Are squared hull/transoms essential for a planing catamaran hull?

[Will Gilmore]

Before a boat can plane, it needs to power up the bow wave. Planing happens when the CG clears the crest of the wave and the boat flattens back out over the top.

Move the CG aft and the stern falls more naturally into the wave trough. You get a little more speed over hull speed before you labor the engine. However, you have to drive it farther up the bow wave before it clears the crest.

Move the CG forward and you don't have to drive the boat up the wave as far to clear the crest, but you have more initial effort just to reach hull speed.

Perhaps a movable weight, such as your body, can improve the dynamics by starting out aft and jumping forward to help get her up. As kids, we use to do this when a boat was under powered, we'd send two of us up to the bow to force the boat over the top of the bow wave.

-Will (Dragonfly)

 

[sandhammaren05]

Hi, I am trying to make a planing catamaran from a windsurfer board cut in half and a 2.5hp outboard.
The board has a rounded stern and I am wondering if it would work better if I was to square it off. It currently has a top speed of just over 8mph and doesn't quite plane. I'm thinking of a cut between the two toe strap fastening plates.
Any thoughts?

NB: I have the recent original thread about other aspects of the design in the 'design' section entitled
2.5hp Catamaran - power delivery issue

View attachment 158807

Yes, square it, but get a bigger motor. An older 2-stroke OMC 7.5 will guarantee planing.

 

[sailhand]

Lots of interesting comments here. But somewhat distressed to discover that there is no medal for going the fastest with a small motor. Oh well I guess Im off to discover the real meaning of life. Hmm lets start at 42 and work from there and see what happens.

 

[dustman]

Hi, I am trying to make a planing catamaran from a windsurfer board cut in half and a 2.5hp outboard.
The board has a rounded stern and I am wondering if it would work better if I was to square it off. It currently has a top speed of just over 8mph and doesn't quite plane. I'm thinking of a cut between the two toe strap fastening plates.
Any thoughts?

Have you considered the prop pitch to be a potential limiting factor? Also, maybe try moving your weight fore and aft to change the trim, may make a difference.

In my studies of automotive aerodynamics, it would seem that a sharp end has significantly less drag compared to a rounded end, at least at any meaningful speed. A sharp stern has significantly more drag until the water separates, then the drag reduces significantly. That's my understanding anyway.

Cool project, by the way.

 

[fallguy]

That thing is sinking aft. You basically can cut it off straight and get a performance pickup, unless the weight of engine and crew is too great; in which case; the stern needs to be squared off further back.

And, the bottom of the stern needs to be sharp and not rounded as well for a couple percent more speed...

 

[otseg]

Next >The aft 33% of the bottom needs to be straight buttock lines and squaring off the rounded chine aft would be helpful too.