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Thread: solving a mystery
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7th March 2011, 01:14 PM #46
Howdy,
You misinterpret the problem. Someone who knows sailing will be aware of what the thread is discussing in quite a different way.
There are boats which measured by the standards of metacentric height and vessel stability should theoretically be a lot less stable than the boat being discussed here.
But they are satisfactory sailing boats.
So it becomes an interesting question to try to work out exactly why there are possibly specific problems with this boat or whether it can be changed by some detail or rigging or setup or sailing technique.
For example, I wonder what you have to say about the metacentric height of this.
This type of boat ended up with the final dimensions of 11ft long and about 12" wide. Later they sprouted hydrofoils, but that is another story.
The people involved in this thread have the knowledge and ability to wrap their heads around this type of sailing with a bit of practice.
So just maybe looking at what is being discussed here with an open mind is the best approach.
Best wishes
Michael Storer
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7th March 2011, 11:36 PM #47
Thank you MIK,
Hoping that discuss about boat's stability can be continued without obstacles ...
There is an illustration of kayaks/sailing canoes stability (at sea and for my planed trip also).
Kayaks which proveniences directly from eskimo umiak have low metacentric height (centre of weight positioned near waterline) and they are really stable at sea:
[ame="http://www.youtube.com/watch?v=4ZYe0h3EypE"]YouTube - Flinders Island Sea Kayaking, Tasmania[/ame]
There is a my BETH "YuanFen" - centre of weight is high that weight of spars and skipper are higher than previously. Than metacentric height is bigger. To this we add the power of the wind and the heeling moment that active hiking is needed:
General dimensions of those boats are a bit similar - kayaks are longer and smoother, initial stability of BETH is better, but in the final analysis - thanks of bigger metacentric height - kayak is really more stable at sea.
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12th March 2011, 11:22 AM #48
Robert,
If I contributed anything of value, it was from confused ignorance rather than informed wisdom.
Now... my bouyancy bags (gave up on sourcing thick vinyl to glue up and am using car inner tubes) are in the pointy bits only... a capsize test in shallow water shows them being of use only when the canoe has a lot of water in it. e.g. when they are in the water.
Side bouyancy is on my to do list, along with more practice on getting my 110kg in a bit more gracefully (think 'slide and wriggle' rather than muscling my way back in).
BTW, I now know that I should have modified MIK's design to allow for stern/bow bouyancy tanks as that would be lighter than the car inner tubes. The benefit of tubes is that you can pick the inner tube size and blow them up to give the 'right' amount of bouyancy, whereas a deck tank is 'static' in dimensions.
Benefit of hindsight I guess...
apologies, this post is related to the thread, but could also go in the drop in sail or QC tips threads... I may be sidetracking things
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12th March 2011, 07:44 PM #49
Clinton,
I'm not an expert
As you've seen the video - side buoyancy are important for restoring the correct position - much less water is inside the boat.
Once we (in my country) used motorcycle inner tubes as a side buoyancy - they are easier to use (inside the cockpit arrangement) than the automobile inner tubes.
You can use styrofoam bloks also - they can be cut into appropriate shapes and fitted into the boat.
BETH (Raid41 also) has built in air (buoyancy) chambers but no foam there (attachement - sorry for Polish description) - you can see them red hatched, and waterproof luggage compartments green hatched - so not much water remains inside the cockpit.
***
For my planned sailing on the sea I consider adding air or styrofoam buoyancy in the cockpit to reduce the amount of water thrown by the waves.
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18th March 2011, 01:05 AM #50
so, returning to page 1 of this thread (which has wandered OT just a little...)
As I understand it, the thinking is that R41 capsize was likely a combination of
slab side of the fine bow 'tripping' on the water, multiplied by narrow bottom
plus high crew weight, happening well forward of a really grippy centreboard.
And probably some other factors like rudder lift opposing the broach vector,
but actually assisting the capsize vector.
Theory is then, using OH&S principles, eliminating one or more of the
contributors is likely to reduce/eliminate the problem. Can't do much about
the high crew weight due to needing dry sleeping space, so...
Possibilities...
1. A Rainbow scow style pram bow changes the bow dynamics - brings the
tripping hazard aft to where it is negated by the centreboard, but keeps the
simplicity of concept. Also increases stability forward. Penalty is a shorter
waterline length, and a noisier, wetter ride in slop. Also noisier to sleep in but
hey, if it doesn't fall over... Sleep-time noise-cancelling by warping a sheet of
foam or carpet under the bow ?
