Desirable and Undesirable Characteristics of Offshore Yachts

[DDW]

This is sort of on my list of things to do. One side benefit of (at least the Raymarine) modern autopilot is the flux gate compass sensor has been replaced by a 9 axis MEMs sensor, and all of the effort needed to develop artificial horizon software was long ago sunk by the drone and cell phone folks. So roll, yaw, pitch, and heading are available at a very high frequency on the NMEA 2K bus. As well rudder angle. There are now cheap devices that record this continuously. My boat is clearly not an UL lifting keel, but it should be easy for someone to collect the data. Go out in the next F8 with your Pogo, run for awhile keel down, then keel up. You could compare roll and raw excursions against rudder angles and probably learn something. 

For the ultimate question, we would need 100 volunteers in 100 Pogos, half with keel up and half down, running in extreme conditions and see how many in each reference class die. But that is an experiment unlikely to get funding. 

I'm a little curious about the structural integrity of keel up operation. Not familiar with the details of the keel  mechanism, but for some that I am familiar with, keel up would not be great. For that matter on some keel down isn't good either, but that is a separate keel integrity discussion. 

 

[Panoramix]

Can you find a stability curve for the RM10.50?  I would be curious how it compares to a Pogo curve, which honestly does not look so terrible with the keel up - more tender certainly, but I also note shaggy's comment that it moved weight distribution aft, which I think, (again I'm not at all any expert on this aspect) would help running in strong stuff - when I was racing light boats we certainly tried to trim weight aft in those conditions..

I would be quite interested if you can find any discussion which included a 'designers comments', say in an interview with an experienced sailor. That would likely be more insightful than a warning in a user manual, which often have a huge legal CYA component to them. I can well imagine the lawyers wanting this to say 'put the damn keel down when you are sailing', and that that might or might not represent best practice in the edge case (downwind in big waves) which most used dont get themselves into. 

I remember seeing it written in Voiles et Voiliers in an artcle comparing shallow draft options where architects were interviewed but I would need to spend a lot of time to find it. I tried briefly to Google it earlier without success.

May be we are collectively wrong but here it is common knowledge.

 

[MikeJohns]

The dynamics of breaking waves calls into question the notion of "tripping" on the keel. I think the Southhampton studies illuminated this. Simplistically, the boat is turned over by a water surface that remains still and parallel to the waterline in the boat's frame of reference. Tripping while broaching has different dynamics.

Yes the whole tripping on the keel is just a meme that perpetuates. It's an urban myth that circulates with no basis in fact.  It's more accurate to say that boats trip over their deck edge, not their keels.

It's not the wave orbital that inverts the boat it's the breaking wave jet. The orbital is not a significant factor.

The boats that were noted by various researchers as completely immune to capsize at one end on the design spectrum were narrow, very heavy and with a massive keel area. The ratio of immersed profile area to topside area is also significant in inversion dynamics.

There's a lot of data and research online now by a variety of researchers with access to facitlies with wave tanks. In the UK, various at Southhampton, Japan (Osaka uni, Nomoto), Netherlands (deKat), and Australia (Renilson). 

 

[Panope]

.......The boats that were noted by various researchers as completely immune to capsize at one end on the design spectrum were narrow, very heavy and with a massive keel area. The ratio of immersed profile area to topside area is also significant in inversion dynamics........

Just to clarify, are we talking hulls like the old pilot cutters and "plank on edge" cutters?

Lore as it that they could stand to ANY weather.  Maybe true? 

 

[Jud - s/v Sputnik]

A lot of this recent discussion is, I humbly admit, going over (well, has already gone ) over my head.

Maybe I need to dig into “Oceanography and Seamanship” (which might more properly be titled “Physical Oceanography and Seamanship”, since it deals with that particular branch of oceanography as applied to seamanship, and not the other two branches, chemical and biological oceanography).

I got the book a while ago and never started it.  Written by a prof at Scripps Institute of Oceanography, who’s been to sea in everything from ships to small boats - the book ranges from understanding global ocean circulation and wave action on shorelines, to yacht design and stability characteristics, etc.

