belly tank/stringer conflict

erikgreen

erikgreen

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Re: belly tank/stringer conflict

Short answer: Yes, it's possible to do. However you will need to be very careful, and unless you have a marine architect review your new design for big $$ you're taking a risk.

The center stringer would probably be the #1 most important structural element for the boat other than the transom.

If you cut it, you'll have to:
  • Create a "box" from two short bulkheads and two side stringers around the tank
  • Make the side stringers the same thickness as the keel stringer
  • plan on tabbing the box sides and bulkheads to the hull and each other and then covering them with glass
  • I'd recommend doing all this with epoxy
  • Under where the tank goes I'd try to add extra glass to reinforce the hull some, with a shortened (vertically) center stringer to support that section some more
  • If possible the deck surrounding the fuel tank hatch should be glassed to the hull sides and epoxy glued and glassed to the tops of the "box" you make to reinforce some more.
How big is your existing center stringer, and where exactly will you cut it?

For a fuel tank that size there are plenty of other options for getting more capacity and moving it out of the way, cutting a main structural element should really be a last resort.

If this is a lake only boat that won't ever be in any kind of waves, rivers, or storms you might be safer than on a big lake or the ocean... where will you use it?

Erik
 
chrishayes

chrishayes

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Re: belly tank/stringer conflict

I will use it for lake only. Maybe the occasional trip to lake Erie(very occasional). The stringers are already out. I am redoing the boat interior completely. I would love to move the weight forward but the only logical way of doing the below deck tank is to have the inlet, pickup and other incidentals all the way at the rear of the deck popping out at the bilge under the splashwell. Then run the filler and breather out the back of the transom.

My thoughts for construction were the same as you noted for the stringers and bulkhead. If epoxy is the right answer here then that would help make the desicion of epoxy vs poly that much easier. I planned to build a flat plywood deck for the tank to sit on supported by a short stringer underneith and tying the box for the tank back into the main stringer down the center of the keel all the way up to the bow. In the rear of the tank would be boxed in strigers back to the transom tied into the two side stringers that go most of the length of the boat. And of course all this would be glassed with the same 18 oz biax as the stringers.

On the other hand, are there other options that you know of for my situation? Im open to ideas. I would love not to have to do all the extra work but with a 150hp merc 1500 the 2 6 gallon portables are not getting it done.
 
erikgreen

erikgreen

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Re: belly tank/stringer conflict

Show us some pics of the boat layout and measurements. You can get bow tanks, side tanks, skinny long OEM tanks and hide the tank in interesting areas.

Since it's a plastic tank the plywood deck idea will be fine for support, add some straps to hold the tank in place. Make sure the tank can expand about 5 percent as it fills, plastic tanks do.

Epoxy is guaranteed to have more strength than poly, and since you're altering the boat's structure I'd recommend that insurance. But you still need to do things the right way, and it sounds like we're on the same page there.

Erik
 
erikgreen

erikgreen

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Re: belly tank/stringer conflict

Yeah, I mean pics of the stringers :)

Erik
 
chrishayes

chrishayes

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Re: belly tank/stringer conflict

cameras dead:mad: card is fine. SOon as I get a new cam I will post them. SO in the meantime, is there other solutions that you know of to my fuel issues? Like can you put 2 smaller tanks in between the main stringer and the outer 2 stringers then somehow link them together?
 
Mark42

Mark42

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Re: belly tank/stringer conflict

I see that Eric is suggesting extreme caution if the stringer is to be cut and new boxing done. Although I agree that caution and a good plan are needed (and with respect to Eric's sound boat knowledge), I do believe that this is a fairly straight forward modification that does not require extensive re-engineering.

First determine the thickness of the hull at the keel. Simply taking a 1/8" drill bit and drilling through will give you the answer. If its 1/2" or thicker its plenty strong to support the boat for the 50 inch length that the stringer will be removed. If you want more assurance, build up another 1/2 inch from the inside about 6 inches wide.

Then box in the area, make good connections to the stringer fore and aft (if not right up against the transom) of the box.

Remember that once the deck is installed, the new box stringers will be extremely strong, being securely fastened to both the hull and deck making a strong sandwich construction.

Careful planning, proper materials and attention to details during construction should give you the belly tank you desire without fear of the boat failing when in use.

Remember to read the coast guard regulations on fuel tanks when designing where the fuel fill and vent will go. The most notable is that if the fill overflows, the fuel cannot run inside the boat (and the splash well is considered "inside the boat") . There are also bonding (grounding) regulations, type/grade of hoses and a few other issues that you should be aware of. All straight forward stuff. Check out the links in my signature to find two projects that install aftermarket tanks in my boats. That should give you some idea of the materials needed. Links to the regulations can be found in a sticky post at the top of this forum.

Good luck and safe boating!
 
chrishayes

chrishayes

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Re: belly tank/stringer conflict

Thanks Mark, I was actually referring to your tank upgrade in my previous post. I would use the exact same fill vent and 3 way valve as you did. You were my inspiration for this tank idea minus the placement. The hull of my boat is extremely overbuilt. If you know anything about the wellcraft airslot you know that this was a boat built on a platform that had no rival. Very SOLID build. I cant imagine that adding this new contruction would in any way alter the safety of the boat. But I have asked to see if the consensus would be thumbs up or not. Truly if others think this isnt a good idea and have some data to back that up, I would not proceed with doing this.
 
