Tunnel drive performance issue

I concur with Apex. A value of 230g/kWh is more realistic. If you have values of 180 g/kWh, go tell MTU or Caterpillar etc etc..they would all love to know how you do it. Since not one of these can get close to such values.....ugh!

 

Rick, because you produced a graph titled Draco 2500, it looked to me -and probably others as well - as if you had access to a database or shipyard design documents, but now I understand what you did.

Unfortunately, the assumptions are not in sync with reality.
The Norwegians used quite a lot of glass, resin and wood for their boats so they could cope with the circumstances there. The average weight with twin Mercuisers or Volvo Pentas was 3500 kgs.
At 10 knots, the bow has lifted quite a bit, and although I did not yet measure this, I estimate the angle to be 15 degrees. With that data, your graph will look quite different I guess.

I have been unable to obtain detailed info for this particular diesel engine, but I found a data-sheet for the SD 1.9 industrial engine. Although block, head, displacement and compression are the same, power is limited to 36 kW at 3000 rpm and the torque is 121 Nm between 1800 and 2600 rpm. They give the specific fuel consumption as 256 g/kWh @ 2200 rpm.
Ad Hoc and Apex1 will readily accept that figure.
The vehicle engines have the same Bosch VE pump with a different suffix of the type number; probably just the settings or some springs are different, so they produce 45 kW at 3700 rpm.
The turbo charged (ABL) version with 0.6 bar boost pressure does 55 kW at the same rpm and has only a fraction more torque.

Somehow this doesn't add up. The air mass is at least 60% more, yet the increase in output power is only 20% and torque is almost the same. My engineer's gut feeling tells me there is a lot more potential but for some reason the fuel injection quantity is limited. There may have been a thermal issue.
At 2100 rpm it really is the end of the line, but there is no black smoke so the air/fuel mixture is still well below the saturation point.
At the present, we regularly use the boat, noon temperatures are near 40 degrees C. in the shadows so this is not the time to experiment, but in the autumn I intend to break the seal of the limiter cap to find out what happens if more copious amounts are injected.

 

.....Just a few things from my rank novice perspective. I thought a Tunnel drive would seek to direct the flow of water toward the props? This is a vee hull? Which imparts lateral water displacement? Dictating some device to route at least some of that displacement back toward the props? Plus, would the bow pitch on acceleration add too much air? What could be done to lessen the pitch up?.....But you seem to be on to something if your fuel usage as dropped that much!.......

 

CDK

"...Rick, because you produced a graph titled Draco 2500, it looked to me -and probably others as well - as if you had access to a database or shipyard design documents, but now I understand what you did..."

That was the reason for my post asking which hull?...as yet no reply. Wont get one either!....see below why.

Since to post what Rick did requires, as a minimum:
a) Lines Plan
b) Weight sheet data showing displacement and LCG, VCG
c) Hydrostatics from Lines Plan,
d) series of runs at various LCGs

Without any of this being done, on your hull, the data posted by Rick is totally meaningless. All it does is:
1) It distracts you form the point of the posting
2) Attempts to give the impression he understand your problem and knows what he is talking about, which he doesn't.

The main thrust of your problems, are:
1)mechanical, which baeckmo is doing very well ascertaining the exact nature
and
2) Hydrodynamic, your hull and power ratio do not match expected "speeds" for your hull form and displacement coupled with the inlet flow to the prop is very poor which results in poor prop thrust characteristics, ie speed. Hence nothing is what you have been orginally told or expected from the beginning prior to your noticing the problem.

All of which point to the fact that serious and very major changes are required to overcome your problem, you cannot escape this fact. It is systemic not a band aid approach.

No doubt I shall get hit with the usual neg points for questioning the non-naval architect again....ugh, grow up!

 

Ad Hoc, you still have 299 points left, so you must at least have done something right.
I do not like the concept of negative points and will never hand out any because there is plenty of other opportunity to express your opinion, but alas, until they change the system we have to live with it. Ebay finally removed the possibility to let sellers give negative points to buyers because in most cases revenge was involved.

