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Convert water tank to diesel Morgan 43 CC

Jan 30, 2011
MacGregor 26S Vernon BC
We recently purchased 1986 M 43 CC, and now find less diesel tankage than advertised. Does anyone know if the 75 gallon fiberglass water tank under the sole at the base of the companionway can be safely converted to diesel? It has already been contaminated with diesel by previous owner, and we expected 135 gallons of diesel tankage rather than only the 60 gallons under the port settee. I understand that if the fliberglass resin used is compatible, then it would accept diesel. If not there may be a risk of diesel dissolving the tank and eventually leaking. The boat was advertised as 125 gallons water and 135 for diesel, totaling 260 gallons. However sailboatdata.com lists 200 gallons water and only 60 for diesel, which adds up to the same amount and suggests the 75 gallon tank under the sole was intended for water. How can we determine if this fiberglass is compatible with diesel, or has anyone else done this in similar or same Morgan? I appreciate any advice.
Sep 8, 2014
Catalina 22 Swing Keel San Diego
That is a slippery slope.... Personally (and I'm not currently a big boat owner) I would prefer to have the water tankage and find another alternative to increase diesel tankage. The contamination in the water tank is fixable (how did this happen?).
If you are intent on changing the water tank to diesel my recommendation would be to install a diesel fuel bladder within the existing water tank. This would be the cheapest option and probably the safest. Keep in mind this would require significant plumbing changes, and access to the top of the tank may be required to get the folded or rolled bladder into the water tank. Probably just a larger access port cut into the top of the tank. This method has many advantages, like being reversible for one but also when the tank requires draining and cleaning it can be done by draining and removing the bladder, the cleaning can be done safely and away from the marina.
But since I am recommending a bladder it may be better to install another 60 gallon bladder under the starboard settee to balance out your existing set up. Then, you can focus on 'un-fouling' your existing water tank, which is possible with the correct cleaners in the tank, and safest to replace the plumbing all together.
Of course, going with a diesel bladder in the existing tank, and then replacing you potable water tankage with another bladder in a new location is also an option to consider.
I hope this helps!
Jan 30, 2011
MacGregor 26S Vernon BC
Thank you CloudDiver for your thoughts. We have a watermaker onboard, capable of making 15 gallons an hour when needed, so for that reason I don't need the extra water tankage. However, as we plan to make long passages, additional diesel tankage is essential, which is why I would like to do the conversion. I am fully prepared to change all the attachments to set it up for diesel, but don't want to overlook incompatibility and eventual breakdown of the tank.

You suggestion of a bladder is a great alternative. Unfortunately because of the internal baffles, we could only use about half of the current 75 gallons, but that may still be worthwhile if the fiberglass is not compatible.

I am hoping that another Morgan owner has faced this question before and knows whether the current fiberglass is diesel compatible.

Thanks again.
Jul 14, 2019
Morgan 43 CC Santa Cruz
Where did you install the water maker? I just bought an '85 Morgan 43 and am interested in installing one myself.
Oct 8, 2021
Morgan 43 Anna Maria
Our M43 had the aft lazarette converted into a diesel tank. At some point, they added a shelf to it about half way up, around where the bottom of the boarding ladder mounts are. It was properly plumbed and all. Perhaps a consideration instead of the midship tank.