Prop thoughts.

[kvand347]

Comparing to the old prop is tough, for several reasons: the old one is probably hand made, and therefore less precise. It could have been rebuilt or otherwise modified in its life, and its overall blade area (% of the overall area within a circle that is taken up with blades) is probably much less than a modern CNC Acme prop. My boat had an OJ 14x16 4 blade, hand made prop that had been rebuilt several times. The measured diameter was actually under 13" when I took it off. I put on an Acme 1868, smaller in every dimension except blade area - 12.5 x 14.25 4 blade .075 cup - and it turned almost exactly the same WOT RPM, although much more smoothly, and the overall feel and throttle response was much better. With a 310hp engine and a 1.23:1 transmission, a 422 seemed like the right prop on paper, but it was way under WOT by 400-600 RPM.

The only way to be 100% sure about getting the right one is to try it with your individual boat, using it the way you expect to normally use it. A lot of places will let you try a prop and exchange it, provided it comes back undamaged.

That's interesting! What is your WOT RPM's? What should it be for my boat? How do I know if I'm properly propped?

[Jetlink]

That's interesting! What is your WOT RPM's? What should it be for my boat? How do I know if I'm properly propped?

Not sure if it is stamped on your engine but I have a stamp of some sort on the left valve cover that has a bunch of pertinent engine information among other things is optimal and max RPM range as well as other information like spark plug gap and timing.

[kvand347]

Not sure if it is stamped on your engine but I have a stamp of some sort on the left valve cover that has a bunch of pertinent engine information among other things is optimal and max RPM range as well as other information like spark plug gap and timing.

Good call! I don't have anything stamped on my motor (maybe because it was rebuilt) but I do have the PCM manual and I'll have to check that tonight.

[jsta281]

Comparing to the old prop is tough, for several reasons: the old one is probably hand made, and therefore less precise. It could have been rebuilt or otherwise modified in its life, and its overall blade area (% of the overall area within a circle that is taken up with blades) is probably much less than a modern CNC Acme prop. My boat had an OJ 14x16 4 blade, hand made prop that had been rebuilt several times. The measured diameter was actually under 13" when I took it off. I put on an Acme 1868, smaller in every dimension except blade area - 12.5 x 14.25 4 blade .075 cup - and it turned almost exactly the same WOT RPM, although much more smoothly, and the overall feel and throttle response was much better. With a 310hp engine and a 1.23:1 transmission, a 422 seemed like the right prop on paper, but it was way under WOT by 400-600 RPM.

The only way to be 100% sure about getting the right one is to try it with your individual boat, using it the way you expect to normally use it. A lot of places will let you try a prop and exchange it, provided it comes back undamaged.

I concur with this exactly. And ACME equivalent to an old hand made is usually at least 1" drop in pitch.
An Acme 1868 may also be the right prop depending on how strong the OP's motor is running. If it is down on power at all 15" pitch could still be to much.

[SquamInboards]

On the old carbureted PCM / Ford 351 @ 240 hp, max WOT RPM was 4400. I have the GT-40 351 that ran from 1995 - 2001 or so, with a max RPM of 4800. I can only get up to 4800 with 1/4 tank of gas and only me in the boat. But max SUSTAINED recommended RPM is only like 3800, which I can easily turn with the 1868, even with lots of people / ballast.

Edit: looking at your signature, Kevin, I see you have the H.O. @285hp. Not sure the WOT RPM on that one, I can look it up though. Someone will probably beat me to it.

And to the power loss issue, we've had to put the 1868 on a boat that previously ran a 422, maybe it had some water weight, engine was tired, or a combination thereof. But it needed the drop to a 14.25" pitch to get up to 4800 RPM.

[inair]

Comparing to the old prop is tough, for several reasons: the old one is probably hand made, and therefore less precise. It could have been rebuilt or otherwise modified in its life, and its overall blade area (% of the overall area within a circle that is taken up with blades) is probably much less than a modern CNC Acme prop. My boat had an OJ 14x16 4 blade, hand made prop that had been rebuilt several times. The measured diameter was actually under 13" when I took it off. I put on an Acme 1868, smaller in every dimension except blade area - 12.5 x 14.25 4 blade .075 cup - and it turned almost exactly the same WOT RPM, although much more smoothly, and the overall feel and throttle response was much better. With a 310hp engine and a 1.23:1 transmission, a 422 seemed like the right prop on paper, but it was way under WOT by 400-600 RPM.

The only way to be 100% sure about getting the right one is to try it with your individual boat, using it the way you expect to normally use it. A lot of places will let you try a prop and exchange it, provided it comes back undamaged.

After all my talking to the folks at ACME, Skidim, and Nettles plus doing a bunch of reading, I think this is all true. Even after I did my own research and called ACME again to talk prop theory, he stuck to his original recommendation (422/66. There are so many variables that making a final definitive choice that fits everything perfectly is more art than science. ACME keeps a data base of what props are sold for what engine /boat combos. So after they do their own engineering, when someone tells them that a certain boat/motor/prop works well for some application, that is added to the data and can change what they recommend to someone who calls. So since they are so generous with their return policy, I will take the opportunity to run the boat with what they sent me for a bit and then make adjustments if necessary.

Geeez! Winter seems to be lasting a long time this year

[jsta281]

Please return when spring finally gets here and give us an update. As a matter of a purely mental exercise I would like to know how it all turns out.

One other thing I think to keep in mind is when selecting a prop it's usually a matter of good better and best. When you put the 668 on compared to your old handmade prop I have little doubt that it will feel like a major improvement. If it feels great and you're happy with the performance then that's all that matters. However, the "best prop" will see your peak horsepower range on the RPM curve. If you don't see that RPM range then you're leaving performance on the table.

Definitely log your rpm's at various speeds. Log RPM and speed at surf speed and wakeboard speed skiing speed and cruising speed and Top Speed.

Also remember

.5 in pitch will change RPM about 200. Reducing pitch will increase RPM.

I look forward to seeing how it all shakes out.

[kvand347]

On the old carbureted PCM / Ford 351 @ 240 hp, max WOT RPM was 4400. I have the GT-40 351 that ran from 1995 - 2001 or so, with a max RPM of 4800. I can only get up to 4800 with 1/4 tank of gas and only me in the boat. But max SUSTAINED recommended RPM is only like 3800, which I can easily turn with the 1868, even with lots of people / ballast.

Edit: looking at your signature, Kevin, I see you have the H.O. @285hp. Not sure the WOT RPM on that one, I can look it up though. Someone will probably beat me to it.

And to the power loss issue, we've had to put the 1868 on a boat that previously ran a 422, maybe it had some water weight, engine was tired, or a combination thereof. But it needed the drop to a 14.25" pitch to get up to 4800 RPM.

Per PCM Manual for my boat: Max (Intermittant) RPM = 4400. Cruising is 3600. I may have to take a look in the spring to see if I am getting to 4600 or not.

[skiman_61]

Did you happen to speak with anyone from OJ? Just curious. I'm a skier as well. Some places will indeed let you try a prop and send it back. Let us know how it turns out?

[inair]

Skiman 61, I didn't talk to any one at OJ. But as to ski props.....well there is more to ski props than jsut the regular prop theory. From looking at other boards more oriented toward the ski crowd, some props put up more or less rooster and at varying distances from the back of the boat and to make it even more interesting speed plays a role as well. Some even said that other wake characteristics changed with certain props and everything can change from boat to boat. Seems like a lot to keep track of but trial and error mixed with some good advise and knowing what you are looking for in terms of WOP RPM and wake shape at speed you ski at will bring the best results.

I'll let everyone know how this all takes shape as we get into some warmer weather.