Fuel tank placement ???

[Todd j]

Does this look okay for placement using the given volumes?   Volumes will be slightly overestimated since I didn’t account for the thickness of the tanks.  All dimensions are to the outside of the tanks. 

[Brian.Dixon]

The best place for the fuel storage center of gravity is about 60% of the way back.  For example, measuring from the tip of the bow to the top edge of the transom, a 28' boat would be 336 inches long.  60% of this is about 202 inches.  If you balanced your fuel weight over a line drawn at 202 inches aft of the bow, then the boat will trim right.

THAT said, there are several things to note (as you keep the 'general rule' above in mind):

• The boat is forgiving of some variation here
• Stern regions tend to 'gain weight' over time ... captains load big coolers of ice back there, fill fish wells with fish and water, take half a dozen people (and their gear) with them and they all run to the stern at the same time, motor brackets tend to move the boat CG aft, etc
• The longer (and wider) the boat, the more forgiving it is.  If you're building a 25-footer, then follow the rules more closely and keep the outboard closer to the transom (short bracket or jack plate).  If building a 30-foot Kodiak, then you can vary the rules more.

When you're deciding your arrangements, length of the pilot house, fuel load and where it's center point is ... just try to be reasonable, given the 60% rule above, and take all heavy items into consideration ... motor bracket length/number of motors/motor weight, fuel system CG, accommodations, water storage, average location of passengers, and go for it.

Given all the above, I would say that your fuel tank arrangement looks fine.  What are your plans for powering and brackets?

Brian

[Todd j]

I built for a single main. I would like to do a 250 with drive by wire.  4-6” electric jack plate. And a 9.9 kicker.    If money was no object, I could win the lottery tomorrow right!   I would closely consider 115 twin etech with Idock.  I can’t remember the differences in the transom.  It may be too late for twins.

[Todd j]

Okay Brian.   New Calcs hot off the press.  My design 60 percent COG is 6 feet 8 inches aft of the cuddy or 189 inches aft the bow.  That said, my fuel  centerline  is 1 foot 6 inches  aft of the 60 percent COG.   If I use the forward tank profile(just for the sake of arguement) and build a water tank 12 inches long.  I will have the 20 gallons of fresh water I want to keep aboard.  That shifts my tanks COG to within 6 inches aft of the design 60 percent COG.
  I think I'll Take it!  What do you think?   Even a blind squirrel finds a nut once in awhile.
    Of course these figures assume the tanks all have the exact same dimensions and so forth.  I expect the forward tank will need a small adjustment for floor support above the tank

[Brian.Dixon]

Okay Brian.   New Calcs hot off the press.  My design 60 percent COG is 6 feet 8 inches aft of the cuddy or 189 inches aft the bow.  That said, my fuel  centerline  is 1 foot 6 inches  aft of the 60 percent COG.   If I use the forward tank profile(just for the sake of arguement) and build a water tank 12 inches long.  I will have the 20 gallons of fresh water I want to keep aboard.  That shifts my tanks COG to within 6 inches aft of the design 60 percent COG.
  I think I'll Take it!  What do you think?   Even a blind squirrel finds a nut once in awhile.
    Of course these figures assume the tanks all have the exact same dimensions and so forth.  I expect the forward tank will need a small adjustment for floor support above the tank

That'll work.

Re Transoms:  For all outboard combos, the motor board (and below) assembly is identical and should never be cut into or interrupted.  Everything on the transom from the motor boards UPWARD, it's just different cut-outs .... you can add/subtract wood and fair it in if you want to change your mind.  I'm with you on the lottery answers... never works out for me either!

[Dan Boccia]

Awesome info from Brian, love having direct access to the designer like this!

Todd, I have a few strong recommendations - you're doing a great job, hopefully this helps:
1. Eliminate the small flat at the bottom of your tank and just make it a V-shape. Whoever builds your tanks will bend the tank with a brake, and saving them a bend will be helpful and will have little impact on volume or clearance and will likely save you a bit of cash and save the builder some headache.

