Marine Engineering - Practical Tips & Tricks

A place to exchanges questions and ideas of a technical / procedural nature. Go ahead, try to stomp us !
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Merlyn
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Re: Marine Engineering - Practical Tips & Tricks

Postby Merlyn » Wed Jul 08, 2015 12:56 am

Very interesting stuff, I do remember reading a lot of it in the past. Didn't know about the SG being different in th Great Lakes though. Will have to educate myself re Canada. But the million dollar question remains, up or down? Coarser pitch or finer? Wonder if anyone in Canada has ever heard of this speed loss? If you bought a powerboat in the Great Lakes area and shipped it elsewhere would there be a loss of knots?
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Re: Marine Engineering - Practical Tips & Tricks

Postby D Winsor » Wed Jul 08, 2015 3:45 am

If you bought a powerboat or yacht designed for optimum performance on the Great Lakes I expect you will experience an increase in speed and performance at sea level and in salt water.
The water in the St. Lawrence Estuary above Quebec City which included the Great Lakes is considered fresh water and fresh water is less dense than salt water. The density of salt water is 1.028 Kg/L while fresh water the density is 1Kg/L
Ships transiting from salt to fresh water usually experience an increase in draft of up to 6" putting the propeller deeper in the water but the propeller becomes less efficient as propeller slip will increase. Discounting the effects of changes in altitude, Lake Superior is 602 ft, Lakes Erie, Huron & Michigan 575 - 580 ft, and Lake Ontario 243ft above Sea Level I have witnessed ships loose up to a knot in speed on a deep water open lake transit with little or no loss of engine performance or measurable power output.
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Merlyn
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Re: Marine Engineering - Practical Tips & Tricks

Postby Merlyn » Wed Jul 08, 2015 5:05 am

Blimey then, we live and learn all the time. Nothing like that goes on over here to my knowledge, but what do we do to maintain speed when transiting from salt to fresh? Prop up or down?
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Re: Marine Engineering - Practical Tips & Tricks

Postby D Winsor » Wed Jul 08, 2015 12:02 pm

I expect you could get the same effect when transiting from the sea up into a fresh water loch in Scotland.
You can wait for a flood tide when transitioning from salt to fresh water as the current created by the incoming salt water will help propel you forward with little or no loss of speed.
If you are talking about a powerboat increasing the full speed the water plain angle by increasing the weight at the stern to put the prop deeper in the water to produce more thrust and reduce the amount of hull contact with the water might work.
On a large ship with a fixed pitch little can be done about it but increasing the pitch on a ship with a CP Prop can help reduce the amount of speed lost.
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Re: Marine Engineering - Practical Tips & Tricks

Postby JK » Thu Jul 09, 2015 5:09 am

When the Cap Rouge ll capsized in 2004 in BC, it was transiting from the strait to the Fraser River. So the FV would have left SW, been in brine, then into FW. She was overloaded in the SW and would have had her draft increased as she entered the FW area. If you read the TSB report, there is no mention of that, probably as it was overshadowed by the fact the vessel was compromised in stability by retrofits over the years. It would have been just another piece of kindling to the fire.

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Re: Marine Engineering - Practical Tips & Tricks

Postby Big Pete » Thu Jul 09, 2015 6:39 am

One thing nobody is mentioning is that when the Density of the Water goes down, the vessel will sit deeper in the Water, increasing the Wetted Surface Area and the Volume of Water to be displaced as the vessel moves forward. So, presumably, the power requirement for the same speed, will increase.

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Re: Marine Engineering - Practical Tips & Tricks

Postby D Winsor » Thu Jul 09, 2015 7:45 am

Pete
I did mention the change in draft when transiting from Salt to Fresh water in an earlier post. "Ships transiting from salt to fresh water usually experience an increase in draft of up to 6" putting the propeller deeper in the water but the propeller becomes less efficient as propeller slip will increase."

The average 730 ft 30,000 Ton Laker loaded with iron ore to a draft of 26' 0" in Sept Isles or Port Cartier Quebec will have a draft of 26' 6" at St. Lambert the first Lock in the St. Lawrence Seaway. St. Lambert is in Montreal Habour well above any tidal effect created by salt water in the St. Lawrence River and is therefore 100% fresh water.
When ships are loaded to such tight tolerances to maximize cargo even the sun can cause problems with draft. It is not uncommon to have heat from the sun playing in the main deck to cause a laker to hog 3" or more putting ship deeper, sometimes over the maximum allowable draft for the seaway, than desired at the ends and lighter in the middle. To help counteract this water sprinklers are used to keep the deck cool
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Re: Marine Engineering - Practical Tips & Tricks

Postby JollyJack » Thu Jul 09, 2015 10:14 am

I don't think the change in water density played a very big part in the Cap Rouge case, she was quite small. Overloading and that bloody great net roller which had been retro-fitted had a far greater effect. There was no requirement at that time for transverse stability on small fishing vessels. The report is here:

http://www.bst-tsb.gc.ca/eng/rapports-r ... 2w0147.asp
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Merlyn
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Re: Marine Engineering - Practical Tips & Tricks

Postby Merlyn » Tue Jul 14, 2015 12:56 am

The answer I was seeking concerning propping up or down is that entering the Med from the Atlantic in order to maintain top governed rpm / speed is that a finer pitch prop/ s would be required. I have personally been involved with this but upto what length ship this would apply I am not sure? Would this apply to bigger ships, can't honestly say I have ever noticed or been involved in this, variable pitches might require slightly different settings perhaps?
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.

