Stringer Questions

Discussion in 'Fiberglass and Composite Boat Building' started by what2be, Aug 24, 2020.

  1. what2be
    Joined: Aug 2020
    Posts: 5
    Likes: 0, Points: 11
    Location: Eureka, ca

    what2be Junior Member

    Hi , I came across this forum on a search for boat repair and was hoping somebody could answer a few questions I have.
    Some background on this boat, it’s a 1982 19’ eliminator that had been neglected and sat under some redwood trees without a cover for about 15 years until my friend gave it to me in 2015. It was full of water above the stringers for a good three or 4 feet from the transom forward. There was no plug in the boat but at some point the redwood needles plugged the drain hole and it started holding water but I have no idea how long it had been full of water.
    As you can see in the pictures there are a couple cracks, One on the top of each stringer running from the transom Forward. On the port side it’s about a foot long crack and on the starboard side it runs about 3 feet.
    Any idea what caused these and is it a common issue? My local boat shop sold me a west marine fiberglass repair kit and from what I’ve read I was told to grind around the crack at 8:12 to one slope apply resin into the crack and then apply steips of woven Matt over that. Does this sound correct?
    IMG_7034.JPG IMG_6831.JPG IMG_4750.JPG IMG_0237.JPG IMG_2449.JPG IMG_4284.JPG
     
  2. Cajunpockettunnel
    Joined: Aug 2020
    Posts: 224
    Likes: 136, Points: 43
    Location: Franklin, LA

    Cajunpockettunnel Senior Member

    If I were in your position, I would get some cab-o-sil, mix it with epoxy and form a thick paste, then add the hardener and apply it to those cracks. Then I would used chop strand mat or CSM and resin to cover it. Although CSM is thirsty, it's strong. I'm no expert, this is my opinion. Wait for others to chime in.
     
  3. ondarvr
    Joined: Dec 2005
    Posts: 2,636
    Likes: 389, Points: 83, Legacy Rep: 506
    Location: Monroe WA

    ondarvr Senior Member

    Check closely for rotten wood, although I do see some nice light colored wood in there.

    Rotten wood from water finding its way into the stringers is common.

    If there's no rot, then glassing over what a there is fine.
     
  4. Cajunpockettunnel
    Joined: Aug 2020
    Posts: 224
    Likes: 136, Points: 43
    Location: Franklin, LA

    Cajunpockettunnel Senior Member

    What he said^^^^!
     
  5. what2be
    Joined: Aug 2020
    Posts: 5
    Likes: 0, Points: 11
    Location: Eureka, ca

    what2be Junior Member

    Thank you for your reply. Any idea what causes these cracks to begin with in the first place? And is there anything i can do to prevent them from happening in the future?

    Thanks
     
  6. ondarvr
    Joined: Dec 2005
    Posts: 2,636
    Likes: 389, Points: 83, Legacy Rep: 506
    Location: Monroe WA

    ondarvr Senior Member

    The cracks are from water soaking the wood and it swelling.

    Don't let it fill with water.
     
    fallguy and Cajunpockettunnel like this.
  7. missinginaction
    Joined: Aug 2007
    Posts: 1,031
    Likes: 215, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Before we get into the reconstruction of the stringers (which certainly looks possible) something else stands out to me. What are those pieces of aluminum plate running from the top of the transom (inside) and bolted to the stringer? Were they part of the original build or an addition at a later date? It looks to me like those pieces are there in an attempt to keep the top of the transom from twisting from the forces applied when the engine accelerates the boat. As power is applied the prop will push the lower unit of the outboard forward. This will cause a twisting force to be applied to the top of the transom, the force would try to twist the transom top aft.

    Those aluminum plates are only bolted to the stringers using some thin fender washers. As the stringers aged (and rotted) the plywood former was no longer able to handle the compressive load required to hold those bolts in place. It looks to me as though the entire rear portion of the stringer will need restoration. What you'd want to see with those aluminum plates would be solid aluminum backing plates, not washers. Ideally, I think that the transom needs attention. The transom and it's connection to the stringers should be strong enough to carry the loads on it's own.

    Is there any fiberglass/resin on the inside of the transom? It almost looks like bare plywood in the photos but that can't possibly be the case. Notice the junction of the stringer and the transom. That area should be heavily laminated with roven woving or heavy fiberglass cloth. The laminate should be a generous radius where the stringer meets the transom in order to spread the load out.

