Artemisia's maiden voyage - first stop Saturday night in Antibes

jfm said:
Thats all a bit bollox portofino. All your explanation of forces and vectors is back to front and the gyro bit was a million miles or so off the mark.

Prestige 680 ran nicely, due to a good hull design on this score. I guess her speed in that video was about 18 knots.

@kingfisher: The wrap was done by grapefruit graphics, who come highly recommended by several customers on here including me.
Click to expand...
Read it again - the normal on paper or ( blackboard in lecture - ) vectors will try and push the bow down -agree ,i just skiped that bit a but the prop shaft angle -makes the top of the prop because its tilted /sloping back and /or wrong pitch - lead to "squatting "this exherts agreater force than, the on paper bow downward vector force ,so bow despite the right way round vector trying to push it down goes up -
That why you see many crap designed hulls with bow pointing at the moon:)
The pod props are ideally angled -when the boats running in a reasonable sea perpendicular to it -no very little chance of any" squatting " -so the boat stays relatively flat ,which is what you see with pod drive boats .

If the propshaft vector which I agree pushes the bow down .
Then why do they ride bow high ?
Why bother with tim tabs ?
As I said the stern drops into "squats " in the hole the top edge of the angled ( arguably wrong pitched ) prop creates .
It's more comman if the prop is further forward , cos the area behind can fall in to the hole it's created .
Less likely if can place the props as far back as poss ,cos then they just chop up the sea ,no boat to "squat" into .
Also the shallower shaft angle the less chance of the top of the prop on a revolution cavitating and loosing grip and destroying the downward vector ,or more chance of the bow actually being pushed down without +ve flaps and more of the prop action is actually spent moving /propelling the boat forwards not digging a hole behind for the boat,s arse to fall into ,and loose shed loads of speed .
This guy had it sused -
null_zpsvcxbbvs7.jpg


Below is more conventional shaft setup actually Jez,s -Here you can see the props more fwd .
So if the boat has the wrong props too much pitch ,or too much shaft angle or a bit of both then ---
1- it will try to "squat " ride bow high (desite the force vector on paper attempting to lower the bow !)
2- the aft rear sections behind the props will need to be flatter to gain more lift -to counteract the arse dropping into churned up sea -arguably inferior seakeeping
3- a lot of prop effect will be lost in actually moving the boat fwd,s -slow speed cruise

Or the set is correct
1- The prop is not too heavely pitched ,just so - but ave speed no race boat .-unless higher rpm engines /gbox ?
2- the top of the prop (dispite the angle ) is not washing out .
3- v little "squatting" more natural bow down vectoring
4- hardly needs trim +ve
null_zps4w9y61ae.jpg


In theory with IPS there should be v little "squatting" caused by duff upstroke of a less than ideal prop angle
And the aft sections tend to be flat to make pod placement easier .
How ever props are not perfect and if , I say if at v higher rpm if the pitch starts to air ate then the stern will sink and the bow will rise .
One pitch for the rpm range that's the prob -but generally IPS posed boats run flat -
Hoping i,ve explained why I said that .
 
Last edited:
Portofino said:
Hoping i,ve explained why I said that .
Click to expand...
Portofino, I appreciate your effort, but aside from not having understood all of your explanation (which might well be my fault), I have a funny feeling that you are missing the forest for the trees.

I'll try to simplify: let's consider the conventional shaft setup of the F150 in your pic.
The thrust vector direction is obviously aligned with the shaft, i.e. diagonal to the water surface.
What moves the boat forward is the horizontal component of that vector, while the vertical component is an efficiency loss (so to speak), but contributes to increase the natural stern lift of the hull.

It's that simple - forget squatting, gyro, Arnesons... :confused: :)

...with apologies for the o/t!
 
MapisM said:
Portofino, I appreciate your effort, but aside from not having understood all of your explanation (which might well be my fault), I have a funny feeling that you are missing the forest for the trees.

I'll try to simplify: let's consider the conventional shaft setup of the F150 in your pic.
The thrust vector direction is obviously aligned with the shaft, i.e. diagonal to the water surface.
What moves the boat forward is the horizontal component of that vector, while the vertical component is an efficiency loss (so to speak), but contributes to increase the natural stern lift of the hull.

It's that simple - forget squatting, gyro, Arnesons... :confused: :)

...with apologies for the o/t!
Click to expand...

