Improving IOR Stability?

[jakmang]

I got a hold of an old San Juan 24 very cheap. These boats suffer from the IOR "death roll" downwind usually ending in a round down and wipeout. I would like to single hand this boat downwind long distance and am interested in doing some experiments to try to make the boat more stable. I have tried do the research on the instability problem. Marachaj and others layout the causes, but no one talks about fixes besides getting another boat. This is probably what I should do, but I figure I might learn a few things by trying to fix it and at the price, I won't ruin much.

The typical damping operation used by sailors are to "twing down" or lower the spinnaker sheeting angle and move weight aft.

Possible fixes that I have considered are aimed at adding stiffness and/or roll damping:

- Adding some weight to the keel.
- Extending the boom to balance the sail plan.
- Daggerboards. I read somewhere that twin keels worked on IOR boats, but they were against the rule.
- Longer rudder. Once the boats roll far enough, they lose steerage.
- Adding a skeg.
- Asymmetric Spinnaker on deep reaches instead of running with symmetric spin. Though Marachaj said reaching on some boats made things worse.

My impression is that there are several flaws to this kind of boat that compound to create this problem:

- Majority of sail too far forward.
- Pinched ends.
- Lack of appendages to give directional and/or rotational stability. "An arrow without fletchings".

If you look at something like a Swan from that era, it would have a similar hull shape, but with a normal mainsail and a skeg.

It should be clear from this post, that I'm not an engineer or an architect. I would appreciate any thoughts you might have. I know that this is kind of a crazy idea, so I would like to avoid a bunch of flaming posts telling me so.

Thanks,
-jak

 

[Petros]

I am not familiar with this issue on these boats, but as an engineer I do know something about stability. I would try the deeper rudder, not just deeper but also larger. and perhaps the asymmetric spinnaker (one that has less camber). With either of these you do not alter the hull, so you can change it back without much effort.

I do not see how extending the boom would help, the problem seems to be an unstable interaction between the hull/water and the spinnaker. Not enough dampening. As an oscillation starts, it sets off some other forces that overshoot the neutral position, and than it overshoots back. More dampening always helps if not eliminates this kind of behavior. A skag is the next thing I would add.

some large spinnakers have a tendency to shed a vortex first off one side, and than the other, causing it to oscillate back and fourth. I am speculating here, but it could be when this happens it will pull your hull to one side, and than the other. Without good directional control than the hull overshoots the input from the spinnaker. asymmetric spinnaker does not have this tendency. Perhaps putting some vent holes along the edges of your existing spinnaker, to stop the vortex shedding, as a simple experiment would teach us something.

to make matters worse the bow is likely being driven down which in turn lifts the stern, reducing rudder effectiveness. Moving the sails forward or aft will not change that when going down wind (it would if off the wind). Other than putting more volume in the forward hull (major rework), I do not see an easy fix to this problem. Lower the spinnaker height if possible? extending the forefoot of the keel forward should increase yaw dampening, but it will also make it directionally less stable unless you increase the size of the rudder at the same time. That is why I would increase the size of the rudder first. you might see if you can move more of your heavy storage further aft, it may raise the bow somewhat when not going down wind but that should help keep more of the rudder in the water.

do you have any pictures of the hull/keel configuration to post?

 

[michael pierzga]

that generation of IOR boat, with pinched transoms and skegs in front of the rudder were a handful downwind.

Expensive to change the shape of the boat. A better rudder might help with keeping a bite . Keep weight out of the bow and crew weight aft will help. Obviously aggressive sail trim will help. With the old Peterson IOR boats we sailed with two guys on the tiller...one pusher, one puller.

The SJ24 was a fast boat in light wind . Best to just sail it for what its good for. Keep a Clean polished bottom , good light air sails and enjoy.

 

[gonzo]

The overall design makes those boats broach and pitchpole in rough weather. They developed into fair weather, round the buoys boats. Those flaws spelled the end of IOR.

 

[jakmang]

San Juan 24 hull pictures

I have attached a couple pictures. The stern shot shows the uninterrupted roundness that is one of the big contributors to the problem.

 

[gonzo]

It is not the roundness, but the displacement distribution. Most of the displacement is in the middle with little displacement and reserve flotation at the ends. Those designs were made to beat a rule and not to produce a good sailing boat.

 

[Paul B]

I got a hold of an old San Juan 24 very cheap. These boats suffer from the IOR "death roll" downwind usually ending in a round down and wipeout. I would like to single hand this boat downwind long distance and am interested in doing some experiments to try to make the boat more stable. I have tried do the research on the instability problem. Marachaj and others layout the causes, but no one talks about fixes besides getting another boat. This is probably what I should do, but I figure I might learn a few things by trying to fix it and at the price, I won't ruin much.

