[PSUBS-MAILIST] Tech Talk...

1. All MINN KOTA and MOTORGUIDE units, including those equipped with double cup seals and an O-ring as stock equipment, are rated for only one additional atmosphere beyond surface pressure: a depth of approximately 33 feet. Double seals provide an added margin of safety within design pressure limitations only. At excessive depths, when seal failure occurs, pressure sufficient to implode the outermost of a double seal arrangement will also be enough to implode the inner seal, which has no greater strength than it's counterpart. Thus, double seals do not enable or support operations at depths beyond those intended by the Manufacturer.

2. The Manufacturers recognize that, for many years, people have sometimes operated their motors at up to 3 additional atmospheres pressure, or a depth of approximately 99 feet, without leakage. This is attributable to safety margins inherent to design, but there is no guarantee such will always be the case. Accordingly, for use beyond one additional atmosphere, pressure compensation is recommended.

3. When operating under excess depth-pressure conditions, the use of some additional watertight means at the electrical port and housing bell (nosecone) O-ring is also recommended.

4. Both MINN KOTA and MOTORGUIDE lower end units may operate when flooded. There is now some debate amongst the techs as to whether or not a flooded housing will engender rapid brush degradation. However, they all agree that leakage and/or flooding can result in an electrical short circuit ,and/or cause the brush springs to rust. (This is especially important to those operating in salt water.) A short, or failed springs, will terminate motor operations altogether. Again, pressure compensation is recommended to avoid these potential problems, and frequent inspection of internal components is advised.

5. Accepted compensation methods include the introduction of air pressure, or oil, within the lower end unit. If oil is to be used, the prime consideration is the elimination of any voids (air spaces) within the housing. One means of accomplishing this is to first impart a slight vacuum to the motor housing , and then filling it with oil at ambient surface pressure.

6. BUSBY's chart (page 332) describes the use of "V V - I - 530a Transformer Oil" as "questionable for motor immersion", and gives better marks for known use and possible applications to "MIL-H-5606B Aircraft Hydraulic Fluid". However, techs presently working in the business do recommend "just about any kind" of Transformer Oil for pressure compensation in both MINN KOTA and MOTORGUIDE units; citing low viscosity, excellent lubrication, and non-conductive properties as reliable advantages.

7. Those considering the use of MOTORGUIDE's 107# thrust unit should understand it does not come equipped to switch current and produce reverse thrust; instead, it is designed for bow-mounted boat applications, where reverse is accomplished by manual thrust vectoring. The MG-107 can be modified for reverse by including a CURTIS MOTOR CONTROLLER (or the like) to the circuitry. However, the MINN KOTA 101# thrust MAXXUM unit DOES come equipped for reverse, and won't require that additional modification.

8. While economy and availability make trolling motors attractive for limited submarine use, they weren't intended for this purpose. The diameter, pitch, and rotational speed of trolling motor props was designed for surface boats. Submarines do better with large diameter, coarse pitch, slower-rotating props that take a bigger bite out of the water and generate less cavitation at lower velocities than trolling motors do. Those presently using trolling motors in their submarines may want to consider the application of gear reduction and an alternative prop (enabling the motor to operate at it's "sweet spot" while allowing the screw to turn at it's optimal rpm) to enhance performance and ensure motor longevity.