Just one far-fetched point of view about my boat's performance:
The two props are one 2 ft apart and I already mentioned that the port engine is a bit lazy when I open the throttles, but at maximum speed both engines run at 2100 rpm. At high idle, both reach 4000 rpm, but could it be that under load the starboard engine is doing most of the work and the port one is happily spinning along?
If you have two alternators charging one battery and the threshold voltages are not exactly equal, one of them behaves also like that.

 

Long time ago I already wrote -in a post in a thread about this present modification and its problems- that when the shafts are at the level of the bottom of a planing boat you got a surface drive. No trick will ensure a clean flow of water on the propellers, which one half is practically above the dynamic water line when the boat approaches the semi planing speed...Very hard to raise a big flow of water about 4 to 7 inches above the waterline without sucking air.

Furthermore the actual tunnels are hydrodynamic brakes, and even modified they will be very costly in drag and energy. Tunnel drive is an evil accepted when you deal with requirements of draft, or of beaching or of protection of the propellers. Often tunnels are a source of problems ( vibrations, noise, loss of efficiency) even when they are largely under the dynamic waterline and nicely designed.

The tunnels of the small catamaran-like boats used in very shallow waters are not a good example as these tunnels direct a mix of water and air to the propeller which are designed for this use.

Apart the engines problems the simplest solution without going to a total revert of the boat to classical drives is to go to surfaces propellers and thus to promote aeration of the propellers with the exhausts and surely air pipes. Maybe I'm getting old but I repeat for the 1000th time that surface drives are nor only for speed boats and work perfectly in medium speed boats.

Rudi Scholtz (of Levi Drives http://www.levidrives.com) makes a surface drive for small fishing boats that works nicely, so he has 17" propellers at decent prices which could fit on your boat. I have not commission from Mr Scholtz so I feel very free to give his name. You should consult him, he knows perfectly this domain and works with Renato Sonny Levi who is the great pioneer of fixed surface drives; that will be less expensive than to follow the present wandering and you'll have results, I mean a boat with a decent speed..

Take care of the engines -easy task- and leave the hard hydro and drive task to a specialist, he will love the problem.

Calculate the ponies of the engines from your consumption divided by a specific consumption between 205-225 gr/hr/HP, 180gr it's a bit optimistic with a small auto engine. The best truck engines from Man reach 155gr, and the 315HP 6 cylinders from Cummins 165-170gr, all turbo with intercooler and computer.

 

CDK
I don't count the points, but others do as it is there only means of credibility!

High idle...is that clutch out you get 4000rpm?

IV's comment on surface drives very well worth looking into. Especially if your hull is prismatic aft.

 

Ilan do´nt forget to say "at optimum load in a extreme narrow band of rpm"!
CDK´s engine is far away from that @2100 rpm, as the given figure of 256g has shown.

 

CDK
I have set her down somewhat and also looked at different trim angles. I have only shown 0 and 5 degrees on the curve but you will see it does not have much impact at 10kts.

The linesplan is included so you can see what I am basing the drag on.

The curve now contains the drag for two cases, the combined prop thrust and individual prop power demand at 1050rpm. If you follow the red arrows you can see how to determine the power produced by each engine at 10kts. You will see the absorbed power for each prop is 12kW.

The fact that the thrust curve intersects the drag at less than 10knots means the water flow into the prop flow field is 90% of the boat speed. So you have a wake factor to consider. That is accounted for by the short arrow to the left.

I have also included the Savitsky estimate for 3.5t. It starts at 15kts and is close to the displacement drag at this point - before the lift is significant.

One point to note is that you show the waterline across the stern at 540mm. If I use this value with the keel level the displacement is 4.4t. So I have assumed your weight estimate is accurate and she sits slightly bow up.

You will see that the 14X17 prop can absorb a maximum of 16kW at 1050rpm. So even if the drag curve is out a lot you are not producing more than 16kW on each engine.

The BSFC of 180g/kWh is about the best I have seen. I suggested this figure as it would give the most optimistic power level for your engines if you do the fuel consumption determination to back calculate power. The best figure I could find for the 1.9l TDI is 197g/kWh.

Any diesel I have ever seen starts pumping out black smoke if you make it labour. If your engines are getting their full fuel supply at full throttle but they are not revving out then I would expect them to be producing black smoke.

Rick W

 

Rick, I have difficulty interpreting Draco_Drag_Power.pdf.
According to the graph prop shaft power and twin prop thrust seem to decrease with speed and approach 0 between 16 and 17 knots....

