Chainplates job and hull strengthening - SailNet Community
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post #1 of 9 Old 12-30-2015 Thread Starter
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Chainplates job and hull strengthening

Hi everyone,

I'm looking for advise concerning a job I am planning to do this coming spring. But first, let me tell you the story.

When I bought the boat in 2012 (my first one), I was still a novice. After a 500 nm. round trip on the St. Lawrence river on the first summer, a guy at the marina told me it could be a good idea to take a look at the chainplates. Further investigation let me think that beautiful Calypso has probably kept an eye on me during our first date. I discover that, due to lack of care from previous owners, not only that chainplates were leaking, but that the wooden blocks attached to them were literally disintegrating in little pieces, and the bolts that fasten the chainplates were so rusted that they were not bigger than toothpicks in the middle (no joke). It is useless to mention that I was planning major chainplate work for the next spring.

During the winter, I was questioning myself on how to do the job properly. The original chainplates were of the encapsulated kind. As they were (and still are) designed in a through deck configuration, the original setup was : 1) the SS chainplates were bolted through a wooden block of the appropriate thickness to fit the hole on deck (but they were bolted through the "width" of the block, so, if I take a 2x3 as an exemple, the chainplate was sitting on smaller side of the block and bolted through 3 inches of wood, while the larger side was the one that filled the "gap" between the chainplate and the interior side of the hull). 2) Then, the block and the CP was glassed on the hull, while the block itself was sitting against the interior side of the deck. 3) Additional bolts were passing through the glass capsule and the block. It is important to note that, if there is a bulkhead located between the front and the middle CP on each side, the CP themselves were not (and still are not) glared or attached to it in any way.

At the time, I found this setup to be possibly not the best one, considering that the glass prohibited me to have a look at the actual state of the chainplates (from the outside, except for some external rust, everything look fine). So I considered changing it. The way I did it was to have the new CP bolted through the hull. Now, from the outside, I have 1) a SS backing plate, 2) the hull, 3) two wooden blocks, one over the other (installed this time on the "larger" side for more tension distribution), 4) the new SS CP (which are now very easy to inspect). Because the interior of the hull was not fair, I've also made a bed from bare epoxy to give an appropriate surface to the wooden blocks (see pictures for details).

It seems to me to be fine and stronger this way, but I now think that the hull was not strong enough to support it. Even if the wood surface that apply the load on the hull is now approximately three times larger than it was in the original setup, I have noticed flat spots (it bends on the inside) on the hull from the outside this summer (nothing "very" apparent, but sufficiently to be noticed at a close look). I haven't measured it, but at first look it seem that the depression is something around an ⅛ of an inch in the worst place, and maybe 3-4 inches around each outside backing plates (see picture). I assume that the glass that previously hold the wooden blocks in place were also acting as load distributors, and the new block, by themselves, are consequently by far to narrow.

Even if there is no crack on hull's paint/gelcoat, I find this situation to be one that necessitates more attention. Now I am planning to redo the job this spring and I am looking for tips. Now, I am considering 1) to strengthen the hull by laminating new layers of glass from the interior; 2) glassed in three horizontal stringers on each side (one for each bolt hole) that will be long enough to reach the three CP at the same time and give them adequate support (that will replace one of the two wooden block); 3) put back the last wooden block and bolt everything back so it makes, still from the outside : a)Backing plate, b)hull (now thicker), c) stringer, d) one wooden block, e) CP.

I don't think it would be necessary (unless for aesthetic reason) to apply new glass from the outside of the hull.

What do you think of that ?

Thanks in advance, and sorry for my not-up-to-the-task English
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Grand Duc - Hughes 29 (hull #13) - Yanmar 2GM20 - Club nautique de Longueuil (Québec, Canada)
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post #2 of 9 Old 12-31-2015
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Re: Chainplates job and hull strengthening

Have you not considered building up the inside of the hull with multiple layers of glass and moving the chainplates to the outside of the hull? Stainless that is encapsulated is prone to corrosion as you have seen. Having them outside eliminates encapsulation and allows better inspection.


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post #3 of 9 Old 12-31-2015
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Re: Chainplates job and hull strengthening

You can extend the back up plates down the inside of the hull, to spread the load more.

That plastic lumber they make from recycled plastic doesn't rot, and has a lot of uses on a boat
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Re: Chainplates job and hull strengthening

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post #5 of 9 Old 12-31-2015
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Re: Chainplates job and hull strengthening

Whether inside or outside of the boat, this is an ill-conceived alteration. As installed, this is a much weaker installation than the original installation and one that is prone to have a shorter lifespan than the original installation. I say this in part because the original installation included knees which stiffened the hull and spread the loads over a much larger area. But also the original installation oriented the chainplate so that the moving axis of the shroud loads were closer to parallel to the long axis of the chainplate, thereby reducing flexure in the chainplate, and minimizing work hardening a more serious problem with stainless steel than corrosion. Additionally, the original chainplates were in double shear rather than single shear. Single shear can be made to work but the chainplates and the bolting surface need to be much thicker or else have way more bolts than were present in the original chain plates.

At the very least, if you chose to orient the chainplates in the direction that you have them in your photos, you should glass in knees perpendicular to the hull in order to reinforce the hull, lengthen and thicken the build up of glass on the interior of the hull, and lengthen and thicken the chainplates themselves, and lastly add more bolts and perhaps backing plates.

A stronger, cheaper, and more reliable solution would be to build knees, orient the chainplates the correct direction, and bolt the chainplate to those knees.