2. A fuller bow to float better, resisting diving, & hopefully less prone to tripping?
3. A wider bottom increases static stability, and creates a fuller bow which
-may- float rather than dig-in. BUT loses the ease of on-land handling. And
does nothing to assist re-boarding, which IIRC is why the project was canned.
4. A 45degree chine panel tapering from 0" near midships up to maybe 3-6"
high at the bow complicates the build, but would make it vastly less likely to
dig-in. Would need to make the bow above it fuller, and reduce rocker below
it to compensate for loss of volume. Both of these might be good things?
5. Bring the main mast aft a foot or two & aim for a more "balanced" balanced
lug to reduce broaching thrust vectors ?
6. A bigger rudder might resist a tripping bow better, but would also 'lift' the
bottom in the direction of capsize... possibly the two cancel out completely
yielding no benefit. so...
7. taking a leaf out of Bolger's "Dart" and "Cartopper" desgns - move the CB
forward & reduce its size, compensating for reduced 'lift' with a larger rudder.
see attachments below.
Result as noted by Bolger in the case of Dart, is a very unconventionally handling vessel.
May not work without also applying one or more of opts 1, 2, 4 or 5,
Which my gut feel are the best options anyway.
Does any of this make sense regarding the actual capsize Brian?
Do any of these (or combinations thereof) make sense to you Michael ?
What would least harm the performance?
You are the one who knows the numbers.
I'm just feeling the 'vibe' of the thing... Mabo & The Castle & all that.
Still got to figure out the re-boarding.
cheers
AJ
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20th March 2011, 10:20 PM #51
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21st March 2011, 03:20 AM #52SENIOR MEMBER
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Thanks so much for your post AJ. You pose questions, suggest answers , just what I was hoping for when I started this thread.
I currently feel your suggestion No4, the chine panel sounds the most promising. I think that bow digging in is what I was getting at with my opening thread. I actually considered sawing the hard chine off and adding such a chine panel.
Here's something that puzzles me about the RAID41 design.
Normally a design is drawn and displacement calculated so the hull sits on the water in the way it is supposed to.
With Raid41 the length is just less than 2 sheets long. Fixed.
Her beam was just under 4' so she would fit one sheet width. Fixed per say.
Sides are vertical so no way to slope hull sides inwards from the deck to reduce immersed hull volume.
So, how do you design her to sit on her waterlines? Increase rocker, the only variable left.
This ends up with the hull below the mast in the air, not even in the water at all. And if I go forward on a down wind run, just how high is the rudder out of the water.
I think a PDR at 8' can be immersed by 90kg man and 40kg hull. Twice as long and 50kg and 90kg man cannot immerse her properly. She sat on the water like a leaf. Two people on board, or water ballast filled and she was completely different.
PDR at 8' immerses and works, Beth being narrower immerses and works, Goat has angled sides and more load normally and works and does not have the rig in the bows.
Poor Raid41, with such good bloodlines.
Brian
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21st March 2011, 04:22 AM #53
G'day Brian
As it stands, I think Raid's bow needs to be kept well clear of the water.
As you describe her - the leaf sitting on rather than in the water.
been drooling over videos of Welsford's Houdini (which I fully expect to be my next build,
despite having 2 sets of foil blanks for Oz-Racer / Raid41, and plans for Eureka & outriggers...)
[ame=http://www.youtube.com/watch?v=ZoFro_gzFi8]YouTube - Launch day 3[/ame]
Houdini tends to run bow down, which may be ok with the huge rudder JW designed for it.
Adding the chine panel on Raid41 would lose bow volume & tend to cause same, but I'm not
sure Raid41s smaller rudder would cope. I think the chine panel would need to be complemented
with a flattening of the rocker forward, and probably fuller bow sections.
By comparison with that fiddle, turning it into a scow might be the simpler & more effective solution.
Still got the re-boarding problem. For which the only solution I see is a ladder.
And/or compartments which flood when capsized to keep it more docile for reboarding.
If the above could be figured, I'd prefer to build it ahead of Houdini.
Would be fun to give most of the Milang - Goolwa fleet a hiding in an "old-fashioned" boat.
