Aluminium hull and superstructure

davids0865

davids0865

Well-Known Member
Joined
23 Nov 2010
Messages
550
Location
Valencia, Spain
www.houseforsalelimousin.com
Hi all,

Are there any special considerations when considering an ally hulled boat, I guess that unlike steel the hull thickness does not deteriorate with rust or lamination, but anything I should think about and check specific to this material?

Thanks

David
 
Aluminium is a wonderful material for building boats with. It might sound obvious, but make sure that a boat has been built with a marine grade aluminium - non marine grades will deteriorate very quickly.
Zinc anodes are important, and you dont really want to park your aluminium boat next to a big steel structure and then connect the two electrically, as your boat might then become an anode for the steel.

They dont need a lot of care and attention - we built a 49' power cat here 12 years ago, and she has needed minimal maintenance during this time. She only needs paint for cosmetic reasons, and we have never had any problems with the aluminium corroding.

Copied below is a photo of her (as per my avatar)
KayakKatunderway01.jpg
 
davids0865 said:
Hi all,

Are there any special considerations when considering an ally hulled boat, I guess that unlike steel the hull thickness does not deteriorate with rust or lamination, but anything I should think about and check specific to this material?

Thanks

David
Click to expand...

If its not a new boat and its got a coat or 3 of antifoul make sure no stoopid buggers dropped a copper coin or any copper wire in the bilges.
 
She only had two 70 hp 4 stroke outboards when she was first launched (this photo was taken soon after), but she still managed to do 16 knots at full chat with about 6 people on board. She now has 2 x 150 hp 4 stroke Yamahas, and she managed 26 knots on sea trials after they were fitted, and she cruises comfortably at about 12 knots with 40 odd passengers on board.

Apologies David, for digressing - is there a particular type of aluminium boat that you are interested in?
 
Plus one for looking out for copper, coins, wiring off cuts etccan quickly go through a hull from the inside.

I have seen a workboat requiring partial re plating due to drying salt water inside the hull causing a strong brine solution with copper wire off cuts.

Also consider thermal insulation and noise, aluminium hulls do resonate.

Look at external electrical corrosion.
 
Aluminium Hulls

Hi David

I am a great fan of metal boats and aluminium does not suffer from corrosion as per steel hulls or de-lamination in GRP. I was fortunate to buy my boat which is all aluminium in 2004. It was built in England by a knowledgeable and affluent Dutchman in 1973 to Lloyd's 100 A1. The pr-purchase survey showed hull thickness was as built! When I installed a larger echo sounder transducer in December 2012, the piece of hull removed was the same thickness as the as built drawings and the inside hull plating is still shiny. Dissimilar metals will always be a problem so zinc anodes are essential but a proper hull bonding system deals with on board electrical problems. Stray currents at poorly maintained jetties will eat the anodes. A great material imho but for a one off design the cost is a draw-back.

Phillip
 
Bajansailor said:
She only had two 70 hp 4 stroke outboards when she was first launched (this photo was taken soon after), but she still managed to do 16 knots at full chat with about 6 people on board. She now has 2 x 150 hp 4 stroke Yamahas, and she managed 26 knots on sea trials after they were fitted, and she cruises comfortably at about 12 knots with 40 odd passengers on board.

Apologies David, for digressing - is there a particular type of aluminium boat that you are interested in?
Click to expand...

Hi All,

Thanks for the first batch of comments, and no problem with drift!

Yes, there is a particular boat that has caught my eye, it's a 1984 Direcktor 66', in Greece on Yachtworld. I have the spec and photos from the broker and she looks well maintained, spacious and well equipped, a bit overbudget, but the main thing is, for me , ally is an unknown quantity.

For me, intending to be a full time liveaboard when I retire (soon) she has many desirable qualities, huge fuel tanks 9000ltrs, bow thruster/holding tanks/watermaker already fitted, big old detroits (simple to maintain, according to Alf), extra cabins for storage, etc etc.

All the boats so far that have appealed have been more mainstream, hence the question.
 
I wouldn't over emphasize the stray current problem but I did retrofit a galvanic isolator, but the boat had lasted from 1973 to 2004 without one without any damage or corrosion. However an aluminium hull in the same yard where I overhauled in 2012 was in poor shape as stainless steel exhaust pods had been direct welded to the aluminium hull!! It comes down tobuild quality and Direcktor meets the test! My recommendation is forget the age, if the boat has been built properly there should be no problems and a great bargin compared with a newer boat. I live aboard for extended periods and single hand most times. 43 foot is big enough for me and you need to develop a berthing scheme that will deal with such a large boat.
 
Bajansailor said:
Aluminium is a wonderful material for building boats with. It might sound obvious, but make sure that a boat has been built with a marine grade aluminium - non marine grades will deteriorate very quickly.
Zinc anodes are important, and you dont really want to park your aluminium boat next to a big steel structure and then connect the two electrically, as your boat might then become an anode for the steel.

They dont need a lot of care and attention - we built a 49' power cat here 12 years ago, and she has needed minimal maintenance during this time. She only needs paint for cosmetic reasons, and we have never had any problems with the aluminium corroding.

Copied below is a photo of her (as per my avatar)
KayakKatunderway01.jpg
Click to expand...

Davids, Apologies for hijacking your thread......

