Re: "death roll"
I believe, if I recollect correctly, that this has something to do
with a change in the type of sail aerodynamics at play, and a change
in LIFT from that generated by airfoil attached laminar and
turbulent flows to that generated by vortex flows. A totally
different way.
Firstly airflow reverses and in a triangular sail a vortex on the
lee of the sail will appear from the clew to the tack. This
powerfully lifts the foot, and in a dinghy if this doesn't dump you
to windward be ready for what happens next! It reverses.
Heeled well to windward, the first vortex disappears but now
another appears from the leaning mast top, from the head to the
tack. This now powerfully lifts what would ordinarily be the luff
quickly heeling you to leewards. If this doesn't dump you to
leewards, get ready, for if not remedied the cycle will repeat, and
with increasing force.
The rocking sideways movement of the sail quickly increases the
apparent wind from that of the usual downwind stalled or nearly
stalled flow and excites or self induces the oscillating vortices
and hence the rolling. This can happen in fairly light air and in my
experience wave action can add to it, but it can occur in relatively
flat water.
A quadrilateral sail like a sprit or gaff may also have vortices
operating from peak to throat in this situation.
When sailing by the lee air flow is reversed and the clew
becomes something like the tack of a crabclaw sail, spinning off a
vortex like a delta airfoil and increasing drive.In this case the
usual luff becomes the leech.
This doesn't just dump dinghies quick as a flash, but can capsize
large keel sailboats in a few sidesways rocks too. Hence the
name "downwind death roll". Vortex lift is very powerful and is used
to advantage in delta wing planes like Concorde, and in the Pacific
or Oceanic lateen or crab claw sail as used on proas. C A Marchaj
in 'Sail Performance' found that on a reach the driving force
coefficient of the crab claw is about 190 per cent of a Bermuda rig.
So you'd get about the same power from a sail half as big! The crab
claw or pacific lateen has 2 vortexs on the lee of the sail, 1
generated by the yard and 1 by the boom. The usual or Mediteranean
lateen, like on PCB&F's Zephyr and Storm Petrel will have only 1
vortex generated by the yard, if at all.
Frank Bethwaite ( or Peter or Julian, any way the father) designer
of the Olympic 49er in his comprehensive 'High Speed Sailing' tells
exactly what self induced oscillation is, how to avoid it, how to
get out of it, and how to use it. He also, I remember, tells how in
light air to get more power from sailing by the lee.
regards
graeme
- Inbolger@yahoogroups.com, "Wayne Gilham" <wgilham@h...> wrote:
with a change in the type of sail aerodynamics at play, and a change
in LIFT from that generated by airfoil attached laminar and
turbulent flows to that generated by vortex flows. A totally
different way.
Firstly airflow reverses and in a triangular sail a vortex on the
lee of the sail will appear from the clew to the tack. This
powerfully lifts the foot, and in a dinghy if this doesn't dump you
to windward be ready for what happens next! It reverses.
Heeled well to windward, the first vortex disappears but now
another appears from the leaning mast top, from the head to the
tack. This now powerfully lifts what would ordinarily be the luff
quickly heeling you to leewards. If this doesn't dump you to
leewards, get ready, for if not remedied the cycle will repeat, and
with increasing force.
The rocking sideways movement of the sail quickly increases the
apparent wind from that of the usual downwind stalled or nearly
stalled flow and excites or self induces the oscillating vortices
and hence the rolling. This can happen in fairly light air and in my
experience wave action can add to it, but it can occur in relatively
flat water.
A quadrilateral sail like a sprit or gaff may also have vortices
operating from peak to throat in this situation.
When sailing by the lee air flow is reversed and the clew
becomes something like the tack of a crabclaw sail, spinning off a
vortex like a delta airfoil and increasing drive.In this case the
usual luff becomes the leech.
This doesn't just dump dinghies quick as a flash, but can capsize
large keel sailboats in a few sidesways rocks too. Hence the
name "downwind death roll". Vortex lift is very powerful and is used
to advantage in delta wing planes like Concorde, and in the Pacific
or Oceanic lateen or crab claw sail as used on proas. C A Marchaj
in 'Sail Performance' found that on a reach the driving force
coefficient of the crab claw is about 190 per cent of a Bermuda rig.
So you'd get about the same power from a sail half as big! The crab
claw or pacific lateen has 2 vortexs on the lee of the sail, 1
generated by the yard and 1 by the boom. The usual or Mediteranean
lateen, like on PCB&F's Zephyr and Storm Petrel will have only 1
vortex generated by the yard, if at all.
