MapisM
Well-known member
Yeah, but that brings us back to my post #53... 
jfm
Well-known member
I meant to reply to this ages ago. Yep, now you say it Jimmy, you're dead right, music aficionado that you are :encouragement:I don't know, I quite liked the sound - it's almost dubstep
On the main event, I haven't got hold of the court's judgement yet. Still trying.
jimmy_the_builder
Well-known member
That's awesome
. Both my truck and van are in the shop this week for audio upgrades (I think nick_h has previously 'enjoyed' the audio system in the truck, I'm sure he'll appreciate another upgrade), I might take this along as a reference track when we do the setup. I quite like the stabiliser actuator-cum-MRI scanner effect
Nick_H
Active member
pdryan
New member
I'm probably joining in the thread a little late but I do have some comments to make on the question of drag.
My history in fin stabilizer design goes back to the 1960s when I led the design team at Vosper that came up with the Mini-Fin, the first production/"low cost" stabilizer system. (Just establishing my creds!)
The comments about wetted area , etc. are all comparing drag in a straight line in a calm sea but the whole point of a stabilizer system is to deal with rougher conditions. Almost all yacht designs these days have a transom as opposed to a canoe stern. As a result, when rolling, there is a large interaction between roll and yaw caused by the fact that the waterplane in the rolled condition is no longer symmetrical about the centreline of the boat so that it is caused to rotate in the yaw plane. (Consequently, increased work for the autopilot or helmsman depending on who is in charge.) A yawed hull has greatly increased drag relative to the same hull running in a straight line with zero yaw, and certainly a lot more than any differences due to presence or not of fins. Given this premise, it is easy to see that the use of stabilizers in anything but flat calm conditions will give rise to a reduction in overall drag and therefore more efficient operation.
I can quote an example from the days when my company was the master distributor for Naiad when they first started. Two nominally identical new Hatteras 53s (except that one was stabilized) were being delivered from the factory to Fort Lauderdale via the offshore route. The stabilized yacht ran almost 200 revs lower on the mains to keep station with the unstabilized one.
I hope that these comments help to clarify things.
Regarding electric fin stabilizers, I would expect that a considerable difference in installation costs would accrue from the reduction of hydraulic installation.
Regarding gyro stabilizers, I can see some drawbacks, the biggest being cost.
My history in fin stabilizer design goes back to the 1960s when I led the design team at Vosper that came up with the Mini-Fin, the first production/"low cost" stabilizer system. (Just establishing my creds!)
The comments about wetted area , etc. are all comparing drag in a straight line in a calm sea but the whole point of a stabilizer system is to deal with rougher conditions. Almost all yacht designs these days have a transom as opposed to a canoe stern. As a result, when rolling, there is a large interaction between roll and yaw caused by the fact that the waterplane in the rolled condition is no longer symmetrical about the centreline of the boat so that it is caused to rotate in the yaw plane. (Consequently, increased work for the autopilot or helmsman depending on who is in charge.) A yawed hull has greatly increased drag relative to the same hull running in a straight line with zero yaw, and certainly a lot more than any differences due to presence or not of fins. Given this premise, it is easy to see that the use of stabilizers in anything but flat calm conditions will give rise to a reduction in overall drag and therefore more efficient operation.
I can quote an example from the days when my company was the master distributor for Naiad when they first started. Two nominally identical new Hatteras 53s (except that one was stabilized) were being delivered from the factory to Fort Lauderdale via the offshore route. The stabilized yacht ran almost 200 revs lower on the mains to keep station with the unstabilized one.
I hope that these comments help to clarify things.
Regarding electric fin stabilizers, I would expect that a considerable difference in installation costs would accrue from the reduction of hydraulic installation.
Regarding gyro stabilizers, I can see some drawbacks, the biggest being cost.
D
Deleted User YDKXO
Guest
D
Deleted User YDKXO
Guest
Thanks for your interesting post, pdryan. Are you able to suggest average fuel consumption improvements across a range of different types of boats and speeds between stabilised and non stabilised?
MapisM
Well-known member
Better late than never! First of all, welcome to the madhouse.
