View Full Version : 18~X18 / Two Fuel Tanks ?

03-09-2011, 06:16 PM
Does anyone here have,~~~ or have you ever seen, ~~~~a Donzi 18 or X-18 etc. with two fuel tanks as an original factory option?

Our old early 1970s X-18 with a Berkeley Jet/ Olds 455 had two front to back fuel tanks direct from the Miami Donzi factory.
It was the Philadelphia boat show boat bought right off the show floor .

Each tank had it's own deck fill & hull side vents .

The two fuel lines ran to a multi position valve just behind the rear seat.
You could run the boat off of either tank~~~or combine both tanks.

This seems to have been a popular set-up with the Jet-Drive X-18/ 18 .
There are at least two other known original Donzi X-18 Jets listed on threads on the forum here~~~
Each of those also have the twin fuel tank option !

My uncle used to call the jet-Donzi X-18 ~~~~~~~~~~

"The floating Gas Tank !

Were twin fuel tanks also an option on the I/O Donzi 18/ X-18 ?

I know there was also a single larger fuel tank option on the 16 & 18 .

03-09-2011, 07:15 PM
2 tanks are great for buoyancy! Check this out.

The float of the bathyscape, Trieste, is filled with gasoline because this is readily available, buoyant, and for all practical purposes, incompressible. The incompressibility of the gasoline means the tanks can be very lightly constructed as the pressure inside and outside of the tanks equalises and they are not required to withstand any pressure differential at all. By contrast the crew cabin must withstand a huge pressure differential and is massively built. Buoyancy can be trimmed easily by replacing gasoline with water, which is denser.

The Trieste was purchased by the United States Navy from Italy in 1957. It had two water ballast tanks and eleven buoyancy tanks holding 120,000 litres of gasoline.

In 1960, it reached a final depth of 35,798 feet in the Mariana trench. No other manned vessel has ever repeated this feat.

03-10-2011, 07:02 AM
The jets needed as much capacity as possible, they blow fuel through at a much higher rate than any other type propulsion.

03-10-2011, 07:47 AM
Where were the two tanks located?

03-10-2011, 07:52 AM
There've been some smaller Donzi fish boats w/two tanks. I like the idea.
You can do a lot w/trim in a really high HP boat. Also, a little extra range is always a good thing. I think a 300 mile range would be a really neat thing to have.

03-10-2011, 09:46 AM
My 22 now has two tanks, the stock 50+gal bow tank and a 40 gallon just ahead of the engine. With just the forward full I lose 2mph @ WOT and have an annoying porpoise, with the forward empty and the aft full I get my best speed and have no porpoising what soever.

03-10-2011, 12:14 PM
The tanks on the 18/X-18 jets were back to back.
The foreward tank was placed about 1 1/2 feet further foreward.
You then had the aft tank behind the stringer's & middle bulkhead just behind the foreward tank.
This cut-in a bit into the deck floor's storage bin area~~~

It's been so long since my uncle owned the X-18 jet in the early to md-1970s that I forget the exact tank sizes.

They both shared the very same fuel gauge with a two position switch to "flip" between both tank fuel level senders .

I have also seen this same twin tank set-up on several X-18 Jets~~~
There are also two with twin tanks posted on X-18 jet threads on this forum.

Since my uncle bought his X-18 jet new off the old Philadelphia Boat Show
floor I do not know if this was a jet option or standard X-18 jet drive equipment?

I have often wondered if it was also an option on the I/O 18 & X-18 also ?

Do the old Donzi price & option lists confirm any of this ?

The jet drives Donzi sure do use more fuel at the lower to mid range speeds .
I can tell you this~~~First hand !
As a teenager all my summer job money went to supplying fuel to the Donzi X-18 jet !

I used to burn-up about 1000 gallons per summer at approx $ 0.35 /gallon.
Wow~~~Those were the days !
Those cheap gas prices are really a thing of the past !

As the pump's wear band thrust ring collar wears from ingesting sand, and the impeller to collar gap clearance gets wider your pump's output thrust goes down and your fuel usage also goes up~~~rapidly !

Greg Guimond
03-11-2011, 07:17 AM
If you felt the need to use ballast as a tuning tool, you could always have the dual tank set-up serve as ballast as well as extra range for a longer excursion.