Frank Bethwaite ( or Peter or Julian, any way the father) designer
of the Olympic 49er in his comprehensive 'High Speed Sailing' tells
exactly what self induced oscillation is, how to avoid it, how to
get out of it, and how to use it. He also, I remember, tells how in
light air to get more power from sailing by the lee.
regards
graeme
- Inbolger@yahoogroups.com, "Wayne Gilham" <wgilham@h...> wrote:
> I once had a good 'ol "death-roll" experience, with a (four-cornered)
> double-spritsail, sailing mostly downwind (about 150 degrees),both head and
> clew held out by sprit-booms, and I can assure you the lower yardholding
> the clew was DEFINITELY set up to be self-vanging (purchase offorward-end
> of that yard or boom came quite high up the mast / luff of thesail...).
> Interesting that the boat finally rolled over, yep spilled me andthe s.o.
> right out, INTO (not away from) the wind!swings so
>
> I asked Phil about the experience, he noted (if I can remember this
> correctly) that what can happen is that the upper yard's top end
> far out to windward until it's even kinda forward of the boat,that the sail
> up near the top develops a force-vector ACROSS the bow,essentially opposite
> the direction that the wind's coming from -- and this pulls therig INTO the
> direction that the wind's coming from behind, instead of away fromthe wind,
> setting up that weird and frightening "pumping", each-swing-worse,till the
> kinda-forward position of the upper yard and the upper part of thesail
> eventually overpowers the lower portion of the sail in one of thoseever "lost it" by
> oscillations.
>
> Would like to know if others with "death roll" experiences
> rolling the boat to the windward side rather than to the leewardside when
> going downwind but slightly off the wind, as I described?
>
> Wayne Gilham
>
> [Non-text portions of this message have been removed]
--- Inbolger@yahoogroups.com, "Peter Lenihan" <peterlenihan@h...> wrote:
Schooner at Kingston???
;^)
Scott Calman
> --- Inbolger@yahoogroups.com, "Wayne Gilham" <wgilham@h...> wrote:Is it true that Bruce Hector plans on re-enacting this with the TIMS
> > Would like to know if others with "death roll" experiences
> ever "lost it" by
> > rolling the boat to the windward side rather than to the leeward
> side when
> > going downwind but slightly off the wind, as I described?
> >
> > Wayne Gilham
>
Schooner at Kingston???
;^)
Scott Calman
I used to have a problem with windsurfers. In strong wind, the mast
would get a bend (I think) and the top of the sail would suddenly lift
the wrong way and just dump me in the water. It wasn't a rhythmic roll
and I seem to recall it happened while close hauled.
would get a bend (I think) and the top of the sail would suddenly lift
the wrong way and just dump me in the water. It wasn't a rhythmic roll
and I seem to recall it happened while close hauled.
>MWayne Gilham wrote:
>snip
>
>Would like to know if others with "death roll" experiences ever "lost it" by
>rolling the boat to the windward side rather than to the leeward side when
>going downwind but slightly off the wind, as I described?
>
>Wayne Gilham
>
--- Inbolger@yahoogroups.com, "Wayne Gilham" <wgilham@h...> wrote:
been "scared to death" a few times,many many moons ago,when first
teaching myself to sail:-)
However,I wonder if sailing a yawl rigged boat(like a Micro) down
wind with the mizzen sheeted opposite to the main,doesn't sorta
cancel out the effect as you described,even if it means "sailing by
the lee" with the mizzen.
Once the mizzen jibes,I know to look out for my main and either
change heading or adjust the mainsheet :-)
I'd like to hear what other yawl sailors have experience too.
Sincerely,
Peter Lenihan,ex-Micro sailor and Elver sailor who can not imagine
owning a sailboat with a mizzen..........
> Would like to know if others with "death roll" experiencesever "lost it" by
> rolling the boat to the windward side rather than to the leewardside when
> going downwind but slightly off the wind, as I described?Can't say that I have ever experienced this "effect" although I have
>
> Wayne Gilham
been "scared to death" a few times,many many moons ago,when first
teaching myself to sail:-)
However,I wonder if sailing a yawl rigged boat(like a Micro) down
wind with the mizzen sheeted opposite to the main,doesn't sorta
cancel out the effect as you described,even if it means "sailing by
the lee" with the mizzen.
