Bouba
Well-Known Member
Re: As promised in another thread....
http://www.powerandmotoryacht.com/refit-and-upgrade/splash-out-spray-rails
Hull shape how aware are you ?
- Thread starter Portofino
- Start date
Portofino
Well-Known Member
Re: As promised in another thread....
On pic 4 the VZ16
Chines -( that the flat bit where the boat is resting on the stand to hold it up -see pic # 4 folks )
1- Hard chine in a planning boat -- agree ?
2- produces lift ,it's primary function found in fast planners - not necessary on say a D speeder -- agree ?
3- if it produces lift the flat bit -- then why not the other two flat bits lower down ?
Do they know all three of them how deep they are ?
Are they all basically submerged ( normal planning not serious wave bouncing ) ?
The primary function of the hard chine is produce lift in a fast planing hull .
Other functions too like deflect spray --the hard chine can do this as well .
The lower two in pic #4 are totally submerged no spray to deflect As such .But they do delect water over that 90 degree sharp edge breaking up the flow as it moves up the V . This reduces the drag --agree?
Less drag = more speed --agree ?
More speed = more hydrodynamic lift
Hence the name "lifting strips /strakes "
They certainly are not spray rails in the rear submerged part of the hull .Also they deflect water to water from the lower to the middle .From the middle to the last one called the hard chine --then its spray -- only the upper hard chine acts as a spray defector.
This is to stop water clinging up the sides of the hull and creating drag .
So if they ( the two submerged one s are not -deflecting spray -- that just leave the flat bit -- creating hydrodynamic lift 1st ( just like the hard chine ) and secondly reducing drag as said by water breaking over the sharp edge --sure = more speed = even more lift than a hull without them .
I think they do contribute to creating lift ,the flat bits ,like the flat bit of the hard chine ,but in the vz16 x3 effect .
We will just have hold our seperate views on this ,nowt wrong with that -- .
They have other functions too ,but the issue here is do they create dynamic lift --- yup like the chine thay do .
Any how at least I picked a good hull from form alone the VZ16 .
I would arrange a sea trial in that .
Pity I did not see the Riva Rivale 52 before sea trial,?
Doesn,t look like a 40 knot boat --Imagine that skirted up at a boat show or in the water .
But I spotted it and know why more importantly -- thx MapishM -- just gets to show PYB was right 95 % of the time
It's the one I would sea trial 1st
http://www.yachtworld.co.uk/boats/2003/Vz-16-2870538/Virgin-Islands-(US)#.WOKIg2t5mSM
Btw MapishM -- how about a rear stern hull pic of your DP56',s lifting strips ---- guessing its got some .
Will you file
them off as per JFM,s advice .? -- only kidding .
On pic 4 the VZ16
Chines -( that the flat bit where the boat is resting on the stand to hold it up -see pic # 4 folks )
1- Hard chine in a planning boat -- agree ?
2- produces lift ,it's primary function found in fast planners - not necessary on say a D speeder -- agree ?
3- if it produces lift the flat bit -- then why not the other two flat bits lower down ?
Do they know all three of them how deep they are ?
Are they all basically submerged ( normal planning not serious wave bouncing ) ?
The primary function of the hard chine is produce lift in a fast planing hull .
Other functions too like deflect spray --the hard chine can do this as well .
The lower two in pic #4 are totally submerged no spray to deflect As such .But they do delect water over that 90 degree sharp edge breaking up the flow as it moves up the V . This reduces the drag --agree?
Less drag = more speed --agree ?
More speed = more hydrodynamic lift
Hence the name "lifting strips /strakes "
They certainly are not spray rails in the rear submerged part of the hull .Also they deflect water to water from the lower to the middle .From the middle to the last one called the hard chine --then its spray -- only the upper hard chine acts as a spray defector.
This is to stop water clinging up the sides of the hull and creating drag .