2. Raise your deck at least 2 1/2" above the top of the stringers. This has many benefits and only one small disadvantage: Benefits are your tanks can go to the top of the stringers (more volume) and there is still plenty room to run fill, vent, and pickup hoses/fittings without having to cut the stringers; This way the top plate of the tanks can be extended as I did with mine to support the tanks on top of the stringers, keeping the bilge clear - no need to place struts on the bottom of the boat to support the tanks, which clutters up the bilge and is a LOT of work comparatively (Dave in Homer and I both did this); this deck height also puts the scuppers above the water line adequately so water does not normally come on board when you're standing at the transom. Disadvantages are that your clearance into the cabin is reduced compared to inside because of the roof extension (just account for this when deciding cabin height).

3. Use 3/16 aluminum - it is plenty strong, 1/4" is way overkill and just adds unnecessary weight

4. Wherever the tank lands on support, bed the ENTIRE area with sealant so zero water/air can penetrate this area. That will prevent pit corrosion.

5. Leave about 3/4" clearance all around for air to circulate

6. Strongly consider 2" or bigger gimble holes at the bottom of all your bulkheads so you can really ventilate bilge fumes out of the boat through passive or active means (you could do a dorado vent in front and back, or install an explosion proof bilge blower). Also, have a way to plug the forward bulkhead so fumes do not go into your cabin, and have the rest of the floor over the bilge completely sealed off, at least inside the cabin. When I finally figured that out, my cabin became completely fume free and it's wonderful.

7. Make darn sure your fuel fill and vents are at least 6" apart, and 8" or more is better. My aft tank has them about 1 1/2" apart and I get fuel out the vent, so I get to cut into my deck this spring and install a new vent tap which is going to be touchy work. While you're at it, put a vent on both forward and aft of the tank so no matter how your boat is sitting on trailer or water, it will vent properly, especially as it's getting full

8. Have the tank fabricator give you a tab on both tanks tapped for a #10 screw for a bonding wire

9. Place the tank pickups in the aft end of the tanks so you get all the fuel - you're always slightly nose up when running.

10. Fight for every gallon you can get from each tank. You never have enough fuel. I already wish I had more. With the 250, plan on 3 nmpg, no more. 2.8 if you like to run at 30 knots.

Some of the above I got right and am super pleased, a few I missed and had to do a lot of painful rework. Fuel tanks are important, aluminum tanks DO fail with in a few years if not done correctly, and there's surprisingly little good advice available online. Hopefully you can find someone knowledgeable to fab the tanks for you. Have fun and good luck!

[json]

Great info Dan, thanks for that. I wanted to mention the dual etec setup, I thought about doing that type of thing with a couple g2 etecs and the result would have been way over the recommended max weight on the transom (they are about 400 lbs each for the 115, so overweight even before adding a hundred pounds twice with a couple brackets). g1 etec is about the same. I would love to have dual setup if for nothing else than having insurance to get home (joystick control would be a seriously awesome addon) but it seems like it's not in the cards... Maybe I am wrong, as I sure would like to open the door on that option again on my build.

[Todd j]

Awesome info from Brian, love having direct access to the designer like this!

Todd, I have a few strong recommendations - you're doing a great job, hopefully this helps:
1. Eliminate the small flat at the bottom of your tank and just make it a V-shape. Whoever builds your tanks will bend the tank with a brake, and saving them a bend will be helpful and will have little impact on volume or clearance and will likely save you a bit of cash and save the builder some headache.

2. Raise your deck at least 2 1/2" above the top of the stringers. This has many benefits and only one small disadvantage: Benefits are your tanks can go to the top of the stringers (more volume) and there is still plenty room to run fill, vent, and pickup hoses/fittings without having to cut the stringers; This way the top plate of the tanks can be extended as I did with mine to support the tanks on top of the stringers, keeping the bilge clear - no need to place struts on the bottom of the boat to support the tanks, which clutters up the bilge and is a LOT of work comparatively (Dave in Homer and I both did this); this deck height also puts the scuppers above the water line adequately so water does not normally come on board when you're standing at the transom. Disadvantages are that your clearance into the cabin is reduced compared to inside because of the roof extension (just account for this when deciding cabin height).

