# Fuel/Water Tank Placement...

Discussion in 'Powerboats' started by monrosm@shrewsb, May 21, 2010.

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### monrosm@shrewsbJunior Member

Ok Im having trouble finding answers to my problem so if anyone has any info it's much appreciated...

Im a university student doing yacht Design working on a design for a 16m cruiser and need help with placements of fuel and water tanks...

Things that need to be taken into account are that the engines are quite far back so put a lot of weight at the stern, also remember that fuel tanks will reduce in weight (fuel being used) when the boat is in use, the water tanks also get lighter but a lot of the water will flow into the grey and black water tanks...I have attached some pictures of the package drawing which should help people understand!

Engines should be pretty obvious, Red blocks = fuel tanks, Light Blue = water, Dark Blue = Black water, and Brown = Grey Water....

What is the calculation for this??? There must be ways of working what goes where with some level of intelligence...its driving me nuts!!!

Thank-you Everyone

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2. Joined: Aug 2007
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### CDKretired engineer

There is no specific calculation. Draw a vector diagram with equally distributed loads for the fixed objects and find the center of gravity. Than add the weight of fluids (and people) as point loads and find the CoG again. From the distance it shifted aft you can calculate the approximate change in angle of the waterline. If that is dramatic you must relocate the tanks or use smaller ones.

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### baeckmoHydrodynamics

.......and fuel tanks should not be low and wide; that will cause aeration and sloshing problems.

4. ### apex1Guest

In addition to the comments of my peers which i fully back.

The easiest (not cheapest though) way to keep such finicky design in balance is a watermaker refilling / balancing the consumed water / fuel weight.

Regards
Richard

5. ### tunnelsPrevious Member

Something unrelated !!
THE exhausts from each motor will be a major problem if you dont change them !! I hope you have thought that one out better because what theres is a huge problem in the making just waiting to happen .

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### monrosm@shrewsbJunior Member

Fuel/Water tank placement

Thankyou everyone for your comments...very interesting and surprising amount of quick feedback...

@CDK, im on the same lines as you with the COG and have been able to model this in a CAD model, the trouble is that as fluids are used or distributed elsewhere the COG changes (by very little i admit)...how does this actually relate in real terms...is performance/stability seriously affected by small changes in the COG...

@baeckmo, thanks for comment, it is just a concept plan and im playing around with new ideas here, I am primarily focused on the design/styling aspect of yachts and my knowledge is limited when it comes to the more technical specifications....What is the norm for fuel/water tank placement of this size vessel?

@Apex 1, I have been told your suggestion by another expert in the field and will look further down that road, but like i said it's only a concept

@Tunnels, very intrigued by what you said...would like to hear more...all useful stuff although the pictures attached are more for placement of components rather than a complete Package Drawing....

Thanks Everyone...

7. ### tunnelsPrevious Member

Its the simple basic things that sink dreams reguardless how many dollars they cost .

Having been into power boats for a number of years its the simple thing that cause the most damage and can simply completely disable a multi million dollar boat because things werent done properly from the very begining . Exhausts are a good example of this . Sailed on a 65 foot yachts and when we tried to motor into a shelted anchorage the motor wouldnt even turn over!, was panic for a while till we finally dropped anchor and lifted the floor boards to fon water trickling out of the air intake manifold . The water lock muffler as higher then the motor and a baffle seporating the water from the exhaust has vibrated and cracked and water run back down the exhaust pip into the motor and with a 6 cylinder there is always a intake and exhaust valve open so filled a cylinder with water and hydrauliced the piston and water trickled into the intake manifold till it was full and it started running out down the side of the motor .

Another case was a cruising boat that left Papeete for there return trip home to Canada and was back in 3 days with a motor filled with water but this was something completely differnant but the exhaust was simular to what you have drawn and could cause a simular problems specially with 4 pipes
Exhausts need a water lock to stop the water from getting back to the motor . You will need to do some searching for the answer !.
Because the cruising boat was a yacht and the motor sea cock had not been turned off so they could simply start and motorsail into a lagoon there was one chance in millions that the water got past the pump and filled the exhaust pipe to the mufler and back flowed to the motor and filled one cylinder and same again hydrauliced the piston .A simple thing like one paddle off the rubber water pump impeller had broken off with age and time and the motor had stopped in the wrong place .
When i spoke to the owner of the boat they had had the same problem5 years previous but no one hade ever been able to find the cause . I tumbled to it as soon as i saw how the pipe and mufler had been installed and set up , only because a friend had the simular problem and that was what they found eventually after days of searching through there whole system .
Another case was a brand new 38foot yacht in the water 3 days and over night the mast and rigging was all that was sticking out at the marina the next morning . Was a shower with the outlet at water level and the shower base below water level and a pump that had been used and never allowed to completely drain the pipe out , the water reverse syphened back into the shower and just kept going all night two things at fault , skin fitting to close to the water and not loop witha anti syphening valve fitter at the highest point . If you do get to design a proper boat do some research and do it right first time .
Good luck

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### StumbleSenior Member

Monrosem,

One of the things I noticed is the odd shape/size of all of the tanks on board.

