Humphree stabilisation system?

[BartW]

Bart, have you experienced any meaningful loss of top speed, or increase in fuel burn, since fitting the stabs to BA?

as you remember my observations from 2,5 y ago, I didn't notice any meaningfull difference in speed nor fuel burn after fitting the stabs,
the only occasion that I sailed her at full throttle that I remember, was at the end of last years season, (with the broken vibration damper as a result ) and I could still reach 26kn, in a not perfectly flat see, but still approx the same speed like during the first sea trial in 2011.

I always did my range calculations on 300l/hr /20nm (or 15l/nm), also during the long trip from Rome to Montenegro, for the P parts, and that alway's worked
last season, I think I was slightly over that, (no accurate data) but we have a lot more weight onboard; .
appart from all the extra's we've build, we have a heavyer tender, permanently 8 scuba tanks, 100kg dive weights, 50% more anker chain, and on average a lot more fuel in the boat (we top up each occasion that we leave Montengro with taks free fuel).
I agree with your that more weight has a bigger influence on fuel consumption.
but unfortunately no real accurate figures, I can't bother the effort for installing fuel flow measurement instruments.
We regularly sail at D speed, the region invites or sometimes obliges to do so (all the Kotor bay is max 7 or 10kn)

 

[Deleted User YDKXO]

Have you spoken to Seakeeper about the results you'd achieve with a model 9? My gut feel is it would still have a big effect, as a model 8 with 15% less angular momentum worked well on a 36 tonne Squadron 65. Also worth a pm to A&K to see what kind of deal they negotiated for theirs.

Thanks Nick. I did actually speak to Seakeeper on their stand at Dusseldorf last year but the guy I spoke to was very unimpressive so we didn't actually get to the point of talking about suitable models. My boat at 40t is quite heavy for its length which is why I thought I would have to go to a 16 but if they could assure me that a 9 would give acceptable results, that makes it much more attractive

Is it A&K who have the Sq65 and did Seakeeper actually advise them that an 8 would be adequate?

What model do you have in your boat?

 

[Deleted User YDKXO]

...otoh, I can't think of where Ferretti stick their OEM gyro in the e/r - Either I don't remember your e/r spaces very well, or it must be pretty small!

They stick it at the aft end of the e/r in the centre and as far as I can see from the few pics I've seen, it does take up a fair bit of space and makes getting around the back of the engines much more difficult. IMHO the main reason that Ferretti don't fit the gyro in the lazarette under the bathing platform ('spoiler' in Ferrettispeak) is that they 'sell' that space as a storage space for a jetski which means that owners can carry both a RIB and a jetski. Yes it is a salty wet environment but I'm not sure its much saltier or wetter than the engine bay. I'm fairly convinced that most of the water in my spoiler leaks through the hatch as the rubber seals are not in great condition - job for Andrea in the winter I think

 

[Nick_H]

Thanks Nick. I did actually speak to Seakeeper on their stand at Dusseldorf last year but the guy I spoke to was very unimpressive so we didn't actually get to the point of talking about suitable models. My boat at 40t is quite heavy for its length which is why I thought I would have to go to a 16 but if they could assure me that a 9 would give acceptable results, that makes it much more attractive

Is it A&K who have the Sq65 and did Seakeeper actually advise them that an 8 would be adequate?

What model do you have in your boat?

Don't get me wrong, I'm sure Seakeeper would advise a 16 on your boat, and a 9 would be a compromise, but it may be one that's worth making if you still got worthwhile results at a lower cost. Also, as MapisM said, the metacentric height is highly relevant to gyro performance, so if you could provide that measurement for your boat then Seakeeper may be able to advise what performance to expect.

The Sq65 was a BCU supplied boat that took over my previous charter. The charter captain is a friend, and told me that the results with the model 8 were still very good, but I didn't get any more specific info. If you wanted more detail then I'm happy to speak to him again, or even put you in touch directly. He's Swedish, but speaks perfect English. I don't know if Seakeeper were involved in the sizing decision.

There's also a test report here for a Model 8000 on a Princess 64 which shows excellent results, though the waves were small for the test, and the boat is a fair bit lighter than yours at 31,500 kgs. http://www.nemo-ms.com/telechargements/trial-report/princess-p64.pdf. Nemo Marine fitted mine and A&K's, and they seem a good company and I think they work all over the W Med.

