fuel tank questions

Designs for inboard or outboard power

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gmb
Posts: 10
Joined: Wed Nov 04, 2009 1:25 pm
Location: Billerica, MA

fuel tank questions

Postby gmb » Thu Dec 17, 2009 7:56 am

I am building a 19' center console. My intent is installing an I/O. The plans I have do not provide the CG for the boat. I would appreciate any help or location to find answers to the following questions. I have used the "search" in this forum, however have found no answers to my questions. My intent is placing the fuel tank under the floor.
1. how do I determine the location of the fuel tank?
2. how do I determine the size of the fuel tank?
3. how is the tank anchored into the boat, I am of the understanding that placing foam around the tank is not a good idea because the tank expands and contracts. If that is the case, how do you secure a tank in the boat, such that it doesn't move, but can expand and contract?
4. what is the best material for the tank to be made of, aluminum, fiberglass, poly/plastic??
5. after the tank is installed, is the floor installed such that the tank can be accessed if there is a problem with it, if so, how is this accomplished?
6. what type of "fill" piping should be utilized, diameter and the location?
7. what are the specifications for venting?

thanks for any help or direction......... Gregg from Billerica, MA........... currently 15 degrees, great boatbuilding weather!!

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Dave Grason
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Re: fuel tank questions

Postby Dave Grason » Thu Dec 17, 2009 8:06 am

gmb wrote:....what is the best material for the tank to be made of, aluminum, fiberglass, poly/plastic??


Well, I can't really answer ALL your questions but I can tackle this one. Really, the very BEST material for gas tanks would be stainless steel. I'm currently building a Zip but as soon as finish it, I'll be starting on the Mist Miss. The M/M calls for a 20 gallon round take in the stern. I found a custom shop that could make the tank to the exact dimensions from S/S or aluminum. It'll be a little pricey but worth it I think.

Aluminum will also be a fine material but it can oxidize in a salt water environment.

Plastic is what I believe will most expansion and contraction. But when fully contracted only the thinnest and cheapest of plastic tanks will actually display any kind of real distortion. Ratcheting straps will work very well holding the tank in place regardless of expansion or contraction.

I hope this will get this discussion kicked off.
Isn't it amazing!! The person that never has the fortitude to pursue his own dreams, will be the first to try and discourage you from pursuing yours.

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Mr Hot Rod
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Location: Chelsea, Quebec, Canada
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Re: fuel tank questions

Postby Mr Hot Rod » Thu Dec 17, 2009 11:56 am

Most of your questions concerning fill/vent piping and installation can be found in Glen's book Inboard Motor Installation (Glen L. Witt and Ken Hankinson)

Here's a link to an article written by David Pascoe describing some of the issues with aluminum tanks and how to install them :


Click here to see how we fabricated our fiberglass mounting decks.

Hope this helps !
____________________
Paul Kane Chelsea, PQ

Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com
Last edited by Mr Hot Rod on Fri Dec 08, 2017 8:04 pm, edited 1 time in total.

gmb
Posts: 10
Joined: Wed Nov 04, 2009 1:25 pm
Location: Billerica, MA

Re: fuel tank questions

Postby gmb » Thu Dec 17, 2009 12:06 pm

thanks to both of you. just ordered the book, hopefully it will set me in the right direction.

Kevin Morin
Posts: 687
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: fuel tank questions

Postby Kevin Morin » Thu Dec 17, 2009 1:18 pm

gmb,
gmb wrote:1. how do I determine the location of the fuel tank?

Since you haven't said any differently, I'll assume the tank is a below the deck tank and therefore, with an aft engine you can make a few assumptions without too much risk.

First you could say the tank will work best under or forward of the console because that is about the center of gravity of the boat and at or near the center of flotation, so adding the wt there reduces its trim effects full to empty.

So I'd recommend you consider the tank just forward of the console, and use a false panel of a few inches thick on the front of the console to put the fill and vent hoses. This would put the after end of the tank under the forward few inches of the console and that section of deck could be a 'hatch' even if it were more or less permanently sealed.

In a boat this small, one crew member walking fore and aft is equivalent to about 200lb crew/ 6.4 lb gallon or 30 gallons of fuel all the way forward or all the way aft. If that few hundred pounds is at or near the center its trim by the bow or stern will be minimized.

gmb wrote:2. how do I determine the size of the fuel tank?

Take the gallons per hour of your engine's consumption, and mulitply it by the number of hours of running at top end you want= gallons of fuel needed. OR; figure the fuel use and a speed -for example at 22 mph you'd use 4.5 gallons per hour [or whatever you figure for that engine an boat wt.] and in five hours running you'll travel 100 miles. So 5 hr.s run * 4.5gal/hr. =22.5 gal. and adding some spare; bump it up to 25 gallons? You'd have to set the distances and other goals to make use of this idea, but that is how you can plan your tank size.

gmb wrote:3. how is the tank anchored into the boat, I am of the understanding that placing foam around the tank is not a good idea because the tank expands and contracts. If that is the case, how do you secure a tank in the boat, such that it doesn't move, but can expand and contract?

