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Heave Compensation

Michael J. Purcell and Ned C. Forrester, Bobbing Crane Heave Compensation for the Deep Towed Fiber Optic Survey System, Society of Naval Architects and Marine Engineers, New England Section, Fiftieth Anniversary Proceedings, May 6-8, 1994

The Fiber Optic Survey System (FOSS) is an underwater vehicle designed for high resolution observation of the seafloor. Towed with the standard 17.3 mm (0.68 inch) fiber optic, electromechanical cable at speeds up to 1 m/s (2 knots), it may be deployed at depths from 100 meters to 6000 meters in wave conditions up to sea state 4. These parameters result in large variations in deployed cable length, towing angle, static cable tension and cable/vehicle dynamic response to wave induced ship motions that are transmitted into the cable. Computer modeling of the cable/vehicle system indicates that certain combinations of towing parameters result in excessive vehicle motion or high dynamic cable tension. This paper addresses the effectiveness of heave compensators, particularly a bobbing crane, in reducing induced motion and cable tension. Both a passive and a combination active/passive system have been considered. Modeling results indicate that the passive system reduces high dynamic cable tensions, but eliminating induced vehicle motion requires an active component.

» Text of Heave Compensation Paper

Alvin Electric Propulsion

Roger H. Maloof, Ned C. Forrester and Charles E. Albrecht, A Brushless Electric Propulsion System for the Research Submersible Alvin, IEEE Oceans '86 (presented but not published), Sept. 23-25, 1986

Alvin has been fitted with a new propulsion system using multiple thrusters directly driven by brushless electric motors. The high efficiency of direct drive out weighs the use of small thrusters on a large vehicle. Analysis and test data support the improved reliability and performance compared to Alvin's old hydraulic propulsion system.

[NB: In the paper we assumed Alvin to be a piece of plywood with thrusters. The analysis we used from PNA (ref 1) is that for a jet boat, which draws its inlet water through a duct in the hull at right angles to the direction of thrust, and thus must first accelerate the water to vessel speed (by pushing with the walls of the duct). In other words, we are assuming that the action of the thruster is to accelerate wake water, BUT that it thus forces more wake water to be generated as a result.

Cal Gongwer (Innerspace Corp. maker of the thrusters) disagrees, and claims that the availability of wake water is an advantage, and that the system should be analyzed as a "wake adapted propeller". See:
Steady Motion Dissertation from Innerspace Corp.

I still think our original analysis is correct, but I do not have data to prove that. (NCF 2007)]

» PDF of scan of paper, 20 pages.

Linux Fedora Core 3 on an HP nw8000 Laptop

Ned C. Forrester, Linux Fedora Core 3 on an HP nw8000 Laptop

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