Aluminium vs Steel

D'artois, what is not a fact?

 

The way you calculated the weight reduction - payload stands out of the comparison alu vs steel. The maximum weightsaving you may achieve bu using aluminium is about 18.5% - I illustrated this with an comparison between two existing boats of the same design. In clear deadweight. If you make such comparisons that it is not correct to make thos on weight for weight basis. That would be too easy.

Steel: specific gravity 7.8 - deduct: reinforcement and ballast;
Aluminium: specific gravity 2.7 - add: ballast, reinforcement, increasing dimension of plating.
Under the black line there is only a difference left of 18.5% in weight.
- the question is does that weigh against the increased costs of material and manufacturing.

 

Nicely said Brien,

What alu guys seems to over look, is that the plating on a aluminium hull and deck goes up by about 50% (eg. 4mm steel = 6mm alu - 3mm steel = 4.5mm alu) in thickness over steel. A similar scenario with framing etc etc.
Although alu is much lighter, the weight saving is reduced considerably, and then all the other things you pointed out reduces it further.

 

No,... and yes

No, if anyway you look at a moderately heavy cruiser, where even optimized you would find yourself with a weight saving of, say, 8 to 10 percent on the total. (Although 10 % of 10 tons is a lot of additional beer you could carry for the same DWL )

But YES, if the objective is performance and you really drive the design, shaving of weight (and cost) on rigging, sail area and so on and find yourself with a different shallower hull and less displacement and probably less cost

This means really that
- both weight and cost differences in the total finished boat for a given identical design get very small and could be neglected
- it probably does not make sense to build a boat that was designed for steel in alloy instead, unless there are other considerations that would justify that choice
- it anyway would be silly to build a boat in steel that was originally designed for alloy.

There is not a choice between materials for one design, only between different designs optimized for their respective material.

 

...?
Seems to me that if you take a ballasted hull, and replace the steel hull & deck with lighter alloy, the VCG moves down, meaning higher stability for the same amount of ballast. Or, replace an alloy shell with a steel shell, your VCG moves up and you would have to increase ballast to get the same righting moment - or did I misunderstand something here?.

 

How many big Russian and American Ice Breakers are built of Aluminium. Big tankers, cargo, car carriers, cruise ships. Those people really pinch a penny if it is possible. Why do they avoid Aluminum? Large naval ships.

 

If I may chime in here,

It is just the other way around
If you change a specific ballasted steel hull & deck to aluminuim, she will be lighter and hence floats higher with the same amount of ballast. IOW, the VCG rises.
If you change a ballasted aluminuim hull & deck to steel, she will be heavier and will floats deeper with same amount of ballast. The VCG lowers.

 

Wynand, what is lower is the VCG with rapport to the waterline, but not within the boat itself.
I see where you are going there, but i assumed that the boat would be identical (including the beer to bring it on it's WL ) in which case it would float on its lines with a lower VCG for the lighter hull. There is no way that by increasing the weight of the deck you would then be allowed to reduce the ballast because of "ballast ratio"
If you accept a different WL you have a different boat alltogether - see my post above.

 

Just a few points Gerd: the illustration I gave i.e. the comparison between two boats of the same design is not a theoretical one but an excisting one.
There are builders that prefer steel above alu and vice versa, therefore a number of architecs - also with an eye on the DIY market - are offering both options.

Now the point of the differences in Ballast we may arrive to the following conclusion: It is not only the keelweight that adds to the righting moment but also the weight of the hull below the COGwet as well as the remaining mass.

For my example of the 37' yacht the difference in ballast is 400 kilo's in favour of steel. The alu construction has to add with 400 kilo's extra because the weight under he waterline is less.

For example the deadweight of a Madeira 44'is in alu is 12000 kgs and in steel
14.300 - a mere difference of 2300 kgs; an almost neglegible difference save for the racing yachts and not justifying the double costs of manufacturing.
In my 2#ct opinion.

 

also... (sorry, can't stop it now ) take a steel hull and heel it 90 degrees. your CG is now laterally displaced with rapport to your CB, the righting moment resulting from the horizontal distance (the arm) and the mass.

Now replace the steel with alloy, all other things equal.

You will find that your CB is vertically very much where it was for the steel hull (small variations due to the shape of the immersed side of the boat would be possible) although a bit higher (less displacement), but your CG is horizontally nearer to the CB, the arm is longer and the mass smaller.


I would say that the righting moment is smaller, no?
EDITED: got all mixed up in the case of the alloy-hull the arm is longer of course, and the righting moment higher. But then the mass is smaller...
Right.
Now, if the arm is longer and the mass smaller - are we getting a draw??

