Spreader positioning

Hi everyone.

How critical is the vertical positioning of the spreaders on a mast?

Here's my position....

I have a wooden 54' motorsailor with no mast. The drawings call for a mast 50' above the deck, with a single pair of spreaders at 30'. The typical two shrouds from right below the spreaders run to a fwd and aft chainplate, and a single shroud runs from the masthead to the spreader endes to the middle chainplate.

I have a chance to buy a suitably heavy 49' mast, with hardware, at a good price. It has one pair of spreaders, right in the center - i.e. 24' 6" above the deck. A rigger told me that to use it I would need to re-block the mast at 30' to put the spreaders where they are supposed to be. This seems counter-intuitive to me. Surely the position of the spreaders is set by the design of the *mast*, not the boat? Shouldn't I leave the spreaders where they are? (I will be using the boom - and mainsail - that come with the mast.)

Any comments?

Richard

 

Don't forget most masts - metal ones anyway - are just made of a constant section aluminium extrusion so there is no real design in the main mast fabric, at least unless they are tapered. The hardware is just put where it needs to be.

 

Masts, rigging and spreaders are complex indeterminate truss/beams, they are usually designed to various "rules" or simple formulas, rather than by engineering analysis. Presuming the design of the mast and rig was done properly I would stick with the spreader arrangement that came on the mast.

If you are so inclined you can verify the design from any modern Yacht design text, or have a naval architect take a look at it if you want to be sure.

If your sail plan, or the sail plan of the orignail yacht of the mast you bought is very unusual, having a NA look at it might be worth the cost just to be sure. Otherwise, if they are similar, the loads should be similar as well, I would say to stay with what worked on the other boat.

 

What you mean under "suitably heavy"?

If MOMENTS OF INERTIA of cross section are not less than specified in original drawings, spreaders in the middle instead of higher will do no harm - quite the contrary.

If required moments of inertia are not known, you need either source this bit of information, or ask a Naval Architect for calculation, supplying him with maximum righting moment, shroud base, and rigging details.


regards

 

If you are using the same sailplan as the mast originally had you should not need to make any changes to the mast and stay placement. The main thing you must do is make sure the chainplates are at least as far away from the centerline on your boat as they were on the original. Also there is the issue of balancing the sail area with the area of lateral resistance, which I am sure you are aware.

 

....have a read of this too....all good knowledge
http://www.riggingandsails.com/pdf/selden-tuning.pdf

 

Remember the hull under sail (on a sloop) has 5 main points where loads are applied to move the hull. The fore/aft chainplates, the mast step area, and the two sets of side plates. The rest is rigging design to maximize the load the hull will take (keel, rudder, etc all relate to this and the distribution of the loads).
Back to spreader location, your guy may have a point as I assume bolted ss chainplates are designed to have a certain angle and by changing it you may be applying a moment and bending them to the point they crack and fail.
I would assume the original design for such a boat had an upper chainplate and two middle plates on each side. For a 50' mast not to have more than top rigging sounds silly, as the spreader would only cause more mast deflection than limiting it.
It might be wise to find a good set of sails that would fit the mast and rigging then adjust the rigging and spreader height to match the sail shape.

 

The separation of the shrouds at deck level are the most important measurment to determine the height of the spreaders. I think the rigger is right. You don't know what that mast was designed for.