Hello All,
Yesterday I did a capsize test on my Weekender. I had my friend Ted video the proceedings and it came out pretty well. Take a look. I'll write something up about it later. Do a search on YouTube for Weekender Knockdown.
Al
That's pretty much what I've suspected. I ran some calculations and few years ago and came up with a 67 degree positive righting arm, before she flops over. This is pretty typical for a boat shaped like a Weekender. Sealed seat boxes would help recovery and going turtle. As would a sealed forepeak and aft deck area.
(08-15-2013, 08:33 PM)Bob Booska link Wrote: [ -> ]that was much easier to stand back up than I thought it would be . . .
That's because it had a lot of water it in. You'll notice it was also very unstable, once upright and easily capsized again. This is one reason you want floatation chambers on small boats, because they tend to down flood quickly which very quickly decreases stability.
That 67 degrees seems about right from underneath the sail. I was surprised how hard I had to fight to get it to fall on me. I'm a big guy (220) and it was quite a pull. Once it started to fall, it came fast and kept going without slowing down until the mast head hit the bottom. I knocked it over a total of three times and climbed back in twice. The first time I got very little water in the cabin mostly because I hurried up and stood her up right away. The next two times I wanted to see how quick and how far it would flood. I think that re-boarding might be best over the transom when flooded as far as I took it. With very little flooding, I was able to climb over the side without even getting the rub rail wet.Â
My "water tight" covers were a mixed bag. It turns out that the forward bulkhead covers worked like a charm. The seat back covers were a complete failure. All the water in the forward compartment came in through the hatch cover, and maybe some through the mast boot. I am in the process of replacing the seat back covers with water tight deck plates. If I were to build another Weekender I would not cut out the holes in the forward bulkhead, instead I would put the biggest commercial deck plates I could find in there. Later I will post a pic of the forward covers that worked so well. they were simple to make and used up some of the scrap from the build.
Al
Paul: I'm not much of a physicist or an engineer. The 67 degrees is from the vertical (heeled past half way to a knock down) right?
Dave
I have two other videos of the proceedings. They are kind of messed up so I didn't post them. On the first knockdown, the boat came right up without a fight. I was surprised how easy it was to get it upright. All I had to do was stand on the keel and up she came. It took a minute for the sail to clear the water, but once that happened, blam, up she came. I think that if I could have kept the main sail on the opposite side of the boat somehow, I could have climbed in the last try without it coming over on me.Â
Yeah Dave, I take it that the 67 degrees is from the vertical.Â
Al
When we do stability calculations we start at a static position (boat sitting dead upright) and usually run through 10 degree increments up to 180 degrees. I'm going from memory, but 67 degrees is what I remember for a Weekender, meaning the mast was 22 degrees past 45 degrees, when she lost all of her righting ability (AVS) and flopped over in the numeric model. In fact at 55 degrees, she was dramatically losing ground on her RM as the curve starts to fall off. This is typical of a lot of flat bottom craft, where they have a high initial stability, but then reach a point where their RM (Righting Moment) falls off a cliff. If you think this is bad, you should see a typical beach cat. It has a huge RM for about 20 degrees, then it flattens out until about 30 degrees, then it drops like a stone and over she goes.
Attached is one of my flat bottom design overview pages and you can see (at the bottom of the page) she too falls off a cliff once she reaches her max RM, but she does retain enough RM to have positive stability at 90 degrees (which a Weekender doesn't). You can see the hull rotating through 180 degrees along the graph at the top. She keeps some RM because proportionately she's higher sided and has a tall deck structure (not shown), which helps. You can see the influence of these deck structures on the up side of the stability curve. Simply put, she'll stand back up if severely rolled, but most flat bottom boats don't have this "reserve" ability, so they get to a point and just flop over. The point where they just flop over is the AVS (Angle of Vanishing Stability), which is to say they haven't any more RM to work with and on a Weekender this is about 67 degrees.
I spent the night anchored out yesterday and spent that time thinking up ways to make things better leak wise. So, yesterday I re-sealed the forward hatch with the foam tape that people use to mount pickup toppers to the bed of the truck. It is about 1 1/4 inch wide and 3/16 thick. I have sealed the hatch cover with this stuff before with mixed results. (witness the flooding during the capsize test) This time I attached it to the bottom of the cover. I made the cover 1/8 larger all around to allow for easy opening and closing. Because of this I left the foam with a 1/8 inch overlap inside the cover so that the foam would be compressed by the coamings as well as being jammed down by the cover. I always dog down the cover when I am under way. I am now thinking up a way to test this system without having to bail the whole boat again.Â
Oh, by the way I figured out a way to have mosquito netting over the hatch with out doing too much. I bought a 4 X 6 sheet of netting from Campmor. I simply tape the whole sheet to the hatch cover along the six foot side and when I pull the cover closed, I reach up and pull the netting down and let it hang. Keeps the bugs out real nice.
AlÂ
Al,
I suppose that your Weekender was completely empty and your spars of solid timber.
I stepped many time as you did, but I never capsized, and I am quite big too (90 kg in summer clothes, in winter much more, expecially after Christmas holidays...), even if I never looked for it and instinctually I shifted my body CG to the boat center.
I admit that my Weekender sides are 5-10 cm higher than original, but my cabin roof and my mast are higher too, so I think it is the same .
Anyway I take in great consideration your test!
My solutions against sinking are an inflatable roller (those used to pull boats on the beach)Â tied to the floor below the transom under the rudder mechanics and a generous extra fender inside the fore bay.
Gianluigi