Looks like a serious but interesting read - interesting for sailors because it’s applied (physical) oceanography.  I’ll report back when I’m done and hopefully add something to this discussion on stability.....or not...   (This second edition was issued after the Fastnet Race disaster, so incorporates some observations from that.)

 

[estarzinger]

 Dorm . . . . 

Dorn is interesting

as is the Wolfson Unit (and other) tank testing.

Such 'theoretical' stuff needs to confirmed in the 'real world'.

In this case - we seem to have two different schools of 'real world' experience with lifting keels - from the South, and what Panoramix describes as 'common knowledge' in France.  That does not make either necessarily wrong - I would guess they represent quite different sets of designs and quite different common conditions.

As to keel tripping . . . as I said above, the more important/primary issue to me seems to be directional stability on steep wave faces.  I guess I think that is clear.  But I personally would not be comfortable writing 'tripping as a secondary/contributing cause' off as a complete myth - perhaps, but it seems too facile for a complicated hydrodynamic situation.

My understanding of the " narrow, very heavy and with a massive keel area" was that they did not stay inverted; NOT that they were immune to knockdown.  In fact, I believe the conclusion was definitive that ALL designed can be rolled.

Jud - I actually dont know - is your boat a lift keel? If so, what sort of design for the keel and housing?

 

[shaggybaxter]

I'm a little curious about the structural integrity of keel up operation. Not familiar with the details of the keel  mechanism, but for some that I am familiar with, keel up would not be great. For that matter on some keel down isn't good either, but that is a separate keel integrity discussion. 

Yep, my dislike about keel up mode under sail as it seems illogical from a forces point of view. My keel head is only a single bolt through an eralyte bearing, so the mating of the keel box and head have to be structural, ie: it must 'fix' the keel in place.

In the perfect world, if I was sailing keel up I'd want the same structural integrity from the keel /keel box, and that, I assume, would be a tall order when supporting the resultant forces from a couple of ton near horizontal in the water.  



Apologies for the thread drift, this is a really good thread. 

 

[ProaSailor]

As to keel tripping . . . as I said above, the more important/primary issue to me seems to be directional stability on steep wave faces.  I guess I think that is clear.  But I personally would not be comfortable writing 'tripping as a secondary/contributing cause' off as a complete myth - perhaps, but it seems too facile for a complicated hydrodynamic situation.

As far as I know, it's been common lore for 50+ years(?) that bluewater multihulls, when sailing downwind or in rough seas from unfavorable directions, will raise their main (forward, deepest) daggerboard(s) to improve directional stability.  It has no effect on their righting moment since they have no ballast.

 

[Veeger]

As far as I know, it's been common lore for 50+ years(?) that bluewater multihulls, when sailing downwind or in rough seas from unfavorable directions, will raise their main (forward, deepest) daggerboard(s) to improve directional stability.  It has no effect on their righting moment since they have no ballast.

My experience with my 40' cat with daggerboards and no stub keels was that, under power with the boards up, there was noticeably less directional stability (i.e. the helmsman or autopilot worked much harder to maintain a stable course... even in flat water).  Consequently, I always ran with about half boards when under power or running downwind.

 

[MikeJohns]

Just to clarify, are we talking hulls like the old pilot cutters and "plank on edge" cutters?

Lore as it that they could stand to ANY weather.  Maybe true? 

The Bristol channel pilot cutters particularly, noted historically and by more modern researchers as being a capable and seaworthy craft. They needed a turn of speed but still able to hold station in rough weather. Variations on the full keel such as cutting away the forefoot were tried but weren't popular with the operators.

The type was easily handled  and had an extreme operational requirement;  able to face the full force of an Atlantic gale and could be hove-to on station in all weather while awaiting an incoming ship and getting the pilot on board. 

None was ever noted to have been lost to the sea, only to collision usually being run down by a ship they were hove-to waiting for !

