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Canarianfisher, I think you are right to question new technologies and you raise good points. No one should accept new things on face value.
I am not a CFD guy or an aerodynamicist, so I cannot really comment on the relative merits of the tab systems, but it does come down to drag vs. righting moment. Whichever system (conventional tabs or Q tabs) delivers the greatest fore/aft rotation with the least drag is the winner. I might add that the pockets required for conventional tabs can create some problems if they are not adequately ventilated, to the extent that some well known hulls have trouble planing off due to the excessive negative pressure in the tab pockets.
I’d guess that the Volvo system is more efficient, but it’s just a guess.
By the way, I believe Karl2 who has posted in this thread is a frequent contributor from BoatDiesel.com. If so, we are fortunate to have him with us, he is a recognized expert on Volvo diesels. I would listen very carefully to what he has to say. Welcome Karl.
One comment on the IPS/forward facing prop controversy, this will never be solved on this forum or any other, and Cummins does a good job playing on the perceived vulnerability of front facing props. I believe Volvo has a patent on the forward facing design, and the competition is left to try and create a “story” around the remaining technology that is left.
I think there is little question as to the efficiency of forward facing vs. rear facing pods, and I would add that in my view, the aircraft comparison is entirely valid.
As to the comments that there is no debris in the air as opposed to the water, I think a guy named Sully Sullenberger might respectfully disagree. He is the airline pilot that suffered severe bird strikes during takeoff, and had two simultaneous engine failures as a result, bringing the plane down with 155 persons on board. He was able to successfully land in the Hudson river with no loss of life.
I think there is little question as to the efficiency of forward facing vs. rear facing pods, and I would add that in my view, the aircraft comparison is entirely valid.
As to the comments that there is no debris in the air as opposed to the water, I think a guy named Sully Sullenberger might respectfully disagree. He is the airline pilot that suffered severe bird strikes during takeoff, and had two simultaneous engine failures as a result, bringing the plane down with 155 persons on board. He was able to successfully land in the Hudson river with no loss of life.
The other point is that Engineers work on the situation they face the 'majority' of the time, not the absolute minority. Thus the efficiency of forward facing props far outweighs the remote chance you hit something.
Yes it is possible you might hit something, but the rest of the time having the props in clean water makes a huge difference to performance and thus in the Engineer's mind it outweighs the probability of hitting something.
In any case if you hit any of the debris that was mentioned, even if the pod has rear facing props, you will still be in a world of hurt!
I knew SOMEBODY would drag Sullenberger into this sooner or later. Whatever, Jethro's point (which you agree with) about relative efficiency of the two systems is, i think, valid, and i agree too. However, I most strongly disagree with his (and your) analysis about crap in the water vis a vis crap in the air. There is simply no comparison between the occasional (yes, very occasional....believe a few hundred bird strikes a year in all U.S. airspace with all types of equipment considered) vs. how many in the water? Hundreds of thousands? More? Relative levels of water borne debris compared with the levels of bird activity which threaten aviation is extremely obvious, statistically.
Granted, the incidence of threatening, floating, crap varies from boating area to boating area, but overall there is no comparison to debris (including live debris) in the air.
Last note. Volvo has a bunch of patents, but I am far from sure that they have a patent on forward facing props. If they do, it is almost certainly invalid. Forward facing pods? Perhaps, but not forward facing props.
Fair enough, but the aero guys have been over this again and again and the conclusion is that it’s worth the efficiency gain to mount forward facing. No question the occurrence of mid-air objects is lower in air than water, but on the other hand, the consequences are much more serious for an airliner than a pleasure boat. I’d say severity trumps frequency.
I’m not taking sides in this, but it does seem like the Volvo guys have come to the same conclusion.
Regarding the patents, I haven’t looked at the USPTO site for the specific claims, but they do publish in their product literature that the system is patented. I have been told that the pod system, and specifically the forward facing/Duo Prop combination claims are heavily patented. Someone may want to check the patent office site if curious, but if true, this to me anyway explains a lot of the boat show “stories” emanating from the folks making rear facing pods.
