Hull material for arctic cruising

At the moment plans are to start at the bottom end, around the Chilean channels. I'm not aiming for a NW passage, but Norway, Greenland and Newfoundland are on the list, and the Bering and the sea of Okhotsk if I'm still enjoying it.

 

Ice is melting away from the arctic so NW passage is soon ok to go atleast during summer months..

 

IMO This safety related observation is not based on valid observation.

When considering small vessels monohulls will always be considered more seaworthy without exception. Even poor stability curve monohulls rolled to 180 degrees don't sink from this condition. A well designed mono properly battened down will right in seconds with no flooding at all. {We can write chapters on "well designed" }.

Smaller monohulls can be designed to a very high level of reliability for severe heavy weather. This type of powered craft is often designed to be able to continue to operate regardless of knockdown pitchpole capsize or even severe collision.

It's easy enough to make a monohull as 'unsinkable' as a cat. I just did this exercise on a yacht, watertight bulkheads and sailable with any compartment flooded, (for the military its any two adjacent flooded). Most smaller metal hulls don't need this much anyway since they are so tough in collision.

By the same token that a 50 foot multi is hard to invert ditto for a monhull and a very low risk for the mono particulalry if designed accordingly.

A monohull totally rolled and even dismasted has more options of picking up survivors in the water eg fast uncomplicated deployment of easily accessed equipment. Designed with safety in mind the auxialliary can be fully operable too.

Cat (and unstable mono) inversions in heavy weather have a high incidence in the loss of life possibly because they lack these abilities. Both mono's and multis that are stable inverted tend to historically have a high probability of killing some of the people aboard if inverted, some drown because they are trapped or unconcious, others drift away becasue they had to sever the lifeline to surface.

The reality of the desirability of a stable inverted platform may not be as attractive as people lead each other to beleive.

 

there have been many successful adventure sailings to Antartica, many in alloy boats Blake chose such a boat for his charters down there
What is the use being in an inverted boat, weeks from rescue, when you could sail in a mono , where you roll so fast you wonder, "how did that happen" and then all you worry about is, "now where are me specs" So a watertight mono, , and that means with a fire house on all hatches and companionway , is IMO they only way

 

Fair comment on the NW passage Teddy. With the way things are going I may duck through the northern sea route as well

Perhaps I should have watered down that statement Mike. I didn't mean this to turn into the eternal mono vs multi debate. I certainly didn't mean to imply that I think a well designed mono isn't safe, or that the average production cat is safer than an expedition mono.

I will rephrase to say that I think I can design and build an adequately safe cat that meets my requirements more cheaply and easily that the equivalent mono. I know this is against conventional wisdom, and it will be interesting to see how quickly I prove myself wrong.

It is unarguable that a mono which rights undamaged if preferable to a capsized catamaran, but the flip side of the coin (forgive the pun) is that monohulls are knocked down more frequently than cruising catamarans.

I agree that equipment access, MOB pickup and entrapment are important, but I believe that these can be dealt with by careful design regardless of the type of craft.

The deciding factor for me on safety is fatigue. Almost all fatal boating accidents could have been foreseen and avoided by a competent (and awake) crew. I personally find catamaran sailing less tiring. Other people find the jerkier motion unbearable and would make the opposite choice for the same reason.

If I frighten myself too much at sea trials I'll sell the cat and buy a heavy mono with the proceeds ;-)


Back on the materials selection I think Alloy is a fantastic boatbuilding material, and its what i've settled on for this project.
However, my internet research has found a few alloy boat sites which contain statements which I think are completely false (not on BD.net of course) and that bear correction, for anyone researching aluminium for boat building in future:

1. False: "Aluminium does not suffer from fatigue failure".
Aluminium alloys are just about the most fatigue susceptible materials available for construction. All materials weaken with stress cycling, but alloy does this faster than steel, wood and most composites. As with any material attention to detail by the designer and builder should ensure that stresses are so low that this doesn't become an issue until many decades of hard service have passed, but it is something that all aluminium boat builders and owners should be aware of.

