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Old 27 September 2005, 11:48   #31
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Quote:
Originally Posted by codprawn
Actually thrust doesn't imply they accelerate you!!! Depends which direction the thrust is coming from!!!
I think it does......Up
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Old 27 September 2005, 11:50   #32
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Conventionally it does. Thrust implies an applied force (whether accelerating or otherwise), drag implies a resistive force. You are right on the up and down tho' - direction is pretty meaningless - it is just a force.

ANYWAY - whatever - thing sticks down, back goes up (of the boat ), nose comes down, speed goes up, everyone's a winner!!

I'd be interested to know what sort of difference you notice on a 9m RIB, it's not something I've considered before because of the hassle, but if the volvo QL ones improve things (esp rough water handling) then it would be worth thinking about.

When do you think you'll be getting the RIB?

D...
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Old 27 September 2005, 12:31   #33
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Good links Cod.

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. Cool. I knew the turbulent effect a protrusion would cause but I never imaged the forces to be so high.

It was interesting to see the sectional diagram of a trimtab and it's effect being greater than the tab area and further forward than the tab itself. Presuming the diagrams to be correct, of course.

At http://www.maritimedynamics.com/motion.htm page, although these are just examples, it's interesting to see the vertical acceleration on the hull to be near zero when running in a following sea at 30 to the waves.
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Old 27 September 2005, 12:55   #34
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Do you think those examples would be with fixed tabs, or with dynamic ones?

D...
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Old 27 September 2005, 12:59   #35
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I presumed them to be adjustable because their blurb referred to the hydraulic equipment needed to control them. I'll need to carry on reading their info.
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Old 27 September 2005, 13:06   #36
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Sorry JW - I didn't mean that!!

Are they manually adjusted, or part of some computer controlled system - both examples are big commercial (40m and 100m) high speed boats/ships - I've no real experience of trim tabs (or big ships) but would have thought that the lag times for constant adjustment by the driver would be too long on something that big.

I would guess that response on a RIB/Sportsboat would be relatively quick, like when you trim the leg in/out.

Dylan...

P.S. You edited that while I was typing!!
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Old 27 September 2005, 13:16   #37
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The effect sounds similar to that produced by "The Lursson Effect" which was put to good use on WWII German E boats with semi displacement hulls. Basically, the twin rudders were cranked inwards by 5 degrees at speed, creating a large area of turbulent water under the transom. 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.
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Old 27 September 2005, 13:36   #38
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Now I am confused...

We trim engines up to lift the bow and go faster.

Now we stick things into the water to bring the bow down to go faster - I know I am missing something here (brain) does it lift the stern.

So... if you trim the engine out and put in your volvo thingies?

Perhaps if I left a patch off the antifoul in a line on the stern enough narnicles would grow to create this without having to spend any money.

And (this is to the enlightened ones) are these better than conventional trim tabs? Drag?
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Old 27 September 2005, 13:50   #39
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Well put!

If we lift the stern, we reduce drag and go faster. Transom drag is a big power/speed killer. If there's too much bow in the water as a result we go slower so we trim out to lift the bow to reduce the wetted area and go faster. The trick is to run with only the prop and about 12" of hull in the water!! (preferably at the stern!) Race boats often run with only the prop in the water. The QL's will help lift the stern and the trimming out will pivot the boatup/down around that point. It's a balancing act which changes with the sea conditions, and the load in your boat which is why so much is written/spoken about it. Every occasion is slightly different from the last.
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Old 27 September 2005, 14:13   #40
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Yes I can't wait to play with all my toys - by the time I have set the jack - adjusted engine trim and fidlled with the trim tabs and throttle settings I will have probably hit something!!!

On the big ferries I am sure the tabs are computer comtrolled - they talk about using them as dampers as well.

Maybe if i am quick enough I can get rid of chine walk with the tabs - up down - up down!!!!
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