Ring Frames (design techies please!)

[Meerkat]

What exactly are they? I have a general idea, but I'm not 100% sure.

* Do they have to be engineered into a design?

* Can they carry the load of a mast?

I'd like to avoid a compression post or mast smack dab in the middle of the forward double berth on a smallish boat. Heeeeelp please!

[John E Hardiman]

Think of a ring frame as a built in bulkhead with most of the middle removed. They can be used to support masts, but they need to be engineered into the design to ensure that the frame itself is thick and deep enough and doesn't overload the skin/decking where it is attached. There are also structural stability considerations (i.e. section buckling stability) but for wood most of those are minimal due to the thickness needed to develop full structural strength of the wood.

[Bruce Hooke]

My understanding is that a ring frame is basically a "frame" that goes full circle and thus includes the deck beam. So, rather than two pieces that come together at right angles at the intersection of the topsides and deck, the frame would curve smoothly right into the deck beam.

It would certainly need to be "engineered" or at least designed based on the boat it is going into. I would not expect miracles from a ring frame regarding providing support for a mast. A ring frame will certainly be quite strong regarding loads that stress the connnection between the deck beam and the frame, but the mast load is pretty much straight down and so it's mostly loading the deck beam portion as a simple beam. So, I would expect that the deck beam portion of a ring frame would be fairly similiar in size to a regular deck beam designed to carry the same load. HOWEVER, one way a ring frame could gain you something is by effectively reducing the length of the deck beam portion of the frame. Depending on the flare in the topsides and the layout of the space inside the boat you might be able to gain a fair amount by curving the ring frame in away from the planking well below the deck and thus reducing the length of the deck beam, which would greatly assist in making the deck beam strong enough to support the mast.

I've sailed many miles on a 23' sailboat with a deck stepped mast without a post under it. It is certainly results in a more flexible system than it would be if there were a post under the mast, so the rigging ends up being looser than a racer would like, but for the non-racer it works just fine. The beam that supports the mast is not exceptionally large, but did crack and need to be repaired about 15 years ago.

[Meerkat]

I hope you'll forgive the limitations of "ascii art"...

I was under the impression that a ring frame: ()
could carry a mast's compression load down to the keel just like a mast or compression post would: I

I can understand that it might be thicker fore/aft than a deck beam (not quite sure why though) and would have to have more vertical depth maybe?

I also thought they didn't have to be absolutly round...

[Paul Scheuer]

The King's Cruiser has a set of two that are bent oak connecting large knees in the corners between the hull and deck and between the forward cabin sides and roof. The head room is a little dicey, but the two full length berths remain clear. The mast is stepped on a fore and aft steel channel that spans the two arches.

[Bruce Hooke]

Originally posted by Meerkat:
I hope you'll forgive the limitations of "ascii art"...

I was under the impression that a ring frame: ()
could carry a mast's compression load down to the keel just like a mast or compression post would: I

I can understand that it might be thicker fore/aft than a deck beam (not quite sure why though) and would have to have more vertical depth maybe?

I also thought they didn't have to be absolutly round...Starting from the end, no, it does not need to be absolutely round. It would generally follow the hull and deck except near the sheer line where it would flow into a curve to join the "deck beam" portion of the ring frame with the "frame" portion.

In any beam increasing the width will increase the strength in direct proportion to the increase in width. A beam that is twice as wide can carry twice the load. On the other hand, increasing the depth increases the load carrying capability by the square of the depth. Make the beam twice as deep and it would carry four times the load. So, as a general rule it is better to increase the depth rather than the width of a beam.

I stand by what I said before -- a ring frame will be a bit better than a regular frame and deck beam at carrying the mast load down to the keel, but I don't think you can expect miracles.

One additional factor that does probably help make a ring frame better at this task than a regular frame and deck beam setup is that the ring frame can better resist the portion of the downward force of the mast that translates to trying to push the sides of the boat out.

I would start the design process by figuring out what size beam would be required to carry the straight vertical load. If that portion of the frame cannot carry the straight beam load then the rest is academic. On the other hand, if you can support this load with a reasonable size beam then I think you are onto a reasonable solution.

As I think about this I am realizing that there is another advantage of a ring frame. A normal beam is basically assumed to be sitting on end supports that support the ends but do not prevent them from rotating slightly as the center of the beam diflects. A ring frame, on the other hand, resists such rotation at the end points, which makes the beam stronger.

In the end, all of these small advantages of a ring frame do probably add up to a significant benefit. The trick, of course, is getting the engineering right so that the ring frame is strong enough to carry the loads, but not so heavy that it takes up excessive space inside the cabin.

[merc412]

Gougeon Bros. on boat construction has a section on I beam frames. Basicly, the deck forms one flange of the I beam, the frame the center and an inner flange is added in laminations. Mast compression puts the inner flange in tension. Build up a section and mount it in a hydraulic press to load it up to see if your frame will take the load.

[George Roberts]

Meerkat ---

You don't say what type of construction is being used or how big "smallish" is but ...

Everthing that has been said sounds reasonable.

I will only add ...

The ring frame need not be the same thickness all the way around. You might design it as a bulkhead and then remove what you need to to make a passageway.

Don't forget to resize the mast.

[john welsford]

In a smallish boat, designed from scratch to include this feature, it is possible to use the forward cabin "wall" , the one going across the front of the cabin, as a beam to support the mast. It needs to carry right out close to the gunwales and to be well supported by the frame underneath.
In little boats, of a size where the mast coming through is likely to be an issue in the accommodation, the loads are not so high as to be difficult to handle with a pair of bulkheads cut away to suit the layout and there are many possible variations on that. But beware that its not so easy with a free standing mast as the spar needs a goodly distance between the hell and the top partners and the distance between deck and keel is usually about adequate.
John W
But "ring frames" are more often used to spread the chainplate loads