Home-  Sailboat-  Submarines-  ROVs-  Metal Working-  Other Stuff -  About Us

Why Build a Sub
   Submarine 101
   Wet Sub
   Planing Wet Sub
   Dry Ambient Sub
   Submarine Yacht
Design Evolution
Cutting Aluminum
Building the Hull
Kort Nozzles
Battery & Chargers
Engine & Jet Drive
   Engine Tune-Up
   Hull Cooler
   Wet Manifold
   Engine Mounts
   Engine Box
   Jet Pump Valve
Helm Controls
Ballast Sled
Landing Gear
Trim Tanks
Wiring Harnes
Tow Truck
Rules of Thumb



Engine Box

Now that the engine has it's permanent mounts it needs a compartment that will keep it dry. The oil cooler and engine coolant heat exchangers are built into the hull and the connections to the engine are inside the engine compartment, but several other lines must enter the compartment. Fuel, electrical, raw water from the jet pump to inject at the exhaust check valves, both of the exhaust lines and most perplexing is the air intake.

Planning the Engine Compartment

When the sub was first designed, I intended to draw air from the snorkel just behind the cabin, but that is 14 feet forward of the engine compartment and too much air duct to easily accommodate. The new plan is to build a tail on the the transom and draw fresh air down the tail and into the engine compartment. The intake duct needs to be about 5 inches in diameter to accommodate the diesel's high demand for air and that is a lot of pipe and a lot of water to clear when it's time to start the engine.

The engine compartment is 29.5 inches wide, 36 inches long and 36 inches high. That is 38,232 cubic inches of air or 22.125 cubic feet. A cubic foot of sea water weighs an impressive 64 pounds, so the total displacement of the compartment is a whopping 1,416 pounds. Incredibly that is more than the weight of the jet pump, gear box and engine combined. Together these parts weigh about 1,200 pounds, leaving me 200 pounds short of sinking. I use to think getting a metal boat to sink would be easy, but building a sub has changed my mind. In the final analysis the 200 pounds displacement is just about right. There is at least that much in the hull, frames, and tail snorkel that will be flooded when submerged. In addition I actually need a little up force in the rear of the sub, because the ballast sled can not be positioned directly under the cabin and it's 26.8 cubic feet of displacement. The lift from the cabin, the lead and batteries in the ballast sled and the smaller lift from the engine compartment create a sort of inverted set of balance scales where the ballast sled is the fulcrum. Think of it like a board with a big balloon lifting one end of the board and a small balloon lifting the other end. A weight is then placed on the board near the big balloon so that the board remains level. It is this balancing act that will be the trick to maintain.

The bottom 12 inches of the engine compartment can flood without any harm.  The oil pan is about 4 1/2 inches off the hull, but there is no harm in letting the oil pan get wet. The starter motor and main bearing at 12 inches are the biggest points of concern and even a little splashing will not harm these.

Several inches of water will routinely  be draw in when surfacing and the snorkel is cleared.  About 5 inches of water in the engine compartment will amount to 200 pounds.

(1) Look down at the beginnings
of an engine compartment.

(1) For the past two nights I have not done much more than stare at the beginnings of the engine compartment trying to decide how it can best be sealed and more perplexing, how to connect the air duct from the snorkel.

One idea is to use a 6 inch check valve, actually a backflow valve normally installed in sewer lines to prevent sewage from backing up into your house during floods. This would allow the duct to be connected at the top of the engine compartment. Once the engine is shutdown the flow of ambient air into the engine compartment will cause back pressure to build in the compartment forcing the check valve to close. A flood valve would be installed at the bottom of the snorkel so that the snorkel gradually flooded when submerging. The danger is that the check valve may fail to properly close and an open valve at the top of the compartment would cause the entire engine compartment to flood.

The other option is to run the intake duct to the bottom of the boat then turn and enter the engine compartment and then turn again and travel up 10 inches or so, inside the compartment. The air duct would act like the "p" trap under a sink and isolate the air in the engine compartment. Once the engine is shutdown and ambient air is flowing into the engine compartment, a flood valve will be opened at the bottom of the duct. The water would rise in the duct inside of the engine compartment until it found it's equilibrium with the air pressure. The duct outside the engine compartment would flood completely. This option has more duct but no check valve up high. The valve to flood the snorkel is smaller, less expensive and down low where it will not compromise the entire compartment should it fail. It will be prudent to install a small bilge pump at the bottom of the duct in order to help clear the duct before the engine is started. If the bilge pump fails the engine will simply draw the water remaining into the engine compartment where it will be expelled by the bilge pumps inside the engine compartment. Having a bilge pump in the duct will also assure that the air way is completely open, allowing an unrestricted flow of air. The valve used to flood the snorkel could also help drain the snorkel when surfacing and once the hull was free of water it could be opened again to allow any water trapped in the bottom of the snorkel to drain out into the hull where it would be picked up by the hull bilge pump.

I have switched back and forth between the above two options more times than I care to think about, but I am definitely going with option two. But I found a great price on a check valve. No! Option two!

(1) Air intake

(2) Raw water connections into
the engine compartment.