Also Andy Claughton of Wolfston in his "Investigation of stability of sailing yachts"  and his other "yachts in breaking waves" noted with repeated tests that the full keel models surfed with excellent directional control in large breaking waves. They also showed no tendency to invert in breaking waves that well exceeded the beam, waves that inverted every fin keel models tested, usually with a height close to the crafts beam .

It dispelles "keel tripping" as a factor.

In the 98 Sydney Hobart coroners inquest Andy Claughton was an expert witness. He upseat a few people with his comments that people had been told often enough specifically what makes a boat un-seaworthy and that they just don't seem to get it !  

 

[DDW]

It's not the wave orbital that inverts the boat it's the breaking wave jet. The orbital is not a significant factor.

Of course the orbital would not invert the boat, but it will heel it some amount - and in a modern hull form that could be quite a bit - making it more vulnerable to the jet. Very hard to separate the two factors, as the orbital is always present, so "not a significant factor" is a difficult thing to demonstrate. 

There are so many differences between a BC cutter and a modern hull form, it would be hard to single out one as the cause of any difference in characteristics. Among them the beam is narrow compared to length and roll moment of inertia, so wave height compared to beam may lack something as a metric to compare. Another might be keel depth which might be double on the modern hull form. 

 

[DDW]

 

[Jud - s/v Sputnik]

Jud - I actually dont know - is your boat a lift keel? If so, what sort of design for the keel and housing?

No, not a lift keel.  (Fortunately - as convenient as those maybe, seems like a potentially big maintenance problem). Big fat fin keel.  (With features both desirable —holds lots of fuel; supports boat standing up, with drying legs— and undesirable: it’s not exactly a modern foil shape with lots of lift  )

 

[Panoramix]

In this case - we seem to have two different schools of 'real world' experience with lifting keels - from the South, and what Panoramix describes as 'common knowledge' in France.  That does not make either necessarily wrong - I would guess they represent quite different sets of designs and quite different common conditions.

Don't get me wrong, with a centreboard, I would lift it as people who owns centreboard boats do... I am aware of the interest of minimising "foil area" in big weather, it is just that when it is a matter of choosing between righting moment and "less foil in the water", I choose righting moment.

The concept of adapting the centre board to the conditions is also present here, a 1980s boat that still sells at high prices pushed the concept far and is well liked :

 

[estarzinger]

Also Andy Claughton of Wolfston ....  showed no tendency to invert in breaking waves 

I

Really, (as I said above) you are rather overstating that - they concluded that all the designs could be rolled  - here is a scan of the key section of the summary . . . 

 

[CapDave]

Also Andy Claughton of Wolfston in his "Investigation of stability of sailing yachts"  and his other "yachts in breaking waves" noted with repeated tests that the full keel models surfed with excellent directional control in large breaking waves. They also showed no tendency to invert in breaking waves that well exceeded the beam,

So - I've owned such a boat (see 1982 picture below) and it was 10 tons of an utter pig to sail for 3,000 miles. I never had her in breaking waves, but I clearly recall that dead downwind in rough weather, the waves overtaking the boat pushed the rudder around and made the tiller a menace - you couldn't really hold it against the water pressure, and the boat became progressively harder to steer. 

And (I think for the second time in this thread) I'd point out the 1957 experience of Miles & Beryl Smeeton in their 46' full keel ketch Tzu Hang who pitchpoled the boat twice in two successive attempts to round Cape Horn, only succeeding on their third 1968 try (shakes head....). Here is a link to an article about it:

https://dragdevicedb.com/drogues-on-monohulls/dm-1-monohull-bermuda-ketch

Also worth noting that on their first try they had with them John Guzzwell, who amateur built a 20' Laurent Giles yawl and leaving Canada in 1955 sailed it singlehanded around the world in stages over four years.

These folks along with Humphrey Barton (1950 Atlantic crossing in a Laurent Giles Vertue 25), the Hiscocks (first trip around in 1952 in an engineless 30' Laurent Giles sloop), and a handful of others are some of the real pioneers of small boat ocean cruising back when people doubted it was possible - they tend to be forgotten behind the more publicized younger sailors who stood on their shoulders. 

