BC23 stability

[Martijn]

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.

[Johan Ellingsen]

Does this displacement include water ballast?

[Martijn]

Does this displacement include water ballast?

Yes it does. Forgot to mention that.

[Paul vR]

Hi Martijn, did you indeed succeed in measuring the mass of Riff Raff with a bathroom scale?

[Martijn]

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.

[Martijn]

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.

[Martijn]

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.

[Graham W]

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.

[Rob Johnstone]

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.

[Martijn]

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.

[Martijn]

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.

[Sea Simon]

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.

[Martijn]

See the below 2 images with updated stability curves for the BC23, based upon information obtained by a inclination test this spring.