(3) Close up of the snorkel water
level meter.

(4) Bottom section of the engine
compartment with bilge pumps,
exhaust pipes, direct acting
solenoid flood valve and water
depth meter in place.

Here is a better picture of the
direct acting valve take on final

(5) Drive shaft cover.

(6) Assembled drive shaft cover.

(7) Assembled drive shaft cover

(8) Cross brace was added to
engine bed to help maintain the
drive shaft alignment.

(9) Engine compartment top.

(10) Casting load plates.


(12) Thru hull fittings.

Building the Engine Compartment

(1) Building a P-trap snorkel is progressing along, slowly as usual. It is a tight squeeze to fit a 5 inch ID air duct to the bottom of the compartment and work it around the drive shaft and between the gear box and fly wheel. The arrangement provides for a 5 1/2 inch by 2 1/2 inch opening when the duct passes into the compartment providing 13 3/4 square inches of opening, which is about same as the intake on the engine. The duct is never smaller than this so as to not overly restrict air flow. Once inside the engine compartment the duct widens out to 13 inches and slims down to 1 5/8 inches as it rises up 10 inches and ends just below the drive shaft. This part of the duct will have to be welded in after the main section is welded so that access to the welds is not restricted.

(2) Barbed hose connectors for 3/8 inch ID hose were turned on the lathe and welded into the panel. Additional ports will be added for bilge pump hoses and ambient air exhaust.

The ambient air exhaust from the engine compartment will be 3 inches just above the top of the snorkel's P-trap opening into the engine compartment.  This will allow air in the engine compartment to vent to the variable ballast tanks before the pushes back through the snorkel and up the tail.

When diving, especially in the first 33 feet, the air in the engine compartment will compress by 50 percent. Ambient air will constantly flow from the cabin at about 3 cubic feet per minute but the high compression of air volume in the first 33 feet of the dive will cause the water level to rise inside the snorkel as the ambient air pressure inside the engine compartment slowly builds back. Any water that floods into the compartment will eventually be expelled by the bilge pumps after the ambient pressure is reestablished.

(3) A water level indicator was built to to monitor the water level in the snorkel. The depth gauge is made from 10 reed switches. One reed switch is positioned every 1 1/4 inches inside of a 5/16 inch brass tube. A ring magnet on a float can slide on the brass tube, and as it does it closes the reed switches near the magnet. At least two switches are closed and any one time, and these will turn on the LED lights on a 10 step LED bar graph that will be mounted in the cabin. The brass tube and float are housed inside a 1 inch aluminum pipe. The top of the pipe has fitting for and small air hose that will connect it back to the engine compartment. This will allow the air in the top of the gauge to equalize with the engine compartment as the water rises in the pipe. The bilge pump connected to the air intake will be used to help clear the intake when surfacing. The air intake must be flooded when starting to submerge so that ambient air is allowed to build in the compartment and so that the large amount of displacement is not lost when water finally floods over the top of the air intake snorkel. A 1 inch direct acting solenoid valve is connected to the air intake path just behind the air intakes bilge pump. It will be opened when submerging so that water can gradually flood into the snorkel. A strainer connects to the rear of the flood valve to help prevent objects for entering that my cause the valve to fail. The flood valve like the bilge pumps is also mounted to the hull by that allow for it the easily be removed.

(4) I am taking a few weeks off to for study, moonlighting and a little vacation, but I was able to get in a days work and finish the lower part of the engine compartment. I lowered the engine into place to position the exhaust pipes. They are staggered because the port side exhaust ports on the 7.3L diesel are staggered. Two bilge pumps are mounted inside the compartment. These will exhaust thought the pipes welded into the rear of the compartment and they will be connected to 1/2 inch check valves before dumping into the hull. The wide opening is the air intakes opening into the compartment. It enters 10 inches below and the widens out so that it will clear the engines fly-wheel. The 10 inch rise will allow 10 inches of water in the air intake path before it can flood over the top and into the compartment. It is the job of the ambient air flowing into the compartment to keep the water level below the 10 inch mark. Connected to the bottom of the air intake path is a small box that houses a water depth gauge and another bilge pump.

Five weeks have gone by now since my son Carl was seriously injured in Iraq (full story) but he is home now and doing well, and I am back in the shop.  In Carl's words: "Drink water. Drive on."

(5) (6) (7) So after clearing away lots of dust and cob webs I started building a drive shaft cover that will enclose the drive shaft between the gear box and the engine compartment. A 5 1/2 inch diameter pipe with a flange will be bolted to the gear box. The outside of the pipe was placed in the lathe and polished with sand paper so that o-rings would seal against it. I then cast a ring to hold o-rings using lost foam in Petro-bond sand. A short 6 inch diameter piece of pipe will be welded to the outside of the ring and then welded to another flange that will bolt to the engine compartment. This design allows for the flange to be unbolted from the engine compartment to exposed the universal joints on the drive shaft for inspection, lubrication and disconnection. It also corrects for the change in alignment between the gear box and the engine compartment since the rear of the engine compartment is vertical. The 6 inch diameter pipe around the universal joint also allows for the engine to shift on it's flexible mounts and still clear the enclosure.

