DIY tunnel drive

Looking at the "lines drag" curve; are you really trying to make us believe that this Draco would be completely without a resistance hump in the Fd range around 1.5?!! Please note my sincere doubts!

Referring to my comments on Savitsky algorithms in another thread, that is another strong cause for questioning the validity of these calculations. Have you made any references to full scale towing tests in this velocity range? As I mentioned before, I have been struggling myself with resistance prediction methods. Thus it would be interesting to know if you are using any corrections for wetted bow, aspect ratio or friction factor et c. in this speed range. And, of course what verification tests you have performed.

In addition to the clean hull drag, we must not forget the drag, caused by flow detachment in the tunnels. Here a rough estimate:

The flow that enters the tunnel sections at 11 knots is about 1.38 m3/s in total. With a 30% loss in mean velocity, the resulting pressure loss is nearly 8 kPa. This equals an extra fluid power of 11 kW, to be supplied by the props. With the propefficiencies we have here (~60%), the tunnel losses cost 25 hp in addition to the reduced propeller efficiency due to the variations in inlet flow, both directionally and velocity-wise.

So let us take the opportunity here, and see if we can improve our predictions in this speed range, I feel we have a black hole to fill there!

 

I am not using an empirical method for calculating the drag. However I have to make manual corrections for the trim. The Savitsky calculator actually gives a drag hump as you would expect for such a heavy boat relative to planing surface.

The prop can absorb a maximum of 29kW at 1300rpm. This occurs when the flow presented to it is at 5kts. If the flow is any slower than this, the blades stall out and the power demand actually drops off.

The issue for the tunnels is not if they slow the water flow onto the prop compared with the open water case but if they allow air into the props. Any air will change their operation dramatically. The slowing flow simply introduces a wake factor.

Hopefully the trim is much worse than I am using and there is a hump in the drag curve as a result and it will reduce when the boat gets on the plane.

Main point is that the motors were not delivering full torque before and they are now doing a little better. Many had written off the tunnels as a complete mess up. A few calculations, even with incomplete data, can lead to the correct conclusions and right course to correct.

Rick W

 

That's a no then!

 

.....well actually a whole load of no's....!

And, you are "....not using an empirical method.....". What method are you using to get results like that, then?

 

PLAYSTATION as apex mentioned several times! And not to forget "assumptions" as Rick mentioned several times! Alltogether "muppet show" results, nice to impress some non educated poor members looking for a supporting hand!

Sasha

 

This is one of the most interesting threads to follow, a lot of knowledge and ways to break down a problem into possible causes.
So why do some waste their time to misconduct or be sarcastis over some of the partcipants...who is right or wrong...CDK will hopfully have the answer in a near future.
So let the creative thoughts flow, read and learn abot different ways to approach problems.

And written comments are not always what they seems to be....sometimes written with a smile...sometimes in anger.....you don´t see that in the text....so bear that in mind.
rgds
Olle

 

Im still waiting!!

 

Some measurements made today. The KTY81-110 temp sensor was pressed to the marked area of the Garrett turbine. Pressure was measured with a fancy boost pressure indicator of which the dial can only been seen with the illumination on (totally useless design feature).

 

I assume you put a tee in the pipe that feeds the wate gate actuator ....then rev up ... ..pressure rises then peaks and drops back when the waste gate opens ...... 6 psi seems a bit low ..you carnt have too much turbo pressure ( well you can like 20 psi) as the fuel burns in excess air ...I would have thought you would have had at least 12 psi..

 

No I didn't, there is a small brass stud on the manifold with a rubber cap over it. I guess it's there for service or the vans had permanent pressure gauges.
The measurements were done at high idle only, so with much less fuel injected than under normal operating conditions. That also explains the low turbine temperatures. I assume that under load there will be much more pressure, otherwise VW would not have spent money on a waste gate.

At the present I cannot freely do what I want to: there are tourists everywhere, police, government inspectors etc. In this season all kinds of laws are in force, like no noise, no speed over 5 mph near the coast etc.
And the daily temp. near 40 C. also doesn't help.

 

Why dont you try dis connecting the waste gate see what happens. Its not going to run away because you have the throttles in your hand.

It finishes the argument of poor boost immediately. Racers don't use waste gates. 6 psi is nothing

 

I pressed thin brass sleeves in the banjo bolts and drilled one 0.4 mm hole in them. The results are disappointing. I now need not pull the start levers anymore to obtain 2600 rpm, and the engine respond better to a change in throttle setting, but the maximum rpm did not increase and there was still no black smoke.
When the floor boards were away, I also checked the functioning of the throttle actuators and found that the small electric motors are developing axial play. The power levers cannot reach the end of their stroke anymore, so "full throttle" in reality is somewhere between 90 and 95%. Changing that is a major operation that I will postpone until the autumn.
The port engine is developing an oil pressure problem: after prolonged running at full power the oil light goes on below 1200 rpm.

Below are a few pictures of the wake at different rpm settings. I temporarily clamped an inclinometer to a side window, leveled it with the boat at rest and saw that the hull angle reaches 6 degrees at 2600 rpm.

 

Cornelis, how can you be so focused on that minor problem? Your bench upholstery needs a refurbish, THAT is important.

Richard

 

You are quite right Richard. It is not the upholstery but a cover that was marine blue when new, now bleached out to ash gray. The dark patch is where the solar panel lies.
I'll tell my wife!

 

KISS .....its a diesel ..enough fuel to produce the power and more than enough air will burn it with out smoke ....so its the fuel then ...KISS