Improving IOR Stability?

[sharpii2]

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.

This is the fix I had in mind (in my earlier post on this thread). It modifies the hull minimally, keeping the main shape of the hull, but eliminating a rule beating tuck.

I wonder about the rudders of that era.

I would think the incentive would be to keep the rudder area as small as possible to get the least friction drag. And the best way to make do with less rudder area is to have a high aspect ratio rudder that is elliptical in profile.

Was that the general practice back then?

 

[Nick.K]

See Mike Johns' post here No 221
http://www.boatdesign.net/forums/stability/seaworthiness-14860-15.html

 

[Petros]

Were all of the yachts built to this troubling IOR standard at the time? Do IOR compliant hulls suffer the same issues?

What was the rule that caused such poor hull shapes? I found the set of rules, but they are a complex set of dimensional requirements, so not the rule itself, but how was it that pinched bow and sterns gave you an advantage under the rules?

 

[Paul B]

The regulations for spinnaker poles forces the sail to be really full on the upper side.

Nothing about spinnaker pole measurement has anything to do with how full the head of a spinnaker is. As long as the SF and SMW do not exceed JC, and the SMW>0.75*SF, the sailmaker can make the spinnaker as full or flat as he wants.

By the way, J24s are known for nose diving.

What does this have to do with the topic?

A: The J24 was not designed to the IOR rule.

B: The J24 is NOT known for "nose diving". I sailed them quite a bit back in the late 70s and early 80s. Worked for the local dealership. Sailed the North Americans, Midwinters, the West Coast Circuit (which we won overall), and offshore. Sailed in 30+ knot puffs in a big ebb in SF Bay, where half the fleet would crash at once in a big sting. Never been on a J24 that "nose dived". Never saw one do it. Saw many round up, round down, and even one sink after a roundup.


I wonder what will be the next bit of disinformation that you post?

 

[Paul B]

Were all of the yachts built to this troubling IOR standard at the time? Do IOR compliant hulls suffer the same issues?

What was the rule that caused such poor hull shapes? I found the set of rules, but they are a complex set of dimensional requirements, so not the rule itself, but how was it that pinched bow and sterns gave you an advantage under the rules?

The IOR rule went through changes every year, so it is difficult to make a blanket statement.

But you can say basically all boats designed to the IOR rule had issues due to the girth measurements. Basically you had the mid girth measurement. This was taken at the point of maximum beam and set the baseline. Then you had the forward girth station where the girth was a % of the midgirth. Lastly you had the aft girth station, where the girth was a larger % of the midgirth.

The distance between the forward girth station and the aft girth station was the length between girths. This was very powerful in the rule. So the designers would try to move the fwd girth aft (fine bow), and the aft girth forward (fine stern).

Add in the depth measurements, especially at the aft inner girth station, and you ended up with the classic IOR "bustle", and corresponding inflection at the AGS.

Water likes to flow over a nice, fair surface. It doesn't like to make abrupt changes in direction, and making it do so right where the rudder sits is not a very good thing.

 

[Petros]

I do not see where the advantage was to make such a hull as a "rule beater", or was it just a bad set of rules?

 

[Paul B]

I do not see where the advantage was to make such a hull as a "rule beater", or was it just a bad set of rules?

Why do F1 designers design such slow, fiddly cars? Surely a car can be made to go faster?

Why are golf club manufacturers building clubs that don't make the ball go as far or as accurately as they can?

Every attempt at making a rule for racing sailboats has been "flawed". The rules are usually drawn up with the existing fleet in mind. Then the smart guys start picking at it and finding ways to make the water see the boat one way, while the rule sees it in a different way. Eventually things get out of hand, people throw up their hands and walk away, and the rule dies. Then the next rule rises and goes through the same life cycle.

 

[sharpii2]

Were all of the yachts built to this troubling IOR standard at the time? Do IOR compliant hulls suffer the same issues?

What was the rule that caused such poor hull shapes? I found the set of rules, but they are a complex set of dimensional requirements, so not the rule itself, but how was it that pinched bow and sterns gave you an advantage under the rules?

Boats were measured on a theoretical Waterline. The shorter this WL, the lower the rating. Hence the pinched bow and concave stern butts.

There was a good article about the rule in 'Good Old Boat' magazine. Don't remember which issue, but it was last year.

IIRC, they used the 0.15 buttock. The length of that was your rated WL. Pinched ends and hollow stern Buttocks shortened this Buttock length considerably, while changing the actual WL length minimally.

The intent of the rule appears to be to encourage designs that had some cruise ability, and to discourage out and out racing machines, that were designed to plane, and/or had very high ballast/displacement ratios.

The fore triangle and the 'aft' triangle (estimate of the size of the mainsail) were measured, which was a somewhat effective way to limit sail area. The masthead rig was an effective dodge around that, as the jib could extend well past the mast, and was really the most effective sail anyway. This led to back stays, to keep the head stay as tight as possible, and the mast moving further and further aft, to make the jib a larger and larger portion of the rig.