Anyway, MIK is the numbers man, & I'd love to hear his rough calculations on the matter.
cheers
AJ
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21st March 2011, 09:26 AM #54Senior Member
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Take a look at my recent video's over at: Texas-GIS:
https://www.woodworkforums.com/f169/t...3/index30.html
It shows a view from the stern of our GIS and you can watch the rudder just before and during the capsize.
I would say that we had a bow down attitude because my daughter was way up front. With her behind the middle seat our bow would of been just above the water.
"the leaf sitting on rather than in the water"
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21st March 2011, 09:36 AM #55SENIOR MEMBER
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John, what a fantastic video.
I have to say, I cannot believe how clean Goat's wake is. Unbelievable. No wonder she is so quick. I am astounded a boat can have such a clean wake.
Wow.
Will have to watch video loads of times. At 1.31 you are distracted by beginning the gybe, some extra heel and it all goes wrong from there on.
Thanks, John. The camera on a stick has never been better applied.
Brian
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21st March 2011, 01:40 PM #56
What Brian said.
In common with Brian's Raid41 capsize, it didn't look like particularly strong wind.
But I don't think this was the 'tripping' sensation that Brian experienced.
Unfortunately, there are no landmarks visible to detect whether the boat slewed as it went over.
To me it looks like you sheeted in to set up for the gybe, causing the wind flow to suddenly
change from stalled in the run, to flowing "backwards" across the sail. Driving the mast to the
left faster than you could react with weight or steering. I notice the rudder came clear of
the water fairly early on, so there was loss of steerage at a time when it might potentially
have still been recoverable.
I wonder if maybe there's a different technique to be followed gybing these light narrow boats with big, powerful sails.
Normally, one sheets in first to reduce the uncontrolled gybing arc, and allow a controlled finale onto the new tack.
If the partially sheeted sail generates big by-the-lee lift, then there will be 2 periods during the gybe when there is
strong heeling force opposite to what one would normally expect. So maybe Raid didn't 'trip' at all ?
Watching vids of Lasers gybing, they seem to make a radical turn towards the sail to start the gybe,
then an equally radical one the other way as the sail swings across. Little or no apparent effort made
to soften the gybing boom. I'm guessing that this is, as MIK? commented earlier, to steer the boat under
the mast & stay upright.
Maybe GIS & Raid both need to be handled in this way ?
just thinking out loud....
AJ
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21st March 2011, 02:37 PM #57Intermediate Member
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IMHO all unballasted boats have a propensity for swimming because they all eventually fall over unless the skipper is able to balance the wind and wave forces. The only difference is in the compromises made in the design. fast boats fall or trip over more often. slow boats dont but are no fun.
One other point needed to be remembered is that an unexpected swim at the gybe mark is a precurser to becoming a world champion. All champions will admit to having unexpected swims at one stage or another at the gybe mark. Its just that as they sail more and more their second sailing brain in their bum learns and gets better at telling them to take action and do something now to avoid a swim.
The last point is that up until the above post we haven't really discussed the main sheet and the head of the sail. A lot of small wooden boats dont have stays, vangs or travellers. So if the mainsheet is loose the head of the sail and indeed the boom sometimesis able to lay off the wind. And as we all know the head of the sail also likes to initiate the swimming event and is able to get involved because of its leverage. The cause of many a learning event on the way to becoming a champion.
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21st March 2011, 04:07 PM #58
theory is, and as seen in the video, the balanced lug is self-vanging.
In the light winds seen, sail twist -shouldn't- be a big issue.
But these are narrow-bottommed boats without much in the way of hull form righting moment,
so seem to be more sensitive to minor events. More akin to recreational riding on a racing bike
than a Sunday drive in the Kingswood.
I also note that the Lasers seem to get crew weight right aft for gybing.
As seen in John's capsize, the rudder came out of the water at about 30deg of heel.
With weight aft, there would have been more boat width in the water to aid stability,
and more rudder immersed to control direction. And less bow in the water to trip over.
So possibly gybe technique needs to be the same for Raid & GIS.
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21st March 2011, 04:38 PM #59Intermediate Member
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Yes thats another factor. Weight aft helps when sharp bowed, fat sterned, flat bottom especially chined hulls are in that uncontrolled round up position with the rudder losing traction. On a lot of sports boats when going downhill in breeze there is often an invisible line on the deck near the mast called the FBL. No fat boys are allowed forward of that line.
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21st March 2011, 05:47 PM #60
for me that line would be located somewhere just aft of the tiller handle...
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