80 Year Old Trawler - Nice Lines

[Rainer]

Hello Mike,

the Harzquerbahn has a 1000 mm gauge this is a common narrow gauge width here in Germany. Second standard is 750 mm.

Smallest bend at the Harzquerbahn has a radius of 60 meter what is a challenge for an engine with 5 driven axels
http://en.wikipedia.org/wiki/Harz_Railway

All other normal gauge is in Europe are 1435 mm = 4′ 8,5″

[Bob Cleek]

I'm still trying to wrap my head arouind this whole steam thing, from an engineering point of view. (The technical information in this forum is fascinating!) I have a lifetime of traditional wooden boat experience, though. So, at the risk of being hated for raining on somebody's parade, I'll just offer a general observation that has held true in every instance. You can't simply "scale down" a design. You CAN, perhaps, stretch or compress a design, depending on the design, by adding or subtracting a small amount to the frame spacing, but that's about it. The problem is that the relationships of the various factors in a hull design aren't linear. (For instance, volume varies by the cube, while waterline length is linear, and displacement is something else again.) Speaking generally, I think most naval architects would agree that you'd do a lot better to select a hull design that was designed to be the size you wanted to end up with, rather than scaling down an 80 or 90 foot hull to 20 or 30 feet and expecting a "scale" result.

[fredrosse]

In fact, a displacement hull form (as defined by her lines drawing) can indeed be "scaled". That is exactly what is done at numerous marine design testing tanks, a scale model is towed in the tank to determine hull character, with proper scaling functions. These model hulls are scaled with a ratio (model dimensions/prototype dimensions) of far less than 30% presented on the first post within this subject. Another example is the training hulls at the "supertanker school" that is used to train personel about maneuvering 1000 foot long oil tankers. These models are about 4% size (a 1000 foot tanker would be modeled at 40 foot length), and are exact scale models of the larger hull, prop, rudder, etc. These model ships have the proper scaling laws applied to give response similar to those in the real ship, hopefully so the capitans and pilots will learn not to smash up the big ships.

It is correct that the relationships are non-linear, but if the proper functions are applied, the exact scaling of a pure displacement hull is a workable option. These relationships were developed by William Froude who pioneered model towing tests in the late 19th century. The Speed/length ratio was originally defined by Froude in his Law of Comparison in 1868.

That is not to say that there could be no problems with a scaled hull, for example, the trawler hull presented in the first post has a very round bottom, implying a metacentric height that is not as "self righting" as a hull with more full lines. That is why I stated that it probably could make good use of bilge keels. However the trawler hull closely follows the form of typical steam launch practice, but somewhat more "seaworthy" for use in rough seas, which is one of the reasons I posted it here.

[Bob Cleek]

I quite agree with much of what you have said. However, I think it is important to note that while a hull may be reduced (or enlarged) in size, the sea cannot. Testing tanks are themselves an artificial representation of the sea, being classically "flat calm," although some now, I believe, can replicate various wave heights and periods, more or less to scale. Similarly, the pilotage schools you mentioned do not accurately replicate all aspects of handling the vessels represented. (For instance, they don't accurately duplicate the effect of windage on the handling of the full size vessels.) I've been told by a couple of my pilot friends who have studied at such facilities that they are really great for honing skills, especially on the supertankers and container ships, but overall bear about as much similarity to the real vessel as a Link trainer did to actually flying a WWII plane. Some of the features are near exact scale representations, while others are not.

Some hull shapes are more susceptible to scaling down than others, to be sure. As you observe, though, while scaled down hull speed test results might be consistently achievable (Froude, et al.), stability is another matter and that bears much more directly upon seaworthiness than does hull speed, particularly in a vessell that is going to have the majority of its weight invested in a very compact (and possibly topheavy) pile of iron amidships. (Why would one bother to put bilge keels on a tender hull if a stiff one could be built in the first place?) Why would a scaled down trawler hull be "somewhat more "seaworthy" for use in rough seas" than a well designed hull of the same size which has not been scaled down? I do agree she has especially beautiful lines for a vessel her size. For my money, though, if I were going to build a launch hull, I'd be looking at something thats already been built and been proven to be a good performer. (Anything by Fred Martin, although I don't believe there is any of his work extant beyond hull lines. Pete Culler has a couple of very nice fantail launch designs in the 30-34' range with complete construction drawings available.)

[Lopez Mike]

I avoid taking my boat out in any sea state that a pancake with a hair lip couldn't spit over.
Mike