Fairline squadron 58

[MapisM]

FP, I think you're working on the premise that a flatter hull generates more Newtonian lift than a deep Vee, for the same horizontal component of hull surface area. But it doesn't.
On an AOTBE basis, the lift of the flat hull is surely the same as the deep Vee, isn't it?

I see your point J, and as I said I'm not even trying to argue on the physics theory.
But don't you think that in my previous empirical example, the "slamming" of the flatter hull is just the evidence/result of a higher Newtonian lift (which is what I hinted at by mentioning "action-reaction") upon wave impact?
Or to put it the other way round, what else do you think that can explain the higher slamming, if the dynamic lift would be exactly the same, as you are saying?

At a guess, the fly in the ointment of your theory is that fluidodynamics work differently depending on the AoA of any surface (and also other factors like speed and density, but that's neither here nor there in this context).
Otoh, my lazy old brain gets warmer just at the thought of going into a theoretical debate about that...

There's another empirical example popping to my mind, though: when we were kids, we instinctively picked the flatter stones we could find, to make them jump over the water.
Now, let's imagine a spherical stone with the same horizontal component of its surface as a flat stone.
Why does the latter work better than the first, for the purpose of "planing" over the water?
I can't say that the one and only reason is the higher Newtonian lift created by the different AoA in the surface of the two stones (as opposed to their H component, which as per assumption is exactly the same).
But if there's another reason, I can't for the life of me think what it is.
If you can, I'm all ears.

 

[stillwaters]

FP, I think you're working on the premise that a flatter hull generates more Newtonian lift than a deep Vee, for the same horizontal component of hull surface area. But it doesn't. On an AOTBE basis, the lift of the flat hull is surely the same as the deep Vee, isn't it?

Now I know I shouldn't bite on this, however as I imagine you must know this isn't the case, have you by now reached the wind-up stage, maybe? In which case I really shouldn't have bitten on it, should I?

 

[jfm]

Mapism, thanks, and I expected you would see the point. Happy to be argued against: I'm just saying it as I see it and am not wishing to get into detail on the complex aspects of fluid mechanics. I'm concentrating merely on the basic Newtonian aspects, plus mentioning in passing the drag reducing effect of spray/lift rails (but that is a different point from the basic lift of a planning hull). I'm trying (possibly not with much success!) to dispel a myth that a flat surface makes more lift than a V surface, AOTBE. Everyone remembers the mercury outboard promotion pic of the upside down table planing!

1. The slamming is (I think) easily explained by the rate of change in lift. If a boat is steady-state planing and the bow hits a wave, the additional lift of the flat hull where it hits the wave occurs 100% instantaneously = slam. With a V hull the wave first hits the point of the Vee making a small amount of lift, then as the boat continues to drive through the wave the surface area creating the lift increases gradually, so there is less slam. Like jumping onto a mattress not concrete. Indeed that is surely the very point of a Vee hull so far as smooth ride is concerned.

2. The skimming stone is easy. The spherical stone could only creates the same lift as the flat stone if a near-hemisphere of stone is submerged, and the drag of that stops the stone instantly. Conversely, the flat stone can generate same lift with almost zero appendage drag (so to speak), so it keeps on going. There is an additional effect not relevant to P boats which is that the trailing half of the "hull" of the spherical stone is half a hemisphere pointing the wrong way so far as Newtonian lift is concerned, so it creates less or zero lift. Both these are significant but I expect the "appendage drag" aspect is more important.

If you think that's rubbish I know you will tell me :encouragement:

 

[jfm]

Now I know I shouldn't bite on this, however as I imagine you must know this isn't the case, have you by now reached the wind-up stage, maybe? In which case I really shouldn't have bitten on it, should I?

I don't do troll-ish wind ups

 

[stillwaters]

I don't do troll-ish wind ups

Fair enough. Unfortunately I'm busy elsewhere for a while, however, time permitting later I'll give you some real life/non myth/ non-books reasons why that just aint so.

