Desirable and Undesirable Characteristics of Offshore Yachts

[Panoramix]

For points above beam (hove to, fore reach, lie ahull) - Heavy boat/low AR barn door rudder is better.

Going upwind, if you have a fin keel boat with a modern rudder, best option seem to sail the boat very slowly (probably just the storm jib or staysail up) but fast enough to have directional stability.

 

[Fah Kiew Tu]

That's a bit of a left field option but I would consider an unstayed rig :



You get rid of lot of potential failure points by removing the stays!

Yeah and you increase the section size/strength of the masts to compensate.

TAANSTAFL.

Friend of mine has an unstayed junk rig. My rig is bigger and considerably lighter, with stays, and I could likely pull 50kg or more off of my rig mass by swapping from galvanised wire to dyneema.

FKT

 

[plenamar]

I can attest. My previous boat was nearly as good as glass gets, a Hinckley, carefully built and very well sorted.

My current boat is cold molded by Brooklin BY, and, so far as I can tell, better in almost every way. Plus beautiful beyond imagine. 

She's not everybody's cuppa. Since my size limit on boats is driven not by money but by what my wife and I can easily handle by ourselves. She's a 48' with the pricetag of a 60' and the accomodations of a 40' or less, but she is perfect for us, now, while I am still doing a little offshore racing, but mainly cruising with my love of 47 years. 

She finds the aesthetics, inside and out, of the woody to be simply irresistible. I find the sailing characteristics downright addictive. With this  boat I have a boat that sails like I think a boat should sail, looks how I think a boat should look, treats her crew how I think a crew should be treated. 

Back to the original topic, the book really only has a couple of sections on hull design. Jim McCurdy, designer of my Hinckley, I think wrote the section on scantlings, while we have much advancement in materials science, that is still worth reading.

A much neglected area in modern boats, I think, is interior ventilation. My current boat has 4 large dorades and 6 opening hatches with integral bug screens. The Dorades have interior plugs so they may be sealed from inside in the event of truly horrendous weather. The result  is a boat is a joy to be abaord during rainstorms. A stuffy boat is not my idea of fun, but I see so many with only hatches for ventilation. We have a spray over our forward hatch, which allows us to keep it open in moderate conditions offshore, and makes sailing quite pleasant

Photo please !

 

[olaf hart]

The boat is 1100 nm away at the moment, best pic I have on file.

Hard to see the soldier stay, it goes from the inner forestay to a couple of feet aft of the lowers.

 

[olaf hart]

Actuallly, iy we include the lowers, the mast is supported at three levels.

 

[MikeJohns]

Aspect ratios in rudders on normal sailboats range from about 1 to about 8 or so. The rapidly varying region is in the lower numbers. I'm talking here about real AR, not a doubled version that some speak of. The mirror effect requires gap sealing rarely seen on boats. Some of the data from tests that are published is polluted by rake, causing spanwise flow. In classic wind tunnel testing going back to about the Wright Bros., very low AR foils have a lower lift slope and stall at (at best) the same or (usually) lower CL. This has been shown to be because the tip vortex wraps to the low pressure side, having two effects: it delays separated flow on the low side and releases pressure from the high side producing the measured effect. Load or lift is proportional to CL, area, and dynamic pressure; lower CL means lower load.

Now, when operated well beyond stall angles, you may find the lower AR foil producing more load because the flow separation is soft, and filled in by the vortex. At 35 deg angle of attack, an AR 1 rudder may produce more lift than an AR 5 rudder. This is how lifting strakes on jet fighters, crab claw rigs, and hang gliders can fly at very high angles of attack. If you are frequently operating your boat's rudder like that, you have a serious rig balance problem or are under ruddered. 

I'll agree that in a seaway the flow is not an ideal free stream but it isn't that far off either. Most normal rudders behave in practice just as aerodynamic theory suggests they should. 

In a boat that's moving well in relation to the sea, the inflow angles will be close enough even in rough seas.

But if we consider Robust control in rough seas, then a lower AR rudder is going to be better in a seaway at preventing for example a wave  induced broach.  The significantly higher inflow stall angle is the factor.

From empirical data and I'll use Molland (Std NACA foil tc 0.1  free stream):

An aspect ratio of 6 develops 15% more lift than an AR of 2 but stalls at only 18 degrees while the AR 2 stalls at 28 degrees. The lower AR rudder remains effective at a much greater range of inflow angles.

