Prop Selection

[jeremyjohnolson]

I have a 2004 Tige 20v Switch and want to get a better prop for wake surfing. Right now it has the ACME 515 3 Blad 13" x 12" Propeller and it's not enough to get up to speed when I have the boat weighted down with a couple thousand pounds of ballast. I was going to go with either the 525 or 911 3 blade props since that is what wakemakers suggests for my boat, but then a guy at a local shop here in Phoenix, AZ told me I should go with a 4 blade prop. I am new to boating, so I need some advice. The guy at the shop told me to take my boat out on the lake and see what the RPM's are on wide open throttle at full speed with the current prop, then that will give him the info he needs to properly size me with a 4 blade prop. Do you think the 20v Switch should have a 4 blade prop? If so, then why doesn't wakemakers suggest any 4 bladers? Any suggestions on a good 4 blade?

[chpthril]

I wouldnt trust either source at this point, heres why;

With that gen v-drive, tige used 2 different gear ratios. This has a direct impact on which type of prop you should go with, a 3 or a 4 blade. Both of those recommendations makes the assumption of which gear ratio you have.

Step 1, determine the v-drive gear ratio. Its either the 1;1 or the 1.46:1. There should be a tag riveted to the v-drive with the gear ratio stamped on it.

[h2o-ski]

I could be wrong, but with those prop options the v-drive would be a 1:1. Those were the exact same props I ran on my 22i. Otherwise you would have no problems getting out of the whole, but absolutely no top end.

[jeremyjohnolson]

It's a Walter Gear Drive 1:1 ratio per the stamp on the side of the box. The guy at the prop shop told me the elevation matters in prop selection as well. Is that accurate?

[chpthril]

Yes, elevation is part of the equation. As elevation increases, horse power drops. So we can compensate for that by adjusting the pitch when making the prop selection. What is your lakes elevation?

With the 1:1, you would want a 3-blade. A 4-blade would run like a 3 legged dog.

[jeremyjohnolson]

Lake elevation is 1700' at Lake Pleasant, AZ. Saguaro Lake is 1506'. Lake Tahoe is 6224'. About 90% of the time, I will be at Lake Pleasant, maybe 5% at Saguaro and 5% at Tahoe.

[jeremyjohnolson]

After talking with Wakemakers.com, I went with the ACME 911 3 Blade 13" x 10.5" Propeller.

[WakeWes]

congrats on the prop, let us know how it rides!

[jeremyjohnolson]

I know it's been a while since I posted here, but as a follow up, the 911 works great. I haven't gone back and tried, but I bet the other prop (the 515) would have been just fine as well since I finally figured out what my actual problem was. It wasn't the prop, it was my pitch angle of the boat due to the weight distribution when I fully filled the ballast bags in the rear of the boat. I was too rear heavy and needed more weight up front to get the boat to be a little closer to a level plane, I mean, not level, but just not so much nose sticking straight up in the air like it was when I had all the weight in the back. Once I leveled the boat out just a little bit more by adding about 400lbs to the bow of the boat, then I was able to get up to speed easy even when all ballasts were full. It makes sense if you think about it because the angle of attack of the prop is already angled down even when the boat is just floating completely level to the water, then you weigh down the back (stern) of the boat and now the angle of the prop is pointing nearly straight down into the water with the force of it pushing straight up into the air in a vertical position. OK, maybe not completely vertical, but vertical enough that the force of the prop is pushing up to the sky too much like it's trying to lift the boat out of the water more than propel it forward on a horizontal plane. So the angle of the boat needs to level out to make it so the prop is pushing the boat forward on the horizontal plane a little bit more and then boom, everything works, I can get up to speed just fine. I know it wasn't just changing the prop that made the difference, because I had the same problem with the new prop until I finally figured out the physics of all this and how it's working based on the angle of the boat.

[chpthril]

So it was asked to explain more about why a 1:1 cant run a 4-blade surf/wake board prop, so here goes.

Lets talks about props in general, first. When you go down in pitch, RPMs go up. This is because the prop literally threads through the water. When you go up in diameter, RPMs go down, because it takes more power to spin the increased surface are through the water. The convers is true also, up in pitch, RPM drop, down in dia, RPM go up.

To improve hole shot, we need to go down in pitch. However, the down side is the increases engine RPM. So we can only go down so much in pitch before the RPMs are just too high. In order to gain the benefits of the lower pitch and keep the RMP increase to minimum, we also go up in diameter.

Why does this matter with the 1:1 v's the 1.46:1? Gear reduction is a torque multiplier. So an aggressive 4-gblade surf prop that runs great on a 1.46:1, is too much for the 1:1 to spin. So the engine bogs.

The newer, larger gen boats run 1.7X:1 and 2.0:1 ratio. These may not be the exact numbers, point being, they are using even great ratios. These boats are running 16 & 17 dia props.

IF you could even fit that size of wheel on a 1.46:1, you would likely get the same results as 1433 4-blade on a 1:1

[jeremyjohnolson]

Thank you for that in depth explanation! I really appreciate it.