Fuel tank placement?

We are in the process of converting a 28' Topaz express from a twin inboard to a twin outboard (225's) setup. The hull is basically stripped , we installed new stringers and built up the transom to accept the transom bracket. The problem we now face is the position of the fuel tank. We made provitions for it to be mounted low and on the centerline of coarse, but how far from the transom should it sit? The outboards with bracket are about 1350# and the fuel tank full(200 gals.) about 1500#. Any suggestions or input would be appreciated.
Thanks,
Capt. Chris

 

Chris,

Basically you have to look at the weight distribution. Because you move weight aft, you have to compensate by moving weight foreward. It's calculating the moments and making them equal but opposing eachother.
Tricky thing in this is the fuel tank is not a permanent weight, but it changes. and so will your CG and trim.

Hope this helped

 

Fuel tank placement

Hi Captn Chris

To do this, you need to know four things:

1.) The wieght and placement of the old engines as well as their length.

2.) The weight and placement of the old fuel tank. (most likely centered above the Center of Bouyancy)

3.) the weight and placement of the new engines along with their length.

4.) the wieght and placement of the mounting brakets and their approx long cg.

Once you have that, use the following procedure.

1.) Measure half the length of the old engines from their placement from the transome and multiplie the distance * weight

2.) Measure half the length of the old fuel tank from the transome and multiplie the distance * weight.

3.) add the results of steps one an two together. That is your old moment from transome or OMT.

5.) Measure half the length of your new engines and their placement from the transome and multiply the distance * weight in negative numbers.

6.) Do the same with your mounting brackets but use their long. cg.

7.) Add steps 5 and 6 together. Remeber, they are both negative numbers.

8.) Add to this the weight of the new tank * half its length. Hopefully, you will get a positive number.

9.) Divide the result of step 8 into the result of step 3 and you will get the approximate placement of your new tank from the transome. This may not be exact, but it should be close. Hopefully, the guesses and the fudge facters will cancel each other out. The exact cg of the engines, both old and new are merely being guessed at here as well as the cg of the new and old tank.

I hope this helps.

Bob

 

Fuel tank placement

Sharpii2,
Thanks for the info!. You either really know what you,re talkin about or you,re trying to pull my leg with these calculations! I really appreciate the input. I'm fabricating a transom bracket soon and plan to make it taller than some of the ones on the market, hoping to add some bouyancy. How critical is the placement of the smaller gear(batteries, fresh water tank etc.)?
thanks again,
Chris

 

You might need to go one step further than this. If your calculated (simplified) LCG, as Bob has described, is further aft with the outboards than with the old engines, you may need to look at shifting some other onboard weights - batteries, genset (if there is one).
As the others have suggested, fuel tanks are - as a rule - not a good way of moving the CG. The exception to this is to have two tanks: one fwd & one aft. As long as there's enough fuel in the tanks, a transfer pump between the two allows you trim on the move.

 

You Know if you palce the fuel tank directly and evnely over the center of gravity location in theory the weght of the fuel should never affect your GC location thus eleminating your k plain adjustments through the variences of fuel levels! so that is what I would do! Also if you nee a custom aluminum or steel tank made I can help you with this! prefer aluminum cheaper to ship!

Good Luck! and post some pics sos the rest of us can see what you got!

 

OOOPs

Hi Captn Chris

In my little calculation program I gave you, I left out two additional steps.

The first one is to multiply the result of th last step by the weight of the new fuel tank, engines, and bracket.

The last one is to divide the above result by the weight of the tank.

This will (at last) tell you how far you have to move the cg of the tank from where it presently sits.

Sorry about that.

Bob

 

Thanks for all the input. Can't wait to get to the shop tomorrow armed with ruler and calculator! We have alot of "before" photos of the Topaz, they are just a bit too embarrassing to post,but shes getting there.It's amazing what thousands of dollars and endless hours of labor can do for an old wreck.
Thanks guys
Capt.

 

Alum. Weights?