3. Use 3/16 aluminum - it is plenty strong, 1/4" is way overkill and just adds unnecessary weight

4. Wherever the tank lands on support, bed the ENTIRE area with sealant so zero water/air can penetrate this area. That will prevent pit corrosion.

5. Leave about 3/4" clearance all around for air to circulate

6. Strongly consider 2" or bigger gimble holes at the bottom of all your bulkheads so you can really ventilate bilge fumes out of the boat through passive or active means (you could do a dorado vent in front and back, or install an explosion proof bilge blower). Also, have a way to plug the forward bulkhead so fumes do not go into your cabin, and have the rest of the floor over the bilge completely sealed off, at least inside the cabin. When I finally figured that out, my cabin became completely fume free and it's wonderful.

7. Make darn sure your fuel fill and vents are at least 6" apart, and 8" or more is better. My aft tank has them about 1 1/2" apart and I get fuel out the vent, so I get to cut into my deck this spring and install a new vent tap which is going to be touchy work. While you're at it, put a vent on both forward and aft of the tank so no matter how your boat is sitting on trailer or water, it will vent properly, especially as it's getting full

8. Have the tank fabricator give you a tab on both tanks tapped for a #10 screw for a bonding wire

9. Place the tank pickups in the aft end of the tanks so you get all the fuel - you're always slightly nose up when running.

10. Fight for every gallon you can get from each tank. You never have enough fuel. I already wish I had more. With the 250, plan on 3 nmpg, no more. 2.8 if you like to run at 30 knots.

Some of the above I got right and am super pleased, a few I missed and had to do a lot of painful rework. Fuel tanks are important, aluminum tanks DO fail with in a few years if not done correctly, and there's surprisingly little good advice available online. Hopefully you can find someone knowledgeable to fab the tanks for you. Have fun and good luck!

Dan   I need to address each of your points one at a time and let you pick my plan apart.   You have a lot of good info and I have wet epoxy to get back to.  Back is sore so I'm taking a quick break

[Djeffrey]

This might help. I put these strips down on the bottom of the hull, glasses them in. I covered them in a this pile of 5200 and layed the tank down in the wet 5200. Allows for air around the tank, and lifts the tank out of potential water.

[Djeffrey]

More food for thought. In the picture below you can see the white blower motor. I made the fuel tank in a completely closed compartment with a bulkhead on both ends, between the stringers. I put the blower on the front bulkhead and a flexible hose on the rear bulkhead which allows me to blow any potential fumes out the back of the boat.

[Todd j]

One at time in order!
1)  eliminating the flat on the bottom of the tank make sense from a " having to pay someone else to build it perspective ".  Fwiw I like Brian's design with it in the bottom of hull.  I can still build air tight bulkheads for the fuel tanks, but maintain a drainage path under the tanks without introducing moisture into the fuel compartments. 

2)  I  built the hull 4" taller than the design height.   I added 3" to the stringers inside the house.  So, I have a built in extra inch of headroom.  I tapered the rear stringers aft for drainage.   I did the smiley spray rails instead of the design.   This puts my deck 1 1/4" inch above the top of the spray rails at the transom.   Maybe more a foot ahead of the trsnsom. 
  My biggest problem following your recommendations is the fact that my extra stringer is already glued and glassed to the top of the designed stringer, thus defining my deck height permanently and eliminating the possibility of hanging a fuel tank from it.  Your suggestion is far superior to what I am left with.   I could have some bars welded across my fuel tames and notch  my secondary stringers to accomplish the same thing?

3)  gotcha weight matters

4)  easily done, thanks for the tip

5)  another easy to do, good idea

6)  I don't want fumes of any kind in the house!  Does gas fumes settle to the bottom?

