Conversion of a plywood boat design to a steel build ?

Note: this thread is a split off, since I was afraid this would derail original thread.

The below quotes come from the thread: Sail Boat Plans 34 to 40 feet Round bilge → post #7 - 8 - 9

Thanks for the answer Mark,

I've asked because I was wondering about a 36' design that according to the designer can be build in multi chine marine plywood (stitch-and-glue), or aluminum, or steel.

Since steel is having about 20 times the density* of okoume plywood, I was thinking the plywood hull skin thickness needs to be divided by 20 to get the same weight of the hull skin, to be able to build the boat with the same dimensions and the same ballast ratio and the same displacement, floating on the same waterline.

( * Steel: 7,750~8,050 kg/m³, okoume plywood at ± 12 % humidity: 330~410 kg/m³ )

Now I guess for this boat the hull planking thickness would be somewhere between ½" and ¾" for the plywood version.

( plywood ½" = 12.7 mm = in metric sheets 12 mm, ¾" = 19.05 mm = in metric sheets 18 mm thick )

When assuming the thickest expected variant of ¾" for the plywood hull planking, then this would call for 19.05 mm (¾") / 20 = 0.95 mm (20 gauge) steel sheet thickness to get the same weight for the hull skin.

( Edit: later I saw in the below attached drawings the standard plans indeed call for hull planking with ¾" plywood, now underlined in the construction drawing. )

It seems to me that this boat in steel would need a hull skin thickness of at least 3 mm (11 gauge ~ ⅛"). And of course much thicker for the box keel sole for strength reasons, and to supply a great deal of the required ballast, which also will eliminate the need for frames on the box keel bottom, which is good for gaining a bit more headroom.

Now the question is; would this conversion from plywood to steel be possible without excessive increment of the beam to get sufficient displacement to carry the extra weight . . ? ?

↓ Reuel Parker Marine → Sea Bright 36 → Specs


BALLAST: 3,483 lbs (1,580 kg) — 2,000 lbs (907 kg) lead foil laminated in keel — 250 lbs (113 kg) trim — 633 lbs (287 kg) water ¹ — 600 lbs (272 kg) fuel ²

¹ fresh water: 75 gallons (284 L) - - ² fuel: 80 gallons (303 L)​

Note: the fresh water and the fuel are not always there, so it's a bit too optimistic to calculate with that for ballast.



A bit more about this boat can be found at: WoodenBoat → Sea Bright 36, which has three more (clickable) drawings, and some additional info in the text.

The below bulkheads drawing is of a Sea Bright 33 (specs) which is much skinnier, which makes her tender I'm afraid, this is to keep it within max trailerable width without a permit.

Couldn't find that drawing for the 36, so I've got it from this blog post with six drawings of the 33, which as per plans is only to be built from plywood.

 

That boat is about half the displacement of what would be comfortably built in steel for a sailboat.
The only way I would investigate the possibility of building in steel would be with its original plywood house and deck but even then, its a light boat.

Murielle

 

Usual disclaimer I have no experience I'm just speculating.

In this paper they compared glass wood and aluminium of a 64' yacht and even aluminium came out heavier mostly because of the deck size so that they had to increase sail area to compare. They used 8mm for bottom hull and 6mm for side and 5mm for deck and bulkheads. That is very close to the 1/4" and 3/16" steel plates mentioned, I guess simply for stiffness.

Maybe you can build just the hull in steel and reduce the ballast without changing the center of gravity much.

If you'd assume 66m² for the hull with 5mm steel that would be about 2.5 tonnes. That's more than your ballast.

 

Dave Gerr's book "Elements of Boat Strength for Builders, Designers an Owners" gives a method of determining the scantlings of various boats built in fibreglass (solid and cored), wood (plank on frame, epoxy and plywood) as well as aluminium/aluminum and steel.

Essentially he treats the hull like a box beam, asking for the depth of the hull interior, its length and breadth. From that he calculates a scantling number and this is a constant for all materials.

To determine the sizes and thickness of the materials, he gives a number which, when multiplied by the scantling number, gives the size of the component both in metric and imperial units. For example, from the book, for a steel vessel, the topsides plating in inches is calculated by 0.12 x Sn to the power of 0.25 where Sn is the Scantling Number. He also gives rules for adjusting the scantlings for different speed vessels (e.g. for up to 7 knots, plate thickhness = 0.12 x Sn(power 0.25), between 7 to 12 knots, multiply that number by 15%. for up to 30 knots, multiply by 25% etc.). Similarly, he scales the numbers to account for pleasure boats, work boats and hard usage types.

He gives minimum sizes and frequently states "the calculated value is under 12 so will be too light" with plenty of worked examples to give an excellent insight into the method.

It isn't a direct "steel is heavier than plywood so to get the same weight divide by the densities" but calculates the required thickness of the shell plating (for example) based on the required strength for the dimensions of that boat. If you are considering building in a different material, it is well worth getting a copy and studying it.

