WATER DENSITY AND ITS APPLICATIONS 



accelerated. The flow lines which this water follows interact with 

 the channel bottom and cause a marked increase in the "squat" or 

 "bottom effect". A 2-foot minimum bottom clearance is a require- 

 ment for safety. This is a precaution for avoidance of screw dam- 

 age from sunken logs or other debris possibly carried into the chan- 

 nel. Further, it helps to minimize displacement of bottom sand by 

 the propeller, which might easily scoop out a foot or two of sand and 

 pile it up a short distance away to ground the next vessel passing. 

 It can therefore be seen that a 28,000 DWT supertanker or bulk 

 carrier with a 33-foot 5 inch sea water draft would require a 40-foot 

 channel for safe navigation in fresh water. 



In actual practice of seamanship, mariners usually use the hy- 

 drometer for measuring the specific gravity or density of waters to 

 predict draft. The density of fresh water is 1000, and the average 

 density of sea water is about 1025 ounces to the cubic foot. In some 

 areas of exceedingly high salt content, such as the Suez Canal, the 

 hydrometer will often float at 1040 or even higher. The water used 

 for determining specific gravity should be drawn in a bucket from 

 over the ship's side, well away from any overboard discharge pipes, 

 and its temperature immediately observed and recorded. The 

 instrument should be spun slightly in the center of the bucket so 

 that the scale will quickly loose any vertical motion, and be read 

 before the turning motion has entirely ceased. 



Samples of the mechanics involved to determine draft frequently 

 found on U. S. Coast Guard examinations follow: 



Problem: — Find the draft to which a ship must be loaded along- 

 side a pier where the density of the water is 1006, that she may float 

 at a draft of 20 feet in sea water. 



Average density of sea water 1025 



Draft at sea X20 



Density of water at pier 1006) 20500 (20 feet 



2012 

 380 



Inches per foot X 12 



Density of water at pier 1006) 4560 (4.5 inches 



4024 



5360 



5030 



330 



Answer:— The ship must be loaded to draw 20 feet 4.5 inches. 

 Hence, she would rise 4.5 inches on passing into sea water. 



Problem: — A vessel's draft is 24 feet 6 inches at a dock where 

 the hydrometer floats at 10. What will be her draft when she gets 

 to sea where the hydrometer floats at 25? 



Rule: — As the difference between salt water and fresh water is to 

 the difference between salt water and the water in which the ship 

 is being loaded, so is the allowance in fresh water to the required 

 allowance. 



Solution: — As 1025: 1010:: 24 ft. 6 in.: draft at sea. 

 Draft at sea = 1010 X 24.5 

 1025 



Density at pier 1010 



Draft at pier X24.5 



Density of sea water 1025) 24745 (24 ft. 



2050 

 4245 

 4100 

 145 



Inches per foot Xl2 



Density of sea water 1025) 1740 (1.7 inches 



1025 

 7150 

 7175 

 Answer: — 24 feet 1.7 inches 



REQUIRED CHANNEL DEPTH 



1. Salt water loaded draft 33 feet 5 inches 



2. Added draft in fresh water 8 inches 



3. Drag (trim down at stern) 1 foot 8 inches 



4. Open water "squat" at 10 knots 7 inches 



5. Added "bottom effect squat" 1 foot 8 inches 



6. Minimum safe bottom clearance 2 feet inches 



TOTAL CHANNEL DEPTH REQUIRED 40 feet inches 



Figure 3. Typical depth requirement. 



DENSITY CURRENTS AND SILTATION 



Density currents in relation to the siltation of docks are of prime 

 importance in many areas. An example of this is the siltation 

 problem in Queen Elizabeth II Dock on the River Mersey in England. 

 As shown in Figure 4, the lock and dock are alongside the Manchester 

 Ship Canal with the canal having separate entrance locks. The dock 

 is designed for the use of oil tankers and will accomodate ships of 

 650 feet in length. The lock connecting the dock with the River 

 Mersey is fitted with sliding caisson gates and prevents run-in at 

 spring tides. A typical salinity at high water springs is 26 parts 

 per thousand by weight (26 °l„ „ ) and the estuary tidal range at East- 

 ham is approximately 30 feet. Water levels in the Queen Elizabeth II 

 Dock as well as the Manchester Canal are maintained slightly above 

 the average of high water spring tides. Fresh water tributaries 

 constitute the primary source of water for the canal, supplemented 

 to some extent by run-in at high spring tides through the locks at 

 Eastham. Water level in the dock is maintained by the use of bal- 

 ancing culverts which supply water from the relatively fresh canal. 

 An average salinity in the dock is 21 "/. . , about 5 "/o - less than that 

 in the estuary, but differences of up to 9 7=° have been noted. 



The origin of siltation has two possibilities: from the balancing 

 culverts connected with the canal or through the lock connected 

 with the River Mersey. Investigation has proved, however, that a 

 negligible amount of silt enters from the canal. 



Conditions outside the lock are very favorable to continuous silt 

 settlement from suspension. A mobile layer of unconsolidated silt 

 with very high concentrations of solids has frequently been ob- 

 served just outside the lock entrance. As the bed level of the 

 estuary in this proximity is somewhat higher than that of the lock 

 the mud layer gravitates, to some extent as a turbidity current, 

 into the lock when it is opened. The effect of differences in salinity 

 or density, however, is of somewhat greater interest. Locking op- 

 erations are normally undertaken during the period of high water. 

 However, this is also the time of the maximum difference in salinity 

 between the lock and estuary. 



When the outer lock gate is opened, denser and more saline 

 water near the bed flows in carrying a large quantity of mud in high 

 concentration. Consequently, there is a corresponding outflow of 

 less saline and relatively clean water over the top of the more dense 

 water and into the estuary. On completion of this process, the outer 

 lock gate is closed and a similar condition develops when the inner 

 lock gate is opened into the dock. That is, silt is carried into the 

 dock by the strong in-flowing density current near the bed. 



The rate of siltation in the dock varies from 3,000 to 8,000 cubic 

 yards per week. This high rate of siltation is further complicated 

 by the difficulties involved in using dredgers in an oil dock. 



Several solutions have been suggested to help alleviate the situa- 

 tion. The most promising appears to be the use of vertical jets just 

 outside the outer lock gate. These jets would be brought into action 

 prior to the time the outer gate is opened. The mud lying in high 

 concentrations near the bed would be carried in suspension in the 

 upper layers of the water and, on opening the outer gate, would be 

 carried away from the lock by the out-flowing surface current. 



The fluctuation of water density is the prime factor in the 

 siltation of tidal basins open to an estuary. Usually, the estuary 

 water is heavy with silt in suspension, concentrations being particu- 

 larly high in the lower layers during the early stages of flood tide. 



73 