2. False(ish): "Aluminium is a high strength boat building material". All common boatbuilding materials are "Structural" materials and could be said to be "high strength" but, for equivalent weights aluminium is weaker in tension and bending than both wood and fiberglass. On the bright side, because metals are isotropic Alloy is more resistant to puncture than fiber based materials; it is also less brittle than FRP, a big plus when you hit things.

3. False: "Welds in aluminium are at least as strong as the metal they join". Aluminium alloys used in boat building are stronger than their chemical composition might suggest because they have undergone treatments that affect their crystalline structure, increasing strength and hardness. They are either heat treated or, more commonly, cold worked (eg 5000 series). The heat from welding damages this microstructure in an area around the weld (the heat affected zone or HAZ). A typical figure may be around 20% tensile strength reduction in the HAZ, depending of course on how the welding was done and what alloys are used. The weld itself may compensate for lower strength by being thicker than the material beside it, but no one has yet found an easy way to restore the HAZ.

I don't want to discourage anyone planning to use alloy. As I say above I think Al alloys are a good material with some great features (unbeaten low maintenance and good workability).

I've included this here because misinformation irritates me and this is where I happed to be pointing when I went off

 

some of what you say is true, some is way off the mark(see my gallery)
I can build a yacht with much shape in the forefoot,
I can belt the thing with a 20 lb hammer as hard as I can, and hardly make a dent, try that with MILD steel, or wood or anything else
i once built a fast charter fish boat, they ran it into rocks at 15 knots, it pushed the panel up 2 inches bewtwwen girders, before the 6mm bottom split, the chines pushed up and held, even in weld areas
mild steel is like putty compared with 5083 , which springs,, mild steels is soggy in the extreme
You take a length of 6061t6, and lay it between two saw horses then tap it, it takes awhile to settle, when you form frames, in a press you have to go past what you would think was the ideal point because it will spring back
Alloy is wonderful material, people dont spend 100 mill for nothing on a superyacht except(Mirebella) sp,
you watch a jumbo jet, the wing, going through millions of cycles without cracking, that does not happen in a boat, let alone a sailboat
it has also been proven that alloy gains strength in extreme cold, dunno why , not a metalurgist
\ you are correct in one thing, weld strength 60% of parent usually, but try breaking it with a big hammer
it is also proven that alloy gains strength in extreme cold

 

If you want some numbers on this for 5086 Hxxx
From 20 degrees to zero degrees the UTS increases by 5% the yield strength only by 1 %. So there's a slight increase in "toughness" to ultimate failure but it will still dent the same.

Re fatigue; yes it does appear to ne more of a problem with alloy.
We have had problems with several alloy fishing boats and the new police patrol boat splitting welds in plating from fatigue due to slamming .
Also the locally built fast ferries have had their 'repairs and rebuilds' during and after rough Pacific-Atlantic delivery trips.

Alloy needs better attention to detail for good long term reliable design, and good pro welding by experienced hands. Look for a good designer and a good fabricator with a proven track record.

Fatigue cracks are usually easily seen with visual inspection and the areas are easily enough beefed up and repaired once access is gained to the area.

Remote temporary repairs even to hulls plate can be done with offcuts carried for the purpose, self tapping screws and polyurethane sealant and are easier/stronger to jury rig repair on the fly than many other materials.

I think it was Maclear and Harris that were designing/building Alloy sailing cats in the late 60's in the US. One of these vessels at 60 feet has crossed the Atlantic many times and later was in regular charter in the Med, it was for sale recently, it apparently surveyed very well.

 

Although steel is the medium I had built most of all my boats, I must come out of the closet and affirm that I would have preferred to build with aluminum for various reasons.
Why did I not? Cost Marine grade alu cost on average 5 times plus more than steel in my country....and this puts it out of reach for must except the rich crowd who can afford it...

 

I know diddly-bo-squat about boat design, however why build a yacht to arctic spec when you´re going to presumably spend just 20% of your time down there?

