Power Cat Rocker

[JEM]

Been having e-mail conversations about design of power cats and their rocker. Keep in mind that the e-mails having been just conceptual ideas. No sketches even made. Just talking about "stuff". But that's how ideas get started.

Discussion turned to issues with producing variable length fiberglass cat hulls using transom dams in a mold. The idea was a 40' mold that could produce hulls from 24-40' work-boat type cat hulls.

One issue I see is the rocker. Power cats have some significant rocker. I'm assuming this is to compensate for the hard tracking of the hull.

Here's my thoughts:
If the rocker was designed for a 40' hull and you reduced the length by almost half via transom dam, you'd take away almost all of the rocker. But a shorter hull may not need as much rocker.

I'm no large power boat designer or nautical engineer. So I'd like to ask for an open discussion of the idea of a long mold for variable length hulls.

Is a practical? Too radical? Is the rocker as big an issue as I think it may be?

Any and all thoughts welcome. Want to learn.

[KCook]

Power cats have some significant rocker.

I was surprised to see that. Considering the reduction in resistance due to the long LOA/beam ratio of each hull, I would have thought there would be less need for rocker. If you want the poor thing to turn, don't make it a multi-hull in the first place

Kelly Cook

[JEM]

Good point. Do power cats even need so much rocker?

Our discussion about cats having lots of rocker is from examples of hulls seen around the net.

Nothing is absolute. Opinions on that?

[KCook]

Well, small cat rig sailing catamarans can have a LOT of rocker. Just so the poor things can try to tack across the wind. As you alluded to in your initial post. Also gives them a sporty look sitting on the beach. But that is a special case, nuthin to do with power cats.

Kelly

[Frosty]

I have a 44 foot semi displacement power cat with massive rocker. The boat is capable of 24Kts with 2 x250 deisels and surface props. I too am not very good with hulls and design stuff but I know engines. When I fit the motors I was really worried about the rocker. I think it helps with some thing called wake fraction. Again not sure but I was told by a guy who did that this was to kind of get the wake to slow down so as to produce a perfect place for the propellor. If you wanting to produce a work boat would the rocker be so important?