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Are Steel Yachts Too Heavy...?

The 32' Tug Yacht - TERRIER - Kasten Marine Design, Inc.
Image Copyright 2002 Lena Kasten

Article Copyright 2002 - 2011 Michael Kasten

Countless dockside "experts" have said:

"The corrosion factor that must be designed into a steel boat for the ongoing rusting that occurs both inside
and outside the vessel causes small boats of 100 feet and less to become excessively and unacceptably heavy.
This extra weight requires larger engines and more fuel capacity - all of which are non-optimum when you
want to make a small ocean going vessel as efficient as possible."

 

With a properly designed steel boat, is this claim actually true...? Or is this simply dockside "BS"...?

In order to address this question, we shall consider several factors here as they relate to a boat's design, including the rationale behind any design; economy of ownership; correct thinking; and the real story regarding the relative weight of a steel vessel.
 

CORRECT THINKING

It is true that when comparing a steel vessel to a similarly configured vessel built of aluminum, the steel vessel will end up weighing more, will have greater displacement, and will require more power and fuel to be propelled through the water.

Shall we therefore imply that a steel vessel is inherently inferior...?

I think not. To make such a statement vastly oversimplifies the picture, and is ultimately extremely misleading.

Every boat is developed for a given purpose, a given budget, a prescribed service life, and for a specific individual. Those are only a few of the many interconnected considerations and the myriad of complex decisions to be made curing the design of a new cruising vessel. To permit any one single factor to override all of the others is incorrect thinking (even though such lack of rigor is rather common...!).

If we take the quote at the beginning of this article about steel boats to its logical conclusion, we would necessarily want to build boats using the lightest possible material. If we pursue lightness as our only goal, we quickly stray toward exotic materials like carbon fiber. Not only is this stuff rather expensive, but we've seen a few of the BOC and America's Cup boats built with carbon fiber simply break in half...!!

In a family cruiser, this kind of behavior is decidedly unwelcome.

Shall we then proclaim that all such racing craft are "bad boats?" No, not at all. Those racing vessels were designed for one purpose only: to win the race. They were not designed to be "economical" or "safe" or "seaworthy" or "sea kindly" or "comfortable" nor even to be all that "pretty" when all is said and done. They were designed only to win - and sometimes to win only one race...!

We really should be asking ourselves, "Why do we so often emulate racing types for our family cruisers...!!??"

By this racing boat example, we can see that are better served if we strike for the middle ground. We must balance the myriad considerations inherent in creating the design of any family cruising boat so that the resulting vessel can be both well-rounded and capable.
 

PURPOSE OF DESIGN

After looking at the vastly differing kinds of boats that have been created over time, we eventually observe that there is no such thing as "the perfect boat."

There are however many boats that are well suited to their intended purpose. This all-important pre-assessment of a vessel's "intended use" is often lacking in the minds of many a prospective boat owner.

When we are presented with the variety of already completed production boats, we often note that each one of them attempts to claim the title of "the perfect boat..." We must immediately ask ourselves, "Perfect for what...?"

In most cases a vessel's "intended purpose" is left to our imagination. In other words, rather than a vessel's 'intended use' being our foremost consideration, we're ordinarily asked to bend our actual 'intended use' to the vessel's existing capabilities.

Does this not place the cart before the horse...!!??

It is the specific goal of Custom Design to address the question of function first, and then to create the boat. With any new yacht design the very first task is to determine the owner's intended purpose for the boat, the available budget, the intended duration of ownership and a number of other general parameters. Once those requirements have been understood, the designer's job is then to create an integrated design solution that fits those pre-established criteria.

As a boat building material, steel offers many advantages. The primary advantages of steel are economy, ease of construction, incredible toughness, improved comfort (roll moment of inertia), survivability in extreme conditions, longevity, and ease of maintenance. In terms of our peace of mind on the water these are all very favorable characteristics.

We will then be confronted with the inevitable question, "Aren't steel boats ugly...?"

All too often, the unfortunate answer is yes... However, if designed with finesse in terms of aesthetics, a steel yacht will be every bit as elegant and graceful as a boat built with any other material, bar none.
 

ECONOMY OF OWNERSHIP

We have observed, both in our own lives and in the lives of others, that "Form Follows Budget." Given that criterion, steel has a lot to offer!