    There is a lot that needs to be addressed. I'd also steer clear of chopped strand mat. Mat is used under gelcoat in hull layups in order to prevent "print through" of the weave from roving or heavy fiberglass cloth. Since the glass fibers are all chopped up it has little strength compared to cloth or roving. Using mat will give you a heavier layup as it really absorbs resin but it will also be weaker than cloth or mat. Cloth or roven woving will get you a lighter, cheaper layup that's also much stronger.

    Welcome to the forum and thank you for posting so many photos. Very helpful.

    Regards,

    MIA
     
    Last edited: Aug 24, 2020
    bajansailor likes this.
  8. Blueknarr
    Joined: Aug 2017
    Posts: 1,124
    Likes: 245, Points: 63
    Location: Colorado

    Blueknarr Senior Member

    The 3rd pic shows crushed black plywood and de-resined while glass at the motor bolt.

    I would give all the wood the ice pick test
     
  9. what2be
    Joined: Aug 2020
    Posts: 5
    Likes: 0, Points: 11
    Location: Eureka, ca

    what2be Junior Member

    Good eye and thanks for the comments. To answer your questions,

    The pieces of aluminum plate from the transom to the stringer were original as far as I know. If the mfg didn't put them in there then i'm guessing it was whoever installed the mounting plate for the outboard engine. And as far as I can tell, there is little to none additional laminate around the stringer where it joins to the transom. I'm no boat expert, (I don't even play one on tv lol) but you would think that would have been a good idea during the boat construction. Maybe someone familiar with Eliminator boats could answer that. However, the rusty twisted 1" flat bar (you can see them in the first picture I posted) on each side that run from the bottom of the aluminum plate where its bolted onto the stringer to the center of the ski tow mount was installed by the original owner. When I took the bolts out of the bottom of the center ski tow bracket the screws were bent and there was also no backing used on the mount when it was installed. I would guess by the bent screws that there was too much pressure being applied to the ski tow mount, which very well could have been my doing. I have only used the boat in the past for single person tubing and never have had a issue but over the 4th of july this year I purchased a 3 person tube and while my wife was driving and towing our 2 kids she made a pretty sharp turn at low speed and I heard a loud crack run through the boat. I think the crack I found yesterday a few feet higher up on the stringer on one side can be attributed to that noise but i'm not positive. All I know is that the original 2 cracks on the stringer that start near the transom and go forward were there when I acquired the boat. In my opinion, I don't think that 3 point tow bar is sufficient for anything other than water skiing and am planning on removing the 1" flat bar supports and the mounts and will purchase something like this that mounts to the engine plate. XXL TurboSwing Ski Tow Bar https://turboswing.com/xxl-turboswing-tow-bar.html
    As for the thin fender washers, you make a excellent point. Why in the hell would whoever mounted the engine plate only use brackets on only one set of bolts on each side instead of both sets of bolts on each side? Im going to buy some additional 1/4" x 2" aluminum angle tomorrow and install those as well (as long as the wood in the transom hasn't rotted, which another poster mentioned could be the case). Thanks for the education on chopped vs woven as well, very informative.
    Inside plate mount1.JPG Inside plate mount2.JPG Outside plate mount 1.JPG Outside plate mount 2.JPG Outside plate mount 3.JPG Ski tow 1.jpeg Ski tow 2.JPG Ski tow 3.JPG IMG_9818.JPG
     
  10. brendan gardam
    Joined: Feb 2020
    Posts: 368
    Likes: 44, Points: 28
    Location: east gippsland australia

    brendan gardam Senior Member

    the easiest thing to do is replace the stringers completely, you don't want to be worrying about rotten stringers when your doing 60 mph, they provide the stiffness to the hull bottom as well as transferring engine thrust through the boat. its not a hard job, probably easier than putting patches everywhere.
     
  11. missinginaction
    Joined: Aug 2007
    Posts: 1,031
    Likes: 215, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    As is the case with just about all older boats, it looks like you're facing some previous owner modifications that are not necessarily good engineering.

