Hi Adam.
Without a Tach you need to rely on sound and feel. The speed of your boat under auxiliary power is dependent on the condition of your hull/keel/rudder, the prop size/pitch, engine size/performance and how much fuel you feed the engine.
It is normal for newish owners to go lightly on the throttle of their new to them boat. The thinking is that if I treat my diesel with light use it will give me a long and enjoyable experience.
This is not the case with a diesel. They need to run at 80-85% of WOT (wide open throttle). Running them an 45-50% will foul the cylinders and shorten the engine life.
Tach’s are not hard to install. In absence try to go out on s calm day and experiment pushing the engine/boat at various speeds. Monitor a GPS speed and the sounds/vibration of the engine. While not perfect you can find a sweet spot in the upper speed range. You are striving to reach about 6 knots in calm no current conditions
if not possible it may be time to examine your prop. There is a great maker of propels near Comox, BC. West by North. I found them very helpful.