 

[MikeJohns]

Presented like this, this plot would tend to show that there is no strong relationship between being rolled and being on the safe side of this graph! But then these are just 5 data points, so statistically not very robust, it would be more interesting if boats that didn't capsize had been included. This is from a report, may be there was another graph somewhere in there showing the full picture.

Also I am a bit wary of a graph effectively saying you can't go offshore in a yacht less than 7m long as experience shows that it is possible.

It's called casualty analysis there's a lot more casualty data that Wolfston used for it's code.

It shows that they (Wolfston) were pretty much bang on with their risk of inversion criteria when applied to the S-H .

There are two interesting factors if you look at the capsized boats one is the length the other is the stability. Apart from one that's close to the line, they all lie clearly below the minimum stability criteria for their length.

It also suggests that boats within a certain range of length were at risk more than smaller or larger boats. I'll look for the full data if you are interested.

But conditions were miserable. The Young Endevour commander ( Naval Officer on a sail training Ship) recorded 6 m waves with a 6m cross sea and winds of 56 knots . Very unpleasant conditions.

 

[MikeJohns]

Going upwind, if you have a fin keel boat with a modern rudder, best option seem to sail the boat very slowly (probably just the storm jib or staysail up) but fast enough to have directional stability.

But with high AR rudders and lighter boats there's a real danger of the boat being set back on the rudder.

 

[DDW]

That's a bit of a left field option but I would consider an unstayed rig :

You get rid of lot of potential failure points by removing the stays!

There are several instances of boats with unstayed rigs doing a barrel roll and the rig surviving. In fact I am not aware of one that has failed that way, though I imagine there must be an example somewhere. With robust cruising specs, I believe a carbon unstayed rig is lighter, and has a lower CG, than a stayed rig. The components critical to failure are reduced from perhaps 100 to only one. 

 

[estarzinger]

, I believe a carbon unstayed rig is lighter, and has a lower CG, than a stayed rig. 

Sounds nice . . but why are they generally not used for racing? There are open classes where there would be no rating rule to get in the way.

 

[DDW]

In a boat that's moving well in relation to the sea, the inflow angles will be close enough even in rough seas.

But if we consider Robust control in rough seas, then a lower AR rudder is going to be better in a seaway at preventing for example a wave  induced broach.  The significantly higher inflow stall angle is the factor.

From empirical data and I'll use Molland (Std NACA foil tc 0.1  free stream):

An aspect ratio of 6 develops 15% more lift than an AR of 2 but stalls at only 18 degrees while the AR 2 stalls at 28 degrees. The lower AR rudder remains effective at a much greater range of inflow angles.

But it isn't rudder angle that steers a boat, it is rudder lift. And the rudder lift of the high AR rudder is higher. From experience steering both types downwind in a seaway, I can assure you that the more efficient, high AR rudder makes control easier on both helmsman and autopilot. It has more lift, a steeper lift slope, and lower hinge moments, all of which are beneficial. To generate the same steering forces, the lower AR rudder will require much greater movement with higher forces, and ultimately cannot generate as much. 

As a single example, when I rebuilt my rudder, it went from AR 1.82 to AR 2.47, and the area went down from 12.8 to 10.8 sq ft. Everything about the steering improved, even with a 16% reduction in area. 

 

[estarzinger]

Winston Churchill springs to mind, a Percy Coverdale wooden cutter that IIRC was immaculately prepared and loved heavy weather, it was simply in the wrong spot.     

BTW - just ran across this description of the specifics of the Winston Churchill incident while looking for something else. Is rather consistent with your understanding of the incident.

Richard Winning said:-
"We had a mean wind speed prior to losing the vessel of about 55 knots, the highest reading I saw was 60, and I would have thought the mean was around 55 knots. The boat was doing very well, proceeding approximately 180. We were steering and doing about five and a half, six knots, the boat was handling it well, in my opinion, everything was quite satisfactory, I was quite happy the way the boat was going. We then came, we were more or less quartering the seas I suppose at that stage, some were larger than others but none, none particularly frightening, until we got this one breaking just in the wrong spot."

