| Basically agreeFebruary 26 2008 at 5:44 AM | Bill (Login billinstuart) |
Response to Resulting prop calculation for a 2.0 gear reduction set up |
| I basically agree with your calculations and assumptions,with one exception. A faster spinning prop does not necessarily have MORE slip..in fact, the reverse may occur. Here's why.
Let's carry this to the extreme..a prop slips 100% at very low rpms. However, the faster you spin it, the more efficient it becomes. As and example, I went from a 14x12 to a 14x10 with cup on my Blackfin with 1:1 final drive. My cruise/top speed increased, and my slip figures now run 10-15% consistently. The surface finish is VERY important to good prop performance, btw. Another approach is to figure prop tip velocity..anything over 175 mph. is borderline for cavitation inception. wow, 175 mph. I hear you say? Yep. That's why surface condition is important. That's also why many 4 blade props have a lower top speed even if the pitch is the same..blade FRICTION.
The stock marine 454 Chebby big block is not real happy at extended rpms over 3600 rpms. Weak stock valve train, cast pistons, etc. They usually produce peak torque at maybe 3200 rpms.
My feeling is..if you can get 4200-4400 rpms flat out, and cruise at 3000 rpms with the secondaries CLOSED you'll be pretty close to optimum.
so Paul, to help keep you sharp. please calculate pi to 250 places for me, will ya?
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