Diesel Tank on Bridgedeck?

[GRIT]

Originally Posted by 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.


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

I agree 100%. Our last boat had both main tanks mounted higher than the engine. Never an issue, and worked a treat. Easy to bleed the air from the filter too!

Cheers.
Paul.

[donradcliffe]

The return line dumps into the top of the fuel tank. No problems with siphoning or air bubbles whatsoever. The top of the tank would have to be more than 5 meters above the injectors for any problems with excess pressure to be considered.

[Robot]

Thankyou for all your suggestions.
It sounds like it may be feasible to use the forward tanks if we return to them.
I am trying to resolve this and other defects with the builder.
Unfortunately with the lockdown in NZ Was unable to travel to SA for the sea trials.
The boat is light at about 6 tons, and is stern heavy unloaded.
shaft drives 29hp.
I have an excellent Cat builder in NZ doing other repairs

I Can post post some photo's etc but will hold off while awaiting some type of resolution.
thanks again

[mark_morwood]

Originally Posted by 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.

ditto

[Bulawayo]

Hi Robot, unfortunately, in the last few years I have noticed the number of 'experts' on this forum that I do not believe have the experience that they allude to.
There is absolutely no issue with having a fuel tank bridge mounted. I installed my fuel tank on the bridge deck of Island Spirit 35ft cat which was the first cat I built (bought as HDB). We blue water cruised that cat for over 30,000NM. Think how many monohulls do not have their fuel tanks in the keel. Look at cats that are produced having their fuel tanks bridge deck mounted (eg Voyage). Personally, I do not like having the weight of fuel tanks mounted aft. Good luck sorting out your concerns.

[Robot]

Thanks boatpoker.
You are right, I had a online tour of the voyage cats, they do have the diesel tanks in a bridgedeck locker.
My bridgedeck is a little higher but this looks feasible, my engines are Nanni 29 hp.
Cheers

[Talbot]

My Privilege 37 had two 80 litre tanks aft from new. Previous owner added two additional tanks right fwd as he tended to motor a fair bit. These have a cut off valve on the tank, and are then led aft to stbd engine room. They each go through a racor filter to a set of valves selecting which tank feeds to the pump, then to another set of valves which select which aft tank is being filled. I tend only to use these fwd tanks in winter when I can increase my tank capacity for my diesel heater.

[Bill Gibbs]

I have a 46’ cat carbon light cruiser. My fuel tank is under the mast at the front of my main cabin, and well above the engines. I have shutoff valves at the tank, as well as in the engine compartments on the input and return lines. All works fine.
“Sitting” a bit down at the sterns can be just fine. Of more concern is your attitude under sail, as dragging transoms is not fast. The driving force of your sails naturally depresses the bows and raises the sterns.

[navalejo]

No issues with having the tank forward, the engines will work fine.
A real fix however will be to extend the transom a bit aft for extra buoyancy, everything else remains the same

[pbmaise]

If you will be cruising long term with fuel, then having extra fuel tanks can be a good thing.

Someone already answered your question

"The concern I have is the diesel return would be “uphill”..

Again it is not a concern. Your engines will pump uphill to a tank. Or from a low tank to a tank that is already almost full.

One trick I learned to do cruising in areas with pretty bad fuel is to always have fresh diesel delivered into my starboard tank. I then had a fuel polishing system that extracted fuel from the starboard tank, passed it through a large 2 micron filter, and then to the port tank. When the starboard tank was low, I would set it on recycle and filter the residue in the starboard tank several times. In this manner my engine and the Racor filters never saw fuel that wasn't already filtered one time.

One thing I could also do is supply the engine from one tank and send recycle back from the engine to a different tank.

You have to pay attention and be very careful with not distractions when doing this type of fuel transfer. If the high speed pump ran dry on the fuel polishing system, then I needed to replace the impeller. I learned to have spare impellers for that fuel pump since they came all the way from a supplier in Florida.

Also the transfer of fuel from one tank to another via the recycle when running the engine is a very slow process. You need to be very aware this can overflow your tank.