"About 4.30 I should've, I should say was around the time we got hit by this wave. I, I, I'm not, not, never been good at judging the height of waves so I couldn't say what it was except to say it was a good deal higher than the top of the mast so, you know, it'd have to be 60 feet, I should think, not so much the size of the, the wave that concerned me as, as its steepness. It was a very steep wave and breaking at the top when we started to climb up it. We got about halfway up, my intention was to try and just get up as quick as I could and nip right over the top of it, but we just didn't have the pace for that and the shape of the wave didn't, wouldn't have allowed it anyway unless we were going a good deal faster than we were. It picked us up, threw us down on our side. At that stage I was steering, John Dean was on watch with me, sitting beside the helm. That wave picked the, picked the boat up and just threw her down on her side, broke on top of us, John and I were swept over."

The surviving crew members of "Winston Churchill" were all of a like opinion, that is, until the actual wave that caused damage to "Winston Churchill", there had been no cause for alarm and the yacht was handling the weather conditions well.

"Winston Churchill" was not rolled by this wave but was knocked down so severely that serious damage was sustained by her. John Stanley described it as like "hitting a brick wall." "Winston Churchill's" three coach house windows on the port side were smashed. The port side bulwark had been damaged to the extent that approximately two metres (6 feet) had been carried away in the vicinity of the chainplates. But of greater concern was that "Winston Churchill" had been damaged below the waterline on the port side. There is no firm evidence on precisely where or what this damage was. However the survivors believe it was below the port side chainplates, the mast being stepped about one third of the vessel's length from the bow.

 

[DDW]

Sounds nice . . but why are they generally not used for racing? There are open classes where there would be no rating rule to get in the way.

But we were talking about cruising boats?

Racing as you know is very much a strangely faddish yet traditional game. I don't know of an open ocean racing class that does not practically require a jib, do you? In small development classes - dinghies - which are truly open, jibs are long gone, standing rigging is used more to tune sail shapes than to keep the rig standing, and a low CG is not an advantage in a dinghy. Jibs are problematic on a rig with no stays as it is flexible. So that objection which is often lofted my way has many explanations. It took big advances in material science and a huge departure from tradition to have fin keels and spade rudders generally adopted, this occurred over about 50 years, beginning at least 50 years after they were acknowledged superior in performance. As a performance device, an unstayed rig is only practical with carbon and modern sails - so ask me again 50 years hence. 

Or, next time you are in California stop by for a demo.

Do you happen to know the weight of the rig on Hawk? Mast, all standing rigging including jib furler? It would be an interesting comparison, because Hawk and Anomaly are similar mission, similar spec boats. 

 

[Zonker]

the whole school of plexus pan liner structural construction (that ball was then just totally ignored/dropped dropped by the industry).

You want a cheap boat or a good boat.   

I always figured the odds were quite high I would need to do a jury rig after  a roll.

Yes, I figure maybe 1/3 or so survive a roll.

As a young dude I spent a lot of time surfing waves in a long sea kayak. Totally unsuitable for surf activities. Zero rocker. Hard to turn. Super tracking. It really helped my thinking about a boat behaves in breaking seas.

If you're actively steering downwind you might be able to avoid a lot of the breaking crests and thus a knockdown. But only if you're moving fast. If you are moving slow you are just a target.

If your boat has a deep forefoot it will bow steer, lock in and it won't turn as the stern is lifted.  You will have very little control.

Punching your bow into a sea works pretty good until the wave gets too big (say 1/2 the LOA of the boat). Then it just picks you up and throws you backwards upside down. This is not good.

 

[shaggybaxter]

BTW - just ran across this description of the specifics of the Winston Churchill incident while looking for something else. Is rather consistent with your understanding of the incident.

Thanks Estar, glad I hadn't just imagined it. I remember getting hammered on a Swarbirck 42'  in that area for a sold 24 hours, the waves were like falling off cliffs. One thing that stuck with me was how good the dual canted rudders were for bite. As you come over the wave crests the boat would get pinned on its side from the force of the wind and yet the dual rudders gave you enough bite to keep steerage and brutalize the bow over the crest and then fall off the other side.  

My recollection now is it would've taken a hell of a single spade rudder to keep control at those heel angles. 

 

[olaf hart]

Estars Winston Churchill post makes my point, in some circumstances a rollover is inevitable and what matters is how the boat and crew survive.

