Re: Potter design ?

JBlumhorst@aol.com
Sun, 12 Sep 1999 03:09:09 EDT


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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In a message dated Sun, 12 Sep 1999 12:53:08 AM Eastern Daylight Time, Sukoshi1044@aol.com writes:

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> West Wight Potter Website at URL
> http://www.lesbois.com/wwpotter/
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> In a message dated 9/11/99 9:48:43 PM Pacific Daylight Time, keithhub@jps.net
> writes:
>
> << I am considering buying a Potter 19 but have a few questions. Does
> anyone know why they don't use spreaders on the mast stays and why
> don't have a rear stay? >>
>
> Hi Keith:
>
> The answer to the first question is that with the relatively short mast a
> spreader is not needed. The backstay is an option <or better yet;
> 'aftermarket'>/used to be standard on HMS 18.
>
> Bill de Ment P-15 #1044 'Sukoshi'
> Eagle, Idaho

Hi keith (and Bill),

Bill is right on target with his answers, and there's more. The loads on the mast are quite adequately borne by the side shrouds that are anchored to chainplates aft of the mast. This location supports the mast column against forward forces as well as lateral forces.

I'd like to quickly address the issue of backstay first:

As Bill pointed out, the rig, as originally designed by Herb Stewart when the boat was called an HMS-18 included a backstay. Nowadays, the backstay is available as a modestly priced "option" from the factory.

I guess the factory's thinking is that they want to keep the entry level boat affordable -- and to tell the truth, the lack of a back stay has never been reported to lead to rig failure.

Nevertheless, the backstay makes a huge improvement in downwind stability and upwind pointing ability. Downwind, it keeps the mast from "pumping". Upwind, it keeps the forestay from sagging under the load of the high apparent wind velocity -- the entry of the jib is much straighter, the headsail doesn't get a large belly, and the boat sails much higher and faster into the wind.

WHEN DOES A RIG NEED SPREADERS? WHY DOESN'T A P19 NEED SPREADERS?

The discussion of spreaders is a little more complex, but one of my books covers it nicely in about 2 or 3 pages. I'll try to make it even simpler.

The purpose of spreaders on most rigs it to get the angles between the mast and the shrouds to at least 12 degrees.

Here's the answer: The Potter 19, has a relatively short mast combined with a wide beam. You don't need spreaders to achieve that minimum 12 degree angle. And with the angle greater than 12 degrees, the 1/8" wire is plenty strong enough to resist the strain placed on it, the chainplates can be reasonably small, the strain on the deck/bulwark attachment for the chainplates is reduced, and the mast column is adequately supported.

With a shroud angle greater than 12-15 degrees, all those components of the boat can SAFELY be made much lighter and less beefy -- and they last longer too, because the forces that cause fatigue and failure are reduced.

If you put a tall mast on a narrow beam boat, that angle would be less than twelve degrees. With an angle of between 1 and 12 degrees, a lateral load on the sail of exerts an enourmous strain (pulling force) on the wire and chainplate. With a shroud angle of less than about 12 degrees, the mast is likewise under enourmous compressive forces and could buckle when a lateral load is placed on it by the wind.

For some examples, lets consider a 100 pound lateral load on a sail at the top.

With a 1 degree angle between the mast and the shroud, the strain on the shroud and chainplate and deck is about 10,000 pounds. At 2 degrees, it'a about 2,400 pounds. At 5 degrees, it's about 1,000 pounds... at 7 degrees it's 800#, at 9* it's 700#, at 11*, it's 600#, and at 13*, it's about 450#

The graph looks something like this:
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Well, that's a very crude graph, but my point is that at around 12 degrees or greater, the curve flattens out and the strain on the shroud is minimized. Same thing goes for the compressive loads on the mast column, they are minimized.

So designers of rigs like to get that angle between the shroud and the mast to around 12-15 degrees. The 12 degrees is considered critical to designing a reliable rig. With a tall mast on a boat with a narrow beam, they have to use spreaders to get the angle wide enough. Without spreaders, they'd have to use ridiculously thick and strong wire, and the chainplates and decks would have to be 2 inches thick!!

So on a Potter 19, you simply don't need spreaders to get a nice wide angle between the side shrouds and the mast. There's no need for the extra weight, complexity or expense of spreaders. The mast is spec'd adequately to resist the compressive and buckling loads with the shrouds stabilizing it, and the 1/8" wire shrouds are more than adequate to handle the strains.

In fact, you don't even need the lower shrouds to support the mast laterally. (Please don't remove them though!!) The top shrouds can handle the strain all by themselves. As I've written in previous lengthy posts, the main purpose of the lower side shrouds is to limit forward bend of the mast when it's tensioned by the backstay, vang, or tightly close hauled mainsheet.

To repeat, the answer is pretty staight forward: The potter is wide and the mast is not tall. You don't need spreaders to get that critical 12 degree angle!

Best,
Judy B