Overall, after I had made the changes, and always washed and dried jerry cans with soapy water, I never had a fuel quality issue.

Regarding being stern heavy, one factor is how much fuel and water you carry. During normal short day sailing, you don't need to have a lot of fuel and water aboard. I loaded my tanks full only when I was making a long passage. Despite the size of my vessel, this represented about 5 inches difference as observed from the stern.

Therefore, I guess my question back to you is how loaded up is the vessel before judging it is stern heavy?

[markiobe]

Nice boat @Bill_Gibbs! Out of interest what does she weigh lightships?

[sailingharry]

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.

It's accepted and at least somewhat common practice (my Sabre had this) ABYC addresses it. For a tank where any of the fuel line is below the top of the tank (ie, if cut and the tank full, it could siphon), the line must have a special anti-siphon valve. It is essentially a check valve with a heavier than ideal spring -- when pumped, it will open, but if the engine is off, the spring seals the valve. It is installed right at the outlet of the tank.

It is not normally required on the return line, as that line doesn't terminate in fuel -- it dumps into the top of the tank.

[Bill Gibbs]

Originally Posted by markiobe

Nice boat @Bill_Gibbs! Out of interest what does she weigh lightships?

In racing trim, she weighs 13,050 pounds without crew and perishables.

[catsketcher]

Bummer about her being stern down when light. That means someone didn't do something properly.

For the last couple of days I have been foaming up the aft 1/3 of my little 7m cat. It is easy to do upsidedown.

I have seen this done before by an owner who did not understand cats. He shoved heaps of weight into the end of the boat during the build, including a huge watertank in the aft beam, and then complained the boat did not sit right. Well that one was definitely his fault as there were two other sisterships, built by the same builder, who sat really nicely.

So the owner of the heavy boat, got another builder to help him construct some cedar strip "hull pans" in situ under the stern of the boat. It must have been a pain but in the end it was done and the boat sat flat and sailed well, but not as well as the sisterships.

Most of the time people just put an extension on and don't worry. If you look at a few Aussie production cats from the late 90s you can see bridgdeck bottoms that slope up going forward. A 37 footer that got stretched was easily visible with it chamfer panel under water aft and well clear forward. BUt most owners never knew there was an issue with the stretched boats as the extended stern was well installed.

If the volume loss is serious, then a good builder can make you a set of hull pans from CAD alterations and then install them. It would be easier than building them in situ, but still a bugger grinding and fairing it in. I have never been happy with my little cat sitting poorly and so am happy foaming her up. I would suggest the designer sends you plans for extra volume pans (you work out how much volume is needed by sliding sailboards or similar under the stern till she gets to where needed. Then you tell him how much volume is required - add on some more for the weight of the addition and get some plans.) Build the pans in a nice shed and get them put under the boat. Ensure that the pan joins under the waterline, insist on this, otherwise the fairing will be much harder.

I would not recommend foam up a large 46 footer. It would be an awful job. Much better to make a nice foam sandwich hull pan, that has a few ribs inside and that is made directly from DXF files of the extra volume calculations done by the designer. Then your boat would only be out of the water for a short while ot get the pre-made additions installed. The designer may need to make them quite large - it depends on the volume required. I am foaming up the aft third of the hulls because I tried something novel with my hull shape but going back under the hull is essential if you want to lift the sterns, and not just smooth the flow.

In reality, adding transom extensions was easy back in the 80s and 90s. This is because lots of cats had wide, stumpy sterns. Nowadays with thinner sterns with steps, adding extensions with any real volume is probably impossible.

It will be a bugger, but in reality, done offsite and well in CAD, it shouldn't be too hard.

cheers

Phil

[Robot]

Hi Phil
Thanks for that insight.
I did sail on a TS42 with a transom extension and one without, what a difference.
The one without squatted and the turbulence was quite loud.
The extension improved this a lot and was 1-2 knots quicker over 10 knots boat speed.
They “glued” the extension directly on.
I thought the additional buoyancy was due to the volume displaced regardless of whether the interior had foam in or not.
Cheers