I have an 8000 model on my 57' 26T boat, and it almost completely removes any roll at anchor, most of it at displacement speeds, and still has a significant effect at planning speeds

 

[A&K]

Hi Mike, we have a Sq58 - so virtually identical to Nick's P57. We have the seakeeper 9 fitted. Very happy with the results and also the fitting by Nemo. They do good work - esp in the context of the SoF where the workmanship can be mixed! Happy to chat further if you'd like.

 

[Cheery]

The pdf is very interesting but it gives a very different figure to this test.

https://www.youtube.com/watch?v=SNJ88mBusbE

 

[Deleted User YDKXO]

There's also a test report here for a Model 8000 on a Princess 64 which shows excellent results, though the waves were small for the test, and the boat is a fair bit lighter than yours at 31,500 kgs. http://www.nemo-ms.com/telechargements/trial-report/princess-p64.pdf. Nemo Marine fitted mine and A&K's, and they seem a good company and I think they work all over the W Med.

Wow thats an impressive report in terms of the roll reduction data. In your experience, given that many boats (like Princesses) have a lazarette aft of the engine bay, is that where the gyro is usually mounted? I seem to remember that at the time you fitted your gyro you reported that the extra weight had no effect on performance or handling. Is that still your view now?

 

[Deleted User YDKXO]

Hi Mike, we have a Sq58 - so virtually identical to Nick's P57. We have the seakeeper 9 fitted. Very happy with the results and also the fitting by Nemo. They do good work - esp in the context of the SoF where the workmanship can be mixed! Happy to chat further if you'd like.

Thanks A & K

 

[Nick_H]

Wow thats an impressive report in terms of the roll reduction data. In your experience, given that many boats (like Princesses) have a lazarette aft of the engine bay, is that where the gyro is usually mounted? I seem to remember that at the time you fitted your gyro you reported that the extra weight had no effect on performance or handling. Is that still your view now?

I certainly don't claim to understand all of the complex physics in the report, and to be honest the way it's added reminds me of a shampoo advert. The claimed 95% roll reduction is at the resonant frequency, whereas you could equally draw the conclusion that reducing an average 5 deg roll to 1.5 deg is a 70% reduction.

Having said that, it's the resonant roll that really matters, because that's the one that sends plates flying, locker doors open etc., and in those situations having a gyro vs. not having one is like night and day. We have sat comfortably on the back of our boat with wine glasses on the table watching the occupants of similar sized boats hanging onto guard rails or bracing themselves against furniture when a big swell comes through. We've also been the only boat of our size that has stayed in an anchorage when everyone else headed back to port.

Yes, many Princess models have a lazarette just aft of the engine room and this is an ideal place to fit the gyro. I do remember that Seakeeper said the units can withstand salt spray, so they don't need to be in a perfectly dry environment, but of course they are expensive units and long term durability is therefore an important issue, so the more protected the better.

If you're seriously considering a gyro then I would direct your technical questions to the factory in the US, not via dealers. As pleasant as Nemo Marine were to deal with, I'm not convinced they have the in depth technical knowledge to comment on borderline applications like yours would be with a model 9.

If there was any reduction in speed after fitting the gyro then it was small fractions of a knot.

 

[Veejayroy]

Wow thats an impressive report in terms of the roll reduction data. In your experience, given that many boats (like Princesses) have a lazarette aft of the engine bay, is that where the gyro is usually mounted? I seem to remember that at the time you fitted your gyro you reported that the extra weight had no effect on performance or handling. Is that still your view now?

Hi Mike, The Gyro can be mounted midship to aft, wherever you can find the space. It does not need to be aft of the engine bay. Princess does it that way, other manufacturers, Azimut, Fairline etc. put it somewhere in the engine room, sometimes even the crew cabin. It is a space/access consideration not a performance based call.

One of the factors to consider through in a retrofit more than the weight addition is the weight balance. If you install the Seakeeper off centreline, maybe move the battery pack or watermaker to the other side to balance the weight.

The weight affecting performance, I would say for a 40 ton boat you are looking at a Seakeeper 5. At 350 kilos, that is pretty much like 370 Ltr of Fuel or a few fat guys like me on board.

 

[Deleted User YDKXO]

If you're seriously considering a gyro then I would direct your technical questions to the factory in the US, not via dealers. As pleasant as Nemo Marine were to deal with, I'm not convinced they have the in depth technical knowledge to comment on borderline applications like yours would be with a model 9.