 

[Petros]

IIRC, they used the 0.15 buttock. The length of that was your rated WL. Pinched ends and hollow stern Buttocks shortened this Buttock length considerably, while changing the actual WL length minimally.....The fore triangle and the 'aft' triangle (estimate of the size of the mainsail) were measured, which was a somewhat effective way to limit sail area. The masthead rig was an effective dodge around that, as the jib could extend well past the mast, and was really the most effective sail anyway. This led to back stays, to keep the head stay as tight as possible, and the mast moving further and further aft, to make the jib a larger and larger portion of the rig.

Now this makes sense, thanks sharpii. Unintended consequences resulted in a twitchy down wind ride. I have seen yachts with this kind of hull, and watch one broach right in front of us once in a club race (really scary!). I never did understand why anyone would make a hull this way, it too had a top rigged spinnaker, must have been the old IOR rules. We were in a tiny Thunderbird 26 and actually won that race, despite having yachts much larger than us in the race. It was rough conditions (November in Puget sound), and lots of wind. Perfect day for a broach I guess. It was lots of fun.

 

[souljour2000]

downwind I am sure these are fun boats...and in many other applications no doubt depending on your intended use...but I came across two stranded race sailors and their beached San Juan 24 one day on a nearby beach along Sarasota Bay... It was a blustery Fall day with gusts certainly into the 25 mph range I recall and they had opened up her hull under the vee-berth area from the oil-canning/pounding....they had made it to shore with one guy constantly bailing out the forward bilge...other guy on the helm...they had raced her hard that day apparently...these boats might benefit from a couple added light stringers over those panels in those fore-foot areas...

 

[sean9c]

Sailed a lot of different IOR boats back in the day. Ya, they were flawed but I remember it as a pretty exciting time, lots of new ideas, lots of boats being built, lots of great racing. I'm not sure there has ever been as much activity and intensity since then. There are also a heck of a lot of old IOR boats still sailing and still winning today. Sjre some of the boats were bad but a lot of them were also great sailboats

 

[gonzo]

The later boats were worse than the early ones, in general. Early designs had more balanced hulls. Also, IOR accepted older boats and gave them an extra handicap reduction to account for their age. I raced very successfully a 43' Austral class designed by German Frers in the late 70s. It was built in 1956 or so and we could beat any new boat in wind directions up to 55 degrees or so. The rig was modified to take advantage of the foretriangle measurement and was a topmast with the clew reaching almost to the stern. When tacking I would grab the clew and run forward, around the mast and then aft.

 

[sean9c]

I don't think that's true. Any study of the IOR boats shows that as the years went by rule changes and designer creativity allowed the boats to get wider sterns, less distortion and fractional rigs which had better balance.
How did the 43' get built in 1956 or so when it wasn't designed until the late '70's? How does a topmast have a clew?

The later boats were worse than the early ones, in general. Early designs had more balanced hulls. Also, IOR accepted older boats and gave them an extra handicap reduction to account for their age. I raced very successfully a 43' Austral class designed by German Frers in the late 70s. It was built in 1956 or so and we could beat any new boat in wind directions up to 55 degrees or so. The rig was modified to take advantage of the foretriangle measurement and was a topmast with the clew reaching almost to the stern. When tacking I would grab the clew and run forward, around the mast and then aft.

 

[gonzo]

It was built in 1956 and designed in 55, but I raced it in the 70s. All triangular sails have a clew. On headsails is where the sheets attach to. The rig was modified in the early 70s. The name was GypsyIII with Argentinian flag.

 

[Perm Stress]

The hull with pinched ends at hull speed is quickly loosing form stability:

*the bow wave pics up the buoyant volume at upper part of narrow stem
*the stern wave pics up the buoyant volume at upper part of narrow stern
*the trough between them is loosing the buoyant volume at wide part of hull close to static waterline, only narrow part of lower hull remain immersed.

The end result is that:
-static waterline, diamond shaped, some 6.7m long and 2.6m wide
is changed to
- strip shaped, some 8.5m long and 1.8m wide one.

as righting moment at small heel angles is approximately proportional to beam cubed, decrease in initial stability is dramatic; hence rolling "rails underwater" and all the other joys... .

Wide sterned hull, in contrast, has a stern wave at wide stern, when at hull speed, so initial stability is decreased much much less.
_____________

summa summarum, there is almost nothing radical that could be done to "downwind sailing" roll behavior:
The kind of hull necessarily generate huge bow and stern waves with correspondingly deep trough in between.
Smoother, lower stern, as in post #26, will help somewhat.
(would be interesting to hear first -hand account about results of this modification).

Increased rudder, aft daggerboards, twin rudders, all will help with directional stability, increase roll damping somewhat.

In my humble opinion, that is about all what could be done here.