DIY tunnel drive

The experiment continues.....

 

Hi there CDK, was just wondering.....! Now add the final part to the tunnel exit also, before you dip her again.

Good luck!

 

Baeckmo, this is as far as I can go at this time.
I planned to do more, but just after I prepared the old derelict trailer for a dive in the Adriatic, a local fisher begged me to use it for his wooden boat. He had hit a rock at the bow and his boat was sinking. So I lost one whole week with high tide and had only one week left to do my conversion. Last Sunday was the last opportunity to lower the boat, so she is afloat again.
The tide here is mainly ruled by air pressure distribution. Two weeks ago the news mentioned flooding of the Piazza Marco Polo in Venice, exactly west of us. That gave us enough water over the poorly built trailer ramp to handle larger boats. By now the sea-level has dropped over a meter and will probably stay there for months.

I have prepared a stainless steel cover that fits between the tunnels and can be bolted on under water on a quiet day. But first I intend to lift the port engine and find the reason for the low oil pressure it showed during the test runs this summer.

Any idea how I can remove a large iron bollard from the pier? The Italians put it there around 1935 when the harbor was deep enough for large vessels. It serves no purpose anymore and blocks access for motorized vehicles.
With the bollard removed I could order a small mobile crane or Caterpillar to lift the engine instead of building my own contraption.

 

Several test runs today under ideal circumstances, with disappointing results. I am contemplating to sink the boat where the depth is over 100 m. but have to figure out how to come home and what explanation to give later to the authorities.

At 8.4 knots (GPS) and approx 2500 rpm the port prop looses its grip on the water. Rpm increases, speed drops. Let's call it cavitation...
Always at the same speed, always the port prop.

 

And tunnels still the same length as shown in note 181 above?

 

Yes Bodo, the tunnel length is still approx. 2 ft.
If adding the platforms would have improved performance it would encourage me to make new modifications, but now I have the feeling it leads nowhere.

There are still a few points I need to check before reaching a final conclusion, like measuring rpm and speed at full throttle for each individual engine. It may also be that we were carrying a lot of rainwater in various compartments. There were too many people on board yesterday.

 

I told you what was the prob pages and pages ago.

I can't find it myself.

Your 4 rudders are a waist of time too, and could be contributory but not primary, 2 rudders is ample.

 

Sorry to keep nagging Cornelis, but in my note #159 I rounded off by mentioning the importance of avoiding ventilation from aft of the tunnel!!!! I have seen multiple examples of this problem, all due to a misunderstanding of the flow and pressure field after the propeller.

For instance; a couple of years ago, the local power company bought a 12m workboat with a semirecessed prop, power ~400 hp, prop dia 26", built somewhere on the Swedish east coast. Tunnel proportions were fairly similar to yours, but all inboard. For some time they were rebuilding transmission regularly, three times a year, plus weld repairs to plating around tunnel/bottom joint and rudder tube, due to wild vibrations. Speed was far from what should be normal according to power/weight ratio.

Finally the crew asked for assistance; they had had enough. I added a short "outboard" extension, something like 400 mm, following the calculated propeller flow. Problem completely gone, running smoothly and speed increase from <12 kn to 18+ with load! Never a weld repair or tranny break-down thereafter!

If you, for some reason insist on keeping the truncated tunnel you just HAVE TO use propellers with blade profiles for supercavitation/ventilation. This kind of short tunnel is in fact used to extend the working range for supercavitating propellers towards lower speeds. With this ventilation, a normal prop simply cannot produce thrust, its as simple as that!

So, come on now, don't fall down before the finish line; just add a foot of tunnel top (3 mm al or whatever) fixed with self tapping screws, its topline sloping aft at about 4-5 degrees.

 

Of course I do not give up; there certainly was disappointment and I felt the need to express my feelings.
We've planned a camper trip through Europe next week and I will come back with some aluminium, bent to the proper radius if possible.

Like I suspected, two large compartments in the aft cabin were full of water plus quite a lot under the main cabin floor. Removing that visibly altered the waterline. It has again been raining here for the last few days, with more to come. so I may find out how it enters the boat now.

