Calcuation to estimate planning speed

[PI Design]

Originally Posted by Petros

I know about the idea that cats can not plane, there are actually some pretty old threads on this forum about that, but it is false.

I didn't say they couldn't plane, I said they didn't generally make good planing hulls. Speed onto the plane is most certainly a function of beam/length (wetted) aspect ratio, where cats do very badly (but then they have much less wave making drag, so its swings and roundabouts). Yes, you can use stepped hulls (like seaplanes do), but as Chrid points out, they have their drawbacks. Sea planes use two floats for stability and structural reasons, not because the cat configuration allows them to plane well.

[Petros]

Originally Posted by CT 249

Are you sure, and is the comparison a perfect one?
.....

The early creators of seaplanes have great difficulty in "unsticking" until canoe and yacht designers like Linton Hope etc did their hull designs. But isn't it to be expected that a shape designed to work with lift from wings and 40-60hp is very different from a shape that will perform better with no aero lift and much less power, and therefore comparisons between the two could be fraught with danger?

Yes quite sure. You can not pick up speed on a float plane unless the wings aren at zero angle of attack, and you come up on plane on the hulls. You have to rotate the aircraft AFTER you reach take-off speed to lift off the water, or you will just stay stalled and never lift off. This is a well understood process, you can not take a light aircraft off the water without being up "on step" of the floats (planing) so you can do the take-off rotation. ON land planes the location of the landing gear is critical for the same reason (as is the location of the float step), so you can rotate once you reach take-off speed.

[CT 249]

Okay, thanks for the correction regarding the take off roll.

However, surely the main point stands - the type of hull that performs best under 40-50hp is very different from the sort of hull that performs best under sail.

Interesting that you find the comparison between seaplane floats and small cats is perfect. Does that mean you think trimarans should have hulls like a Catalina or Sunderland flying boat?

[Petros]

This is not quite true, the same type of hull will perform in similar ways in similar conditions. With a sailboat you have to adjust the size and shape of the surfaces to the available power (sails), speed and the weight of the craft, hence my reason to for the question, how do you size a planing surface? From my professional experience in fluid mechanics I know this must include a lift coefficient, a surface area, and a speed to calculate the lift, what is not clean to me how this works on the surface of a fluid vs. inside the fluid (as a wing on an aircraft, or a rudder or center board). The thing that is not clear is how does the interface on the surface affect the equation.

And yes, a properly sized planing hull on a cat will be faster when in planing mode, the problem with the traditional planing design is that they have way too much drag when in the displacement mode. For a sailboat to be fast in all modes of sailing, you have to make compromises, which means likely a planing hull is not a good choice for most sailing conditions.

I have been toying with ideas for a shape for a planing cat hull that still has reasonable non-planing (displacement) performance. I will have to build at least a small version of it see if it works. But I need to be able to size the planing surface for the speeds and weight I anticipate, and will take the rest as it comes.

[Guest625101138]

I have found both Michlet and Savitsky give similar results for drag under planing conditions. Even with hulls down to L/B of 4. This link gives a quick result for Savitsky:
http://illustrations.marin.ntnu.no/h...ing/index.html
Working out the thrust line might be a bit of a challenge with a sail. The rest shouid be OK.

I have also seen an analysis of planing that gets good results on the basis of the surface being half a foil. So the Cd would be half of a flat plate under the same conditions and the L/D is the same as the plate. This seems to give reasonable results per attached JavaFoil screen dump.

The fastest all round sailing hulls I have seen have very flat sterns that plane easily going downwind. They are almost hard chinned so they have a "V" hull that reduces slamming going to windward. So the angle of heal is used advantageously to get very different hull forms.

Rick W.