For myself, I would be very cautions about selecting a design in one material and trying to build it in another as, unless the designer originally intended this OR the displacement is heavy enough to absorb the change in material (and even then, the ballast ratios are likely to be wildly different), it is difficult to say if the resulting vessel will be a successful design in service.

 

Phil,

I agree - there are so many good steel or aluminum design why not just pick one of them. Hopefully they include complete plans to allow NC cutting !

BTW, we used Dave Gerr's book for our scantlings. We checked against ABS and it's a bit heavier, but that's fine with our boat.
Mark

 

Amazon says there is a newer / second edition for this book and also his propeller handbook but it's "currently unavailable". Is this just a glitch in their database or is there a new edition? Anyone know?

EDIT: Amazon error, no second editions yet.

 

According to his website, the first edition is still the valid one with no mention of a second edition:

Books http://www.gerrmarine.com/Books.html

However, he has published an update and errata correction in 2018:

ELEMENTS OF BOAT STRENGTH http://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH.html

My question is, why select steel and take the time and effort to convert a hull designed for a different material to steel?

IF (and it is a VERY big IF - my mother did not raise stupid children!) I wanted a round the world live aboard cruiser then it would need to be heavy displacement to carry the stores and spares etc. to make the vessel self sufficient. Steel does not have any particular disadvantage there. A big bonus is that any damage can be repaired in the remotest part of the world. Everywhere you will be able to find steel and a welder of some sort to patch the hull and repair whatever is needed, no matter how poorly the appearance and quality of the weld, it will be strong enough.

Aluminium/aluminum, not so much as it requires special welding machines and a skilled welder. You would definitely have difficulty away from civilisation.

Wood and glassfibre, both equally fragile under those circumstances and about the same difficulty of repair (OK, specific cases will differ but in general ...).

Similarly for a work boat/trawler/fishing vessel, steel every time as initial cost, maintenance and strength and the ability to stand up to service conditions are very favourable for the material. Aluminium/aluminum is the same provided you have modern fabrication facilities where you are operating.

Either select an existing steel design or ask the designer of the boat to redesign it - it WILL differ from the original. See Dudley Dix's salty Dog design in steel which was, I believe, originally designed in wood/epoxy for the differences. Abou the only similarity is the profile and interior layout.

 

For a small boat -and 36-40 feet is a small boat- steel has several inconveniences. the first point steel is damn heavy. The second one is that metal yachts need a lot of insulation, and that's a lot of weight, work and money. The third is maintenance; rust and corrosion on a commercial ship are not a real problem, on a small yacht these are plagues. For me steel is unsuitable in these sizes.
A 36-40 feet steel yacht is very heavy, asking for a lot of sail, a big mast, a big engine if you want to move it at a decent speed. I'm sorry to say that the old fashion heavy displacement yachts are pains to sail. Or they were pitifully slow or they needed big clouds of sails and big arms. And often not very seaworthy.
Aluminium is very good, but requires insulation and when good quality is wanted it's too technical for 98% of the amateur builders. Pretty expensive also.
When you have tasted and tested a ballasted dinghy medium displacement (in french dériveur lesté) able to pass in 2 feet of water, to be left on the sand at low tide, and best able to sail steadily at a decent speed whatever the direction of the wind without big sails or big efforts, you become addict of this kind of boats if you are a monohull sailor. There are plenty of recent plans in France and Holland, of modern shapes, god habitability and nice performances.
In these sizes GRP sandwich and wood composites are the clear winners.
As GRP sandwich has some requirements hard to meet by an amateur builder, remain the plywood/epox/glass fiber and the composite wood/epox/fiber strip plank which are manageable for an amateur with not previous experience.
Strip can be tedious but this method give very durable and very strong yachts of any shape with minimal frames.
Plywood although limited to hard chines shapes is an excellent material, easy to work and strong when used as composite (stitch and glue).
A plywood hull when enough glass and epox are used outside is rather impervious to scratches and does not require more maintenance than a GRP.
Plywood is pretty easy to repair anywhere.

 

Thanks all for the replies so far, I'm a little short of time right now, will respond later, thanks all for sharing the insight . .

 

Just FYI Dave Gerr said it's an amazon error and any second edition is a long time off.

 

Old thread but a relevant thought...check out the Atkin Liza Jane for an example of a very small boat in a very wooden boat sort of style but adapted to steel construction by a respected designer. The full chapter in Practical Small Boat Designs explains that the inspiration for the boat was, in fact, a local amateur who built a couple of steel boats based on wooden boat plans, one of which proved so rugged that, after a storm reduced the rest of the boats in the harbor to kindling, the steel knockabout was repaired and back in the water in a week or so.

With the relatively common availability of CNC sheet metal fabrication, even for steel sheet, I would think that a steel boat is actually far more attractive for the amateur builder than in the past. A local shop should be able to create a CNC-cut kit of parts for the amateur builder that would greatly reduce the labor involved in getting the hull together and modern epoxy finishes greatly improve the corrosion resistance. Or you could go the route of having the bare hull professionally built, blasted, and finished at least up to the primer stage and then do the rest yourself.

 

Hi,

CNC steel boats have been around for quite some time !

Cheers,
Mark