Towline Pull Static Requirements (English
Units)
Description:
This template computes the
static (metacentric height) towline pull requirements for vessels that are
equipped to tow. It is based on 46CFR173.095(b) of Subchapter S -
Subdivision and Stability. Several drafts,
over the vessel's range of operation, are evaluated in this template.
These calculations apply specifically to vessels equipped
with Z-Drives. This includes tractor tugs, OSVs, workboats and fish
boats. They do not apply to tugs, offshore supply vessels,
workboats, fish boats or tractor tugs that are conventionally shafted, or
with vertical axis propulsion (Voith-Schneider) tugs, or with paddle
wheels.
Note there are also dynamic (moment area) requirements
contained within 46CFR173.095(a). A tow vessel may alternatively
comply with these dynamic requirements. These dynamic requirements
are covered in another template. The dynamic criterion is often less
stringent than the static requirements contained in this template.
Electronic
Document Type: Microsoft Excel
spreadsheet
Cost:
Free
Number of
Pages: single sheet
Inputs:
-
The following inputs are required for each draft:
-
T, Draft, feet
(over operational range of tug)
-
D, Displacement, long
tons
-
KMT, Height from Keel to Metacenter,
feet
-
Characteristics of Vessel inputs, irrespective of
draft:
-
B, Moulded Beam, feet
-
d, Depth to top of Freeboard Deck, feet
-
h, Maximum vertical distance from center of
propeller shaft to towing bitts, feet
-
D, propeller diameter, feet
-
HP, shaft power per shaft, at the engine,
horsepower
-
h, efficiency
of Z-Drive between engine and propeller, in range of 0.95 to 0.98
according to McGowen & Meyer paper listed in references below
-
N, Number of Propellers, non dimensional value
-
q, Angle between
lines shown in Figure F1A
(plan view), degrees. First line is drawn between vertical axes
of Z-Drives. Second line is drawn along the closest slipstream
edge of the Z-Drive unit behind the unit facing directly athwartships
(transverse).
-
Name of Vessel
-
Length of Vessel, feet
-
Name of Firm Operating Vessel
-
Characteristics required irrespective of vessel:
Outputs:
-
f, Minimum Freeboard present, feet, for each
draft specified
-
s, Modified Factor that applies specifically to
Z-Drive Tugs, non dimensional value
-
P, shaft power per shaft, at the propeller,
horsepower
-
GMr, Required Metacentric Height,
feet, for each draft specified
-
KG, Maximum Allowable VCG (Vertical Center
of Gravity), feet, for each draft specified
Suggested
Reading:
Subchapter S - Subdivision and Stability, Title 46 Shipping, USCG
Procedure H1-04, MSC Guidelines for Review of
Stability for Uninspected Tugboats (C), dated 3/21/00, USCG,
Washington D. C.
COMDTINST M16000.9, Marine Safety Manual, Volume IV,
Sections 6.C.1, 6.E.1b. & 6.E.2, United States Coast Guard, Washington,
D. C. McGowen,
John F., & Meyer, Richard B., Has Stability Delayed the Delivery of
Your Tug?, Marine Technology, January 1980, 6 pages,
SNAME, Jersey City,
N. J..
NAVIC 12-83, Intact Stability of Towing and Fishing
Vessels, Research Results, dated 15 Nov. 1983, United States Coast
Guard, Washington, D. C. -
Principles
of Naval Architecture, by
SNAME, Jersey City, N. J.
-
MSC Marine Technical Note - MTN 01-01 Towline Pull Criteria for
Vessels Equipped with Z Drives, dated September 17, 2001
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