DIY tunnel drive

It is all, but really all said here in the thread. It is good practice to read a thread before contributing.

 

Come on now CDK, your propeller is just a torque converter working in- and cooled by huge amounts of water. The questions are: What rpm's does your engine reach with full throttle at zero speed ahead, and with fully submerged propeller? And what is the accompanying boost pressure and xh temperature? There is nothing strange or risky about this test, as long as you can steer 90 degrees to the jetty, it is common practice, just get it done! And don't worry about geographic dislocation.......!

To "powerabout": boost pressure is never max at torque max, see earlier notes on this subject!

 

Did the line break at the knot? Do the bollards have any surface defects? Very difficult to get 100% strength conversion efficiency from single strand to multi-strand.

I calculate 2000N thrust at 1500rpm on the engine. To give an idea of the engine speed sensitivity, if it was 1600rpm then the strain would be no more than 3200N.

Rick W

 

Baeckmo, it isn't the part in the water I'm concerned about, nor is it the thrust bearing that can take 3,5 tons in its oil filled chamber. But it leans against a cover that is held in place by 4 long M8 bolts and nuts.
I know, M8 stainless survives over 800 kg static load, and there are 4 of them, but still.....Now I wish I'd have used M10.

But I'll humour you and measure rpm and boost pressure. Xh temp I cannot promise because I don't think it is feasible to press a sensor against the turbine housing on a rocking boat without getting your fingers burned. But I'll look for something I can temporarily attach.

 

The fishing line broke somewhere and in more than one place. It connected 2 spliced rope slings, not the bollard. Contrary to my expectation it did not break at the knot, but I used a fisherman's knot where the line is wrapped several times around itself.

 

If it did not break at the knot then it means there must have been a weak point elsewhere. If it broke in more than one place then this also means the strands were not sharing the load. The friction over the rope slings must have been enough to cause significant variation in individual strand tension.

A fisherman's knot will achieve about 80% of the filament strength at very best. So the failure was less than 80% of the rating. Was the line dry when the test was being carried out? Was the line moist when the knot was preloaded.

Rick W

 

You do´nt know the Dutch have a tradition in seafarers usances? Do you? They must not learn from computer players how to distribute loads on ropes! They breathe Ocean when they have their first glimpse at the cruel world, and they fought the sea more and better than any!

Ähh, just my two cent of course (some may not know)

 

Damn it Baeckmo, look what happened!

There are between 2000 and 2100 rpm, the boost pressure gauge said +0.6 bar.
Air temp. 38,5 degrees, the exhaust I couldn't measure.

 

With your setup so far (and what is known about engine power characteristics), the engines should be able to reach something like 2600 rpm at Va=0, producing a bollard pull of <~4 kN each. So if you made the threads ok there should be nothing to worry about!

I feel pretty comfortable that the engines will be performing ok now, maybe some final adjustment of fuel flow and throttle linkage has to be done, but nothing "hidden" anymore.

Now to the wet jobs! your boat weighs in at 3600 kg you said. That is nearly one tonne more than it was advertized (and designed for) in mid 80:ies..... Each time you go onboard, there is a bolt or a nut or an ?? dropping from your pocket; take an hour and clean her from surplus weight!!

That done, she still needs all the planing (NOTE: NOT PLANNING) surface she can get. To this end, continue the original bottom (no twist, no hook) between inner tunnel sides as far back as to the very leading edge of your rudders. NOTE: Inner tunnel sides to be extended vertically down until they meet bottom extension! Use longitudinal stiffening in line with "jet box" sides and centerline. No flexing allowed!

This way you get 1) more planing area, 2) virtual LCG when planing is increased, 3) air leak path from inside into tunnel is broken, 4) the resistance at 16 knots is reduced ~20 % and trim reduced ~2.5 degrees!

The next step depends on how far your ambition can stretch..... , but one thing is certain; the propellers are far too close (axially) to the tunnel ends, they are ventilated from behind! Even if you don't want to put more midnight oil into a radical revision of the tunnels, at least add 150-200 mm to the tunnel length, with the top sector inclined ~half the shaft inclination!

 

CDK
Prop absorbed power under these conditions is 16kW. Gives engine torque of 73Nm.

Rick W

 

In addition to baeckmo's excellent precise, don't forget to reduce the angle of the 'inlet' from its 45 degrees to the hull bottom, or so, to around 20~25. This also helps with the 150~200mm and shaft inclination as he noted too.

 

This particular boat was built in 1979 and delivered March 1980. The Draco company was a bit vague about the weight because the boat was available with different engines. If I remember correctly, they stated 2800-3000 kg depending on motorization.
There is a 300 ltr fuel tank and a 100 ltr water tank.
I added a large GRP roll bar, radar, stainless anchor, 30 m. chain, electric winch, davits and carry an inflatable in the aft cabin. The propulsion is lighter than the stern drives, but the box at the stern is an extra 70 kg.

Maybe I exaggerated by 100 kg or so, but I remember than once in an Italian marina the mobile crane operator stopped lifting the boat and positioned the crane closer because an alarm sounded, telling him that the load exceeded the 3 tons limit for that beam angle.

 

Wow, did you cross any national borders while pushing around??? Cruising the archipelago has certainly got a wider meaning....

Now, for the propulsion; the boost pressure indicates that your engine is not too far from its nominal power characteristics at this operating point. Propeller data indicate same; most computer programs rely on algorithms that fail at low speeds of advance. So back to daddys pencil, propeller diagram and a calculator... (see that, Frosty?).

With an average figure of kq=0.12 for this kind of prop, water density 1000 kg/m3, 1050 shaft rpm and a transmission efficiency of 94 %, we have a power consumption of 33 hp. This is for a prop in an unlimited volume of water; the flow restriction of the tunnel will increase power further, say at least 10 %. The resulting total developed power is then ~36 hp (~27 kW), which is very close to the preliminary engine data collected earlier!

As in all engineering processes, be it electronics, mechanical or else, trouble shooting in marine engineering most of all demands a logical testing sequence, based on knowledge of loading characteristics of the machinery involved (and calibrated instruments......). Add some sceptic attitude to flashy (fishy??) computer wizards, and you are well off!

So, lean back now, have a cold beer with me, and try to figure out how to proceed. I suggest that for now, you let the engines be as they are, and focus on the tunnel issues as outlined in my previous note. With the davit and dinghy in mind, stretching the whole bottom, also between tunnel and chines, should be contemplated. And, as a final step of course, a total reshaping of tunnels, according to Ad Hoc's and my previous suggestions.

 

Consistently good advice again baeckmo....nice to see someone else with their 'sharp' pencil being used too ...mine is always by my side at the ready..

I'm not a beer person, but i'd certainly enjoy leaning back..admiring the view and chatting about...well, plenty really, with the two of you

 

While you were composing your reply, I converted one of the 1th drawings to represent the approximate current situation.
Would it be advantageous to extend the side skirts all the way to the edge of the transom, following the bottom's incline?