So if they ( the two submerged one s are not -deflecting spray -- that just leave the flat bit -- creating hydrodynamic lift 1st ( just like the hard chine ) and secondly reducing drag as said by water breaking over the sharp edge --sure = more speed = even more lift than a hull without them .
I think they do contribute to creating lift ,the flat bits ,like the flat bit of the hard chine ,but in the vz16 x3 effect .
We will just have hold our seperate views on this ,nowt wrong with that -- .
They have other functions too ,but the issue here is do they create dynamic lift --- yup like the chine thay do .
Any how at least I picked a good hull from form alone the VZ16 .
I would arrange a sea trial in that .
Pity I did not see the Riva Rivale 52 before sea trial,?
Doesn,t look like a 40 knot boat --Imagine that skirted up at a boat show or in the water .
But I spotted it and know why more importantly -- thx MapishM -- just gets to show PYB was right 95 % of the time
It's the one I would sea trial 1st
http://www.yachtworld.co.uk/boats/2003/Vz-16-2870538/Virgin-Islands-(US)#.WOKIg2t5mSM
Btw MapishM -- how about a rear stern hull pic of your DP56',s lifting strips ---- guessing its got some .
Will you file
them off as per JFM,s advice .? -- only kidding .
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MapisM
Well-Known Member
Re: As promised in another thread....
Besides, the F165 is an early 90s project, while the F681 is much more modern, and in both hulls they stopped the rails well before the shafts.
They just went from 3 each side to 2 somewhat larger ones (don't ask me why).
Bottom line, I believe that you can sleep well!
Well, half way rails don't seem to have affected Ferretti growth too badly, anyway...
Besides, the F165 is an early 90s project, while the F681 is much more modern, and in both hulls they stopped the rails well before the shafts.
They just went from 3 each side to 2 somewhat larger ones (don't ask me why).
Bottom line, I believe that you can sleep well!
MapisM
Well-Known Member
Re: As promised in another thread....
And even 40mph/35kts sounds extremely optimistic anyway, particularly with 6cyl engines.
A nice boat she is, anyway. In fact it's one that I did consider, but the DP56 interiors are much better organized, on top of a few other minor things I didn't like on the VZ 16.
The truly great VZ model is the 18, but that's actually a 19+ m boat - possibly the main competitor of the SL62, back in the days.
Ref. the DP 56 hull, just look at #2 above - it's almost identical.
And nope, I'm not going to grind off the external rail - the only one out of three which goes all the way to the transom.
In fact she isn't. Your link points to a boat in the US, and over the Pond nobody knows mph from kts.
And even 40mph/35kts sounds extremely optimistic anyway, particularly with 6cyl engines.
A nice boat she is, anyway. In fact it's one that I did consider, but the DP56 interiors are much better organized, on top of a few other minor things I didn't like on the VZ 16.
The truly great VZ model is the 18, but that's actually a 19+ m boat - possibly the main competitor of the SL62, back in the days.
Ref. the DP 56 hull, just look at #2 above - it's almost identical.
And nope, I'm not going to grind off the external rail - the only one out of three which goes all the way to the transom.
jfm
Well-Known Member
Re: As promised in another thread....
Haha - I sould have looked more closely and realised #8 is the aicon. Apols to all at DP. And I cant tell diff in those pics between DP 72 and 80 so have swapped them. So here is revised list, changes in red:
a) VZ 16 4
b) DP 55 2
c) Ferretti 165 1
d) DP 58 3
e) Abbate G 36 5
f) Ferretti 681 6
g) DP 80 10
h) Aicon 56 8
i) DP 72 9
j) Riva 68 Ego 7
Haha - I sould have looked more closely and realised #8 is the aicon. Apols to all at DP. And I cant tell diff in those pics between DP 72 and 80 so have swapped them. So here is revised list, changes in red:
a) VZ 16 4
b) DP 55 2
c) Ferretti 165 1
d) DP 58 3
e) Abbate G 36 5
f) Ferretti 681 6
g) DP 80 10
h) Aicon 56 8
i) DP 72 9
j) Riva 68 Ego 7
MapisM
Well-Known Member
Re: As promised in another thread....
