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Jonathan G
02-02-2011, 07:19 PM
I have a 50 year old Cheoy Lee sailboat. Fore of the mast (about 8 inches) is a bronze rod that traverses from the mast base through the deck. On deck it has a nut. What is its purpose? Where can i find information on it?

Peerie Maa
02-02-2011, 07:24 PM
Pictures would help.

It is probably to resist the compressive stresses from the rig pushing the mast out through the bottom and opening your garboards, but without pictures that is just a guess.

Hwyl
02-02-2011, 07:24 PM
It's to counteract the pull of the shrouds and other rigging tensions that are on the deck, such as turning block, if you visualise the forces and think of the deck as a beam it should become obvious

Jonathan G
02-02-2011, 08:21 PM
It looks like it could be a compression rod? it would be to displace the loads on the deck down to the keel. Does that sound right ? Any other insight?

Vinny&Shawn
02-02-2011, 08:24 PM
This is a tie rod installed to counteract tensions as described in previous posts. Murray Peterson prescribed these to be added to many of his designs. I built one into a Crocker yawl that I once owned,this went from the mast step through the mast partner. Also on two other vessels of our ownership I added what is called a Pardey tie rod developed by Larry Pardey, this is a large turnbuckle attached to the mast and is connected to a deck beam that is closest to the mast and partner. In all instances there was a noticeable reduction in hull movement.

Jay Greer
02-03-2011, 12:20 AM
All of the above comments are correct. The compression rod, in the way of the mast, is a major component in helping to lesson hogging of the hull.
Jay

Ian McColgin
02-03-2011, 09:28 AM
I don't see anything in the term "rod" that leads me to call whatever this is a compression post. Especially since there's a nut on deck and no mention of one under. And even more especially since it's not right under the mast but 8" away.

But we also have troublesome terms like "mast base" with no idea as to whether this is the mast's butt or a step on deck.

I think Gareth has it right - if a correct interpretation is that it's a rod from mast step or keel up through the deck. Then it is a structure in tension - like a stay - rather than a structure in compression, like a stayed mast. And most especially if the deck is even slightly cambered.

Since the stays are out from the mast, there are two vectors operating at the chainplates. One vector is up, of course. But the other is in, against the deck and such. Given that this boat could easily have two lowers and one upper on each side, that's six closely spaced locii for what amounts to squeezing the hull inwards. That inward strain is resisted by the athwartships strength of the deck. Now a cambered deck is a good thing - sheds water and is a gentle arch that resists any point loading of people walking about. However, a lot of boats rather gently ease into the gunnels coming up and in at the chainplates. This pinching in can also leave enough room for the bow and stern, those stays subtly 'slacked' by the shrouds' movement, to sag. Hoging is a droop of bow and stern relative to the middle. One of the three or four causes of hogging is just this - the timbers that go into the partners are not fully resisting the pinch.

(Two other causes that spring readily to mind are bad blocking if the boat is out for a winter or for a year or two, or for a sailboat leaving her in the water with no rig up or with the rig slacked.)

So, if it's a keel stepped mast, this is NOT a compression post. A comression post simply translates the thrust of the mast straight to the keel rather than out along the deck to the sides and then down. If a boat needs a compression post, the need is grossly obvious from the deck sag and subtly visible from the opposite of hoging, a thrusting out at the gunnels athwart the mast and drooping of the bow and stern.

If the mast is deck stepped or if the interpretation that the "rod" goes to the keel is wrong, they from the description thus far, I don't know. So take pix.

