Hears a Q for you Jay somthing ive wondered about. Tow props measure same pitch exactly ones a 4 blade ones a 3 blade. If the 4 blade runs the rpm as the 3 blade witch will be faster. I was always under the imprestion the 4 blade would be at the same rpm. Never really tryed that before. I know theres a rule that some use to apply to 4 blades that you loose some rpm with a 4 blade. So if you could use that to match the 3 blades rpm would that gain you speed in a 4 blade? Also another thing about Slip.:help Is the formula the same for a 3blade as a 4 or 5 blade prop in the calculation?
Pretty interesting thread if ya ask me. ive learned lots. I think the biggest thread is the Darris says no to 300 on a GS though?:big grin OR the one about a 24ft Grand Sport doeing close to 90 with a stock 225 promax and a 27 trophy.:big grin
Whipper, As far as I know there is only one calculation for prop slip. Yes when going from a 3 to a 4 blade rule of thumb is to drop 1 pitch. theres a lot to consider on this subject like diameter, most 4 blades drop in diameter vs. a 3 blade like the Trophy thats a 13 3/4 or the Turbo TXP thats 14 3/8. Pitch can be replaced by diameter and a prop that has alot of cup can replace pitch, adding cup to the trailing edge is the same as adding pitch, it will also lift your stern.
I've always been a firm believer that a 4 blade is the better prop for extreme surface running, the more blades the better it holds. To answer your question about will a 4 blade as fast as 3 blade, I say yes it's possiable, especially with Todds props, Why? because of the amount of blade area his prop has along with lab finishing.
Tow props are normally what we call "straight pitch" meaning from the leading edge to the trailing edge the pitch is the same. What we run are called "progressive pitch" like Todd explained earlyer. You are correct about 3 vs. 4 blade, on our style of boats (Allisons) a 3 blade will be the faster prop but for handling purposes most of us prefer the 4 blade. I know I do.
Now, on heavy boats or boats that are carrying a heavy load like a big Ranger a 4 blade will be the faster prop because of it's weight carrying ability, of course over time this has also changed, Yamaha has a 3 blade called the T-1, it's a 15 1/8 diameter, this is a fast prop that will carry the weight like a 4 blade, it's blades are huge.
One thing I have learned since being an Allison owner and being on this site is Allison owners are different meaning an average Joe running a Triton or Ranger just wants his prop to run. An Allison owner wants his boat to run with a full detail of how and why it's doing what it does....I guess I'm in the right place cause I'm the same way...