# 15 Stepped Hulls

Lorne CampbellTechnical

Steps are all the rage these days in planing craft, and particularly so in the continuing development of the RIB.
They were introduced at the beginning of the 1900s and used in the fastest racing powerboats from around 1910 up to the late 1930s but then fell from popularity.
Adding steps to existing hull designs is not new, many of the 1920s American Gold Cup racers which were originally step less were modified to multi-steppers at the beginning of the 1930s and instantly became quicker.
These days stepped hulls are not confined to specialised racers. They usually come from modified standard deep vee moulds for craft used in recreation and cruising and this allows direct comparison between the two types. Basic advantages are lower drag and better pitch control in rough water but the degree of advantage depends on how good the original hull was. Reduced drag can give a speed improvement of 5% to 10%. Speed increase comes from a reduction in wetted surface, meaning less drag. Lift depends on area, angle of attack and speed.


To get the Centre of Lift (CL) correct there will be more wetted area than necessary.  A stepped hull has two or more contact patches both sides of the CG. The individual angle of each step can be set during design and a very small change of overall trim will create a large change in contact area between steps.


More speed will cause a stepped hull to rise bodily since the lift centre hardly moves; the patches just get smaller as speed induced pressure increases. A non-stepper reduces trim instead, otherwise hull rise would cause an aft shift of CL; wetted surface stays the same or increases, so the drag is higher.  Shifting lift quickly from one step to another rather than changing trim to move the wetted surface means much less pitch change for the modern stepped hull in a seaway.

Old style stepped hulls carried their weight forward and used to fall into the trough beyond waves.  Modern ones lightly loaded forward will keep their bows lifted for a fraction of time after wave impact before the aft step or steps lift the stern and bring the boat level again.
Non-stepped hulls carry on pitching up while the wave rolls aft along the bottom until the whole hull has changed trim to an angle which launches it into the air.
Three possibilities are:
(a)  reduced roll stability – careful design can lessen this – due to the lower wetted surface;
(b)  increased drag off plane and at low planing speeds, which is not really a problem with relatively fast planing craft such as RIBs and

(c)  steps must be ventilated properly to avoid step suction although deep vee hulls do not usually suffer from this since the outboard edge of the step cuts the chine well above the surface when planing.

Marine Craft Designers and Naval Architects