Trim - Page 2

jwalker · 27 September 2005, 13:36

Thanks Charles. I went to the Volvo site and read just that but there's no explanation of how they function or what movement the blades make. From what I can tell, it appears the blade protrudes down from the transom about 50mm. The pictures appear to show two blades, one on top of the other. I wondered whether there was a pivoting action as they moved downward. I can't suss it.

Scary Des · 27 September 2005, 13:58

It got me intrigued too, so I looked at the site and could find nothing informative

So I rang my local dealer, clueless wasn’t the word, I quote, 'Volvo make what.....oh.... I didn’t know that'

So I rang Volvo they knew a bit more but not a lot. Full vertical stoke is 35mm

(not much) by 300 or 450 (two width sizes)

it works by inducing lift at the back of the boat, me 'do you mean lift or thrust

' Volvo '.......I’ll send you some literature'.
Volvo Couldn’t answer what effect they had on speed or what comparative work they had done

but did say (several times) that they were very good, based on what I don’t know.

I'll wait for the Info and see because if they are good I’d be interested Des.

codprawn · 27 September 2005, 14:54

The blades just move in and out about 2" - not a lot but it creates quite a bit of lift to the stern by creating a "step" of water ashead of them - this obviously lifts the bow and pushes down the stern - just like a normal trim tab.

The advantage is they are so neat and ahve very little chance of going wrong - they also seem cheaper than most!!!

control panel £42

9m 6 core cable £50

control unit £58

other cable £28

Interceptor £83 each

£344 ex vat for a system using the 2 biggest interceptors. Up to 4 can be used on bigger boats - this is from Volvo's retail price list!!!

One clever part of the system means you can actually steer the boat if your main steering fails.

Volvo could be onto a winner with this but they just aren't bothering to market it!!!

codprawn · 27 September 2005, 14:59

Interceptors aren't really new.

For more info see this

They are used on quite a lot of fast ferries etc - can also be used to dampen roll etc - from the companies conclusions they aren't as effective as normal trim tabs but a lot easier to install and cause less drag etc. Also they are far harder to damage - no problem with grounding etc.

http://maritimedynamics.com/mdi_msi_brochure.pdf

http://www.maritimedynamics.com/index.htm

Scary Des · 27 September 2005, 15:59

Quote:

Originally Posted by codprawn

The blades just move in and out about 2"......

Volvo say 35mm which is 1.4”

Quote:

Originally Posted by codprawn

..... creates quite a bit of lift to the stern.....

I think you will find it is thrust, lift is what you get going into a low pressure area such as above a wing.

Quote:

Originally Posted by codprawn

......this obviously lifts the bow and pushes down the stern - just like a normal trim tab......

It's the other way around.

Not getting at you Codders

it just helps to get the facts right.

You are right that it is cheap and looks low maintenance

Additionally it doesn’t clutter the transom area with horrible sharp SS blades that bend when hit

Des

codprawn · 27 September 2005, 16:19

Quote:

Originally Posted by Scary Des

Volvo say 35mm which is 1.4”

I think you will find it is thrust, lift is what you get going into a low pressure area such as above a wing.

It's the other way around.

Not getting at you Codders

it just helps to get the facts right.

You are right that it is cheap and looks low maintenance

Additionally it doesn’t clutter the transom area with horrible sharp SS blades that bend when hit

Des

Of course that is what I meant - currently have about 15 windows open and 3 phones going - I need to get a grip!!!

I

DGR · 27 September 2005, 16:21

Not sure that lift and thrust mean quite the same thing in hydrodynamic terms - water being incompressible. If trim tabs or these fin things raise the back end (and lower the nose) - it must induce some drag rather than thrust. Thrust implies that they accelerate you, which they can't.

I can see how they could level the hull out and therefore give more speed, i.e. the additional drag of the tab could reduce the overall drag of the hull. The tabs must induce drag though - otherwise they wouldn't/couldn't have any effect.

...all IMHO

...

D...

codprawn · 27 September 2005, 16:30

of course they cause drag - stick a match into the water and it will cause some drag - a step of water will build up under the hull at the back which will lift the stern and drop the nose - call it lift - call it thrust - it is pretty simple really - words just complicate it!!!

DGR · 27 September 2005, 16:33

Quote:

Originally Posted by codprawn

of course they cause drag......... - call it lift - call it thrust - it is pretty simple really - words just complicate it!!!

Fair enough - but it is much easier if people use the same simple words.........

codprawn · 27 September 2005, 16:37

Actually thrust doesn't imply they accelerate you!!! Depends which direction the thrust is coming from!!!

Scary Des · 27 September 2005, 16:48

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

DGR · 27 September 2005, 16:50

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...

jwalker · 27 September 2005, 17:31

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.

DGR · 27 September 2005, 17:55

Do you think those examples would be with fixed tabs, or with dynamic ones?

D...

jwalker · 27 September 2005, 17:59

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.

DGR · 27 September 2005, 18:06

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!!

hard1 · 27 September 2005, 18:16

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.

Rodan · 27 September 2005, 18:36

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?

hard1 · 27 September 2005, 18:50

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.

codprawn · 27 September 2005, 19:13

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!!!!