Unmanned stability

[john.burnett]

This is my first post, so please be gentle.

My question: Is it uncommon for the weight of the boom be able to tip an unmanned boat? In my (scant) design calculations, I only considered stability with the weight of the skipper included.

I recently went for my first sail in a boat that I designed. It is essentially a Bolger Windsprint with the tail cut off to make the overall length about 11'-6". For some reason, I thought it would be a good idea to shorten the beam to 3'-6" from Windsprint's 4'-6". I'll admit that this was influenced more by intuition and 'looks about right' than good design.

In any case, I put the boat in the water next to the dock and rigged it. When the boom was loosely blown over the far side of the boat as I stood on the dock, the boom caused the whole boat to start to tip away from the dock. I had to put my foot in the boat to prevent it from flipping.

With some apprehension, I got in the boat and maiden voyaged across the lake in a light wind and came back dry.

How concerned should I be?
Two design factors contributed to this tippiness: the short beam and perhaps-too-heavy mast. See photos for to get a feeling for the boat.

http://www.johnburnett.net/album/v/Boat/

[daiquiri]

It would be interesting to kow what scenarios did you take into consideration for the stability calcs.
For example: both the boom and the skipper at the same side of the boat...?

[john.burnett]

daiquiri:
My calcs were far from rigorous, and I did them 2 years ago when I started the boat. But I included the skipper seated in the center of the boat. I believe that I (hopefully) included the mast, but I omitted the boom altogether. As I recall, the boat would right itself up until the water started coming over the side.

That seemed reasonable, so I moved on. I never calculated stability without a person in the boat, though.

[PAR]

Can you provide a brief description of your stability calculation efforts?

Of course any weight moved off the centerline will affect the trim of the boat, but wholesale changes to the fundamental parameters of the boat's basic dimensions, seems unwise at best, if preformed by a novice.

[daiquiri]

Originally Posted by john.burnett

This is my first post, so please be gentle.
...
...
How concerned should I be?

Well, let me be gentle - based on what you wrote here, you should be pretty concerned. Don't use it without wearing a PFD (well, that's a general advice, but in this case more valid than usually).
Now, the boat has been already constructed, so very little can be done to improve its stability apart from adding a ballast in some place as low as practically possible, or equipping it with amas. Making a lighter mast is also a good option, considering that right now your sailing will be limited to very light wind conditions anyways.
That boom looks very light:
http://www.johnburnett.net/album/v/B...97192a3a66734d
if it managed to flip your boat, it means that her stability margin is incredibly small.

[Guest625101138]

John
How does it sail?

Wind surfers fall over all the time. I have seen lasers blow over on the beach even with loose sheet in a gust.

If it will not stay upright of its own accord beside the dock then you simply need some way of securing it at the dock while you park the car. I think that is what children are used for unless they are old enough to drive and then you can hold the boat while the offspring parks the car.

It reminds me a little of a modern moth hull. Nice and slender and I bet reasonably fast. All you need now are the foils and the tramp. Self bailing helps too. If it goes over often you could make it self bailing.

Rick

[john.burnett]

Rick:
It sailed okay in the light wind that I had. I have sailed only half dozen times, so it is hard for me to say. I think that I just need to be a little more nimble than if I had made it wider. Some people sail canoes, right?

I will be adding a rear cleat to help tie it to the dock, in lieu of having children at the moment..

Thanks for your comment about the Laser, that makes me feel better that it happens to "real" boats sometimes.

[Guest625101138]

Your boat is no doubt a little tender but all dinghies rely on crew weight to perform at their best. Also sailing in a dinghy means you are bound to go over at some stage. Where you might find it difficult is boarding it once it is swamped. This is something you should practice near to shore. It is difficult to get a boat any distance if you are in the water trying to keep it upright.

Rick

[daiquiri]

Originally Posted by Rick Willoughby

all dinghies rely on crew weight to perform at their best.

Definitely yes. What they usually don't do is to flip over under the weight of their own boom.

[wardd]

maybe rerig it with a smaller bermuda rig and take the performance hit. may help

[TeddyDiver]

Boomles main might solve it

[hoytedow]

I must ask. Is your mast hollow or solid?

[m4tt]

Im not a NA or anything, but most modern skiffs do not float upright unmanned. They do have the advantage of planing and are designed to be stable at such speeds. But in light wind or no wind, they naturally float upside down. Looks like you used a fence post as a mast, perhaps i lighter version could lower your cog. Good luck! dont forget a lifejacket, flippers perhaps.

[gonzo]

Basically, you screwed up a good design. That's one way of learning. We all do things like that. Next time you'll have more experience.

[Doug Lord]

Originally Posted by john.burnett

This is my first post, so please be gentle.

My question: Is it uncommon for the weight of the boom be able to tip an unmanned boat?

==============================
A boat I grew up sailing and racing- the 15'6" Windmill- behaved just like that. You could not leave it unattended at a dock with the sails up w/o an almost certain capsize if there was any wind at all. But it was one of the finest boats I've ever sailed and very fast. You can make adjustments to your thinking since thats where you're at. I wouldn't try to change much more ,particularly if the boat performs well. Good Luck!