Adding Logitudinal Stringers

[bristol27]

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 stringers?
-> 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 more notes 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 which start at the forward chain locker and move through the boat all the way to the lazarette (or visit my project site - http://www.bristol27.com):

Chain locker

Forward v-berth w/ kerosene tank installed

Aft v-berth, area where water tanks will be placed

Cockpit bulkhead and battery compartments/lockers

Port sea locker

Lazarette locker w/ yet to be installed "splitter bulkheads"

Ok, thank so much for everyone's input and please let me know if I can clarify anything.

[Rachel]

I'll be interested to see what knowledgeable folks say here.

One intuitive-type thought I had, while reading, is that continuous (full-length or nearly) stringers would strengthen "the boat" whereas maybe ones that only run between bulkheads (discontinuously) would be giving you more "panel strength" (i.e reduce oil-canning) than whole boat strength (?) I wonder if the discontinuous ones could make undesirable stress risers at their ends?

There are pre-made stringers of various shapes made by [argh, can't remember company name right now], and they are made of closed cell foam with fiberglass over and under it that then runs out into tabs. Edited to add: Okay, it came to me, it's Compsys/Prisma. They have some installation notes here that might be of interest:

http://www.preforms.com/docs/PRISMA_Bea ... lation.pdf

Another note is that I think Dave Gerr's "The Elements of Boat Strength" addresses this to some extent (I can take a look).

[bristol27]

Thanks for your quick input Rachel. I've responded to a few of your comments below and I'll keep an eye out for the wise minds on this forum:

One intuitive-type thought I had, while reading, is that continuous (full-length or nearly) stringers would strengthen "the boat" whereas maybe ones that only run between bulkheads (discontinuously) would be giving you more "panel strength" (i.e reduce oil-canning) than whole boat strength (?)

-> Very good points. At this point, I think the best I can do is to gain panel strength. The only areas at this time where I could add the longest (e.g. "full length") stringers would be from the transom, through the lazarette, cockpit and terminating at the first mast-step bulkhead. Unfortunately, I've come to the realization of adding stringers a little too late to go full length as I've proceeded a little too far in construction of other areas. It's not necessarily too late, but I am facing some design challenges with the current state of things.

I wonder if the discontinuous ones could make undesirable stress risers at their ends?

-> Would this be reduced if the stringers were tapered at their end?

Another note is that I think Dave Gerr's "The Elements of Boat Strength" addresses this to some extent (I can take a look).

-> I also looked at David Gerr's "Elements of Boat Strength" and perhaps I missed it, but he didn't go into too great of detail about adding stringers in pre-existing boats and the pluses and minuses of full v. partial. If you find a section I can refer to, just let me know as I have the book on hand.

[Rachel]

I also looked at David Gerr's "Elements of Boat Strength" and perhaps I missed it, but he didn't go into too great of detail about adding stringers in pre-existing boats and the pluses and minuses of full v. partial.

Okay, you are probably right then. I didn't know you had the book to refer to, and I was going from memory, as I don't have the book to hand.

[mitiempo]

bristol27

I don't think for the most part it is necessary. If you add stringers at all I would concentrate on the forward cabin area, both under the V-berth and the hull sides above the berth maybe as well. I can't see the advantage of stringers aft where the hull will never contact something. The head and main cabin area should be quite stiff with the cabinetry glassed in properly without stringers. And I don't think I would taper the ends before a bulkhead but instead blend the stringer into the bulkhead somehow.

As far as which material to use, whatever is chosen it is only a former for the fiberglass over it. If you are using epoxy, as you should, any foam that is flexible enough to follow the curve and closed cell so it doesn't absorb resin to excess is fine. Use hot melt glue to hold it in place before glassing. With 2 or 3 layers of 1708 over it they will be more than strong enough.

If you really want to bang ice a metal boat is a better choice, as abrasion can be a larger issue than ultimate panel strength.

[LazyGuy]

Personally, if you do not have access to the whole length of the boat, I think you might be doing more harm than good. Not only as pointed out earlier that you are merely increasing panel strength, you are now going to concentrate what little twist the boat will have to the area under the mast. The boat should function as a unit, not as 5 different sections with varying strengths.

