How are props sized?
- The prop size has two numbers; diameter and pitch. Pitch always follows the diameter when describing a propeller.
What is Diameter?
- The diameter is the distance across the circle that the prop makes measured in inches.
What is Pitch?
- Pitch is the distance the propeller would move forward in one revolution with no slippage, measured in inches.
What is Cupping?
- Cupping is a small lip at the trailing edge of a prop that can help get better bite in the water, sometimes better holeshot, and top end speed.
What is Ventilation?
- Ventilation is the prop grabbing air from the surface or from exhaust gasses. If a motor is trimmed too high, your prop may start breaking the surface and start grabbing air, raising your rpm rapidly and possibly losing speed. If your motor is mounted too high on the transom the same effect can take place. On many props there are ventilation holes that you can plug or unplug allowing more or less exhaust gas around the prop during holeshot. If your engine can’t get high enough rpm during holeshot you can unplug holes and allow the prop to ventilate slightly to get your engine to a more powerful rpm. If you unplug to many holes, you will loose traction and holeshot will suffer.
What is Cavitation?
- Cavitation is boiling water with your propeller. It can be caused by a number of reasons; incorrect pitch, style or damage. It can result in a “burn” on the backside of the blade. Just a burr on the leading edge of the blade, can reduce pressure enough to boil the water going across the backside of the blade resulting in a burn mark.
What is WOT?
-Wide Open Throttle.
How high or low should I mount my engine?
- As a rule of thumb, the cavitation plate of the engine should be level with the bottom of the transom. In case of euro transoms or brackets, the cavitation plate should be one inch higher for every twelve inches back from the transom. The lower your engine the more drag, However if your engine is too high, the propeller will ventilate, and water pressure to the engine may be an issue. Engine height is not an exact number, you need to experiment to get it right.
What are the advantage/disadvantages in a three, four or five blade configuration?
-Think of a propeller as a tire on a car, a wide tire has more surface area, therefore more traction on the start, but more drag at top end. A narrow tire has less traction on the start, and less drag at top end.
A three blade prop has less surface area in the water therefore, less drag then a four or five blade prop. As a general rule it has higher top end speeds. A three blade prop “normally” gets better fuel efficiency at higher speeds then a four or five blade prop. However, holeshot on a three blade prop may not be as good as a four or five blade prop because a three blade prop has less traction. Three blade props normally have more “bow lift” than four or five blades.
A four blade prop has more surface area, normally giving it better holeshot over a three blade. It will normally ventilate less then a three blade because of the added traction. This can allow for better handling in rough seas. A four blade prop lifts the stern of a boat more than a three blade prop, which can achieve lower plain speeds. Four blade props can have better efficiency at midrange speeds than a three blade, but not be as efficient at high speeds.
A five blade prop has more traction than three or a four blade prop. This gives a smoother and faster holeshot. It also provides sternlift and lower plaining speeds. This can be helpful for skiing, or anyone that wants a quick plaining speed. Top end will normally suffer with a five blade prop, because of the large surface area in the water. Five blades can also allow for higher motor mounting for high horsepower boats resulting in less motor drag, but adequate traction.