Thanks again Nick. It seems that the factory watches this forum because I've just been contacted by them by PM

 

[Deleted User YDKXO]

Hi Mike, The Gyro can be mounted midship to aft, wherever you can find the space. It does not need to be aft of the engine bay. Princess does it that way, other manufacturers, Azimut, Fairline etc. put it somewhere in the engine room, sometimes even the crew cabin. It is a space/access consideration not a performance based call.

One of the factors to consider through in a retrofit more than the weight addition is the weight balance. If you install the Seakeeper off centreline, maybe move the battery pack or watermaker to the other side to balance the weight.

The weight affecting performance, I would say for a 40 ton boat you are looking at a Seakeeper 5. At 350 kilos, that is pretty much like 370 Ltr of Fuel or a few fat guys like me on board.

Thanks Veejayroy but how does that square with this statement on your website?

The Seakeeper 5 is designed to achieve 70 to 90% roll reduction on boats up to 20 tons, vessels 30-50'

 

[Veejayroy]

My bad Deleted User, It was quite late at night and for some reason I thought it was a 40 foot boat and not a 40 ton boat.
You are absolutely right and the website is also correct. I apologize again.

 

[MapisM]

The data shows that a pair of 1msq fins requires about 100bhp at 25 knots

I'm returning on this point because I was a bit surprised by this number the first time I read it, but I didn't want to argue with it based on gut feeling alone.

Now jfm, may I ask you how was that measured?
I suppose the only accurate way to do that would require using the very same boat, exactly in the same conditions (fouling, fuel load, sea state, etc.), with and without the fins.
But removing the fins (not to mention the shafts!) is not a trivial job by all means - I can't imagine that being done within the same day of testing.

Anyway, the reasoning which revamped my initial gut feeling is somewhat complicated - bear with me, because I couldn't find any major flaw in the following logic, but maybe you can.
Yesterday I met an old friend in Lake Como. He began building small(ish) speedboats well before GRP was around, though he's only using GRP nowadays, obviously.
Currently, his larger boat is a 15m open cruiser, for which a few clients asked to fit some sort of stabilizers, which is why we ended talking of them.
I mentioned him the fin drag ballpark you mentioned, but he made by heart a sort of sanity check, with interesting results.

First of all, one premise: according to him, in any decent P hull, the power requirement at constant cruising speed is strictly driven by its wet surface.
I argued that afaik weight is equally important, and I had some numbers in mind to prove it, based on some first hand experiences.
But his objection was that the only meaningful effect of weight (remember, at constant speed) is the higher hull pressure on the water, and/or a different AoA. Regardless, both affect the overall wet surface.
Which brings back to his point: the required power, at any given speed, mostly depends on the wet surface.
I must say that in spite of the apparently simplistic reasoning, I think he has a point.
Surely that's his conclusion anyhow - strictly based on experience admittedly (as opposed to CFD or other hi-tech systems), but we are talking of almost 40 years of it, no less...
Btw, coming to think of it, all the tricks used in modern speedboats (stepped hull, notched transom, air entrapment, padded keel... You name it) at the end of the day are all meant, one way or another, to reduce the wet surface.

Anyhow, the rest of the reasoning he made for the following "sanity check" is based on the previous premise.
1) in terms of wet surface, a pair of 1sqm fins add 4sqm overall.
2) 4sqm is just about the wet surface of one of his most successful boats, a 25 footer, once steadily planing.
3) This boat is normally equipped with either a single 300hp or twin 200hp engines.
4) According to him, 100hp are nowhere near enough to keep her steadily cruising at 25knots. He was unable to give me an accurate number by heart (though he told me that he can check that, by digging in his files), but he thought that it's bound to be nearer to 200, rather than 100.
5) Last but not least, he confirmed me that acceleration aside, where the 2x200 version of the same boat obviously has an edge, the max speed is practically identical with the single 300.
But, and it's a big but, the weight difference (hence the wet surface increase) is almost negligible, and could be compensated by running the single engine boat with a full tank against the twin with a lighter load.
So, where are those 100hp disappearing? There are obviously more overall losses in two 200hp engines+transmissions, compared to one 300, and this is one factor.
But according to him, most of the difference is due to the higher drag introduced by a second outdrive.
In fact, he believes that when it comes to any appendices stuck inside the water, the drag increase is a multiple compared to the simple wet surface rule that he uses for the hull alone.
In other words, no matter how hydrodinamically optimized a lower leg can be (and obviously Merc, VP, ecc. put all their best efforts in that), its drag is much higher than their wet surface alone would imply.
The reason being that the hull surface is only "sliding" against the water, with no single parts actually creating a direct resistance opposed to the water flow (aside from transducers or seacocks, which you also mentioned but are obviously much smaller in size), while the outdrive cone must actually "drill a hole in the water", to use his wording.
Along the same lines, the frontal surface of each fin, even if obviously much smaller than the overall wet surface, is bound to introduce an additional drag which is even higher than the one implied by the wet surface alone.
According to him, such effect might be worth at least doubling the above mentioned effect, at 20 knots. But he believes that it would be at least four times that, at 30 knots - these being the typical speeds he's more used to work with; I suppose it's safe to say that it'll be somewhere in between at 25 knots...
Bottom line, his strictly experience-based (as opposed to instrumentally measured) estimate is somewhere between 300 and 400 hp, for the power required to push a couple of 1sqm wings through the water at 25 knots.

