Three;
No, four-No five, how about Two? Maybe six? What are we talking about here? How about this. Propellors? Yeah those things that hang under a boat, stir the dickens out of the water and make the boat move forward.
Ha.Ha, We're not talking about the number of props hanging under the vessel though. Nope! We are talking about the number of petals( Blades) it has. Look at submarines of today. Five, Six petals, Whatever works for them I guess. Why am I going on about propellors anyway?
Nope, we are talking about these items for one reason. On Paper(Card) models you have to make them out of some very small sometimes many pieces of the cardstock. If you haven't ever built your own prop in any material, you are in for an interesting experience.
Why? Well, First, the size of the prop in relation to the vessel. Two, How many petals? Three, Left or Right hand? Four , what color? Now here's another thought Do you want the props turning in opposing directions. On a two or more prop vessel this is very important.
Do you know how, on most multi prop vessels why they work. Simple. They either turn Inwards at the Top or the Bottom. Yup, counter-rotating. Why? Well if you ever get the chance get on a boat, start the outboard or inboard engine.
Now, Put it in Gear. Let the boat ride easy with minimum throttle. The stern will eventually make a complete turn in the direction the prop is turning. This is called Prop Walk. To combat this on bigger vessels they added another, each one turning in the opposite direction either inboard at the bottom or top.
This affects the way you build them in Card and the scale too! First you have to make the center or core of the prop. This can be a plain little ring that is cut to simulate a prop spinner or not. Then the petals. Now this is the important part. First off how many petals and Which direction does the prop turn?
The reason this is important is simple. You cannot just leave the petals flat at an angle on the model.Why? Think how stupid and childish that would look. You have to shape the puppies to cut the water in the right direction.They are this way to reduce or eliminate cavitation.
What is that? Look at a prop going through the water. Now think of all those bubbles. So a trimmed shaped blade designed to eliminate those bubbles during Operations is needed. Those bubbles are cavitation. A sure sign of a poorly designed prop. Period.
Many iterations of props exist, Some for speed, some for Strength in long term use and some for pure power transfer in the liquid medium. This so they can pull or push efficiently. All this as efficiently as possible. A petal is curved with the leading edge situated to the front edge of the blade based on which direction the blade turns when going forward.
This edge on High speed boats(Ski or Wakeboard) and High performance boats, is usually straight on that edge. It allows the prop more efficiency at speed. Remember if you could see the pattern of the water you would know why they are called screws.
In paper you are talking usually a prop center less than an eighth of an inch thick. Then you have to cut out ana fasten the petals to the center and this is where you can make or break a model. They have to have the right number of beautifully shaped petals,nThey have to indicate turning direction and they have to be the right color because of the material if they are unpainted.
Then the shaft housing has to be modeled in a streamline type if you can. Then there's the outboard strut which is near the end of the shaft where it sits in for stability and balance when operations commence. Build them right and your vessel will pop, believe me>