Boom, Winch, Knuckle, Swing, Extend...
The Ups and Downs of Four and Five Function Marine Cranes
by
Bob Bernstein
(previously published in National Fisherman magazine)
A hundred and fifty years ago there wasn’t much choice when it came to hoisting something aboard. You had a yard arm and a system of ropes and pulleys and not much else. Fishermen would saddle themselves to a length of cordage and heave-ho in unison, maybe to the tune of a 19th century Scandinavian hauling song like, “Halarvisa”:
[chorus]
Karre, varre, vitt bom bom!
Nicke, dicke, dickum, plutt!
Fortunately, the typical hand-over-hand team work of the past has been replaced by solo crane operators manipulating joy sticks in heated or air-conditioned control cabs. And hauling “sing-outs” like the one above have given way to headphones and digital audio CDs.
Well, maybe not all hoisting or lifting operations are that luxurious, but there’s no debating the fact that today’s deck cranes have made life on fish boats a whole lot easier and safer.
Choosing a Crane
Lance Washburn, Sales Manager for Hydra-Pro, Inc., a crane manufacturing, design, and sales company based in Seattle, Washington, with fixed boom crane capacities up to 2000 ton-meters and telescoping and knuckle boom crane capacities up to 300,000 foot-pounds, describes the different types of marine cranes as follows:
(1) The Fixed Boom Crane is the simplest. It’s a three function crane that can boom (luff), swing (slue), and winch (hoist).
(2) The Telescopic or Extension Boom Crane is a four function crane that can boom, swing, winch, and extend.
(3) The Knuckle Boom Crane, or Articulated Crane, is another four function crane that can boom, swing, winch, and, as the name implies,“knuckle.”
(4) The Knuckle-Extension Crane is a five function crane that can boom, knuckle, winch, swing, and extend.
A further bit of clarification comes from William Morgan, Owner of The Morgan Company in Fortuna, California, fabricators and distributors of marine cranes up to and including offshore cranes with capacities of 1,000,000 foot-pounds and telescopic and knuckle boom cranes up to 300,000 foot-pounds. According to Morgan, the true articulated crane folds into a figure four. “Many knuckle or articulated cranes may not fold completely into a figure four and not be true articulated cranes,” he says.
Whether you choose, fixed, telescoping, or articulated depends a lot on the application and the fishery, but also on personal preference and how much versatility you need. For example, while you’re likely to find knuckle cranes on the majority of Alaskan crab boats and extension cranes on most factory trawlers, Washburn insists,“There’s no hard and fast rule.” Trawlers will use knuckle cranes, and some crabbers will opt for extension cranes.
“Crabbers prefer the knuckle or knuckle-extension crane,” says Washburn, “for the simple reason that it gives them more control of the pot. “The knuckle crane,” he explains, “allows the operator to keep the tip of the crane closer to the load, which makes it safer on a crab boat. On a crab boat you [also] have a pot guard or a crab cage at the tip of the boom. It’s a kind of a tripod arrangement with a rubber tire that allows you to compress the tire with the pot right at the sheave head.”
Knuckle or articulated cranes have a several other advantages. True articulated cranes can fold up and stow in less deck space, and, in general, knuckle cranes are faster to operate. As William Morgan explains: “A telescoping crane is usually operating in one plane, one function at a time. They’re very slow because the operator winches, then booms, then swings. Articulated cranes are much faster. They can move in three planes at once, like an extension of your arm.”
Morgan also points out that the articulated crane is the highest capacity crane for its size, weight, and cost. “A telescoping crane runs at lower pressure, so it has larger cylinders,” he says. “The knuckle crane has higher pressures and smaller cylinders, and it’s made of higher strength steel. Overall it’s a lighter crane and it lifts more.”
The fact that Morgan says the articulated crane is less expensive will probably come as a surprise to many people shopping for their first crane. “You take the same capacity crane -- telescoping versus articulated -- and your talking up to twenty five percent less money for the articulating crane,” says Morgan.
The reason for this, as he explains, is that manufactures build more articulated cranes than telescoping cranes. In addition, the majority of telescoping cranes use a ball bearing for rotation. “Our articulated cranes,” says Morgan, “use a rack and pinion system for rotation. The rack and pinion, which actually has higher torque, costs less.”
Rack and pinion systems are also used to extend the booms in some telescoping cranes. “The only cylinders we have [in our cranes] are the lift cylinders,” says Mike Atkins, Sales Manager for The Techcrane Global Corporation in Covington, LA, manufacturers representatives for EBI fixed boom and telescoping cranes. “We use lift cylinders,” he explains, “but instead of an internal extension cylinder -- the part that extends the boom in and out on most all other cranes -- ours have a rack and pinion system. This eliminates any reason to get inside the boom for maintenance or repair, for example, if you had a broken seal. With our system you never have to get internal to repair a leak.”