I recall seeing a paper recently that described the conditions that lead to that sort of rogue wave, it suggested cross seas at 120 degrees was the likely cause.

 

[MFH125]

There are several instances of boats with unstayed rigs doing a barrel roll and the rig surviving. In fact I am not aware of one that has failed that way, though I imagine there must be an example somewhere. With robust cruising specs, I believe a carbon unstayed rig is lighter, and has a lower CG, than a stayed rig. The components critical to failure are reduced from perhaps 100 to only one. 

Hey @DDW, on your boat, are there retaining rings or similar mechanical devices to keep the wing masts from sliding off their bearings should the boat be fully inverted?

 

[estarzinger]

But we were talking about cruising boats?

yea, sure - I was just trying to understand what the drawbacks were and thought that question might reveal them.  People have certainly tried unstayed rigs - freedom and others - and then there was the aero rig which got big PR with quite a number of new highish profile boats built, but then they all seem to fizzle out with little enthusiasm. I dont know anything about it, never sailed one, have no opinion.  It seems like a great idea, and I'm sure your boat sails very well.  But given the history, I guess there must be some significant practical downsides somewhere.

Do you happen to know the weight of the rig on Hawk? Mast, all standing rigging including jib furler?

I don't but I guess Hawk's was quite a bit heavier.  You probably did actual elegant engineering, while we just did 'extra simple and extra strong'.  We also had to deal with outright fraud from our rig supplier - they did not provide the sections I ordered (and paid extra for) but rather some tubes they had sitting in their warehouse (I only discovered this some years after we launched - when I took the rig out of the boat for some work).

 

[estarzinger]

I recall seeing a paper recently that described the conditions that lead to that sort of rogue wave, it suggested cross seas at 120 degrees was the likely cause.

cross waves and/or current (oh yea, and/or odd bottom contour - even in reasonably deep water) are the typical situation . . . but (going a bit far afield) there is also some quantum mechanics theory suggesting that even without those factors individual waves can steal wave energy from the surrounding wave field and become 'rogue size' (eg more than twice significant height).

 

[DDW]

Hey @DDW, on your boat, are there retaining rings or similar mechanical devices to keep the wing masts from sliding off their bearings should the boat be fully inverted?

Yes. The mainmast is retained by the lower bearing which has a retaining collar. The mizzen does not rotate but has features to retain it which I have never actually hooked up. It sits in two applications of Spartite (heel and parteners) and is typically pretty stuck when I try to pull it - one time we lifted the back of the boat off the stands and that mast only weighs 125 lbs. Both are retained secondarily by the halyards, reefing lines, and sheets which turn on the deck to run to the cockpit. 

yea, sure - I was just trying to understand what the drawbacks were and thought that question might reveal them.  People have certainly tried unstayed rigs - freedom and others - and then there was the aero rig which got big PR with quite a number of new highish profile boats built, but then they all seem to fizzle out with little enthusiasm. I dont know anything about it, never sailed one, have no opinion.  It seems like a great idea, and I'm sure your boat sails very well.  But given the history, I guess there must be some significant practical downsides somewhere.

Large unstayed rigs (in modern form) are in a very early state of development compared to a sloop which is quite mature. Nonsuch sold over 1000 boats, so you could call that a success - not many series production lines are more successful - and they have held their value better than most. Freedom sold a lot of boats too, but they churned the design a lot making the fleet pretty disparate in capability and performance - some sail OK, others not so much. Many of those designed by big names as their very first effort into a cat type rig, without much of an understanding of it. Wylie is still dabbling in the space, there is a new 60' daysailer being built with a una rig.  

Aerorig is an interesting one, the rigs they built were exceedingly heavy as they did not understand carbon engineering/construction. I know of one example, a 60' monohull, where the Aerorig supplied spar weighed 3000 lbs, replaced by a Composite Engineering version at under 1000. 

As in anything in boat design, the unstayed rig forces compromises, the main one being a jib is inefficient. The cascade of decisions starting there leads to a non traditional looking rig, and anything non traditional is a hard sell in yachting. In small truly open development classes, jibs have been gone for 50 years. Larger boat racing rules explicitly or practically require a sloop rig, cruising boats follow racing trends, I think that is the short answer.