J

jonesg

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Re: belly tank/stringer conflict

I'm right in the middle of this with my boat, its straightfwd enough, mine was already designed and I just followed what was there.

I have a heavy 20' center console from 1974.
Down the center keel they put a half pipe, take 4 inch plastic pipe, cut it in half along the length , wax and lay heavy glass cloth over it, keep excess to a minimum to eliminate need to trim or grinding before installing.
Glass this half pipe to the keel, doubles as a drain under the tank.

A tube is stronger than a sq shaped stringer.

Over the half pipe lay planks from side to side, about 4 planks is good for a 48inch tank. Planks on mine are 1 inch thick, 6 inches wide teak or other such heavy wood, glassed both siides and glassed to the hull.

Box around the fuel tank, on mine the rear is open to the bilge so its just a 3 sided box, 3/4inch ply, glassed both sides and edges resined heavily, glass to hull, make sure to bed in some putty, not wood to hull directly.
Just aft of the tank is a vertical 2x6 cross brace, glassed as usual and also has alum angle brackets thru bolted with stainless bolts. This I think is to prevent the hull sides from panting in and outwards.

the critical joint will be the fwd keel stringer to the face of the front of the box. I'd leave at least 2 inches space around the tank if possible and it will require the plastic venting tubes, if that system isn't already on your boat I'd consider a fan driven system because the tube will have to be ducted under the deck either side (one in one out) with the associated intake and exh's mounted under the gunnels, easy if the cap is off, a swine if the cap is installed.

epoxy is great if you can afford it,
I'm a big fan of unwaxed poly, not just for price,
I love the speed I can work with it.

btw, if you haven't ordered your fuel hoses yet, follow Mark's suggestion ,
http://foreandaftmarine.com/
those guys have very cheap prices. I had to get 10 feet of fuel fill hose, only cost $54. !
 
chrishayes

chrishayes

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Re: belly tank/stringer conflict

huh, I hadnt thought of the venting. Are there kits or could you direct me to some reading I could do on this? All this talk is exactly what I wanted when starting this thread. lots of good ideas here. My brain is starting to wrap around what is all involved. keep it coming...
 
chrishayes

chrishayes

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Re: belly tank/stringer conflict

I havent ordered my material yet, so would the 17oz cloth that I planned to use on the stringers with epoxy work well here? I plan to do two layers or tabbing with 8.5oz tape overlapped then a full dressing of 17oz.
 
Mark42

Mark42

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Re: belly tank/stringer conflict

chrishayes said:
huh, I hadnt thought of the venting. Are there kits or could you direct me to some reading I could do on this? All this talk is exactly what I wanted when starting this thread. lots of good ideas here. My brain is starting to wrap around what is all involved. keep it coming...
Click to expand...

I'm not clear on what venting is needed, other than the tank itself.

Don't forget to make passage for any water that might leak in forward of the tank to drain to the rear where the bilge pump should be. Basically this means two holes at hull level in the bulkhead (cross piece in front of the tank) so water on either side of the center stringer can drain aft and under the tank to the bilge.
 
J

jonesg

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Re: belly tank/stringer conflict

look at any boat with tanks under the deck, there are air vents on the outside of the hull, one scoops air in because it faces fwd, the other faces bakward and draws air out. they are ducted to the tank compartment.
the air ducts are the same as you see on clothes dryers, plastic with wire coil.

you can't just stick a tank under the deck if the ducts aren't there.
no big deal but like everything else it needs to be done to uscg standards.
 
J

jonesg

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Re: belly tank/stringer conflict

Look at these pics

http://boston.craigslist.org/nos/boa/1169778097.html

it looks like he installed air ducts on top of the center console, they look like 2 red headlights on top of the console in pic#4

the pic of inside the console shows the ducts going down.
actually one should be facing rearwards to draw air out methinks.
But anyway this is an example of a simple way to retrofit venting.
 
Mark42

Mark42

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Re: belly tank/stringer conflict

Thanks! I learn something new every day here.
 
chrishayes

chrishayes

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Messages
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Re: belly tank/stringer conflict

I gotcha on the venting, does seem like a good idea. Simple. My only concern is that I would have to secure the hoses somehow as I am leaving the stern side of the tank box open to the bilge. I would think that this would alter the efficiency of the air evacuation but on the other hand wouldnt it need less air due to it being open to the bilge?

Mark, I will of course be cutting half moons out of the perpendicular stringers to allow water to free flow back to the bilge.

My next question would be how to provide access to the tank for futre servicing needs. Is there a premade hatch that I can install? Would the hatch need to be larger than the tank for removal if necassary? Really, the tank design that I will get will have all the incidentals that would need servicing at the rear most part of the tank that will be open to the bilge. Would I still need to put a hatch down? I really dont like the idea of a homebuilt hatch. After taking all the time to get everything sealed up I would hate to have a place that water would always run to other than the bilge. Seems like a liabilty to me.
 
chrishayes

chrishayes

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Re: belly tank/stringer conflict

I finally got the pics off the card. Here is the layout from the factory
http://i631.photobucket.com/albums/uu37/chrishayes_photo/boat026.jpg
boat029.jpg


boat030.jpg
 
chrishayes

chrishayes

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Messages
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Re: belly tank/stringer conflict

Eric or Mark, do you see any problem now that the pics of my stock stringer placement has been added? I am about to this point in the project.
 
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