Meanwhile, regardless of each crane companies proprietary engineering (and there are many more crane manufacturers and distributors than those contacted for this article), there’s no question that of the 4 generic types of cranes available, the knuckle or articulated is the most complicated. It’s structurally more complex and also more complex to operate. “You need an operator who can think [faster] because the crane is operating in three planes,” says Morgan.
What this means is that not every owner or captain wants the additional complexity of an articulated crane. For example, a factory trawler, where deck space may not be at a premium, may only need a crane to launch and recover a tender in and out of a fixed cradle. Or maybe the crane is needed to move nets on and off the boat two or three times a year. In cases like these, the relative simplicity and ease of operation of a fixed boom or telescoping crane might be more appealing.
Running the Numbers
Once you’ve picked a type of crane, the next step is to spec it out. In this regard, the most important variable is what Washburn refers to as the “Defining Moment.”
“The first thing you have to do is figure out how much you want to lift and at what radius,” says Washburn. “Those are the determining factors -- lift and radius.”
The particular ‘moment’ Washburn is talking about is a quantity defined as the cross product of linear force and the distance from the point of rotation at which that force is applied, otherwise known as torque. Torque, or the moment of force, is usually defined in foot-pounds or tons-meters.
For example, say you want to lift 3000 pounds at 50’ and 20,000 pounds at 20’.
You multiply 50’ x 3000 lbs., and 20’ x 20,000 lbs., and you come up with moments of 150,000 foot-pounds and 400,000 foot-pounds respectively. Clearly, the defining moment is the one for which the product is 400,000 foot-pounds. That’s the one the crane has to be designed and built for.
“The defining moment determines the bearing and the pedestal,” says Washburn.”[Here at Hydra-Pro] we deal in Single Row Ball Cranes. This [design consists] of a single row of balls that ride in a race mounted in between the pedestal and the turret.”
The turret and pedestal are where most of the stress and fatigue will manifest (see illustration). In fact. according to a recent Crane Safety Workshop conducted by the Offshore Technology Research Center and the Minerals Management Service of the U.S. Department of the Interior, the majority of marine crane failure incidents are pedestal related. This is why the design and construction of the pedestal and turret of a marine crane are so important.
On land, the lift is static, but at sea, where the vessel is moving side to side and fore and aft, the lift is dynamic. For example, an empty crab pot weighs about 700 pounds. Loaded with product it might weigh as much as 1300 pounds. With the vessel rolling and pitching, that weight can increase thirty to thirty five percent, adding another 450 pounds to the pot. However, as Washburn points out, the dynamic doesn’t only affect the load: “The dynamic acts on the entire crane. You’ve got the weight of everything to consider, crane structure as well.”
According to Washburn, normal accelerations will be at a dynamic of less than 1.33, i.e. 33% over static load. (1.0 is considered zero dynamic.) Hydra Pro cranes, and cranes from other established marine crane companies, are designed with a minimum of 1.33 dynamic.“Cranes built with 1.33 to 2.0 dynamics are relatively common,” says Washburn. Although, he adds, Hydra Pro, as well as other companies, often supply cranes that meet or exceed specifications set forth by national and foreign regulatory agencies.
For example, Morgan cranes are constructed by PM of Italy to meet DIN Standard 15081H1B2. “It’s a German engineering standard,” says Morgan. “It specifies a dynamic of over 3.0, and [includes standards for] how long a crane should last and how it should be built.”
API (American Petroleum Institute), U.S.C.G., U.S. Navy, ABS (American Bureau of Shipping), Lloyds, Rina, Det Norske Veritas, and OSHA, are some of the other entities that establish (or share) standards for marine cranes. For example, you may note that your crane meets API-2C specifications, which states, among other things, that the static load is 1.5 times the dynamic load.
Of course, you can get whatever you want. If you want to lift a dynamic load of 6000 pounds, and you want a 3.5 dynamic, you’re going to get a crane that can lift a static load of 21,000 pounds. “We had a customer who requested a 3.5 dynamic,” says Washburn. “He had an offshore application, and he had damaged three cranes in previous situations. These [the old cranes] had been cranes designed and built for shore applications.”
Knowing how and when to conduct a given lift is where the load chart comes into play. In fact, in order to be compliant with regulatory agency standards, every crane on every vessel should have load/radius charts for static and dynamic lifting posted in plain sight of the operator.
For instance, the EBI TC10-24-40 telescoping boom is 24’ in retracted mode and 40’ in extended mode. A quick glance at the load chart shows it has a static lift capacity at 20’ of 9750 pounds and a dynamic load capacity at 20’ of 7200 pounds. The static load is 1.5 times the dynamic load.
Does this mean an operator has carte blanche to lift 7200 pounds in extended mode in any sea state? Not according to Atkins.
“To be honest,” says Atkins, “you can’t just make a statement that this thing can lift 7200 pounds in any sea state. There are a lot of factors involved, and a lot of it has to do with the stability of the vessel. You should check with a naval architect.”
