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Old 14 May 2015, 19:39   #1
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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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Old 15 May 2015, 02:50   #2
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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.


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Old 15 May 2015, 03:26   #3
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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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Old 15 May 2015, 03:35   #4
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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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Old 15 May 2015, 04:45   #5
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Quote:
Originally Posted by Zippy View Post
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.
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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Old 15 May 2015, 05:34   #6
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Quote:
Originally Posted by Zippy View Post
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.
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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Old 15 May 2015, 09:21   #7
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I imagine they are difficult to launch and get on and of roller coaster trailers???
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Old 15 May 2015, 12:00   #8
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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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Old 15 May 2015, 12:48   #9
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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


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Old 15 May 2015, 15:07   #10
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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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