Ten minutes per hour! That is slow. American-made spring driven clocks are poor timekeepers and if yours loses or gains a minute or two a week that is just about the best you can expect.
After having worked on the case a few months ago it is past time to work on the movement.
The clock was in running condition when I acquired it though it did manage to run its rated 30-hour cycle.

Something is definitely amiss. Wear, a good cleaning or something more serious. Read on and we’ll find out what is slowing this clock down.
A Ship’s Bell clock is a type of marine clock but in clock circles, there is a distinct difference between it and the clock you see here.
Marine clocks either tell the time or strike the hours like a normal domestic clock and because they have a balance wheel escapement they can be placed on a moving object such as a train or a boat whereas Ship’s Bell clocks originated in sailing ship days when the crew of a vessel was divided into Port and Starboard Watches, each on duty for four hours, then off for four hours.

One stroke of the ship’s bell indicates the first half hour of the watch. Then an additional bell is struck for each succeeding half hour for a total of 8 bells.

Based on a quick inspection I see nothing amiss other than a crimped hairspring on the balance escapement. I wonder if that might be the issue?

Now, let’s take this apart and see what we have. Look for a future article as I detail the servicing of the movement.