I am not versed enough with the IPS system to be a proponent or opponent, though I do not believe any credible argument can be made as to its advantages in efficiency. If you want to argue about it as a system, that is less protected, that is another issue unto itself. Cummins Mercruiser had to counter the Volvo offering and did so with Zeus, though it is much less of a proprietary system than IPS (ZF has a pod system very similar to Zeus). Maybe CM did a rear facing drive because they think its better, and maybe it is because there are patent issues, I do not know and do not really need to know. I would simply allude to this: volvo developed, patented and introduced the first twin prop drive, the Duo-Prop, and had patent protection for 7 years. Mercruiser scoffed at it, ridiculed it, and did everything they could to belittle the idea. When the patent protection ran out, what did Mercruiser do? Introduce the Bravo 3, which then they of course claimed to be far superior (I have run both countless times, and I can tell you that both are good, but the B3 is in no way superior to the DP). Now if they ridiculed it and the very idea of the technology, why then did they decide to copy the idea? Forward facing pods are found all over the world on larger modern ships, ferries, etc. Are they the "one answer" to propulsion in boats? NO, because one answer does not exist. There are different designs because there are diffferent applications. Much like there is no one design of fishing or cruising boat that works for everyone. If there were, every builder would be building that design. Competition breeds innovation, and the end result is better offereings to the end user. Rather than trying to cut down a design, most of you should welcome new innovative designs even if they do not fit your needs, because the next innovation by the company that builds your power system might just be the cats meow fo you.
, but after seeing their " sort of trim tab product" I am forced to believe that there has to be something wrong with them
I have the Volvo QL tabs and they work great. I certainly haven't seen anything to suggest that they hurt performance - I believe Volvo claims that they provide less drag. ?? Either way, I'll happily put them on my next boat.
Thank you for holding me in such high regard, although I'm not sure it's completely deserved. I've not had any hands on experience with the Volvo system for trimming boats but I have looked at it and seen the literature.
My take is that either the QL system or conventional tabs would create a very minor amount of drag compared to the overall drag of the wetted surface of the hull when trimmed down by either system. Whatever incremental difference in the amount of drag created by the two systems would likely be close to immeasurable.
It could be argued that the conventional tabs add wetted surface by virtue of their surface area, therefore they could create more drag than the QLs. I'm not saying that is a fact, just that it's another viewpoint.
I think the QL trim system is counterintuitive and that often throws people off. Without test data comparing one system with another I would not venture a guess as to which would be better as far as drag is concerned. I would, however, point out that the Volvo QL system has been out for some time now and the conventional tab mfg's don't seem to be denying the Volvo claims. A quick look at the Bennet and Lenco sites show them bashing each other in the hydraulic vs electric cylinders area but they don't mention the interceptor type system. Perhaps they don't have a good argument against the interceptors so they choose to remain silent.
Well this is why THT is so great ..... reading about other people thinkings makes us see things thorough other peoples minds, despite being a civil engineer myself my speciality has nothing to do with boats but being a glider pilot has taken me close to some aerodynamic elemental studies and to see the gliders airbrakes operating in person which are very effective you just have to deploy them a little bit and the glider sinks in the air almost instantly and gess what? most glider´s airbrakes are just a very small blade that gets inserted into the airflow on top of the wing, EXACTLY like the interceptor type of trimming devices do, them being volvo or humphreys thus there being a very high similarity, since when airplanes want to dissipate some of their kinetic energy they insert surfaces at angles close to 90 degrees into the airflow and this is what made me though this could be interesting to discuss here as has been proven by the many members taking their time to express their opinions on the subject. I will try to find in the intenet a picture of the wing of a glider with airbrakes so you all can see the similarity ...
I wonder who is qualified, studied as deeply as Volvo's engineers and can prove (not just guess) that their IPS and Qtabs are wrong.
I would refrain from saying whether their 2 systems are efficient or not. Simply because I would feel an idiot talking about something I have not much clue about, and would leave the comments to the qualified professionals
(...) despite being a civil engineer myself my speciality has nothing to do with boats but being a glider pilot has taken me close to some aerodynamic elemental studies (...)
Try to compress or expand liquid water, and you will know the difference with air.
Without talking about surface tensions, viscosity,...
Yes, Mr XV2PS, liquids are not compressible but this has not stopped scientists to develop physic laws for fluids which have a lot in common, sharing the theory and formulas that define their behaviour precisely, that is why gases and liquids have a common generic name: Fluids.
On the other hand I do believe is fair and healthy to question technical solutions offered by the trade, we should not forget the many times absurd products have been offered by engineers and companys only driven by their desire to obtain profit.
By the way, all of us here in THT talk and question many design features about boats, engines, reels or indeed anything so I do not see why or since when to question about the goodness of any product we must be engineers specialized in that field of knowledge.