This is not to say that all the stringers should go the length of the boat. My boat has two pairs of stringers, one that run 12" (or so) below the sheer and two stringers that start about 1/3 the way back (sort of where the full keel starts, that angle up to be about 4' apart at the transom. but many times the builders located the bulkheads where they were needed to support the deck or to minimize oil canning. At times even to support the overhangs to prevent hogging. By adding stringers, potentially you are making what was the strong point, the new weak point and the area where there are no stringers the weakest of all.

[Rachel]

Just curious: Does/did the Bristol 27 have any stringers to begin with? (I realize other things can function as stringers too, like cabinet backs or shelf supports.)

[bristol27]

Wow, lot's of good input here. Thanks everyone. I'm getting a more clear picture and have answered your comments below:

If you really want to bang ice a metal boat is a better choice, as abrasion can be a larger issue than ultimate panel strength.

-> 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 :)

Personally, if you do not have access to the whole length of the boat, I think you might be doing more harm than good. Not only as pointed out earlier that you are merely increasing panel strength, you are now going to concentrate what little twist the boat will have to the area under the mast. The boat should function as a unit, not as 5 different sections with varying strengths.

-> I see. I guess that does 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. I agree that full would certainly be ideal. Lazyguy, could you you could go more into the idea of things functioning as a "unit"?

This is a little hard for me to explain exactly, but I feel like the panel areas I wont' have access to won't turn into a weak point. The reason I don't believe this is because the panels I no longer have access to (inside the kerosene tank, the head compartment and the icebox area) are well built up. The icebox and head compartment have over-built chainplates. The head compartment has a sole for the head, as well as shelves. The kerosene tank is likely my weaker point, however it's stil built quite strong. Here are some images so you can take a look at the construction methods and hopefully, if you think they won't create a weak point, I can continue thinking of adding stringers in other areas of the boat.

Head compartment chain plates

Head compartment sole & shelf (somewhat like stringers)

Icebox chainplates

Icebox area - icebox counter, shelves

Inside of kerosene tank reinforcement. The tank has two 3/4" meranti bulkheads, plus PVC drainage running below it. I built the supports for the lid with cleats which are epoxied and fiberglassed to the hull. I also added a layer of 1708 biax to every side of the tank.

Sorry, these images probably aren't 100% perfect for what I'm trying to convey here. That being said, they should give the basic idea. And here are some specific links to a full gallery of the areas in discussion (hopefully this is OK to share these):

+ http://www.bristol27.com/projects/icebox
+ http://www.bristol27.com/projects/head-compartment
+ http://www.bristol27.com/projects/chainplates
+ http://www.bristol27.com/projects/tank-kerosene

Does/did the Bristol 27 have any stringers to begin with? (I realize other things can function as stringers too, like cabinet backs or shelf supports.)

-> No stringers in the original boat. To assist your question and the discussion at large, I've included the original B27 technical diagram below:



One last thing - everyone mentions oil canning. I basically understand what oil canning is, but does anyone have images of a boat damaged by oil canning? Or perhaps someone could succinctly describe the type of forces that will create oil canning.

[Carl-A259]

The freighter that ran into the ocean liner Irish Princess in the St Lawrence seaway years ago was built this way, with the main structures built horizontally , they are not called stringers but I can't remember what they are called. The freighter sustained very little damage and the Irish Princess was stoved in to where door number plates from the inner cabins where found on the freighters deck. I think she sunk in about 10 minutes. I'm just a hobby derelict boat restorer that thinks Bristol are a sturdy built boat. If I were to add a "stringer" I think I would butt the bulworks with a knee type brace off the bulk head and not taper it down as already mentioned. I would go the entire length, if your going to do this, cuz boats don't only run into stuff, they also get run into.

[mitiempo]

Carl

Longitudinals is the term you are looking for I think.

My thought is that for one, the Bristol is a pretty strong boat with no major issues in structure of the hull that I know of. For another, fiberglass does flex, and that is part of its ultimate strength in a situation where something is hit. If a panel is stiffened a large amount that flexibility is lost. On the other hand, if the panel is too flexible oil canning can occur, most common in the forward part of the boat when beating to windward in seas, and that can lead to interior structure coming loose.

I remember on Sailnet a few years ago a post about crossing the Atlantic in a Beneteau (I think) and the forward sections were oil canning - a crew member watched the panels move in and out from below.

[Rachel]

If I were to add a "stringer" I think I would butt the bulworks with a knee type brace off the bulk head and not taper it down as already mentioned.

Could you say this another way? I can't picture what you mean. (Not that it's my thread, but I'm still following along.)

