How much percent?

[fisherman]

How much percent of forward thrust is lost due to shaft angle on a shaft-driven boat compared to like a boat with a sterndrive which is set at (0)zero degrees trim which has 100% forward thrust?

[SparkyDog]

% thrust lost = 100 x (1 - cosine (shaft angle*))

* angle in degrees

[nevd]

If the boat is planing or semi planing the boat will travel at a positive angle to water (trim angle). This will mean that the angle in the equation above is shaft angle to keel + keel angle to water at operating speeds - ie shaft angle to horizontal.

nevd

[dereksireci]

Here's another point to keep in mind: There is a difference in efficiency between the two setups. Although the stern drive thrust may be in line with the direction of travel of the vessel, the outdrive may cause a greater loss of horsepower between the engine and the prop.

Another factor to consider in the inboard powered vessel besides just the shaft angle is the center of gravity of the boat in relation to the line of the propeller shaft. This will affect running trim angle and speed.

I think the answer to your question is that there is no simple answer.