Gas Tank Coffin

[Curious]

Does anyone know why the gas tank coffin on my 78 22' FF is deeper in the front and shallower in the stern?  It goes from about 13" deep in the front to about 9" deep in rear.  The tank is a little over 8" tall so it is really close in the rear, especially with the vent tube closer to the back of the tank.  I wanted to increase the size of the tank to gain a few more gallons and install with the Pascoe method but that isn't going to be possible without losing about an inch of tank height.





I guess it's kind of hard to see in the photos but if you look close it is deeper in the front and shallower in the rear.

[seagate]

might be to keep weight forward,and for hoses to fit.

[love2fish]

They might have also had the idea to keep the tank level/flat while the boat was floating so the gauge reads accurately.
(Level when the boat was new, with the original engine specs and transom set up, and dry foam).

[wingtime]

I agree my coffin was the same way.  I also believe it is to keep the bottom of the tank level.  Now if that is level while at rest or running on a plane thats another story.  Also you have a self bailing deck..  so it has to be higher in the front to allow the water to run to the stern.  So maybe it is a combination of the two.  

That is really strange that your vent is in the aft end of the tank.  I would think you would have a tendency to have fuel in the vent while running with a full tank or even while filling the tank.

[wingtime]

Here's a pic of my tank with the coffin cover off, you can see mine is deeper in the front too.  The vent in the front and the pickup is in the rear of the tank.

[Curious]

Wow, I wish my tank looks like that.  There's a blueprint pic in the photo gallery that shows the tank level but with the same angles as ours so I assume it has something to do with keeping it level at rest.  I thought that the vent was in an odd location, too.  Doesn't make a lotta sense to me.  Also the pickup is about in the middle of the tank with a pickup tube inside running all the way aft.  I guess I'll have to keep the size as is and reinstall with a method other than Pascoe.  Suggestions? Refoam?  Coal tar epoxy coat and restrap?

[wingtime]

You should have seen my old tank!  Talk about nasty.  I built that tank with the help of a friend of mine that is a marine welder.  I'm not familiar with the Pascoe style on install.  Mine is installed on 3/4" strips of starboard stuck to the bottom of the tanks with 5200 for spacers and then strapped in place.  This way air can circulate around it so any water can dry out.

[Curious]

You've just described the Pascoe method!  Except the starboard gets glued to the base of the tank coffin with 5200 so there are no straps to hold moisture.  Problem is I don't have the clearance to add the starboard strips.

[love2fish]

Wingtime- did the 5200 actually stick to the Starboard? I didnt think anthing actually bonded to Starboard. did you have to do anything special to it?
Thanks!

[imonna 19 6]

curious,

I had same issue, same installation idea, and same problem.  i decided that if i moved the vent and fill to the front and the pickup about 2/5 towards the front, i could free up some clearance and increase the height of my tank by 1/2 inch, which translated to a few more gallons and then i did use the pascoe method.  here are some pics.

IMHO i wouldnt want the vent towards the back (because you will lose fill ability  before the tank is full)








best of luck
Bill

[wingtime]

I sanded the starboard to help with adhesion.  The 5200 was really there as a seal between seal the tank and the starboard to keep water from being trapped between them and causing corrosion.  That is also why the starboard was placed in from the edges of the tank so any water running down the side of the tank would drip off to the coffin and not contact the starboard/tank.  It's all theory of course.  I can tell you the original 29 year old tank only had corrosion where it something in contact with it that could hold water or trap water against it.

[Curious]

Bill, thanks for the info/advice.  I will definately move the vent up next to the fill like yours.  This looks like the original tank and if so I can't think of any good reason why the vent is so far back.  Anyways, even with the vent moved forward I don't think I can add much height, if any AND do the Pascoe method.  One of the tank quotes I got included an exterior epoxy coating.  Is that enough of a barrier to prevent the crevice corrosion issue so i could technically refoam?  I also thought about bedliner or the coal tar epoxy coating.  Then there is no way water can contact the aluminum so there should ("should") be no corrosion, right?  

By the way, what is the red material your strips are made of?
Thanks again, Dan

[imonna 19 6]

I will only be the first one to say it, but with regards to how to install a tank, opinions are like^%^$%   everyones got one and mine is just an opinion.  

You will find massive posts on this and other websites with convincing arguments for and against every conceivable tank install and none are seen as fullproof, all are seen as decent.  Don't get to lost in figuring out which is perfect, just pick a method and be true to it.  

seems like most people agree....just dont leave crevices for water.  epoxy, could chip and might leave a crevice. foam is seen as a big nono becasue it will trap water.   Even these methods will probably last a couple decades before failure.

In all my research the two methods that seemed most reliable over time was a thick coat of coal tar, so water cant get to the surface; and pascoe install.  

that material is FRP or fiber reinforced plastic.  it is what is reference in the pascoe article.  5200 adheres nicely to it (so i've been told).  i got the whole strip from McMaster Carr for about $80 bucks and it arrive in 3 days.  I used the extra pieces for backing on my through holes...worked great.





http://www.mcmaster.com/

Bill

[gran398]

Good stuff folks.

One premise of the Pascoe method, per JJ's old post, is that the weight of the tank when glued to the coffin floor via the strips prevents movement.

I like his thinking, he's astute for sure, and makes great points. But can't for the life of me feel good about installing a tank with nothing to prevent movement fore, aft, and side-to. Fiberglass 2X2 girders will lock ours in from the top.

We are still scratching our heads as to the best method to chock the bow and stern corners, mid-ships too. One idea is to pour foam in thick-walled plastic containers...similar to the shape of quart milk bottles, but heavy, black plastic. Set the tank on the runners, glue everything up...then two days later, foam the plastic containers.