Fuel, water, black water tank construction

[kudu]

Looking for feedback concerning various materials used in fuel, water and black water tank construction. What are the most durable, safest (from a health standpoint), yet cost effective materials to use? Thanks

[gonzo]

It depends on the size. For smaller tanks, plastic is the best and cheapest.

[kudu]

Hi Gonzo...The fuel supply will be divided into 3 or 4 tanks, total amount equals 528 gal. Water will have 3 possibly 4 tanks, total 600 gal. Black water maxium of 200 gal, hopefully one tank, two if needed.

[gonzo]

For those sizes the tanks need to be metal. Marine grade aluminum is the best for fuel and water. For black water, because it is corrosive, it needs to be coated or made with stainless. Fuel and black water tanks have to be certified.

[kudu]

Darn, I would have thought stainless steel better was better for drinking water. Also, would the Coast Guard certify a holding tank constructed by an individual? I was planning on building it out of heavy gauge polyethylene, welded construction. Since I will be using a composting head, the tank will just be dealing with shower, lavy and kitchen sink water.

[gonzo]

I'm not sure if you can legally build a holding tank. A water tank is OK.

[kudu]

I'm wondering if there should be some sort of cushion/ protection between these tanks and the fiberglass hull. Considering the amount of weight, will there be an abrasion possibility even though they are fastened securely?

[Willallison]

Professional Boatbuilder magazine had a very thorough article regarding tanks - their construction and restraints. It was a few months back, but you can easily order back issues via their website http://www.proboat.com/

[Portager]

Kudu;

I'm wondering why you plan to carry 600 gallons of water. That is roughly 5,000 lbs of water. You’ll need effective anti-slosh baffling. Most builders utilize a water maker when the water requirement exceeds 100-200 gallons.

Regards;
Mike Schooley

[Willallison]

Reckon I can answer that one Mike - $$$$$
Personally, in a displacement vessel, where weight is less of a concern than in a planing hull, I'd go the big tank route too.
The 36 CheoyLee that I've mentioned to you before carried 2 1/2 tons of water (and 3 1/2 of fuel). True, all that extra weight may have cost us 0.2 of a knot or a hundred extra rpm, but that's way cheaper than installing a watermaker - and way less ongoing maintenance too.

[Portager]

I think your looking at it bass ackwards Will.

Assume the empty weight of your boat is 75% of the fully loaded weight (this is true for Portager). So if I compare a boat with 600 gallons or 5,000 lbs of water to another boat with 100 gallons or 830 lbs plus a watermaker, lets assume a Stectra Ventura 150 (http://www.spectrawatermakers.com/ ) which produces 150 gallons of water per day and weighs 150 lbs, so the weight difference is 5000 – 830 – 150 = 4,020 lbs. Since for each pound of payload weight I need 3 pounds of inert weight to carry it, the total empty weight difference will be 12,000 lbs. Now Michael Kasten uses a cost of $10/lb for the cost of a new boat, so the boat with 600 gallons of water will cost $120,000 more than the boat with the watermaker. Since the cost of the Spectra Ventura 150 is less than $5,000, the net cost savings is $115,000. My finders fee is 10%.

Now I’ll admit that this is an extreme example, but I think that it illustrates that even though watermakers are expensive they will pay for themselves by allowing a smaller cheaper boat to provide the performance and capability of a much larger and more expensive boat.

So once again I ask, why do you want a 600 gallon water tank?

Regards;
Mike Schooley

[Willallison]

Now that's an interesting way of looking at it! But I can assure you that our CheoyLee didn't cost us an extra US 125K 'cause it has big tanks!! In your example you assume that the extra weight will require an extra 12,000lbs of material to support it - clearly this isn't true.
"Kapala 111" (the CheoyLee's name) has an unloaded mass of about 11 tons. Using your methods, installing smaller tanks would result in a dry weight of about 5 1/2 tons. The fill level of the various (fuel and water) tanks makes little difference to the draft - though their location does mean that differing loads can effect trim.

If ultimate economy / speed / trailerability / very low draft are your goals, then yes, I'd agree: the watermakers is the way to go. If not - gimme the simple and cheap bigger tanks....

[kudu]

Hi guys...If it's of any interest, I will be cruising worldwide and have opted for the extra water with a watermaker as backup. Prefer not to depend on a mechanical device that may or may not function as intended. With a displacement weight of 46,500 lbs. it should not be too much of an issue. In addition, I will be carring 560 gals. of fuel. Thanks for your input!