Righting moment

[TreblePlink]

As I restore Charisma: https://astro.temple.edu/~bstavis/pr/783-charisma.htm

I am trying to get a handle on keeping the righting moment under control. At about 7800 lbs, she has 3000 lbs of lead keel, ending about 3.5 ft. below the waterline, and a steel extension that pivots down about an additional 3 feet.

As I work out placement of heavy items like dingy motor, anchor winch, water tank, radar, extra fuel containers etc. I want to make sure she will right herself if knocked down. Rhodes was obviously an excellent designer, but I have no hard data on the design (I've inquired to the folks at Mystic seaport, where the Rhodes data is, but have not heard back after two months.)

I understand the difference between static roll moment and dynamic, but I don't know where even the roll pivot point is. Without winching the mast into the water, how do I know how much margin is there? I'm very interested in self-righting.

Chris

[TreblePlink]

Wow. 100 views and zero suggestions. I must really be SOL.

[carstenb]

There is a formula for figuring out the roll point and the knock-own point. off hand I forget (it is not something yo use every day) it. but it can be looked up. Try reading this - it will give you some pointers

Understanding monohull sailboat stability curves | M.B. Marsh Marine Design


otherwise google "righting moment of a sailboat"

or

"sailboat stability calculations"

Thye should lead you to something you can use

[Polux]

It is important for you to understand that type of sailboats boat with those characteristics (like most sailboats) will not re-right themselves alone. I mean if the boat capsizes (inverted) and there is no waves the boat will remain inverted.

But that sailboat will not turn turtle by wind alone and only breaking waves can do that. Do the stability characteristics of that boat it will be needed a much smaller wave to right it than the one that rolled it. So it will be a question of time till the boat gets up on its feet again, normally around 1 to 2 minutes, depending on the AVS and the inverted stability.

It is important to have the boat closed and the seacocks closed because if a significant water enters the boat the stability would be compromised and the boat can take a lot more to re right itself or even sunk.

Regarding what you want to know it is not the righting moment but the stability curve. It is possible to draw one with some inclining tests and the software to calculate it. Short of talking with a NA you can talk with IRC or ORC measurers.
ORC - World Leader in Rating Technology

[Benz]

Seat-of-pants boat engineering rule #1: keep weight low.
Small boat rule #1: eschew unnecessary items. Consider: do you really need an anchor windlass for a 28', light displacement boat? I can usually pull my 5/16" chain and 45# anchor up by hand. Surely that tackle on your boat will be far lighter.
Also, consider whether you can get by without a dinghy engine. Oars are far lighter, and stow easily on a side deck. I have cruised a lot with only rowing dinghies.
Also, consider NOT bringing extra fuel. A Rhodes 28 oughtta sail pretty handy. I used to cruise with an 80-mile motoring range--enough to get me in and out of port and a little extra.
Just some thoughts from one small boat cruiser to another...

[Suijin]

I have to agree with Benz. You're over thinking this, in that you are over-planning on what you're going to carry. You don't need a windlass or 200' of 5/16" chain, or 10 jerry cans of fuel. Get the boat restored, get it in the water and start sailing her. You'll quickly get an appreciation for her tenderness and stability.

More weight on a boat is bad. Keep it light and only add stuff that your personal experience or immediate plans require.

[TreblePlink]

Good suggestions all. My experience with a powerboat with significant windage was that pulling up a 25 pound anchor when the wind was blowing could be very difficult without a 2nd person running the motor to take up the slack. I have a 35 lb anchor and mostly nylon rode for Charisma, and was thinking a standard sheet winch might make it a lot easier. The dingy motor is a question though.

[barnakiel]

Originally Posted by TreblePlink

As I restore Charisma: https://astro.temple.edu/~bstavis/pr/783-charisma.htm

I am trying to get a handle on keeping the righting moment under control. At about 7800 lbs, she has 3000 lbs of lead keel, ending about 3.5 ft. below the waterline, and a steel extension that pivots down about an additional 3 feet.

As I work out placement of heavy items like dingy motor, anchor winch, water tank, radar, extra fuel containers etc. I want to make sure she will right herself if knocked down. Rhodes was obviously an excellent designer, but I have no hard data on the design (I've inquired to the folks at Mystic seaport, where the Rhodes data is, but have not heard back after two months.)

I understand the difference between static roll moment and dynamic, but I don't know where even the roll pivot point is. Without winching the mast into the water, how do I know how much margin is there? I'm very interested in self-righting.

Chris

Chris,

I will by-pass you RM worries and share some first hand experience from knockdowns (one about 135 degrees, the other about flat out) in a boat that is 26' and alike (but not identical) your design. Here we go.

1) The boat will come back. You have same draft as we do but your slab is lead while ours is cast iron. Your boat will come back sooner and from more heel.

2) Other than winching that windex into the water, at the dock, a real life knockdown is a DYNAMIC event. Your RM may count but your roll inertia will count twice as much. One of the things you do not want to know about roll inertia is that outboard on that rail actually SLOWS down your roll and makes it more difficult for a big wave to roll you. So does that radar dome 4ft above the deck. Read first hand reports from boats that lost weight aloft (lost masts) in bad conditions - they nearly always get knocked down again and again, as soon as they shed the weight aloft (lost the stick).

3) Your main concern will be that HUGE cockpit. It will fill and the boat will list heavily. Measure the volume and come back. How much volume did you get, uh? One ton of water, two tons? Next comes that huge companionway. Huge and open. Wish washboards that may ... wash overboard, perhpas. You have been warned.

4) Your second main concern will be the dynamics of the wipe out - spreaders dislocated, heavy objects flying (batteries, engines, galley stoves, etc.