G'luck

Michael D. Storey
02-03-2011, 09:44 AM
suggest that you look up the mast for other fittings, say that would be the other end of some sort of contraption. I am struggling to conjure an image, here. Part of a self-tending staysail? Hey, how bout a pic?

chuckt
02-03-2011, 10:36 AM
What is a picture worth? Oh yeah I remember now. ;)

Jay Greer
02-03-2011, 10:49 AM
You guys are making a basic principal of boat construction, needlesly, complicated. Ian has, however, stated the differance between a "compression post and a compression rod", very clearly. While there are many other reasons that a mast stepped incastrae as opposed to being stepped on deck is superior for both performance and ease of construction that should be obvious to those who have both a bit instinct and common sense. Please note that no insult is implied or directed at anyone.
Jay

JamesCaird
02-03-2011, 11:38 AM
Hi Guys-I tend to think of the described arrangement not so much as a "mast rod" but rather as a "deck rod' or "cabin house rod". As described by Ian, if you imagine the horizontal component of forces at the chainplates trying to squeeze the hull together there near the mast, the deck would bulge up (or down) if allowed. The rod described would resist that tendency of the deck to try to bulge up. Sometimes the rod is seen from mast step to cabin trunk roof where mast penetrates there. Without this rod often the mast wedges and partners (or the mast itself) will be heard to squeak as the boat is driven to windward , for example, indicating movement at the partners-it's not the mast so much as the deck moving! Sometimes a horizontal compression strut is placed athwartships near the mast-with the adjustable ends lodged into sockets at the carlins. (This strut is often removable for cabin convenience) Obviously in some designs a whole bulkhead or web frame is located at this position, all for the same purpose. Further, I have seen arrangements where chainplates are internally tied directly to the mast step with cable and turnbuckles, all these things together trying to reduce the tendency of the rig loads to squeeze the hull. My understanding/Cheers/JC

(Hey-just noticed-How did I get to be a "Senior Member"?)

Ian McColgin
02-03-2011, 12:16 PM
Cabeling the chainplates to the keel is a solution for a poorly engineered boat with tired fastening. It spreads the vertical force vector away from the hull and planks. It also exactly doubles the inward horizontal vectors. It's a solution that can get a boat past her first century but like all stress solutions needs some engineering thought since it can also simply move the problem to a new weak link.

redbopeep
02-03-2011, 12:51 PM
We dealt with installing new posts of this sort in December of 2007 when re-doing the deck. Here's my web log post on the matter of compression/tension as well as a pic of the bronze post being driven into the deck beam---LINK (http://blog.mahdee.com/2007/12/08/compression-post-driving/)

Here's the post in front of the main mast as depicted on the plans. It is a metal post with a wooden split post surrounding it.

http://farm5.static.flickr.com/4119/5413825650_3c07003fb7_z.jpg

Jay Greer
02-03-2011, 07:10 PM
My Common Sense sloop design has both a compression strut and the upper chain plates wrap under the mast step with turnbuckles on either side of a saddle that runs under the step.
jay

floatingkiwi
02-03-2011, 10:07 PM
I have a 50 year old Cheoy Lee sailboat. Fore of the mast (about 8 inches) is a bronze rod that traverses from the mast base through the deck. On deck it has a nut. What is its purpose? Where can i find information on it?

It IS, to prevent the deck from lifting or pumping from forces exerting by rigging and mast, in a keel stepped arrangement..
Don't take my word for it. See for yourself.
http://im1.shutterfly.com/proctaserv/47a1dc22b3127ccefc13304658da00000037100AZtGbhu3cs2 QPbz4

wizbang 13
02-03-2011, 10:14 PM
Lemme get this straight,
It's a "rod stay" that tranfers floor load into the partners... thus , the deck?
That certainly would tend to tighten up a macaroni rig set in a carvel hull.
or, is it a gizmo to keep the deck DOWN, as in, the mast partners are caulked very, very tight?

floatingkiwi
02-03-2011, 10:20 PM
We dealt with installing new posts of this sort in December of 2007 when re-doing the deck. Here's my web log post on the matter of compression/tension as well as a pic of the bronze post being driven into the deck beam---LINK (http://blog.mahdee.com/2007/12/08/compression-post-driving/)

Here's the post in front of the main mast as depicted on the plans. It is a metal post with a wooden split post surrounding it.

