carried with the present channel depth, and at practically no additional expense. The saving would thus be represented by the watertransportation charges, at the existing rates, for the additional amounts of phosphate rock carried on each ship. The amounts of the additional cargoes might vary from 500 to 5,000 tons per ship; at the approximate rate of $4 per ton for water transportation from South Boca Grande to North Atlantic ports, the saving for the additional cargoes would range from $2,000 to $20,000 per ship.

24. A large proportion of the prospective savings can, however, be realized in the near future without provision of the improvement requested at South Boca Grande. The existing project for a 30-foot depth in Tampa Harbor will probably be completed during the current year; when this is accomplished, vessels desiring to carry larger cargoes of phosphate rock than can now be handled at South Boca Grande may be diverted to Tampa. Since the land-transportation rates from the phosphate mines to Tampa are the same as those from the mines to South Boca Grande, the prospective savings for the improvement requested, as outlined in the preceding paragraph, could be effected at Tampa, diminished only by the expense of the additional run between Tampa and South Boca Grande. Justification for the improvement requested should therefore be based upon the extra expense for the diversion to Tampa rather than upon the economy of larger cargoes at South Boca Grande.

25. Vessels of a size that would require a channel 30 feet deep when fully loaded cost between $30 and $50 per hour to operate, depending upon the speed as well as the size. The average operating cost for this type of vessel is conservatively estimated to be $35 per hour. Ships that carry full loads of phosphate rock usually sail to South Boca Grande direct from the Atlantic Ocean. Diverting them to Tampa would thus require sailing an additional distance of about 190 miles for the round trip. The speed of the larger ships now engaged in the phosphate trade ranges between 10 and 13 knots. Assuming an average speed of 12 knots, the average saving per ship would be 16 hours in time, or $560 in money.

26. Traffic at South Boca Grande is at present of two distinct types although both carry the same class of commerce. One type consists of vessels that load phosphate rock either to the limits of their capacties or to the limit of the channel depth. The other type consists of vessels that load phosphate rock as a low-rate bottom cargo, proceeding to other ports for higher-rate top cargoes. During 1935, there were 55 trips made by vessels of the former type and 48 by the latter type. Of the former type, there were only four vessels that were loaded to the limit of the channel but not to the limits of their capaci ties, and which therefore would have carried larger cargoes if sufficient channel depth had been available. Of the latter type, there were 28 trips made by vessels of sufficient size and capacity to require a channel depth greater than 27 feet if fully loaded.

27. It is possible, however, that some vessels may at times engage in both types of traffic. During 1935 at least one vessel of a size that would require a channel deeper than 27 feet when loaded, made one trip loaded to the limit of the channel, and on another trip carried only a small bottom cargo. This same vessel also made at least one trip from Tampa during the year with a bottom cargo of phosphate rock. The explanation of this occurrence may be that on the occasion

when the vessel carried a full load of phosphate rock, it was unable to obtain a higher-rate top cargo. When higher-rate top cargoes are readily obtainable, vessels obviously would not carry full loads of phosphate rock, but undoubtedly there are times when top cargoes are not available in sufficient quantities to bring in a total revenue greater than could be obtained from a full load of phosphate rock. It therefore seems likely that any of the ships that usually carry only bottom cargoes from South Boca Grande might occasionally find it more convenient and profitable to carry a full load of phosphate rock. 28. It is claimed that provision of the deeper channel would result in the replacement, by larger ships, of the smaller ships that handle full cargoes but do not require a channel deeper than 27 feet: To determine the extent of this replacement would necessitate a canvas of many ship operators located in various parts of the world. It seems probable, however, that such a replacement would actually be effected, at least to a small extent. It is, in general, a fact that whenever sufficient commerce is available, deeper harbors attract larger vessels, within the limits of economical operation.

