Another Prop/Rpm/Speed thread

[roffey]

I have a question here. A 19 inch prop is faster than a 17 inch prop, correct? The reason why the 17 would be faster is the RPM would be higher and higher RPM would be more RPM on the prop? I only ask as my neighbor on the lake as a 4 blade 19 inch, my prop is a 23 inch. He wants me to get a 19 inch so my boat will be faster but I say the 23 inch at 4900 RPM with my 4.3 I/O is the correct setup. A 19 inch would be to high a RPM.

Please correct me if I'm wrong, hate that when I'm wrong, LOL.

[THEFERMANATOR]

Originally Posted by roffey

I have a question here. A 19 inch prop is faster than a 17 inch prop, correct? The reason why the 17 would be faster is the RPM would be higher and higher RPM would be more RPM on the prop? I only ask as my neighbor on the lake as a 4 blade 19 inch, my prop is a 23 inch. He wants me to get a 19 inch so my boat will be faster but I say the 23 inch at 4900 RPM with my 4.3 I/O is the correct setup. A 19 inch would be to high a RPM.

Please correct me if I'm wrong, hate that when I'm wrong, LOL.

The correct prop to keep the engine in its powerband that works with your hull and needs will be the fastest. Theres alot more to propping than just pitch and rpm's. You need to prop your boat to run at the correct wot rpm's, but you also need to pick the correct number of blades, rake, cup, blade design, and such for your boat to run at the correct attitude under power. A higher pitch can slow you down if your engine can reach enough rpms to reach peak power. The searay I did had a 5.7l tbi engine, and it picked up 9 mph and 600 rpms at wot by having the prop repitched from a 17 to a 16. Its not just about pitch, its about setting the boat up.

As to yours, 4900 may mean you could use another inch of pitch, or might be dead on. Without knowing the exact engine, I can't say where it makes peak power at. And without lnowing how the boat rides out and planes, theres no way of knowing if you have the correct style prop.

[roffey]

I thought the pitch on the prop was how far the boat will move with one revolution or rotation of the prop. So a 19 inch prop will move 19 inches and a 23 inch prop will move 23 inches. The limiting factor will be prop slip and over or under revving the motor. But as you say there is a bunch more involved and I know I am over simplifying it. I will not hijack this thread any longer, LOL. Thanks for the reply.

[THEFERMANATOR]

Originally Posted by roffey

I thought the pitch on the prop was how far the boat will move with one revolution or rotation of the prop. So a 19 inch prop will move 19 inches and a 23 inch prop will move 23 inches. The limiting factor will be prop slip and over or under revving the motor. But as you say there is a bunch more involved and I know I am over simplifying it. I will not hijack this thread any longer, LOL. Thanks for the reply.

In simple terms yes, pitch is the theoretical distance a prop will move itself with one revolution. But as you said that whole prop slip factor is a HUGE factor. Then throw in the problem of some hulls tend to popoise and need a prop to hold the bow down, others ride bow high and need to lift the stern, some run stern high and dig the bow in needing more bow lift. So when you throw hull drag into the equation, on top of getting the engine into it's power band, prop slip, and so on, propping can get confusing. Some people put on trim tabs to make a boat work and run right, but this adds drag from the tabs being used to plant the bow. It's all about setup. I've gone from a 15 pitch in one style prop on a boat, to a 19 pitch with another styke prop on the same boat, with both turning the same WOT RPM.

[roffey]

Thanks for the info. I was thinking of re propping my boat but think I will keep what I have. My boat on a nice day with a little ripple on the water will run with about the last foot of the 20 foot hull in the water and is smooth to the point it is smother at WOT than another speed. My RPM is 4900 and 54 MPH on the GPS. Motor is a 4.3 TKS (2 BBL). Once again sorry for the hijack and thanks for the input Therm.

[THEFERMANATOR]

Originally Posted by roffey

Thanks for the info. I was thinking of re propping my boat but think I will keep what I have. My boat on a nice day with a little ripple on the water will run with about the last foot of the 20 foot hull in the water and is smooth to the point it is smother at WOT than another speed. My RPM is 4900 and 54 MPH on the GPS. Motor is a 4.3 TKS (2 BBL). Once again sorry for the hijack and thanks for the input Therm.

You might could pick up some speed by propping to drop your WOT RPM's a touch as I think that engine makes peak power around 4600-4800 RPM's, but your so close you chance going to far and ruining your performance.