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Random Quote: GOD does not deduct from a man's alotted time, those hours spent fishing.
I'm certainly no engineer...
But just trying to visualize the forces that would be applied with either type of tab. Seems that the conventional tab, because of the extension and angle would provide more stern lift, which is the whole point of adding tab.
The Volvo tabs, in my feeble mind, would want to make the stern of the boat "pull" left or right instead of "up", when only one tab is applied. Seems that they create more drag than anything else- doesn't seem it would be any different than shoving a flat-bladed shovel down into the water- how efficient would that be??
Either type creates drag, the idea as said above is to improve the running angle so that despite the added drag, the boat runs at a more efficient attitude. My boat is the perfect example. Stern heavy with the diesels (most of them were fitted with big-blocks and probably designed that way), I need full tabs when running for the best attitude, highest speed, and best fuel efficiency. A well-designed boat wouldn't need that...they should cruise at their most efficient attitude without the aid of tabs.
Not sure why you guys work yourselves into a lather. To lift the stern of your boat by any amount takes the same amount of force - no matter how you do it. The volvo tabs will create the same drag as any other type - the mechanism is different - thats all...no hydraulics, external resevoirs, or pumps. The ONLY advantage these things have it they are simpler mechanically, and smaller physically (won't stick out behind your transom) - thats it -
so...if someone doesn't like the volvo tabs, then don't buy them for gods sake, buy bennett or anyone else's tab that will make you feel better -
not sure what is gained by the rants on this board...mostly by guys that don't (apparently) even own this product.
There are no strategies, I am just a boater and have nothing to do with any brand, something I believe it is not the case of some of the writers in this thread which for some reason, ( probably a commercial link with Volvo) pretend that this doubts about a Volvo product are just bullshit while there are honest reasons to doubt about the design of the volvo product in question.
Here you can see that at least in airplanes when the desire is to cause drag as much as possible the way chosen is to insert a blade 90 degrees into the airflow exactly what volvo engineers have chosen to do.
As the history has proven, engineers like any other human does make mistakes and there is no need to bring up any example because sure all of us do now about quite a few mistakes made by very prestigious brands ....... so there is no need to blindly believe in whatever engineers say ( and I am one of them )
There is no lack of scientific data or papers on this topic in the public domain:
Try and Google Anthony F. Molland - Marine Rudders and control surfaces, or Brizzolara - Hydrodynamic analysis of interceptors, or Tsia J.F - Study on the compound effects of interceptors and many more...
Written for the Naval Architect or Hydrodynamic expert and Hokus-Pokus for most of us but leaves no doubt as to the functional merits of the concept.
There are no strategies, I am just a boater and have nothing to do with any brand, something I believe it is not the case of some of the writers in this thread which for some reason, ( probably a commercial link with Volvo) pretend that this doubts about a Volvo product are just bullshit while there are honest reasons to doubt about the design of the volvo product in question.
Here you can see that at least in airplanes when the desire is to cause drag as much as possible the way chosen is to insert a blade 90 degrees into the airflow exactly what volvo engineers have chosen to do.
As the history has proven, engineers like any other human does make mistakes and there is no need to bring up any example because sure all of us do now about quite a few mistakes made by very prestigious brands ....... so there is no need to blindly believe in whatever engineers say ( and I am one of them )
The glider plane comparison with interceptors on a boat is not entirely relevant although I understand the point you are trying to make.
Since the interceptors are positioned on top of the wings (and this is no accident, they are on top of the wings for a reason) ask yourself what it is they are trying to do in the glider example: Increase drag or reduce lift ?
I submit the purpose is to reduce lift (and probably very successfully). The reverse would happen if they protruded below the wings - The lift would increase - Similar to extending interceptors vertically down from the transom on a planing boat - Also very efficient.
Is there drag associated with interceptors - Absolutely. However, for the same amount of lift there is less drag than if you angle a large surface into the water (or air) flow.
As consumers we have the right to (and should) be suspicious of unsubstantiated claims. In the US after 2.00 am there is lots of stuff sold on cable TV that one should be suspicious of.
Although interceptors as a trim device may not be for everyone and for all applications it does not fall in the snake-oil category
Thank you Karl for pointing us into some releavant documented information and making reasonable thoughts about the working principle of the volvo tabs instead of insisting into derailing this thread by being constantly picking irrelevant parts and phrases like some volvo related plain ignorants have been doing for quite a while right now.
