DIY tunnel drive

[CDK]

To fully appreciate this project, you should look at http://forums.boatdesign.net/forumdisplay.php?f=34 "Why are all prop shafts water lubricated?".

For my 26 ft. Draco Twincab, now being equipped with Mercruiser gasoline engines and Berkeley jets, I designed an experimental tunnel drive system that will be powered by two small turbocharged diesels.
The boat was originally equipped with sterndrives, so the engine bay location and lengthare unsuitable for a conventional prop shaft construction. Also, the height is insufficient to use V-drives.

At the present I am preparing two GRP tunnels, 17" diameter, about 2 ft long that will be laminated to the transom once the jet drives are removed and the engine area is cleaned and degreased. The stainless steel construction is already finished.

The prop shafts are enclosed in stainless steel tubes with welded on flanges that will initially be bolted to the transom. Then the inner tubes will be fully laminated in the 2" transom wall to obtain a solid construction.
The outer (ball)bearing is a sealed, grease filled type, mounted behind two marine oil seals that are neoprene coated. The gap between the two seals is filled with a mixture of lithium grease and silicone oil. I added silicone oil because it has unique properties as a water repellant. The gap between the 2nd seal and the ball bearing receives oil through a small opening at the top of the bore where the bearing is pressed in. The bearing carrier is only held in place by an O-ring between the carrier and the cover, located in a V-shaped space. The ring was pressed outward against the tube when the 4 bolts were tightened. When loaded with a 500 lbs force it did not visibly move.
The short inner tube has two back to back mounted tapered roller bearings, thrust rings and a neoprene oil seal, held in place by a steel cover plate and 4 bolts. With the latter the bearing preload can be adjusted.
A small stud with a threaded hole will be connected to a transparent oil reservoir mounted well above the waterline. The whole tube is filled with SAE 15W40 engine oil, making water intrusion virtually impossible.

Here are some photo's and drawings of this part of the project.

[Frosty]

Are you using surface props?

[CDK]

The props are still a big question mark.
To test the drives for stability and steering at low speed I will start with 3-blade 14x17" normal props. I hope to be able to determine the speed at which the tunnels start to draw air. If that is near my 20 kn. target, I will try to improve the bottom section in front of the tunnels. Does it occur much sooner, I will have to use sp props, in which case I shall also feed the exhaust gases to the tunnels.

[c-cat]

Be sure to vent the tunnels via. tubing.Otherwise probably won't acheive planning speed.Thought about doing this to some merc.sportjets but reverse would be an issue. You also may think about halo rudders behind the props, traditional rudders tend to chatter in turbulence, and stop working all together at @45-50mph.

[TerryKing]

CDK, nice mechanical work there....

Can you help us visualize what the bottom profile and tunnels look like, from the side?? I'm not sure how this all goes together...

[CDK]

Progress is a bit slow due to the weather. The tunnels are attached to the (double) transom and the prop tubes are in place. Also the ugly bottom holes from the jets are closed with over 1/2" of GRP but work outside is hampered by rain, salt deposits after NE wind and low temp. As soon as the weather permits I will clean up the mess outside and make pictures.

-"You also may think about halo rudders behind the props, traditional rudders tend to chatter in turbulence, and stop working all together at @45-50mph."-
If I will be able to reach half that speed with my two small diesels I will be more than satisfied....

[PetterM]

Interesting project and very nice mechanical work.
How much power do you have?
What is expectation of speed?

[CDK]

Interesting project and very nice mechanical work.
How much power do you have?
What is expectation of speed?

Twin VW 1,9 ltr. turbodiesels, 75 HP in a VW t-4 transporter, perhaps a bit more after marinising because there is no EGR anymore and the injection pumps can easily be adjusted to deliver approx 100 HP for a short time.
Speed target is 20+ knots.

[Pericles]

CDK,

Have you see this?

http://www.dbdmarine.com/CD-300-Marine-outdrive.htm

Pericles

[Frosty]

Personally I am well aware of dbd. Passed from some place or another my enquiries were thought of as being not serious when I actually wanted to buy 2.

I was also told that they were'nt actually in construction.

Have things changed?

[CDK]

CDK,

Have you see this?

http://www.dbdmarine.com/CD-300-Marine-outdrive.htm

Pericles

No, I have not.
Looks good though, clearly inspired by Arneson and/or Q-SPD.
The ball shaped CVJ under water is an expensive construction that requires regular maintenance and also the submerged tiller arm seal is something to worry about.
Anyhow, my DIY tunnel drives are almost ready....

[CDK]

Here are some pictures of the work in progress.
The camera of course is very unforgiving and shows every irregularity, but I comfort myself with the thought that once ready, the whole structure will be submerged and after a few weeks will look like a giant broccoli anyhow.

The tunnels are made from 16 layers of glass and resin, a little over 1/2" thick. Inside they are smooth, the outside is covered with resin mixed with a liberal amount of talcum powder. The local hardware stores here offer only one type of resin and large bags of talcum powder to obtain any viscosity between syrup and butter. I watched local fishermen make new boats by covering the old ones with GRP and a powder filled top coat. The commercial boatbuilders flatten and fill the whole surface, the private ones only polish the dry area, the hasty ones just paint it and go fishing.

