Alumunium 5083 thicknesses

[adamgoldberg]

For those with Al experience, is the following sufficient, overkill, underspecified, etc?

• 6mm plate 5083 marine-grade aluminium bulkheads
• 8mm hull plate 5083 marine-grade aluminium below chime
• 5mm hull plate 5083 marine-grade aluminium above chime
• 4mm plate aluminium 5083 marine-grade aluminium superstructure
• 26mm aluminium bar on the keel of each hull and plate reinforcement of each bow

[dannc]

Originally Posted by adamgoldberg

For those with Al experience, is the following sufficient, overkill, underspecified, etc?

• 6mm plate 5083 marine-grade aluminium bulkheads
• 8mm hull plate 5083 marine-grade aluminium below chime
• 5mm hull plate 5083 marine-grade aluminium above chime
• 4mm plate aluminium 5083 marine-grade aluminium superstructure
• 26mm aluminium bar on the keel of each hull and plate reinforcement of each bow

It *** LOOKS *** adequate but the "strength" of the boat is determined by not only the plate thickness but by the scantlngs behind the AL plate.

Gerr goes into this in his book, The Elements of Boat Strength: For Builders, Designers, and Owners, https://www.amazon.com/Elements-Boat...=UTF8&qid=&sr=

Kasten discusses this on his website, Metal Boats For Blue Water - Kasten Marine Design. There is quite a bit of information on Kasten's site if one is interested in boat design and especially metal boats.

Later,
Dan

[noelex 77]

What size and type of yacht is this for?

A lot of the strength of an aluminium boat is in the framework so plans detailing these elements are essential to make even a crude assessment of the overall strength.

Can you post plans please?

[adamgoldberg]

Originally Posted by noelex 77

What size and type of yacht is this for?

45-50' Al blue water liveaboard catamaran.

Originally Posted by noelex 77

Can you post plans please?

Alas, no.

[noelex 77]

For a 45 foot catamaran those skin and bulkhead thickness are typical or very slightly on the generous side. To assess the structure properly the spacing and construction of the bulkheads, stringers etc needs to be considered, but there are no red flags in the limited details available.

[adamgoldberg]

Originally Posted by noelex 77

For a 45 foot catamaran those skin and bulkhead thickness are typical or slightly on the generous side. To assess the structure properly the spacing and construction of the bulkheads, stringers etc needs to be considered, but there are no red flags in the limited details available.

THANKS, that's the sort of answer I was hoping for. I trust the designer/architect, but am reviewing a build spec contract...

[dannc]

Originally Posted by adamgoldberg

45-50' Al blue water liveaboard catamaran.
...

Since a catamaran is more weight sensitive than a mono hull, using 5383 or 5059, MIGHT allow one to have the same "strength" but thinner plate, thus lighter weight. It depends on the scantlings and designed parameters.

See Kasten's website for a good discussion, Determining Scantlings According to the ABS Rule.

There is a nice table about AL yields on the link which is important to fully understand this quote from the page:

We can see from the above that there is quite a variation in the strength of each one of these alloys depending on their "temper". There is also a considerable variation in strength between alloys regardless of their "temper".

For my own designs, in order to make use of the most conservative strength values I use the fully annealed material strength as is most often specified within the ABS rules. In addition to using the fully annealed condition, for the scantling calculations I generally assume the lowest strength alloy (5086) will be used unless it is KNOWN for certain that a specific higher strength alloy will be available and will be used. This approach allows use of 5086 throughout if that is all that's available to the builder. Then if a higher as-welded-yield strength material is used, the vessel will simply be that much stronger, however in all cases the ABS rule will still have been satisfied.

This can result in heavier weight than is necessary if 5383 or 5059 plate happens to be available. Therefore in a weight-sensitive application, there is plenty of room for optimization of the structure to suit the higher fully annealed yield strength of 5383 or 5059 alloys.

There might be an increase the AL plate cost, if the 5383 or 5059 is available, but it is something to be considered in the design spiral.

Later,
Dan

[Matt Johnson]

You'd have to verify, but I believe a Mumby 48 is 4mm below waterline and 3mm above. So just based on the numbers in your OP and having no other data, it looks like it's less of a performance cat and more rugged explorer.