So, I guess what I am trying to say is study the RM but do study the dynamics up next. A knockout is a DYNAMIC event.

One example of how you can take advantage of the nature is to distribute heavy stuff ACROSS the boat. The heavier the edges, the more roll inertia your boat will have. You cannot place an anchor winch athwartships, but you can incorporate two water tanks under the berths, perhaps.

If you think you do not have enough ballast there, place the batteries in the bilge ... but if you trust Cecil Rhodes ... place the batteries one each side of the centerline.

While distributing stuff across the boat, keep the heaviest objects lowest. Batteries remain heavy throughout the passage. Water tanks get very light when empty. Consider such things when laying out the stuff.

Another example is filling up the cockpit well. Then it cannot fill up with water. Some sailors fill them up with sailbags tied down.

Another example is locking the spreader ends. Then they cannot be dislocated by water pushing on them, nor by suddenly slackened rigging.

Etc.

So. My 2c. I hope this will somehow add to your road to sailing happiness.

Cheers,
b.

[Don C L]

Well, not sure if Cecil Rhodes ever designed a boat, but Philip would probably say keep it all low of course!
So, Treble, I have a boat similar to yours, I think, but no centerboard. But I am a little confused because the Rhodes 28, which is actually called a Rhodes Ranger 29, I am familiar with does not have a centerboard. Are you sure of the make? Have any photos?
In my case with displacement of 7500# and a ballast of 3120# and a 4' draft, I am not concerned about recovery from a knockdown. In fact I have been knocked pretty far and it popped right back. If you look at photos under my avatar you'll find a letter from Olin Stephens to the original owner about the metacentric height, which relates to the subject you have brought up. The thing to worry about is the weight in the boat, any boat, that is able to move, at all. So for example, batteries; if they are not mounted in a way that they will stay put in the event of a capsize, they become a real LIABILITY instead of an asset. Same goes for lumps of chain. As Stephens points out, once the boat is over on its side, it is crucial that water cannot enter or it becomes a "free surface" and will flow to the lowest point and hold the boat from righting easily.
Olin Stephens Letter - Description of metacentric height. Cruisers & Sailing Photo Gallery
You had another thread I think asking about cruising with your boat. If you have the Rhodes Ranger 29, I would not be TOO concerned about 40 knot winds or larger waves for the most part. In ANY boat, those kinds of conditions should get your attention, but the design is a very seaworthy one IMO. The boat will likely handle conditions you are talking about well. As far as whether you should run for cover or run for open sea when faced with rough weather, the tried and true answer is: it depends. In my boat or MOST boats, I would NOT run for an anchorage or harbor unless I knew it and its entrance very well. Anchoring in 40 knots without a really powerful engine, would be tough I think, to say the least. Not at all worth the gamble IMO unless it is somehow your absolute last option. You'll likely only get one shot to get it right with a smaller boat like ours. I did it once engineless in my old Columbia 24 in 20 to 25 knots in anchorage sheltered from swell but not wind. Fortunately I had everything prepped and the anchor set in the first grab. If it hadn't, I did have an escape route (which is always a good thing to have), but it was a few moments of high blood pressure. Haven't tried yet in 40 knots I confess. No plans to.

I am not sure if you were referring to a squall or storm.. the main difference being wave height, period and whether they are breaking too.

OK, edit: Sorry I didn't click on that link to your boat. I am puzzled because I hadn't seen that one. It is actually not listed as a Rhodes design but I see it was on the Rhodes site. In any case, I see the design does not have as low a center of gravity as the Ranger 29. I'll look at what specs there are for it.

[Don C L]

OK, the closest design I can think of is the Tartan 27
TARTAN 27 sailboat specifications and details on sailboatdata.com

It has a very good rep. I have not sailed one, but there are folks here with them. I'll keep looking.

[Don C L]

Check these:
Centerboard Boats: swing keel = no blue water?
http://www.cruisersforum.com/forums/...ios-78528.html
http://www.cruisersforum.com/forums/...oat-76879.html

[TeddyDiver]

Originally Posted by TreblePlink

As I restore Charisma: https://astro.temple.edu/~bstavis/pr/783-charisma.htm

I am trying to get a handle on keeping the righting moment under control. At about 7800 lbs, she has 3000 lbs of lead keel, ending about 3.5 ft. below the waterline, and a steel extension that pivots down about an additional 3 feet.

As I work out placement of heavy items like dingy motor, anchor winch, water tank, radar, extra fuel containers etc. I want to make sure she will right herself if knocked down. Rhodes was obviously an excellent designer, but I have no hard data on the design (I've inquired to the folks at Mystic seaport, where the Rhodes data is, but have not heard back after two months.)

I understand the difference between static roll moment and dynamic, but I don't know where even the roll pivot point is. Without winching the mast into the water, how do I know how much margin is there? I'm very interested in self-righting.

Chris

Working out the weights is the easy part. From there it's simple to calculate the height of the center of gravity. What's more of a task is to get the center of flotation, in several degrees of heel, let's say at 0, 45, 90, 135 and 180 degrees. This will give enough data for approximate GZ curve.. If you can get a copy of Larsson et Eliasson book of boat design (principles of... if I recall it right) it helps a lot, and Skene's "element's of..." to learn Simpson rule to calculate volumes.

BR Teddy

[barnakiel]

Originally Posted by Don C L

Well, not sure if Cecil Rhodes ever designed a boat (...)

Sure he did. And one that sank all the way to the bottom of Africa too! ;-)

Thanks for finding this one out!

Cheers,
b.