Hull Material Tradeoffs: Wood/FiberglassPlastic/Aluminum/Steel

[MichaelMc]

In light of the Conception tragedy, understanding hull material tradeoffs seemed appropriate, relative to wanting the walls and hull not burning, short of really high temps. For boat operator laypeople, what is the big picture? Realizing coats, drapes, pillows, laminate cabinets, etc still burn.

Is there a rough cut estimate of the cost difference between a wood/fiberglass and aluminum or stainless steel hull boats? Like aside from engine, electronics, etc. a steel hull costs X, wood costs Y. Steel for hull, walls, decks, some fittings, etc. I presume steel is heavier? So it would need/want a bigger engine. Aside from the hull costs.

Pick your size boat. Small crew of 2 plus 4-6 passengers/guests. Crew of 4-6 plus passengers of 12-20-30...

How does a Coast Guard 47' Aluminum lifeboat ($1.2M) compare to a comparable size and engined wood hull fishing boat/cruiser?

How big are the upkeep differences?

@Wookie, It seemed as if you had experience with metal hulls. Any, just rough, thoughts?

A 2008 thread, Wooden boats or steel boats which one is more economical in building?, suggests not a big cost difference. Wood material is cheaper, easier to shape, but more labor intensive. Steel material is more costly but faster to weld. Wood is more suited for small trailering of the boat. My layperson cut from that other thread. Some other good comments there.

A 2006 blue water sailing thread on materials: steel hull vs other materials. It depends. For longer/deeper boats steel starts doing well. Many people can weld steel, not many can weld aluminum.

On the melt points of aluminum/steel. A reformatted post by Akimbo:

Aluminum melts at 1,221°F or 660.3°C.
Steel melts at around 2,500°F or 1,370°C (varies by alloy).
Stainless Steel melts between 2,540 and 2,642°F or 1,394 and 1,450°C (varies by alloy).
I'm guessing that steel wasn't mentioned because it requires paint, which burns and outgasses toxic chemicals well below 1,100°F. Of course they both are much more malleable at these temperatures so are highly compromised structurally

And burn points of organics:

1100F is the high end when of where you'll get flashover. At 1100F everything organic in the room is emitting burnable gasses and just needs enough oxygen to ignite. In this fire it seems there was no shortage of oxygen. Actual major league structural fires are apparently significantly hotter, up to around 1000 C in a room undergoing flashover per NIST. ASTM E119 (a 'standard fire' chart) shows 538C (1000F) at 5 minutes, 704C (1300F) at 10 minutes, 843C (1550F) at 20 minutes.

- 90' R/V Coral Sea that Akimbo mentions with steel hull: R/V Coral Sea — HSU Marine Lab

I'm not in the market for a boat, just curious.

Edits: several to add and refine.

 

[Akimbo]

There's a lot more to consider than construction costs. In particular the type of work, fuel, and maintenance. This explains the popularity of FRPs (Fiberglass Reinforced Plastic) hulls under the 20M/65' range. New wood hulls are pretty rare these days.

The only steel hull California liveaboards I can think of were Glenn Miller's two boats, the Emerald and the Coral Sea. Both operated out of Santa Barbara. The Coral Sea was sold to Humboldt State after Miller was killed in a helicopter crash. I heard they chopped her in half and added 20'.

 

[Eric Sedletzky]

I had a marine repair business between 2002 to 2009 (officially, and named Pacific Coast Marine). I still have one client with a 57’ trimaran sail boat that I service when he’s not out sailing the far corners of the Pacific.
As far as boat construction and materials:
I started doing fiberglass and gel repairs on small craft which lead to fiberglass and paint repairs on larger craft which lead to repairs and refinishing on even larger wooden craft, which lead to some limited work on large steel craft mostly refinishing. But I understand all of it and know construction methods and materials pretty well.
The big trimaran was glassply (glassed over plywood) which was a very popular method of construction from the late 50’s all the way up through the 70’s and even 80’s.
Wood was reasonably cheap but the main attraction was the availability of
boat plans. For instance, Arthur Piver of Mill Valley sold a set of full size plans and patterns for anybody to build a sailboat in their front yard.
Back in those day it was easy to obtain the best marine plywood which was soaked in arsenic to prevent rot. Wood such as white oak and mahogany for frames and vertical grain fir were easy to get for framing and structure. Silicon bronze ringed boat nails and screws were easy to get. Nowdays non of that stuff is easy to get and in the case of ultra premium marine plywood forget it. One thing we have now that they didn’t have then is epoxy resin. They had resorcinol glue.
The longest lasting boats I’ve ever seen are old solid wood planked boats from 100 years ago. If they are taken care of and maintained, some of those old wood boats from the teens thru the 40’s like the old Monterey fishing boats are still in great shape. Salt water pickles the wood and they stay preserved indefinitely while they are in the water. Take them out and dry dock them for a year and they are toast.

Glassed over plywood is my least favorite material. All it takes is for a pinhole leak somewhere through a crack in the skin and fresh water intrusion and you have a big rotten mess. By the time there are any visible signs on the outside of the hull it’s guaranteed the damage is way involved when you tear into it.

