Wind Generator Affects Steering

[Ianm]

When going downwind without sail in 35 knots I was surprised to find that my Aries windvane would not steer the boat. I tried hand steering and I could keep the boat on course, but only with extreme concentration and pushing the tiller to extreme angles. When I stopped the wind generator steering returned to normal.
Has anyone had a similar experience?
The boat is a Brolga 33, a 1969 offshore racing boat similar to an S&S 34 of the same vintage. It was doing 5kts without sail. The sea was only a couple of meters.
The wind generator is an Ampair 100 with a rather short tail and the unit does tend to be rather unsteady around its vertical axis.

[Simes]

I think that I would also be surprised. Is the Aries in the air flow from the Ampair?
I would not expect it to be as you say that you were sailing downwind. There probably was nothing in the wind path to the Aries.
Without seeing your installation anyone would struggle to come up with an explanation for you.

Difficult to see any connection at the moment.

The Aries should drive your 33 footer with ease in 35 Knots of wind, Ours drives our 45 footer down wind with ease in virtually all sea / wind states.

Simes

[joeblogss]

I can see no reason why that should happen, unless the windy is somehow interfeering with the air flow over the vane ?
sounds odd, I have Impala SS 35 1978, tho quite a bit diferent to the 34.
My aries steers the boat through all conditions and all wind angles.

[barnakiel]

Sounds interesting. The only idea I will have off the top is a coincidence or something related to the gyro effect.

Will come back to see what others say. Most interesting post.

b.

[skipmac]

The fact that the boat was hard to hand steer as well as not steering with the vane indicates the problem is not related to the vane but the wind gen.

The boat is relatively light and assume as an offshore racer it has a fin keel? If that is the case then the wind on the wind gen all the way aft is putting a lot of force on the stern of the boat. With a fin keel the boat will easily pivot around the center of lateral resistance about amidships. So pushing on the stern causes the boat to try to pivot around the keel, making it out of balance and really hard to steer.

[senormechanico]

Skipmac has it. The center of effort is way aft, causing the boat to want to swap ends.
If the wind generator was on the bow, you would have perfect tracking. In fact, you might experience difficulty in turning the boat away from DDW.

[Ianm]

Thanks for all the comments and suggestions.

First a bit more detail about the circumstances. We were running downwind with the genoa furled to No4 size on a pole. Aries was steering and maintaing quite a straight course. I completely furled the genoa, in preparation for setting the pole on the other side. With no sail up, Aries could not steer.

Yes, the boat has a moderate fin keel and a skeg hung rudder, but it is not light. It is at least 7 tonnes.

I do not think that Ampair 100 is interfering with the air to the vane.

What prompted me to stop the Ampair was not the thought of the windage aft but its directional instability. The steering was very erratic and I connected this with the erratic movement of the wind generator. It was difficult to quantify, but I estmate that I could steer within 20 degrees and the wind generator was moving through 50 degrees. I do not know if other brands of wind generators are more stable.The tail fin of the Ampair was fixed with its maximum dimension in the vertical, rather than the probably more stable horizontal option.


Now I tend to agree that the principle cause of the steering difficulty was simply the windage of generator.

However, if the steering problem was due to windage alone, and not related to the Ampair’s instability, I find it puzzling that the problem has not been widely reported.
.

[senormechanico]

With no sail up, the area swept by the wind generator blades becomes a significant part of the total windage and its location. In effect, you launched a spinnaker the size of the wind generator swept area in the worst possible spot for going downwind, i.e. your stern.

[skipmac]

I think the problem has not been widely reported as it would not occur except under specific conditions like you report; strong wind from aft with no head sail up on a relatively light boat (not lightly built for her size but not a large boat like 45' or so).

Most of the time sailing downwind you would have some headsail up which would counteract the force on the wind gen aft so no problem. Other points of sail the force on the wind gen would be more towards the side so you would not have that force trying to push the stern around. You probably aren't the first person to experience this. Bet you a $1.00 or a cold beer someone sooner or later will report the same problem.

[sailorboy1]

well this had me all worried, then I found it was one of those less than 1% conditional things

but it is worth filing away for someday and am glad it was memtioned

[barnakiel]

This would only be the case for a boat nearly halted. But with 35 knots of wind and the boat GOING downwind the forces on the hull far outweigh the forces on the rig. The boat should steer easily as long as the speed was enough for this design to gain sufficient steerage.

If the boat was going too slow then definitely any stuff hoisted or lowered fore/aft will affect how she behaves.

b.

[Jim Cate]

But, if it is a windage problem, why does shutting the thing off make such a big change?

Wouldn't think that the windage would change that drastically when stopping the blades (sorta like the old "better to freewheel or lock the prop when sailing" conundrum).

Interesting situation!

cheers,
Jim

[senormechanico]

When the blades are "flying" the windage is very nearly the same as a solid disc vs. just the area of the individual blades.

I have a wind generator on a piling on my dock which charges a big battery bank in the house. When it's really blowing, I can see deflection of the metal pole attached to the piling, but if I shut it down from the house, the same wind pressure doesn't bend the pole nearly as much.

[Ianm]

Rather belatedly, I have thought how to put some numbers to the forces:

The Ampair 100 has a blade diameter of 0.93m, area 0.68m2. Assume that, for the generator spinning, force on it is the same as the wind force on a flat disc of similar area. For a 35kt wind I calculate that the force on it is 25kg. (density of air = 1.2kg/m3. 1kt =0.5m/second. Force = 1.2x0.68x(35x0.5)2 N = 25kg). For 30kts, 19kg.

If the yacht yaws 20 degrees and the wind generator is 6m aft of the Centre of Lateral Resistance, the moment tending to yaw the yacht still further is 25x6xsin20 = 51kgm.

If the rudder has no hydraulic balance (mine has a very small balance area) and the tiller length is 1.5m, the force required on the end of the tiller is 51/1.5 = 34kg

When the generator is not spinning the area presented to the wind is less than 20% of the disc, so the force required on the tiller is less than 7kg.

.
Assuming the above assumptions are good enough and that my calcs are correct, they support the opinions that the affect on the steering is simply due to the windage of the spinning generator.

[barnakiel]

Originally Posted by Ianm

Rather belatedly, I have thought how to put some numbers to the forces:

... the force required on the end of the tiller is 51/1.5 = 34kg

...When the generator is not spinning ... less than 7kg.

Assuming the above assumptions ... the affect on the steering is simply due to the windage of the spinning generator.

I think your calcs are along the right lines.

However, you assumed a static hull - omitting the forces from the water onto the moving hull. With the hull moving, the forces created on the rudder/tiller are mostly the result of the water flow along the foils. Then the forces of the windage and those on the moving hull/appendages balance out (or not).

Look at the foils of a VOR or a VG boat (an extreme case - only to illustrate the idea). Then look at the sail area related to the foil area... If we were to think windage only, these boats would not be steer-able. And they are, with a tiller, in a 60-70 ft boat.

The windage forces may be as you described them. Then the hydrodynamic forces will balance them as the boat moves, and the faster we go, the less we will sense the windage on the tiller.

barnie