Propeller engineering involves a combination of science, gut instinct based on real world experience, and trial and error. Regardless of the project, the latter part of this equation is always present – even on military vessels, when billions are spent to get it right, you cannot get away from trial and error.

With Wind Horse, we’re just on our second set of props, which have been slightly modified from their original design. By adding a slight amount of cupping we have gotten close to where we want to be. (Adding a trailing edge cup to a propeller works like a flap on the back edge of an airplane wing. The cup/flap increases the lift of the foil.)

With the second set of props, we are now operating very close to our original design estimates which were set at the beginning of this project. Fuel burn at various speeds in smooth water is right on prediction, and significantly better than with the first set of props.

The one negative is these props are a little noisier than the previous set. So we are discussing our options with propeller guru Don McPherson of HydroComp.

Here is a photo of the trailing edge of one of the props, after it was modified with cup. Don advised adding a #5 cup to the trailing edge. These sizes are tied to gauges which Michigan Wheel supplies to their prop shops. Notice how the cup does not extend onto the tip of the propeller.

With this photo we’re trying to show you what a #5 cup looks like. It is a very tiny change, about 1/16″ (1.5mm). Yet that is enough to drop our wide open throttle RPM by 12%.

The noise we are hearing – and it is low by powerboat standards – is from an interaction between the blades and the hull. As the blade tips become vertical, they enter a region of boundary layer water that is moving more slowly than the water they have just passed through. This can set up a vibration/harmonic. If this vibration is at a frequency which is sympathetic with the surrounding hull structure, the vibrational energy is then efficiently transmitted to the interior.

It is almost impossible to predict how this will work in advance. Tip clearance – the distance between the prop tip and hull – has an impact. More tip clearance is always better, and we typically use 50% more than industry average. Propeller configuration affects vibration frequency, and hence has an impact on how it interacts with the hull’s natural response rate. In some cases four blades work better than five – and often just the opposite is true. In our case, the original five-bladed props were better in this respect than the four-blade design we now have. Wind Horse has other advantages compared to the norm. The structure ahead of the props is very streamlined, so that it has minimal negative impact on water flow. And the hull itself has a very thin boundary layer. Still, noise and vibration are things you feel, and once you are accustomed to a certain level, no matter how favorable these levels may be, they can be come irritating. You might say we’re chasing perfection on this score. We are happy with the efficiency, as previously stated. But, we’d like to get closer to the vibration levels of the previous five-bladed propellers.

Before changing props there are other refinements on these props we can test. We are going to try modifying the pressure distribution on these blades, to change their natural harmonic and how this interacts with the hull structure. We will do this by removing a bit of pitch from the outer areas of the blades, and then compensating by adding cupping onto the tips of the propeller blades. We will then see how this feels inside of the boat.

Just to confuse the issue a bit further, we have one test we need to complete before we make these changes. The props have not yet been trialed driving into big seas and lots of wind. We have been waiting for the right conditions – the type of day one usually wants to avoid – to make these tests. With winter at hand we expect to have a nice gale before too much time has passed. We want to make sure that the present configuration does not excessively overload the engines when driving uphill in gale/storm force conditions.

Then we need to find a local prop shop that has computerized propeller scan gear so we make sure the changes are done correctly.

We’ve never gone through anything like this with our many sailing designs. But on the sailboats we do spend a lot of time during sea trials working with sailmakers. Removing sails for recutting is unavoidable, if you want them just right. Propeller tuning is a lot easier, and in the end the cost/benefit relationship seems the same – if you are trying for perfection.