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14 May 2015, 23:39
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#1
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RIBnet admin team
Country: UK - England
Town: The wilds of Wiltshire
Boat name: Dominator
Make: SR5.4
Length: 7m +
Engine: Yam 85
MMSI: 235055163
Join Date: Jul 2005
Posts: 13,067
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Stepped hulls. Advantages/disadvantages?
It occurred to me today that I've never driven a stepped hull.
What's the advantages/disadvantages?
I understand and can visualise the drag reduction, which in theory I guess equates to improved top speed and improved fuel consumption at normal speeds.
If this is the case,why don't we see many stepped hull Ribs? Particularly why don't we see them in the sub 6m category?
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15 May 2015, 06:50
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#2
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Member
Country: UK - England
Town: South Yorks
Boat name: Black Pig
Make: Ribcraft
Length: 5m +
Engine: DF140a
MMSI: 235111389
Join Date: Feb 2008
Posts: 12,109
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Maybe there isn't enough length of hull to fit the steps in on sub 6m, by the time you have got the bow down to the keel, there's only a couple of metres before the stern starts to form.
.....sh1t happens.......
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15 May 2015, 07:26
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#3
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Member
Country: UK - England
Town: portsmouth
Boat name: Hullabaloo
Make: Humber
Length: 8m +
Engine: 225 Optimax
Join Date: Mar 2011
Posts: 980
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As a Dry Stack owner, stepped hulls are "the work of the devil"! And I suspect that just like putting a large spoiler on the back of you average family saloon car, they look flash but priovide little advantage for everyday use!
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15 May 2015, 07:35
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#4
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Member
Country: UK - England
Town: New Forest
Boat name: Charlie Brown
Make: Scorpion
Length: 8m +
Engine: 275 Verado
MMSI: 235069179
Join Date: Oct 2004
Posts: 1,082
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Stepped hulls are good at increasing efficiency of the hull in terms of allowing greater speed for lesser HP.
Our old boat was a fairly heavy 9 meter inboard rib with stepped hull and was easily capable of 45 kts with a relatively small 256 HP.
The down side is they can be more unpredictable in turns, i was always worried when novices were helming that boat. straight line and sensible turns were fine but start throwing it around and the back end would slide and step out of line if you were not careful.
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15 May 2015, 08:45
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#5
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Member
Country: UK - England
Length: 7m +
Join Date: Aug 2009
Posts: 1,619
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Quote:
Originally Posted by Zippy
Stepped hulls are good at increasing efficiency of the hull in terms of allowing greater speed for lesser HP.
Our old boat was a fairly heavy 9 meter inboard rib with stepped hull and was easily capable of 45 kts with a relatively small 256 HP.
The down side is they can be more unpredictable in turns, i was always worried when novices were helming that boat. straight line and sensible turns were fine but start throwing it around and the back end would slide and step out of line if you were not careful.
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Great response, ive read some articles on stepped hull design from various mags but your brief paragraph sums it up very well. If you look at most race boats they have stepped hulls for the speed adavantage but for what most of us want to do for most of the time a normal V hull with no steps is the better option. Just consider the fact you dont see RNLI Ribs with stepped hulls (I bet someone comes along and states im wrong LOL)
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15 May 2015, 09:34
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#6
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RIBnet admin team
Country: UK - England
Town: The wilds of Wiltshire
Boat name: Dominator
Make: SR5.4
Length: 7m +
Engine: Yam 85
MMSI: 235055163
Join Date: Jul 2005
Posts: 13,067
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Quote:
Originally Posted by Zippy
Stepped hulls are good at increasing efficiency of the hull in terms of allowing greater speed for lesser HP.
Our old boat was a fairly heavy 9 meter inboard rib with stepped hull and was easily capable of 45 kts with a relatively small 256 HP.
The down side is they can be more unpredictable in turns, i was always worried when novices were helming that boat. straight line and sensible turns were fine but start throwing it around and the back end would slide and step out of line if you were not careful.
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Thanks-sums it up nicely. I guess you wouldn't want a sub 6m stepped hull anyway then. Sub 6m boats get thrown around a lot!
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15 May 2015, 13:21
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#7
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Member
Country: UK - Scotland
Town: glasgow
Boat name: arriba
Make: marlin
Length: 7m +
Engine: suzuki 300
Join Date: Jun 2007
Posts: 315
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I imagine they are difficult to launch and get on and of roller coaster trailers???
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15 May 2015, 16:00
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#8
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Member
Country: UK - Scotland
Town: Isle of Lewis
Boat name: Macleod Special
Make: Mako Thundercat
Length: 4m +
Engine: Yamaha 70ces
Join Date: Jul 2010
Posts: 1,256
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I think the hulls themselves are heavier, due to the greater surface area? So on a smaller RIB the increase in weight might be more than the reduction in drag is worth
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15 May 2015, 16:48
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#9
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Member
Country: UK - England
Town: swanwick/hamble
Boat name: stormchaser
Make: custom rib
Length: 8m +
Engine: inboard/diesel
Join Date: Aug 2007
Posts: 3,848
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Should leave the steps to the race boat, where you keep stepping up like that you shorten the transom height, not to much of a problem on a hard boat because you can alter the top moulding, on a rib however you have to follow the line of the tube, much more difficult
Sent by mobile thingy
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15 May 2015, 19:07
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#10
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Member
Country: UK - England
Town: winchester
Length: 7m +
Join Date: Jul 2014
Posts: 2
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I was lucky enough to have the use of a new 7.4m rib last summer. Had a stepped hull and was very quick top end. I had planned to post on here to ask why the handling was odd..
Leaned heavily to the left under hard acceleration and stayed that way unless I trimmed the motor quite a long way up.
Unexpectedly, it was almost impossible to induce cavitation in tight turns, and would take almost full throttle before slipping.
I assume the outboard is mounted too low - which might be causing the lean at speed ?
Boat was quite 'flighty' at WOT though, which is probably the stepped hull, and I'm not sure if this would become worse if we raise the outboard??
BTW - how do you know from a basic principle point of view if the engine is at the right height with a stepped hull?
Any advice would be good.
Thks
Matt
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