    I have to put in a disclaimer. I'm an inboard guy, always have been. Since inboards are directly mounted to a boats stringers those stringers need to be very strong. In my small cruiser, the transom was originally only about 3/16" thick. Just solid fiberglass. Think of a pre-fabbed fiberglass shower for your bathroom. Not strong at all but it didn't need to be. It was just keeping the water out.

    In your case though the transom needs to be extremely strong. It must carry all of the load that the engine produces and other loads like those you mention hauling skiers and tubers around. Your original post asked about the stringers and they certainly need attention. As I look at your photos though I'm more concerned about the transom's ability to do it's job.

    I'll post a few "before and after" photos of the small cruiser I restored a few years ago that will give you an idea of what you're facing with stringer replacement.

    There's my engine back in 2007 when I started.. Stringers rotted. Engine out of alignment, beds all sagged out. See those fender washers? There were a dozen of them trying to hold up about 1000 lbs of engine plus the prop pushing on everything. What a mess. See those fender washers above the muffler there? They held up a swim platform that just had one or two people on it at most. Plywood crushed, those flat washers now concave because they were not up to the task. Remember this is a boat manufactured by Silverton Marine in 1973. This is a so called professional build. Well, it lasted for a decade or so before things started falling apart. I restored it because I loved the design, not the execution.

    [​IMG]

    Out goes the engine.....geez....what a mess. Those blue vertical posts you seed were a previous owners attempt at supporting the back deck. It didn't work, but that's another story.
    [​IMG]

    Fast forward to new stringer installation. A jig was made to straddle the engine stringers so I'd know where to locate the new engine beds. The outboard stringers were solid and didn't need replacement thank God! But take a look at the lousy job of glassing the outboard stringer to the transom. Right at the back of the fuel tank on the right. There was just a thin layer of glass in there, and not much resin. They got away with that because there was no appreciable load applied in that area and it stayed dry under the aft deck.

    [​IMG]

    New stringers going in. Those stringers have been in my boat for 10 years. I just removed the prop shaft for maintenance. The engine never moved, no sagging no shifting in hundreds of hours of operation. You'll probably be surprised at how those stringers are made. The former material is Foamular 250 from Dow Corning. Yep, just 2" thick rigid XPS foam insulation material. $35.00 at your local Home Depot. You might think, how did he build stringers out of foam? Well, once you learn how stringers work, you'll understand that core of a stringer (the former) is simply some material that is used to hang the fiberglass cloth on. The core provides absolutely no strength to the stringer. All of the loads are carried by the fiberglass laminate that is laid-up over the core. So that laminate has to be strong.

    You'll notice four wooden inserts built into the stringers. Those sections are necessary in order to provide compressive strength. When finished the engine beds are located at those spots and the wood absorbs the compressive loads that the bolts impart. It's hard to see but you might also notice that there are no sharp corners on those wooden inserts. everything is radiused. This eliminates what are called "hard spots" in an assembly, it spreads the load forces out across the laminate/assembly rather than concentrating the forces in one spot. BTW, what looks like oil stains was permanent, the entire area was degreased and abraded.
    [​IMG]

    Finished stringers with beds installed. You can't see it but there are matching aluminum plates (3/8") on the outside of the stringers to take the load of the beds. 1/2" stainless steel machine bolts hold each bed in place. Note the radius where the stringers join the forward floor (the transverse bulkhead) which was also replaced as part of the restoration and the inside of the hull itself. Everything flows, no sharp transitions. the stringers, the floor and the hull itself are taking the forces generated by the propeller. I tried to eliminate any areas where force would be concentrated and cause problems. It worked.
    [​IMG]

    Down she goes......I can't believe this picture is a decade old.

    [​IMG]

    Here are links to a couple of books that I found very helpful. The first one is general in nature, the second is more technical. Dave Gerr is still a professor at the State University of New York Maratime College in New York City.

    https://www.amazon.com/Nature-Boats-Insights-Esoterica-Nautically/dp/007024233X

    https://www.amazon.com/Elements-Boa...f Boat Strength&qid=1598361587&s=books&sr=1-1

    Good luck with your project.