 

[[2068]]

FP, I think you're working on the premise that a flatter hull generates more Newtonian lift than a deep Vee, for the same horizontal component of hull surface area. But it doesn't. On an AOTBE basis, the lift of the flat hull is surely the same as the deep Vee, isn't it?

On smooth water, the upwards force on a uniformly flat hull will be the same as the upwards force on the uniform deep vee. The deep vee will simply settle at whatever point the forces from the water / weight / flotation etc. all balance.

But this isn't the point.

In the real world, most boats do not have an even weight distribution front to rear.
They are not required to be able to go 35kts backwards as well as 35kts forwards.
They need to be able to cope with waves - to not stuff the bow in a following sea, nor to crash and bang upwind.
So you can create a finer deep vee at the front that produces less lift relative to the flatter sections behind, the deep vee part having a smaller cross-sectional area in the water.

As an exercise, try drawing a 10m boat looking from underneath and divide it up into 1m sections lengthwise.
Mark out the wetted surface area.
Now try and design your boat such that it has a pointy bow and roughly equal cross-sectional area in contact with the water in each 1m section.
Let us know how this looks

If this doesn't go well, we might move to agreeing that the area in contact with the water might vary along the length of the boat.

.

 

[jfm]

On we might move to agreeing that the area in contact with the water might vary along the length of the boat.
.

Not wanting to be difficult, but I totally don't follow the point you're trying to make and we might be at crossed purposes. Ref the above, we don't need to move to agree; I already agree with you and don't think I've said otherwise. However, I have always referred to the horizontal plane component of the hull's surface area, which is obviously different from the hull's actual surface area that you seem to be dealing with in your "exercise".

The only point I'm making is in that there is no material difference in the basic Newtonian lift of a flat hull and a Vee hull (AOTBE, especially the horizontal plane component of hull surface area). Thus, the idea that a boat planes "on" the flat strip surfaces of its spray/lift rails and its chine flats is a myth.

 

[[2068]]

1. The slamming is (I think) easily explained by the rate of change in lift. If a boat is steady-state planing and the bow hits a wave, the additional lift of the flat hull where it hits the wave occurs 100% instantaneously = slam. With a V hull the wave first hits the point of the Vee making a small amount of lift, then as the boat continues to drive through the wave the surface area creating the lift increases gradually, so there is less slam. Like jumping onto a mattress not concrete. Indeed that is surely the very point of a Vee hull so far as smooth ride is concerned.

I think this is correct.
"With a V hull the wave first hits the point of the Vee making a small amount of lift"
In flat water or small waves, the point of the Vee isn't generating much lift relative to the rest of the hull.

 

[[2068]]

Not wanting to be difficult, but I totally don't follow the point you're trying to make and we might be at crossed purposes. Ref the above, we don't need to move to agree; I already agree with you and don't think I've said otherwise. However, I have always referred to the horizontal plane component of the hull's surface area, which is obviously different from the hull's actual surface area that you seem to be dealing with in your "exercise"..

Okay, try doing it with roughly equal sections of the horizontal plane component of the hull's surface area ... you will have the same challenge. It will vary from stern to bow. So "for the same horizontal component of hull surface area" doesn't really apply, because on a real life hull, it isn't the same at the stern as at the bow.

 

[MapisM]

If you think that's rubbish I know you will tell me :encouragement:

LOL, yeah, no worries. I'm sure you would reciprocate the favor, btw!
Ok, forget (2). In hindsight, I agree that there aren't enough similarities with a hull to qualify it as a proper example.
But ref (1), your objection seems to assume that time is a secondary factor, while it's essential, when talking of dynamic/Newtonian (as opposed to static/Archimede) lift.
Sorry, I must rush out now and I'll be driving for the rest of the day, but just a quick thought: as I understand, you agree that landing on a mattress is different than on concrete.
In this respect, rails surely contribute to make a hull more concrete-ish, don't they?

