Technical question regarding speed and nautical stuff

[sorahl1]

This way no one gets that whiplash feeling when the ship takea off or slows abruptly..

Thanks for this post... I like to imagine it is the kind of info I would have gotten from discussions with my granddad had he lived longer and we could have had 'adult' conversations about his years at sea.

John

 

[Neptune's Fork]

Thanks Capt BJ for that fascinating and informative post. Sort of reminds me of those movies on a jetliner where the pilot crew gets killed and they ask the passengers - Anybody know how to fly an airplane? If you were a passenger on a ship and something happened to the crew, you could step right in and sail it home.

 

[Capt_BJ]

During his first cruise, a DCL cruise, Inaugural year of MAGIC, my son suggested I should get the job as Captain of the ship ......

I did another 10 years with USCG . . .

 

[Donna M]

I was on a cruise ship when it made a turn to shelter the pilot boat from the wind. I saw some people freaking out since they felt the ship "tipping" due to the tight turn. When the pilot was getting picked up, the captain made an announcement first to prepare the passengers for the "tipping." I thought it was all very interesting.

 

[Neptune's Fork]

I was on a cruise ship when it made a turn to shelter the pilot boat from the wind. I saw some people freaking out since they felt the ship "tipping" due to the tight turn. When the pilot was getting picked up, the captain made an announcement first to prepare the passengers for the "tipping." I thought it was all very interesting.

Was this leaving out of Port Canaveral?

 

[Capt_BJ]

Was this leaving out of Port Canaveral?

it can happen at Canaveral, or any port ....

if the seas are rough, you (the master) can use the mass of the ship to create a 'smooth area' in what we call 'the lee'

To make the water easier for the recovery of a pilot (or recovering a ships boat that carried a boarding team to another vessel) ......

You turn the ship broadside to the seas and on the downside the seas are flat(er) aka 'in the lee'

if you sit here very long the seas hitting the ship broadside make it ROLL considerably ... so there is a timing factor ....

the general term is SEAMANSHIP

 

[truck1]

Was this leaving out of Port Canaveral?

Its happened a few years ago coming out of Ft Lauderdale I believe. A different line left and a inexperienced bridge crew member mis read a guage and ordered the computer in to a tight turn as the engines were ramping up. He injured several dozen people if I remember correctly and the ship turned a round with a tug escort and went back into Ft L.

When we got married on the Magic, just as the sailaway party was ending and the ships horns were playing, some one on the bridge hit the thrusters too hard and I saw and felt the Magc heel to her port side as we moved away from the pier. She rolled back and forth 2 or 3 times before she settled down. Its a law of physics. For ever action there is a complete and opposite reaction.

 

[Capt_BJ]

Its happened a few years ago coming out of Ft Lauderdale I believe. A different line left and a inexperienced bridge crew member mis read a guage and ordered the computer in to a tight turn as the engines were ramping up. He injured several dozen people if I remember correctly and the ship turned a round with a tug escort and went back into Ft L.


I think you may be remembering this one (summary from NTSB Investigation report)

Heeling Accident on M/V Crown Princess



Executive Summary
Executive Summary
On July 18, 2006, the cruise ship Crown Princess, which had been in service about a month, departed Port Canaveral, Florida, for Brooklyn, New York, its last port on a 10‑day round-trip voyage to the Caribbean. Slightly more than an hour after departing, while on a heading to intersect its track to Brooklyn, the vessel’s automatic steering system began a turn to port. In an effort to counter the effects of a perceived high rate of turn, the second officer, the senior watch officer on the bridge, disengaged the automatic steering mode of the vessel’s integrated navigation system and took manual control of the steering. The second officer turned the wheel first to port and then from port to starboard several times, eventually causing the vessel to heel at a maximum angle of about 24° to starboard. The heeling caused people to be thrown about or struck by unsecured objects, resulting in 14 serious and 284 minor injuries to passengers and crewmembers. The vessel incurred no damage to its structure but sustained considerable damage to unsecured interior components and to cabinets and their contents.


Probable Cause
Probable Cause
The National Transportation Safety Board determines that the probable cause of the Crown Princess accident was the second officer’s incorrect wheel commands, executed first to counter an unanticipated high rate of turn and then to counter the vessel’s heeling. Contributing to the cause of the accident were the captain’s and staff captain’s inappropriate inputs to the vessel’s integrated navigation system while the vessel was traveling at high speed in relatively shallow water, their failure to stabilize the vessel’s heading fluctuations before leaving the bridge, and the inadequate training of crewmembers in the use of integrated navigation systems.

full report at: http://www.ntsb.gov/investigations/AccidentReports/Reports/MAR0801.pdf

 

[Neptune's Fork]

This came up recently in a conversation with a couple boat people, I think there's a few people here that may know, what is the average RPM of the main props when travelling at 20 knots?

