Brent, I have not heard of this effect, but it makes sense intuitively if the thruster is too close to the hull surface. I wonder if that is particularly true of propulsors mounted within accelerating or decelerating ducts (Kort nozzles), as is the case with the azimuthing thrusters you linked to? I have always understood that the propulsor should be mounted with its axis parallel to the upstream water velocity - this would obviously be affected by hull form in the immediate vicinity, but for a straight fore-and-aft hull section which extends at least a couple of propulsor diameters fore and aft, with a corresponding parallel fore-and-aft thruster, I would expect the upstream flow to also be parallel with the exception of a small boundary layer adjacent to the hull surface. If the thruster was mounted so close to the hull surface that it falls partially within the boundary layer flow, then there could be some effect on the effective upstream flow direction. More likely, however, and only in the specific case of a ducted thruster, is an interaction between the boundary layer flow around the hull with the accelerated/decelerated flow around the outside of the duct shroud. The flow direction around the outside of the shroud will depend on the specific duct geometry - this will be specified by the designer to achieve a particular torque/power relationship, but in any case will not be parallel to the propulsor axis unless the shroud is nothing more than a straight tube to protect (or provide protection from) the propulsor - not very common. As the duct shroud changes the flow direction around it, it needs to be canted accordingly so that in close proximity to the hull (and this will change depending on proximity), the flow around the outside of the shroud is in the same direction as the flow along the hull. Too much toe-out, and the discharge thrust acting against the hull makes the configuration inefficient. Not enough, and the reduction in pressure could lead to separation of the flow boundary layer at the hull and vortex shedding. All of this requires an intensive fluid flow analysis which is probably not feasable for your average PSub builder. In our case, simply moving the thruster outboard from the hull should minimize any detrimental flow interactions, and we can then assume effective flow direction parallel to the hull.
On a related note, thrusters on both submersibles and ROVs are often mounted aft and as far outboard as possible with toe-out approaching 30 degrees in some cases, in order to increase the effectiveness in turning. The ideal angle for turning is to have the thruster perpendicular to the center of rotation, but then you lose effectiveness when both thrusters are acting in tandem for forward propulsion, so this is a trade-off depending on the vehicle requirements. If you mount your thusters parallel to the fore-and-aft line, at a position directly abeam of the center of rotation of the vehicle, you get the best of both, although you then need to contend with some additional drag of the appendages required, as well as place the thrusters in a position not well-protected.
A question in another post asked about limiting thruster rotation to 90 degrees (versus 360, since the motor can be reversed). Be aware that a thruster will not have the same efficiency in both directions unless it is a symmetrical tunnel thruster, so operational requirements need to be considered.
-Sean
On Jul 8, 2009, Brent Hartwig <brenthartwig@hotmail.com> wrote:
In looking at how I wish to configure my side thrusters, I'm considering tilting my trusters towards the point of the bow when horizontal 3 to 7 degrees. This is to negate the friction or thrust attachment as the outflow of the thrusters hit the hull. A principle they refered to as the gonda (SP) effect. I don't have the correct spelling, so my searches have not come up with any relevant data. Perhaps one of you gentlemen know.
After looking at the below data I would be interesting in your guys input.
Flowserve Data
Pleuger Type WFSD Azimuthing Thrusters
http://thruster.flowserve.com
The usefull thrust of a thruster, can be adversely affected by thruster hull, and thruster thruster interaction loses.
They use a 7 degree tilted axes to there thrusters to reduce thruster loses by up to 15 to 25% due to friction or thrust attachment. A principle refered to as the gonda (SP) effect.
Even when mounted at a mid point of the longnitudinal portion of a ship thruster loses can be 10 to 15%.
Flowserve uses a 97 degree bevel gear that creates there 7 degree tilted axes thruster angle. This eliminates thrust attachment and associated losses.
The 7 degree tilted axes thrusters signifiagantly reduce conditions for adverse inflow conditions when dealing with thrusters pointing toward thrusters."
http://www.youtube.com/watch?v=PzjFEe47bzA
Regards,
Szybowski