 

[jfm]

Okay, try doing it with roughly equal sections of the horizontal plane component of the hull's surface area ... you will have the same challenge. It will vary from stern to bow. So "for the same horizontal component of hull surface area" doesn't really apply, because on a real life hull, it isn't the same at the stern as at the bow.

Honestly not trying to be difficult or evasive, but I am not seeing your point. Sure, it will vary from stern to bow, but what has that got to do with the only point I'm making which is that flat and V hulls make the same lift aotbe (ie a boat doesn't plane "on" its flat rails or flat chines)? Ref your last sentence we might be at crossed purposes: I have never said that HC of hull area doesn't vary from bow to stern; of course it varies, but that has nothing to do with my point.
Sorry if being thick in not getting your point.

 

[jfm]

LOL, yeah, no worries. I'm sure you would reciprocate the favor, btw!
Ok, forget (2). In hindsight, I agree that there aren't enough similarities with a hull to qualify it as a proper example.
But ref (1), your objection seems to assume that time is a secondary factor, while it's essential, when talking of dynamic/Newtonian (as opposed to static/Archimede) lift.
Sorry, I must rush out now and I'll be driving for the rest of the day, but just a quick thought: as I understand, you agree that landing on a mattress is different than on concrete.
In this respect, rails surely contribute to make a hull more concrete-ish, don't they?

No worries - happy driving. I hope in a nice car :encouragement:
Ref (1), time is a factor. The V creates a slower acceleration of the bow (upwards) as the boat hits the wave, thus reducing slamming. Time is essential to that analysis - the dynamic Newtonian force (or indeed the buoyance force) ramps up over time in a V hull thus creating a lower acceleration when hitting the wave. Mattress/concrete.

Yes i suppose rails are more concrete-ish (for the split second they contact the wave surface) than mattress, but that effect is (a) tiny compared with the drag reducing benefits (and convenience benefits) of separating the water flow from the hull, plus (b) so small that it could easily be compensated by other (also small) complex hydrodynamic benefits of the rails

 

[[2068]]

Honestly not trying to be difficult or evasive, but I am not seeing your point. Sure, it will vary from stern to bow, but what has that got to do with the only point I'm making which is that flat and V hulls make the same lift aotbe (ie a boat doesn't plane "on" its flat rails or flat chines)? Ref your last sentence we might be at crossed purposes: I have never said that HC of hull area doesn't vary from bow to stern; of course it varies, but that has nothing to do with my point.
Sorry if being thick in not getting your point.

My point is that saying that "flat surfaces provide lift while angled ones by implication (the surface of a V hull) don't" is incorrect, is misleading in the context of an actual hull that people might buy.

Back to the real world: if you look at the hull of a Broom 42, it has an almost entirely flat section at the stern, and a reasonably deep forefoot, it planes readily at low to moderate speeds due to the stern sections generating relatively more lift vs. the bow section. If you were to put massive engines inside and go for 50kts, it would be un-driveable: the bow would stuff.

Now consider a similar boat where the Vee section only flattens off a little towards the stern ... It won't plane so readily, but with a but of trimming and plenty of power, the stern lift vs. bow lift will be more balanced, mainly due to the horizontal component of hull surface area not varying so drastically from stern to bow.

i.e. the Vee sections provide less lift not because they are Vee shaped, but because they alter the shape of the contact area between the hull and the water as the speed varies.

.

 

[jfm]

My point is that your assertion that "flat surfaces provide lift while angled ones by implication (the surface of a V hull) don't" whilst not exactly incorrect, is at best, misleading ...

OK, we are at crossed purposes. That assertion is not my assertion. It's the very opposite: it's the assertion I am disagreeing with!