http://farm5.static.flickr.com/4119/5413825650_3c07003fb7_z.jpg

Mate. Your bewilderment about the term compression post is irrelevant as it isn't the thing you're installing that people refer to when saying," compression post", but a continuation of the mast below the deck, on a deck stepped mast. In fact, a lot of people are actually unaware of this tension rod existing ,( especially those having only owned small boats), let alone knowing what it is called.
And your blog mentions the DECK being in tension which is incorrect aswell. The deck is ,( in this situation), under compression from the sides, although the urge for it to bulge upwards could be thought of as being pretty inTENSE. Like Arabs are inTENTS, and when raising them in high winds, can become very inTENSE.

floatingkiwi
02-03-2011, 10:44 PM
Lemme get this straight,
It's a "rod stay" that tranfers floor load into the partners... thus , the deck?
That certainly would tend to tighten up a macaroni rig set in a carvel hull.
or, is it a gizmo to keep the deck DOWN, as in, the mast partners are caulked very, very tight?

See, even knowledgeable olde wizbang hasn't heard of them.(no offence mate).
Here, pretend the eraser is the deck and my thumb and forefinger are the hull and chainplates.
http://im1.shutterfly.com/proctaserv/47a1dc22b3127ccefc131e6f58e400000037100AZtGbhu3cs2 QPbz4a

OK. Now push down on the mast and up with the rigging and the hull wants to compress the deck inwards, resulting with the already curved up at the centre, deck, wanting to lift. See that?
http://im1.shutterfly.com/proctaserv/47a1dc22b3127ccefc13f723184400000037100AZtGbhu3cs2 QPbz4aThe rod resists that deck lifting , which is apparent mostly,( and usually only when using ones eyes), at the mast, because the mast is close to it and doesn't move.Please note that this is in a keel stepped mast arrangement.
I reckon the descriptions offered by Ian and JamesCaird are the best, although not the only possibly correct, explanations.
Apart, of course, from my little pictorial display here, heh heh.

Roger Cumming
02-03-2011, 11:22 PM
The tie rod is in tension, not compression. Just as the photos directly above illustrate, the shrouds exert a force like the thumb and index finger that squeezes the deck causing it to bulge upwards. The tie rod resists this by holding the deck down and preventing it from bulging up, putting the rod in tension.

Canoeyawl
02-03-2011, 11:48 PM
The tie rod resists this by holding the deck down and preventing it from bulging up, putting the rod in tension.
Thus keeping the sides of the boat out and ends of the boat up, preventing hogging.

floatingkiwi
02-04-2011, 02:06 AM
I just, coincidentally, found this whilst surfing the net just now.See the tension wire in front,(or back), of the mast?http://www.divaroyal.com/pics/cwfolk_pic3.jpg
The reason I have shown a deal of interest in this thread is because I have been through it thoroughly after finding a pair of wires with swaged ends and fittings that fit perfectly between partners and mast step, on another folkboat I was working with. Sparked quite the debate, it did.

Wooden Boat Fittings
02-04-2011, 07:27 AM
Rods or wires in compression don't work, they buckle -- as George Burns famously said (although in a different context,) it's like trying to play pool with a rope.

A timber compression post under a deck-stepped mast could possibly work in tension as well as compression, but its fastenings would have to be pretty cleverly designed. It's much easier to rig a parallel rod or wire as described to take any tension stress.

A compromise solution for a deck-stepped mast is to use a length of steel pipe, as Sanderling has, to take both compression and tension. It can be fastened easily and properly using screw-on flanges bolted to deckhead and keel so that it resists tension forces, and (unlike the wire or tie-rod) its slenderness ratio can be large enough to allow it to properly resist compression loading as well.

When I first saw Sanderling's steel compression post I didn't like the look of it a great deal, and I thought of changing it for an oregon one instead. But when I considered that it was also acting in tension I refrained. While timber to me would have looked far nicer, it would have only been able to work satisfactorily in compression. (The compromise was to box the steel pipe in oregon siding.)


