Trim

[jwalker]

Originally Posted by hard1

.....Basically, the twin rudders were cranked inwards by 5 degrees at speed..... Far from creating drag, it created a high pressure area and thus created lift and enabled the hull to gain another 5 knots or so. Clever.

I would expect it to create a low pressure area between the rudders. Are you sure it wasn't a slightly different effect?

----------------------
Just googled it. Apparently, the rudders turn out rather than in and they’re special trim rudders. Interesting, I’ll read on.

http://www.modelwarships.com/reviews...2-S-100-p2.htm

[hard1]

Originally Posted by jwalker

I would expect it to create a low pressure area between the rudders. Are you sure it wasn't a slightly different effect?

Hmmm, I think you may well be right. I'll creep down to the the archives and dust them off. The net result was the hull kind of running on what was tantamount to an air bubble with resulting increase in speed. One would like to imagine that this was entirely coincidental, with the dockyard guys getting the rudder alignment wrong, and just before they were about to shoot him, the delighted skipper yelled "Nein nein, nix schotten! it mit fasterwerker! Ach! too late..."

Apologies

[codprawn]

Just found this on a great site

http://www.bmpt.org.uk/boats/S130/index4.htm

Another key innovation was the addition of a special rudder arrangement beginning with S-2 (1932). Port and starboard of the main rudder were two smaller “rudders” that could be angled outboard to 30 degrees. By generating what became known as the “Lürssen Effekt," at high speed, the angled “rudders” drew a ventilation air pocket slightly behind the three screws, increasing their efficiency, reducing the stern wave and keeping the boat at a nearly horizontal attitude.


Three shafts, main rudder and “effects” rudders


A wedge was added to the lower stern beginning with S-18 (1938). This deflected the water flow slightly downwards, counteracting any tendency for the hull to settle into the water as speed increased.

[hard1]

Originally Posted by codprawn

Just found this on a great site

http://www.bmpt.org.uk/boats/S130/index4.htm

Another key innovation was the addition of a special rudder arrangement beginning with S-2 (1932). Port and starboard of the main rudder were two smaller “rudders” that could be angled outboard to 30 degrees. By generating what became known as the “Lürssen Effekt," at high speed, the angled “rudders” drew a ventilation air pocket slightly behind the three screws, increasing their efficiency, reducing the stern wave and keeping the boat at a nearly horizontal attitude.


Three shafts, main rudder and “effects” rudders


A wedge was added to the lower stern beginning with S-18 (1938). This deflected the water flow slightly downwards, counteracting any tendency for the hull to settle into the water as speed increased.

Ach, so!

For "wedge" read QL Boat Trim System, a trim revolution!!! Which conclusively proves that there is nothing, but nothing ever new under the sun...

[codprawn]

Yes I always say that!!!

I quite like the look of the QL tab indicator

[Charles]

Originally Posted by jwalker

So, at high speed, (and they emphasise high speed) the turbulence created be the protrusion becomes vigorous enough to form a firm, virtual wedge considerably larger than the protrusion itself, which gives the effect of a hooked hull. I knew the turbulent effect a protrusion would cause but I never imaged the forces to be so high.

Fantastic, you have explained that very well. The ex-Volvo chap over here said the same thing in a much more complicated way. I also asked what I thought a reasonable question and that is one of the principles of Bennett or traditional tabs i.e.they also 'add' to the hull length which obviously greatly assists trimming the boat more effectively - would the QL's not 'lack' in that department as they clearly do not stick out horizontally. The answer was to think outside the box as the principle is not the same?!

[jwalker]

I've been back to Cod's link for a bit of swatting and this PDF is just the info we need. It's based an a 7mtr model...spot on!

http://www.maritimedynamics.com/products/trim_tabs.htm

click on; View the MDI Data Sheet on Trim Tabs (pdf) at the top of the page.

[Scary Des]

Originally Posted by hard1

.......For "wedge" read QL Boat Trim System, a trim revolution!!! Which conclusively proves that there is nothing, but nothing ever new under the sun...

Aaaah but it is a virtual wedge so maybe it is new everything is virtual these days what worries me is when someone comes up with the idea of a virtual salary Des

[Rodan]

Basically back to smart tabs then... ?

[Mark Halliday]

Originally Posted by Scary Des

a virtual salary Des

I've virtually got one of those.

Des. I think your hat is safe. Mr Cods boat is going to have so much clobber on it, it will carry on down the ramp after it has left the trailer, never to be seen again. Virtual certainty.

[Cookee]

If it says that they are effective up to 50mph, has anyone any idea what they would be like on a 60 - 70 mph stepped hull RIB? Thought about fitting trim tabs to the race boat, but they are heavy and bulky - these seem neater and more compact - maybe lighter as well?

[Scary Des]

The fact that they work fast might be interesting for racing

I was thinking about splashing the idea and designing my own

My boat always leans to the right almost regardless of wind, wave etc so one 'virtual wedge' might be the answer it could be manually operated with a lever Des

[Pete7]

Des now your talking, manually operated with a normal 33c cable, we just need a look at the volvo design and then have a small engineering firm make one up.

In fact make half a dozen in S/S or Ali and sell the rest. One of those nice billet control boxes for the control ends.

Pete

[Scary Des]

Originally Posted by Pete7

... One of those nice billet control boxes for the control ends.....Pete

Hadn’t thought about cable operation but a good idea

What do you mean by a billet thingy Des

[Pete7]

The cable idea would mean that you have the ability to adjust them, so one could be fractionally lower than the other to correct your slight list to port. It would also be instant, grab a handfull of lever and the tabs are up or down.

The race lads have nice machined control boxes for throttles and gearbox controls and look very sexy. Cookee will know what I mean. Graduations would show how much trim tab is below the boat or when they are fully clear.

Picture isn't quite what I had in mind, but gives an idea.

Pete

[Scary Des]

Aaah neat idea

I was thinking of something very simple that looked a bit like a slues (?) gate threaded bar with a handle on the top for up and down. Des

[Pete7]

Hmm, just thought, water pressure going to be quite high and would push against cables. Not sure if it would work with cables in compression as tabs might move up. Think the water pressure is about 80 lbs sq inch (unless anyone knows differently). Multiply by the area of the exposed tab is alot of pressure, afterall you can ski barefoot at 20 knots, thats an adults weight supported by a very small footprint.

Fancy going halves:

http://cgi.ebay.co.uk/Teleflex-Morse...QQcmdZViewItem

Slues gate ? is that like a vice ?

[Mark Halliday]

Originally Posted by Pete7

Hmm, just thought, water pressure going to be quite high and would push against cables.

Stick a fulcrum in, so pushing - pulls and pulling - pushes.

[Edit] It will also multiply the force on and reduce the movement of the tab for the much larger lever movement. Super fine control.

[Pete7]

Possibly, but still think the water pressure might over come the friction of the cable and lever. However that would mean they automtically raise at a given pressure, say planning speed.

Is there a draughtsman in the house to draw one. Could be a very lucrative contract asuming the the tall one only takes a small cut for using his forum for biz

[Mark Halliday]

Originally Posted by Pete7

Is there a draughtsman in the house to draw one. Could be a very lucrative contract

Yep, and he dont seem to want to do what he is ment to be doing at the mo.