Varifold – What on earth are those?
From over 10 years of testing in cavitation tunnels, academics and consultancy, I have to say that there is nothing quite like a folding sailboat propeller. I know King Propulsion is the US distributor for the Varifold range of folding sailboat propellers but I still think they are special. So for a short introduction onto these elegant and lets face it, rather clever designs, let me give you a little background.
The group behind the Varifold is Bruntons propellers (www.bruntons-propellers.com) based in the UK. The company designs and supplies propellers of the most advanced designs to navies and commercial shipbuilders around the world and their in house team of Designers, Engineers and Naval Architects who work on those projects also bring their undoubted expertise to Varifold and our other leisure propellers.Varifold, in all its versions – two, three and four blade, are an exceptional low drag racing propeller, so how is this achieved? Well really there are three types of sailboat propeller, first you have your fixed pitch propeller. This is a standard no frills off the shelf unit. Powers great but when under sail it will windmill, i.e. rotate on its own if not locked off. This rotation or even locking of the propeller all costs energy. So aside from wave making resistance, frictional drag inherent in any vessel you are not getting the full effect of the wind in your sails as you are effectively dragging a bucket of water behind you.
So how do we reduce this? Well the second, and fairly popular option is to use a feathering propeller such as a Variprop. This type of propeller thinks like an aircraft propeller. The pitch is set dynamically depending upon the rotation of the propeller when under motor. When the shaft is not turning the blades take the least path of resistance and align themselves with the flow effectively weathervaning. So this is good, we have reduced the drag somewhat so what is the catch? Well to feather effectively the blade needs to present a small area to the flow that does not induce lift or turbulence. The blade shape of these propellers are like those on a turbo-prop plane, they are straight with no pitch distribution, or twist if you like, to the blade. While this works great in air, water is sticky stuff. Now excuse my lapse here but… as you move to the tip of the propeller modern marine propellers reduce the pitch to “offload the tips”. Why do this? Well a large pitch at the tip causes greater loading. This loading is the weather moving faster over one side of the blade than the other (and creating lift). Now for all those physics classes you took, as you increase speed in a fluid, the pressure drops, once the pressure drops below the vaporization pressure of the water it will “cold boil” or cavitate. Think Hunt for Red October and submarines, all those bubbles! Well the bubbles are like your car wheels spinning and getting nowhere, again wasted energy. But it is also noisy and with lots of vibration. So for this design, under sail is better but under motor they are prone to noise and vibration from cavitation, and well other phenomena.
So how do we fix that? Well the third way is to use the benefits of both in a folding sailboat propeller. Take the blade section, shape and pitch distribution of the fixed pitch and get the blades to move out of the way under sail. So imagine a fixed pitch with the blade able to pivot at the root and you have the Varifold concept.The Varifold performance is achieved by state of the art blade design (true airfoil sections) and proper blade pitch distribution to offload the tip. As I mentioned this tip off-loading radically reduces cavitation and, in combination with the skewed blade design, greatly reduces noisy pressure pulses against the hull that are responsible for the vibration and noise when under motor. As far as I can see no other manufacturer offers this full package on such a wide range of folding propellers.
The Varifold range of folding sailboat propellers has grown from a single two bladed model to include three and four blade versions and are now being fitted to Swans, Baltic Yachts, Leopard Catamarans for engines from 20hp to 750hp. So check out the couple of images below and that is how they fold!!