8m demountable cat design - feedback wanted

[Coyote Boats]

I’m looking for feedback on a hull shape I’ve drawn for an 8 m demountable catamaran. The design intent is local lake sailing with occasional trailering for multi-week cruising. Think Stiletto 27 with a bit more stability and load-carrying capacity, or a Stiletto 30 that’s a little more travel-friendly.

Specs so far:

• Displacement: 1800 kg
• Length: 8 m
• Beam: 5.7 m
• LWL: 7.38m
• BWL: 0.7m
• Foils: Daggerboards and cassette rudders (the boat will be moored in ~3 ft of water)
• Construction: Strip-planked below the knuckle; 6 mm ply on frames above the knuckle
• Prismatic Coefficient: ≈ 0.64

I haven’t drawn a cabin yet—may keep a flush deck and skip a cabin entirely. I also haven’t started on the rig or foil design yet; at this point I’m only looking for feedback on the hull shape.

The hull has a fairly flat rocker with a slight hump aft, similar to Shuttleworth and Grainger designs. Sections are V forward, U-shaped in the mid-sections, and flat-U aft.

My questions:
1. What can/should I do to further refine the hull shape below the knuckle before I start building?
2. Is the hump in the rocker too pronounced?
3. What should I ask that I'm not asking?

Some screenshots for context:

Center of buoyancy:

Waterplane section:

Body plan:

If someone knows how to add an STL to a post I'd be happy to include that too.

Thanks!

 

[oldmulti]

Coyote Boats. The shape looks OK but have you any calculations of where the EG the aft fore and aft PC's are to judge the amount of asymmetry there is in the design etc. These sort of numbers can be obtained by using eg Hullform 9 or DELFTship hull design software (both are for free on the web). Each of these have hull drag sub routines which give an idea if one hull shape has a lower drag than another. All the numbers are just indicative, and you may have a reason for a fuller stern etc. The Grainger bump in the stern depends a lot on the expected speed of the hull and he soften it out in cruising designs. There is no perfect answer for a hull shape and factors such as intended purpose of the design and the general speeds the boats will sail at control the hull shape general outcome. Your shape seems reasonable but at 8 meters crew weight will influence the hull shape and the wider stern may be needed to support the weight aft. Below are some jpegs of the Shuttleworth 8 meter Cheetah design done in about 1990. The design was fast and sailed well on all points. It had a finer stern shape than your proposed hull shape which reflected the thoughts of the time. Not suggesting anything. Good luck and have fun.

 

[Coyote Boats]

@oldmulti - Thank you very much for the reply. I appreciate the Cheetah information and will digest it over the next couple of days. I had not seen DefltShip previously, I've downloaded it and am now trying to find a way to import the Fusion 360 surface to DelftShip for further analysis.

Thanks!

 

[fallguy]

I can’t offer the insights of oldmulti or others on hull shape for sailing anythings.

I can tell you planning the hulls without or with the cabin is an error. My boat cabin resulted in a trim problem as it was not correctly weight studied. And it isn’t always easy to place the cabin anywhere, so I advise deciding up or down on one.

 

[Coyote Boats]

@fallguy - great point. I really do need to get far enough along with the design to complete hydrostatics before starting in on building the hulls. I'll post more information as I devote time to the project.

 

[rberrey]

Something like the K - Design 900 with an optional pod for extended cruses with load as the priority -v- speed