ROTATING CARBON MAST

Why should we care about a rotating mast?@September 13, 2014 4:40 AM (GMT+2) / DOUG

Outremer offers 3 mast options on the 5X. Fixed aluminum, fixed carbon fiber and rotating carbon fiber. Having no experience with rotating masts, my initial reaction was that it seemed unwise to add the complexity of a rotating mast to a boat being used for long distance cruising. As with all boat decisions there are pros and cons, and so I needed to find out if the benefits of the 5X rotating rig are worth the extra cost and complexity.

Outremer 5X under sail with the carbon fiber mast rotated. Note the radar dome installed on the spreader.

The first two fixed mast options are pretty easy to understand, as it’s a simple question of weight. The carbon fiber mast weighs 280 kg (616 lbs) less than the aluminum mast. Taking weight out of the boat is important, and it’s especially important to save weight up high, as this has the biggest impact on the pitching motion of the boat. Less weight aloft = less pitching = more comfort and more speed.

So, we know we want a carbon fiber mast, so the next question is fixed or rotating? To figure that out, we need to look at why Outremer has gone to all the trouble of designing a rotating mast for a cruising boat in the first place!

It turns out the benefits of a rotating mast are not just theoretical, and I discovered that for myself when I did the test sail on the 5X Addiction. We were going upwind in a light breeze of about 7 knots just after completing a tack, with the rotating mast set straight on the center line of the boat. Once we had settled onto the new tack, we rotated the mast into the wind and I could literally feel the boat surge forward! Tests at different wind speeds and angles confirm that there is a 10-15% increase in performance with the mast rotated. This is great, but how does it work?

When a boat is traveling with the wind coming from in front of the beam (<90 degrees) the sails operate as airfoils in much the same way as an airplane wing.

Rigid wing airfoil

When the wind strikes the front edge of this rigid wing, the air is separated and must travel a longer distance in the case of particle A vs particle B. This creates a higher velocity on the top surface and a corresponding area of low pressure. So the wing is pulled upwards due to the lift force developed. This force is called aerodynamic lift.

Flexible sail airfoil

In the case of a sail, there is no rigid bottom surface, so it is less efficient than a rigid wing, but it still forms an airfoil because the two air particles A and B must travel different distances, and so a low pressure region of lift is created in the same fashion as a rigid wing. Around 2/3 of the driving force of the sail comes from aerodynamic lift, with the remaining 1/3 generated by the force of the wind striking the inside (bottom) surface of the sail.

This is the case for an ideal airfoil, but on a sailboat there is a mast in front of the leading edge of the sail. The bigger the boat, the larger the mast cross section has to be to handle the force of the sails, and this becomes a factor influencing the shape of the airfoil we are able to present to the wind.

Fixed mast airfoil

This diagram shows the effect that a fixed mast has on the airfoil. Since the wind must make a tight turn around the mast, a turbulence zone is created which reduces the amount of lift being generated by the forward section of the sail. It also drives the lift force direction slightly aft, reducing the ability of the boat to sail upwind.

Rotating mast airfoil

By rotating the mast into the wind, we can clean up the leading edge of the airfoil and eliminate the turbulence. This increases the lift force and moves the lift angle  forward, giving us more speed and better pointing ability (how close we can sail, or point, into the wind direction).

There are other benefits to a rotating mast, regarding reefing the mainsail. Normally when reefing, you turn the boat into the wind to take pressure off the front edge of the sail in order to lower it. This puts the headsail into a luffing mode which is uncomfortable and potentially damaging to the sail. With a rotating mast, you can turn the mast into the wind and lower the mainsail. This allows the headsail to keep drawing during the reefing process and is easier and places less stress on the rig and the crew.

Because the rotating mast is in fact a rigid airfoil, it acts as an additional 12m2 sail, so by rotating the mast to the centerline position as the wind increases, you have the ability to depower the sail, which in effect, becomes an additional reef point.

And for the mathematically inclined:

12m2 mast / (12m2 mast + 125m2 mainsail) + 6% lift improvement  = 15% performance increase from a rotating mast vs a fixed mast of the same size. And conversely, straightening the mast when the wind increases will de-power the mainsail by 15%.

So what’s the catch?

As always, all this goodness comes with a price, and in this case there are three issues that have to be considered:

For us to be able make the decision to choose the rotating mast option, we needed to find a solution to each of these. Here’s what we came up with:

Mechanical compexity

This one was actually pretty easy. Outremer has done a nice job of designing a simple and robust system for securing the mast, and operating the rotation controls from the cockpit. It does add a little more complexity when sailing, but to me it’s negligible, and since I am a committed sail tweaker anyway, I am looking forward to having another power control on the boat. Our conclusion: Outremer’s system is fine for our needs and has been proven over time on a large number of their other boats. We are happy to install it as designed.

5X Rotating mast

Rotation control lines led back to cockpit

Wind angle error

When the mast is rotated, the wind angle measured by the sensor at the top of the mast will be incorrect. This is because the wind angle instrument measures angle with respect to the mast center-line. So if the mast is rotated 20 degrees, the wind angle will read 20 degrees less than than actual apparent wind. This is a problem, but it can be corrected in software by the instrument system as long as we can provide an accurate reading of the actual angle of the mast.

To read the angle of the mast, we need another sensor:

NKE mast rotation sensor

This rotation sensor from NKE has been used by many offshore racing boats and has proven very reliable. The only concern I had was the cable that connects the mast sheave to the sensor body. If it breaks, there is no way to fix it without taking the mast off. NKE claims a 10 year life for the cable, and Outremer has never had a failure, but they add a second spare cable at the mast base that can be fitted if there is a failure of the original cable. Our conclusion: The benefits outweigh our concerns over the reliability of the sensor. If the sensor did fail, it will only affect the wind angle reading, which is a non critical data point, so we’re OK with this.

Radar image error

Most sailboats install the radar dome on a spreader located at the top section of the mast (see the first photo in this post). A high elevation for the dome provides greater radar range and minimum interference. On a rotating mast, the radar will provide significant errors when the mast is rotated. For example, if you have the mast rotated and a ship is approaching in the dark and headed straight for you, it will appear as through the ship is actually approaching from the side. This is not good.

As I write this, in September 2014, there is no reliable solution to this problem. The radar image angle should be able to be corrected in software in the same way as the wind direction, but in practice, other owners have experienced system failures where the correction angle is lost, so the radar reverts back to a non-corrected image. Although this can be resolved by rebooting the radar software, there is no way to tell if and when the system has stopped processing the angle correction input. I expect this will be resolved in a future version of the software, but it still leaves us vulnerable if there is a fault in the mast angle correction sensor.

We feel that a reliable and accurate radar, is an essential safety element when voyaging offshore, so installing the radar dome on a rotating mast is not acceptable to us. Our solution is to install the radar dome on a carbon fiber pole at the back of the boat, keeping it fixed with the vessel centerline. We will lose some range due to the lower mounting location, but it won’t be enough to compromise our safety at sea.

So all in all, the rotating mast option is a good one. It does add some complexity, but we found ways to deal with that and we are happy to be able to take advantage of the significant benefits that a rotating rig provides.