whats the difference

[Rogue Wave]

This question was just asked on Ribnet

whats the difference between a stepped hull and a normal Hull.

I nearly answered that the stepped hull was faster but more wobbly, but I thought there has got to be more to it than that!

please let me know the difference

cheers

[K9 racing]

ones got steps in it!

imho the stepped hull tends to be faster thats why most offshore boats have them but i dont know what the steps do.

john

[Gav]

A normal v hull a certain amount of drag, whilst planing, the less drag, the quicker the hull.
With a stepped hull, ie 2 steps........ as soon as the first sep is breached, the hull is then planing on its rear step..thus less drag.

I'm sure Jonny "Boat Boy" Fuller will be able to give a much better explanation

over to you Jon.............

[JamesM]

I think as the water runs past the step an area of low pressure is created directly behind the step, in order to fill the void created air is drawn in from the sides. The hull immediatley after the step is therefore running on air enriched water (bubbles) and so creates less drag than if it were running on normal water.

The step or steps are normally at the back of the hull as this is the area the boat runs on when at speed. However by creating less drag at the back inorder to go faster in a straight line you are also reducing the levels of grip at the back when cornering. As a result stepped hulls can be more prone to hooking.

Ultimately on a light boat on mirror flat water steps would be no faster as if trimmed right the boat will be running on only the last 6 inches or so of the hull (Scream and Fly style) and so the steps would not be in contact with the water anyway.

[BluFin]

Nicely put.
One point to note If done incorrectly they can actually cause suction & hold the boat down, defeating the whole point, hence the venting.

[MrRob]

Originally posted by Johnny Boat Dude
I think as the water runs past the step an area of low pressure is created directly behind the step, in order to fill the void created air is drawn in from the sides. The hull immediatley after the step is therefore running on air enriched water (bubbles) and so creates less drag than if it were running on normal water.

The step or steps are normally at the back of the hull as this is the area the boat runs on when at speed. However by creating less drag at the back inorder to go faster in a straight line you are also reducing the levels of grip at the back when cornering. As a result stepped hulls can be more prone to hooking.

Ultimately on a light boat on mirror flat water steps would be no faster as if trimmed right the boat will be running on only the last 6 inches or so of the hull (Scream and Fly style) and so the steps would not be in contact with the water anyway.

So if the step is there pruely to produce air bubbles under the hull surely you could add this feature to any hull.
if you had small holes in the rear of the hull which were vented at the front of the hull then they would suck air in and create air enriched water.

I would 'imagine' a stepped hull to be more stable than a regular one because you dont have to run on the last 6" of the hull to get great speed.

[John G]

The two basic principles behind the efficiency gains of stepped hull designs are;

· Their ability to maintain the planing surfaces of a hull at an angle of trim that is, or is close to, the optimum for that particular deadrise
· The introduction of air to the hull water interface which acts as a lubricant and prevents suction drag

The relative inefficiency of an un-stepped hull is due to the fact that whilst retaining a realistic, and practical, weight distribution the hull is unable to remain in equilibrium throughout its speed range if the optimum angle of trim is maintained.

Consider a craft planing through the water at its optimum trim angle. If the speed of the vessel is increased the lift force generated per unit area of the wetted hull increases causing the craft to rise out of the water to maintain vertical equilibrium. The vessel is planing at an angle to the water (the trim angle), as it rises the wetted area of the hull moves aft. This then disrupts the longitudinal equilibrium of the craft as the centre of lift moves aft with the wetted area. The craft must then adopt a smaller angle of trim to bring the centre of lift forward again if it is to maintain its increased speed and its longitudinal equilibrium. This reduced trim though is below the optimum trim angle and therefore the planing surface is less efficient at producing lift thus requiring more wetted surface to produce the necessary lift.

The division of the planing surface of a stepped hull into several smaller surfaces along the length of the craft overcomes this problem. When the speed of the vessel increases the associated rise of the hull does not produce such a dramatic shift in the centre of lift as the average change in the shorter planing surfaces is much less than that seen in the large single surface.

The theory of air lubrication is often used to explain entirely the performance advantage of stepped hulls. The idea that the vessel is ‘riding on a cushion of bubbles’ is easy to convey to the boating public. It is true that some benefit is gained from the same effect seen on the surface of a golf ball or the skin of a shark. In these cases a turbulent layer of fluid, be it water or air is trapped next to the object creating an alternative interface with the body of fluid through which the object is travelling. As the friction between fluid and fluid is much less than between solid and fluid the resistance experienced by the object is reduced. In the case of stepped hulls it is air that is introduced to form an alternative interface to the hull water one.

