Do you have any data about the maximum boat speed that could be obtained with the previous 200-HP engine?
Generally boat speed at maximum power is related to the square-root of the the power-to-weight ratio. If the power decreases to 175-HP from 200-HP, then the boat speed would tend to decrease in proportion to (175/200)^0.5 or by a factor of 0.935, assuming the total weight of the boat did not change. For example, if the boat could reach 40-MPH with the 200-HP engine, the expected speed with 175-HP would be 40 x 0.935 = 37.4-MPH.
With this as the target speed, the propeller pitch would be selected so that at the maximum recommended crankshaft speed of the 175-HP engine, the boat speed would be 37.4-MPH. The engine gear ratio must also be known and included as a factor. The propeller SLIP can be estimated at 10-percent. A propeller calculator can then predict a PITCH value.
Yamaha-brand propellers are generally good propellers and will be a good fit for a Yamaha engine. Propellers from an engine OEM usually are more expensive than aftermarket brands.
If you provide the following information, a propeller pitch can be estimated:
- prior top boat speed at full-throttle with 200-HP engine
- weights of new and old engines
- maximum engine speed recommended for new engine
- gear ratio of new engine
I anticipate that a 17-pitch propeller may be a workable starting point.
Often with the sale of a new, loose engine for re-powering a boat, the selling dealer will provide the engine buyer with advice on propeller selection, and often provide test propellers to verify the choice. Dealers with experience as seller of a particular engine brand and that brand's particular engine models usually have experience in how those engines will perform on boats of a particular hull design and weight.