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Towbody System Design Support

VDS Tow Body
VDS tow body scaled model

Our client asked us to support the development of a sonar tow body prototype for a proposal.  For the prototype, they needed a vehicle that could be towed at speeds exceeding 25 knots and reach 600 ft depths.  The goal was to propose a system that would have superior sonar performance while minimizing towing loads.  Because of our extensive experience with tow body design, our client asked us to provide our expertise for their prototype.  Our contribution to the project demonstrated Einhorn Engineering’s innovative design work.

We managed a variety of aspects of the project from advising our client on equipment arrangements to predicting tow loads.  We started by working with our client to develop an ideal equipment arrangement for the prototype.  We helped them select a compact arrangement that would provide the best chance of minimizing towing loads.  The equipment geometry dictates the final shape and size of the vehicle so Einhorn Engineering’s contribution in defining it was vital in achieving the aims of the prototype.

With the equipment parameters defined, Einhorn Engineering worked with tow cable manufacturers to complete a tow cable design. We then conceived two concepts that could accommodate the equipment.  We presented our client with images of concept models and predicted weights to communicate our work.  We thought the best approach was to evaluate and compare the two concepts and they agreed.

Hydrodynamic Design

A tow body design must consider hydrodynamic principles in order to “fly” through the water smoothly.  Hydrodynamic drag and body weight are crucial parameters because they significantly impact tow cable size and strength.  A robust tow cable requires a correspondingly robust tow winch and handling system.  Tow cable size also impacts the loads transmitted to the ship deck. For the system to be feasible we needed to minimize those predicted loads.  To determine which of our two proposed concepts would be best, we performed CFD (computational fluid dynamics) analyses to compare hydrodynamic characteristics as shown in the figure below.  We quickly determined the “boot” design had a superior form factor for minimizing towing loads.

Tow body CFD comparison

To optimize hydrodynamic stability at various angles of attack, we continued our CFD analyses on the boot tow body.  A stable vehicle has a strong restoring force when perturbed.  The image below shows a CFD analysis of the tow body at an angle of attack with respect to yaw.

Tow Body angle of attack CFD
Tow body CFD analysis at angle of attack

The CFD analyses helped us to improve the vehicle geometry and its corresponding hydrodynamic characteristics. Through our analyses, we found that the vehicle would need a depressor wing to reach the target depth.  We determined how much downward lift was required to reach the target depth and designed the wing and mounting arrangement.  In addition, adding an aft stabilizer wing improved the tow body’s stability for pitch angles of attack.

To accommodate the sonar equipment and the subsea electrical system, we designed an internal mounting frame.  We needed to minimize the weight of the frame while ensuring adequate structural strength.  To that end, we used both finite element analysis as well as classical mechanics analysis techniques to design it.  Only after iterative optimization and clear communication with our client, our team was able to complete the tow body prototype design.

Fabrication & Testing

After we completed the tow body design we built a scale model.  As part of that effort, we worked with our fabrication partners to see that our designs are practical, budget-friendly, and easy to machine.

Once the prototype was built, our client tested it at the NSWC Carderock tow tank. The tests verified the tow body performed as designed and met our client’s expectations.

Tow Body Tank Testing
Tow body testing