Asymmetrical hull for 3m planing cat?

[Andrew Fish]

I'm looking to build a 3.0m long planing cat that will have a 6hp outboard. Parallel inside hulls (don't know the proper term sorry) seem to be the way to go so the water isn't trying to compress between the hulls when not planing. And have different turning characteristics. Pic attached for refence to what I'm talking about. Are there any disadvantages to completely asymmetrical hulls?

 

[fallguy]

I'm looking to build a 3.0m long planing cat that will have a 6hp outboard. Parallel inside hulls (don't know the proper term sorry) seem to be the way to go so the water isn't trying to compress between the hulls when not planing. And have different turning characteristics. Pic attached for refence to what I'm talking about. Are there any disadvantages to completely asymmetrical hulls?

The primary consideration in a small cat is the loss of displacement for the same waterline length; especially as cats are already weight sensitive. My honest opinion is for a 3m cat with only 6hp; the loss is not worth any upside to less wave interference.

Further, with only 6hp, the boat will often operate at displacement speeds.

If you are designing for less wsa to reduce drag; that would be another consideration, but at 6hp does not seem to be the main reason.

 

[Andrew Fish]

Thanks fallguy,

As far as I can work out, and correct me if I'm wrong, the loss of displacement mainly relates to sailing cats, because of the extra rocker and hull shape. The drawing I attached is the actual shape I'm proposing for my small cat, the hulls have flat bottoms and rectangular cross sections. The design displacement is only 160kg, ie one person, the boat, outboard and fishing gear. With 6hp it will mainly be planing unless sea conditions don't allow. I guess my main concern is the turning ability of this design. I'm probably over-thinking it but inflatable cats tend to have a lot of leeway because of the round hulls, but mine (which will be ply) might have the opposite problem and want to flip if turned too sharply at speed.

 

[fallguy]

For the same hull configuration; if you flatten one side; this reduces the waterplane and thus; displacement. This is not to say it cannot be adjusted back up.

Have you done power calcs? Do you know the prop you’ll run? Can the prop be changed enough on the 6 hp to suit the needs?

I don’t think your displacement needs and this assumes weights of 150kg are going to plane out the cat well.

 

[Andrew Fish]

I don't have the software to do power calcs. I'm really just assuming it will plane as well as the inflatable designs considering it has a way wider and flatter underside than the inflatables. This is an example:

 

[fallguy]

The TrueKit Discovery 330 only weighs 80 pounds!

Your wider and flatter planing surfaces will be heavier and have more drag.

 

[HelmutSheina]

Kiwi Fish there is an Aussie dude who goes via the handle @sailhand on here who built a ~10' catamaran dinghy that planes with a 3hp, fwd & reverse. You can search his posts and here is a video. He was selling plans cheaply at one point.

 

[HelmutSheina]

What you say about asymmetrical hulls and water compression is news to me and I feel it will add complexity, cost, loss of volume (displacement) and will make the boat more tender; for near zero gain.

Here is a free set of plans for a small cat tender by talented multihull designer and builder Mark Gumprecht that might have some ideas for you.

L'il Nip Free Plans

L'il Nip Plans

duckworks.com

 

[Andrew Fish]

Thanks everyone,
fallguy - you are correct and I didn't appreciate how much more drag the extra surface area has. I will add a chine to reduce the planing surface area.
HelmutSheina - I've been in contact with sailhand, he has given some invaluable advise. I should have added the design brief, my cat will need to go through surf so the idea is a self bailing unsinkable design. I have another thread with more details: Ply or foam cat inspired by 3m inflatables https://www.boatdesign.net/threads/ply-or-foam-cat-inspired-by-3m-inflatables.71160/

 

[BlueBell]

3m, 160kg, 6HP catamarans don't plane.
Maybe you should be considering hydrofoils.

Self-bailing? They'd be sealed sponsons/pontoons/floats/amas, there is no bailing.

 

[Andrew Fish]

What's the takacat with 6hp in the above vid doing? He states it's 'effortless' planing (1:10). The 160kg is the total weight including me, the boat, and gear. Not just the boat, that will weigh 40-45kg. The bridge deck will take on water (it will be a tub shape with openings at the back) and the water will run straight out the back, that's what I meant by self bailing. Just as like the above inflatables do. Sailhands cat above does 12 knots with 3hp. It weighs 47kg.

 

[HelmutSheina]

There is a 3.5m homemade cat in the "magic carpet" video above planing in both directions with a 3hp Yamaha.

 

[BlueBell]

Andrew, the graphics in your original post were misunderstood.

Best of luck.

 

[HelmutSheina]

How about an Hawaiian surf canoe by a bloke who lives on Coromandel Peninsula by the name of Gary Dierking (who is also on this forum). It would use a 2-3hp and paddle you past the breakers. Gary has plans available and it is also in his book.

Building Outrigger Sailing Canoes, Modern Construction Methods for Three Fast, Beautiful Boats by Gary Dierking : Free Download, Borrow, and Streaming : Internet Archive

Building Outrigger Sailing Canoes, Modern Construction Methods for Three Fast, Beautiful Boats by Gary Dierking

archive.org

Build your own 18' (5.4M) outrigger sailing canoe

Length overall 17’ 9” (5400mm) Hull width 18 ½” (470mm) Overall width 6' 7" (2000mm) Dra...

uluaoutrigger.blogspot.com

 

[IronPrice]

BlackDogCats make asymmetrical aluminium PowerCats in NZ. Their smallest model is 4m. A lot more boat than you are considering, but they might provide some inspiration.

I test drove one. They stay level in turns and track nicely. They are horrid in a short chop though.