Diesel Tank on Bridgedeck?

[Robot]

Hi I am new to the forum.
I have just had delivery of a new cruiser/racer 46 foot carbon Catamaran.
It sits a little stern heavy, so the builder placed additional diesel tanks in the bridgedeck.
There were fuel delivery issues on the delivery sail.

It’s still a little stern heavy so I wonder whether we can just use the forward tanks.
The concern I have is the diesel return would be “uphill”.
Would a small reservoir in the aft tank for the return, then a pump to return this to the forward tank work?

If we need to use the aft tanks I think we may need a transom extension for optimal performance .
I’d be grateful for your thoughts.
Cheers

[Orion Jim]

A new 46’ carbon cruising/racing cat that has any issues whatsoever should be rectified by the builder with the designer providing the plans. The acceptance sea trial should have noticed any major issues and the builder should have rectified them before you accepted delivery and made the final payment.

[fxykty]

If it is truly a performance cat then it shouldn’t need much fuel either. Stick to just the aft tanks.

As Orion Jim pointed out, if the cat is stern heavy as per original design and launch, that looks like a design and/or build error. Adding diesel tanks a long way away and well above the engines is a ridiculous idea.

Are there any other fixed weights that can be moved forward, such as batteries/inverter/charger, generator, water maker, etc? Water tanks should be near the centre of buoyancy so their state of fill doesn’t affect trim.

Many cats can benefit from stern extensions as designers/builders generally prefer to make their cats too short for their displacement, then compound that by placing engines as far back as possible to get them out of the accommodation spaces.

Do you care to disclose the model?

[GRIT]

Congrats on your new purchase! I'm sure you'll be pleased with her once you get the kinks out.

I'd contact the engine manufacturer to see if it'll pump it uphill. Though it's probably not that far up, perhaps a metre? That would be my first thought. Second option is as you say, just return the fuel to the aft tanks, and pump it forward on occasion.

Heavy in the stern is quite common on lighter boats. Is it heavy in the stern with a dinghy on the davits, or heavy even without the dinghy on davits? How low is your stern, compared to what you'd expected?

As Fxykty says, move as much as you can forward.

Cheers.
Paul.

[Montanan]

Of keen safety concern with having fuel stored "uphill" of the fuel rails of the engine is that fuel could flow downhill by gravity inducement if a check valve or fuel line fails and then you have a flood where you sure as heck don't want it.

Always preferrable to have fuel pumped up from its storage to its points of distribution and usage, never the other way around.

I suspect that is likely a standard marine safety requirement.

[ggray]

Fuel tank (as well as water tank) on our cat is on bridgedeck at and just forward of the mast. No problem with file return. Nor fuel delivery.

Like Orion Jim, I would be surprised that a boat made with such attention to keeping weight to a minimum (using carbon fiber) would be "out of balance". Are you fully loaded with provisions? Anything changed from the designer's plan such as larger engines?

[GRIT]

Montanan: For gasoline, yes. Against ABYC. I saw no such requirement for Diesel.

I have seen several production cats with tanks higher than the engine. Though none of them were US built cats, which is primarily where ABYC is used.

Cheers.
Paul.

[boatpoker]

Originally Posted by GRIT

Montanan: For gasoline, yes. Against ABYC. I saw no such requirement for Diesel.

I have seen several production cats with tanks higher than the engine. Though none of them were US built cats, which is primarily where ABYC is used.

Cheers.
Paul.

ABYC H-33 requires diesel to be returned to the same tank from which it was drawn. A shut-off valve would also be required if a gravity leak could occur.

[Ballsnall]

There shouldn't be any issue with returning the fuel "uphill" and is quite common in work/fishing boats. Measure the height and check with the engine manufacture if you have concerns.

What were the fuel delivery issues?

[noelex 77]

Welcome to the forum, Robot and congratulations on the new boat.

Like many workboats and trawlers, our yacht has a day fuel tank installed higher than the engine.

One minor advantage of this system is that the fuel delivery is “downhill”, which makes bleeding simple and allows the engine to run if there is a failure of the lift pump. Obviously the fuel return is uphill. This is not an issue. The diesel fuel lines have to be under very high pressure for the engine to operate.

