Mast Assembly

[Henri Hali]

I've just installed my new mast step. In the process I'll be cutting off about 1 1/4'' of my mast.
I had a hell of a time reinstalling the tie rod to the new mast step since it is 1 1/4 '' higher.
Well it is done, but it got me to thinking what the damn thing is for in the first place.
The instructions tell me to install the rod "hand tight". Okay, I did, but what does it do?

Henri

[S/V Deo Volente]

It's to keep the deck from flexing, it helps to counter the upward stress from the shrouds.

[Dale Tanski]

Henri,

A very good question indeed!  I had similar thoughts when I reinstalled mine but figured it must be there for something but what?  I sure am hoping that the hull doesn't flex in from shroud pressure and that rod is there to keep the deck from pushing up.  If the shrouds exerted that much pressure that the hull began to flex inward causing the deck to bulge and rise, that 3/8" or what ever rod would probably not do that much to stop it.  Additionally, shroud loading would only be on one side at a time.

I would venture a guess that rod is there to counteract the upward forces if one were to install turning blocks at the base of the spar to lead halyards and or reef lines back to the cockpit.  It it takes 500 pounds to hold up the mainsail when the mainsheet is on hard upwind, then there would be a 500 pound load in the up direction from the main halyard turning block if it was mounted on the deck at the base of the mast. Currently that up load is absorbed by the spar where the main halyard winch is mounted. 

So, 500 pounds up on the mainsail by the main halyard would equal 500 pounds in the up direction of the deck at the base of the mast thus the need for that rod.  Either Bill Shaw figured that someday someone would install halyards running aft and it would be a great addition to the boat before the deck buckled skyward or its there to ground the mast collar?

Dale

[S/V Deo Volente]

Actually it is combined forces of shrouds pulling up and the mast pushing down that wants to force the deck up. Dave Gerr explains it in "The elements of boat strength: for builders, designers, and owners" I know it seems hard to believe a Pearson would flex that much, but it's a common design practice.

[Henri Hali]

I mentioned the tie rod question to a friend who is a PE.
He often sails with me and was baffled as well.  Well.......... he's kind of like a dog with a bone when an engineering question comes up.
His research found the following. (As the German soldier from the old TV show Laugh In said, "Feery interesthing")

Tie Rod:
Prevents panting of the deck, pumping up and down as the hull becomes alternately narrower and wider depending on how the water is bearing on the hull through rough waves. EFFECTIVELY CUTS THE SPAN OF THE DECK BEAM AT THE MAST IN HALF, thereby making the deck beams way stiffer for the same sided dimension. For example four feet on an eight foot beam boat. Like putting a pier under two shorter bridge spans, the beams' cross section can be smaller than on one long one for the same flex under load. Same boat with a deck stepped mast with either a cross beam or compression post under the step will not have this tie rod.

So it looks like the rod has nothing to do with the mast at all.

Cool!

Henri

[barrylab]

A little related experience here:

I just obtained a Loos & Co tension gauge and decided to tension all the shrouds to 20% of working load to see if it made a difference in performance. It turns out I was tuning the mast to about 10% working load, and found that it's common to go for 25-30% on racing boats.

During the tuning I heard a pop, and after investigation found the tie rod had pulled free of the badly tapped threads in the aluminum mast step. I also found the door to the vee birth wouldn't close without scraping the bulk head.

I installed a Helicoil, de-tuned the mast, tightened the tie rod then re-tuned the mast to the original 20% tensions, and the door now closes properly. It seems to work just as designed. Funny how engineering works 

[EdHouston]

OK you say you tightened the tie rod how tight?

Ed

[barrylab]

I spun the nut down until it was as tight as I could get it with my fingers, then gave it a half turn with a box wrench. I put a second nut on to lock the first, then re-tuned the rig.