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New waterproof connectors from Blue Trail Engineering

I’d like to introduce our Cobalt Series waterproof connectors to the BR community. Click on the link for pricing and options. We’ve been producing these for about five months now and they have generated tremendous interest and a lot of satisfied users.


We also make underwater servos rated to 100 meters.

Damon McMillan


Hi Damon,

Although your connectors are mentioned briefly within a few other discussions within the forum, I cannot believe this discussion has not got more replies since this release of connectors appears to be some kind of revolution with some unique balance between price and performance.

Since I am ignorant in electricity, could you please explain how the female connector rating (20 amps for the thruster cable termination kit) and the male bulkhead connector rating for 3 pins (12 amps) are compatible with the T200 ratings itself (24amps nominal and 32amps maximum).



Hi Eloi,

I agree, these connectors are some kind of revolution! But perhaps I’m biased…

The current ratings get a little complicated, but here goes:

The contacts used in the 3-pin connectors are rated by the manufacturer at 11.2 amps assuming a 10-degree C temperature rise in air. This means that if you pass 11.2 amps through the contacts while they are dangling in mid-air, their temperature will go up by 10 degrees C. The contacts themselves, being made of metal, can of course handle much more heat than that. But the manufacturer knows that in many cases the contacts will be housed in plastic, which can be quite heat-sensitive. So the manufacturer states a very conservative 11.2 amp rating.

In the case of Cobalt connectors, the contacts are housed in PEEK plastic. The glass transition temperature of PEEK is 140 C, compared to about 60 C for nylon. So the PEEK is MUCH more heat-resistant. That’s why I bump the rating up a little to 12 amps. Perhaps I should bump it up a lot more, but I’m being conservative at this point.

Now regarding the thruster… The thruster has three wires. The ESC splits the current coming from the two supply (battery) wires into the three thruster wires. So each of the three thruster wires actually only sees about 2/3 of the current of the supply wires. An electrical engineer might argue about the specifics of that, but that is roughly what happens. So if the 3-pin connector is rated at 12 amps per pin, that is essentially the same as an 18-amp rating (18 * 2/3 = 12) for a thruster or similar application. However, I have done dedicated testing of the T200 thruster hooked up to a 3-pin Cobalt connector. I have run it at 24 amps through the supply wires long enough for the temperatures to stabilize (at least 10 minutes), with no visible degradation on the connector contacts. To be conservative, I thus give the 3-pin Cobalt connector a rating of 20 amps when used with a thruster.

The T200 can draw more than 20 amps, though, as you pointed out. That’s why I try to be very clear about the current rating of my connectors. So far I have not had any customers report damaged connectors due to high currents, possibly because they’re only experiencing higher currents for short bursts.

Hope that helps!



Hi Damon,

Thanks for your reply.

We are almost always driving the ROV in fast currents. Therefore we are on 100% gain with full throttle forward or sideways for up to 30 minutes.

In that case, would it actually make sense to use the 4 pin model (20 amp rating instead of 12amp for the 3 pin)?



Hi Eloi,

Yes, if you know you’re going to be going at full throttle for an extended period of time, it would definitely make sense to use the 4-pin model.


I would add something to this and say these manufacture ratings were also hanging in free air. As soon as you put these into service on an ROV, and especially in one being used in moving currents, the water is going to take out a LOT of heat.