All that said, I still would rather have fins in any P boat with a reasonably high roll period (which as a rule of thumb means well above 70'), particularly if I were thinking to use her at cruising speeds not exceeding the 20kts ballpark.
But for anything with a shortish RP and/or meant for faster cruising speeds, gyros are a much better compromise imho.
And fwiw, the above builder I spoke with is not even remotely considering fins, for his 15m boat that he's being asked to fit with some kind of stabs.

 

[jfm]

Mapism:

The ~100hp (per fin pair, btw, not per fin) is a ballpark number but Sleipner (and I expect the others) do have much data computed from CFD. It is a widely observed fact that fins remove only a knot or so of top speed typically on a 24m boat with 1msq fins. My boat hits the same speed as other sq78 with cat C32, within a knot. Princess insiders report the same for their fleet

I can see the guy's thinking but he is missing so much that he has a seriously wrong answer imo. P boats drag is very clearly not just a function of surface area and friction. P boats, except at very high speeds, are substantially held up by Archimedes even while planing, and the hull drills a huge hole in the water to use your guy's analogy, or mills a large slot in the water to use an even better analogy. The "pumping energy" needed to move quickly that much water is the major part of P boat drag, not just surface area friction drag. That as I see it is the mistake in your guy's thinking.

If he were correct that "somewhere between 300 and 400 hp, for the power required to push a couple of 1sqm wings through the water at 25 knots" then BartW and I for starters would see that. These boats are ballpark 1800hp total in cruise, and you couldn't not feel a substantial change if 3-400 hp got taken out. Bart reports above no meaningful speed difference on BA after the fins were fitted (ok they are 0.6m but still significant)

That's how I see it on a quick bash-the-keyboard reply. Happy to hear contrary view but I do firmly disagree that P boat power requirements at 25knots are based mainly on surface friction drag- P boats are substantially held up by Archimedes and not by waterskiing effect

 

[MapisM]

I do firmly disagree that P boat power requirements at 25knots are based mainly on surface friction drag- P boats are substantially held up by Archimedes and not by waterskiing effect

Well, but what's the difference?
I mean, when waterskiing, your effort to stay attached to the line very much depends on your weight, AOTBE.
And when a 50kg boy or a 80kg adult are using the same ski, that difference is eventually reflected in a higher wet surface (more of the same ski touching the water) - what else?
Not saying you don't have a point, but I fail to understand it.

 

[jfm]

what else?

The what else is drag other than just surface area friction. The ski is a foil like a plane's wing, and the 80kg guy will have more foil area at same AoA, or the same area but with bigger AoA, or some combo. Either way, that means more drag, and it is not surface area drag it is water pumping drag. I mean if 80kg and 50kg both have 80cm of the ski in the water, so same surface, but Mr 80kg has more AoA, then the harder pull/increased fuel burn for mr 80kg is ALL due to water pumping effort not surface friction. Or if same AoA but mr 80kg has 90cm of ski in water and 50kg has 60cm, then there is more water pumping work done by Mr 80kg's ski, and the surface are drag of his extra 30cm is neither here nor there

This isnt a perfect analogy with P boats because they have Archimedes effect too, but I think (hope!) answers your q, or at least is some more food for thought!

I'm signing off for a bit... --->airport--->Antibes now

 

[MapisM]

All the best for the weekend weather then - afaik it's pretty awful atm, around Liguria/SoF...

But in the meantime, a few further thoughts, because I perfectly see what you mean, and that's indeed some food for thought.
Btw, I think that also your analogy is correct - in fact, I believe that Archimedes didn't even consider what happens dynamically (=at P speed), where I suspect that the relevance of his principle is close to nothing - also for boats, not just for "pure" foils.
Shouldn't we ask Newton rather than Archimedes to explain us why P boats behave like they do?

