# Measuring Deflection at the top of a mast whilst sailing

Hi All,

I've got a challenging project and I was hoping I can pick some of your brains for the optimal solution.

As the title suggests I'm trying to measure the deflection at the top of the mast whilst the boat is sailing.

Here are two ideas which I came up with:

• Infrared camera & marker: place 3 cameras around the boat with a marker at the top of the mast then triangulate. Water might wash over creating lots of noise.

• Please two IMU's (Accelerometer + Gyro) one at the bottom of the mast and the other at the top. Compute the angles and then get the difference which gives us an angle which we can use to get deflection using Pythagoras. Would the top IMU create much higher reading due to higher inertia?

I'm not after a perfect absolute measurement, rather I'm looking at relative values between each of the masts we're trying out.

Thanks
Charles

I think I'd put some strain gauges on the mast .

That's how aircraft do it.

Allan

That's a good idea however it sounds like a destructive approach which I'd only take as a last resort.

CharlesDesign:
That's a good idea however it sounds like a destructive approach which I'd only take as a last resort.

It isn't

lg, couka

Strain gauges are bonded to surfaces - completely nondestructive.

How would you remove a bonded gauge without damaging the surface?

You probably can't. Leave it there. It's cheap.

Allan

IMUs would probably not work. Under the best of circumstances (perfectly still with lots of averaging) you can expect perhaps +/- 1 degree tilt angle accuracy, and when being whipped around by wind and water, it would be hopeless to measure a few degrees relative deflection.

Are you interested to distinguish backwards-forwards from side-to-side deflection? If not, the tension in the stays (if there are any) would be more informative.

Given that the mast is made of material which obeys Hookes law ( a spring) it should be pretty good. You could place strain gauges in pairs of 2 : one fore/aft, the other port /starboard.

You could then calibrate with a mechanically induced deflection - eg by pulling on a rope attached to the masthead.

You'd then get both force/deflection if a spring balance was in the rope, both against strain gauge reading...

Allan

The vector sum of the 2 strain gauge readings then gives you the deflection....

I bet the big racing yachts do something like this

Allan