Secondly, many thanks for a very interesting insight on fin stabs operations - I never considered the point you raised, but it does make good sense.
And having myself a 53' Naiad stabilized vessel (albeit not an Hatt), I was glad to read your conclusions! :encouragement:
Actually, in my experience, it takes a rather rough sea to make the boat tracking more straight when stabilized, but it does make a difference anyway - depending also on the course vs. waves, quartering seas being the worst.
May I ask you in what conditions were cruising those two Hatteras?
MapisM
Well-known member
My understanding of pdryan post is that the two vessel were able to keep the same VMG (to steal a sailing term), with the stabilized vessel running at 200rpm less.
Now, I can't estimate by heart the difference in fuel consumption (though it's possible to make some calculations), but surely 200rpm is a helluva difference, with engines that quite likely were DDs...!
D
Deleted User YDKXO
Guest
Yes a helluva difference. On my boat a 200rpm reduction in rpm at planing speed is equivalent to a 20% drop in consumption. At displacement speed, the reduction is even greater. And this is why I asked the question because the difference seems huge. The accepted wisdom seems to be that fins cause drag at speed and therefore increase consumption but pdryan seems to be suggesting the opposite. It does make sense in principle though
BartW
Well-known member
I've alway's been told, (as written in one of my posts earlyer in this threat) that due to the boat making a more straight course, (or less yaw.... !?)
that overall / average fuel consumption is slightly lower.
while we all know that fins create extra drag, wich creates more consumption
In my case, I don't have enough accurate data to give a accurate report from our experience on the stabs only;
there are many other paramaters that influence consumption, some of them a lot more,
such as fuel weight (fully loaded or not), fouling, additional weight from upgrades, the way you use the boat, etc..
But my general feeling / experience is that our stabs didn't cause a noticeable increase nor decrease of fuel consumption.
Before installing the stabs, the aspect of more fuel consumption was alway's in my mind,
now that I have them and didn't experience a noticeable difference, I completely forgot about this aspect.
MapisM
Well-known member
Precisely. My train of thoughts on pdryan post was as follows:
a) From New Bern to Ft.Lauderdale, it's an offshore ocean trip well in excess of 500nm.
b) Those 53' boats surely were LRC style boats with a SD hull, not fishermen.
c) Depending on power installed, they might cruise at up to 18 knots or so, but at the cost of an horrendous fuel burn - probably above the range of those boats.
Therefore, I guessed they were making it either at D speed, or not much above it.
Also because (obviously) the faster the speed, the less relevant the yaw effect (and its off-track implication) is.
Bottom line, I perfectly see the point pdryan made, but I'm still convinced that on a 35kts boat the fins drag (minus) can be more relevant than the tracking stabilization (plus) in the overall efficiency balance.
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pdryan
New member
Thank you, Deleted User, MapisM, and BartW for your thoughtful replies to my post.
When I wrote the post I didn't know how much interest it would generate so I kept it pretty basic. The theory of roll/yaw interaction is undeniable; my degree was in Aeronautical Engineering which included, as part of mechanics of flight, the interaction between all of the degrees of freedom. The marine equivalent is a lot simpler since the boat is much stiffer in the longitudinal plane, but in the yaw plane the effects of roll are quite marked for the reason I quoted before. That said, the effect will vary considerably from hull to hull due to different design, displacement and different hull types.
The example I quoted does rely on hearsay; I was not present on the trip. The vessels involved were Hatteras 53 Motor Yachts which had a hard chine hull designed by Jack Hargraves with DD 8V92 main engines and had a top speed around 18 knots. They were probably cruising at 15/16 knots; certainly not displacement speed. I don't know the sea state but obviously it wasn't flat calm or the effect wouldn't have occurred. However, I was present on a sea trial of a Hatteras 42 LRC, a displacement hulled boat, where, in relatively minor seas (2 to 3 feet) the wake straightened visibly when the stabs were turned on. I was also present on the post-installation stabilizer trial of a hard chine Rybovitch sport fisherman which had a higher calm water top speed after the stabs were fitted, presumably because the neutral angle of the fins was adding some lift and thus reducing the wetted area of the hull. The first installation of Vosper Minifins in the US was at the Chris Craft plant in Pompano Beach and on sea trial of the 47 Commander hard chine hull the actual stabilization was less than impressive but the reduction in steering correction was immense, about 70% less with stabs on than otherwise.