    MIA
     
    Last edited: Aug 25, 2020
    bajansailor and Rumars like this.
  12. Rumars
    Joined: Mar 2013
    Posts: 1,145
    Likes: 511, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    I would say you are looking at partial stringer and transom replacement just from what I can see. It's probably simpler to do a full replacement then patching it all, access is better. If you use wood for stringers and transom core, laminations should be done with epoxy resin. Buy an oscillating multitool and a good orbital grinder with dust extraction.
     
  13. Blueknarr
    Joined: Aug 2017
    Posts: 1,124
    Likes: 245, Points: 63
    Location: Colorado

    Blueknarr Senior Member

    I don't often disagree with MIA.
    But your stringer cores are structural.
    6oz of twill over a 2by is not enough glass do not be relying on the wood for structure.

    I'm being interrupted. I will continue shortly.
     
    Rumars likes this.
  14. missinginaction
    Joined: Aug 2007
    Posts: 1,031
    Likes: 215, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Maybe we're both right depending on how you look at things. I used Dave Gerrs book "The Elements of Boat Strength" as my primary guide. When I was in the early stages of this I didn't know what I was doing. So I read books. Gerr's approach seemed reasonable to me. If the OP gets Gerr's book and knows how to use a scientific calculator, he'll be fine as Gerr's engineering formulas are pretty easy to understand.

    I remember having a discussion with Paul Ricelli many years ago. Paul made the observation that Gerr's "scantling rule" which is used to derive all kinds of dimensions for various parts of a build (including laminate thickness) were on the heavy side. Paul meant that Gerr tended to err on the side of overbuilding, in Paul's opinion anyway. If I remember right Paul told me that I could have gone lighter with my stringers because I used epoxy. He felt that Gerr's formulas made an allowance for the use of polyester resins that were not as strong. That might be true. Nevertheless, I followed Gerr's formulas and haven't had any problems with my stringers. In my case Gerr's formula recommended a laminate thickness of 0.2 inches for the engine stringer and 0.25 inches at the engine beds. So I figured out how much glass I needed for a 0.25" layup and went to work.

    On page 43 of his book Gerr states "....It is accurate, however, that the core is a non-structural former. You can use any low cost foam that is convenient, as long as it is compatible with the resin and cannot absorb water. (Solid wood cores at the engine mounts is always required.)"

    That's where I come up with the idea that the core of a stringer carries no load. But there are other ways to make a stringer.

    I suppose that you could build stringers out of solid wood. In that case the stringer would have to be structural as there would be little to no laminate covering the stringer.

    I remember some people at the boatyard telling me I was crazy. Gerr's method worked for me.

    Regards, MIA
     

  15. what2be
    Joined: Aug 2020
    Posts: 5
    Likes: 0, Points: 11
    Location: Eureka, ca

    what2be Junior Member

    Your statement about the core of a stringer is interesting and makes sense, although I had always assumed the wood works in conjunction with the fiberglass laminate to help transfer the loads applied to them. the fact that you can use a foam core proves me wrong obviously. I have had prior boat experience via a few flat bottom vdrives and have quite a bit of experience in engine building, but know very little about outboard engines, especially 2 strokes. My transom feels good using the ice pick test a previous poster suggested. I am going to prep and then add some laminate to the transom areas where the previous owner did not apply any brackets for backing and then install new brackets. I am not going to replace the stringers at this time, although it wasnt bad advice at all. One question I have (and a bit off topic) is how is the horsepower rating determined for a boat such as mine? Does the MFG rate it or just tell people to go put whatever they want on it? If i keep this boat much longer, im going to have to get rid of the 2 stroke and buy a 4 stroke, so I was curious if a 200 would be too much for this boat or if I should just stay with a 175 like it has.
     
Loading...
Similar Threads
  1. H2OHOII
    Replies:
    21
    Views:
    11,609
  2. WalleyeSniper
    Replies:
    8
    Views:
    299
  3. alyons05
    Replies:
    20
    Views:
    1,144
  4. Martin Upton
    Replies:
    44
    Views:
    3,108
  5. nrraiders35
    Replies:
    1
    Views:
    808
  6. Martin Upton
    Replies:
    63
    Views:
    2,786
  7. Shawn Barlow
    Replies:
    2
    Views:
    1,160
  8. atengnr
    Replies:
    46
    Views:
    2,056
  9. E350
    Replies:
    38
    Views:
    3,621
  10. chowdan
    Replies:
    9
    Views:
    2,223
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.