29. It therefore appears that the prospective benefits can be definitely evaluated only in part. If the present traffic at South Boca Grande is used as a basis, it may be considered as fairly definite that the four vessels which were loaded to the limit of the channel in 1935 would carry larger cargoes in the future and would thus be directly benefited by the improvement. In consideration of the fact that as many as 28 ships are capable of benefiting from the improvement if conditions should warrant their loading full cargoes, it is believed that at least two more vessels may be safely included in the estimate of prospective benefits. At a unit saving of $560 per vessel, as determined in paragraph 25, the total saving for the six vessels would be $3,360 per year. There is very little definite evidence upon which to base a more extensive evaluation of benefits; nevertheless, when consideration is given to the amount and character of the commerce involved, and to the possibilities of increased traffic by larger vessels, it seems reasonable to expect that more than six vessels annually would take advantage of the economies afforded by a deeper channel at South Boca Grande.

VESSEL TRAFFIC

30. The trips and drafts of vessels that called at the port of South Boca Grande during the calendar year 1934 are shown in the following table:

Boca Grande Channel, trips and drafts of vessels, 1934

The ships range in length from 250 to 435 feet and have capacities of between 4,000 and 14,000 tons. This table includes only oceangoing vessels; it does not include boats of less than 10-foot draft that used the Boca Grande Channel en route to Punta Gorda.

DIFFICULTIES ATTENDING NAVIGATION

31. It has been reported that many of the larger ships have considerable difficulty in navigating the turn in the channel. Although the project width at the turn is 500 feet, shoaling has encroached upon the channel so that the present controlling width between the 27-foot contours is only about 200 feet. Shoaling at this point appears to be more rapid than at any other point in the channel. The narrow controlling width, together with the turn of about 24 degrees, constitutes a real hazard to navigation, particularly for deeply loaded vessels departing on an ebb tide. Many of the ships are forced to wait for a flood tide or a full tide. It is believed that increasing the width a the bend to 700 feet would provide a usable width adequate for the needs of any commerce that would require a channel depth of 30 feet. In 1934, 48 vessels were loaded to a draft of 24 feet or more, and might have been delayed while awaiting a favorable tide. If it be assumed that 10 of these were actually so delayed an average of 5 hours each. at an average operating cost of $30 per hour, elimination of such delays by widening the bend would result in average annual saving of $1,500.

SURVEY

32. In connection with proposed maintenance operations, a survey of the Boca Grande Channel at Charlotte Harbor was made in September 1935. Rock investigations were not included in the survey, but in view of the fact that in previous surveys depths between and 32 feet have been found in many places throughout the entire length of the channel, it does not seem likely that any rock exists above an elevation of 32 feet below mean low water. Results of the survey are shown on the map accompanying this report.

PLAN OF IMPROVEMENT

33. The plan of improvement considered herein provides for deepening the present channel to 30 feet and extending it at each end to s natural depth of 30 feet. Since considerable difficulty has been experienced in navigating through the bend in the existing channel. it is also proposed to widen the bend on the southeast side to a total width of 700 feet, decreasing to the normal channel width of 300 feet in a distance of 1,500 feet on each side. An allowance of 2 feet overdepth throughout, and sideslopes of 1 vertical on 3 horizontal are used in the estimates.

34. In preparing the estimates of quantities and costs for the improvements considered herein, it has been assumed that the maintenance of a depth of 27 feet plus 3 feet overdepth is already provided for under existing authority, and that this depth will therefore exist whe the improvement dredging starts, or that removal of material to this depth will be paid for from maintenance funds and will not be a charge

against the cost of the new work. On this basis the estimated cost of the above-described improvement is as follows:

Deepening and extending channel: 340,000 cubic yards soft material, at 121⁄2 cents.............

Increased widening at the bend: 70,000 cubic yards soft material, at 121⁄2 cents..

Engineering and contingencies, approximately 15 percent..

Total....

$42, 500

8,750

7, 750

59, 000

35. The unit cost used in the foregoing estimate is based upon that developed during the operations of the Government-owned seagoing dredge Benyaurd on maintenance work at this project, plus an allowance of 25 percent for the added cost due to the increased depth to which the dredging would be carried and to possible difficulties due to changes in material, unfavorable weather, etc.