 

[Capt_BJ]

dunno specifics for any cruise ship, but there are many variables ....

IME on CG cutters the shaft rpm for a ship designed to go maximum of 20 knots using diesel power was maximum 300 shaft RPM. Normal cruising speed I'd keep srpm in the 180 - 240 range typically (this was an efficient setup wrt fuel consumption) .... on a larger cutter with a split plant the diesel speed was still a maximum of 300 shaft RPM which would get us to 20 and to go faster we'd switch to gas turbine power and shaft rpm increased significantly giving us a speed up to 30. "My" turbines where directly geared to the shafts; not a turbine/electric setup. I would note that these ships used a controllable pitch prop so "stop" was where the shaft was still turning at over 100 RPM, but the prop was at zero pitch, like an airplane engine at idle. To go forward at slow speed the pitch was increased rather that shaft speed. This is a very 'responsive' system in that to go from ahead to astern one does not stop the engine and 'change gears' but simply changes pitch while everything keeps spinning. It is a complicated system because like flying a helo, the system must always be concerned about the load (power/torque) the engine needs to produce. When pitch is reduced the load goes down and the engine can overspeed; when pitch is applied engine load increases and it needs 'more gas' lest it stall.

Most cruise ships today run some form of diesel electric plant. A few have gas turbine, still turning generators to feed electric 'prime mover' for propulsion.

Large merchant ships tend to have very large diesels that turn 'very slow' intended to run at a steady speed for long periods of time making them very sluggish in responding to speed changes.

 

[Neptune's Fork]

dunno specifics for any cruise ship, but there are many variables ....

IME on CG cutters the shaft rpm for a ship designed to go maximum of 20 knots using diesel power was maximum 300 shaft RPM. Normal cruising speed I'd keep srpm in the 180 - 240 range typically (this was an efficient setup wrt fuel consumption) .... on a larger cutter with a split plant the diesel speed was still a maximum of 300 shaft RPM which would get us to 20 and to go faster we'd switch to gas turbine power and shaft rpm increased significantly giving us a speed up to 30. "My" turbines where directly geared to the shafts; not a turbine/electric setup. I would note that these ships used a controllable pitch prop so "stop" was where the shaft was still turning at over 100 RPM, but the prop was at zero pitch, like an airplane engine at idle. To go forward at slow speed the pitch was increased rather that shaft speed. This is a very 'responsive' system in that to go from ahead to astern one does not stop the engine and 'change gears' but simply changes pitch while everything keeps spinning. It is a complicated system because like flying a helo, the system must always be concerned about the load (power/torque) the engine needs to produce. When pitch is reduced the load goes down and the engine can overspeed; when pitch is applied engine load increases and it needs 'more gas' lest it stall.

Most cruise ships today run some form of diesel electric plant. A few have gas turbine, still turning generators to feed electric 'prime mover' for propulsion.

Large merchant ships tend to have very large diesels that turn 'very slow' intended to run at a steady speed for long periods of time making them very sluggish in responding to speed changes.

So these cruise ships we're talking about are roughly 3 or 4 times bigger than the largest cutter, would that mean prop rotation speed would not be quite as high (because of bigger propellers)?

 

[Capt_BJ]

reasonable assumption. Consider the mass of a large prop ..... spinning at "just" 100 or 200 rpm generates a bunch of inertia ..... stopping it is a problem ... and if there is a vibration issue ..... WOW. In general I'd guess the speed to be much nearer 100 than 1000.

 

[Neptune's Fork]

reasonable assumption. Consider the mass of a large prop ..... spinning at "just" 100 or 200 rpm generates a bunch of inertia ..... stopping it is a problem ... and if there is a vibration issue ..... WOW. In general I'd guess the speed to be much nearer 100 than 1000.

I can certainly see why they say that you don't want to be in the water anywhere near the prop area. I would imagine that creates very strong suction.

 

[jrabbit]

That would be from in the cabin. There is a channel on the in-cabin television that constantly show the ship's progress and wind direction. It's pretty cool!

Also on the "big screen tv" in the Cove Cafe (adults only)

Do these monitors also display the current speed, wind speed and direction, too?

Yes, course, speed, Lat, Long, direction and "Apparent (effective) wind speed across the deck and the direction of the wind

 

[Neptune's Fork]

I noticed in the medical emergency on Fantasy thread there was mention that one of the lifeboats had to medvac a passenger, so was just curious as to what speeds are the lifeboats capable of?