I will try to find the pointed information despite the songs of ignorance sang by some while others were trying to help clarify the technical aspects of this interceptors.
On the other hand, it is not only me who find counterintuitive the working principle of those tabs, simultaneously, I do admit intuitive feelings might be wrong, but it should also be admittted that there are many drag causing devices that work precisely by moving surfaces perpendicularly to the direction of the movement, another well known example would be a parachute.
Also, all control surfaces in aeroplanes them being in the tail or wings or even in submarines or boatīs rudders or even formula 1 or Indy cars despite their goal being to cause a force that drives the craft ( or parts of it ) into the desired direction you can not find a single one working by the interceptor principle, all of them in any of those crafts works by gently and smoothly deflecting the surrounding fluid and not by doing it so abruptly at 90 degrees to the flow so, if the system causes less drag why no one else uses it ??
The intuitive feelings of Mr TOPBNR are felt by many others, me included and his intuitive reasoning about there being a pull towards the rear caused by the interceptors Is also felt by many as logical which is by the way why I started this thread that from my point of view needed clarification, regardless of when the product was introduced or the holiness or not of the cows .....
Also, despite you being right about the fact that in gliders the spoilers are in the top and that they do try to break lift, I would say that their purpose is dual, to break lift and to dissipate the kynetic energy of the aiplane, at the same time there are gliders that deploy their spoilers or airbrakes into both, the top and bottom surface of the wing, the mission of any type of spoiler or airbrake as you probably know being to slow the plane once in the ground ( and break lift when in the air to produce a sink without an increase of speed).
Must be clear that as you said they might be ok, it is only that they do seem to have a suspicious design and that to illustrate the point of their suspiciousness someone must play the devilīs advocate role which only have the purpose of starting a debate that could lead into clarifying the working concept of the product so the purpose was to enlighten us all by thinking together and arriving into some reasonable conclussion not to start bizantine arguments about the correction of my english that because of not being my native language should not be expected and should be irrelevant as is not the main point of this thread
Last edited by CANARIANFISHER; 11-07-2009 at 12:57 AM.
Reason: spelling corrections
It seems to me that several people are getting pretty worked up over nothing (or at least next to nothing).
We know the QL system works as an effective means of trimming to boat. The argument seems to have come to one of whether they produce more drag than the conventional style tab. As I said in my previous post, the difference in drag between the two systems is most likely minimal, especially when compared to overall drag created by the wetted surface of the hull.
Just for an exercise in logic let's look at some numbers. I'm rounding numbers here so don't hold my feet too close to the fire. Let's say your boat, the Drag Queen, Is 22' long with an 8.5' beam. It's waterline length is 20' and waterline beam is 8'. when you've got the Drag Queen up on plane, trimmed up and running good about half the hull is in the water, maybe about 64 square feet of surface area is wet. Trim her down and all the hull is in the water, about 128 square feet or 18,432 square inches.
Those trim tabs you used to push the bow down, typical 9" x 12" blades, have a surface area of 108 square inches or just a little more than 1/2 of 1% of the hull's surface area. How much impact do you think those little tabs have on drag now compared to the whole hull? Do you really think the QL tabs could be that much worse?
Do the tabs add to the overall drag? Of course they do, either system. Would there be a big difference in total drag between two identical boats, the Drag Queen equipped with conventional tabs and the Drag King equipped with the QL system? I doubt it.
Disclaimer: If you see emoticons, that means the above is written in jest. It is for entertainment purposes only. It is not meant to represent any actual persons, places, or things. It is the opinion of the poster that it is funny. Your opinion may vary. No animals were harmed in the typing of this post but a few egos may have been bruised.
Yes, spoilers being on top of aircraft wings is no accident. Contrary to popular belief, placement there is not so much about adding drag, but more about reducing lift. (Ref: Bernoulli's equation.)
Back to boats..... Since drag squares with velocity, the QL design is not so plausible with go fast boats, but it is so with cruisers. In fact, contrary to what one might initially envision, total drag with this type device is actually less than with conventional tabs in the lower speed regimes.
