According to the Boston Whaler test data from
http://www.bostonwhaler.com/boat_graphi ... 4114PM.pdfthe top speed of the 170 MONTAUK with a 90-HP engine is 40.9-MPH. If the engine power is increased to 115-HP from 90-HP, then we can anticipate the boat speed will improve at the square root of the power-to-weight ratio. For simplicity, I assume the 115-HP engine adds no extra weight. Then we figure the new top speed to be
40.9-MPH x (115/90)^0.5 = 46-MPH
Now we need to know the maximum RPM of the new engine. Let me go find that data. Okay, found it at
https://www.mercurymarine.com/en/us/eng ... ourstroke/The maximum engine speed is 6,300-RPM.
Now we need a propeller calculator. The very wonderful propeller calculator from CONTINUOUSWAVE is still not fixed, so I will use Mercury's almost identical version which they implemented recently. It is at
http://www.mercuryracing.com/prop-slip-calculator/(Note that there is a link to it in the announcement article at the top of the PERFORMANCE forum about propeller calculators.)
To use the calculator we need to enter values for the following:
--RPM
--RATIO
--BOAT SPEED MPH
--PROPELLER SLIP
and then the calculator will crunch some simple algebra and compute PITCH, The calculator is not an oracle. It just takes the data you give it and manipulates it according to very rigid and simple algebra. There is no magic in the calculator.
Here is our data for this application:
--RPM = 6300
--RATIO = 2.38
--BOAT SPEED MPH = 46
--PROPELLER SLIP = 10
SLIP as a value of 10 is just an estimate of what we will expect from the typical propeller when running at full throttle. SLIP may be lower in some cases. SLIP also depends on how the PITCH has been stated.
Now we push the CALCULATE button to get PITCH:
The calculator says PITCH = 20.4-inch
On that basis, I would begin your propeller testing with "the 19-pitch propeller" you mentioned in your post earlier. I think you said it was a LASER II 19-pitch. That should be a good propeller. It is stainless steel, and designed for good boat speed performance on light, fast boats.
Report the results from testing with the LASER II 19 propeller. Then that actual data can be used to make more informed suggestions.
There are four assumptions in this calculation. First, we calculated the new boat speed target based on the usual performance of moderate v-hull planing boats as described by naval architect George Crouch many years ago, i.e., that speed is proportional to the horsepower-to-weight ratio by the 0.5-exponent. This is just an approximation, and for a particular hull there may be some deviation from that. Next we assumed that the new engine develops its maximum horsepower at its maximum engine speed. This is typically true for many four-stroke-power-cycle outboard engines, but it may not be for the specific engine under discussion here. The engine might actually deliver its peak horsepower at a lower engine speed. Third, we just plugged in a SLIP value of 10-percent. This is a reasonable estimate. Generally, one uses the propeller calculator to calculate the SLIP factor, as implied by it being the right-most field in the calculator. Finally, I did not include any allowance for a change in boat weight. Even though the new engine may weigh a bit less than the old engine, we don't have the total boat weight of your boat. We could use the Boston Whaler boat test weight of 2,089-lbs, but a change of only 40-lbs is quite minor. For this estimate I have ignored it. These four assumptions are generally reasonable assumptions, and when the calculator cranks out 20.4-inch PITCH, that is probably a good place to start.
The LASER II 19 is close enough to the calculated PITCH to be a good choice for an initial trial. Also, propellers with a lot of blade cup tend to behave as if they had more pitch. Whether or not Mercury has included any effect of blade cupping on their calculation of the LASER II pitch rating is hard to know. Generally a propeller with a lot of blade cup will need to have one-inch of pitch added to make the calculator more accurate. That is, in effect the LASER II 19 may actually act more like a 20-pitch propeller. Give it a test run and let's see.
Also, note how the new engine has a big gear reduction. This is probably intentional. It allows the engine to turn the usual pitch propellers and also lets the engine speed accelerate well above 6,000-RPM, which it probably needs to be able to reach to make its rated power. The propellers are designed for a certain propeller shaft speed. They know nothing about engine speed. With these high-revving newer four-stroke-cycle outboard engines, the outboard uses a higher (numerical) gear reduction in order that the propeller shaft speeds will be the same as they were when the propellers were designed back in the days of two-stroke-power-cycle outboard engines that typically did not run above 5,500-RPM.