In your case the solution adopted by builder sounds like a slightly clumsy fix, but having a diesel day tank has many advantages in helping to provide cleaner fuel to the engine so perhaps it can be turned into an advantage.

[catsketcher]

As someone who has designed and built a cat that sat low at the back end - bad job designer and builder.

If your cat is low at the bum when young it will only get worse later. I would be worried about whether it is a design issue (lack of bouyancy in the bum) or build (too much weight in the bum - adding extras that no one told the designer about). A weight study is a vital part of initial planning and getting it wrong is a serious stuff up - or someone kept adding stuff during the build - a bit here and there adds up.

No matter what - adding extra waterline doesn't do much for rectifying the situation. Adding an extension just glosses over the problem - a lack of bouyancy aft so that the boat squats to move the CB aft. Adding a little bit of foam at the transom will have almost no effect on the bouyancy - it will clean up the transom flow but add very little immersed volume. To get more bouyancy, you have to be pretty radical. Add foam from forward of the transom and rejig the waterlines in the aft third/quarter of the boat. Then you can get some serious volume back there. Lots of work, but maybe you could just do it below the waterline and reduce fairing in a major way.

(I have just returned from getting my little 7.2 metre cat craned upside down so that I can do this - foaming up the aft quarter because I drew her with an anaemic bum- cranes are costly!)

I would get about 8 friends - weigh them and get the tanks empty. Then ask your friends to walk about with your dinghy off the back and nothing in the aft compartments. Maybe she float well when your friends are forward and you will need to get watertanks and fuel forward (but never in the front third) and you can buy a much lighter dinghy.

I tried to do some clever stuff with volumes so that the stern would not overpower the bows. But that was dumb. The hulls should be balanced in volume fore and aft - and we get greater volume forward by increasing height.

I don't really understand why a carbon cat would have this issue. I would expect everyone to be very switched on about weight and keeping it central. I don't understand why any tanks would be aft in a performance cat - unless they are small.

Maybe you can just use the aft tanks as small day tanks and have big tanks forward of the mast beam that pump to the day tanks as required. Get the water up front too. Surgery is hard and expensive, foaming up under the waterline is not too massive, but going above the waterline and increasing volume above waterline too is a HUGE job. Moving weight and getting a light dinghy is cheap in comparison.

cheers

Phil

[tamicatana]

Sorry to hear your boat isn’t ok.

My 1992 Catana’s fuel tanks are well forward at effectively the mast x-beam in her deck lockers. That’s how the boat was built some 30 years ago. No issues.

[GordMay]

Greetings and welcome aboard the CF, Robot.

[Montanan]

Originally Posted by boatpoker

ABYC H-33 requires diesel to be returned to the same tank from which it was drawn. A shut-off valve would also be required if a gravity leak could occur.

And an anti-siphon valve needs to be installed and those do trend to become problematic as to getting stuck and constraining flow.

Also the return line has to be set up so as to avoid drain back that will induce air bubbles else there will be fuel supply problems. Many engine manuals provide for explicit instructions as to how the lines have to set relative to the tanks to mitigate this issue. Bubbles in Champagne are fine, in fuel not so fine.

Fighting gravity adds complexity and distinct failure points.

Be sure to recognize that this could be inherently problematic and when the engine begins to have failure trace to each of the thence known points of prospective failures so as to be able to fix the issue. Engine problems do have that nagging issue of arising just when one doesn't need such, such as passing a harsh bar, dealing with current, exposed to a lee shore, etc.

[noelex 77]

An anti-siphon valve is not needed.

A shut off valve should be fitted to every diesel tank irrespective of its location, and as pointed out, the return should return to the tank that is used, which except in an emergency would be the day tank.

There is nothing strange about a diesel day tank, virtually most large trawlers have one installed. Of course these vessels depend absolutely on their diesel supply for all propulsion. Yachts have some redundancy so arguably they are overkill, but I am surprised they are not fitted to more yachts. The increasing use of biofuels and common rail engines makes fuel cleanliness more important than ever.

Originally Posted by Montanan

Fighting gravity adds complexity and distinct failure points.

Many yacht engines are fighting gravity to suck up fuel. A day tank removes this.