http://www.woodenboatfittings.com.au/boats/sanderling/intl-fwd-1.jpg


Despite the comments about through-deck masts being superior, a mast stepped in a tabernacle is better for boats that, for instance, need to shoot bridges (or possibly power-lines) --


http://farm4.static.flickr.com/3424/4002415769_4277b9d314.jpg


or are trailed --


http://www.woodenboatfittings.com.au/boats/caoimhe/caoimhe5.jpg
Mike

redbopeep
02-04-2011, 12:37 PM
Mate. Your bewilderment about the term compression post is irrelevant as it isn't the thing you're installing that people refer to when saying," compression post", but a continuation of the mast below the deck, on a deck stepped mast. In fact, a lot of people are actually unaware of this tension rod existing ,( especially those having only owned small boats), let alone knowing what it is called.
And your blog mentions the DECK being in tension which is incorrect aswell. The deck is ,( in this situation), under compression from the sides, although the urge for it to bulge upwards could be thought of as being pretty inTENSE. Like Arabs are inTENTS, and when raising them in high winds, can become very inTENSE.

Thanks, so much, for pointing out my error in the blog post stating the deck was in tension when I meant the post (typing fast does sometimes cause problems when one blogs); however it is clear from the rest of the blog post, I do understand that the deck is being forced up by the rig load/squeeze of the hull and putting the POST in tension. But, thanks, I corrected my 3 year old blog post today.

Now, the fact that many, many, many professional woodenboat builders (yes, professional shipwrights) call this post a "compression post" is darn confusing to me, yes.

And it is actually relevant here, too since other folks may hear the same term incorrectly bandied about. The purpose of the metal rod is not confusing to me at all but the fact that I've heard several shipwrights call the thing a compression post is very weird and confusing. My former boat with a deck stepped mast had a compression post. That was my only exposure to real compression posts. However, the incorrect use by shipwrights/boatbuilders and professional marine industry personnel of "compression post" for this rod is entirely confusing.

Ian McColgin
02-04-2011, 01:01 PM
I never heard of such a rod, even hidden inside a pipe of some sort, called a compression post. Not anywhere from Long Island to Maine, but then I've not been in the baylinercondominiums much.

In our usage, a compression post is directly under the load and takes compression. It might have some screws at top and bottom so that when the load is off - like a deck stepped mast taken down for winter - it does not just fall aside.

Similarly a tension rod (nothing with tension gets to be a post) could have a tube around it such that when the strain is off - again as with a mast down - and people are still about on deck and such you don't have the deck sag down 1/64" or so.

There are other places where tension rods can help. I recall a racer around here that was so lightly built that the bows panted underweigh. There was just enough convex (on the outside) to the shape that tensioning rods from gunnel to stem were just the thing - kept the shape like a strung bow.

Hwyl
02-04-2011, 03:34 PM
One of the 12's. I think it was American Eagle/War Baby had rigging wires with turnbuckles from the chainplates to the mast step, when it was retrofitted to a cruiser racer one of these went though the head compartment and in close proximity to the head. I can't quite remember if you had to straddle it to sit on the head, my memory is fuzzy, I think probably you had to sit on the head with your legs askew and to one side. It always made me smile, I hope it made Ted Turner smile too.

John B
02-04-2011, 03:39 PM
This thread has been really bugging me. A tie rod, which is what this thing is, ties things together. Thats why it has washers and nuts on the ends.
It's in tension to stop the deck / beams/ partners lifting at the partners due to the camber of the deck and the force exerted on them by the mast sideways.

floatingkiwi
02-04-2011, 04:18 PM
This thread has been really bugging me. A tie rod, which is what this thing is, ties things together. Thats why it has washers and nuts on the ends.
It's in tension to stop the deck / beams/ partners lifting at the partners due to the camber of the deck and the force exerted on them by the mast sideways.

No, not the mast sideways but the gunnels being forced inward due to the mast pushing down on the keel and the rigging pulling the chaimnplates up and, well, in, as the virtual triangle btween the keel and c'plates and up to the masthead tries to straighten out.
Another thing that isn't mentioned, unless I missed it, is that this tension rod or wire, is not necessary in a deckstepped arrangement. If there was something like this in a deckstepped mast, it would only work if there were two wires from keel to c'plates to stop the gunnels lifting. Cripes. That would probably mean one was babying tired fastenings .
http://im1.shutterfly.com/proctaserv/47a1dc22b3127ccefc13304658da00000037100AZtGbhu3cs2 QPbz4

John B
02-04-2011, 05:05 PM
I'm sure thats a component Kerry , but you still need a tie rod or bulkheads that do the same job on a freestanding rig where all the rig loads are side/ forward/ back oblique, and thats stress perpendicular to the mast.