As for IPS vs ZEUS, IPS is more efficient. There are a couple head to head tests out there. At least one is on BoatTest, where two identical Doral Cruisers were tested.....one with IPS, and one with ZEUS. The numbers generally speak for themselves.
Anyhow, while it seems like the potential for damage from debris would be enhanced with the IPS design, I have a couple friends with IPS boats. Between them, they have hundreds of hours on their boats, and not once has this been an issue. Best to have good insurance though, no matter which one prefers.
Oreely; If I understood it right you are only considering the amount of wetted surface which is not an adequate approach to the doubts in question, I would say that the shape of the trim tab is where we should focus, we all know that a square block moving in a fluid will have a lot more drag than for example a bulbous rouded shape of the same surface that is why the fron of cars and indeed any other moving object are not shaped square. conventional trim tabs have more in common with a bulbous shape than volvoīs due to them deflecting water in an gentle way as aopposed to doing it abruptly at right angles.
I would feel comfortable believing what V1row says about volvoīs tabs not being suitable for high speeds admitting they certainly have the advantage of being much more compact and less interfering with fishing lines but on a fast cc of my own No Way ... I would install them
If would you stop treating hydrodynamics and aerodynamics as the same thing you would begin to understand something instead of talking in circles for 3 pages of useless posts that resort to your same "conclusion."
Your "gentle way" (I like that one) of deflecting water doesn't create the pressure wave needed to provide lift in the transom. Cant you read the explanations???? If you don't like it ... fine . But understand something, People with a LOT more understanding of hydraulic principles than you engineered these products and they do work.
For F%$#s sake, find a boat with a pair and go for a goddamn ride instead of "just thinking" they dont work.
I would If I could find one ...... but there seems to be very few, in fact I have not seen even one fast boat fitted with those, by the way if you do not like the thread do not read it ........ as simple as that
I am not Oreely or Darbikrash, but have you ever noticed that 99.999% or propeller driven airplanes use the tractor configuration and not the pusher configuration? Yes, the props are the first things to get struck but one usually assumes that nothing is going to get struck, even though this sometimes happens. From a design standpoint the tractor config is much more efficient than the pusher configuration since the props are in clean air/water.
I am old enough to remember when turbo jets became turbo fans (with the addition of additional fan blades within a shroud) and people accused the designers of being "nuts". Now look at almost every jet engine made. It will be a turbo fan.
I am going now to study their trim tabls to see what the fuss is all about.
Skymaster (push-pull) by Cessna has a propeller facing front and a propeller facing rear. It performs a lot better on just the rear prop, than it does on just the front prop.
The best performing TurboProp by far is the Piaggio Avanti and it has two pusher props and is much faster than Tractor style TurboProps with more horsepower.
Everyone is comparing 'q' tabs and Trim Tabs to aircraft speed brakes (spoilers). A more logical approach would be to compare Trim Tabs to the Wing Flap system on an Aircraft, which while increasing Drag, also increases Lift. Speed Brakes are all Drag with no increase in lift.
I am no expert in the field, I do not claim anything, but this is just my own idea not statement:
Props, wings, spoilers,... have the same fluids on both sides.
QLtabs have same fluids when boat in displacement QL tabs have different fluids (air/water) on each side when boat on plane.
I have made a rough sketch.
What I know is that turbulent flow creates a huge drag.
On the front of a spoiler or a QLtab, a fluid cell may appear to divert front flow, avoiding turbulent flow (a bit like on golf balls). Easier even for air that can compress and may have a reduced or no cell.
At the rear of the spoiler, turbulent flow (most drag) does what it is meant for: slowing the plane.
On a QLtab on displacement, I would expect the same: turbulent flow behind the tab just as for all transom.
But on planing, in the front would be the water cell diverting downward the flow, but in the back, there would not be turbulent water slowing, but just air which in comparison to water creates insignificant drag.
Again, no statement, just sketches of what I explained earlier.
You are probably right, Mr XV2, I appreciate your effort to clarify the subject and hope that others do as well, thank you. Maybe I posted this in the wrong forum mainly oriented to boat users and this subject probably needed some professionals that probably are more aboundant in other forums. IN this one when you arise a technical theorical question you just get bashed too much and too many memebers get irritated easily when you question about the goodness or not of a product derailing the thread. Thank you again for taking your time to make the drawings and to think about it.
