Hi Mark,
That is undoubtedly a Hectoperm👍 Easily recognisable as the case extends right back to only a thin edge of the brush end cap. The Decaperms have a shorter body and different, wider, brush gear end cap with an earthing tag sticking out.
Pics attached of both Hecto (pic 1) and Decaperm (pic 2) courtesy of Jotika😁. Also of my Decaperm (pic 3), now sitting on the shelf after long since being removed from my 1960s 28" 2kg Sea Scout, where it provided very sedate performance🤔. Pic also shows current fit of a Propdrive brushless. Much zippier😊
I assume that your Hecto is the 6V version, based on your preference for onboard power stated below, and the 6V 6.5(?)Ah SLA battery in the background of your photo.
Therefore it's current draw at max efficiency ON load should be around 8A.
These Marx motors are built for torque and pulling power not for high revs and speed!
So I reckon your 6V Hecto should be fine to get your your 6kg freighter on the move.
Freighters of that era were not known for speed, maybe 16 - 18kn.
Accepted formula for max Scale Speed is the square root of scale x the max speed of the original in knots.
You didn't tell us the scale but I'm assuming 1:100, or 1:96 if imperial units.
For simplicity assume 1:100 and max original 18 kn.
Thus Max scale speed = √1/100x18 = 0.1x18 = 1.8kn.
Which equals a gentle stroll of 2.714mph😉
Or if you prefer 3.33kph, or roughly 1 metre per second😁
That hull can probably take a 6V 7-8Ah SLA without problems (better useful ballast than 'dead lead'😉) so I reckon you'll get well over an hour cruising (like I do with my 1.35m destroyer; 2 540s running on 2 6V 7Ah SLAs in series), or are you a 'pedal to the metal' kinda guy?😁
Hope this helps Mark.
Looking forward to the Sea Trial report👍
Stay safe🤞 and don't run aground!☹️😭
Cheers, Doug 😎

Hi Mark,
The picture of a motor looks like a decaperm or hectaperm these are high quality motors and very efficient for a DC brushed motor.
they did two versions of each, a 6V and a 12v,
with a big prop use the geared end to connect to the shaft.
I have a 6v Decaperm and it moved a 90cm very heavy boat about 6kg around for over 30 mins at 4mph on a 6v 4ah battery. normally pulls 4 amp underload.

Regards Roy

First, Which Marx motor is that you have? If it is the big one then that maybe a bit of overkill, as well as a significant battery draw. See the link to Marx motor specs. http://www.gtmbc.org.uk/engines.html
From my experience, I would guess that you would be looking for a motoor providing around 3500 rpm and drawing NO MORE than 5 amps. I have used Johnson 970 motors, which provide this, in My 1/48 destroyer, a 4' tug and a 5' ARS. All very successfully. Unfortunately, they are about as common as hen's teeth and they are 12 volt.
One draw back you will find with 6 volt motors is that the amp draw will be much high( almost double) that of a 12 volt motor. That said, it would not be hard to use 12volts for your motor and step down the 12 volts to 6 volts for the rest of your boat's systems. Just an option to consider.
One excellent source for motors is the Component Shop https://www.componentshop.co.uk/
I have had great success with their motors and their service is first rate.
I hope others will pipe up with suggestions for you as well.