Tunnel Tabbing Suggestions?

Discussion in 'Boat Design' started by helluvaboater, Mar 13, 2016.

  1. helluvaboater
    Joined: Jul 2015
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    Location: Maui

    helluvaboater Junior Member

    I am currently installing a tunnel in my fiberglass fishing boat.

    Boat Specs:

    • LOA: 27 feet
    • LWL: 25 feet
    • Weight (Fully loaded): 8000ish lbs
    • Engine: Cummins 6BT 210HP or 370HP
    • Gear: 1.56:1 ZF63IV
    • Shaft 1.75"
    • Prop 20 x 19 3B bronze
    • Rudder : 20 x 16 bronze w/ 1.5" shaft

    Tons of pics at: http://fiberglassin.com/pics/2016-03-12/

    [​IMG]

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    http://fiberglassin.com/pics/2016-03-10/20160309_190452.jpg

    http://fiberglassin.com/pics/2016-03-10/20160310_001013.jpg

    The keel of my hull is about 1-inch thick. It's about 1/2 thick a foot or two outside of the keel and it gets thinner as you go out toward the upper parts of the hull.

    I was going to make the tunnel a little over an inch thick or more even.

    http://fiberglassin.com/pics/2016-03-10/20160309_235543.jpg

    I will be using using 1.5 oz mat and 18 oz woven roving because I have a ton of those materials already.

    Today, I finished making the mold to glass the tunnel shape into the hull.

    Here is my plan for tunnel installation, step-by-step:

    1: Finish preparing area around tunnel area in hull by grinding area with FRESH 16-grit grinding discs

    2: Wet sand the mold down to 2000 grit

    3: Polish mold with mold release wax

    4: Spray mold with PVA

    5: Align tunnel mold under boat perfectly

    6: Fill the crack between the mold and the hull with cabosil/q-cell thickened VE resin in order to make a nice fillet

    7: Do a few layers of mat over the mold to get the shape of the tunnel connected to the hull

    8: Remove mold once there are enough layers to keep the shape of the mold

    9: Cut squares in the top of the new tunnel shape and make flat/recessed areas into the tunnel roof for the rudder and struts

    10: Once flat areas for rudder and strut are in place, I will glass the hell out of the inside of the tunnel with VE resin, tabbing into the hull, transom and stringers as much as necessary.

    11: Once I do some more layers on the tunnel, I will do some more overlapping layers over my stringers again, then back to the tunnel for more layers, then maybe back to the stringers for more layers. I want everything to overlap as much as possible.

    Does all this sound logical??

    My questions are:

    Any tips you guys can give me on shaping the tabs or fiberglass pieces?

    How thick should the tunnel be?

    How far out should the tabs really go?

    Advice on the shape of the tabs?

    Should I start small with the tabs and get larger and larger?

    How would you guys go about this?

    I probably should have glassed the tunnel in completely before the stringers went in, but I can always do more layers on the stringers and I plan to do so. I left a 1" gap between the top tunnel and the transom that way I can "tuck" fiberglass for the tunnel under the transom itself, instead of just connecting the tunnel to the inner transom wall.
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yes, the tunnel should have been completed, before the stringers went in. It doesn't seem reasonable to make the tunnel 1" thick. !/2" is likely all you need.

    Tabs should be 12 times the laminate thickness (generally), though often you don't have enough room for this. As to the shape of the tabs, well this seems odd, place them along every edge and contact point. Take the first layer right up to the stringer, maybe "climbing" up it a few inches. From fairing and resin/fiber ratio point of view, the largest piece of tabbing goes on first, with progressively smaller pieces over, using an appropriate stagger.

    Yes, wrap the tunnel laminate around the outside of the transom and tab the inside to the inside of the hull shell.

    Is that a wooden core on the tunnel? How thick, what species, how much fabric on each side?
     
  3. helluvaboater
    Joined: Jul 2015
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    helluvaboater Junior Member

    The lighter tunnel shape you see in this pic is the tunnel MOLD pushed up inside the hull:

    http://fiberglassin.com/pics/2016-03-10/20160309_235543.jpg

    The mold is made with 1-inch masonite strips, plywood, 1.5oz mat and putty.


    Tunnel mold construction pics:


    http://fiberglassin.com/pics/2016-01-26/

    The actual tunnel will be solid fiberglass connected to the hull. No core.

    Since the tunnel should have went in before the stringers, is it acceptable to do more coats over top of the stringers and even on the other sides of the stringers onto the hull, all of which overlap the tunnel layers??

    The stringers are 1.75" x 8" Weyerhauser microlam LVL.
     