A compression post is fitted to deck stepped masts.because its loaded in compression.

Wooden Boat Fittings
02-04-2011, 07:02 PM
Yes. But as I said above, it can also be loaded in tension (depending on the sailing loads from moment to moment, or shock loading from falling of a wave for instance.) It's appropriate to call it a 'compression post' because most of the time it probably is in compression.

In Kerry's diagram, the two tension arrows in the hull are pointing the wrong way -- the shrouds are pulling up on the chainplates, the hull is pulling down on them, and the resolution of the two forces is an inwards push into the deck. His illustration using the eraser of what happens next if the deck structure can't take the horizontal forces is quite clear -- if the deck has any camber on it, up it goes. You can stop this by having a wire or rod in tension alongside the compression post. Or, as in Sanderling's case, a post that functions in both compression or tension as necessary.

floatingkiwi
02-04-2011, 07:37 PM
A compression post is fitted to deck stepped masts.because its loaded in compression.

I agree. I am not, nor is the basis of this thread, talking about a compression post. A boat that has the mast going to the keel does not even need a compression post.
Once again, this tension rod in unnecessary in a DECKSTEPPED arrangement.

Read Ians last post.

floatingkiwi
02-04-2011, 07:44 PM
http://www.divaroyal.com/pics/cwfolk_pic3.jpgSee the wire in front,(or aft), of the mast. Forget about the mast . The wire. Does that look like it is set up for compression?

floatingkiwi
02-04-2011, 07:47 PM
In Kerry's diagram, the two tension arrows in the hull are pointing the wrong way --.

Actually now you mention it. I could have drawn them both ways top and bottom eh?

Vinny&Shawn
02-06-2011, 10:32 AM
Here is Brian Toss's discription, as I tried to explain in post #5.
http://i419.photobucket.com/albums/pp278/vgeorge1/PardeyTie-Rod.jpg?t=1297006310

wizbang 13
02-06-2011, 10:49 AM
It looks to me like 10 pounds of metal substituted for 100 pounds of wood.
I'll stick with beefy partners.

Vinny&Shawn
02-06-2011, 03:20 PM
That is just a simple drawing, not detailed or to depict any scantlings, it is just showing how to install the tie-rod in a keel stepped mast. read Brian's description.

redbopeep
02-07-2011, 05:19 PM
I don't agree with the Pardey's tie-rod being used to achieve the same goal as the post depicted in the drawing I posted here. The way ours is done, the metal rod ties the deck down to the mast step and the mast step is secured with bolts through the floors which are in turn bolted to the keel. A lot of "in turn" and certainly not bolted through the keel or anything but...fairly secure for keeping the deck "down" when the rigging loads are bearing inwards on the cambered deck pushing it up. In theory, if you use a tie-rod per the Toss illustration and the Pardey's method, at some point, the deck is going to be pulling UP on the mast with that tie rod. So, load up the rig alot and then pull up on your mast...in a tired old boat I wonder if the whole thing could lift the mast out of the mast step? Hopefully not, probably not, but I like the rod as we have it better--even if it isn't convenient in terms of the space it takes up.

CapnJ2ds
02-08-2011, 08:32 AM
Can we forget about compression posts, please!

This sort of tension rod used to be quite common, particularly on gaff rigs where the halliards came down to a pin rail on deck at the partners. It's to counteract the upward pull of the halliards. The advent of deck-stepped masts |:), f--------s hulls :eek:, and internal halliards with inbuilt turning blocks :eek::eek: made these tension rods unusual today.

redbopeep
02-08-2011, 09:31 AM
The upward pull of the halyards is nothing in comparison to the load being imposed on the hull while sailing. The halyards will break before the deck is going to move upwards. Where ever there's a keel-stepped mast and a flush deck (or raised deck) boat, you should see some variation on this rod near the mast.