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Old 01-10-2011, 13:45   #1
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Question Adding Logitudinal Stringers

Hi Everyone,

I'm steadily making progress on the full restoration of my Bristol 27, but before I fully build out the interior, I've been considering adding some fore-to-aft stringers in the boat. I may be using the term "stringer" wrong here, but in my mind, I consider a stringer as a forward to aft reinforcement, fiberglassed directly to the hull which will add add strength and resist torsion of the hull. Below is an image of the stringers I plan to add (the stringer locations are in red):



To further assist, I've broken down my current thinking and questions below. If you'd like to add some input, feel free to address the specific questions I've posited and/or add comments to anything I'm not considering.

+ What material should I use for the stringers?
-> I've heard that one can use halved PVC, foam, wood or even halved garden hose. My issue with the hose or PVC is that there would be a gap inside that could possibly trap water. Since these stringers will likely be installed below a layer of insulation, there won't be an opportunity to drain any condensated water. Adding wood would seam quite heavy, so maybe foam is the best choice? If so, what type of foam?

+ Should I drill through already installed bulkheads in an effort to span stringers between bulkheads?
-> I've read the most effective stringers run all the way from forward to aft. However, it would seem to me that adding them between the major bulkheads in the boat would also add a significant amount of strength and I don't really want to drill through bulkheads to run these stringers if it's not necessary.

+ How important is it to add these?
-> I'm really trying to go all out for strength on this boat. I'd like to go to northern latitudes where ice may be an issue. Obviously I'm not trying to winter over or anything with a fiberglass boat, but I want to build the strongest boat possible.

A few note about this plan:
+ I won't be able to install stringers between the mast-head bulkheads (the head compartment and icebox area), because I've already completed major construction in these areas; meaning I have insulation already installed and no clear access to the hull to install stringers. That being said, these areas are quite reinforced already so I'm not too-too worried.

+ I would add more stringers, however things like the settee tops, interior bulkheads and shelves will act as stringers as well. So, the stringers have been placed only in areas where there will be large "panels" of fiberglass.

+ If you'd like to get a more full idea of how the interior is constructed at this time, see the images below or visit my project site - www.bristol27.com











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Old 01-10-2011, 14:42   #2
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Re: Adding Logitudinal Stringers

Quote:
Originally Posted by bristol27 View Post

+ How important is it to add these?
-> I'm really trying to go all out for strength on this boat. I'd like to go to northern latitudes where ice may be an issue.

Strengthening the bow is the key area for summer/light ice cruising. Doing two things are useful: (a) putting a stainless plate on the outside covering about a 1 above and below the waterline covering from say a foot or two aft from the bow. That keeps the ice from gouging up the fiberglass and spreads out the impact loads. Then (b) a web of both longitundal and vertical stifferners is useful to strengthen the basic structure. The usual is about 50 cm spacing, with the stiffners being about 5cm x 5cm.

The rudder is the second most important area. Don't know if there is much you can do there. Some stiffners on the hull around the rudder post entry, and some extra glass on the leading and trailing adge of the rudder could not hurt but it is usually the shaft that gets bent.

For the rest of the hull - it can not hurt to add stiffners and since you have stripped the hull it will not be hard, but its certaintly not essential.

+ What material should I use for the stringers?

Divinycell is probably the most common foam for this job. You can actually buy pre-asymbled stiffners - costs a bit more than do it yourself. I would be tempted to use wood for the ones right at the bow waterline, as the wood adds significant strength itself and would not add all that much weight.

+ Should I drill through already installed bulkheads in an effort to span stringers between bulkheads?

Probably not worth the effort. Just bond and tape the stringers to either side of the bulkhead.

For sailing/storm loads the most important area is at the mast main chainplate and storm jib chainplate. Some 'ring frames' in those areas mighy be worthwhile if you are doing this sort of structural work.
I don't know if you read 'engineering', but if you do then the ABS polar/ice scantling recommendations (this is very steel ship oriented but gives an idea of ice related structural issues) and for general fiberglass sailing vessels are on our website
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Old 01-10-2011, 15:33   #3
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Re: Adding Logitudinal Stringers

My question is why would one do it? Why not buy a boat that has the proper strength / design for the planned adventure?

Other than this, here are my 2 cents:

+ What material should I use for the stringers?