At the time of building and during a vessel's ownership, steel is at first a very economical material for boat building, and then, if properly built, an easy boat to take care of. You will not for example be repairing stove-in planking nor dry rot nor smashed and ruined fiberglass structure nor fiberglass boat-pox...

In modern times (since the mid-fifties) proper surface preparation (sandblasting) and modern coating systems (epoxy paint) have been well known and have allowed small steel boats to last a very long time. Steel, after all, does not change with time (unlike wood and plastic), therefore its life is simply a matter of how well it has been protected by the builder.

Due to weight considerations, it is not optimum to design small steel yachts with a corrosion allowance of any appreciable thickness. Thus the original application of protective coatings by the builder is quite important.

In terms of maintenance, the proper care of any vessel involves keeping up its paint system. If the paint system has been correctly applied to a steel vessel by the builder, one can expect very much the same maintenance regime with a steel boat as with a fiberglass boat - but without the ordeal of "boat pox" (and generally without the fear of running into stuff). The key to easy maintenance and long life is proper application of the paint system at the time of building.

In order to address this necessity when creating a new metal boat design, we add an 8 to 12 page Paint Specification as a supplement to our usual 40 to 70 page Vessel Specification. In our Paint Specification, the vessel's original coating application and subsequent maintenance are explicitly laid out for the benefit of owner and builder.

For more information on these matters, please see our article on Corrosion Prevention.
 

DISPLACEMENT vs. SIZE

With regard to displacement, we observe that the volume of any object varies as the cube of its dimensions (l x w x h). In order to compare the relative (heavy versus light) displacements of vessels of different sizes, it is desirable to "equalize" them mathematically via some "non-dimensional" means.

For that purpose it is useful to calculate the "Displacement to Length" ratio (D/L). Written discursively in the imperial measurement system, a vessel's D/L is equal to the displacement (in long tons) divided by the cube of one hundredth of the WL length in feet. Written as an equation:

D/L = Tons / (.01*WL)^3. One long ton equals 2,240 lbs.

The 46' Gulliver, one of our steel trawler yacht designs, has a D/L of 227 light, and 315 loaded. In the case of the 46' Gulliver this reveals an optimization of the materials of construction to suit the original client's stated purpose and budget for the vessel, i.e. a comfortable, rugged, affordable, easy to care for liveaboard motor yacht, intended for expeditionary use including a circumnavigation under power.

The DL for Gulliver is fairly typical among our steel Motor Yacht Designs and Sailing Yacht Designs. In particular, check out the relatively small but fairly light displacement 36' steel ketch design, Grace (illustrated below) and the 32' steel tug yacht, Terrier (illustrated above). These boats are considerably smaller than 100' LOA, yet they are fairly light for their size .

It is interesting to calculate the D/L (using an accurate WL and displacement) for a variety of steel vessels, then compare their DL to that of boats built in other materials. Certainly we will find extremes among the vessels being considered, however we will also quickly observe that a well designed and built steel vessel will weigh very nearly the same as a similar boat in any other material.

The inevitable question is then, "What is the practical lower size limit for a steel yacht?"

This depends on many factors, however as a rule of thumb we have observed that steel vessels can very successfully be built in sizes down to around 30 feet or so. Although this is not an absolute limit, if smaller than 30' a yacht will necessarily have a much higher D/L in order to carry the structure, so will seem overly heavy. There are exceptions to this rule of thumb, with many successful steel vessels of even smaller sizes but it is not so commonly done.
 

THE B.S. FACTOR...!

In light of the above conclusions we can see that the claim made at the beginning of this article by our 'dockside expert' with regard to steel vessels is prejudicial at best, as well as grossly misleading and patently false. In other words, the "B.S. factor" is really over the top…!
 

The 42' Sailing Yacht - ZEPHYR
The Steel Yacht, Zephyr

Other Articles on Boat Structure

Metal Boats for Blue Water | Aluminum vs Steel | Steel Boats | Aluminum for Boats
Metal Boat Framing | Metal Boat Building Methods | Metal Boat Welding Sequence | Designing Metal Boat Structure
Composites for Boats | The Evolution of a Wooden Sailing Type