Instruments, Tools (i.e. bow thrusters) and Thinking at the Margin

This is from the thread on single screw tugs,

Tugsailor is thinking about this in the correct way.

Has to do with thinking on the margins, a concept from economics.

A long time Alaskan tugboat sailor having well-honed skills from years of experience is not going to gain much with another month or two of sea time. Far more to be gained, on the margin, by adding a bow thruster to a single screw tug,

Bow thrusters have been common on single screw tugs in Europe for decades.

In fact, a lot of twin screw European tugs have bow thrusters.

There are many good reasons to put a relatively inexpensive bow thruster in a tug, but no good reasons not to, other than cost.

Just because we use to get the job done with WW II era single screw direct reversible tugs that took 15 seconds to shift from ahead to astern doesn’t mean that we should aspire to return to doing it that way.


The first OSV with a bow thruster to appear in Singapore was the Young America:

When she came to Loyang base for the first time it was keenly watched.
The comment around the table at Hilltop Café was; “If you know how to “walk-the-boat” you don’t need none of that BS”.

The concept of thinking at the margin applies to other tools as well. I asked a Sabine River pilot about not having a PPU. He said the pilots there didn’t use them (at that time) but he’d use any tool they gave him. He was an excellent pilot BTW.

It’s often the case that as soon as any tool like this get mentioned the thinking is the tool is a substitute for basic skills rather than in addition to.

Other examples are a compass on a coastal freighter running inside (except to run the iron mike), a anemometer when approaching a TC at night and so forth.

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Ok, what’s an Iron Mike?

Coastlines everywhere are littered with wrecks, not because the people onboard did not know how to ‘walk the boat’, but probably because the conditions were ‘marginal’ and they should not have been there in the first place.
We all stand on the shoulders of the giants who went out before us and did most of the crap that we do now with none, if any, of the tools that we take for granted.
And they got to eat hard tack with weevils too.


“Walk-the-boat” was a term used on “mudboats” with twin screws, no bow thruster.
It involved getting the boat to “walk” sideways to bring it alongside a drillship, or barge, with spread mooring wires/chains not much further apart than the length of the boat.
Done by using one screw ahead, one astern and rudders hard over. Lots of black smoke from the stacks was the sign of a good boat handler.

PS> The good ol boys were not very impressed when I brought a boat alongside, between two other boats, at Loyang by setting a bow spring, rudder hard over and slow ahead on one engine. Comment; “Damn foriner, don’t know how to walk-the-boat”.
No black smoke either.

Good to have a bit of a discussion about shiphandling in the days of DP control where, it seems to me, no-one has much of an idea about what the computer is doing.

I have driven a ship with one CP screw and a small bowthruster which could be made to move sideways with a bit of ingenuity, and I do know the theory of moving sideways with two screws and no bowthruster.

If one engine is put ahead and the other astern, the stern will naturally move in the direction of the ahead engine. Then if the rudders are turned the other way the bow will move in that direction as well.

The effectiveness of the technique depends a lot on the configuration of the propulsion ie, inward or outward turning screws, FP or CP, with or without Kort nozzles, the distance between the screws and the rudders and the size and type of rudders. So it is a matter of giving it a try and seeing how it goes.

If a twin screw boat is being operated and it loses one engine the boats capabilities are going to shrink.

Opposite is true of adding a boat thruster to a twin screw boat, it’s capabilities are going to increase.

The same is true in other cases that have been discussed here. For example such things as the ability to remotely read ballast / fuel tank levels and vessel’s draft. If those things are lost there will be a decrease in vessel / crew capabilities.

Say a company runs a fleet of twin-screw (or single screw) boats.

If that company purchased a boat equipped with a bow thruster it makes the most sense to pick the captain who is most skilled to run that boat rather than a lower skilled captain.

The lower skilled captain might lack the ability to put the thruster on the more capable boat to good use.

On the other hand they could give him some training in its use!

The office does not have anyone that can “train” their best Captain on the use of the bow thruster.

Maybe they could do what the companies did with the first US tractor tugs, fly an experienced European tractor tug Captain over to provide training, and/or send Captains to Europe for training.

I’d assume that any of my captains could run the boat without using the bow thruster, so any of them can run the boat with it. I’d probably give the boat to the Captain most interested in experimenting with the additional capabilities bow thruster, and writing up procedures for using it.

In my time I have gone from single fixed pitch propellers with reversible engines (go Doxford!) to twin variable pitch props with twin 75 degree Becker flap rudders and twin bowthrusters.
Regardless of your thoughts on skill levels, there is no doubt which is faster in getting a ship alongside with minimal effort; the time saving is substantial and not having to use tugs gives a massive cost saving.

Sure, in real life lots more options.

I know you understand this but in principle if the problem is simplified; worker A is 10% more productive the worker B, a new tool is x% more efficient etc than by logic the more productive worker should get the better tool for greater productivity of both workers.

Of course in practice not that simple, but in principle at least, everything else being equal, the more skilled workers should get the better tools.

Thinking about one tool or method as being a substitute for another can be an error.

For example the ship I’m familiar with is single screw direct reversible with a bow thruster. A tug is not a substitute for a bow thruster nor is a bow thruster a substitute for a tug. Tugs provide power, a bow thruster provides precision.

Same is true of ballast tank gauges vs sending a crewmember around with a sounding tape. Tank levels are critical information and need to be vivified at intervals but during cargo operations the cargo officer needs to be able to continuously monitor tank conditions.

Take away the gauges or the bow thruster is adding to the risk, everything else being equal.

We use the bow thrusters all the time up to the limit of what they can cope with, about 30 knots on the beam,
Over that we use a tug as well as the thrusters.

It depends on the situation. A mishandled landing with a large, heavy ship to a pier with poor fendering could be 5 or 6 figures damage. Not much margin for error.

On the other hand I ran a 30 meter catamaran with 4 Hamilton jets, sometimes with high winds but an acceptable landing was anywhere from about 0.5 kts to 5 kts or more, good fenders and a light boat. In the case of a bad landing the damage likely would be under $100.

Our limit for unmooring with winds abeam onshore for the car ship with one tug aft and bowthruster forward is in the neighborhood of 18 kts.

Norwegian seiners started to use side thrusters both at bow and stern to go with their new net hauling power blocks already in the mid-1960s.

The thrusters are used to keep the Seiner from drifting onto it’s own net while pumping the catch from the sein net alongside to the RSW tanks:

Fotokreditt: Roger Nygård / Norges sildesalgslag

Before side thrusters a boat like this, with up to 250Hp engine, was used to tow the seiner away from the net:

Before power blocks the sein net was handled from net boats (manually):

Yes, I agree. For the propose of “thinking on the margin” we can just assume skills, risk and effort are going to be held constant.