There is some logic behind propellers despite the ‘Black Art’ image but they are complicated devices. Lorne Campbell helps to demystify this issue.
Since they are the things which move your RIB they are worth some consideration. The following thoughts are not comprehensive but should give some insight into their workings.
Propellers are sized by diameter and pitch. Diameter is twice the maximum radius (centre to blade tip). Pitch is the distance a propeller would theoretically ‘screw’ itself forward in one revolution in a solid medium.
A 14″ x 28″ propeller would be 14 inches diameter and 28 inches pitch. The diameter is always quoted first. Diameter is mainly dictated by the torque supplied. Power is proportional to torque x rpm. For constant power, torque x rpm stays constant or an increase in rpm gives a proportional reduction in torque and vice versa.
Torque is a force times a leverage length and resistance on a large diameter propeller is further from the hub than with a smaller diameter. The larger diameter prop will run slower for the same engine power. This usually means a greater reduction ratio (mechanical leverage) to supply the extra torque. Since the prop has to be twisted, your RIB twists (rolls) in the opposite direction. Obviously counter-rotating propellers will mutually balance but a single prop needs an offset force to create the balance.
You can supply this by offsetting a weight to one side, use an offset wedge, one trim tab or you can just let the boat heel. Deadrise reduction on the low side will give the counter-torque needed. Sometimes the whole engine is offset but this moves the thrust line to the wrong side. A right-hand prop will paddle to Starboard and turn the boat to Port (but try to spin the wheel to Starboard); a Starboard prop offset means the thrust line also tries to turn the boat to Port.
Turning the steering right to keep straight will counter the roll somewhat since the turning effect is low and tries to roll the boat to Starboard. Torque and paddlewheel effect are worse with larger diameters. A large slow spinning propeller needs more torque than a small fast one, so the small prop will kick the boat around less, but the small fast prop is not as efficient. Thrust is the product of the mass of water the prop grabs and the extra speed added to it every second but the energy used to do this is proportional to mass times velocity added squared (i.e. mass x velocity x velocity); so even ‘though a small mass of water and being thrown fast will supply the same thrust as a large mass thrown more slowly, more energy is used with the small prop because there is a greater change of velocity squared. Which diameter and gear ratio combination to use is a matter of compromise.
A tiny prop at umpteen thousand rpm has minimum torque effect but great inefficiency while a 15-foot diameter one will rotate your RIB instead and be heavier than the boat. Blade shape affects torque required even with a similar diameter. If most blade area is out towards the tip – say a very spoon-shaped chopper prop – then the average load will be further from the hub and torque will be greater than with a
narrow blade tip.
With fast RIBs, there is the further complication of immersion. Setting propellers high to minimize gearcase drag reduces the immersed area. Props with full blade tips can be set higher than narrow tipped blades. Set high they can be of larger diameter (true surface propellers are the extreme example) because less of the total diameter is submerged – at high speed!
My article on propellers generated quite a lot of interest, so I thought it would be only fair to do a follow-up, initiated by Paul Dean, he wrote:
“I read your article on propellers on the hotRIBS website with great interest and found it informative. One thing I’m curious about is what the difference is between a “Cleaver prop” and a Chopper prop”. Can you please describe the differences and what applications the two styles are used in?”
A cleaver prop is named because its blades look like an axe (cleaver). It has a curved leading edge and a dead straight trailing edge. It is used on high-speed craft and runs best in surface piercing mode. The leading edge is very sharp while the straight trailing edge is blunt and ground at 90 degrees to the chord line giving a sharp edge on the face of the blade. Sometimes the edge at the back of the blade is rounded – this is for fine-tuning and varies the lift on the prop. If you slice the blade from front to rear and look at the section exposed it has a bent wedge shape starting thin at the front and progressively getting thicker towards the trailing edge.
The face side of the blade (the surface you see when the boat is viewed from the stern) is concave – this hollow increases at the trailing edge in the form of ‘cupping’ – while the back of the blade is convex. As the blade slices down into the water, it cuts out a chunk of the sea and throws it backwards. Most of the work is done by the face of the blade which is the high-pressure side, unlike a fully submerged prop which has rounder looking blades and an aerofoil shaped section. This type sucks itself along with most of the work done by low pressure on the back of the blade (the side towards the bow of the boat). With a submerged prop the water senses the suction of the prop and starts to speed up before reaching the prop.
With the cleaver, the water is taken by surprise and so there are holes left in the water where a chunk has been chewed out. When the prop is on the surface the holes are filled with air (ventilated); when the prop is submerged the holes are filled with water vapour (steam) – the low pressure ‘boils’ the water. The latter submerged type is known as ‘super-cavitating’ since they are designed to work with constant cavitation – the work being done by the prop face while the back is one side of the cavity.
The chopper type prop is only called thus to differentiate it from the cleaver. The Brits used to call them ‘spoons’ which, to me, is a better description of the blade shape, but the American ‘chopper’ title is now mostly used in the UK too. The outline of the blade is similar to a tablespoon and basically it works the same way as a cleaver. If a blade is sliced from front to rear the face side is similar to a cleaver although the back tends to follow the curve of the face more closely and maximum thickness is usually not at the trailing edge which is usually rounded. The face is hollow and the trailing edge usually well cupped as with the cleaver.
The reason for the two types is in their effect on the boat. Both types run best in surface mode and are similar inefficiency. The cleaver creates lift while the chopper usually pulls the stern down and this can change the efficiency of the hull itself and, therefore, the overall ‘package’ efficiency. The cleaver blade with its wedge section does most of its work when it slices down into the water; the reaction to this is to lift the propeller itself (and the gearbox, drive, transom, etc.) up.
Where you have a boat with a lot of bow lift (like a fast cat with aerodynamic lift carrying the bow) the cleaver helps the balance by lifting the stern.
Most choppers have a lot of rake on the blades (the blade angles back from hub to tip). The sharp spoon shape cuts into the water on the downward slice and then does its work while passing from side to side across the bottom of the arc. At this stage, the force on the blade is forward and downwards. This depresses the stern of the boat. On craft which tries to run too flat for good efficiency, this will help lift the bow. The chopper digs itself in and can have a greater torque effect than a cleaver which is a little ‘mushier’.
On a hull which is sensitive to torque, a cleaver might give a better handling craft. A craft which gets the best speed with a chopper may lose 2 or 3 mph with a cleaver but the improved handling may be worth the sacrifice.