4 blade, 5 blade or 3 blade?

My boat:
1999 Glastron GS180SF with 125 Mercury outboard (2 stroke)

The questions:
what difference does the number of blades make?
what does the number of blades do?
can my motor actually handle a four or five blade prop?
do you select a different pitch or diameter if going from a three to a four or five?
how would I go about selecting a four or five blade prop for my boat.

*I slalom ski a lot on the course. my motor throws a good sized white wash from the motor (is this prop related?)

Re: 4 blade, 5 blade or 3 blade?

1) Many folks get all wrapped up in "diameter" when in fact it has little to do with performance. Diameter is a function of prop design and in fact, only given diameters will actually fit your engine. So pick a prop specifically designed for your engine and then the design will take care of diameter. As prop pitch goes up, diameter comes down a bit and vice versa.
2) More blades, more push but requires more power if pitch remains the same when comparing 3, 4, and 5 blades. Generally, you would select a four or five blade prop for a heavy boat that needs lots of push to get out of the hole. Generally, speed will be better with fewer blades.
3) Prop selection is a crap shoot. If you want to change props, you need to first determine what the "primary" use is. In your case skiing seems to be that use. A four blade would likely be a better choice.
4) You then need to make some wide open throttle runs with what you consider and average load to get baseline RPM and speed numbers. The best prop is one that allows the engine to run at or very near the top of the manufacturers WOT rpm band. That can be accomplished with a 2, 3, 4, or 5 blade prop. Jumping between prop manufacturers can cause very different performance even if the pitch is the same. There's that word "design" again and hence the reason we say it is all a "crap shoot". Research, research and more research.

Re: 4 blade, 5 blade or 3 blade?

Wot your engine with max 2 mates and see max rpm achieved, compare with factory wot engine model parameters, if slight less from max rpm play with pitch same diameter accordingly, to get best top speed for skiing purposes. Sometimes it's better to have 2 props, factory standard and an extra for top speed, skiing, whatever your needs.

Happy Boating

Re: 4 blade, 5 blade or 3 blade?

As Silvertip mentioned, except for hole shots, 3 blades would give you better performance. Buttttttt you can improve your 3 blade hole shot with vent holes. The holes are near the front of the prop, bored in the outer diameter of the hub, just above the leading edge of the blade, one per blade.

What they do is put exhaust gasses over the blades of the prop along with normally flowing water during the hole shot. The gas compresses (unlike water that doesn't) and reduces the load on the engine. This load reduction allows the engine to get it's rpm's up fast which gets the HP up fast and shortens the duration of the hole shot/time to plane out; regardless of heavy boat, or trying to pop a couple of slaloms out of a deep water start. It's like reducing the pitch of your prop by 2-3" roughly during the shot.

Once you get up and going, the pressure of the water flowing by the prop seals off the holes forcing gasses out the rear of the hub in the normal fashion. The prop bites clean water, the engine down shifts in rpm with the load (like going into overdrive in a car) and away you go with your efficient 3 blade design.

Having had one, it is THE only way to go.

My 2c

Mark