Improving IOR Stability?

The first pic (Heaven Can Wait sailing as Jamesons) was already posted and shows a rudder IN the water. The second pic (Ran Tan) is of a non-IOR boat so it shows nothing about IOR designs apart from the fact that they are not unique in broaching.

Yes, the Cha Cha and 2 Farr pics show that AFTER you have broached the rudder can come out, but that is the RESULT of a broach and not the cause of it. My IOR=style 28' boat has a rudder that draws 4'6" - she very rarely broaches but she has done so. A boat of such dimensions could not get the rudder out of the water unless the bow was a long way under. This has never happened.

 

Yes, I didn't notice an improvement in rolling stability, which rather surprised me. There may have been an improvement but if so it was not major.

Distorted buttocks may have made rolling worse by (as Mike says) disturbance of the flow over the rudder and generating a steeper stern wave, perhaps.

Agree that the roll comes before the broach normally when running square (you can broach without rolling on a reach, of course) but the point is that while the roll creates the broach most of the time, you can have boats that roll without broaching.

You note that you are looking for another boat - obviously I don't know your reasoning but you could well find that a modern spin, well tweaked, or some other sail will make the boat handle pretty well. As noted earlier, SMALL mods can make a big difference

 

My understanding of a broach goes something like this:

1.) Due to being over driven, the bow on the sharp bowed boat starts to dig in. This may be because the Center of Buoyancy and the Center of Floatation are pretty far aft on such a boat.

2.) Due to the immersed bow, The Center of Lateral Area (CLA) moves forward. This causes the boat to want to round up. It starts to turn and with the turn, and, due to the high Center of Area (CA) of the sail plan, this starts a roll.

3.) The helmsman senses this and tries to add corrective helm. Maybe he adds too much and the boat yaws to leeward. If that happens, the boat could roll to the other side. If he adds just enough, soon enough The boat straightens out and goes in the general direction it is intended to go. If he adds too little, the boat continues to turn upwind and roll in the direction it started to roll.

4.) If he over does it or doesn't do it enough, the boat continues to yawl, changing the angle of attack on the deep, high aspect ratio rudder, causing it to 'stall', losing at least half its 'lift', making the boat temporarily un controllable, causing it to broach.

This is, of course, an over simplification, as the broaches I have seen, seem to happen after a series of rolls, which increase in severity.

Perhaps rolls are also induced by wave action as well, which complicates the matter even more.

With a short, deep keel and little or no bearing aft, there is little to dampen the roll, which, as far as I have read, is the major criticism of that hull type.

 

A sudden gust can also overpower the boat into a broach. It will be an almost knockdown, the rudder ventilates or comes out of the water and the boat rounds up to about 90 degrees to the wind and sometimes may surf sideways.

 

You need to separate the cause and effect, since there is a difference between mono’s, multihull’s, power and sail driven, when it comes to broaching.

Broaching is basically when the stern has been picked up by a following sea. The bow, with little or no reserve buoyancy cannot counter this trim. Therefore bow is then driven into the trough of a wave since no bouncy to counter this ever increasing bow immersion and the stern is lifted further. There comes a point when the bow, which is now buried, acts as a pivot, and has a tendency to yaw the vessel as the stern is lifted and displaced by the following wave. The result is the vessel yaws and then is left beam onto the waves, which can capsize the vessel with a compromised or minimal stability owing to such motions, especially if the wave amplitude is large; it can swamp the boat.

It is also nicely summarised in this MCA report.

 

My understanding of a broach goes something like this:

1.) Due to being over driven, the bow on the sharp bowed boat starts to dig in. This may be because the Center of Buoyancy and the Center of Floatation are pretty far aft on such a boat.

2.) Due to the immersed bow, The Center of Lateral Area (CLA) moves forward. This causes the boat to want to round up. It starts to turn and with the turn, and, due to the high Center of Area (CA) of the sail plan, this starts a roll.

3.) The helmsman senses this and tries to add corrective helm. Maybe he adds too much and the boat yaws to leeward. If that happens, the boat could roll to the other side. If he adds just enough, soon enough The boat straightens out and goes in the general direction it is intended to go. If he adds too little, the boat continues to turn upwind and roll in the direction it started to roll.

4.) If he over does it or doesn't do it enough, the boat continues to yawl, changing the angle of attack on the deep, high aspect ratio rudder, causing it to 'stall', losing at least half its 'lift', making the boat temporarily un controllable, causing it to broach.

This is, of course, an over simplification, as the broaches I have seen, seem to happen after a series of rolls, which increase in severity.

Perhaps rolls are also induced by wave action as well, which complicates the matter even more.

With a short, deep keel and little or no bearing aft, there is little to dampen the roll, which, as far as I have read, is the major criticism of that hull type.

 

See coments above in RED.

 

if one is sailing directly down wind, what exactly to you mean by "rolling to windward" or leeward for that matter. Windward would be toward the rear of the boat!

 

IOR hull shapes would get rolling in flat water. You're dealing with different issues.

 

You're joking, right?

 

That was sarcasm. I just want to know exactly what is meant by the person posting those words. I have been around sailboat all of my life and I know the way people use these words, I just want to make sure I understand what the people posting here mean when they talk about rolling to windward or to leeward when the boat is moving straight down wind. It is a matter of clarity.

 

I don't think he is. This aligns pretty well with his other contributions on ways to solve the OP's problem:

Add vent holes to the spinnaker
Duct taping 2x4s to the hull
Putting a fence on the rudder
Lowering the spinnaker hoist to 3/4 height on a masthead rig
Using twin rudders angled at 40 degrees (!) outward


He also mentioned that he did not see an advantage for designers to draw "rule beating" shapes under the IOR.

So his not knowing what side is the weather side is not a surprise.

 

Then maybe it's best if we just let him ponder on which is the windward side.

 

If you are going downwind- wind directly from the stern- upwind is not clearly defined. Racing rules say you are in the tack opposite from where the boom of the mainsail is. However, that is a racing rule that has nothing to do with vessel behavior. The question is relevant: What is upwind when you are going dead downwind?

 

4 Questions 4

(1) Aero-Dynamic induced Roll

(2) Aero-Dynamic induced Yaw ... Broach

(3) Aero-Dynamic induced Yaw + Hidro-Dynamic induced Yaw ... Broach

http://www.youtube.com/watch?v=xNRsK2ocfnA

(4) Hidro-Dynamic induced Yaw, Broach and Capsize (Fasnet 1979)