Glass over high density foam. The glass is tabbed, the resin is epoxy.

+ Should I drill through already installed bulkheads in an effort to span stringers between bulkheads?

I would not drill. On our boat they are not continuous. They come all the way to the bulkhead but do not penetrate (the bulkheads sit on foam spacers - they do not touch the hull skin). Our hull is boatyard built to Lloyds, designed 60'ties, built 80'ies.

+ How important is it to add these?

If they are required, they should already be there from day 1. Otherwise, they can be added to limit flexing.

Most interesting questions and project! Good luck and let us know how you elect. If you can post pictures when you are finished this would be great for anybody going for a similar job!

b.
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Old 01-10-2011, 18:08   #4
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Re: Adding Logitudinal Stringers

To the OP . . . one thing to note - add insulation to the hull, 2" would be nice. Otherwise it will sweat badly and be cold.


Quote:
Originally Posted by barnakiel View Post
My question is why would one do it? Why not buy a boat that has the proper strength / design for the planned adventure?
b. boats that are 'properly designed' to do ice are rather rare and usually quite expensive.
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Old 02-10-2011, 05:04   #5
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Re: Adding Logitudinal Stringers

Doh!

Then I would buy an overbuilt steel casco and build it up for ice.

But I understand there are many ways to cruise the ice, as has been proven by people doing the NW passage in kayaks and Hobbie Cats! (And successfully.)

Ice is one of my 'neverending project' dream adventures. Too bad my boat is only 26 and plastic. But should I ever come across a relevant opportunity (read a boat for the job and a sack of gold) ... expect my postings from mid-winter Antarctic (or Arctic) peninsula!

All the best and standby,

barnakiel
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Old 02-10-2011, 07:24   #6
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Re: Adding Logitudinal Stringers

Hmmm!! This is interesting. I like my ice surrounded by Gin and Tonic (with a twist of lemon, for scurvy, ya know), so please educate me.

I have fitted pre-fabbbed hat section, and build-in-place stringers in new and retro-fitted boats.
I would think that some overall length longitudinal strength is lost by breaking the stringers at bulkheads. OTOH, you could create a grid assembly if the str'ger ends are large surface tabbed to the b/heads.
If your fear is basically in regard to passing surface ice along the hull, how does the dfficulty/ expense factor compare between your proposed plan, and laying the hull down in the yard, and applying some high strength outer skins.
I once did work on a "Chay Blyth, British Steel" type of boat, that had doublers along the water line, painted nicely, couldn't be detected from 20 ft.
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Old 02-10-2011, 07:58   #7
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Re: Adding Logitudinal Stringers

I was thinking along the same lines. Maybe something like kevlar could be used to build up skin resistance by laminating on the outside of a GRP hull?

There are some kevlar (Catana) and kevlar energized (Jeanneau) boats around.

Re the lack of continuity I am not sure how much this counts as where the stringer is not continuous the panel is supported by the bulkhead thus no support is lost but rather one supporting member is substituted by another.

There is always the gap between what seems perfect (strongest) and how things are done by boatyards. It is due to costs or due to built limitations. Single offs are built to the best standards, then they end up very expensive. When the world is seen like this, all good boatyards 'cut corners'. The trick is to do so and yet deliver a the boat that is up to her planned job.

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Old 02-10-2011, 12:30   #8
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Re: Adding Logitudinal Stringers

Thanks for everyone's input! I've responded to a few comments below:

Quote:
I would think that some overall length longitudinal strength is lost by breaking the stringers at bulkheads.
-> This seems to get to the root of the question. Is it necessary to have full length stringers to add strength or would partial stringers suffice? I guess I would like more information on the difference between the two types of stringers - partial versus full.

Quote:
My question is why would one do it? Why not buy a boat that has the proper strength / design for the planned adventure?
-> I'm already rebuilding the whole boat, and it's main purpose isn't ice sailing. No doubt a boat that is to be designed specifically for ice sailing would be better, but I only plan on a few ice excursions.

I should mention that I've designed multiple water-tight compartments and collision bulkheads into the boat, so it's getting there to a decent boat.

Quote:
one thing to note - add insulation to the hull, 2" would be nice. Otherwise it will sweat badly and be cold.
-> I'll be adding 1" of armacell insulation to the entire interior. Some of it has already been installed (in the head compartment and icebox areas) and protected with a layer of baltek and 2 layers of 7 oz. cloth.