7)  thanks for passing this on so others may benefit

  yup

9)   yup again

10)  gotcha

Thanks for all the pro tips Dan

[Brian.Dixon]

1. The "flat" below the tanks is technically a "floor" since it is below decks.  It is not necessary from a structural perspective, but it does help stiffen the hull.  You can decide if you want it or not ... I do like the fact that if you drain through the floor, that it provides a route to the stern (bilge) that is separate from the tanks.

2. The number one reason that aluminum tanks fail is water ... in the fuel, tank sitting in water, or standing water on top of the tank.  If you want the cat's meow of pre-installation treatment for an aluminum tank, then primer it with Zinc Chromate primer and paint with quality epoxy paint.  This eliminates most water-related issues.  This is how quality aluminum aircraft are built.

Dan - thanks for the sage advice!  With your permission, I will add your suggestions to the build manual and will attribute the notes to you ... you'll be world-wide famous overnight!

Brian

[Todd j]

It's pretty cool that all the best ways to skin the cat can be a hgv armed so readily.  I wish I had all this info 2 weeks ago. The ship has sailed for me in some regards.  Itll work either way, but I can definitely see the advantages to Dan's build method

[tom e]

Dan,

A couple of questions re. your comments. 

Re. #6, you write, "...my cabin became completely fume-free..."  This suggests that before you made the change, you did have some fumes in the cabin.  Is that right?  In any case, where are your tanks located? 

Re. #7, "...fill and vent are 6" apart..."   Like your tank, mine are just a few inches apart (in the transom end).  I stupidly assumed that the outfit had built and sold enough tanks that they would have figured this out.  Guess not.  In any event, I came across the products below and couldn't figure out how this could possibly work.  Some of them do have a one-way check valve, but I can't see whether these solve a problem, but make a new one or what. 

 https://www.amazon.com/dp/B0000AZ3OJ?tag=duckduckgo-d-20&linkCode=osi&th=1&psc=1

https://www.fisheriessupply.com/perko-0543-vented-straight-neck-deck-fill-1-1-2in-hose-0543dpgblk

Thoughts?

te

[Dan Boccia]

Todd, stick with your plan, and use what you can of my comments. It will be interesting to see how your deck works out!

From everything I have read and heard from the few salts around here that actually have decades of experience, pit corrosion of aluminum fuel tanks is the number 1 cause of failure. This happens when moisture (either vapor or actual water) gets in contact with the tank and has nowhere to go - especially salt water, and typically in small gaps/crevices/cracks between the tank and surrounding structure. That is why, wherever your tank comes in contact with structure, you must be sure it is 100% bedded in sealant. If the entire bottom of your tank is in contact with the hull, I think you're asking for trouble - can you guarantee that 100% of that very large area will actually get bedded properly? Since you're not hanging your tank, I'd go with DJeffrey's approach of a few cleats to set the tank onto, leaving ~3/4" air gap. Bedding those cleats gives you much better chances, and saves a ton on sealant!

Gas fumes are denser than air so collect in the lowest areas. That said, the *smell* of them can rise and permeate everything.

Tom, yes I did have some fumes in my cabin. I have bulkheads both sides of both tanks in the floor, with a 1" limber hole at bottom of each bulkhead for any liquids to drain out to the aft bilge. This allows a path for the fuel smell to enter the cabin through a floor hatch which is unsealed. Rather than trying to seal the hatch, I cut another hatch closer to the bulkhead so I can access the forward fuel tank bulkhead, and I put a plug in the limber hole. That way, the only path for fuel vapors to come out is through the aft bilge. Problem solved, no more fuel scent in the cabin.

And Tom, those fill/vent combo fittings are fine, but do not solve the fundamental problem. If your fuel vent fitting is too close to the fill, as air rushes out of the tank and into the vent fitting, it picks up bubbles/mist from the incoming fuel, and the whole works comes out the vent. It's stupid, I'm cussing myself something horrible for not recognizing that after I spent a LOT of time trying to come up with a good design. There is just limited info on such things to us commoners.