How to get a tall mast under a low bridge.

[Paul Akers]

Ken:

Some, (fortunately, not all) of the bridges on the ICW are too short for some boats to clear and forces the boats outside of the ICW. (Sometimes, until now!)

 

[Bob V]

I'm impressed.

It's one thing for us to figure out how they did it but I want to have a conversation with the genius who thought of it.

 

[Shippy]

call me chicken

On someone elses boat, I sould be very interested and willing to participate. On my own boat, no way unless it was a matter of life or death. I fully understand the physics but I would be worried to death that we would round up at the worse possible moment (like going under the bridge, or some jerk wanted to go flying by in his big poser boat causing one heck of a wake or worse, catching the line on teh ballon and dragging me.

 

[Ranger Paul18578]

Turn sharply

Once the bags are tied to the mast (however they got up there), getting them down onto the water on the desired side should be pretty easy, e.g., if you wanted the bags on the starboard side, unleash the bags and turn sharply to starboard! (or vice versa?)

 

[Chris Connor]

Angle of Loll - Negative GM

If I had to guess at this, I would say the boat has a negative GM. All vessels will theoretically roll about a point known as the metacentre (M). The centre of gravity of a suspended weight is at the point of suspension. As soon as the weight comes on the masthead, the centre of gravity of the vessel will shift towards the added weight. The centre of gravity moves to a point above the metacentre. The vessel takes on a "LIST" that brings the centre of buoyancy vertical with the centre of gravity of the ship - or equilibrium.It would not be difficult to determine how much weight was needed to determine the 'angle of loll'. I did something similar while in college for licenses.Chances are the owner is a naval architect or a very brave senior deck officer!Just the same, when the boat was upright with the weights lashed to the mast, she would be extremely tender and could 'flop' either way depending on wind or external forces if the weights were just let go. Trust me, the problem would not be getting the vessel to roll out!I know of a ship that tested a heavy lift crane with insufficient ballast in the double bottoms. AS they added water to a 40 tonne tank attached to her derrick she developed an angle of loll that resulted in her 'flopping over until the tank struck the wharf! It was quite violent. The owner of the shipyard wharf was not very happy.Probably much easier than unstepping the mast though!Regards,Chris

 

[Paul Akers]

GOOOD Idea, Ranger Paul!

NM

 

[Robert Gainer]

Chris, you may have mistyped when you said

Chris,You may have mistyped when you described the metacenter as the rotation point of the underwater volume of a vessel. The metacenter is a point on the heeled center line which is directly above the center of heeled buoyancy and it is labeled M in naval architecture. The vessel rotates about a moving point which is the center of buoyancy (B). As the boat heels the center of buoyancy moves outboard, more or less depending on the shape of the hull so the hull is always rotating around a point that is itself moving. The all important GZ is the horizontal distance between the center of gravity (G) and the center of buoyancy at the level of G and is measured on the construction line from B to M and that measurement point is called Z. This value (GZ) is popularly called the righting arm. The metacentric height is the distance between G and M and it is called GM and is important because the righting force is proportional to the metacentric height (GM) times the sine of the angle of heel. GM also has a direct relationship to a boat's rolling period in seconds.As far as negative GM goes I can’t think of an example of that in yachts offhand. Even with this example of weight high in the rigging the GM is not negative. Can you provide an example of this negative GM?All the best,Robert Gainer

 

[Stillraining]

Wheeea !

I'm now reminded why I never studied engineering!

 

[Chris Connor]

Angle of LOLL

Hello Robert,I agree with everything you have stated.My thoughts were that perhaps they have managed to raise the centre of gravity of the boat above the transverse metacentre. The G would certainly shift upwards from G with any additional weight suspended at the masthead. It looks like they are using two large water bags which could hold at a guess several tons of water.From Ladage's Stability and Trim."If the centre of gravity lies above the transverse metacentre (G above M), the vessel is in a state of unstable equilbirium, that is, she possesses a negative GM. There is no tendancy for the vessel to right herself at small angles of inclination. An upsetting moment is formed ant the vessel will incline from the erect position. A negative GM does not mean that the vessel will capsize. It merely means that the vessel does not have any initial stability , and that she will incline to an angle where B has moved far enough to the low side of the vessel to be once more in the same vertical line as G.Apparently, then, a vessel can acquire a list not only by having its center of gravity off the centerline but also by having its centre of gravity too high in the vessel"My description of metacentre in my original post was not very well done. I tried to keep it simple to prevent simialr responses e.g. from stillraining. I am not saying that this is what they have done. Without the stability curves for the boat and all of the information - It just a guess.I have commanded drillships in Brazil that had very small initial stability, i.e. small values of GM - it's not a comfortable position to be in. My response is probably way more technical than people are interested in. For this I apologize in advance.Regards

 

[Robert Gainer]

Yes Chris,

Yes Chris, I can imagine G above M on a barge or other vessel with a large Block Coefficient and a heavy deck cargo. And as you say it would not be a comforting situation. But because of the dynamics it’s hard to see this happening on a small yacht without sending her over on her beam ends at the slightest roll. The most frightened I have ever been at sea was on a large commercial ship with a small GM and a very long roll. During some bad weather she would hang at the end of the roll for what seemed like forever and I was thinking she would never come up again. I have been in some bad stuff in small boats and felt safer. I don’t know how you big ship boys can put up with it.All the best,Robert Gainer

 

[Chris Connor]

Capsize recovery

Robert, Very early in my seagoing career, I was assigned as Coxswain on a 44 foot self-righting lifeboat in Souris PEI. They were quite common in the RNLB and USCG. We used them in the CCG as well but added an enclosed deckhouse.I often wondered what it would be like to take that ride. I never did it. I know that the USCG use to capsize them at Cape Disappointment in Washington. Although I did use the seatbelt on the Coxswain's chair at the wheel one nasty night out NE of the point on a SAR mission.It was the only vessel I have ever worked in that had a seatbelt at the helm position.I work on a semi-submersible now that has altogether different stability issues. The unit I am on had very low stability margins until they added blister tanks to improve them.I have a Catalina 310 for pleasure. I don't intend to do stability conditions for her!Regards,Chris

 

[Chris Connor]

Tartan 34

Hello Robert:Although I am very content and happy with my Catalina 310, I nearly succumbed to two footitus this year. I even went so far as to have the local Catalina dealer send me some brochures. My spouse brought me back down to planet earth.I asked him for the Tartan 34 and the Catalina 350. The Tartan looks like an awesome boat. I would imagine that with your experience and background, the fact that it is your current choice of boat says quite a lot about that make and model.I will have to dig out those brochures again!I head back to West Africa in a nine days. I have to have something to think about during my month away.BRgds,Chris