DISCUSSION

36. The claim that provision of the improvement requested would result in the movement of larger cargoes at appreciable savings appears to be well founded. The full extent of the resultant benefits cannot, however, be determined with any degree of certainty. Accompanying this report are seven letters of endorsement, in which representatives of companies having considerable interests in phosphate operations at South Boca Grande express their opinions of the general value of the improvement requested; but all of them fail to state in definite terms the extent to which their companies might be expected to use the improvement.

37. The prospective savings as estimated hereinbefore are based chiefly upon a conservative analysis of present traffic; it is recognized that the development of additional benefits is a possibility of considerable importance. More than 25 percent of the 1935 traffic at South Boca Grande could be considered as prospective users of a deeper channel if conditions should warrant their handling capacity loads; but because data concerning the many factors involved in a prediction of future shipping conditions are not readily available, consideration of that portion of the prospective traffic can be made only in a general way. The estimated prospective savings of $3,360 may therefore be taken as the minimum savings to be expected.

38. If the prospective savings are capitalized at 4 percent, justification of the expenditure of $84,000 is indicated. This figure compares favorably with the estimated cost of $59,000 of the improvement requested. If the existing project for the Boca Grande Channel should be modified to provide for the improvement requested, it is probable that the improvement work could be prosecuted in coordination with maintenance of the present channel at a slight saving in cost for the combined work. Maintenance to the present project depth of 27 feet plus 3 feet overdepth is tentatively scheduled for execution during the current year. When this is completed, the channel will in fact have a depth of 30 feet. Since shoaling usually occurs at a comparatively slow rate, it is likely that the 30-foot depth would exist for some time thereafter without requiring immediate improvement work. When shoaling should progress sufficiently to interfere with navigation, the 30-foot depth could again be obtained by main

tenance under the 27-foot project, and at the same time, improvement work under the modified project considered herein could be conveniently and economically prosecuted to provide a project depth of 30 feet plus 2 feet overdepth, and the increased widening at the bend.

39. If and when the Atlantic-Gulf Ship Canal is completed, the situation will be reversed so far as concerns the advantage in time savings of South Boca Grande over Tampa for ships entering the Gulf from the Atlantic Ocean through the canal. The Gulf entrance of the canal is about 70 miles north of the Tampa Harbor entrance and 140 miles north of the Charlotte Harbor entrance; but because of the greater distance between Tampa and deep water in the Gulf, the actual saving in distance for the run from the canal to Tampa as against that from the canal to South Boca Grande is about 30 miles. Thus, at an average speed of 12 knots, phosphate ships will be able to call at Tampa instead of at South Boca Grande with a saving of 5 hours for the round trip.

40. This would not hold true, however, in the case of vessels carrying phosphate to South America and to the Far East by way of the Panama Canal. Such vessels would, of course, still find it more advantageous to load phosphate at South Boca Grande. About 25 percent of the larger ships now calling at South Boca Grande carry phosphate to the Far East.

41. Furthermore, there may be some doubt as to whether vessels which now ply between Atlantic ports and South Boca Grande wil actually make use of the Atlantic-Gulf Ship Canal because of the small saving afforded. The round trip between Atlantic ports and Tampa via the canal could be made at a saving of only 10.8 hours over the trip between Atlantic ports and South Boca Grande via the Florida Straits. It seems likely that pilotage costs and the hazards of navigating a deeply loaded vessel through the canal might offset this small saving in some cases, at least.

42. Inasmuch as the canal will not be completed at least for severs years, it would have no immediate effect upon the prospective savings for the improvement requested, and in view of the foregoing facts, it is believed that even after completion of the canal, the prospective savings from various sources would still be sufficient to justify the improvement requested.

43. When the existing project was adopted in 1912, the commerce of the port was largely controlled by one company, and its future development was more or less problematical. It was therefore deemed equitable to require liberal local cooperation in carrying out the work. The harbor has now become an important phosphate port, and is used by a number of interests. Any savings in transportation expense for phosphate rock would undoubtedly be beneficial to the industry as a whole. It is therefore believed that cooperation on the part of local interests should not be required at this time.

WATER POWER AND OTHER SPECIAL SUBJECTS

44. It is not practicable to coordinate the improvement desired with projects for terminal construction, power development, flood control, irrigation, land reclamation, or other works so as to reduce the cost to the United States.