Almost there J, but not quite - below in blue the final adjustments.
a) VZ 16 4
b) DP 55 2
c) Ferretti 165 1
d) DP 58 3 -> 10
e) Abbate G 36 5
f) Ferretti 681 6
g) DP 80 10 -> 9
h) Aicon 56 8
i) DP 72 9 -> 3
j) Riva 68 Ego 7
Btw, I must apologize, because you might have been distracted by a previous post of mine, where I said that all DP hulls up to the 72 had no prop tunnels.
In fact, I had in mind that the 58 open was built as an extension of the 55/56 hull, which is true in general terms, but the modifications were more extensive than I thought, and one of them was the semi-tunnels astern, as per pic #10.
Therefore, you deserve the virtual keg anyway, I reckon - also as the only participant to the context!
All that aside, I'd be interested to hear your take on my train of thoughts ref rails increasing lift due to their newtonian resistance to the transversal component of the water flow.
You know, also for peace of mind of Deleted User, who might still be in doubt about whether to throw his boat away or not...
Almost there J, but not quite - below in blue the final adjustments.
a) VZ 16 4
b) DP 55 2
c) Ferretti 165 1
d) DP 58 3 -> 10
e) Abbate G 36 5
f) Ferretti 681 6
g) DP 80 10 -> 9
h) Aicon 56 8
i) DP 72 9 -> 3
j) Riva 68 Ego 7
Btw, I must apologize, because you might have been distracted by a previous post of mine, where I said that all DP hulls up to the 72 had no prop tunnels.
In fact, I had in mind that the 58 open was built as an extension of the 55/56 hull, which is true in general terms, but the modifications were more extensive than I thought, and one of them was the semi-tunnels astern, as per pic #10.
Therefore, you deserve the virtual keg anyway, I reckon - also as the only participant to the context!
All that aside, I'd be interested to hear your take on my train of thoughts ref rails increasing lift due to their newtonian resistance to the transversal component of the water flow.
You know, also for peace of mind of Deleted User, who might still be in doubt about whether to throw his boat away or not...
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jfm
Well-Known Member
Re: As promised in another thread....
1. First, the transverse flow component is tiny. I recall letting my flat stab fins self centre at speed and setting up the curve whereby they would have a different self centre angle at different speeds, and you are talking something like <5 degrees. See also Figure 2 of page 4/22 of the excellent pdf linked to in post #4 of this thread http://www.boatdesign.net/forums/powerboats/stepped-hull-19295.html which suggests the same.
2. Now apply some maths, taking 5 degrees of transverse flow. Tangent 5 degrees is 0.087. Let's say 10%. So a 30 knot boat has 3 knots of transverse flow. Or if you want 10 degress, tan - 17%, so 5 knots of transverse flow. The "trim tab left effect" of a spray rail that is only (say) 60mm wide @ 3 or 5 knots or any single figure knots is a Newtonian lift that is virtually zero. If the idea of a spray rail were to create Newtonian lift you would frankly make them bigger and mount them athwartships, but that has already been invented and is called a trim tab!
3. Next question is whether the rails create a beneficial stepped hull effect in the transverse water flow direction. Well no they don't and intuition tells you that. As you know the benefit of a step is that it changes the aspect ratio of the planing surfaces, to resemble a plane wing (wide and short, not long and narrow). But the benefit of this wide aspect ratio is non existent at 3 or 5 knots. Steps also require that there is a fresh leading edge to each of the two "separate" wings that the step creates. Thus a step cannot work in the completely submerged part at the back of a hull, where it would just add drag, and the spray rails we are talking about here are in that permanently submerged part at the back of the boat. Hence overall these rails produce no step effect and instead they add a small amount of drag.