Once the mizzen jibes,I know to look out for my main and either
change heading or adjust the mainsheet :-)
I'd like to hear what other yawl sailors have experience too.
Sincerely,
Peter Lenihan,ex-Micro sailor and Elver sailor who can not imagine
owning a sailboat with a mizzen..........
craig o'donnell wrote:
the reverse to be true on the laser (very prone to
the dreaded deathroll). What happens is that the
boat swings around a bit and then the daggerboard
digs in and "trips" the boat. Next thing you know
you are lying on the sail. Keeping the centreboard
up lets the boat slew around a bit. Took me a while
to work this out, it is kind of counter-intuitive.
I think someone else mentioned sailing slightly by
the lee. Bring the boom in a little and the sail's
centre of effort moves forward and closer to the
boat's centerline, which helps tame the speed wobbles.
This should work for other sail types as well I would
think.
Bruce Fountain
Senior Software Engineer
Union Switch & Signal
Perth, Western Australia
> Keeping the CB down helps.This might be true on many dinghies, but I found
the reverse to be true on the laser (very prone to
the dreaded deathroll). What happens is that the
boat swings around a bit and then the daggerboard
digs in and "trips" the boat. Next thing you know
you are lying on the sail. Keeping the centreboard
up lets the boat slew around a bit. Took me a while
to work this out, it is kind of counter-intuitive.
I think someone else mentioned sailing slightly by
the lee. Bring the boom in a little and the sail's
centre of effort moves forward and closer to the
boat's centerline, which helps tame the speed wobbles.
This should work for other sail types as well I would
think.
Bruce Fountain
Senior Software Engineer
Union Switch & Signal
Perth, Western Australia
I have an 86sq ft spritsail. I have experienced most "death rolling"
when sailing without a boom dead downwind. This is cured by sailing
slightly off dead down wind. However, I have since put a sprit boom
on the sail and have no troubles with rolling at all. Whenever I
begin to feel a slight bit, I trim the sail in until it disappears.
The sprit boom really tames the sail and I prefer it to a regular
dinghy boom set up. Dennis
when sailing without a boom dead downwind. This is cured by sailing
slightly off dead down wind. However, I have since put a sprit boom
on the sail and have no troubles with rolling at all. Whenever I
begin to feel a slight bit, I trim the sail in until it disappears.
The sprit boom really tames the sail and I prefer it to a regular
dinghy boom set up. Dennis
Sure.
Haven't done accurate statistics, but my feeling is that most capsizes are
to windward. The expected knock-down to leeward is sort of self-limiting
since the further one heels, the less sail area sees the wind.
I mentioned "meeting the boom with the tiller" to stop an incipient jibe.
It works for the "death roll" too.
Roger
derbyrm@...
derbyrm.mystarband.net/default.htm
Haven't done accurate statistics, but my feeling is that most capsizes are
to windward. The expected knock-down to leeward is sort of self-limiting
since the further one heels, the less sail area sees the wind.
I mentioned "meeting the boom with the tiller" to stop an incipient jibe.
It works for the "death roll" too.
Roger
derbyrm@...
derbyrm.mystarband.net/default.htm
----- Original Message -----
From: "Wayne Gilham" <wgilham@...>
<snip>
> Interesting that the boat finally rolled over, yep spilled me and the s.o.
> right out, INTO (not away from) the wind!
>
<snip>
>
> Would like to know if others with "death roll" experiences ever "lost it"
by
> rolling the boat to the windward side rather than to the leeward side when
> going downwind but slightly off the wind, as I described?
>Would like to know if others with "death roll" experiences ever "lost it" byYep. Sure have. Capsize to windward is the classic Dinghy Death Roll.
>rolling the boat to the windward side rather than to the leeward side when
>going downwind but slightly off the wind, as I described?
Keeping the CB down helps. Not letting the yard get forward of the mast
helps. But this can happen even on triangular sails.
--
Craig O'Donnell
Sinepuxent Ancestors & Boats
<http://freepages.genealogy.rootsweb.com/~fassitt/>
The Proa FAQ <http://boat-links.com/proafaq.html>
The Cheap Pages <http://www.friend.ly.net/~dadadata/>
Sailing Canoes, Polytarp Sails, Bamboo, Chinese Junks,
American Proas, the Bolger Boat Honor Roll,
Plywood Boats, Bamboo Rafts, &c.
_________________________________
-- Professor of Boatology -- Junkomologist
-- Macintosh kinda guy
Friend of Wanda the Wonder Cat, 1991-1997.
