Towline Pull Static Requirements (Metric Units)
This template computes the static (metacentric height) towline pull
requirements for vessels that are equipped to tow. It is based on 46 CFR 173.095(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 46 CFR 173.095(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
requirements contained in this template.
Document Type: Microsoft Excel
The following inputs are required for each draft:
T, Draft, meters
(over operational range of tug)
KMT, Height from Keel to Metacenter,
Characteristics of Vessel inputs, irrespective of draft:
B, Moulded Beam, feet
d, Depth to top of Freeboard Deck, meters
h, Maximum vertical distance from center of
propeller shaft to towing bitts, meters
D, propeller diameter, meters
HP, shaft power per shaft, at the engine, metric
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, meters
Name of Firm Operating Vessel
Characteristics required irrespective of vessel:
f, Minimum Freeboard present, meters, for each
s, Modified Factor that applies specifically to
Z-Drive Tugs, non dimensional value
P, shaft power per shaft, at the propeller, KW
GMr, Required Metacentric Height,
meters, for each draft specified
KG, Maximum Allowable VCG (Vertical Center
of Gravity), meters, for each draft specified
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,
John F., & Meyer, Richard B., Has Stability Delayed the Delivery of
Your Tug?, Marine Technology, January 1980, 6 pages,
SNAME, Jersey City,
NAVIC 12-83, Intact Stability of Towing and Fishing
Vessels, Research Results, dated 15 Nov. 1983, United States Coast
Guard, Washington, D. C.
Naval Architecture, by SNAME,
Jersey City, N. J.
Marine Technical Note - MTN 01-01 Towline Pull Criteria for Vessels
Equipped with Z Drives, dated September 17, 2001
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