Improving IOR Stability?

It seems like none of you ever sailed any of these boats downwind in heavy weather. The rudders do ventilate in a reach and the boats round up. Downwind, no amount of rudder size is going to prevent them from burying the bow. They broach not because of the rudder, but because the bow and deck are under water.

 

I sure wish I had ALL the IOR expertise you do.

In the measly 10 years I sailed those boats I was involved in many wipeouts, and watched even more at close quarters. In only very few instances were bows underwater.

In non-IOR designs (such as MORC boats) you would find even finer bows, taller rigs, but fuller sterns. Somehow those boats didn't bury bows and wipe out.

Here are a couple of photos of IOR boats doing the normal ocillations. Both are very close to losing it in a rounddown. Neither has the bow and deck underwater. Both are being sailed by professional crews. If they were sailed by the normal weekend warrior crews they would probably already have spun.

 

yes, it would be helpful to be in one of these boats under those conditions. I have seen it happen on other boats when I was crewing during a local club race, and judging by those pictures, it is not just a matter of burying the bow. It is a loss of directional stablity.

A hull maintains directional stability because of the forces acting on it tend to keep it pointed in the direction is is moving. the hull, keel, rudder(s) all work together so as the water flows over the surfaces it creates forces that keep the pointy end in the direction the hull is moving. this is not quite as obvious as it might sound, because what is happening with these hulls is the forces that normally keep the pointy end ahead of the blunt end go away, and it want to swap ends (broach). So it seems to me the issue is how do you change the shape to maintain those forces that keep it pointed straight ahead.

It does appear the rudder is losing effectiveness and it suddenly acts like the rudder is not there. So, make the rudder deeper and larger, put a flow fence on it about 1.5 feet down from the top to stop/retard ventilation, put two rudders on the transom angled outward about 20 degrees (that way at least one rudder is in the water at all times). I would also try the vortex generator/strake idea, it is simple and cheap to test out. None of these experiments requiring altering the hull. Dagger boards in the aft end at outward angles should also work, but these would require altering the hull, and it seems if the rudder was ineffective, you would not be able to steer the boat when it is heeled over, even if it stayed in a straight line.

But if that rudder lifts out of the water, even part way, you will swap ends. Short of making the hull narrower at the max beam, or making the stern fuller, these ideas all should help keep the pointy end ahead of the blunt end.

 

None of that'll get the thing under control. To get it under control you have to stop the roll. Look at the pics, think about where the load from the spinnaker is going, it's pulling on the mast head that's now a long way offset from where the hull is, and it has a long lever arm. That's why we always talked about keeping the boat under the spinnaker. You wouldn't be able to put on a rudder big enough to overcome the out of whack aero/hydro dynamics

 

I did mention lowering the spinnaker, perhaps to a 3/4 mast height, or just not use it. but other sailboats have masthead spinnakers and do not have this behavior, so why would this one be prone to directional instability and others not? There is something different about this boat.

 

How on earth do you think that could be done on a masthead rigged boat?

 

not very easily.

 

Or heat your angle up about 5%, problem solved

 

Yes that is the easiest and probably only solution.

Since this boat is from very early 70's there is probably nothing wrong with it's stability.

A big masthead spinnaker and a hull that has a very high resistance at those speeds makes it prone to rolling caused by vortex shedding. Tweaking helps a bit, but still most of the spinnaker can oscillate from side to side. Very active steering helps and a more effective rudder helps the active steering, but that is not the solution for single handed nor long distance.

Many modern boats are also prone to rolling in heavy winds while running dead downwind. Deep rudders and faster speeds helps, but very active steering is required on many boats. Just steering 5-10 degrees of dead downwind stops the rolling.

 

Unfortunately, Marachaj says it is not that simple. Apparently, coming up on the wind causes stern waves to become quartering waves. Quartering waves are one of the causes of the roll starting. Actively trying to surf *might* help or it could make things worse.

Another major factor in rolling is too much or incorrect helmsman input.

Eventually a wave in a confused sea will bump the boat, this will start the problem or the helmsman will be surprised and start the problem.

 

Here is an example of how a professional design office modified an IOR hull to make it friendlier.

This was a much later design than the SJ24, so already had a lower counter and wider transom. The modification filled out the stern, lowered the counter even more (I added the red line to indicate the bottom in the IOR drawing), and filled the area behind the bustle.

 

The regulations for spinnaker poles forces the sail to be really full on the upper side. If IOR had allowed double poles of unlimited length, the spinnakers would have a completely different shape. By the way, J24s are known for nose diving.

 

Can you post or put links up to Marachaj's essays on the topic? I did several searches and could not find anything related to sailboats associated with that name.

this is all very interesting, it explains much of reasons hull shapes have changed over the last 40 years.

 

This problem occurs also without waves and on long distance sailing it is quite likely that waves and wind have different direction. Rolling at increasing amplitude is caused by running too close to dead downwind and this problem is solved by not running that deep. Waves are another problem.

Yes running with spinnaker in above 20 knots or so (depends on the boat) will sooner or later cause a broach. Not running dead downwind will make the problem very much less likely.

 

"Seaworthiness the forgotten factor"
A long rant against light displacement boats and especially IOR shaped hulls but some very interesting ideas such as the link between heave and capsize