Few experiments have been carried out to assess the extent to which this introduction of air occurs. There is some doubt though in the authors mind as to whether an effective layer of air is actually attached to the hull surface. The previous examples of the golf ball and shark’s skin differ in that both feature aggressively roughened surfaces intended to induce the layer of turbulence over the whole surface. The main role of airflow within a stepped hull is to prevent the onset of suction drag caused by the low pressure area created as the water flows across the gap from one planing surface to the next. This suction force if left un-ventillated can greatly increase drag and also cause handling problems.

The reason the 'Scream n Fly' boats travel on the aft 6" of their hulls are:

In the case of the cats the hydro-dynamic lift of the hull is supplimented by the aerodynamic lift of the tunnel which is located well forward. The two forces act either side of the centre of gravity in order to balance the vessel.

In the case of the unstepped monohulls travelling on the aft 6" of the hull is partly due to a small amount of aero lift but mainly due to the balancing effect of the extremely trimmed out engines. The line of thrust from the propeller is angled above horizontal creating a turning moment on the boat allowing it to run right on the rear of the hull. Whist this means that the hull surface is operating nearer its optimum angle and consiquently with a small wetted area it is not neccessarily the most efficient way to run the boat as the thrust from the propeller is not being directed solely in the direction you are travelling.

[Jon Fuller]

Are these your words?

[MrRob]

Well that shut me up...

[Jon Fuller]

Originally posted by John G
In the case of the unstepped monohulls travelling on the aft 6" of the hull is partly due to a small amount of aero lift but mainly due to the balancing effect of the extremely trimmed out engines. The line of thrust from the propeller is angled above horizontal creating a turning moment on the boat allowing it to run right on the rear of the hull. Whist this means that the hull surface is operating nearer its optimum angle and consiquently with a small wetted area it is not neccessarily the most efficient way to run the boat as the thrust from the propeller is not being directed solely in the direction you are travelling.

And try it in the rough!

[John G]

Mr Rob was on to something when he said that the stepped hulls might be more stable as the wetted surface is spread out along the boat.

Whilst the overall reduction in wetted area of a stepped hull can cause instability in roll if not designed properly there can also other benefits to the stepped arrangement.

Stepped hulls react in a quite different manner to conventional deep vee hulls when encountering waves.

As a conventional hull passes through a wave the centre of lift moves steadily aft as the fwd end becomes un-wetted. It is therefore some time before the centre of lift has passed aft of the centre of gravity and then started to lift the stern and bring the bow down. This menas that the bow will keep on rising until most of the boat has passed through the wave = lift off / air time / boat flying out of the water!

A stepped hull does things differently. As the wave passes under a step this totally un-wets the planing surface ahead of it. This causes the overall centre of lift (average of all the planing surfaces) to suddenly jump aft. If the steps are positioned correctly then the centre of lift will jump to a position aft of the centre of gravity much sooner than in a conventional hull. This means that a stepped hull will start to lower its bow sooner and therefore stay flatter in the water = less chance of air time = more comfortable = faster, as propeller is in the water more often.

[John G]

Originally posted by Jon Fuller
Are these your words?

Yes. Am pretty busy right now so just cut and pasted from my old Uni dissertation. Resarched it plenty though and talked to other designers so confident it's all right!

[MrRob]

Can you explain the centre of lift to help us less clued up types understand.

[John G]

Originally posted by MrRob
Can you explain the centre of lift to help us less clued up types understand.

Your planing hull produces dynamic lift (as opposed to just bouyant lift in a displacement - yacht type hull) - the centre of lift is the point on the hull at which this lift force appears to act - i.e instead of being spread over the wetted area of the hull you imagine that all of the lift is concentrated in one point - a kind of average. You need to do this to determine how the boat is going to trim - i.e where the = centre of lift is in relation to the boats centre of gravity (where the weight of the boat acts - but you knew that bit already!)

[Carl]

top left pic,in tech terms,riding on her bum

[John G]

Found a couple of old diagrams that might make things clearer - I hope!

[John G]

Stepped hull

[Gav]

what?

[SplitPin]

Its a pity James Beard isn't still around!!!!!

[JamesM]

Originally posted by John G slightly edited by Johnny Boat Dude
un stepped hull = lift off / air time / boat flying out of the water!

stepped hull = less chance of air time / blah blah blah

I know which one I'd choose