 

CDK
Both the thrust and power curve for the prop are given at 1050rpm. The maximum water velocity past the prop for it to be producing thrust is pitch X rpm inches every minute.

If the boat somehow managed to exceed the speed where the thrust and power are zero for 1050rpm then the prop becomes a turbine and will produce a retarding force.

Rick W

 

CDK
"..Rick, I have difficulty interpreting Draco_Drag_Power.pdf..."
Again, I'm not surprised, i don't know why you persist in looking at data that has no meaning nor reference to your boat as she sits in the water. It is a distraction, for well..make up your own mind.

The questions you must ask yourself are in several categories thus:

Basic Data
1) Is this the exact lines of your hull, including the tunnel and the 'other bits'
2) Does your boat have a LCG of 2.5m....but from where? (do you know?)
3) Does your boat have a VCG of 1.0m
4) Does your boat have Cb of 0.38 (I very very much doubt it.!!)
5) Does your boat have a prismatic 10 degree deadrise angle?

Next you need to look at the method of calculating the results.

Results
1) Where is the appendage drag taken account of?
2) Where is the "assumed line of thrust" to calculate the drag? (is it where your engine shaft line is?)
3) Is this real tank test data or just a computer program to give "rough idea" of EHP?
4) The drag curve shows almost no prismatic hump

So before going in too deep, if you can really be bothered (i wouldn't), you have the following:

A lines plan that is not your boat. The basic data of your boat certainly does not appear to be the same (Cb=0.38!!). The method of calculating the Drag is just a very simple program that can never provide 100% results, just indicative for those that have no tank test data available.

This is corroborated by the near zero prismatic hump. A boat of such low L/B ratio shall exhibit a very pronounced pristanic hump, noticable by extreme trim. In other words the results are just a simple exercise for, well, Ricks amusment is all i can tell.... since it has no relationship with your real boat and its data and just muddies the waters.

As for the black smoke. This all too common on waterjet powered boats. The master/capt usually opens up full throttle almost instantly. There is a delay/lag in the response, the engine delivery is beyond the what the waterjet is momentarily able to absorb. Therefore too much fuel is being burnt; in a nut shell. This is easy to explain when you have the engine power/fuel consumption curves, it is self evident as you look at the RPMs and fuel consumption in relation to the cube power curve.

 

Maybe a bit risky to get involved in double threads on the same topic but......
The Savitsky algorithms in their original shapes often generate dubious values when it comes to real world, real size boats. First, surface roughness is in practice far greater than the standard ITTC or similar, that is an integral part of the basic S method. Secondly, the expressions for lift include compensations for planing surface aspect ratio, BUT in the expressions for drag, there is no such compensation involved, contrary to dynamic lift theories in general. Remember that the S method is not strictly built from physics, but are just mathematical fits to a limited number of tests.

So, if you use the Savitsky standard form, you will get far too optimistic drags for the kind of boat we discuss here! To check this, I have measured actual thrust in a number of boats, using outdrive trim cylinder pressures, and found that there has to be an additional, modifying factor, compensating for AR, in the same way as we have for finite span wings.

For CDK's Draco we actually have an real world operating point at 28 knots, that may give a fixpoint for calculation. This speed was reached with ~2x130 hp From efficiency tests on similar drives we get a total overall propulsion efficiency of roughly 55 %. At 28 knots this amounts to 7307 N, to be compared to the thrust figures in Rick's diagram, saying 4600 N! Without knowing what version of Savitsky's method he is using, I can only say that he is unrealistic!

"If the landscape differs from the map, then don't thrust that map!"

 

CDK
Given the apparently low power output of your engines I am wondering if they are locked in limp mode. It is not something I know about but you see a lot of VW and Audi diesel owners complaining about it.

Here is some discussion on it:
http://www.myturbodiesel.com/1000q_how_to/multi/limpmodetdi.htm

Rick W

 

baeckmo

Fully concur, hence my comment "..So before going in too deep, if you can really be bothered (i wouldn't),.."

PS..just wondering whether you'll also get the very sad, hit with negative points now too (as i do), since one cannot question the non-naval architect and his 'status'...ugh!!!