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Big Pete
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Re: Marine Engineering - Practical Tips & Tricks

Postby Big Pete » Tue Jul 14, 2015 9:16 am

I have never seen props changed on "Big" ships. I do remember a story of one ship in the P&O group that had the original prop replaced with one with a lesser pitch because the engine was overloading on Torque before it reached it's rated RPM and so could not develop full power. The original Bronze prop was kept as the spare and lashed down on top of the aft Deck House alongside the emergency steering position. When the ship went for scrap someone remembered this valuable lump of Bronze and it was discovered it been miraculously converted into carved Wood with a realistic paint job. More of the "Dark Side" forces at work?

As a Junior I sailed on the old Merkara aka Strathmeigle, she had the same problem of the prop over torqueing the engine, and could never develop full power or speed, she was on a General Cargo run from the Persian Gulf to Kiwi and Aussie, but was going on to a container run through Suez and they wanted maximum speed. They cropped the tips of the prop blades, increasing the slip and so reduced the Torque at any given RPM enough for the engine to make max torque at maximum RPM and for the ship to reach its designed speed, but at a slightly reduced efficiency.

With CPP obviously there would be no need to change the prop, just increase or reduce the pitch. Usual thing, if the Exhaust temps are too high, you phone the Bridge and ask them to reduce pitch. 10 minutes later they, they think you have forgotten about the exhaust temps and put the pitch up again!

Obviously with CPP there is usually an overload device on the fuel rack which will automatically send a signal to "Shed Pitch" or "Reduce Pitch" when a lever on the fuel rack hits a micro switch or a potentiometer rotates to a certain position, preventing the engine being overloaded. You often see this flashing in heavy weather when the Deck Officers try to force the ship at full speed through heavy Seas. Of course this only works if the individual fuel racks for the pumps all remain set exactly the same as when the pitch reduction system was set up and nobody has changed the lengths or the spline positions on the Governor shafts.

This system would obviously prevent overloading from any cause also it is rare for commercial shipping to run at more than 85% power.

I have also seen on supply boats with big electric bow thrusters and cargo pumps driven by shaft alternators, a system of " pitch limiting" which works entirely separately. For instance if the breaker for one Bow Thrust is put in, the pitch control "desired Value" signal from the Bridge is restricted to a maximum of 80%, if the second BT breaker is put in, it is restricted to 60%, appropriate reductions are also made when the breakers for the heavy consumers i.e. cargo pumps and Bulk compressors are put in. These reductions are related to the power of the engine and the power demand from the consumer, and will often be different from the ones quoted as an example, but will prevent the combination of Electrical and propeller load overloading the engine.

BP
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Merlyn
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Re: Marine Engineering - Practical Tips & Tricks

Postby Merlyn » Tue Jul 14, 2015 11:25 pm

Didn't know you were handy with wood chisels as well as torque wrenches etc?
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.

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Re: Marine Engineering - Practical Tips & Tricks

Postby Merlyn » Tue Apr 04, 2017 1:54 am

Any one out there remember the Cord Ring era?
Barrel shaped liner wear present, can't get replacement liners quick enough, have to get them especially made, long delays all round.
Back in the seventies Cord Piston Rings were the rage.
The top rings were very different, the oil control rings were something else, they used to grip the bores equally in the centre of the liner barrel wear as they did at both the top and bottom end.
Very different set ups with multi rings and sprung spacers, had to be careful when fitting that zigzag centre piece being spring loaded it sat in the groove correctly and the two rings sat either side of it.
Plus the spacer that sat in the bottom of the ring groove providing the outward thrust motion required for the rings to really grip the bores.
When you fitted these it was always a " temporary basis " until new piston/ liners were available.
Some engines I went back to after a Cord Ring fit I had done previously had started to burn oil and compressions down many many hours later the liners were almost broached out to an extent you would never believe.
I can remember measuring liners with the poker gauges ( internal mikes ) and being astounded at the amount removed from the liners between overhauls was incredible.
Haven't done one for donkeys years nor heard of them in our neck of the woods.
We used to fit them in engines in ships that were to be sold but as the norm. they would reappear in the same hands many years later.
The usual " temp." Repair situation that went on for years.
What was all the rage back then over here doesn't seem to be in fashion any more.
How things change / move on, ( or down ?)
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.


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