Author Topic: BC23 stability  (Read 2589 times)

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Martijn

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BC23 stability
« on: 19 Jan 2021, 12:06 »
As most of the winter jobs have now been tackled and spring is still some time away I've taken the lines of Riff Raff last weekend and created a 3D model of the hull. This is part of a project to determine the stability curve with and without water ballast.
The image below shows the progress so far. The hull has been completed and faired, cockpit and cabin are still to be done.
The fully loaded displacement (including mast, sails, engine, fuel and personal items) excluding crew according to this model is 1518 kg, which seems spot on.
BC23 #54 "Riff Raff"

Johan Ellingsen

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Re: BC23 stability
« Reply #1 on: 19 Jan 2021, 18:08 »
Does this displacement include water ballast?
CBL "Lill-Freja"

Martijn

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Re: BC23 stability
« Reply #2 on: 19 Jan 2021, 20:30 »
Quote
Does this displacement include water ballast?
Yes it does. Forgot to mention that.
BC23 #54 "Riff Raff"

Paul vR

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Re: BC23 stability
« Reply #3 on: 20 Jan 2021, 19:00 »
Hi Martijn, did you indeed succeed in measuring the mass of Riff Raff with a bathroom scale?
BC23 #39 "Aderyn"

Martijn

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Re: BC23 stability
« Reply #4 on: 21 Jan 2021, 08:05 »
Hi Paul,

No not yet. I've removed the mast, boom and all other gear for easier access while doing some maintenance.
For obvious reasons I want to know the weight of the fully loaded boat so I have temporarily postponed this.
It is still on my to do list, and once I have done it I will make sure to post the results on this forum.
BC23 #54 "Riff Raff"

Martijn

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Re: BC23 stability
« Reply #5 on: 21 Jan 2021, 08:19 »
Just a quick follow up.
I have completed the model obtained the first results based upon a (very conservative) estimation of the height of the center of gravity for the empty boat. The results below are for a fully loaded boat with and without water ballast.

As expected the righting lever curve shows a significant increase in stability (152%) with the ballast tank completely filled.
Without water ballast the positive range of stability is app. 90 degrees. Filling the ballast tank increases the range with 20 degrees to 110 degrees.

When comparing the righting moment curves (righting lever multiplied by displacement) the stability actually is more than double the stability of the boat without water ballast, increasing from 251 kg*m to 605 kg*m, an increase of 240%.

Again, this is based upon an estimate for the center of gravity of the empty boat. I'm planning to do an inclining test once the boat is back in the water this spring and update the data.

These preliminary results seem close to the results posted in PBO, except for the fact that the PBO results take the buoyancy of the mast into account. This accounts for the sudden increase of stability at 90 degrees which is when the mast is submerged.
BC23 #54 "Riff Raff"

Martijn

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Re: BC23 stability
« Reply #6 on: 21 Jan 2021, 08:29 »
Below 2 images show the empty boat at 90 degrees heel and the ballasted boat at 110 degrees.
The cabin entrance is just above the water.
BC23 #54 "Riff Raff"

Graham W

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Re: BC23 stability
« Reply #7 on: 21 Jan 2021, 14:01 »
Impressive stuff.  What tends to happen to unballasted BR20’s in the case of capsize is that momentum and wind on the buoyant hull overcome any buoyancy provided by the mast and they continue to roll over until turtled.  For that reason alone, it’s best to have the ballast in when cruising at sea.  Racing with crew aboard and with rescue boats nearby is a different matter.

For a whole range of theoretical stability calculations for the BC23 and other Swallows, see the link to the .pdf file on https://www.swallowyachtsassociation.org/?page_id=433.
Graham
Gunter-rigged GRP BR20 #59 Turaco III

Rob Johnstone

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Re: BC23 stability
« Reply #8 on: 22 Jan 2021, 19:53 »
As Graham say, impressive stuff Martijn. As far as the buoyancy provided by the mast, I wouldn't count on it, with the early boats any way. Vagabond (no 10) had a solid plug in the bottom of the mast. When I managed to break it, (the mast ran away from me when de-rigging and snapped on one of the winches), a litre or so of water came out of it. Later models have a "breathable" plug in the bottom, so I assume they drain.
Rob J
Matt Newland designed but self built 15ft one off - "Lockdown". Ex BC23 #10 "Vagabond" and BC 23 # 54 "Riff Raff"

Martijn

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Re: BC23 stability
« Reply #9 on: 23 Jan 2021, 08:57 »
Thank you Rob and Graham.

Personally I prefer not to include buoyancy of the mast in stability but apparently in the PBO calculation for all 4 boats the mast was included.
The righting lever curve below shows the result when the mast is included, just to compare. It matches the curve from PBO rather nicely. The range of stability has increased by 15 degrees.

Once I've calculated the exact height of the center of gravity I will post more accurate graphs which also include CB up/down and the effect of crew.
BC23 #54 "Riff Raff"

Martijn

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Re: BC23 stability
« Reply #10 on: 02 May 2022, 18:37 »
Update.....

It was launching day last Friday, and I found the time to do a proper inclining test in order to calculate the vertical center of gravity of the boat both with and without the ballast tank filled.
I'll post the results here as soon as I can, but unfortunately at the moment I have no access to this forum from Windows 11.
I'm writing this on my daughters' laptop without access to the stability data. :(
BC23 #54 "Riff Raff"

Sea Simon

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Re: BC23 stability
« Reply #11 on: 03 May 2022, 18:21 »
I have no access to a new Windows PC, other than that belonging to my (soon to be EX - retired!) Employer. That is so "secure" that it is almost useless, even for their business!

However, I have a new Chromebook and that works fine here, as (generally) does my very old Android tablet, and my relatively new Android phone.
BRe # 52 - "Two Sisters"  2016. Plank sprit, conventional jib. Asym spinn. Coppercoat. Honda 5. SOLD Nov 2022....
...From Oct 22.
BC 26 #1001. "Two Sisters 2", 2013. Alloy spars, Bermudan Sloop; fixed twin spade rudders, Beta diesel saildrive. Lift keel with lead bulb. Coppercoat. Cornwall UK.

Martijn

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Re: BC23 stability
« Reply #12 on: 06 Oct 2022, 15:48 »
See the below 2 images with updated stability curves for the BC23, based upon information obtained by a inclination test this spring.
BC23 #54 "Riff Raff"