Explaining maritime propeller design in comments doesn't work well, so I give it a shot here.
Given: 70 hp at 5800 RPM engine output, geared down at the prop shaft to 2300 RPM. Propeller with 3 blades, 14" in diameter and 17" in pitch.
First let's write that in SI units: 52.2 kW, 0.3556 m diameter, 0.4318 m pitch. That is an advance ratio of 1.2143. Speed is 38.333 rev/sec. If prop efficiency is 100%, thrust is simply power divided by speed. If the prop has no slip (unrealistic, but provides us with a starting point), the boat speed would be 16.5 m/s. The advance ratio above neglects slip, so the real value is maybe 1 (more for a lightly loaded prop and vice versa). 20% slip would reduce speed to 13.8 m/s. This corresponds to a thrust between 3782 N for 100% efficiency and 1891 N for 50% efficiency. The truth will be somewhere between these two extremes.
How is efficiency calculated?
Yes, prop flow is complex and this doesn't get any less complex when the prop is mounted at the stern of a boat. Flow is not uniform there and if the propeller is too close to the surface, it will operate in two media. Since one is a liquid, it itself can turn into a gas if pressure is sufficiently low (cavitation).
In order to straighten the flow and reduce the impact of the boat on the prop, all kinds of things have been tried (Schneekluth duct, pre- and post swirl fins, different values for rake and skew and even winglets in the shape of Kappel propellers). While aircraft propeller blades can reasonably be approximated by wings, ship propeller blades have tight restrictions on diameter which makes them more similar to twisted disks than to wings. Consequently, much of the flow is influenced by the tip effects and the propeller has a much higher solidity (called the developed area ratio in maritime propellers) so mutual interference between the single blades is also more pronounced. This all has an influence on efficiency.
That should be enough to explain why there is no simple formula. Naval engineers are used to diagrams and coefficients, and in this case use a Kramer diagram (see below, picture source) to calculate the ideal efficiency of a propeller. Ideal means that this diagram does not include viscous losses and assumes uniform flow.
To use this diagram, you need to know the propeller area A$_O$, the inflow velocity v$_A$ (which is normally lower than the ship's speed due to the vicinity of the hull), the thrust T, the rotation rate n in rev/sec and the propeller diameter D. With a known advance coefficient you start at the bottom and move up on the diagonal lines until you hit the horizontal line for the right number of blades. From that point you go straight up until you hit the known ideal efficiency (and read the corresponding thrust coefficient on the right side) or until you hit the known thrust coefficient and read the ideal efficiency from the curves.
In your case both efficiency and thrust coefficient are unknown and we don't even know the boat's speed. So to proceed means a lot of guessing until you go out and measure the speed your propeller-engine combination is capable of, and then return to the Kramer diagram.