Additional Permanent fuel storage in tender garage

Large pleasure motoryachts have tender garages in the stern and there are companies that create custom fitted fuel tanks. How terrible of an idea would it be to completely convert a tender garage into permanent auxiliary fuel tank for the purpose of extended cruising range? This question presumes that adding fuel capacity to an already created boat is cost prohibitive, but input on that is welcome too. This is a purely theoretical question. Thank you for your expertise!

Current Assumptions:

• V-hull planing fiberglass yacht 55000 kg (Examples: Sunseeker Predator 80, Pershing 82, or Azimut Atlantis 77S)
• Tender 800 kg
• Additional fuel 1500 kg

Current Bad Idea Board:

• Bad Weight Distribution : You will ruin the trim by adding tons to the stern.

Alternative Idea Board:

• Install permanent internal tanks midships. How Feasible is this?

 

Welcome to the forum.

Calculate the weight of the anticipated additional fuel load, compared to the full up tender and see where you stand, though my suggestion would be, this isn't the best location for this mass to be located.

 

Thank you! I'm putting weight distribution onto the Bad Idea Board. Let's assume these things:

• Yacht 55000 kg
• Tender 800 kg
• Additional fuel 1500 kg

That is conservatively a 3% change in boat balance. I've very curious how detrimental a 3% change is to a fiberglass planing V hull powerboat.

Also, if anyone thinks of Coast Guard Regulations, structural/support issues, fuel-flow logistics, or anything else that makes this a terrible idea - all ideas welcome!

 

Without knowing your boat (make, model and year) it's tough to tell, but the PPI and moment to trim figures are easy enough to calculated with the basic dimensions of the boat. Without these, I can still say adding nearly two tons of fuel at the very aft end of your boat, will cause major trim issues.

 

Let's say you are doing this to extend the range of a typical "look at me" douchey harbor-queen express like the latest 2015 Sunseeker Predator 80, Pershing 82, or Azimut Atlantis 77S. The type of boat that is not very customizable and where increasing the originally manufactured fuel tanks would be probably impossible.

 

The Sunseeker Predator 80 carries 1,600 gallons of fuel in two tanks just aft of midship for a total of over 5 tons. Now you're looking to add few tons and place it as far aft as practical? You will extend its 300 mile range by about 90 miles, so is this worth losing the garage and trimming down by the stern? You'd be much better off with bladders or more tanks, mounted in and around the cool mechanical spaces

 

Thanks for the wisdom! How practical is it to mount more permanent internal tanks in a boat like this usually? I imagine you'd have to rip-open some expensive teak. At the very least, it would involve some fiberglass modification? I mean, there isn't just empty space waiting to be utilized in modern boats is there?

According to a review of the latest Predator 80, at 5.1 GPH an additional 400 gallons of fuel would give an extra 550 nautical miles. That's worth something to someone theoretically. You could even cross the Atlantic at points, imagine passage-making in a sport cruiser! Stupid idea?

Code:

Sunseeker Predator 80 - Final Boat Test Numbers:
RPM	KNOTS	GPH	RANGE	dB(A)
1000	7.1	5.1	1,986	70
1500	16.9	26.6	906	74
2000	26.1	57.5	647	76
2350	32.9	87.4	537	78
Speeds are two-way averages measured w/ Simrad GPS. 
GPH taken via MAN display.
Range based on 90% of advertised fuel capacity.

Thanks again for entertaining this stupid train of thought. I'm sure many uninformed people who see pretty yachts on the internet have always wanted to ask the experts these same questions.