4. Bottom line I suggest is that (a) there can be no spray shedding benefit of a rail once it is in the aft part of a 30-35kt hull; (b) in contrast there is a spray shedding benefit in a race boat because that is riding above its chines all the way to the transom and therefore cutting off spray at the transom matters; (c) back to 30 knot boats then, the perma-submerged spray rails do nothing to cut spray so provide no benefit, but they must induce a small amount of drag due to increased wetted area caused by the strip and due to interrupting the transverse flow admittedly only at 3-5 knots.
I therefore continue to say that the (big) efficiency benefit of a spray rail comes from cutting the pumping losses caused by spray, which point is not new and is very well understood by hull designers. The excellent pdf above quantifies that in the region of 15% drag reduction at 40 knots but with a 4th power relationship so the benefit at 20 knots is much less. Of course there is a still a "dry boat" benefit at 20knots so spray rails are a good thing in slower P boats (and SD). And I say that a spray rail in the permanently submerged part of the hull ie the aft ~1/3rd of a 30 knot boat gives no benefit but creates a (small) increase in drag - there is zero scientific analysis or theorising that says there is benefit, and plenty that says there is a small downside. Most of the hulls in your pictures (including #5, the abate G36) seem designed by folks who agree all this but three hulls (2,4,9) still take one rail to the transom against - I think - all sensible analysis. Of those three, two are DP, and DP also have two other boats that don't take the rails to the transom - boats 3 and 10 - go figure!
Happy to hear a contrary view if you disagree
(Two previously mentioned benefits of spray rails on here, being that spray rails provide lift because they are flat not angled, and that they "grip" the water to reduce at-anchor roll are so preposterous that I wont bother refuting them again and I'm assuming you agree.)
Hi P. I cogitated on this some, and a few thoughts are below, but overall conclusion/proposal is that any increased Newtonian lift arising from the transverse component of the water flow is ~zero and more than offset by increased drag.All that aside, I'd be interested to hear your take on my train of thoughts ref rails increasing lift due to their newtonian resistance to the transversal component of the water flow.
You know, also for peace of mind of Deleted User, who might still be in doubt about whether to throw his boat away or not...
1. First, the transverse flow component is tiny. I recall letting my flat stab fins self centre at speed and setting up the curve whereby they would have a different self centre angle at different speeds, and you are talking something like <5 degrees. See also Figure 2 of page 4/22 of the excellent pdf linked to in post #4 of this thread http://www.boatdesign.net/forums/powerboats/stepped-hull-19295.html which suggests the same.
2. Now apply some maths, taking 5 degrees of transverse flow. Tangent 5 degrees is 0.087. Let's say 10%. So a 30 knot boat has 3 knots of transverse flow. Or if you want 10 degress, tan - 17%, so 5 knots of transverse flow. The "trim tab left effect" of a spray rail that is only (say) 60mm wide @ 3 or 5 knots or any single figure knots is a Newtonian lift that is virtually zero. If the idea of a spray rail were to create Newtonian lift you would frankly make them bigger and mount them athwartships, but that has already been invented and is called a trim tab!
3. Next question is whether the rails create a beneficial stepped hull effect in the transverse water flow direction. Well no they don't and intuition tells you that. As you know the benefit of a step is that it changes the aspect ratio of the planing surfaces, to resemble a plane wing (wide and short, not long and narrow). But the benefit of this wide aspect ratio is non existent at 3 or 5 knots. Steps also require that there is a fresh leading edge to each of the two "separate" wings that the step creates. Thus a step cannot work in the completely submerged part at the back of a hull, where it would just add drag, and the spray rails we are talking about here are in that permanently submerged part at the back of the boat. Hence overall these rails produce no step effect and instead they add a small amount of drag.