It's a simplification that does not do it all justice in the "field " -sorry on the water
The "on paper stern lift /bow down vector is reduced hence " squatting " due to the props loosing it on the upward stroke cos it's at the wrong angle to the incoming water .Stern sinks bow points to the moon .
The more it sinks sans trim the more adverse the prop angle becomes -that what you see boats looking like there about to take off @ 18-22 knots .
Interesting to see EGT,s and load on theses :)
BTW the Itama pic is a shaft -not Arne -thay put the props as far back ( and engines as mid ) as poss so reduce shaft a angle -= max out on the horizontal -like an IPS ,but crucially minimise any squatting ,cos there's no boat in the airated water ,no boat drop in the hole -now most of the vertical ( want little there is due it shallow shaft angle ) component CAN and is used to - lift the stern -= runs with zero trim
Arse is not dropping .
On the F150 there's 2M of boat behind the props to drop down - which it does if the top of the prop due to the angle starts to loose it's efficiency and the vertical force plummets .-not saying it does -but that's more likely gonna lead to a bow high run -unless you trim it -
Where as the pod is idea the props when running are vertually perpendicular to the water flow -= no squatting tendency = flat ride -that what you see with IPS boats
Just explaining what I see
I know what Darwin felt like now :)-nobody listened - they told him God created the different beak shapes with the finches on the Galapagos islands .
 
Portofino said:
due to the props loosing it on the upward stroke cos it's at the wrong angle to the incoming water ...

,cos there's no boat in the airated water ,no boat drop in the hole -
Click to expand...
Portofino,
The shaft angle produces an upward component in the prop thrust vector, so dropping the bow. This can be seen in practice by trimming an outboard or out drive. The flaws in your argument are the two things quoted above:

1. Sure there is a load difference in the up and down stroke prop blades, but that has nothing to do with bow lift. The rising blade has a lower apparent water speed than the falling blade, but contrary to what you say it is not the "upward" that is wrong; it is equally possible for the downward to be wrong, depending on how the prop is pitched etc. But the cosine of 10degrees (which is where most shaft angles in our sector are these days) is 0.985 which means that with a perfect prop the blades are just 0.75% away from perfect, for about 2x 1/4 of their revolution and are perfect for 1/2 their revolution. That is in effect nothing because proably 99% of props on the planet are already 0.75% from perfect, so "wrong" just doesn't matter here and you get much more pitch wrongness when you compare a boat with its fuel tanks empty and full. Indeed until we got 3d scanners we couldn't even achieve that degree of consistency among the different blades on the same prop. The engineering issue here is pulsating forces on the blades as the prop rotates, but all that is well within the capability of the prop's materials and construction. Basically this whole phenomenon isn't a cause of bow rise. Even if you add 3% of bow rise, so the shaft angle becomes 13degrees, the answer is still the square root of sod all. Above say 15 degrees of shaft angle, and 18 degrees of true angle, it perhaps gets interesting as regards cyclical stressing of the prop metal (because cos18 = .95), but the effect you would see still wouldn't be bow rise

2. In a shaft drive boat, the water abaft of the props might be turbulent but it isn't aerated. There is nowhere for air to enter the system. Propulsion systems that do introduce gas/air (outboards, stern drives, IPS) do so abaft or almost abaft of the transom. So there is no loss of buoyancy abaft of the props, and the rear of the boat doesn't sink or squat for that reason

Incidentally, underwater exhausts can cause lack of stern lift aotbe but that has nothing to do with shaft angle

Bow rise, or eliminating it, is all about hull design and weight distribution, plus a contribution from the upward thrust component in a shaft drive boat. A planing boat is partly waterskiing on its hull (partly, because there is a buoyancy effect too) and as any water skier knows you get skis with different inherent running angles (finer tail for slalom, wider tail for jumps, etc) plus you can alter the running angle yourself by moving your body and redistributing the forces
 
Last edited:
Portofino said:
I know what Darwin felt like now :) -nobody listened - they told him God created the different beak shapes with the finches on the Galapagos islands .
Click to expand...
Well, if it wasn't God, who else in his right mind would think of going to the Galapagos just to fiddle with some bird beaks...? :D :p

Actually, I said that I didn't understand ALL of your previous explanations, but I did get the gist of it.
With the only exception of the gyroscopic effect of duoprops, which jfm very kindly defined "a million miles or so" off the mark, while light years would be a more appropriate measurement unit, imho... :p

But that doesn't really matter. The point is, what of the elements you mentioned are actually RELEVANT?
The stress you put on squatting and hull "drop in the hole", when talking of mid 20s planing boats, imho is neither here nor there, unless the hull is real bad. And I mean AWFULLY bad.
In fact, you reminded me of some debates on offshoreonly forum about Hydromotive cleaver props, whose blade shape alone could increase the bow lift a tiny wee bit compared to some other brands, in turn reducing the wet surface and increasing the speed by one knot or two. But that was one knot or two out of 3 digits speed...! :eek:

Otoh, it's a well known fact that the cruising attitude of shaft driven P hulls depends a lot on the shafts angle, among other factors.
As is well known that P hulls with tunnels, with their lower shaft angle, result in a higher cruising AoA, AOTBE.
And that is strictly driven by the thrust vector projections, more than anything else.
Of course, tunnel hulls also have an inherent lower lift because of their hydrodynamic shape, the V deadrise and its variation along the hull length is another crucial element, the props position more or less astern surely matters, and ultimately I guess that also squatting can have some effect, at least at high speed.

But let me put it this way: if someone would take the above Ferretti or Itama hulls on shafts, and would stick pods exactly in the same place of the existing props (hence obviously with much less or none at all of the vertical lift vector component associated with shafts), do you think that the cruising AoA with pods would be:
a) higher, or
b) lower, or
c) unaffected?

'fiuaskme, even if I wouldn't dare guessing any number, the answer is (a) for good.
It's bound to be, unless physics was reinvented while I was having a nap.
And that's all I meant with my first reply to your post #35 - nothing else...! :encouragement:


PS: I only just read last jfm reply, after writing mine.
It's nice to see that there's some science supporting my gut feeling arguments... :D
 
Last edited:
blimey!

master drifters at work again, ts ts ts!

:p

from an engineering POV and my understanding of vectors forces et al, i'd agree with JFM and MM,


BUT,

do we have any other pics and videos of Artemisia in action?
Is she safe in her home port?

cheers

V.
 
vas said:
do we have any other pics and videos of Artemisia in action?
V.
Click to expand...

Coming up:
These from Antibes Saturday afternoon:
A80CE19D-B72F-4E22-BECF-0C8F01CA745B.jpg

68350A82-121D-47B6-A7F5-4F6867906427.jpg

19EC5D78-3F23-42E0-85E7-CAC5DE76043E.jpg


Artemisia went into my berth in Antibes for Saturday night, which turned out to be very calm and a delightful night so we anchored out between the Lerins islands...
22AF95A3-9941-4EA0-A6B6-E6011FAEB8B2.jpg


On Sunday Artemisia went to Elba in flat calm seas. I have a picture of her in Azzuro but it is mafweiss's so I should let him post it. I guess that today she arrived in Salerno
 
Thx for the pics ( unable to see the vid)
I,am not sold on the colour or the wrap -idea - -but realise many are.
Wish the Op safe / pleasant / uneventful del trip
 
Surely, one of the points of having a wrap is to appeal to your own tastes. Then, once you decide to either sell or have a change of heart about the colour or design you can remove the wrap. If you wanted a Princess V72 with flames licking up the side (like some cigarettes) you can, and then remove them when it's time to sell you return the unblemished hull colour to anodyne white.
 
Cheery said:
Surely, one of the points of having a wrap is to appeal to your own tastes. Then, once you decide to either sell or have a change of heart about the colour or design you can remove the wrap. If you wanted a Princess V72 with flames licking up the side (like some cigarettes) you can, and then remove them when it's time to sell you return the unblemished hull colour to anodyne white.
Click to expand...
Exactly! The Blue on Artemisia looks really good in the flesh. Some wraps can look a bit flat but this blue is really deep and the whole thing is all round pretty striking. And exactly as you say, when the boat is sold it can return to stock beige, if the market demands that, and the conversion will cost peanuts

When considering wrapping a yacht, it is always worth typing "yacht Guilty" into google images for a bit of inspiration:encouragement:
 
Last edited:
jfm said:
When considering wrapping a yacht, it is always worth typing "yacht Guilty" into google images for a bit of inspiration:encouragement:
Click to expand...

thanks John, hadn't realised that this belongs to Dakis, the biggest art collector down here. When I'm next in Pireaus I'll try to spot it :D

cheers

V.
 
vas said:
thanks John, hadn't realised that this belongs to Dakis, the biggest art collector down here. When I'm next in Pireaus I'll try to spot it :D

cheers

V.
Click to expand...
Yup, it does indeed belong to your fellow countryman! Is in cote D'Azur plenty - iirc was at Monaco GP a couple of years ago
 
Cheery said:
I'd go out of my way to try and not spot it!
Click to expand...
Seconded.
Btw, with Guilty it would take much more than wrapping removal to make the vessel half decent for reselling, considering the overall superstructure design... :disgust:
 
You must log in or register to reply here.

Share this page

Top