OK sorry also on my side for picking on you. But here you blame the others to bash. Actually you have been bashing the system first. This is also why I felt a bit upset. But again, sorry for being over sarcastic.
I am not sure my explanation is valid or not. At first I also thought that system was somehow not logic, but my own explanation seems to satisfy me
Now of course, classic tabs and QL both generate drag, and I am unable to say which one is better. However what is sure is that with all the debris encountered here in the water, QL would be knocked out in a day or two, and I do not want to prove it
Re the forward facing props, same story. However, for having recently seen detailed drawings of some bigger vessels, it seems to be more and more common (actually with an electrical engine inside) , and that is also called IPS (Integrated power system or something like this coincidence though). Interest maybe for maneuverability as well.
Discussion is always interesting but it appears that we have now reach the point of flogging a dead horse.
At the end of the day this is pretty simple: In a free market economy we - The consumer - decides. If a product won't deliver on a claim, if the claim provides no benefit or if the benefit has no value for the intended consumer we won't buy it. The product will disappear from the market.
Cool thread... Love when the THTers call each other names.. I always think of when I was in 3rd grade and called a kid pizza face, after he called me Fatso once too often. Terrible, you see the nuns told us you get pimples from playing with yourself
I like both of the products, innovative...
Both been proven to work..
On the tabs; Looks like less stuff to cut off my tuna, and less stuff to paint, less is better.
On the pods; Since they are predominately used in historically inboard engine boats, the only thing making the pods blade mor volnerable is it doesn't have the shaft & strut to hit the object first... Oh, I see where you're coming from... You figure on a straight inboard the top half the prop is protected if you hit something, only the lower half of your prop would be damaged...
Seriously, I think the pod designs are the future for big boats, and not a fad. They work, they make a big boat more user friendly. Even Mom can dock one now.
I was born in the 50's I have been boating since the age of five, flying since 16. Did a stint for 11 months for Lake Aircraft as an Instructor/ Demo/Ferry pilot. (Lake Amphibians utilize pusher props as do Republic SeaBees, the Piaggo Royal Gull, the Avanti and numerous others) There is give and take in all these designs. The Cessna Skymaster does indeed have a better single engine rate of climb on the "pusher" or aft engine, but this is due to a larger diameter prop installed to make up for poor performance with a similar diameter to the front engine. Nose wheel (as opposed to tail wheel) ground clearance is a problem. Turbulence ie the wake from the structure ahead of the pusher causes a loss of efficiency we may have gained by not imposing a high velocity stream on structure behind a tractor prop. Anything that happens to leave the structure in front of a pusher seems to (according to Murphy's Law) manage to strike, or be struck by the prop and 4 or 5 times in my short tenure with Lake Aircraft through some part of the airframe requiring (if you are lucky) minor rework of the prop where hit and a patch if the airframe was struck (see note on Murphy and remember he was an optimist) Turbofans have been Tractors to assure good flow into fan at varying angles of attack for different flight configurations. Testing was done in the late 70's on pusher and tractor open rotor fans and is seeing a rebirth of late with the biggest problem being noise. Surface drives are still the best way to go efficiency wise but have other problems. This might open a can of worms.
I would If I could find one ...... but there seems to be very few, in fact I have not seen even one fast boat fitted with those, by the way if you do not like the thread do not read it ........ as simple as that
I have them. They work. I'm not sure why the reference to "fast" boats?
Quote:
Originally Posted by CANARIANFISHER
...I am forced to believe that there has to be something wrong with them ... I find this comparable to someone trying to sell me a wheel that is not round pretending that it has some obscure benefits.......
The auto level system is pretty sweet. As previously noted, its not brand new - intercepter systems have been used on commercial vessels.
I'm also a glider pilot and I am very familiar with flap and spoiler systems so an intercepter system is certainly not an odd unknown technology... to me.
Once of the biggest issues with the Volvo QL trim tab is having a hull configuration which can accept it. You need a large area behind the unit in the hull to fit the motor and it needs to be installed in an area where the transom meets the bottom without chines. This rules out many hulls.
Like the IPS system, lets see how the rest of the industry responds to this technology... In 10 years from now.
Dwayne
__________________ Stabicraft 829
Scuba, Fishing, and Weekends away with the family.