The typical damping operation used by sailors are to "twing down" or lower the spinnaker sheeting angle and move weight aft.

Possible fixes that I have considered are aimed at adding stiffness and/or roll damping:

- Adding some weight to the keel. This would help the recovery after the fact, but would do little to stop the boat from rounding up/down. It would help upwind in a blow, and reaching. It would also cost more than the boat is worth.
- Extending the boom to balance the sail plan. This could help a bit. The sailplan is so unbalanced that boats of this type used to use bloopers. Extending the main boom is a common modification to this type of boat after the IOR went away. Typically the amount of extension is 10% to 20%.
- Daggerboards. I read somewhere that twin keels worked on IOR boats, but they were against the rule. Daggerboards would not help you, nor were they against the rule.
- Longer rudder. Once the boats roll far enough, they lose steerage. A deeper rudder would be helpful.
- Adding a skeg. This would not help. In fact, it would make things worse. Rudders work better as lifting surfaces rather than as drag.
- Asymmetric Spinnaker on deep reaches instead of running with symmetric spin. Though Marachaj said reaching on some boats made things worse. An ASO will be much easier for you to handle shorthanded. Even if you stay with your SYM kite it is smart to sail slightly hotter when things get hairy downwind (say 150 degrees rather than 170 degrees).
My impression is that there are several flaws to this kind of boat that compound to create this problem:

- Majority of sail too far forward.
- Pinched ends. This is your real problem.
- Lack of appendages to give directional and/or rotational stability. "An arrow without fletchings".

If you look at something like a Swan from that era, it would have a similar hull shape, but with a normal mainsail and a skeg.

It should be clear from this post, that I'm not an engineer or an architect. I would appreciate any thoughts you might have. I know that this is kind of a crazy idea, so I would like to avoid a bunch of flaming posts telling me so.

Thanks,
-jak

Many boats from that era were "fixed" afterward by modifications to the stern, making it fuller.

I watched one boat very similar to yours (a Santana 25 Quarter Tonner) go through this transformation and it made the boat much better. The simple, inexpensive way to do this is to create the new shape you want with ribs glued externally to the aft part of the hull (generally needs to be about 1/4 to 1/3 of the length, faired into the forward existing shape). You fill the area between the ribs with foam, fair it out, then glass over, fairing that.

If that works for you, you can then do something similar at the bow (extending the waterline).

After you do all this you will have a boat that sails better than originally, but cost far too much and is unsellable.

 

[Stumble]

The best of the options to me would be to have a different rudder made. While there are other options, the reality is there is no good reason to spend a significant amount of money on a boat worth what this is. While a good rudder can probably be knocked up in a weekend with some foam and glass.

 

[sean9c]

I don't see how a new larger rudder helps the problem. The problem is hull shape, too much in the middle, too little in the ends. If you leave the hull shape alone and add a larger rudder it'll still roll. Back in the old days when we sailed these things one of the problems was steering too much when they rolled, you can't possibly get the timing just right so you made the problem worse by trying to steer. Yes, ultimately they'd roll enough that they'd go one way or the other and you had to steer. The trick was knowing when to start adding helm. A larger rudder may ultimately help get the thing under control once you've spun out.
The SJ24 is a pretty tender boat and really sails well in light air. If this was mine and I was looking to modernize it I think I'd look at adding boom length like PaulB mentioned and getting rid of the genoa, ya, it's doesn't help the roll but it'd make it a lot nicer to sail. Unless you're racing, once again as PaulB mentioned just don't sail dead down wind when it's windy, it doesn't take much angle change to get them to stop rolling.
Back in the old days I was doing some regatta and the Clarks, who owned SJ, showed up with a SJ24 with more ballast and even more rig. It was like, holy cow, this thing isn't tender enough we're going to make it worse.

 

[Paul B]

IBack in the old days I was doing some regatta and the Clarks, who owned SJ, showed up with a SJ24 with more ballast and even more rig. It was like, holy cow, this thing isn't tender enough we're going to make it worse.

That boat was DYNAMITE. I sailed on it with maybe the third owner.

 

[sharpii2]

My vote would go to increasing the rudder area. The new rudder would be the same depth, but wider. This would make it larger in area and lower in aspect ratio. Lower aspect ratio foils stall later (at greater angles of attack) than higher aspect ratio ones do. This would help eliminate the problem that is easiest to fix.