Quote:
Then I would buy an overbuilt steel casco and build it up for ice.
-> Yes, a metal boat is the only choice for any kind of boat for real ice sailing. However, I don't plan on any ice breaking or anything. I'm thinking of eventually trying a summer run to Antartica or a summer trip to Kamchatka and so on. I would imagine the main issue would be banging into sea ice or something similar. Also, as I will be solo sailing, the more overall strength (ice aside), the better.

A few months ago, I actually had a few weeks of deep thinking with my project and considered dropping my project and building a steel, junk rigged boat. The conclusion I came to is that this boat will meet my current needs, I have too much sweat equity in this boat and once I start sailing on this boat, I will learn sooo much more than I know now. I could go on about this, but let's focus on the topic at hand
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Old 02-10-2011, 13:11   #9
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Re: Adding Logitudinal Stringers

Perhaps you can build two identical panels with respective stringers design. Then destroy them while measuring the forces. You will get both the strength differences and the mode of failure in one.

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Old 02-10-2011, 16:19   #10
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Re: Adding Logitudinal Stringers

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Originally Posted by bristol27 View Post
Is it necessary to have full length stringers to add strength or would partial stringers suffice? I guess I would like more information on the difference between the two types of stringers - partial versus full.

Assuming your bulkheads are sound and well tabbed to the hull . . . you loose nothing by making the stringers discontinuous but well tabbed to the bulkheads. The bulkhead will bridge the load and make them essentially continuous.

-> I'll be adding 1" of armacell insulation to the entire interior.

Armacell is good stuff - will completely stop condensation. 1" is ok for insulation but I would prefer 2". 1" should be ok though if you have some air gap between the armacell and the ceilings.

-> the main issue would be banging into sea ice

Yes, #1 risk (relatively common) is running the bow into growler, #2 risk(relatively rare) is ice closing in around you and pushing/squeezing all around the waterline. The suggestion in a post above to use a kevlar/epoxy layer around the bow (or even as a belt right around the waterline) might be easier for you than my stainless plate suggestion.

The stainless plate is the standard for the arctic crowd as it would generally spread a point load better (obviously depends on the relative stiffness of the plate used vs the kevlar layer used) and not get gouged up when slicing thru a surface layer of ice (occasionally about .5-1" of surface ice in the mornings in anchorages where there is fresh water running into the harbour) and its quite sharp and hard. But a kevlar layer will do ok and probably be easier and look better.
. . . .
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Old 03-10-2011, 15:06   #11
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Re: Adding Logitudinal Stringers

Structural design isn't my field but I would be concerned about fatigue issues regarding discontinuous stringers. Keep in mind that boats flex all the time in a seaway. Remember the MSC Napoli, which almost broke in 2 and sank because the stringers were improperly led through the fore engineroom bulkhead. Then, cracks appeared in this area.

A colleague who is an engineer in stuctures design told me that "stress is like dust: it gathers in corners". Then, I would suggest to take the trouble to make the stringers continuous and avoid "hard spots".

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Old 04-10-2011, 11:30   #12
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Re: Adding Logitudinal Stringers

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(...) Then, I would suggest to take the trouble to make the stringers continuous and avoid "hard spots".

Alain
It would take a length of unsupported panel to create a hard spot at stringer's end. Please note the OP intends to laminate tab stringer to the bulkhead.

Looking at what I can see in my boat, this is an accepted method of executing the stringers.

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Old 04-10-2011, 11:46   #13
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Re: Adding Logitudinal Stringers

Those stringers need to run most of the full length as one piece. Starting and stopping will just create high stress spots at the ends of each stringer. Unless all you're trying to do is keep the hull from oil canning rather than trying to stiffen the entire structure. But why do this on a proven design?
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Old 04-10-2011, 22:15   #14
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Re: Adding Logitudinal Stringers

I have decided against adding longitudinal stringers at this point in the project. Had it been earlier in the process (like, right after I gutted the entire boat) it might be a different story. However, considering the current access I have to the interior, plus the uncertainty of what I would gain from adding just partial stringers, this doesn't seem like a wise design decision or effort.

Perhaps in the next boat I build I will implement this type of thinking earlier on, but now onward to cockpit modifications, seahood construction and other fun things
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