Can you tell or help me figure out how much a fuel tank weighs? It is 48"X60"X20". Built from 3/16th alum plate. Thanks, still tryin to get my figures together for the topaz project.
thanks,
Chris

 

My calculations show that it should weigh around 188.496 lbs sounds a little heavy to me though but should be the actual weight not including fittings and the welds fut the welds are probably under a lb. let me know if this sounds rite or not!

That is a bone dry calculation and is scientificaly dead on.

 

freeagent,
That sounds right on. We only have records from the scrap yard were we junked this tank I gave you the dimensions for. The scrap yard's scale is like the opposite of the butcher with his thumb on the scale, if you know what I mean! (he came up with about 150#) THANKS ,I'll use this weight in the calculations. I have to remember that this whole thing is not rocket science and the fudge factor will apply sooner or later.
Thank for the fast reply!
Capt. Chris

 

I still am a firm believer of placing the fuel cells directly over the gravity center but then that brings in the search for the needle in the haystack "The Gravity Center" well Good luck bro! keep us posted!

 

I am building a boat that has never been built except on paper. SOOO, I finish the hull so it floats. Realizing the deck is fairly evenly spread out in weight, a bag here and there. I then buy 20 --50# bags of whatever from Home Depot and distribute them as piles of machinery. I have the scale weights of eng., trans., shaft, V drive box, rudder assy., etc. As long as it floats level, perfect. When you plug the prop, strut and rudder holes, you could do the same ballesting with bags and only layout 1 set of holes and brackets the first time. You probably still have a water line to reference to ? Piece of cake. If it looks right, it is right.------------------A thought. Your transom is looking at a serious weight increase. You could work a deal with a local scrap metal yard. BORROW 1,400# of scrap railroad track cut to lenghts that equal the width of the 2 OBs in their brackets. RR track goes between 120 to 220# per foot. Put that on the brackets and life gets beautiful. Rest is bag stacking. A little more effort, but it will let you hit it right the first time with no doubts.

 

I usually do this with models! much cheaper and I always go to the extreame in the weight just to make sure everything is gona work out! you can always find your wl after it is rigged! the waterlines in plans that are not modeled are not acurate they are a calcualted factor that really don't matter uless you plan on running a freeboard in a matter that inches of hight are critical! not very common!

 

Mo' bote math

Hi Chris

The first thing you need to do is to turn the dimensions of your tank into feet.

Thus: 48" becomes 4ft, 60" becomes 5ft, and 20" becomes 1.67ft.

The next thing you do calculate the various 'sides' of the tank. There will be six.

They are as follows:

4ft * 5ft = 20sft, for your top and bottom which means 40sft*2or40.00sft,
4ft * 1.67ft = 6.68sft for each end and you have two of those or 13.36sft,
and 5ft * 1.67ft = 8.35sft for each end and you have two at 16.70sft
Now add this all together and you get 70.06sft
for your total tank surface area, but we're not done yet.

Fuel tanks need baffles so that when the boat changes speed suddenly, the contents don't all surge to one end and cause seam rupturing pressures. These baffles will be nothing but plates with holes drilled in them, so the fuel can move from end to end, but not too quickly. We'll guess that there are two running accross the length of the tank, dividing the length into three, and one running the length of the tank, dividing the width into two.

This will in effect, give us two additional ends, at 13.36sft total, and one additional side at 8.35sft. Now our tank surface area is 70.06sft + 13.36 +
8.35sft which = 91.77sft.

Aluminum weighs about 164lbs per cuft and there are 12 1in thick plates in a cuft block and there are 16 1/16in smaller plates within each 1in thick plate and we are using 3 of those for every square foot of tank surface area. Knowing this, we can make the following equation:

91.77 * 1/12 * 1/16 * 3 * 164 = our tank weight. Which is:

approx. 235lbs.

And I say approximately because I don't know how thick the baffles are, or if there are any, for that matter (my long suffering instructor, who shall remain nameless on this post, to save embarasment, insisted on the existence of baffles). I also don't know how big the holes are or how many there are in each supposed baffle. Even so, I think this number is close enough to be useful.

I hope this is helpful.

Bob.

PS- I solute you for taking up such a large project and wish you well. In a world of so many watchers, it is a pleasure to come across an actual doer.