Incidentally, regarding percentages of reduction in roll which have been quoted, it is worth noting that experience over the years has shown that a reduction of 50% is barely noticeable and a reduction of 70/80% has to be achieved before it becomes truly obvious. We would always quote 90% as being "achievable" under optimum circumstances but that the true value in stabilization came from reduction in running time from point A to point B and the efficiency gained.
When I wrote the post I didn't know how much interest it would generate so I kept it pretty basic. The theory of roll/yaw interaction is undeniable; my degree was in Aeronautical Engineering which included, as part of mechanics of flight, the interaction between all of the degrees of freedom. The marine equivalent is a lot simpler since the boat is much stiffer in the longitudinal plane, but in the yaw plane the effects of roll are quite marked for the reason I quoted before. That said, the effect will vary considerably from hull to hull due to different design, displacement and different hull types.
The example I quoted does rely on hearsay; I was not present on the trip. The vessels involved were Hatteras 53 Motor Yachts which had a hard chine hull designed by Jack Hargraves with DD 8V92 main engines and had a top speed around 18 knots. They were probably cruising at 15/16 knots; certainly not displacement speed. I don't know the sea state but obviously it wasn't flat calm or the effect wouldn't have occurred. However, I was present on a sea trial of a Hatteras 42 LRC, a displacement hulled boat, where, in relatively minor seas (2 to 3 feet) the wake straightened visibly when the stabs were turned on. I was also present on the post-installation stabilizer trial of a hard chine Rybovitch sport fisherman which had a higher calm water top speed after the stabs were fitted, presumably because the neutral angle of the fins was adding some lift and thus reducing the wetted area of the hull. The first installation of Vosper Minifins in the US was at the Chris Craft plant in Pompano Beach and on sea trial of the 47 Commander hard chine hull the actual stabilization was less than impressive but the reduction in steering correction was immense, about 70% less with stabs on than otherwise.
Incidentally, regarding percentages of reduction in roll which have been quoted, it is worth noting that experience over the years has shown that a reduction of 50% is barely noticeable and a reduction of 70/80% has to be achieved before it becomes truly obvious. We would always quote 90% as being "achievable" under optimum circumstances but that the true value in stabilization came from reduction in running time from point A to point B and the efficiency gained.
MapisM
Well-known member
Many thanks to you for sharing with us your valuable knowledge.
The one on the two identical Hatteras is particularly impressive, because on those boats, 15/16 knots is already a pretty high speed, which should be enough to create some self-straightening effect, so to speak, in any kind of seas pleasure boaters would normally consider cruising in...
...Then again, delivery skippers might well have been on a tight schedule, and surely those hulls can handle some rough stuff anyway.
As I briefly mentioned, my own experience on a 53' trawler (albeit with a pure displacement round hull, with a deep and long keel, typically cruising between 7 and 9 knots) is that by turning on the Naiads the reduction in the a/p work becomes perceivable with at least 3' waves (if in quartering/following seas), and even worse in beam/head seas (where the rolling reduction is instead VERY perceivable, like day and night!).
I'm also surprised by what you said on the fisherman achieving a higher top speed after installing fin stabs, due to their lifting/wet surface reduction effect.
In this very same thread, I argued with jfm who basically said the same, because if that were generally true, I guess it wouldn't cost a lot to stick to any planing hull a couple of fixed plastic fins, just to make the boat more efficient...?!? But obviously, according to your experience, it can happen.
I'd be tempted to estrapolate that if and when that happens, it's probably the P hull design that could have been better, to start with.
But let's not open that can of worms!
The one on the two identical Hatteras is particularly impressive, because on those boats, 15/16 knots is already a pretty high speed, which should be enough to create some self-straightening effect, so to speak, in any kind of seas pleasure boaters would normally consider cruising in...
...Then again, delivery skippers might well have been on a tight schedule, and surely those hulls can handle some rough stuff anyway.