 

[benjenbav]

No worries - happy driving. I hope in a nice car :encouragement:
Ref (1), time is a factor. The V creates a slower acceleration of the bow (upwards) as the boat hits the wave, thus reducing slamming. Time is essential to that analysis - the dynamic Newtonian force (or indeed the buoyance force) ramps up over time in a V hull thus creating a lower acceleration when hitting the wave. Mattress/concrete.

Yes i suppose rails are more concrete-ish (for the split second they contact the wave surface) than mattress, but that effect is (a) tiny compared with the drag reducing benefits (and convenience benefits) of separating the water flow from the hull, plus (b) so small that it could easily be compensated by other (also small) complex hydrodynamic benefits of the rails

Hi, j; not going to dip my toe or even skim my stones in the fluid mech arguments, but about a million posts back I think someone asked whether the Sq58 was a comfortable means of transport at 10 knots. I stand to be corrected but you might be the only person on this thread who has empirical knowledge of this, being a longtime Sq58 owner with plenty of miles under your belt at D speeds on such a craft and I sorta wondered if you would mind commenting?

 

[jfm]

Hi, j; not going to dip my toe or even skim my stones in the fluid mech arguments, but about a million posts back I think someone asked whether the Sq58 was a comfortable means of transport at 10 knots. I stand to be corrected but you might be the only person on this thread who has empirical knowledge of this, being a longtime Sq58 owner with plenty of miles under your belt at D speeds on such a craft and I sorta wondered if you would mind commenting?

Yep this has drifted - thanks for the call to order. I reckon medmilo has more miles than me, but we are both somewhere around the 1000hours mark in Sq58. I did plenty of mine at d speed, which btw is more like 8 than 10kts in that size boat if you want to avoid the fuel burn as you start pushing the bow wave. Anyway, it was supremely comfortable, but of course that is more a function of picking the right weather than the right boat. It is a quiet boat and a pleasure at those speeds, and has a nice low profile which might make her less of a roller than more modern 60 footers but probably gives her, unhelpfully, a shorter roll period. All that said, I never noticed anything bad, and those sorts of differences can be tiny. She was a delightful boat at d speed due basically to her quietness, long hull (going under the bathing platform) and all round creature comforts. As you know I liked that boat so much that I bought 2 of them :encouragement:
It won't happen again m'lud

 

[[2068]]

OK, we are at crossed purposes. That assertion is not my assertion. It's the very opposite: it's the assertion I am disagreeing with!

ok, the wrong way around ... post edited above. To many negatives flying around.

 

[benjenbav]

Yep this has drifted - thanks for the call to order. I reckon medmilo has more miles than me, but we are both somewhere around the 1000hours mark in Sq58. I did plenty of mine at d speed, which btw is more like 8 than 10kts in that size boat if you want to avoid the fuel burn as you start pushing the bow wave. Anyway, it was supremely comfortable, but of course that is more a function of picking the right weather than the right boat. It is a quiet boat and a pleasure at those speeds, and has a nice low profile which might make her less of a roller than more modern 60 footers but probably gives her, unhelpfully, a shorter roll period. All that said, I never noticed anything bad, and those sorts of differences can be tiny. She was a delightful boat at d speed due basically to her quietness, long hull (going under the bathing platform) and all round creature comforts. As you know I liked that boat so much that I bought 2 of them :encouragement:


It won't happen again m'lud

Thanks. I think I remember a night passage that you filmed and posted on here which looked spectacular: steering straight into the moonlight on the sea and that you mentioned then what a joy it was to have to go at suitable speeds for night-nav. Obviously, the boat helps make these things a pleasure and the ability to turn the taps up when you need to is a great bonus.

 

[jfm]

Well that prompted me to look through dusty corners of photobucket. Couldn't find the night one but here are some shots from cruises on my first Sq 58, Corsica and places, often bimbling along at 9 knots. Nice memories - it was a super boat. Raymarine RL80CRC for navigation :encouragement:

 

[jfm]

And the second sq 58. (Fully back OT now!) ...