(8) I added a cross brace between the forward engine mounts in order to reduce the amount of lateral shift. The clearance between the drive shaft and the drive shaft housing is tight, so I don't want much give in the engine bed.

(6) The outer flange of the drive shaft housing bolts directly onto the back on the engine compartment and the 1/4 inch bolts pass directly through the flange and into the engine compartment. This is not the best way to install the bolts, because one end to the bolt is on the wet side and the other end is on the dry side. This is totally unacceptable for a 1ATM design but on an ambient the seals will only be required hold against minimum pressure and minor amounts of leakage is acceptable because the bilge pumps are already in place to remove water that is drawn in through the air intake when the engine is started and the snorkel is purged.

(9) The top half of the engine compartment is straight forward. A frame that will bolt to the lower section of the compartment was cut from 1 1/2 inch angle then clamped in place, welded together and then drilled for the 20, 1/4 inch bolts.

Because this is the only way into the engine compartment, I wanted to use as few bolts as possible so the spacing between the bolts is about 6 inches, but the compression load from each bolt will be spread by using aluminum load plates on the top and bottom at each bolt.

(10) Randi helped me cut out the foam and cast the first batch of load plates. A hot wire foam cutter was used to make the foam parts from common insulation foam.  Those are then glued with hot glue to a channel and spur also make from foam.  That is then set in sand and molten aluminum is pour in.  The aluminum burns away the foam and fills the void to form the part.  The photo shows two sets that have been cast, the sand still needs to be knocked out of one set.  After they are cast the individual parts are cut from the spur and channel, cleaned up and drilled for the bolt that will pull the two halves of the engine compartment together.

A year and a half has passed since I wrote the the preceding paragraph or worked on the engine compartment. Just 3 short paragraphs back I had returned home with Carl from weeks at Walter Reed where he recovered from his injuries in Iraq. Since then he help me gut an abandoned house we purchased, he and Randi were married, moved to Georgia, Carl completed his Fireman training, Randi worked on her Associates. And now I am now going to be a grand dad. They are going to move back Tulsa so we can help out, especially since Carl will likely deploy to Iraq or Afghanistan in the next year or two. Kay has learned to deal with chronic pain and sets an example of true strength for me every day.

She and I recycled that abandoned house into the perfect home and boat yard, complete with hot tub. We and I have learned to sail and have we have a bare boat certificate. Kay continues to learn more in the shop including learning to weld. And we have amassed 4000 pounds of lead which will ballast the sail boat we are planning to build once the submarine is completed. Sometimes a lot of life can happen between the paragraphs. I could not be prouder of Carl, Randi and Kay or more excited about the coming years.

(11) Knowing that the welds in the engine compartment likely contained a few pin holes I simply applied a coat of epoxy paint (black) to the seams and then sprayed it with silver engine paint.

(12) Thru hull fittings were also added. In this case they go thru the the lower, forward section of the engine compartment. There are two copper pipes potted in epoxy that were formed up with pieces of PCV pipe.  These are for the fuel supply and return lines to the fuel tank. An inlet for the ambient air line was turned from aluminum the lathe and welded in. And then three openings for electrical lines; one for the controls and sensor wires that is a PVC union so that he wiring harness can be completely removed if need; and two 3 inch pieces of aluminum pipe welded in place, one each for the 4/0 welding cable that will connect the starter batteries to the engine. The cables will be fixed in place with epoxy once installed.




(1) Flywheel with U-Joint shaft
resting in place.

(2) Double U-Joint shafts bolted

Drive Shaft Installation

# Jet pump with the gear box were installed first and then the engine is installed with only bolts holding the flywheel in place.  The drive shaft can not yet be connected to the flywheel. 

# Double U-joint drive shaft needs to be unbolted between the two U-joins and the aft half is slips over the splined shaft on the gear box. 

# Inner drive shaft cover is bolted to the gear box and the outer cover with it's o-ring in place is slid over the inner cover and pulled all the way aft.

# Lower aft section of the engine box is inserted between the flywheel and the drive shaft cover. Don't bolt it in, but pull the port side aft to make room for the flywheel driveshaft half.

# (1) Install the drive shaft half onto the flywheel. Don't tighten any of the bolts or nuts on the two studs until all of the bolts are in place.  A little RTV sealant is needed around the bolts to prevent oil leaks because the passages open into the crankcase.  The engine will need to be turned over by jogging the starter in order to make the bolt holes accessible.

# Place the gasket for the outer drive shaft cover over the aft drive shaft half.

# (2) Slide the aft dive shaft forward and install the 4 bolts that join the two U-joints together.  Don't jog the engine until two bolts are in place.  Don't tighten the bolts until all 4 are in place.  Use a large screw driver turn the flywheel by prying on the teeth in order to line up the holes.  Prying the plates apart slightly will also help the bolts fix in place.

# Move the lower engine compartment plate forward and then slide the outer drive shaft cover forward with the gasket in place, and install the bolts that joint the outer drive shaft cover to the lower engine compartment plate.