# IOR /IMS Question

Discussion in 'Sailboats' started by nemo, Sep 6, 2002.

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### nemoNaval Architect

Hi!
I'm wondering what are the main differences between IOR and IMS regulation in boat design, since I'm restoring an IOR class boat, and I would like to know how to improve it in terms of weight distribution and so on, because I want to get an IMS certificate for it.
THanks
Stefano

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### Steve HollisterJunior Member

The IOR was a rating rule and the IMS is a handicapping rule.

What that means is that the IOR based its rating on a large number of point measurements to estimate the effective sailing length of the boat and tried to relate that to the speed potential of the boat. However, the IMS rule uses the full, 3D shape of the boat to determine the speed potential of the boat. The IMS rule tried to eliminate the problems associated with point measurements, such as bumps and distorted hull shapes. Everyone tried to "beat" the IOR rule, whereas the IMS rule avoids these problems. Therefore, the IMS rule is called a handicapping rule because it more accurately reflects the speed potential of the boat and you really can't "beat" it with bumps and distortions.

Well, that's the theory, anyway. In reality, the IMS rule is better, but not perfect. And, what with very fast PCs on everyone's desk, some designers have found ways to trick the IMS VPP formula. This is not as easy as with the IOR rule, but it can be done. My feeling is that there is no such thing as a true handicapping rule. All rules can be analyzed and "beaten" to some level or another. If there is enough demand to squeeze out that last fraction of a knot increase, then there will be enough money to tear apart any rule. [Score one for the box rules and the one-design classes!]

Please keep in mind that although the IMS VPP calculation is pretty good and uses the measured 3D hull shape, it is not a computational fluid dynamics (CFD) program, like some of those used for America's Cup designs. The calculation consist of two parts: the LPP (lines processing program) and the VPP (velocity prediction program). The LPP avoids the old IOR point measurement problems by calculating an effective sailing length using the full 3D shape of the hull. Although this calculation is influenced by hydrodynamic theory, it still reduces the complex 3D shape of the hull to few simple dimensions which generally reflect the speed potential of the boat. For example, if you want to examine the subtle differences in 'U' vs. 'V' section shapes forward, then the LPP/VPP won't do that for you.

Back to your question of what you can do to make your boat faster? In theory, nothing, since the IMS rule is supposed to accurately predict the performance of your boat. But, that's not true. The IMS assumes that your boat is reasonably designed. If you put on a perfectly flat keel (no airfoil shape), then the IMS rule does not give you any credit. The assumption is that you are not sailing a poorly designed tub. Since you probably have a good IOR boat, then the best thing to do is to eliminate any strange IOR tucks and measurement bumps, if possible. The IMS rule doesn't "see" any of those bumps and they make your boat go slower. They may have been worth the drag under the IOR rule, but they don't gain you anything now.

In terms of weight distribution, I'm not sure you can do much except to set the boat up to make it easy to sail in your prevailing wind conditions. The IMS rule calculates the stability for the boat accurately, so there is probably not much you can do there to "optimize" your boat for IMS. If you make your boat more stable, then the IMS rule will "see" that your boat is going to be faster in heavy air. [Perhaps someone else has an idea here.]

Regards,
Steve Hollister

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### nemoNaval Architect

My boat is a Farr 31, I know it has been built also in an IMS version, so I was wondering what was the difference between the 2 versions.
Looking at the hull, the only thing I would change is the skeg next to the rudder. I think it's not necessary, and I would replace the rudder with a semi-compensated one.

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Location: Rhode Island

### Steve HollisterJunior Member

Your best bet would be to contact the Farr office and ask their advice - I'm not really familiar with the design. Since it was built in both IOR and IMS versions, they might have a quick and easy answer for you.

Steve

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### umichNew Member

I am a design student, we have a sailboat design project. Are there any good starting out points(websites) to check out? Also Any info on where i can find good user profiles as to what type of person uses what type of boat. 3 who's Any info would be greatly appreciated! thanks

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