  4. helluvaboater
    Joined: Jul 2015
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    helluvaboater Junior Member

    Planning to have some friends over to fill the crack with peanut butter and then do (hopefully) the first 3 layers of tunnel glass on Tuesday. This is how I am planning to lay the first 3 layers of 1.5 oz mat:

    [​IMG]

    I ripped slits halfway down the sides long ways and also a slit down the middle where it goes over the keel to help it conform. If I have to rip it more once we wet it, then so be it. But, I think it will conform ok with no bubbles the way it is trimmed and ripped now.

    Each layer will be 80 inches long and 38 inches wide. The first layer will run up each stringer 3 inches. Every layer after that will be tapered smaller by 1-inch. Largest fiberglass pieces first...

    I think we will use about 80 to 100 oz total of vinyl ester that I plan on dividing into 4 or 5 batches to extend our work time. If we only get 2 layers down, that will be OK too. Just planning to get 2-3 layers glassed in solid. The goal is to have something solid glassed to the hull so that I can cut out flat spots for both the rudder and strut. Those flat area will be reinforced throughout the overall construction of the tunnel.

    Any more comments or suggestions??? Suggestions please!!
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Mat is a bulking agent and helps bond interlaminate layers. It doesn't have any real strength. I know you have a bunch of it, but you don't need much, besides it drinks resin like I do beer. Make your staggers 2", not 1" and use biax, alternating with mat, unless the biax is a combo fabric (1208, etc.). I'd like to see the first layer go up the stringers higher, but 3" will probably do fine.
     
  6. helluvaboater
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    helluvaboater Junior Member

    Thanks Par, you da man. You always have a place on Maui to stay if you can give me some tips on my boat...

    Once I get a few layers of mat on, I will start alternating 18oz roving with the mat layers. My main objective right now is to get the tunnel shape right onto the hull accurately so I can get the rudder in and calculate the v-drive placement. I didn't go higher on the stringers because I wanted to keep the flat planes of the stringers pretty flat for the engine brackets.

    I might end up taking the first layer or two up to the top of the stringer. I also plan on doing 3-4 more units of 1.5mat/18oz roving over the tops of the stringers once I am done with the tunnel to tie it all together.

    I highly appreciate your help!
     
  7. helluvaboater
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    helluvaboater Junior Member

    If I taper 2 inches smaller for each layer, I will be right up against the edge of the hole within 4 or 5 layers. That is why I was only going to do a 1-inch taper.

    Should I still go with a 2 inch taper and then come back out with bigger layers later on?
     
  8. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    LOL....so true. I'll have to remember that one! :D
     
  9. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    A MAT with 30% glass has a tensile breaking strength of 133 N/mm2. A ROV with 45% glass has a tensile breaking strength of 163 N/mm2.
    Whether or not those are very exact figures, I do not despise therefore MAT capacity to withstand stress.
     
  10. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    I would put the flat spots for the rudder and strut on the mold before glassing it in, it will avoid some messy upside down work later, and possible exposure of fibers to water ingress at the cutouts.
    When they are part of the mold, they will be much stronger, as the layup will include these in every layer, and should have added layers tapered out beyond them.
    They can be easily formed with a flat piece bedded in kleen klay and tooled out nice around the edge.
    I don't see any gelcoat in your layup schedule, might be a good layer to have in there.
    How will you laminate in the 1" gap at the transom? It is a physical impossibility unless you lay up the tunnel before installing it in place.
    I saw your thread on another site, and was concerned about the integrity of the joint in the stringers.
    Correct me if I'm not seeing it right, but It looks like your new stringer butts the bulkhead and stops, with existing stringers continuing on the forward side of the bulkhead? This is a critical spot and it needs a very robust overlap!

    BTW, beer is a bulking agent too!
     
  11. helluvaboater
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    helluvaboater Junior Member

    I did not consider this because I have never used the clay before. Thank you I will seriously consider it. I supposed I have to try and figure out the distance from the rudder to the strut for this for this too. I know the strut needs to be VERY close to the prop along the shaft...

    The inside is cut a little larger than 1-inch and I think we will be able to reach the brushes in there (from the transom side too) until the tunnel is about 1/2 thick. Then we will have to fill the remaining crack with peanut butter. Even though the entire thickness of the tunnel won't overlap, I still think this arrangement will be stronger than having none of the tunnel overlap.

    This is a concern of mine as well because they only run together for 6 inches. I am tabbing the hell out of it with like 8 units of 1.5 oz mat/18oz roving but I still don't think that's enough. This concerned me so much so that I made a post that was basically never answered...

    http://www.boatdesign.net/forums/boat-design/new-stringers-55182.html

    Which basically scared me since usually when no one answers on here, it means you are on the wrong track. So, my plan is to continue the existing plywood stringers that run all the way from the deck to the hull. They will extend aft all the way to the new aft bulkhead.

