fuel manifolding on OA 423 Clkassico

concerned about manifolding on OA 423 classic. Have 3 tanks, one is belly other two are saddle. The manifold only allows return to saddle tanks. the saddle tanks are higher than belly, so gravity will fill (maybe overfill) the belly tank if full. Do you have to use saddle tanks dow to a certain level before using belly tank. i've tried using only belly tank with manifold supply but that put 70 gal/hr into the saddle tanks. If full with 600 gal can all mainfolds be open? Thanks for your help!

I would use the belly tank as a day tank and move the return lines to there.
What size is the belly tank?
That is if it was my boat.

The tank is 300 gal. Cant figure why no returns to that tank and why fuel level of saddle tanks appears to be a lot high than the belly tank level.

300 Gal is a large day tank, but that would solve your issue.

No, using the belly tank as a day tank won’t work because he said there is no return to the belly tank. He said the only returns are to the saddle tanks. If he uses the belly tank as a day tank the saddle tanks will soon overflow unless they are empty to begin with.

NO!

Read the earlier reply where I told him to move all returns to the day/belly tank.

I missed that post. If you do that it would work. But wouldn’t it be easier to just add a return to the belly tank and return fuel to where it came from?

If you did a day tank it would be simpler and you could run a polisher on that tank.

The one area that OA’s appear to be inconsistent is fuel manifolds. I would like to know more about your setup.

here's the set up. I have 3 fuel tanks. 2 saddle tanks that are located in eng rm. They sit on top of the stringers and have a height of 32" and a volume of 150 gal each. The belly tank, also 150 gal, is located under the master stateroom sole, which I estimate the top of the tank is 20" below the top of the saddle tanks.The engines, 330 Cummins return approx 30 gal/hr to only the saddle tanks.When not under way I can open the manifolds to drain into the belly tank. In 30 minutes I drained 150 gal from both tanks. I have thought of opening all manifolds, but when full the level of saddles would cause the the belly to overflow. So I have to get that the level down in the saddles so it won't overflow the belly. Then I can open all valves and run the return fuel back into the belly through gravity.

I understand that putting a return manifold system would be ideal, however, putting return lines into the belly would be major. And somehow owners have managed. When we took over the boat the PO did not use the belly and only partially filled saddle so that the weight balance would not be noticeable.

Mistake in my previous post, the belly tank holds 300 gal.

Well that is a really poor design. In the long run I would bite the bullet and install a return on the belly tank. In the short term I would not use the belly tank and just use the saddle tanks. Otherwise you will be constantly switching the fuel feeds.

If the belly tank is 300 gallons then I would certainly install a return line to it. 300 gallons is too much fuel to pass up using.

I think you're right. Other than drilling a return the belly tank, the rest will be valving. Don't see the need to manifold returns as it will either go to saddles or belly. Thanks for all your insight.

I would also add valves to enable the return to go to the tank you are drawing from. Otherwise it isn’t if you will accidentally overfill a tank but rather when you will overfill a tank. It is more work but the fines for spilling diesel can be staggering.

Bruce,


What do you have for deck fills? 1, 2, or 3?

I am forming the opinion that the belly tank was either installed third party or it was meant as a limited use delivery tank. It’s not OA’s way to do something so poorly. The easy solution would be to pump the belly tank dry and cap it. If keeping the belly tank the the best solution would be do things right and add a return line.

One other possibility, since type of engine is not mentioned, is it possible fuel circulation is unnecessary?

Dont believe that tank was not designed OA because the owners manual contains the drawing from them. However, the narrative in the same manual does not descibe the system correctly.I have checked with an owner whose is a 1995 year and he has a return to the belly tank. Mine is a 2001 and only returns to the forward saddle tanks.if the fuel level of the forward tanks weren't 20" higher than the belly tank I'd be OK. But that means at full fuel levels the belly tank would be full and the fuel level half way up the fuel fill pipe if all fuel manifolds open.

Just to put this thread to bed, I was able to contact a OA dealer in Seattle who told me the set up is used even on the larger OA's and OA has no problem with the fuel backing up the fuel fill on the belly tanks. He said the setup is supposed to have all the manifold valves open unless a particular reason occurs to isolate a tank. I have tank tenders on all the tanks so it is fairly easy to keep track of.

My thanks to all who responded and put me on the road to the explanation.

I have the same setup on my OA 423. I learned the hard way that there is no return line to the aft tank. I close the saddle tanks feed and opened the aft tank to use that fuel. My mistake was not realizing how fast the fuel is pumped thru the engines. I lost one engine when it ran out of fuel on aft tank only. Not sure why one engine keep running long enough to get to a marina.
I now use the saddle tanks as day tanks and move fuel out of the aft tank watching the fuel level in the saddles so as not to overflow them.
Another issue is that the only way to to check fuel level in the aft tank is thur a port in the aft closet floor and a dip stick. The tank tender is pretty iffy. Also need to open this port when filling to check level and vent air from tank during high flow fills.

Welcome aboard, Bob. OA did some pretty crappy engineering on that fuel system. It shouldn’t be that hard to transfer fuel tanks and fills.