As I briefly mentioned, my own experience on a 53' trawler (albeit with a pure displacement round hull, with a deep and long keel, typically cruising between 7 and 9 knots) is that by turning on the Naiads the reduction in the a/p work becomes perceivable with at least 3' waves (if in quartering/following seas), and even worse in beam/head seas (where the rolling reduction is instead VERY perceivable, like day and night!).
I'm also surprised by what you said on the fisherman achieving a higher top speed after installing fin stabs, due to their lifting/wet surface reduction effect.
In this very same thread, I argued with jfm who basically said the same, because if that were generally true, I guess it wouldn't cost a lot to stick to any planing hull a couple of fixed plastic fins, just to make the boat more efficient...?!? But obviously, according to your experience, it can happen.
I'd be tempted to estrapolate that if and when that happens, it's probably the P hull design that could have been better, to start with.
But let's not open that can of worms!
pdryan
New member
Re the higher speed after installation story. I suspect that the fins were installed with their neutral position straight fore and aft ( i.e. parallel to the keel) which is not the correct position on a planing hull. I always installed the fins with at least 3 degrees angle to the keel with the trailing edge out on any planing or semi-planing hull.
MapisM
Well-known member
I perfectly see your point.
As I understand, on some modern systems, it's even possible to self-adjust dynamically the neutral fins center, because for any given hull, the angle you mention depends also on the speed, to some extent.
Otoh, if in a specific P hull the addition of a couple of fins parallel to the keel brings more benefits (in terms of higher lift and lower wet surface) compared to the obviously negative effect of additional fins drag, I can't help but think that the hull doesn't have enough inherent lift, to start with....
As I understand, on some modern systems, it's even possible to self-adjust dynamically the neutral fins center, because for any given hull, the angle you mention depends also on the speed, to some extent.
Otoh, if in a specific P hull the addition of a couple of fins parallel to the keel brings more benefits (in terms of higher lift and lower wet surface) compared to the obviously negative effect of additional fins drag, I can't help but think that the hull doesn't have enough inherent lift, to start with....
rwoofer
Active member
I perfectly see your point.
As I understand, on some modern systems, it's even possible to self-adjust dynamically the neutral fins center, because for any given hull, the angle you mention depends also on the speed, to some extent.
Otoh, if in a specific P hull the addition of a couple of fins parallel to the keel brings more benefits (in terms of higher lift and lower wet surface) compared to the obviously negative effect of additional fins drag, I can't help but think that the hull doesn't have enough inherent lift, to start with....
i suspect the lift drag ratio of a fin is far better than you could achieve with the hull surface. Isn't that the basis of hydrofoils?
MapisM
Well-known member
Well, based on what pdryan told us, your suspicion is obviously correct, at least for some Rybovitch boats.
What I'm saying is that if this would generally be true, most P hulls would have been designed from the beginning with some fin-alike appendixes.
Otoh, I'm not aware of any P boats like that, as well as I'm not aware of hydrofoil boats which weren't specifically designed as such.
Besides, each and every ultra-fast boat, bar none, have as little stuff touching the water as physically possible, for very obvious/intuitive reasons - see my previous post #94 for a practical example...
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rwoofer
Active member
I genuinely think people didn't think of it. You either were a hydrofoil or you weren't and in between wasn't considered. Why put fins on and all that entails if you weren't going to take full advantage of it....
If you look to the sailing world it is only in the last few years that appendages with a vertical lift element, that are not full on hydrofoils, are actually being explored. There is DSS which is a horizontal foil to counteract heeling. There are the new IMOCA 60s that now have extremely curved daggerboards to create lift, but not lift the hull out of the water. These appendages have only become viable as performance enhancers due to advances in hull design and materials. Put a vertical lift fin on a 70s IOR racer and it did probably slow it down.
MapisM
Well-known member
Mmm... I've seen all sort of weird ideas for increasing the lift and/or reduce the wet surface in planing hulls, some of them also rather sophisticated.
It would be funny if such rather cheap and simple addition could increase the efficiency of each and every P hull, and nobody ever gave it a try.
But what do I know? Time will tell, I suppose...