Steel used to he the cheapest way to build a boat. Problem is that steel is heavy. This can be good or bad, depending on the design. If you want fast then not so good. If you want full displacement and don’t mind slow (8-10 knots max) then steel is great. Steel rusts and can be a maintenance pig, but nowadays with the excellent epoxies available this is not as bad of a problem. Steel is the safest as far as fire and collisions snd grounding. Many world sailors will only use a steel hull because of all the trash in the ocean including floating shipping containers that they can hit.
The problem nowadsys is getting marine grade steel at a good price or just trying find something suitable for building a boat. This Chinese crap they sell us now claiming to be legitimate certified product is a complete joke and it’s garbage. There is a designated alloy that is to be used for boat construction and I can look that up in one of my manuals.
Aluminum I know very little about.
However, aluminum seems to be the current favorite material of choice for moderately larger vessels these days. It’s all going to be spool gun welded with inert gas. Many of the newer catamaran ferry boats running in the SF bay are constructed out of aluminum.
Moulded hand laid up fiberglass is the most expensive and labor intensive method of boat building, but you can do a lot of cool stuff. It is said that a properly built solid fiberglass boat is the best option for low maintenance and overall longevity, but at a steep price. Many luxury custom yachts for the mega wealthy are made out of solid fiberglass. The problem is the cost of resin has soared with oil prices, and domestic skilled labor is prohibitive. It was a trend in the 80’s for companies to offshore the hull building to asian countries including Vietnam. The problem is nobody showed them how to properly lay up glass and roll out the trapped air (they are wood workers) and as a result we saw a ton of hulls fail from osmosis water migration, so bad that large blisters would squirt out water when pierced on dry land. In some cases the osmosis was so bad that it would literally split the plies apart under the waterline and condemning the boat. These boats were in the 45’ to 65’ range, twin engine personal LOB yachts.

 

[EricTheDood]

Aluminum loses its temper past 400F. It'll drastically weaken long before it melts.

 

[Eric Sedletzky]

Aluminum loses its temper past 400F. It'll drastically weaken long before it melts.

That’s on 6061 which has a higher silicon content. 5052 is much softer and that is what is mostly sold in sheet form. Most common extrusions like angle, flat bar, rod, box tube, pipe/tubing etc. is 6061. I have no idea why?
I’ve been fabricating/welding up a lot of huge monument signs for business parks lately (for another sign shop) so that’s how I know this.
The only other aluminum alloy that I have used is 2025 which is an aircraft grade sheet. Airstream trailers are made out of this material because it has strength but it’s also pliable and resists sudden cracking and failure from vibration. Aircraft can’t fail.
There might be a special alloy that is best for marine use but I would have to refer to one of my textbook manuals and look it up.
I know of one boat builder locally that uses a lot of steel and they build tenders used in the bay. They said that one marine outfit refuses to use anything aluminum because they have had them fail catastrophically while in use and sink within less than a minute putting workers in the water. These tenders regularly get sandwiched in between barges and take some heavy abuse. They use old tires on the sides as fenders

 

[Eric Sedletzky]

Here are some aluminum alloys used in shipbuilding. This is out of the book Steel Boatbuilding by Thomas E. Colvin c1996 Tiller Publishing.

 

[broncobowsher]

Drop a penny under a seat of an aluminum boat, Wait a year, Fix a penny sized hole I the bottom of the boat. I've seen pontoon boats on salt water, little badging that they are the "saltwater series". Never looked into what makes them special.

You will NEVER see a stainless steel hulled boat. Consider the costs when people complain about the cost of regular steel.

Heavy, fire proof, you do know that they have made concrete hulled boats. Some have been good, some not.

 

[Eric Sedletzky]

Drop a penny under a seat of an aluminum boat, Wait a year, Fix a penny sized hole I the bottom of the boat. I've seen pontoon boats on salt water, little badging that they are the "saltwater series". Never looked into what makes them special.

You will NEVER see a stainless steel hulled boat. Consider the costs when people complain about the cost of regular steel.

Heavy, fire proof, you do know that they have made concrete hulled boats. Some have been good, some not.

Oh yeah, forgot about ferro cement, probably because it came and went so fast and every ferro boat I’ve heard about was condemned and scraped long before I got into the game. It was something they experimented with in the 60’s and 70’s as an alternative to moulded fiberglass for complex compounded shapes on sailboats.
Didn’t work out so well.

 

[rjack321]

As the owner of a aluminum hulled rib I can say that rewelding/repairs are not simple at all.

Basically aluminum corrodes "inside" not just on the skin. It absorbs salts and ions from seawater and there is microscopic dissimilar metal corrosion going on internally. This doesn't really impact the boat's strength or longevity (assuming you diligently replace anodes and scrupulously avoid stray dockside currents)

Repair-wise? Its a royal pain in the butt. That corrosion and absorbed ions make re-welding used aluminum downright awful. I had an elephant trunk added to my hull (basically a giant thru drain on the transom) and the guy had to weld, grind off, reweld, grind off, reweld again about 3 times in order to cook off (vaporize) the contaminants in the aluminum so that he could make a sound weld.

Ps have not see a new construction wooden vessel outside of a very high end specialized craft (small sailboats and canoes mostly) in many years. The knowledge, experience and expertise to even do that on liveaboard charter sized scale doesn't even really exist anymore in the USA and Canada. The train-up/staff-up/tool-up costs for a shipyard to even bid on such a project would be huge and probably kill it before even getting past the marine architect.

 

[Wookie]

Most folks who don’t understand aluminum hate aluminum. I wouldn’t have anything else. Caveat is that it needs to be over 40 years old or with aluminum from Australia, where all of the great modern aluminum catamarans are built and designed. I assume rjack that your boat is a northwest build, maybe a zodiac and maybe 20 years old? Alcoa made a bad run of alloy that acts exactly as you describe. The Seattle ferry bought 2 of them, and Alcoa bought them back. Turns out they removed the manganese in the alloy and that allowed interstitial point defects in the aluminum, and they turned into Swiss cheese. My 40 year old crewboat and my newer 40 year old patrol boat do not have that problem.

For easy reference, plate is 5083 or 5086 and scantlings are 6061 or 6063. I heard of someone buying 5684 the other day, but I don’t know why you would. Weld wire is 5356 and never 4043.