I think was Mark Wildeys account of the difference in handling he encountered when he once had a puncture in the rear section of the tube on one side. No difference at all.
But hasn't Mark Wildey got a Scorpion? If the tubes are above the waterline at speed then why should there be any difference in handling with a puncture?
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Old 25 July 2005, 18:15   #57
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Quote:
Originally Posted by Richard B
But if your RIB depends on the tube inflation to keep afloat, that's bad design.
So, the Navy, Coastguard and about half of the members on here who own, or have owned, an Avon Searider have bought badly designed boats.

While we're at it better get onto the RNLI and advise them of the bad design of their D-class inflatables - they'd best get some sort of hard boat hull fitted.
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Old 25 July 2005, 18:17   #58
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Scorpion- a hard boat with a fender?

My tubes are pretty high - it's almost a hard boat with a fender!

Not sure how much they contribute to the ride at speed.

The Scorpions soft ride is due, I think, almost entirely to its extreme deep V at the bow and the narrow waterline beam.

The bigger the boat the less important the tubes are, in my opinion.
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Old 25 July 2005, 18:26   #59
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Throttle response.

Driving the boat on the throttle is much simpler on a larger boat due to the lower power to weight ratio. You can be pretty ham fisted, and use stabs of full throttle almost with impunity in say a 6.5m+ boat and not get into too much trouble. - things happen slower.

Spent a day driving a 7.8m Ballistic with 200hp - you could really beast the throttle to adjust the boat with nothing too frightening happening. 1300kg, 200hp = 154hp / ton.

This seems to be the case with my 6.5m with a 150hp but you can't hold full throttle for too long. 900kg, 150hp = 166hp / ton.

WOT in my 5.4m Searider with 90hp was almost suicide in a big sea. 500kg, 90hp = 180hp / ton.

So, it's more difficult in a smaller boat, something that those of us with bigger boats should try appreciate, or remember back to when we had smaller boats.
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Old 26 July 2005, 05:41   #60
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But on the other hand, when the power to weight is high, the rib is more responsive. For my own part, I find driving my bigger (heavier) diesel powered rib a good deal more difficult especially if the going gets rough.

Because of the low power to weight, one has to judge more accurately what the sea is going to do and what one needs to do as a driver to cope with it. As a very simple example, let's imagine we are in a following sea and rising over the wave crest; because the boat is heavy and has considerable inertia once it's on its way into the trough ahead we are committed. If the trough is a bit deeper than anticipated there is a difficuty. Opening the throttle to raise the bow will not have as great or as immediate an effect because the boat is heavy and it has less power.

In the case of the diesel engine, when just passing over the crest we would not be able to throttle back very far because there is a need to keep the turbo spinning ready for when the power is needed in the trough otherwise the throttle will be almost, and alarmingly, unresponsive just when it is needed to raise the bow up the back of the wave in front. So we don't have the luxury of the full range of throttle, the engine speed must be kept up. A good throttle technique and fine control are required along with a better estimation of the sea condition immediately in front of the boat.

Similarly, when running a beam sea, a better estimation of what the waves are doing, at that critical distance in front of the boat where it will be in a few seconds time, is required....is that curly wave going to subside and leave you running a trough or are you going to end up right on it's crest or is the following wave going to build and be right on your beam side if you decide to turn behind the first one. It needs to be got right because one can't simply open the throttle and expect to shoot out of the danger area.

So, yes, I do agree that a boat with a high power to weight can be a handful if got wrong but it's responsiveness allows a good driver to use it to the full. But a heavy boat with a lower power to weight requires considerably better estimation of what the sea is likely to do, or, indeed, is doing, out of sight.
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