Jeff


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post #6 of 9 Old 12-31-2015
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Re: Chainplates job and hull strengthening

I retract my previous statement. For some reason I didn't pick up that the orientation had been changed and assumed the photos just showed replacement of rotted materials. I will be silent as this one is over my head.


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post #7 of 9 Old 01-02-2016 Thread Starter
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Re: Chainplates job and hull strengthening

First of all, just to make that clear for eveyone, all the pictures were taken after the first modifications have been made.

Jeff, you said that
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Whether inside or outside of the boat, this is an ill-conceived alteration. As installed, this is a much weaker installation than the original installation and one that is prone to have a shorter lifespan than the original installation. I say this in part because the original installation included knees which stiffened the hull and spread the loads over a much larger area.

Jeff
But don't you think that glassed in stringers will do the job, maybe even more efficiently as I plan to have them long enough to reach the three chainplates at once ?

You also speak about using wooden knees to orient the chainplate in the axis of the shrouds. It think it could be a good idea (and propably a lot of work ) but in the original setup I don't think the wooden blocks were used for that pupose, because they were all, as far as I can remender, just "square" pieces of wood, without any specific angle. This why I thought at first that just new wood blocks should work fine. But probably that original setup was not up to the task as well, because I rember that the original CP were bended when I removed them.

Also, could you give me more explanation on the difference between single and double shear, because I don't understand what you are reffering to. Is it something concerning the thickness of the SS ?

Again, thanks everybody or your help.

Grand Duc - Hughes 29 (hull #13) - Yanmar 2GM20 - Club nautique de Longueuil (Québec, Canada)
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Re: Chainplates job and hull strengthening

Anybody?

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Re: Chainplates job and hull strengthening

Hi again everyone,

First, thanks to Jeff_H for his advises. I've taken a few weeks to read more on the topic and think about it. The principal points I should keep in mind should be this:

1- To allow easy inspection and to avoid corrosion problem, the chainplates themselves should not be covered by fibreglass;
2- To minimize the stress on the chainplates and on the boat, the chainplates should be aligned with their respective shrouds;
3- To avoid exaggerated stress on hull, I should prefer an installation using mounting knees that needs to be glassed on the hull

This how I am now conceiving it:

1- Remove my current installation;
2- Strengthen the hull by adding layers of glass from the inside (and probably also from the outside, to fill the depressions that have been made by the chainplates);
3- On that thickened hull, glassed in wooden knees correctly shaped to sit against the side of the hull and the deck and to give the correct angle to the chainplates and their shrouds (but with no chainplates on them);

Question #1: Do you have any suggestion concerning the type of wood I should use? The backing blocks I've for my current installation (shown on above pictures were made of IPE. I was planning to use the same type of wood, for is resistance to rot and density, even though it would be hard to shape.

3.1- To prevent risk of rot, I will first apply a coat of epoxy on the wooden blocks.

Question #2: What do you think of penetrating epoxy? According to West System website, it doesn't seem to be a good idea, especially if I am using high density wood. (WEST SYSTEM - Projects - Thinning WEST SYSTEM epoxy)

3.2- To also prevent risk of rot, I was thinking that it could no be a bad idea to first drill larger hole for mounting bolts, fill them with epoxy, and re-drill them the correct size.

Question #3: Do you think it is a good idea and, if yes, do you have any suggestion concerning the type of epoxy I should use (pure, with additives and which one?)

4- Flatten the "front side" of the glassed-in wooden knees so as the chainplates perfectly sit on them.

Question #4: Do you think it will work or should I better try to have the chainplate seats already molded in the last layers of laminate when I will be glassing in the wooden blocks?

5- Drill mounting holes for chainplates and their backing plates through the glassed-in wooden blocks and through the hull.

Remark #1: I wasn't sure if outside backing plates were still a good idea, because I was fearing that stress apply from the outside on a relatively small areas (the ones covered by the backing plates) will generate again the same problem (I have considered for a period to do something more like what can be found under "Option 2" there : Replacing The Chainplates | SEA SPRITE ASSOCIATION). But the way I figure it now is that having the wooden knees glued against the hull with epoxy and glassed in will made the whole "one block" that should work together (i.e. the load apply from the chainplate to its backing plate will be distributed to a larger part of the hull, because the backing plate will pull the whole glassed-in knee "through" the hull.)

Remark #2: What decided me to have outside backing plates is that it seems to me the only way to easily remove the mounting bolts to inspect them and change them if necessary.

Question #5: How many mounting bolts do you think I need (is there a formula to calculate this?) and is their a specific pattern to place them (vertically aligned, "Z shaped" pattern, etc?)

Question # 6: Do you have any comments concerning Remark #2 ?

Question # 7: One problems that I see with this is that having the chainplates aligned with their respective shrouds and the outside backing plate sitting against the hull will create a problem with the angle of the mounting bolts (they will not rest at perpendicular angle with the metal piece on one of their side, which, I assume, will tend to bend them). Should I be concerned with that (especially with the lower-shrouds)? To avoid that problem, I have imagined that I could use thicker chainplates and ask the shop to drill-in mounting holes that will be parallel with the hull axis. What do you think of that ?

6- Install chainplates and their backing plates with the mounting bolts, sealing them with appropriate sealant.

Question # 8: Do you have suggestion concerning the material the chainplates, backing plates and mounting bolts should be made of? I've read that 316 stainless steel should be OK.

What do you think of this project ? Am I doing it the right way ? If you have any suggestions of comments concerning the whole project or concerning specifically asked questions, I will be glad to read them.

Thanks in advance to everybody.

Grand Duc - Hughes 29 (hull #13) - Yanmar 2GM20 - Club nautique de Longueuil (Québec, Canada)
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