You anchor metal tanks just like inboard engines, by welding mounting lugs or pads gusseted to the sides or ends of the tank and bolting those to beds, or structural longitudinals in the framing of the boat.

Bedding metal tanks in foam is poor practice unless the tank is heavily coated in coal tar epoxy paint. Aluminum tanks are often attacked by poultice corrosion if bedded in foam but with enough paint and surface treatment they can survive.

Expansion and contraction of a metal tank means that tank wasn't well made. Tanks should be rigid - flexure from filling shouldn't deform a metal tank because it indicates the panel stiffness it too close to elastic limits and the filling emptying cycle will/may result in flex to fatigue cracks.

Plastic tanks are not included in these remarks.
gmb wrote:4. what is the best material for the tank to be made of, aluminum, fiberglass, poly/plastic??


"Best material" is a relative determination, if you're set up to build in metal then tanks for this size boat are 'easy'. If you don't have metal working or welding equipment- that term, 'easy', won't apply to those materials. Each material has proponents and have some problems. If you won't be building the tank then why not find the nearest supplier of each type tank and price them by the volume to make a comparison?

I've built more aluminum tanks than I can count or recall, a few SS tanks and more than a few steel tanks for skiffs up to Gulf of Alaska crabbers, and if done correctly metal tanks are my preference. But I'm a metal oriented builder and have all the required equipment so my opinion is naturally focused on metal.

gmb wrote:5. after the tank is installed, is the floor installed such that the tank can be accessed if there is a problem with it, if so, how is this accomplished?

I'd put a face frame under the surface of the deck just slightly larger than the tank's outer most dimensions. Then using that "inside lip" I'd mount a removable deck section sealed around the opening in the deck.
gmb wrote:6. what type of "fill" piping should be utilized, diameter and the location?

1-1/2" hard pipe as the main downcomer inside the tank with hose and fittings upward to the fill cap. Venting in this small a tank can be 3/8" tube but 1/2" would cover all fill rates you'd expect from gasoline pumps -even at the dock.

Locating the fill and vents in the forward panel of the console is the simplest for a below decks tank on the centerline in a small open skiff. This absolutely must be completely separate compartment from the body of the console- view it more as a cover that mounted to the front of the console forming a removeable shroud rather than being 'in' the console. This would make the fill and vent fittings as high as any point in the skiff (usually the console is higher than the gunwales) and that means spills and most avoidable.
gmb wrote:7. what are the specifications for venting?

Well..... when I was building full time I got busted by the State Environmental sleuths for venting overboard and then the Coasties tried to give us a fine for venting over the deck!! rock and a hard place?

I live a long way from most regulatory inspections- I'll hope the more knowledgeable builders here will answer this more effectively? I sure don't want you to try and tell them "Some heavy-displacement burned-out old welder in Alaska told me......" during a routine boarding inspection and get a fine because I'm uninformed!

When the incoming gasoline is filling the tank you need to be able to allow the gases off the top of the tank to avoid the fill backing up and overflowing. The fitting should all be rated for gasoline (not just 'fuel') and the hose and hard tubing shouldn't get any smaller at any distance from the tank to their end fitting. The rest of the time the vent allows the tank top spaces to adjust to cancel differential pressure between the tank volume and the atmosphere- that means you could get by with a 1/4" tube and be fine, but not for filling operations.

Cheers,
Kevin Morin

gmb
Posts: 10
Joined: Wed Nov 04, 2009 1:25 pm
Location: Billerica, MA

Re: fuel tank questions

Postby gmb » Thu Dec 17, 2009 1:33 pm

Kevin, thanks so much, where in Alaska are you, my Dad's first cousin lives in Alaska, he just retired as the head of the maintenance/grounds department for the University of Alaska, he lives in two log cabins, (has a spare in case one burns down?? so he tells me) and does have an out house, kind of primative for a new england guy used to bathrooms, electricity and running water. He was born and raised in MA, however after the Korean war, ended up in Alaska and has stated he'd never come back here to live, and he has remained true to the statement for the last 50 years.

Kevin Morin
Posts: 687
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: fuel tank questions

Postby Kevin Morin » Sat Dec 19, 2009 4:10 pm

gmb,
I live on the Cook Inlet in the South Central [coastal] part of the state. Your Uncle is 300 miles north- it gets cold there- but thankfully not here***. He lives the 'old Alaskan' life if he still has an outhouse! Cabins are too much upkeep for someone as lazy as me, but each to their own.

Let me know if you have more tank questions, I'm not much help except in metal but I'm willing to give my 2cents if you need 'em.

*** by comparison

Cheers,
Kevin Morin


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