The boxlike transom I added 3 years ago to obtain enough engine room length for Berkeley jets and Mercruiser engines.
Although it lost its function now, I decided to leave it in place because the original transom had so many holes it would have needed a complete renewal. Now it provides addition strength to the tunnels.

There are 4 small rudders (3 shown) with a wing profile, made from solid GRP with a stainless steel insert to attach them to the steering shafts. Levers and a tie-bar still have to be made, and so is the contraption to connect it to the Morse steering cable.

The far too shiny red paint is a first layer of copperbased antifouling, applied minutes before the pictures were taken. This type of paint is banned in some countries and it does not completely avoid marine growth, but I also tried several modern anti-foulings that were even less effective. The broth we call the Adriatic sea contains life forms that attach even to teflon. I experimented with silicone oil but gave up because it slowly washes off and people kept staring and commenting at the oil film around the boat.

[CDK]

This concludes the DIY tunnel drive project. There is still some paintwork to be done to the dry part of the transom, but that is not essential.
The 4 rudders pivot on ss shafts with ball bearings behind neoprene seals, greased for life. Their angle is limited to 40 degrees, otherwise the propeller tips come too close. One of the middle rudders is connected to the long shaft with a steering arm that is pushed in and out by a bar that has ball joints at both ends. Because the geometry dictates approx. 3/4" sideways movement and 1/4" vertical for the bar, the transom hole had to be fairly large and elliptical. It is covered with a ss plate and an angled tube with a neoprene bellows. Not an easy part to make, but it's there.

Because the prop shaft ends inside the boat have no provisions like splines or keyways - I have no tools for such a job - and the Spicer 1350 universal joints need 1-1/4" 10 spline stubs, adapters had to be made.
It proved impossible to buy such splined shafts without a gearbox or large electric motor attached to the other end, so I called in the help of an old friend far away who has access to machines I can only dream of. He cut the SAE-B splines in piece of high tensile strenght shaft, I drilled holes that exactly fit the prop shaft ends. The half disks that are held together with 4 very strong Allan bolts compress the stubs around the prop shafts so they can take of the torque of the turbocharged diesel engines that are already waiting.

[CDK]

At least 2 months behind schedule, today we made the first test run with diy tunnel drives and diy turbocharged diesels. Not yet at full speed of course, there were enough troubles at less than half throttle already.

The good things are that the engines make no more noise or vibration when compared with the old Merc's. In fact if they didn't smoke like a woodfire after startup, nobody would suspect there were oil burners under the floor.
Another good thing is that the boat at speeds under 12 kn. tends to go straight ahead, so I don't have to do much steering to keep her on a heading, just compensate a bit for crosswind.
And an engineering experiment I had bad feelings about proved to be an excellent idea: The raw water, after passing through the heat exchanger and the gearbox, is injected IN the exhaust end of the turbocharger that now forms a swirl chamber. An enormous difference in temperature between the turbine and the end where the exhaust hose is clamped, but the drop forged steel housing can take it.

But there are bad things as well. The steering with 4 small rudders at the end of the tunnels is so ineffective that running on one engine is out of the question unless the intention is to drive around in circles.
With 2 engines at the same rpm small corrections are possible with the steering wheel, but for anything else increasing one engine's rpm is the only solution. As the purpose of dual engines is extra safety in case of an engine failure, the rudders must be able to keep the boat at least on course with only one engine running.
On the way back to the harbor the port engine suddenly became very noisy but that was caused by the coupler disk that touched an insufficiently tightened nut and can easily be corrected. The steering problem is much more serious.

[FAST FRED]

The whole tube is filled with SAE 15W40 engine oil, making water intrusion virtually impossible.

Seals leak and in time the Sheen Police may be after you.

There are commercial stern bearing lubricants that are not petroleum based , and are "green"at least in terms of being fined.

FF

[CDK]

The same would apply to all the outboard engines, sterndrives and jets on this planet. I'm sorry, but I have more urgent problems to take care of.
Btw, between the TWO outer seals there is a chamber filled with silicone oil: water repellent, non-toxic, not (quite) petroleum based.

[PetterM]

Very impressive project. :)
Your rudders look a bit small for the speeds you are doing.

[CDK]

Thank you for your admiration.
That the rudders are too small became manifest in the first turn, adequate steering is only possible with engine rpm.
I already ordered some ss plate and shaft material and will make new rudders with 40% more surface. These I can enlarge by welding: the current ones are GRP with an ss insert to attach them to the steering pins.

[CDK]

The new stainless steel rudders are in place. They really make a difference: running on one engine it is now possible to stay on course and even make a port turn with only the port engine engaged. With both engines at the same rpm, steering is sufficient for all situations except cramped spaces; with one engine in reverse the boat can be turned on the spot in any direction.
So far we've made only short tests covering 26 miles and there is still some work to be done on the engines and instrumentation, but it is already clear that the efficiency has vastly improved when compared to the stern drives the boat was originally equipped with. Not 10 or 20% better, but probably over 50% more mpg!

[PetterM]

any pics? :)

[CDK]

I'm sorry, no. I was so focussed on improving the performance that I didn't even pull the boat out but installed the rudders while submerged. A hell of a job because every small piece you drop is lost.
They are just stainless steel plates, corners rounded and a piece of shaft welded at 1/3 of the 12 inch length.