it was my experience of many years in San Franisco bay that the SF pilots had a grading system A - C for tug horsepower and bollard pull so they could properly mate the displacement of the ship to the correct horsepower and bollard pull of the tug.
I thought this was universal but apparently not
You are referring to this incident?:
It may also be wise to ensure that Pilots understand the limitation of the tugs they are assigned.
Even a modern Tractor tug has limitations:
That rating system only factors in bollard pull and has absolutely nothing to with what happened here. Bollard pull and maximum speed are not necessarily directly related to each other.
Regardless, 10 kts for making up anywhere other than, perhaps, centerlead aft, is too fast, IMO.
There was another recent tug/ship collision in Houston where excessive speed was a factor, this time when making up centerlead forward, also at close to 10 kts.
https://www.ntsb.gov/news/press-releases/Pages/NR20230726.aspx
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They know. Some just don’t care.
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It’s also incumbent on tug operators to tell the pilot “no” when being asked to do something excessively dangerous. I know in some places that can feel like a hard thing to do, but the tug captain/mate is ultimately first responsible for his own vessel and crew.
Relationships between tug operators and pilots vary from port to port but the more each side understands what the other is going through, the smoother and safer the operation will be.
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And then you have the kiss ass that will say yes to every pilots request then the pilots expect every other tug to put themselves in the same dangerous scenario because, well why can’t you do what so and so was capable of?
Say no and then you have to explain it to the office, because they also want to bend over backwards to make the pilots and the ships happy.
But dont forget…everyone has the “authority” to stop a job if they feel unsafe.
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Same story, different link to workboat magazine’s summary.
I never did ship docking with a z-drive but I did lots of it with conventional tugs. You had to be very careful working forward of the break or aft of the rumble if the ship had any way on it. On the big, slow, ungainly ocean going TowBoat I was on you simply never left the flat side.
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Yes well I read that report and from what I can tell the operator attempted to a slick maneuver that was clearly beyond his skill level.
Almost any operator can spin the boat 180 while underway and continue driving in reverse. However, to try that maneuver right in the suction zone along the quarter of a ship is for an advanced operator only and really I’m not sure why you would risk that there. Smart and safe thing to do is perform the maneuver ANYWHERE BUT right off the quarter, (Abreast midship and well off, aft of the quarter in open water behind the ship, even abreast of the quarter but well off of the ship, wherever) and once you have her well in hand, then work her in towards the quarter and put your line up. But to try and whip her around right in the suction zone, then in the midst of that instability, try to land her backwards on the quarter at 7 knots or whatever, yeh not surprising he lost control.
I see now on second reading he also tried to land her very far aft and got sucked in. Landing on the cut away after flare of a ship, at speed, is perilous.
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That’s what I got out of it as well.
I’ve never been to Corpus Christie but I imagine it’s a fairly narrow channel so the ship would be sucking a lot of water from the shallows while moving along at any speed adding to the suction effect.
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In has to do more with horsepower and perhaps the area where the pilot positioned the boat
there is always this suction at the speed the ship was going.
had the tug been positioned differently and had more HP this would not have happened
I’ve no experience with ship-assist but I’ve seen (apparently) inexperienced boat handlers have problems coming alongside.
The hydrodynamic forces involved in interactions between vessels is not linear but increases exponentially proportional to speed.
The hydrodynamic forces at 10 kts could be double (more or less) the forces at 7 kts.
As the tugboat attempted to move into position, its speed increased to 11.6 knots, just 1.4 knots less than its maximum rated ahead speed, according to the NTSB report. Investigators noted that “higher speed reduces the amount of reserve propulsion power available to the operator.”
The tug in question is among the newest and most powerful types in use in the US for ship docking. In my opinion it was an issue of an ill advised maneuver, improper positioning by the operator, performed at too high a speed.
Attempting to land on the after flare of a cut away ship (with your tug going ahead or astern) at speed is perilous and I’ve seen it punish plenty of operators. Once you get into the stern suction point of no return, you either have a choice to ram the ship to protect the rest of your boat, or make a dramatic correction that throws the trailing end of your tug under the quarter flare. (You choose what damage you want) Either way you are going to land hard and possibly damage both your boat and/or the ship.
All the operator had to do was land a little more forward on the parallel mid-body (it doesn’t take much to get ahead of the suction zone) and then slide back into position.
Also, 9.6 knots is towards the upper limit of the speed by which you want to bring a tug alongside, much less try a risky maneuver on the quarter.
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Looks like the discussion quickly veered off into the wisdom of making up a tug while running at 10 knots.
To answer the original post… having a table that purports to match tug power to ship displacement is problematic. Displacement is not the only factor. Other considerations are current, wind, difficulty of maneuver. On a fine day with the ship head out and no one in any hurry, no wind, no current, one 1700 hp tug can ease the bow off the dock enough to get headed outbound. The same is not true on a windy night with the ship in ballast that has to be backed down a long slip. The danger of committing to a table is a tug dispatcher might assign an underpowered tug to a difficult maneuver causing delay or an attempt with less than adequate assistance. There are other equally undesirable possibilities.
As to the collision… if the ship was running at 10 knots it was clearly not anywhere near the berth. The tug had plenty of time to wait until the ship slowed down to maneuver. Most pilots just tell the tug where they’d like her made up… maybe a few words about the intended maneuver like which direction they intended to turn - stuff like that. If I needed a tug while running 10 knots that would be worth a lot of explanation.
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I can see why a table could be problematic.
Might not be useful in practice but an argument can be made, based on human factors, that in principle a table would assist in some situations.
It would likely require a disclaimer of some sort based on a legal and practical review etc, maybe some documented formal training etc.
A table might assist to make a first approximation but ultimately of course it’s going to be a matter of expert judgement.