Anyway, theory aside, I agree that the heavier skier could still float by increasing more the AoA than the wet surface alone, but I suppose that this was already considered by the guy I was discussing with, when he specified what I translated as "any decent P hull" in my previous post.
In other words, the heavier skier would be a poor one in your assumption, because the optimal AoA of the ski remains the same regardless of the skier weight, and shifting the weight back instead of finding a proper balance by flexing the knees as required is a basic but very big mistake.
In fact, that's NOT what you would expect from any proper P hull, whose AoA (once over the hump) is almost constant.
Which means, at the end of the day, that for any given hull shape (with its characteristic AoA), a balanced weight increase/decrease is bound to be reflected in sitting deeper/higher in the water, hence with a larger/smaller wet surface.
And in this respect, I still can't see a fault in the reasoning of that chap.

Otoh, it's rather the equivalency of the wet surface effect between a hull and a fin which, coming to think of it, is a bit of a stretch.
I mean, at least when a fin is in its "neutral" position, the surface drag alone must be somewhat lower compared to a hull, which by definition is never "neutrally" sliding against the water, because no matter how low its AoA can be, it obviously can't be zero.

Yep, it's more complicated than that - also because the fins, while working, are constantly changing their AoA. Which surely is even higher than the one of the hull sometimes, but not always.
Otoh, the effect of the frontal part of the fin, which has to cut its way through the water, is surely (and always) more relevant than the surface drag, but it's hard to estimate it.
Oh, well. It was an interesting speculation anyway.
Also because, if nothing else, the risotto with persico fish that I had yesterday at lunch with that chap was excellent!

 

[BartW]

Otoh, the effect of the frontal part of the fin, which has to cut its way through the water, is surely (and always) more relevant than the surface drag, but it's hard to estimate it.

well this was the only comment I had during this whole discussion about wet surface,
in that respect I've been wondering where is the best relation between fin length and fin width;
I think that a longer fin (from front to tale end) has more effect at zero speed,
and a smaller fin width (a shorter shaft) has less frontal surface, so less drag ?
but I guess that most fin manufacturers have done their homework and calculations for this,
most of them have fluidodynamic 3D simulations on their webside, so they have done the maths.

 

[jfm]

Hi mapis. That was interesting stuff. I don't buy it all and it is complex, plus I'm sitting on a plane on iPhone only

The main issues are that I think you and lunch partner think Newton not Archimedes floats a p boat. I don't think that's anywhere near correct until you get to fast stuff. As long as the hull is underwater not skimming, as is mostly the case on a p boat apart from at the bow, Archimedes is at work. You can't stop him. Newton is also at work but he is perhaps taking 15 tonnes of a 50 tonne boat at 25 kts, leaving Archimedes holding 35. A hydrofoil boat would be all Newton but not a plastic fantastic cruiser

Regarding this.....
<<First of all, one premise: according to him, in any decent P hull, the power requirement at constant cruising speed is strictly driven by its wet surface.
I argued that afaik weight is equally important, and I had some numbers in mind to prove it, based on some first hand experiences.
But his objection was that the only meaningful effect of weight (remember, at constant speed) is the higher hull pressure on the water, and/or a different AoA. Regardless, both affect the overall wet surface.
Which brings back to his point: the required power, at any given speed, mostly depends on the wet surface.>>
... I think lunch partner is very mistaken. The first sentence is profoundly wrong IMHO. First, the higher hull pressure on the water is achievable by increased a of a and to the extent that doesn't happen enough the Archimedes simply works harder as the hull sinks deeper into water that has higher pressure. With Archimedes, drag increases due to drilling a bigger hole ie pumping losses. With Newton, if a of a increases you also have more pumping losses as the higher a of a pushes more water out of the way. Surface friction drag is the smallest thing to consider. Hence my big problem with first quoted sentence above

It is observable that barts and my boats haven't slowed down much with fins. I have a wealthy friend with an x yachts 55 and he had his rough roller-applied anti foul paint changed for very expensive spay applied smooth anti foul. It was like spray matt white paint. Speed difference would need a laboratory process to be noticed

The drag of fins is all Archimedes hole-drilling drag with only a tiny surface friction component.

As a geeky comment Newton drag and Archimedes drag can overlap of course but I don't think that changes anything

I think!