Couldnt you build a wooden boat as you seem to prefer and attach a thin steel apron 3" either side of the waterline to protect against thin razor like pack ice and maybe an apron over the bow 12 inches either side?

When you´re not chasing icebergs you could store the pieces and save weight into the bargain.
++

Ok guys you heard it here first, copyright is mine.

 

Thin metal sheeting in a wooden boat is arctic specs allready so you late..
Some wooden ships were also sheeted with copper plates to protect also against more tropical hazards ages ago..

 

some of my early
boats cracked, but only because I did not design the structure correctly. once I started using girders, and deep floors, I was and still can give 10 years warranty in boats that are charter and go to see in slamming weather

 

RHP, I do have a fondness for wood, but I'm not a fanatic. Ally is winning for me for these reasons:
1. Faster construction. The only way wood could be faster is hard chine ply. I have no objection to chines myself, but resale value on a round bilge ally hull is definitely higher, for much the same price to build
2. Unpainted topsides. Not only do I save time and money not painting, but I also don't have to touch up the paint when I scratch/chip it.

Lazeyjack, from your experience do you favour frames welded to plate, or stringers proud of frames and a gap between plate and frames?

Mike, would you care to recommend any fabricators as particularly noteworthy in Tassie?

Wynand, by having relatively narrow and low hulls without accomodation, and a small central living pod out of wood/composite my metal weight is calculated to come under 2.5tons, so it shouldn't break the bank.

 

Auto
the yard has to be very good for the first, really good
i think I would choose wide spaced frames with stringers(t,s) flush with frames suitable notching the frames, if I were an owner looking to have it built by average yard
But if YOU are going to build, then with care you could set up for frames only, the Germans and Dutch do very immaculate jobs, if you look under WELDING A STEEL BOAT thread you can see how good it can be, there is pic of frame only steel boat there
limbers and stress relief notches(no triaxial wels) are important
Frames only are cleaner by far, joining the floors etc
if you have doubts go the stringer way
Another overlooked method is HEAVY plate and wider spaced frames, especially worth a look at on chined boats, remember 3mm mild weighs as much as 9mm alloy, , WITH 8MM ALLOY YOU CAN BUILD A VERY FAIR BOAT,, if you make a mistake the palte will show you)

 

this is one bh nautika and I are doing, you see no stringers, but beltings are external stringers
Stringer are messy, specially if there is a lot of integral tankage
working with duel monitors
structure is my speciality, I would be happy help for free!!

 

Wynand ........ and everyone else.

I'd like to give steel a boost here

Actually for your type of boats alloy is possibly a better choice . Alloy suits light and medium displacment construction. Particularly if chasing light weight with thin steel plating.

In heavier workboats and medium to heavy displacement craft, steel comes out the clear winner IMHO.

Although Alloy has a high ultimate strength for its weight it is not very stiff, the youngs modulus (E) is around 70 compared with steels 200 so it is 1/3 the weight and 1/3 the stiffness. In otherwords an alloy structure deflects the same as a steel structure of equal dimensions with only 1/3 the applied force.

When designing to 'deflection' criteria alloy loses much of its advantage and if the Section Modulus needs to be so much higher then the weights have far less between them and also the structural sections are more prone to buckling. Then it needs more brackets and frame supports, which people tend to overlook.

Then steel is the clear winner as it has a much higher fatigue endurance and is a gift from the gods when it comes to weld strength ( very close to 100%).

The lack of stiffness in alloy needs carefull consideration, and It needs more material and more complex assembly to stiffen the hull. Such as Lazyjacks deep floors AND longitudinal girders combined. Some designers don't appreciate this and 'soft' decks are a good initial indication.

Also in design stages consider carefully the loss of strength with each and every weld. This all relates to the problems with fatigue as well. It is no material for unskilled or inexperienced designers or fabricators.

In my own design work I am currently looking at using alloy watertight bulkheads and doors and superstructure in otherwise steel construction. Not for workboats since the dissimilar metal issue is too risky in dirty unkempt vessels but it would work well private yachts.