 

[kinardly]

Please forgive the diversion. I can certainly see how a following wave would yank that rudder around like a barn door in a hurricane. I wonder if a semi balanced spade would fare better? 
 

OK, I’ll shut up now. Great thread BTW.

 

[CapDave]

As far as I know, it's been common lore for 50+ years(?) that bluewater multihulls, when sailing downwind or in rough seas from unfavorable directions, will raise their main (forward, deepest) daggerboard(s) to improve directional stability.  It has no effect on their righting moment since they have no ballast.

We've found our Atlantic 57 steers better with a little daggerboard down off the wind. 

 

[CapDave]

Also Andy Claughton of Wolfston in his "Investigation of stability of sailing yachts"  and his other "yachts in breaking waves" noted with repeated tests that the full keel models surfed with excellent directional control in large breaking waves. They also showed no tendency to invert in breaking waves that well exceeded the beam, waves that inverted every fin keel models tested, usually with a height close to the crafts beam .

One more full keel story. My first real offshore trip was on an amateur built engineless steel Thomas Colvin 48' Pinky Gaff Schooner called Papillon. I joined the owners in the Fall of 1979 in Southwest Harbor, ME to sail to Cape Henry. We sailed out into a 3-day SW'ly blow (hey, I wasn't the captain, but I did see and not like the forecast and went on anyway....).

When it got too rough to make headway we hove to with a reefed foresail on a starboard tack. As the wind built we furled the foresail, hove to under bare poles - but really we were more lying ahull at that point. At the height of the storm there was so much water falling on the boat that it was basically 2-3 feet underwater for about five or six hours, never clearing the decks. If we'd blown a hatch off we'd have been fighting for our lives - 1 point for steel. Throughout the experience I always felt like the boat was managing OK, the motion while violent made sense to me - except for about two hours at the utter peak when it seemed to me the boat was starting to struggle a little to stay rightside up in the bigger waves/heavier water.

The weather subsided, the captain shot a few sun sights - we'd missed Cape Sable by 30 miles, and we were 150 miles closer to Bermuda than Cape Henry. Like 1/4 of the way across the North Atlantic! Everything that had been lashed on deck (there was a lot of stuff!) was gone except the tender.

So we headed for Bermuda. When we got there, turns out the steel pipe masts were too heavy, the boat just fell over close hauled, and we couldn't make progress upwind at all to enter the harbor. We attracted attention with a signal mirror to shore, and the Coasties came out and towed us in.

How much wind/wave? Really I've no idea. By FAR the roughest sustained weather in my subsequent 70K+ miles of sailing. As a swag I'd say we saw lots of 50 knots/20' sustained, at least some 60 knots/25' sustained and definitely gusts to 70+ and some much much bigger waves mixed in. We basically left the boat completely to herself, and she managed.

In 2011 the owner/builder's son put her on the beach at Fire Island, though I understand she was subsequently salvaged and refitted.

 

[ProaSailor]

We've found our Atlantic 57 steers better with a little daggerboard down off the wind. 

I'm not surprised.  Raising a long (9 feet draft) daggerboard to match rudder draft reduces draft and wetted surface substantially.  The significant feature is that very little mass moves in the daggerboard, which could be done on monohulls too, but is a given on multihulls.

Heavy Weather Strategies When Sailing a Catamaran
https://www.sailmagazine.com/multihulls/heavy-weather-strategies-when-sailing-a-catamaran
PETER JOHNSTONE - UPDATED:MAR 27, 2019 - ORIGINAL:JUL 30, 2014

On a performance cat, you can raise both boards and experience perhaps the greatest days of your sailing life. Speed is your friend. A modern performance cat’s bows will rise as she gains speed, and there are no downsides to going faster. The closer you sail to the wave train’s speed, the smoother the ride becomes, and the less chance you have of experiencing a large wave impact. The sail selection can be a deeply reefed mainsail or a jib sheeted to the outboard rail. Even in 50-60 knots of wind, the ride will be smooth and comfortable as you sail at 15-25 knots. Always check helm balance to keep the rudders and pilots lightly loaded.