And with nothing to do with you and to whom it might concern:
Stupidity From Wikipedia, the free encyclopedia
Stupidity, or dumbness, is the property a person, action or belief instantiates by virtue of having or being indicative of low intelligence or poor learning abilities. Stupidity is distinct from irrationality because stupidity denotes an incapability or unwillingness to properly consider the relevant information. It is frequently used as a pejorative and consequently has a negative connotation. The term has fallen out of favor in medical journals as it is seen as a generic term used to describe a wide variety of conditions.[citation needed]
There are many concepts in science that are counter-intuitive, and unless someone posting here has actually performed a reasonably controlled test (or even run a CFD model comparison), then I think the the criticism of the Volvo QL Tabs are pointless. I haven't seen one iota of actual data in this thread, especially from the original poster, to support the assertion that they create more drag than a conventional trim tab.
I am not splitting hairs here; many of you are arguing passionately about something that you really don't know, which is absurd to say the least. Insisting that the QL "must" create more drag, without any evidence, is very Medieval and is not likely to win you any friends amongst people that actually design and test things for a living.
Note that several people don't particularly like the QL on other grounds, and that's entirely fair. But until someone posts something even remotely resembling an actual analysis, I am going to have consider everyone arguing this to be slightly nuts.
Does anybody think for a minute that the engineers at Volvo did not do drag studies on both tabs???
As for the forward facing props; how many people have actually hit debris that significantly damaged their props? Not saying that it can't happen. But in your boating experience, how many have? I've been boating for over 40 years and have yet to hit something that damage the prop, lower unit, strut or rudder with the exception of hitting something on the bottom.
Which comes to mind; I wonder if Volvo has entertained the idea of forward facing props on stern drives and outboard engines?
The unconventional is always a "hard sell" when first introduced.
Well said, Peptide..absurd & slightly nuts....well said.
Canary- Nice definition, glad you got a Websters. But, I dont see "stupiditys" anywhere there. You left out your picture as well.........must be your "strategery".
__________________
"Alright, alright...dont rush me...I'ma thinkin...and my head hurts"- Yosemite Sam
Yes, Mr XV2PS, liquids are not compressible but this has not stopped scientists to develop physic laws for fluids which have a lot in common, sharing the theory and formulas that define their behaviour precisely, that is why gases and liquids have a common generic name: Fluids.
Another difference between aerodynamics and the tabs:
Aerodynamics have gas (air) on boths sides of the wings. On the airbrakes you refer to, the rear sides creates more drag than the front side;
On tabs, one side has water, the other does not have. I think on Q tabs, the drag on the front is compensated by a water cell that acts as a deflector, while in the back, since there is air and not water, the drag is insignificant. The front cell may create less drag than a full length tab.
However I wrote I "think" and I leave the specialists to develop.
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On the other hand I do believe is fair and healthy to question technical solutions offered by the trade, we should not forget the many times absurd products have been offered by engineers and companys only driven by their desire to obtain profit.
agreed to question. But this:
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from you on first post:
I am forced to believe that there has to be something wrong with them,
This is not a question this is a statement without any supporting facts.
There are many concepts in science that are counter-intuitive, and unless someone posting here has actually performed a reasonably controlled test (or even run a CFD model comparison), then I think the the criticism of the Volvo QL Tabs are pointless. I haven't seen one iota of actual data in this thread, especially from the original poster, to support the assertion that they create more drag than a conventional trim tab.
I am not splitting hairs here; many of you are arguing passionately about something that you really don't know, which is absurd to say the least. Insisting that the QL "must" create more drag, without any evidence, is very Medieval and is not likely to win you any friends amongst people that actually design and test things for a living.
Note that several people don't particularly like the QL on other grounds, and that's entirely fair. But until someone posts something even remotely resembling an actual analysis, I am going to have consider everyone arguing this to be slightly nuts.
To be able to provide the proof, a high speed hidrodynamic channel would be needed something which is out of reach for most but a very few universitys or large companys, still I believe it is fair to question about something that does look strange, particularly if it has looked strange to quite a few as it has been the case, Volvo has not provided in their info about the product with any drag comparison chart, only have provided some sales oriented drawings showing conventional trim tabs with an exagerated downward angle which coincidentally is good to promote the image of their product which certainly does look suspicious, if volvo do not provide any data why should we believe thye product is good? just because they want to sell it ??
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 )
Last edited by CANARIANFISHER; 11-06-2009 at 01:36 AM.
Reason: ADDITION OF TEXT
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)
Well, no better argumentation shows all the weakness of yours
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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.
None pretends, we just await your proves as you first jumped in attacking the concept
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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.
Again, you did not understand that drag was stronger due to the rear side, and in the case of Qtabs, rear side is just air not water. But again, do not hesitate to plainly add that explanation again
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As the history has proven
apparently you are not part of history
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, 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 )
Well you be sure that Volvo's competitors did not await your super engineering concepts, and if Volvo systems were so wrong, they would already have been proven as such by competitors.
I am sure you will still find a way to argue on this