Of course it is possible to have them custom made, but the cost to custom manufacture custom shapped tanks could add thousands to the final construction cost. While it is of course possible, practically it becomes a very expensive way to spend the construction budget.

I have also noticed that typical installation winds up with black and grey water being combined into one large or two smaller tanks. Though this may just be a local consideration in the UK.

9. ### tunnelsPrevious Member

Just make glass tanks as a part of the hull constuction simple and save a lots money as well and get more capacity !!!!!.

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Monro...

Beautifully creative work well presented.....thank you....now do it over..

To answer (I think) your question....the hull has a longitudinal center of buoyancy (LCB), if you want the boat to float as designed, your weights must place the center of gravity (LCG) at the LCB. If the LCG is someplace other than the level LCB, the boat will trim until they are centered one over the other. You can investigate this in a simple weights and moments spreadsheet, see the spreadsheet library section of this site under software.

You don't mention speed or power of this boat, but the hull would appear to be the wrong shape for anything above very low speeds. Low speed hulls are fine aft (as yours is) and have the LCB centered close to amidships, thus the weights must be centered as well. Higher speed hulls have the LCB/LCG further aft (roughly station 6 in a 10 station hull) and of course the weights are centered around that. In this case the hull would be much deeper at the transom than you have shown, allowing more space for your engines.

Specifically about some of your features.....the dude can't see where he's going, he needs to be able to see the water 2-3 boat lengths in front, even when trimmed at speed. You need some way to stop the big waves coming over the stern and flooding the boat....this happens! The tanks need to be simple buildable and pressure tested shapes. Tapered rectangles are the norm. You can run a series of tanks across the boat and centered on your LCB...or you can get creative...but remember some fool owner will do it wrong and your wonderful design will float crookedly.

11. Joined: Mar 2002
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Oh ya....

Usual practice is to get the hull to float down by the stern at full load of all tanks. Hopefully it should then still float down (a bit) by the stern with half load tanks, and only down by the bow a tiny bit when empty....boats way down by the bow look bad......

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### monrosm@shrewsbJunior Member

Fuel/water tank placement

Hi Guys, thankyou for your quick and informative response, sorry I diddnt reply sooner iv been away for the weekend!!!!

@ Tunnels, I think you have a very good point there although I was advised to have submerged exhausts as this avoids the horrible plume of black smoke you might otherwise get trailing from the rear end, it also stains and surface of the boat it comes into contact with.

I have read a lot about situations you have described, sounds like they need some careful attention to detail, leaking seals are also the number one insurance claim and the easiest one to fake...I will research this in more detail!!!

@Stumble, these have purposely been shaped this way to break up the design, the Mark 1 version had very basic shaped tanks and I was advised to play with the shapes to give it more of an appeal...I admit I didn't see the point as these are all unseen objects by the potential buyer but if it's going to improve my grade then so be it!!!

@Tad Thanks for the feedback....I am primarily a designer and more focused on the styling and shape of the vessel, the design gets sent to a Naval Architect and engineers, they say 'this is good' 'this is bad' comes back to me and I do it over again....but I have made of note of all you have said as I get slightly irritated about not knowing at least the basics when it comes to LCB and LCG....
Also the 'dude' should be able to see where he is going, the Bow has a cut out with Railings in it's place...this should allot for 5 or so more degrees of vision....trimmed at speed could be a problem though! I was wondering what the legalities are of having say a camera mounted into the bow allowing the driver to see....This is something the Auto industry have started using...Look at Audi...they now have cameras at the back which comes on when you reverse...neat!

Can you point me in the right direction to find a package drawing of a similar sized boat? These things seem to be very illusive!

Thank you all!

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### monrosm@shrewsbJunior Member

TAD, I think you need to adjust sizes of pictures on your website....they are all squashed!

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### Easy RiderSenior Member

Yes .. then it will be worth it to look at Yellow Cedar again.

Easy Rider

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