[PUP] Designing passagemaker - hull

[hannu venermo]

All steel boats are painted. Some(most) alu boats are painted.
In general, all boats are painted, including fiberglass and wood.
Over last 20 years, average re-coat intervals are about 10 years, at 
very low cost due to ease of prep for steel hull.

Its an easy process, that does not need anything special-
Primer, filler (opt) barrier coat, top coat.
Brush, roller, spray, whatever.

There were many many good comments;

Passagemaker hull - suggestions:
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Lets start with hull-
by far, the best boatbuilding material is wood.
But it may not be the best *boat material*. I explain...

It is the strongest material by weight, about 30-50% stronger than 
steel. (Cross-laminated plywood, epoxy glued, ie cold-molded).
Its easy to patch, available worldwide, durable, flexible without losing 
strength, easy to work with.

It is a natural insulator. Will not bleed, rust, get osmosis, or leak. 
Easy to patch later in small or large sizes, temporary patches are easy.
*Emergency repairs are very easy*. Bang a spare piece of plywood onto 
the hull with nails and hammer in case of large hole due to container 
collision etc.

There are many houses and boats over 100 years old.
But ...

Wood requires air-movement ie ventilation and a dry bilge. With these 
two requirements, wood hulls will last indefinitely, provided that it 
has an antifouling coat of some kind.
The only error that will destroy a wood hull is water, like coating it 
with plastic, allowing condensation to develop inside, or leaving 
standing water within the boat/bilges.
Most large ships (sail) were  built with wood and were extremely 
successful. When they sank, it was not due to hull failures (I do not 
know of ANY) but collision with reefs etc. in storm where any hull, 
failing due to natural forces, quickly sinks. When old-time sailing 
ships sunk, they did not have modern engines, pumps, nav instruments, 
storm warning etc etc.

However, I do not advocate a wood hull.
A good wood hull, for lets say 50 ft (I prefer 70 ft - cost is *not* 
related to length but to systems and mass, but we will talk about this 
later)
, should be approximately 50 mm thick. This would not tend to reassure 
purchasers (unfortunately). Due to "market perception" resale value will 
be poor. Work must be done in a low-cost country - but could then easily 
be done to an exceptional quality/standard.
I suggest not using wood due to market perception, lack of skilled 
designers *for large passagemakers*.
Anyone can draw one - but how many large passagemakers have been built 
in last 10 years of wood ?
Wood is a good option, but only suitable for someone who does not care 
about others peoples opinions, and is not worried about ultimate strength.

Aluminium has many of the same good qualities of wood, in that it is 
easy to work with, rel. strong, except that;
- Certain kinds of corrosion are possible. This is not a real problem.
- Aluminium has 3 times the thermal conductivity of steel. It will be 
cold, and must be well insulated. Insulation has benefits, and all 
passagemakers should be insulated anyway.
-Cost
Alu is very expensive in small runs. I therefore rule out alu on grounds 
of cost.

Fiberglass
Long term osmosis/structural issues (theoretical): - as the Nordhans 
show, hull strength/longevity/quality is not an issue.
In catastrophic accidents, fiberglass hulls are the only ones not 
salvageable. Lots of examples. They tear and break apart.
Need large production runs for low cost - not possible in the 
passagemaker market.
Most fbr hulls are flimsy, unlike the Nordhavns, for example. Maybe 
Krogen or Selene are good (no personal experience yet), and certainly 
any hulls can be made well.
-Fibreglass is a good option, except for major repairs and cost.

Steel
Steel is easy to work with, cheap, skilled labour is available 
worldwide, so is steel, its easy to repair, emergency repairs are easy.
Steel hulls, are by far, the strongest of all hulls in passagemakers.

Steel and alu are the only ones that will survive hitting large 
submerged containers and reefs at speed more/less intact and able to 
contine. NO fiberglasss hull in production will survive a container or 
reef.
In hurricane conditions steel and alu are by far the safest.

Of these, steel is cheapest, easiest, strongest. For small run 
passagemakers, an 70k steel hull would be 200k in fbr or 400k in 
aluminium *for equivalent strength* best/suitable for passagemaker. For 
this reason, production hulls are built *far flimsier* and far less 
strong than steel hulls.
Steel also needs insulation, like alu and fbr. Cermis coatings, like 
paints, are available, as  are a variety of insulation systems. No 
commercial craft suffer any problems or costs due to steel hull 
maintenance issues. There just is no problem.

Steel is not heavy. In a large craft, steel hull mass is not appreciable 
heavier. And in general, the heavier the better.
This is also supported to a great extent by the fact that all commercial 
craft (fishing, trawlers, navy, etc) are built in steel. In engineering 
terms, steel is not a structurally heavy material.

Aluminium would need to be very thick to have equivalent strength to 
steel. Extremely expensive makes it a poor choice.

Wood composites would work - needed large amounts of manpower and lack 
of current expertise and tooling make it a non-optimum choice.
Using modern cnc machinery, wood hulls would be cheap and strong, when 
built in the right places, there is just a lack of customer demand.

So, based on these parameters, I suggest steel as the best choice.
Lets continue on this !