Volvo Open 70 Heeling/Righting Moment

[Aly]

Hi,

I'm currently doing a project on a Volvo Open 70 and was working on a VPP. Unfortunately I am getting a heeling moment of about 10 times the righting moment which does not seem right!!

Anyone done a similar project and have any ideas for reasonable value of righting and heeling moments?

Cheers.

[amolitor]

You're calculating the heeling moment applied by wind on the sails, and estimating the righting moment of the hull?

These things operate a pretty high angles of heel. Heeling moment applied by the wind drops off as something like the cosine of the angle of heel. They also have a canting keel, so CG is going to be fairly far to windward. Finally, they reef early and often, lowering the CE in anything but fairly light air, which further reduces the heeling moment.

Can you talk a little more about what you've done so far?

[Aly]

Hey,

Thanks for the reply. Basically the VPP so far uses the IMS VPP aero model to predict the performance of the sails. The values seem to agree with a more basic aerodynamic model I had from a previous project. I figured the only problem could be the heeling moment lever as I have taken in to account the effect of keel cant, and righting moment of hull. I didn't realise the heeling moment reduced so much? Is this due to a decrease in the heeling lever size? Or is this due to another effect? I had thought the effect of heel on the heeling moment had already been taken in to account, but obviously not.

Any more info would be greatly appreciated as at the moment the VPP is useless if I have to assume heel angles, etc.

Thanks,
Aly

[amolitor]

I haven't any idea what the IMS VPP model would include, sorry! I would be surprised if it didn't take angle of heel into account, somehow or other (at least as an input!) It's not that big of a deal (assuming I am right) since at 20 degrees of heel you're at 94% of the heeling moment you'd get upright.

The righting moment of these boats is going to be pretty damn huge, in general. Uh, just swagging it wildly, and hoping I get into the right ballpark, I am getting a metacentric height of around 50 feet, which with a displacement of 27,500 pounds gives us close to 25,000 foot-lbs of right moment per degree.

Fudging and guessing some more, this suggests maybe 200,000 foot-pounds of righting moment at 15 degrees of heel, with the keel centered. They'd probably rotate the keel bulb (at, conservatively, 10,000 pounds) out another 7 feet or so, you're in the general area of 250,000 to 300,000 foot pounds of righting moment.

At 15 knots of breeze, with say 6000 square feet of sail up, you might see a total sail loading 5800 pounds. Say 3/4 of that turns into heeling moment, and say it's applied, I dunno, to a center of effort 35 feet up. Let's see what happens! Looks like 152,000 foot pounds of heeling moment.

Looks like we have an excess of righting moment, so the boat won't be over at 15 degrees, quite. Assuming I got all my wild-assed guesses and my orders of magnitude right. Does any of that feel about right, or point out anything that you might be assuming/estimating differently?

If you search a little in these forums, you find some other VO70 discussion, which might be helpful.

[Aly]

Hey,

Thanks for your help, your numbers were reasonable though I still got a large heeling angle. The main problem appeared to be due to my high wind speeds, I have now implemented reef and flat, as well as a sail wardrobe so my results are far more reasonable!

Thanks again,

Aly