 

Well, yes, it's not the keel but the CG - regardless of HOW it got where it is, and what it is comprised of - and its position with regard to the CB during the roll. What is clear though is that the heavier your topsides, deck and cabin (and they represent in most cases a much bigger part of the weight than the immersed hull) the more you have to compensate with ballast for a given hullshape and WL.
If you had no keel or any ballast at all but a big fat 5 ton bottom plate in your hull, that would bring you back upright in no time.. I had a 6 mm steel bottom on the last boat I built, and this time no keel at all, just inside ballast - and don't get me wrong, I am a steel man myself ;-)

 

D'Artois, with due respect, I think you misread what I said. My point was that the 20-45% weight saving is for the structure alone. The total displacement reduction in using Aluminum vs. steel will be less, as you state. So if one saves 20% in structure, the overall displacement savings will be quite a bit less, such as 15% or smaller.

I have read and done numerous parametric design comparisons for the navy of Aluminum vs. Steel. I can't really disclose too much about it, but trust me, we've done these studies very in-depth. As I said before, much depends on the details of the design. It's not appropriate to say a single number, like 18% what you will save. The range is wide, depending...

 

There, I am with you - but therefore differences are so minimal, or better, so unsignificant. The original topic was actually to establish economic/strength and the value of the non-corrosive properties of aluminium.

A friend of mine, who has a charterbusiness operates an alu schooner of 60' - over the years of hard use the whole hull is dented, dented, and dented.

Itis as you say - much can depend on design, quality of design plays also an important role. Maybe you know the French design "Antarctica" if I have the name right, well that is an iceclass alu design with a hull of 2,5 cm thickness,
doing very succesfully antarctic research. But for the everyday sailor a steel design is the strongest, most affordable (I prefer not to use the word cheap - nothing is cheap in boating) and in the end the easiest to maintain.
The costs - in particular on the large yachts - to protect their expensive and erosion vulnerable hulls with active protection - are high and you may ask yourself, for what?

As I stated earlier, some years ago I was involved in a schooner project and the hull of that schooner was an Iceclass rated hull. The insides were powdercoated, top of the top workmanship - only that hull costed 2,2 mil
Euro's - for a 29 mtr schooner.
The schooner would have had the same beauty for 1,2 mil euro less. And less headache too.

 

Steel this powerboat?

Hi everybody

Time for my $0.02.

A 15 to 18% overall weight savings does not seem like a lot. Indeed, the types of boats I care to design, it really won't matter much. However, with certain classes of boats that weight saving can compound just like interest.

Suppose you have a boat where the engine accounts for at least 20% of the over all weight. If you make the boat lighter, you can make the engine lighter as well. A smaller, lighter, engine needs less fuel for a given range, so now the fuel tank can be smaller and lighter too. Since these two items can make up 30% or more of the boats weight, the hull structure can now be made lighter too. For this reason, I can see a real problem in resorting to a heavier construction material.

Ie. How many full planing steel powerboats do you see?

Also, welding may not be the ideal construction method for Aluminum. Most aluminum boats I see are riveted. But, alas, it doesn't look pretty.

Bob

 

Exatly, Bob .

And from here on we now have to choose: design a boat for this new, lighter displacement, with a diffent hull or, like in d'Artois example build to the same design but out of lighter material.

In the first case it is simply a different boat.

If it's the same hull:
For a cruising sailboat, I would say that 18% additional load carrying capacity on the same lines would indeed be interesting. Especiall as you might act intelligently and stow that low to bring the VCG further down.
But obviously the value this will have to you (and it's relationship to the price you pay for it) depends on the program and the size of boat: 2 people living on a 30 footer would be very happy with 500 kg more to load, but on bigger boat they can carry more than they can eat anyway, and to have two tons more to load won't really change their life. They might choose to give part of that up for a lighter different design and more performance, or - like d'Artois - consider that that's not worth the extra money.

As for price, we have to see what we bring it in relationship to, comfort, or performance, or ease of build for DIY or volume or load capacity etc. In the end, you can't compare anything unless you have specified the program and major requirements.

Maybe we are comparing apples and oranges anyway, when looking at these two materials just because they both happen to be metals. If we talk hardchine construction we might include ply, or if its round bilge, why not to strip planking or carbon epoxy... strange that steel/alloy always triggers these discussions

BTW, I also ran my assumptions from yesterday through some calcs on my own 31' steel project. It's not a draw at all. All other things equal and for the same lines and ballast: The lighter the shell (hull/deck/cabin) the lower the VCG and the higher the righting moments.