Rotating rigs?

[44C]

G'day everyone. A few of you will probably know me from the cruiser's forum, where I've asked this question before.

I'm building an Oram 44C, and getting to the stage where I need to make some decisions, like whether or not I want a rotating rig.

Bob Oram is very much in favour of them, and they do have advantages, mostly performance related, but also the additional safety advantage of being able to more easily reduce or lower the mainsail going downwind.

I guess the downsides for me were the difficulties with mashead wind instruments, nav lights, and also leading halyards and reefing lines back to the cockpit. Also I've heard they can be noisy, with lines creaking at at the rotating spanner.

I think I can either work around or eliminate most of these however.

Bob reckons there is a significant performance advantage with them, but I'm wondering just how much? Has anyone tried sailing with a rotating rig locked centred, recorded the speed, then rotated the mast and checked the difference?

I'd be very interested to know just how much difference there is.

[catty]

G'day everyone. A few of you will probably know me from the cruiser's forum, where I've asked this question before.

I'm building an Oram 44C, and getting to the stage where I need to make some decisions, like whether or not I want a rotating rig.

Bob Oram is very much in favour of them, and they do have advantages, mostly performance related, but also the additional safety advantage of being able to more easily reduce or lower the mainsail going downwind.

I guess the downsides for me were the difficulties with mashead wind instruments, nav lights, and also leading halyards and reefing lines back to the cockpit. Also I've heard they can be noisy, with lines creaking at at the rotating spanner.

I think I can either work around or eliminate most of these however.

Bob reckons there is a significant performance advantage with them, but I'm wondering just how much? Has anyone tried sailing with a rotating rig locked centred, recorded the speed, then rotated the mast and checked the difference?

I'd be very interested to know just how much difference there is.

The only true test is a side by side drag with another boat. A while back a friend on his Tennant Northener 11m with a rotating aluminium mast section very similar to your 16M 235 section (if not identical) sailed a 2nm side by side drag against myself with fixed spreader-less mast in approximately 8-10 knots of wind with sheets slightly eased. Both of us on autopilot and with the northener 11 altering mast rotation to attempt settle this argument. I can report it made **** all difference.

44c , its time to stop procrastinating , your time would be better spent "v-ing up the forward sections.

[Ozmultis]

On my old (as in a few years back) Kerr 9.5, the rotator made bugger all difference to performance, (but then again the sails were crap) but it made a huge difference to sail handling.

[44C]

Thanks for the useful reply Andrew.

I wonder if you could elaborate on the sail handling differences?

I'd say I'm really more interested in this aspect than performance anyway.

Has anyone experienced the difference in ability to reef while running, compared to a full battcar system for instance?

[catty]

Thanks for the useful reply Andrew.

I wonder if you could elaborate on the sail handling differences?

I'd say I'm really more interested in this aspect than performance anyway.

Has anyone experienced the difference in ability to reef while running, compared to a full battcar system for instance?

44c, you will find the largest friction component is the sail/batten friction on your aft swept side stays when trying to reef whilst running, not on your battcar system.

Just as a point of interest, does the battcar system ,you have chosen, use ball bearing cars or rod sliders?

[Nordic]

I guess the downsides for me were the difficulties with mashead wind instruments, nav lights, and also leading halyards and reefing lines back to the cockpit. Also I've heard they can be noisy, with lines creaking at at the rotating spanner.

This is not something I have seen on my rig, the rotator lines are tight. I use 12 mm sheet lines as the forces are low, and 12 mm is easy on the hands.

I think I can either work around or eliminate most of these however.

Bob reckons there is a significant performance advantage with them, but I'm wondering just how much? Has anyone tried sailing with a rotating rig locked centred, recorded the speed, then rotated the mast and checked the difference?

I'd be very interested to know just how much difference there is.


When talking about masts we need to look at the mast + sail together and not alone. For a wing mast we need to look at the chord (length) and profile as well.

The case for a wingmast is pretty clear in my mind for a few simple reasons:

1.) If you agree that minimising the seperation bubble aft of the mast will improve your drive (lift) and lower the drag, then it's a no-brainer.

2.) If you look at the drag or drag coefficient of a mast at say 40 degrees relative, the drag coefficient is close to 1.0. The comparable Cd for a wingmast of equal tickness pointed directly into the wind is lower than 0.1.
So just the windage is a sizeable factor. If you want to calculate the windage, then use Fdrag = ½ x 1,29 x v xv x Cd x Area. (Newtons)
So the drag grows with the square of the relative wind speed.


There are of course many other factors that will influence both the lift and drag of a rig.

An easy way to look at what chord you want on your wingmast, is to look at the length of the mainsail leech and the chord of the wingmast together. If you want the rig optimised for upwind work at say 35 degrees relative, then the chord needs to be about 10% of this length, to have the mast pointing directly into the wind for minimum windage and good lift.

There is alot of good information here: http://tspeer.com/Wingmasts/teardropPaper.pdf

Click on the PDF a bit lower down on the page.

A big factor in the performance of any boat is the windage of the boat and rig, I've started work on a write up on this trying to keep it simple but reasonably correct.

A 12 mm wire at 20 m/s gives about 450 grams of drag per meter! This is about the same windage as a wingmast with a width of 360 mms or around 30 times more....

A wingmast does not add much complexity to a boat, just the rotator controls, but does add a neat way to flatten the main by overrotating it.

So in my opinion, wingmast is the way to go if you have a relatively fast boat, not worth it for the heavy boats.

44C - Bob Oram knows his stuff, and doesn't over complicate things, so if he says it's a good idea, I would tend to believe him. Then read up on it.

Alan