The Hulls

May 21, 2012.

The catamaran is built mostly of 5083 H321aluminum alloy.  The kind of aluminum that resists marine corrosion to he point that it can be left bare, for the ultimate low-maintenance boat that theoretically never needs varnish, wax or paint (except for anti fouling on the underwater parts).

For heat reflection the aluminum surfaces have to be painted white. 

For efficiency, the hulls of a displacement catamaran that are submerged must have a width to length ratio of 1 : 14.  Anything less compromises efficiency.  A ratio of 1 : 12 is barely acceptable, while a ratio of 1 : 8 is only good for a houseboat.  The PH8 has width to length ratio of 1 : 13.3.

The propellers and rudders are fully protected.  A beaching skeg on the bottom of the hulls lets you rest the cat on the ground without a cradle or protections.

The hulls are devoted mainly  to the engines, generator, various systems and tanks.  If some space is left over it can be put to good use for storing supplies, spare parts or other odds and ends.  Before storing too much stuff in the hulls (or for that matter, anywhere on the cat) remember that weight and the longitudinal center of gravity are very important in a catamaran and must stay within specifications.

The hulls are partitioned with bulkheads into watertight compartments for added safety.  Each hull has five compartments that are separated from each other, each with its own bilge pump.  The bow compartments, which are the main anticollision compartments of the boat are completely isolated from the rest of the hulls, with openings for electrical wires kept to a minimum and placed up high.

To be able to replace an engine or to do an extensive overhaul above the engine room there is a bolted hatch through which the engine can be lifted out.




Wherever possible, the boat is built of 5083 aluminum alloy.  Extruded pieces on the inside like stringers and flat bars are 6060 aluminum alloy and a few parts, mostly pipes (propeller shaft pipe, railings etc.) are of 5754 aluminum alloy.


Aluminum alloy, with > 4% magnesium.  

The 5 mm plates of the first batch were thus marked:
5083 H321 - ASTM B209  Alcoa Italia
Th = 5 mm cast no. 81124C4 lot no. 6997-6998


Aluminum alloy, with some magnesium.  The mechanical properties are not as good as the 5083 alloy, but has good corrosion resistance.


Values for extruded pieces, annealed: Rm = 125 MPa, Rm 0.2 = 80 MPa 

  Density (kg/dm3) Melting point  Thermal conductivity(W/mK) RM  (MPA) RP 0.2 (MPA)
5083 2.66 574 - 638 110 - 140 240 125
5754 2.66 590 - 645 132 190 80
6060 2.7 585 - 650 200 - 240 125 80

The certificates provided by Airoldi Metalli,are also part of this section:

Airoldi Metalli spa Aluminum certificates

Airoldi Metalli spa
Viale Grandi, 59
I-22047 Molteno (LC)
Phone: +39 031 3574 111
Fax: +39 031 3572 077


Safety Rails

The minimum height of a safety rail around the boat must be  910 mm, while the distance between the openings must not be more than 380 mm and not less than 230 mm.  The minimum height of the falchetta must be 17 mm. 

To make repairs more easy, the stanchions are inserted into a piece of tube that is welded to the holl.  The weld between the stanchion and the supporting tube is only between the stanchion and the tube, making the repair or substitution of a piece of rail a relatively simple matter.

The rail is made of 5754 aluminum.  The stanchions and the top rail have an O.D. of 35 mm, while the middle rails have an O.D of 25 mm.  The stanchions are supported by a piece of pipe with an O.D. of 42 mm.

Stanchion supports metalluminio round tube O.D. 42 mm, I.D. 36 mm
Top rail metalluminio round tube O.D. 35 mm, I.D. 29 mm
Middle rails metalluminio round tube O.D. 25 mm, I.D. 19 mm

Metalluminio srl
Via Valdilocchi, 6
I-19126, La Spezia 
Phone: +39 0187 50 30 77
Fax: +39 0187 50 30 58

The following catalogue supplied by Metalluminio is also part of this section:

Metalluminio Metalluminio catalogue



All the windows are non opening and glued to the hull. 

Refer to the section Windows for more information. 



Fuel Tanks

The two fuel tanks are part of the hull and go from station 6 to station 8.5.

Each tank has 3 inspection covers bolted to the tank top plate.

The tank area on frame 6 (form level 0 to level 1320) is 1.308m2.
The tank area on frame 8.5 (form level 0 to level 1320) is 1.326m2, averaging 1.317m2.

The empty distance between frame 6 and 8.5 is 2.495 m, minus the two frames 7 and 8, therefore 2.485 m.
The fuel capacity of one tank is therefore 1.317 x 2.485 = 3.272 m3 or 3,272 liters.
The total fuel capacity is 6,544 liters.

Water Tanks

The two water tanks are part of the hull and go from station 9 to station 10.  Each tank has one big inspection cover bolted to the tank top plate.

The inside of the tanks are painted with food grade epoxy resins.

The tank area on frame 9 (form level 0 to level 719) is 0.650m2.
The tank area on frame 10 (form level 0 to level 719) is 0.649m2, averaging 0.6495m2.

The empty distance between frame 9 and 10 is 0.995 m.
The water capacity of one tank is therefore 0.6495 x 0.995 = 0.646 m3 or 646 liters.
The total water capacity is 1,292 liters.

Black Water Tank

The black water tank is a commercially available polyethylene tank strapped down inside the hull.

Refer to the section gray and black waters for more information.


Aluminum and Stainless Fasteners

May 2012

To stop galvanic corrosion when a stainless steel fastener goes through aluminum Nemo indistrie spa suggests to use their Seaguard anticorrosion paste.  Bought one tube for testing.



Refer to the section Welding and Cutting Aluminum for more information.


cat_02_e hulls.htm