 

Bodo, following your advice I've added another foot of tunnel top, made from 3 mm stainless steel. Not fixed with self tapping screws but stainless M5x25.

Even if it doesn't work, the exercise was a memorable one.
Because it seemed a minor job I decided not to lift the boat but install it underwater using a pneumatic drill. Did part of the job disguised as a tourist (fkk) but was attacked by numerous shrimps, part dressed in jeans and a t-shirt until my hands were numb and finished it this morning in neoprene which I should have worn from the start of course.
Freaky weather conditions with a low over Europe and a high lingering over the southern Meds brings so much water in our bay I had to do everything swimming with zero visibility from the tool's air bubbles. An experience I will never forget, but the extensions are in place and looking good.

There is some really lousy weather coming our way and I need time to rescue my tools from the seawater, smoke cigars and drink plenty. Then I will do the decisive test run!

 

A proven method to come always to perfect test results Cornelis!

 

test results

The presence of the 1 ft. tunnel extensions severely changes to boat's behavior. This became clear immediately after leaving the berth where I need to make an on the spot turn by running the port engine in reverse, starboard in fwd.
This was a quick manoeuvre before, but with the extensions it took several times longer, even at higher throttle settings. Once underway, the rudder response seems somewhat better, especially at small rudder angles.

The wake pattern shows much less turbulence, more like the average conventional prop shaft construction.

Positive remarks end here.
At any throttle setting except idle, the speed is slower than before.
I installed a new log/depth sounder near the starboard corner of the platform. It has not been adjusted yet, so the numbers have no absolute meaning. I recorded 5.0 kn at 1500 rpm.

All attempts to plane failed because the bow only lifted a few degrees so the hull's wet area caused too much drag. Top speed on the log was 9 kn. @ 2400 rpm, no cavitation.

Because the tunnel extensions interfered with the stairs of the bathing platform, I went to a sandy beach, jumped in with some tools and removed them.
On the return trip, speed at 1500 rpm was 5.6 kn. Attempts to plane ended in cavitation near 11 kn.

 

Solution in sight?

When designing this propulsion system I read Glenn-L inboard motor installations several times and followed the advice to have the props turn away from the center line of the hull to minimize air drawn in from the side.

That may be true for a conventional system where the props are deep under the waterline. In my design it means however that there is a 12" wide low pressure area between the props, where air is drawn in even from behind the props.

I have now fixed an aluminum plate (orange) that extends the blue bridge between the tunnels to approx. 2" behind the tunnel openings. This is only a temporary cover which doesn't perfectly fit the irregular shape of that area, but the first tests were very promising: there is visibly less air in the wake.
At full throttle there is still evidence of air intrusion, so the cover must probably be a bit longer and precisely follow the contours of the tunnels.

 

So you outward turning props, i.e. RH on right side.
On performance boats although it makes dodgy handling, they sometimes turn then in as you get more bow lift.
Need more bow lift use more rake on the props
Need less slip use more cup on the blades and if you roll that around to the outside edge you will get a small effect like extra rake.

Maybe you also have to remove the platforms on the outside so the transom can go down, bow up and hence head toward planning?
Did you do a calculation that the blade area you have showing under the hull bottom is enough such that the loading is not so great they will cavitate?
you might need 4 or 5 blade props?

Just a thought...where will the air/water come from to fill the top half of the tunnel when you are moving forward?
Do you think that will create a huge suction?
Cheers

 

My design was without the platforms. Cemberci did the prop calculation for this weight/speed/engine hp. At and below hull speed the drives are a complete success: fuel consumption 75% less than with stern drives, fuel cost per hour almost 90% less.

Above hull speed the problems start. White wake, lots of rpm but no power to climb over the hump. Baeckmo and Ad Hoc suggested the platforms and tunnel extensions (these I recently removed).
I think -but cannot prove it yet - that once the boat planes there will be plenty of water to keep the tunnels filled.