40 ft cruising cat design. First steps.

It is a measure, it's not the only measure. Ballast percentage is a measure of weight distribution but there is a huge difference between the COG of a boat with 10 tons of lead in a bulb on the end of a long fin keel and a boat with 10 tons of concrete in the belly below the floorboards.

You can move volume out fore and aft without changing the Cp, but there is a limit to how much you can do that before you start to make the hull really bizarre.

 

For cruising cats Cp = 0,57-0,60 is good. As to weight \structure. I think its OK. At least on my spreadsheet of materials and and equipment weight.

 

My understanding (which may be flawed) was that cruising cats could get away with a lower Cp because the additional displacement for length made them less tender. If you are designing a lightweight cruising cat this might be something to consider.

 

I am with UpOnStands with explanation. Exactly what i have understood while reading about prismatic coefficient.

 

Water-plane coefficient is a better measure what comes to hull form vs pitching IMHO.

 

Yes well imagine the bows of 2 different but similar hulls with the same Cp. One gains its Cp by having a narrow waterline and deep forefoot, the other hull has wider waterline at the bow (higher water plane coefficient) but shallow forefoot. Both hulls displace the same at the bow, but one gains the displacement by being wide, one by being deep. The narrow one will cut through waves better and likely have a smaller bow wave. But its smaller waterplane will mean its resistance to hobby horsing will be worse. But then again being able to slice through waves will mean less moment to initiate hobbyhorsing to begin with?

 

After seeing the cross section I believe a flat or much flatter roof will be better and doable for the sake of steering cables. Surely such a high windage an FP looking cabin top seems a questionable design compromise just so your planned steering mech works. IMO the steering system needs to be looked at if a huge sloping roof is the concession that needs to be made to accommodate it.

A large flat cabin top will be a nice lounging area with cushions sometimes. I certainly hung out on the flat cabin top of my last cat to enjoy the 360 view with a beer in a beautiful anchorage.

 

Layout.

Layout.

 

Steering

Double steering station setup. Conduit and open wire Combination. Rudder synchronisation with aluminium rod.

 

just saw this on the new TAG 50
twin aft helms but the helmsman sits side on - certainly cannot complain about sight lines up the hull sides. Not sure I like the immediate step down to get to the winches.
edit: its likely that the passerelle and swim ladder flip completely over to fill in the gaps while sailing.
Port side passerelle and starboard side swim ladder
They say the rudders are liftable - trying to find more detail.
(originally posted in wrong thread)

 

Steering assembly

Steering assembly left station.

 

Final design

Final design renderings.