[mitiempo]

Here's the Sailnet link to 4 pages of discussion about an oil canning hull. http://www.sailnet.com/forums/boat-revi ... 423-a.html

[mitiempo]

I think he means to blend the stringer into the bulkhead and not end it either abruptly or before the bulkhead.

Oil canning is most common on flat panels, found more on lighter modern boats. As long as the panel has some curve to it strength is increased by a large amount and it is less likely to happen.

[LazyGuy]

Personally, if you do not have access to the whole length of the boat, I think you might be doing more harm than good. Not only as pointed out earlier that you are merely increasing panel strength, you are now going to concentrate what little twist the boat will have to the area under the mast. The boat should function as a unit, not as 5 different sections with varying strengths.

-> I see. I guess that does 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. I agree that full would certainly be ideal. Lazyguy, could you you could go more into the idea of things functioning as a "unit"?

This is a little hard for me to explain exactly, but I feel like the panel areas I wont' have access to won't turn into a weak point. The reason I don't believe this is because the panels I no longer have access to (inside the kerosene tank, the head compartment and the icebox area) are well built up. The icebox and head compartment have over-built chainplates. The head compartment has a sole for the head, as well as shelves. The kerosene tank is likely my weaker point, however it's stil built quite strong. Here are some images so you can take a look at the construction methods and hopefully, if you think they won't create a weak point, I can continue thinking of adding stringers in other areas of the boat.

I am not a structural engineer but as an electrical engineer I work with SEs all the time. It is not that the stringers need to go from bow to stern, it is that they need to be continuous. By stopping and starting at every bulkhead you have a stress riser at the bulkhead. Picture talking a pencil, cutting it in two and gluing it to either side of a piece of cardboard. The two pieces glued to the cardboard are not as strong as the original pencil. Also, if you twist the pencils or try to bend them, 99% of the time the break will happen where it joins the cardboard. If you offset both ends of the pencil (not perfectly lined up) then the stress riser is even higher so there is more of a likelihood that the failure will occur at the cardboard. You have seven compartments (5 bulkheads) all with the stringers offset you are creating multiple stress risers rather than keeping the stresses spread across more of the structures.

The boat did not originally have stringers and I have never heard of a Bristol that has not been heavily modified (for racing by removing structure) that has broken apart. Plenty of BeneHuntAlinas but no Pearson, Bristol, etc... These boats have SOLID fiberglass hulls that are at least ten times stronger than they need to be. The stringers will remove flex that adds to the "sea kindliness" of the boat that might make the ride a little more harsh.

Please let me know if this answers your questions.

[Carl-A259]

[quote="mitiempo"]I think he means to blend the stringer into the bulkhead and not end it either abruptly or before the bulkhead.

Rachel, yes this is what I mean,especially if the intent is collision protection

[Rachel]

I'm not an engineer (of any type) either, so take this in that context.

These boats have SOLID fiberglass hulls that are at least ten times stronger than they need to be. The stringers will remove flex that adds to the "sea kindliness" of the boat that might make the ride a little more harsh.

I was with you right up until that last sentence. I agree that it is likely that the boat is strong enough (although our boats can certainly benefit from improvements if they are well thought out), but I disagree that any flexure adds to sea kindliness or a "smooth ride."

My feeling is that our boats are meant to be (basically) monocoque structures, and any flexing is detrimental and weakens the boat (ultimately). Not that we want stress risers, but neither do we want things flexing. I think that -- beyond the designed shape, which we aren't changing -- it would be things more like keeping weight in or out of the ends that would make the boat more or less sea kindly, wouldn't it?

I still don't know about the discontinuous stringers, although my feeling is that I would either want to 1) Make them continuous; or 2) Add short ones only if a particular panel seemed to need it, and not just throughout the boat. I suppose if it were me and I still felt unsure, I might check in with a professional/structural/designer - for a consultation.

[Crazer]

If I were rebuilding a boat so completely, and I was worried about hitting things, I'd make as many watertight compartments as feasible (ala Baldwin's Atom) but I suspect given your thoroughness that you've already thought of this and perhaps on planning on doing so.

[LazyGuy]

Rachel,

You are right, I take back the sea kindliness comment. That would be more of a hull form and weight placement issue.

[bristol27]

To wrap up this thread, I first wanted to say thanks to everyone who had input. This forum answers in this and other threads proves how great a community exists here.

Secondly, 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 :)