    (shown in orange: http://www.fiberglassin.com/pics/2016-03-19/Untitled-1.jpg)

    I will know where the bulkhead will be placed once i get the v-drive in place. I am also considering continuing the plywood stringers all the way to the transom, but I don't think there will be good enough engine box access for this.

    There are will also be some more plywood added that will run from the hull to the deck, adding lateral support. I did find it interesting that the previous engine stringers stopped on their forward bulkheads...

    http://www.fiberglassin.com/pics/2016-03-19/663634_3e8918c29fa246ed9b96a5c34d2814d6.jpg

    ...and they were all fine with twin Volvo KAD40 diesels.
     
  12. kapnD
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    kapnD Senior Member

    You must have a plan for the layout of all components BEFORE you build.
    Your ER is far too tight to just assume that everything will fit.
    Here's how I do it, not real professional, but a hands-on method:
    Snap a line on the garage floor that represents the inside bottom of the boat, another that shows the top of the engine stringers, draw in the transom, and the tunnel and the forward ER bulkhead.
    Now make profile cutouts of your engine, trans, shaft, strut, prop and rudder, and lay them onto the lines you drew until everything lines up and has adequate clearances.
    Now you know where everything goes, and whether it all fits and what parts will need to be modified to fit.
    Don't forget to give some consideration to the exhaust system, it too requires a good bit of space, especially with the exhaust facing the bow.
    Read up on that at boatdiesel.com, and follow Tony's instructions.

    You don't have to use clay, its just a quick and easy way to make radius and filler for one-off molded parts. Use PVA over it.

    The prop wants no more than one shaft diameter between it and the strut.

    Thanks for the link to your stringer thread.
    No amount of tabbing will make up for lack of overlap. Think about sheer strength.
     
  13. helluvaboater
    Joined: Jul 2015
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    helluvaboater Junior Member

    I don't have a garage floor, it is all dirt around here including my shop. I get what you are saying though. My plan was to build a wood box inside the boat itself that represents the engine, once I get the v-drive in and then build the boat around that.

    I am taking your advice kapnD on the flat spots for the strut and rudder. I got some Van Aken Plastalina clay from the hobby store and I am going to cut some window glass for the flat spots and try to make a nice curve with the clay.

    The flat spot for the rudder port seems easy enough but I am having to estimate for the location of the flat spot for the strut based on where I think it should be and the shape of the tunnel mold itself.

    I did a mockup using the actual rudder and prop with the tunnel next to them.

    http://fiberglassin.com/pics/2016-03-21/20160321_160946.jpg

    http://fiberglassin.com/pics/2016-03-21/20160321_161005.jpg

    http://fiberglassin.com/pics/2016-03-21/20160321_161056.jpg

    http://fiberglassin.com/pics/2016-03-21/20160321_161110.jpg

    http://fiberglassin.com/pics/2016-03-21/20160321_161123.jpg

    The cardboard pieces in the pics represent where I think the flat spots for the rudder and strut should be. Center to center, the strut base is 25 inches forward of the rudder port. The black line on the mold represents where the mold meets the hull. The rudder will protrude just past the transom.

    I think the flat spot for the strut could come back an inch or two, but I think it will be good where it is. If I come back more, the flat spot will go past the main turn in the tunnel roof. A custom strut is definitely in order for.

    How far is the strut supposed to swing back?

    Does the strut mounting location look OK in the pics or should I move it fore or aft?
     
  14. kapnD
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    kapnD Senior Member

    If you don't already have the strut, the "swing back" can be adjusted to allow for some fudge factor in placement of the flat.
    I don't think there is any rule for this, but it does need to be strong, so keep it reasonable.
    Many are built vertical, for simplicity. I think a radical rake is a high speed thing to get clean water to the prop.
    Surfaced plywood (melamine) is great for one off molds, or a chunk of the bottom fiberglass you cut out would do, anything slick and flat. Just screw it in place from the underside, and fill the edges.
    Radius the edges to fit your washer roller, so that there is absolutely no air in your laminate in this critical spot. Some of your photos seem to show air in the knytex lams, and you mentioned using a brush, so hope you are using a washer roller to get the air out!
     

  15. helluvaboater
    Joined: Jul 2015
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    helluvaboater Junior Member

    All the bubbles you see are grinded out. The only bubbles that formed was from where we did a bunch of layers at once and we always stop once the layup gets too hot and we can't get a bubble out.

    Thanks for all your advice.
     
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