Outboard propellers are those props that are situated or positioned outside of a vessel's hub.
The vessels that have propellers usually are yachts, power-boats, and boats that have motors installed on it.
Some boats which are not driven by motor like pedal boats and paddle boats also use propellers.
Propellers are very important parts of your boat, as important as its engine - if your prop is mismatched, or damaged, the boat performance will be reduced greatly.
Even if you have a tiptop engine, but the prop is not appropriate for it, you'll get nothing out of its performance.
How does a propeller work? A prop pushes your boat by its spinning blades that "grab" water and "pulls" itself through the water.
As the outboard propeller starts to rotate, water is pushed behind it, and the boat and yourself are carried forward.
If you notice how a screw pulls itself into a wood piece, this is also the same way a prop works in the water.
Props are sometimes referred to as "screw" because of this similarity.
The performance of outboard propellers are dependent upon several factors.
One is its size.
When we speak of size in props, it refers to 2 things - diameter and pitch.
All props sizes are expressed by 2 figures, e.
g.
, 13" x 19".
The 13" dimension refers to the diameter, and the 19" dimension is the pitch.
Diameter, simply, is the distance across the prop blade tips.
Pitch refers to the distance traveled in a single prop revolution, in the above example, 19".
If you consider that water is not like wood which is solid, this figure should be less, because of slip.
The leading edge of blades cuts the water, and its trailing edge is from where the water flows away.
This is water slip.
The number of blades on a prop also affects performance.
The ideal number favored by water skiers and cruisers is from 3 to 4..
This will reduce vibration and give you smoother ride.
If you use 5 or more blades, the top end speed will be minimized, because it increases the drag caused by the prop.
There is also the blade rake, when the blades of outboard propellers slope or lean, either forward or toward the stern of your boat (aft).
Blades that slope aft gives increased speed.
The blade area also affects performance.
If your prop has a high blade area, it has less blade loading and will suffer from cavitation less.
You can say that cavitation is present when the blades surface are covered with vacuum bubbles.
This results from incorrect prop type/size, damaged and heavily loaded blades.
Cleaver type or pointed blades provide stern lift while rounded, high rake blades provide bow lift generally.
The material from which outboard propellers are made also contributes to the prop's performance.
Typically, props are cast in aluminum, stainless steel and plastic composite.
Plastic is very rarely used, and are fitted only on emergency cases.
This material is low in price and the impact resistance is quite reasonable.
When damaged, plastic cannot be repaired but most prop stores have replacement blades.
The aluminum is the most common because is costs less than stainless steel.
The most durable of all these materials is the stainless steel and can increase prop performance because it can be made into thinner blades that reduce cavitation.
Propellers should be kept in good working condition.
Even the least damage on it can affect performance.
Have your props serviced and repaired by professionals, and never attempt any DIY jobs on it.
It may make your prop beyond economical repair and, in the long run, may cost you more.