Other than that, the hull shape would need to be altered.

If it were my boat, I would alter the stern only, because that has the worst 'bump'. Also, the added buoyancy there would help.

Other than that, I would keep in mind that this boat was of a design rule that encouraged highest average speed for a limited amount of working sail (not including the spinnaker). This means it was really designed to sail best at sub hull speed.

Making it go faster, as trying to win a down wind leg of a race, is asking for trouble. So I would go down wind slower when cruising.

I would also use an asymmetrical spinnaker that was cut quite flat.

The real purpose of this sail would be to keep the bow pointed down wind.

I would sail down wind in tacks.

 

[Petros]

Just thinking out loud here, you have to make the bow and/or stern larger, or at least make it act larger, to have more effect against the water. A larger rudder/skag should help this, and this can also be tested with a vortex generator strake on the sides of the hull.

That is a simple thing that might be worth a test; adding strakes to the sides of the hull. take some 8' 2x4 lumber and cut two triangular shape strips about one inch on a side. Temp glue them on with cheap adhesive caulk (the kind you buy at the lumber yard) and perhaps two continuous strips of duck tape to hold them on. Place them starting at the widest point right about where a hard chin would go at the turn of the bilge. Run them back about 8 ft, you might shape the front so it is more steam line, and just leave the rest as cut.

My idea is as it starts to yaw, these strakes will shed a large vortex on the afterbody of the hull, keeping the flow attached and counter acting the movement of the hull, helping to keep it in a straight line. This will make the narrow stern act like a much wider one. It will cost a bit of drag, but should make the hull much more directionally stable.

You can do with test with less than $20 worth of wood, duck tape and caulk. If it works you can either use a good hardwood and glass them over, and screw and glue them to the hull (this will allow you to remove them with minimal patching if you should ever choose to do so), or just make fiberglass ones.

This is a common fix sometimes used on aircraft when they have a yaw or dutch roll stability problem. The effects of vortex generators are far more powerful than one might expect, and it is simple enough to try out here. It might even be class legal.

 

[gonzo]

These boats bury the bow in the water and either pitchpole or broach. No amount of rudder is going to compensate for the lack of flotation forward, that is madness. Sometimes while surfing they just bury themselves and the wave will poop the boat.

 

[Nick.K]

In Marchaj´s book on seaworthiness he outlines the lack of roll damping as a factor in the tendency to broach. The IOR boats trim down at the bow when they heel lifting the stern. The top of the rudder is exposed at the end of the rolls which is also when the rudder is under most load. Air is sucked down the low pressure side, the rudder looses lift and the boat broaches.
Perhaps a low aspect false skeg ahead of the rudder would help prevent ventilation and also increase roll damping?

 

[jakmang]

Daggerboards

Just thinking out loud here, you have to make the bow and/or stern larger, or at least make it act larger, to have more effect against the water. A larger rudder/skag should help this, and this can also be tested with a vortex generator strake on the sides of the hull.

That is a simple thing that might be worth a test; adding strakes to the sides of the hull. take some 8' 2x4 lumber and cut two triangular shape strips about one inch on a side. Temp glue them on with cheap adhesive caulk (the kind you buy at the lumber yard) and perhaps two continuous strips of duck tape to hold them on. Place them starting at the widest point right about where a hard chin would go at the turn of the bilge. Run them back about 8 ft, you might shape the front so it is more steam line, and just leave the rest as cut.

My idea is as it starts to yaw, these strakes will shed a large vortex on the afterbody of the hull, keeping the flow attached and counter acting the movement of the hull, helping to keep it in a straight line. This will make the narrow stern act like a much wider one. It will cost a bit of drag, but should make the hull much more directionally stable.

You can do with test with less than $20 worth of wood, duck tape and caulk. If it works you can either use a good hardwood and glass them over, and screw and glue them to the hull (this will allow you to remove them with minimal patching if you should ever choose to do so), or just make fiberglass ones.

This is a common fix sometimes used on aircraft when they have a yaw or dutch roll stability problem. The effects of vortex generators are far more powerful than one might expect, and it is simple enough to try out here. It might even be class legal.

This was my idea with twin daggerboards. They would be about 3 or 4 feet long and maybe a foot wide. I was thinking of placing them at about 30 degrees up from the centerline with the trailing edge about inline with the front of the keel. They could be pulled up going upwind so the only drag would be the apertures.

Again, I'm not an engineer, but it seems like it would be difficult to roll with these two "wings". They might need to be asymmetric so they would not create lift in two directions. Would also give some direction when heeled.