Rascal Runabout - Fuel tank Ventilation Question

[Speedboy]

Per the General specifications supplied with the Rascal Runabout plans, I installed a Tempo TP12L 12 gallon plastic fuel tank (cross-linked polyethylene). The tank is located behind the seat beneath the motor well. There is a deck fill directly above the tank and the tank is also vented to a fuel vent on the wall of the motor well. I am concerned about whether I need to install vents on the deck, or install a blower in the bilge area. The Coast Guard regulations seems to indicate that for a permanently installed fuel tank, there must be 15 sq. inches of ventilation area to atmosphere for every cubic foot of net volume in the compartment that contains the fuel tank. I do have area on either side of the seat that is in direct contact with the atmosphere, but I only have 135 sq. inches. With 17.25 cubic feet of net volume, the calculations suggest I need 259 sq. inches of access to atmosphere. I have seen a lot of pictures of Rascal Runabouts that have been built but have never seen any vents suggesting use of natural ventilation or a blower in the bilge area. Does anyone know if the Rascal design requires vents or a blower? I am going for a Coast Guard courtesy inspection in five days, so I would like to be prepared.
Thanks,

Speedboy

[Dale R. Hamilton]

Speed- You have an outboard. No need to worry about vent requirements if motor is outboard the hull. Whoa- just thought of the battery. Must insulate this from gas tank compartment. Should be easy with sealed battery box.

[Speedboy]

Dale, You are correct about the outboard engine not requiring ventilation, it is the permanently installed fuel tank that I am concerned about. Compartments that are open to atmosphere do not require ventilation for the fuel tank. However, my boat does not appear to meet the requirement for open to atmosphere. With 15 net cubic feet of volume in the compartment where the fuel tank is located, I only have 136 sq. inches of are in contact with atmosphere. I need 15 x 15 sq. in. or 225 sq. inches to be considered open to atmosphere. Here is the law I found.

FEDERAL LAW

183.605 - Definitions

As used in this subpart:

“Open to the atmosphere” means a compartment that has at least 15 square inches of open area directly exposed to the atmosphere for each cubic foot of net compartment volume.


A compartment is any space in a boat that has length, width and height. It may be completely enclosed, partially enclosed or have one of its surfaces completely open. An example would be a compartment under a bow deck or a motor well where there is not an enclosing bulkhead. A canvas cover is considered the same as an enclosing bulkhead when in place.

Open areas are openings that are not specified. Open areas may be at the bottom, side or top of a compartment as long as:

the open area equals 15 square inches for each cubic foot of net compartment volume (See Figure 2), and
the opening is directly exposed to the atmosphere.
NOTE:
A number of openings may be added together in order to obtain the required total.
Openings into an open cockpit are acceptable.

So it seems I need a supplementary form of ventilation in the compartment to remove fuel vapors.

Speedboy

[Dale R. Hamilton]

well you convinced me. Why not just add a small blower to be operated when ign key is on.

[Tom Lathrop]

My reading of the Coast Guard requirements in their "Guide For Backyard Boatbuilders" does not get that restrictive. For outboards with an enclosed fuel tank they say that in and out ventilation is required but I don't remember any area requirements. It is the same as all the sailboats that you see with outboard auxiliaries. The manufacturers all put one in vent and one out vent on the aft decks. All of the small auxilliary sailboats I've owned or seen seen are done that way. My powerboat has passed CG inspection withthat set up.

[Speedboy]

I guess I'll install a blower, but I just hate drilling a 2" hole in the deck! for the exit vent.

Speedboy

[mmd]

Speedboy, forgive me for asking what seems to be a dumb question, but my ignorance of the details of the Rascal's construction forces me to ask the obvious: Is the volume (17.25 cu. ft.) you have calculated in the enclosed space that contains the fuel tank the gross volume (the total of the enclosed space) or the net volume (the total of the enclosed space less the volume of the tank? The reason I ask is twofold - 1.) by eyeball, the space behind the seat seems quite small to have that much internal net volume and 2.) the Rascal seems too well designed to have such an obvious error overlooked.

[Speedboy]

MMD, the rough dimensions under the motor well are 18"H x 48" wide x 33" deep. That gives me 16.5 cubic feet. Subtract out the two cubic feet for the 11.5 gal. gas tank and you get around 14.5 net cubic feet. 14.5 x 15 = 220 sq. inches to atmosphere, I've got about 130 sq. inches on the sides of the seat. Short by about 90 sq. inches. You're correct, I have never seen any vents on Rascals built with an outboard, so I am wondering if I am misinterpreting the regulations. Thanks for the help. One option is to reduce the net volume by adding sealed water tanks or some other allowed means of reducing the net volume, not that I need a water tanks, but I could leave them empty for flotation purposes.

Speedboy

[mmd]

I didn't realize that the length was so great - my estimation of the other two dimensions was about right. I agree that the installation of a couple of sealed floatation tanks in the compartment to provide positive buoyancy for the heavy engine in case of a flooding would be a good idea, and would happily solve the ventilated area problem. Check with your local CG inspector to make sure that you can calculate the net volume of the compartment by deducting the air tank volumes before you go out & buy them, though.