cat hulls

[Fluid]

Here's a good explanation.....

There is a lot of misinformation about steps in boat hulls. The referenced link makes the same mistake that most folks do. Yes, steps can "break up the water" but there are better things to do than that to increase performance. Today's full scale marine architects understand how steps work but it wasn't always so...

In short, correctly designed steps are set an an angle of attack to the boat's designed running attitude - they are not parallel to the hull/sponson bottom. This angle supplies lift - a good thing. Planing surfaces are most efficient in developing lift for only a short distance from where the step hits the water. Instead of using a long flat hull bottom - which is not very efficient in developing lift for most of its length - you put in 2-4 steps which only contact the water for an inch or so. This does several things, it develops more lift for much less drag, and the steps can be designed to positively define the best angle of attach for the hull instead of trying to "fly" it at an unsteady attitude.

Incorrectly designed - and many model boats with steps have rather poorly designed steps - and you'd be better off without steps. Simply putting 'breaks' in the hull bottom just increases drag. Steps can also degrade turning performance markedly, particularly in monos. If you don't understand how they work I'd not put them on.

John Hacker was the king of stepped hulls in the 1930s. He understood their design and function better than anyone else did and he built a fleet of GoldCup-winning stepped hydros. Step design languished for half a century and was pretty much forgotten except as a novelty until modern designers took a look at Hacker's designs and re-learned how and why they worked. Today stepped hulls are common on full-scale racing monos and cats.


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