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/ (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]

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/Boat/Boat+rigged.JPG.html?g2_GALLERYSID=b4ea524351f9444af297192a3a66734d
if it managed to flip your boat, it means that her stability margin is incredibly small. :eek:

[Rick Willoughby]

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.

[Rick Willoughby]

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]

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 :D

[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]

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!

[oralpiskin]

hi everybody
ı am a ship engineering student and trying to learn about submarine design so ı need a useful document containing informations about stability and hydrostatic calculations for submarines because ı don't have any computer program. ıf there is one will help me in this subject ı will be pleased...

[Ad Hoc]

Try:
"Dynamics of Marine Vehicles" by R. Bhattacharayya
or
"Mechanics of Marine Vehicles" by Clayton & Bishop
or
"Hydrodynamics of Ship Design - part III" Saunders, SNAME publication

These do a reasonable over view.

[oralpiskin]

thanks for answer but do these books contain about submarines or under water vehicles?

[Ad Hoc]

They cover all...but have either chapters or subchapters related specifically to submarines.
I've just grabbed them off my bookshelf, (big bulky books), to check..and yes, these cover all the basics and a bit more...so yes, these cover all and will suffice, for you.

[oralpiskin]

thanks gratefully...

[karabacak]

Ma huseyin from turkey and ma get some problem about designing submerged vehicle forms You know some software that can help me optimizing & analiyzing about ma ma project body. I need it as soon as possible because I do not have much time about my project.

[oralpiskin]

dear Ad Hoc,
I couldn't find books you proposed cause our library is not enough, do u have any file you may share and if anyone has these infos ı will wait for documents...

[ancient kayaker]

This is my first post, so please be gentle ....


-see, guys, I told ya we were scaring off people ...

...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" ....

Stability (righting moment) is roughly proportional to the cube of the beam and is proportional to the length, for the same draft. That’s not all, obviously displacement is also effected, so if that is kept the same the ratios aren’t exactly as above, nonetheless you have reduced the stability by almost 2/3. This a completely different boat now.

Because the displacement at the original draft is also reduced, but the load is not, it is a few inches lower in the water. It looks like you did not increase rocker, or bottom curvature, to raise the transom. This likely causes the bottom edge of the transom to be immersed. If this is the case, then as it moves through the water there will be turbulence, causing drag and noise, which you may have noticed.

I estimate from the rigged photo that you have about 105 sq ft of sail area. That would suit the original design. However, the ability to bear sail varies as the 2.5 power of linear dimension; so it should also be scaled down to around 60 sq ft for your boat.

Sailing canoes can carry a lot of sail area, up to 100 sq ft on a beam of around 3 ft or so but are usually 15 ft long or more and we are talking of a race boat here, weight kept low in the water and athletic sailor to boot; much less for a typical canoe, more like 50 sq ft.


You should definitely be careful sailing this boat in its present state, especially if you are an inexperienced sailor. I would suggest you reduce the size of the sail, and you should always wear a lifejacket! A lighter mast might help, it looks more rugged than usual, but the rig you have is fairly low. I would not add ballast unless you have enough built-in buoyancy, and I don't see any in the photos. if it feels tippy, and it might, you can strap some plastic foam pool noodles a few inches above the waterline.

[messabout]

AK has told it like it is. One of the ways that you might lessen the tiddly tendency is to lose the lug rig. The yard is a serious liability on a skinny boat. To a lesser extent the crash boat style outwales on your boat are kinda' big. Even that small amount of extra weight will contribute to the stability problem. It does have a rudder assembly that is probably overkill.

AK has mentioned that the stability is roughly proportional to the cube of the beam. We are talking about the beam at the chines in this case. Be careful to note that it is a proportion. Example: compare 40 inch chine width with 48 inch chine width. The cubes of those numbers have a proportion of about 1,74 and sure enough the righting moment of the 48 inch one will be about 1.74 times that of the 40 inch one. You should not bet your life on a simple rule of thumb like this but it is a useful concept that can help the decision making process in the design stage.

[Ad Hoc]

oralpiskin
If you ask your library, they should be able to get them for you, loaned from another library. (Well, that is how most libraries work).
Other than that, you could buy one of those books? I have all of them, not cheap. But if you become a qualified professional naval architect, they are very good reference books which you shall use throughout your career, as such, a worthwhile investment.
There is far too much to scan in...would take many many hours

[oralpiskin]

Ad Hoc,
thanks for your suggestions, I will find....

[karabacak]

Ad Hoc
but it is not the way libraries in Turkey

[yipster]

you may try your luck in university, some, like here in holland that have NA, have some hard to get books that they do not loan out in their library but you can read at their library for a small fee

[JLIMA]

I agree with the fact that your mast is over robust and could be lightened considerably by just using a much smaller diameter mast. I got away with using a 10' 1by as a mast on an 9' gaff rigged cat and it never gave out or gave me any trouble as long as i had the boat.