Hi Bajan,

Love your Cat....
Just wondering what sort of thickness the roof is, and the size/ wall thickness of the frame work?
Can you walk on the roof?
I notice you don't have a Center longitudinal tube on the apex of the roof, is it strong enough without?
Does the curve on the sides provide enough strength with the long. tube located 4" below the side curve?

As you have probably guessed, I want to build something like your roof on my 7.6 mtr Cat.........

View attachment 34266View attachment 34267

K
 
Marine grade aluminium is an excellent boat building material. Just a few more to add to the above:

Sound insulation is a must ..... they usually use spray on foam nowadays as it adheres to the aluminium and hence poultice corrosion is eliminated.

Aluminium corrodes by pitting, i.e. isolated corrosion that works through the thickness of the plate. Therefore you need to inspect all the plate.

Galvanic corrosion is the biggest problem ..... get to know the galvanic series, but also speak to surveyors that are experienced with aluminium boats as the galvanic series should not be taken at face value (aluminium can quickly coat the more noble material an pacify it).

Don't use bleach, caustic soda, or strong acids ..... HCl is an absolute no-no.

Lead ballast is a problem too if it isn't encapsulated in something inert.

Don't let this scare you off. Aluminium is a great material ..... and with a little care will last a long time. Repairs are easy provided the fixtures and fittings can be removed. There are lots of aluminium boats out there to testify .....
 
David, that Derecktor vessel in Greece looks very fine indeed.
http://www.yachtworld.com/boats/1984/Derecktor-66'--2455905/Greece#.UgRO_KyWz-t
She might be rather thirsty though, with a pair of 12V GM's down below.....

And Derecktors in the USA have had a long standing reputation for excellent quality.
http://www.derecktor.com/

Rusty Barge, re the cat we built here, the roof thickness is 18 gauge aluminium, about 1.5 mm - it is riveted to the tubular framework underneath.
The crew have often walked on it - the heaviest crew member must be at least 14 stone.
It does flex / deflect, but it is very strong - no danger of it splitting.
The framework is built using 1.5" (38 mm) diameter aluminium tube (wall thickness approx 3 mm).
She was originally designed to carry kayaks hanging from the roof beams in the stern half, hence these beams were built a bit like an improvised 'I' beam using 1.5" O/D tube, as per the photo below.

(I am not involved with Ocean Adventures in any way - I just designed and helped to build the boat).


PA240585_zpse470c044.jpg


There is a centreline longitudinal 1.5" tube, with an additional pair port and starboard to reduce the unsupported panel size.
Even though the roof sheeting material is only 1.5 mm thick, it still had to be put through the roller to get the curve that 'wraps around' the edge - it was too stiff to try and bend it by hand.


She was originally fitted with a bow ramp for beach landing passengers quickly - they no longer do this, so earlier this year the ramp was replaced with a boarding ladder for swimmers, as shown below.

P3220013_zpsfb9cb7c0.jpg
 
Bajansailor said:
David, that Derecktor vessel in Greece looks very fine indeed.
http://www.yachtworld.com/boats/1984/Derecktor-66'--2455905/Greece#.UgRO_KyWz-t
She might be rather thirsty though, with a pair of 12V GM's down below.....

And Derecktors in the USA have had a long standing reputation for excellent quality.
http://www.derecktor.com/
Click to expand...



Thanks all for the insight.

Thirsty, well yeh, 250lph at 2050rpm for 22kts, however, pootling at say 11kts/1200rpm the chart suggests 60lph or maybe a little less! Still not cheap I know.
 
Rusty Barge, re the cat we built here, the roof thickness is 18 gauge aluminium, about 1.5 mm - it is riveted to the tubular framework underneath.
The crew have often walked on it - the heaviest crew member must be at least 14 stone.
It does flex / deflect, but it is very strong - no danger of it splitting.
The framework is built using 1.5" (38 mm) diameter aluminium tube (wall thickness approx 3 mm).
She was originally designed to carry kayaks hanging from the roof beams in the stern half, hence these beams were built a bit like an improvised 'I' beam using 1.5" O/D tube, as per the photo below.

(I am not involved with Ocean Adventures in any way - I just designed and helped to build the boat).
Click to expand...

Thanks for all that info........
 
davids0865 said:
Thanks all for the insight.

Thirsty, well yeh, 250lph at 2050rpm for 22kts, however, pootling at say 11kts/1200rpm the chart suggests 60lph or maybe a little less! Still not cheap I know.
Click to expand...

Chart will be for single engine ... expect twins to run lighter HP load at low RPM's. Lovely engines (but I am partial) and the 71 series were one of the best from DD ever from a reliability perspective. Key here is for the engines to reach a healthy working temp for clean combustion .... and to give them 10 - 15 min at high load (15 - 1800 rpm) at end of cruise to get rid of any carbon buildup etc.. More mechanical friction in these engines, means higher load & consumption at low loads (before turbo kicks in at approx 1200) when compared with 4-strokes... but those lumps will last foerver wehen given clean fuel, good oil and air.. :) ....
 
View attachment 34308

That's a some boat for €200k......

the great advantage of aluminium hull is that age doesn't matter; would you be seriously looking at a 1984 Grp boat that's nearly 30 years old? But.....the disadvantage is that you need a qualified coded welder to tackle aluminium; what's the hourly rate? I believe a general rule of thumb is a ally hull costs exactly twice the price of a steel one. (but of course not the finished boat)

I think It does really make sense in a boat of this age.....
 
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