_________________________________
It's quite normal Wayne, using any rig.
I have managed this quite neatly sailing a Finn and also a Moth, both
with modern triangular mainsails, with super-tight boom vangs.
When it looks like it could happen, pull the mainsheet in a bit; that
dampens it down if you are lucky!
DonB
I have managed this quite neatly sailing a Finn and also a Moth, both
with modern triangular mainsails, with super-tight boom vangs.
When it looks like it could happen, pull the mainsheet in a bit; that
dampens it down if you are lucky!
DonB
--- Inbolger@yahoogroups.com, "Wayne Gilham" <wgilham@h...> wrote:
> I once had a good 'ol "death-roll" experience, with a (four-
cornered)
> double-spritsail, sailing mostly downwind (about 150 degrees), both
head and
> clew held out by sprit-booms, and I can assure you the lower yard
holding
> the clew was DEFINITELY set up to be self-vanging (purchase of
forward-end
> of that yard or boom came quite high up the mast / luff of the
sail...).
> Interesting that the boat finally rolled over, yep spilled me and
the s.o.
> right out, INTO (not away from) the wind!
>
> I asked Phil about the experience, he noted (if I can remember this
> correctly) that what can happen is that the upper yard's top end
swings so
> far out to windward until it's even kinda forward of the boat, that
the sail
> up near the top develops a force-vector ACROSS the bow, essentially
opposite
> the direction that the wind's coming from -- and this pulls the rig
INTO the
> direction that the wind's coming from behind, instead of away from
the wind,
> setting up that weird and frightening "pumping", each-swing-worse,
till the
> kinda-forward position of the upper yard and the upper part of the
sail
> eventually overpowers the lower portion of the sail in one of those
> oscillations.
>
> Would like to know if others with "death roll" experiences
ever "lost it" by
> rolling the boat to the windward side rather than to the leeward
side when
> going downwind but slightly off the wind, as I described?
>
> Wayne Gilham
>
> [Non-text portions of this message have been removed]
I once had a good 'ol "death-roll" experience, with a (four-cornered)
double-spritsail, sailing mostly downwind (about 150 degrees), both head and
clew held out by sprit-booms, and I can assure you the lower yard holding
the clew was DEFINITELY set up to be self-vanging (purchase of forward-end
of that yard or boom came quite high up the mast / luff of the sail...).
Interesting that the boat finally rolled over, yep spilled me and the s.o.
right out, INTO (not away from) the wind!
I asked Phil about the experience, he noted (if I can remember this
correctly) that what can happen is that the upper yard's top end swings so
far out to windward until it's even kinda forward of the boat, that the sail
up near the top develops a force-vector ACROSS the bow, essentially opposite
the direction that the wind's coming from -- and this pulls the rig INTO the
direction that the wind's coming from behind, instead of away from the wind,
setting up that weird and frightening "pumping", each-swing-worse, till the
kinda-forward position of the upper yard and the upper part of the sail
eventually overpowers the lower portion of the sail in one of those
oscillations.
Would like to know if others with "death roll" experiences ever "lost it" by
rolling the boat to the windward side rather than to the leeward side when
going downwind but slightly off the wind, as I described?
Wayne Gilham
[Non-text portions of this message have been removed]
double-spritsail, sailing mostly downwind (about 150 degrees), both head and
clew held out by sprit-booms, and I can assure you the lower yard holding
the clew was DEFINITELY set up to be self-vanging (purchase of forward-end
of that yard or boom came quite high up the mast / luff of the sail...).
Interesting that the boat finally rolled over, yep spilled me and the s.o.
right out, INTO (not away from) the wind!
I asked Phil about the experience, he noted (if I can remember this
correctly) that what can happen is that the upper yard's top end swings so
far out to windward until it's even kinda forward of the boat, that the sail
up near the top develops a force-vector ACROSS the bow, essentially opposite
the direction that the wind's coming from -- and this pulls the rig INTO the
direction that the wind's coming from behind, instead of away from the wind,
setting up that weird and frightening "pumping", each-swing-worse, till the
kinda-forward position of the upper yard and the upper part of the sail
eventually overpowers the lower portion of the sail in one of those
oscillations.
Would like to know if others with "death roll" experiences ever "lost it" by
rolling the boat to the windward side rather than to the leeward side when
going downwind but slightly off the wind, as I described?
Wayne Gilham
[Non-text portions of this message have been removed]