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Old 08-18-2016, 08:19 AM   #12
City: MA
Country: United States
Join Date: Aug 2016
Posts: 12
It does seem odd. From what I've learned however, determining the correct prop size and pitch is not an exact science. The boat specs are fed into a computer program and the suggested prop specs are spit out. If the wheel does not perform as expected, some experimentation follows. On a boat like ours, a displacement or semi-displacement hull, the shops seem to feel as close to a "square prop" is a good starting point. In other words 26"x 26" or 24"x 24", etc. From there the tweeking begins.

In our case, using the known specs of the boat, a 26"x 24" was determined to be the correct size. Being a semi-custom design however, there are some unknowns. I spoke with the boat builder and he said he recalled (it's been 15 years, so he was going off memory) there was a cavitation issue on some boats and at some point in time, they lowered the prop shaft to give more clearance around the prop. Was this considered in the calculations done by the computer program? Possibly not. One shop said the 26"x 24" was correct but then he said the computer also determined a 26" x 19" was correct. He felt that this couldn't be correct, even though it was specified by the computer.

From what I've read, reducing the prop diameter 1" will increase RPM by 500. Every inch reduction in pitch will increase RPM by 200. This would seem to be an easy way to exactly modify a wheel and eliminate experimentation. However, the shops told me that this is a very, very general rule of thumb and other variables, like prop configuration, boat speed, prop materials, etc. will cause unexpected results.

Why does our boat potentially have the wrong size prop, I don't know. The boat must have performed as designed at the initial sea trials, otherwise the engine manufacturer would not have warranted the engine. Was there a change in diameter and pitch at the request of the previous owner for a reason? Again, I don't know since the boat's history is not available.

When passing along what I've learned about a wheel size that works on another boat with the same specs as ours, the prop shop recommended dropping the diameter by an inch and the pitch by two inches, then see what the results are. If a further reduction is required, then that will be the next step. It's easier to remove material than add it back on. This is the direction we will least for now...
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