# 40 ft cruising cat design. First steps.

Discussion in 'Multihulls' started by bscatam, Mar 28, 2016.

1. Joined: Aug 2012
Posts: 95
Likes: 7, Points: 8, Legacy Rep: 61
Location: Nordland, WA, USA

### Emerson WhiteJunior Member

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.

2. Joined: Mar 2016
Posts: 75
Likes: 3, Points: 8, Legacy Rep: 16
Location: Black Sea

### bscatamJunior Member

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.

3. Joined: Aug 2012
Posts: 95
Likes: 7, Points: 8, Legacy Rep: 61
Location: Nordland, WA, USA

### Emerson WhiteJunior Member

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.

4. Joined: Jan 2017
Posts: 204
Likes: 15, Points: 18, Legacy Rep: 7

### valery gaulinSenior Member

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

5. Joined: Dec 2007
Posts: 2,588
Likes: 125, Points: 73, Legacy Rep: 1650
Location: Finland/Norway

### TeddyDiverGollywobbler

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

6. Joined: Sep 2004
Posts: 1,270
Likes: 27, Points: 48, Legacy Rep: 228
Location: Brisbane

### DennisRBSenior Member

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?

7. Joined: Sep 2004
Posts: 1,270
Likes: 27, Points: 48, Legacy Rep: 228
Location: Brisbane

### DennisRBSenior Member

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.

8. Joined: Mar 2016
Posts: 75
Likes: 3, Points: 8, Legacy Rep: 16
Location: Black Sea

### bscatamJunior Member

Layout.

Layout.

#### Attached Files:

File size:
241.2 KB
Views:
263
• ###### taru J3.jpg
File size:
244.3 KB
Views:
218
9. Joined: Mar 2016
Posts: 75
Likes: 3, Points: 8, Legacy Rep: 16
Location: Black Sea

### bscatamJunior Member

Steering

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

#### Attached Files:

File size:
122.8 KB
Views:
247
File size:
124.7 KB
Views:
228
• ###### dss2.jpg
File size:
164.1 KB
Views:
294
10. Joined: Nov 2015
Posts: 681
Likes: 14, Points: 18, Legacy Rep: 16
Location: Sydney

### UpOnStandsSenior Member

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.

#### Attached Files:

• ###### tag50_catamaran_tag_yachts_sailing_south_africa.jpeg
File size:
139.3 KB
Views:
353
11. Joined: Mar 2016
Posts: 75
Likes: 3, Points: 8, Legacy Rep: 16
Location: Black Sea

### bscatamJunior Member

Steering assembly

Steering assembly left station.

#### Attached Files:

• ###### sta.jpg
File size:
205.8 KB
Views:
189

12. Joined: Mar 2016
Posts: 75
Likes: 3, Points: 8, Legacy Rep: 16
Location: Black Sea

### bscatamJunior Member

Final design

Final design renderings.

#### Attached Files:

File size:
236.5 KB
Views:
270
• ###### NP1.jpg
File size:
198.6 KB
Views:
226
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.