4. Bottom line I suggest is that (a) there can be no spray shedding benefit of a rail once it is in the aft part of a 30-35kt hull; (b) in contrast there is a spray shedding benefit in a race boat because that is riding above its chines all the way to the transom and therefore cutting off spray at the transom matters; (c) back to 30 knot boats then, the perma-submerged spray rails do nothing to cut spray so provide no benefit, but they must induce a small amount of drag due to increased wetted area caused by the strip and due to interrupting the transverse flow admittedly only at 3-5 knots.
I therefore continue to say that the (big) efficiency benefit of a spray rail comes from cutting the pumping losses caused by spray, which point is not new and is very well understood by hull designers. The excellent pdf above quantifies that in the region of 15% drag reduction at 40 knots but with a 4th power relationship so the benefit at 20 knots is much less. Of course there is a still a "dry boat" benefit at 20knots so spray rails are a good thing in slower P boats (and SD). And I say that a spray rail in the permanently submerged part of the hull ie the aft ~1/3rd of a 30 knot boat gives no benefit but creates a (small) increase in drag - there is zero scientific analysis or theorising that says there is benefit, and plenty that says there is a small downside. Most of the hulls in your pictures (including #5, the abate G36) seem designed by folks who agree all this but three hulls (2,4,9) still take one rail to the transom against - I think - all sensible analysis. Of those three, two are DP, and DP also have two other boats that don't take the rails to the transom - boats 3 and 10 - go figure!
Happy to hear a contrary view if you disagree
(Two previously mentioned benefits of spray rails on here, being that spray rails provide lift because they are flat not angled, and that they "grip" the water to reduce at-anchor roll are so preposterous that I wont bother refuting them again and I'm assuming you agree.)
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benjenbav
Well-Known Member
Is it at all possible that these rails exert a lateral resistance to the displacement of water in order to push more of it over the propellers?
jfm
Well-Known Member
I may not be following your train of thought BJB but I think not. You say "more of it" but while the pressure of water changes its density doesn't, so absent aeration the same amount of water flows over the propellers come what may.
As regards moving water about the place, you have to remember that water is a low viscosity fluid and these rails are tiny. They aren't going to make a lot of difference to water flow. Their usefulness in shedding spray is a completely different thing and they are proven to work well at that. Shedding spray is important to dryness/comfort on lots of boats and is important to drag on fast boats (drag from spray has a quadratic relationship to speed )
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benjenbav
Well-Known Member
I was thinking that the 'v' shape of the hull would tend to push the water outwards and the rails might counteract that to a small extent. Like the channelling of water over a mill wheel but less so. Probably very wrong.
If they don't do that what do they do on a hull which is completely immersed? Could they be there simply to add rigidity to the structure?
rbcoomer
Well-Known Member
The water in the mill is channelled between a solid object, gravity and air so will speed up at the contraction point as it's being pushed forward into air (path of least resistance) and thus has somewhere to go. The water under the hull has nowhere to go other than to displace other water, but the resistance to it will presumably be similar in every direction sideways and down? Thus I guess it can only offer a fixed resistance to the hull, irrespective of shape? (Well resistance to gravity anyway as that's what's pushing the hull into the water!) Never studied fluid dynamics, so I'm probably wrong too!
MapisM
Well-Known Member
Re: As promised in another thread....
I had to re-read some parts of that article 3 times, eventually understanding 80% - if that!
But back to your comments, I think they make good sense.
In fact, my thought was more about the principle as such, on which as I understand you don't disagree, than the relevance of the effect.
If I should really try to argue against your reasoning (not that I want to, or that I think it's relevant, but just for sake of not losing the touch vs. hair splitting...), I don't think it's correct to extrapolate only the transversal component of the water flow and compare it to a 3 or 5 knots fluid movement - which of course I agree is neither here nor there in terms of lift, because water at that speed is not hard enough, if I may steal a rough but effective expression which I heard in a Reggie Fountain video.
I mean, the fluid movement relative to the hull still goes at 30 knots or whatever, and I am not convinced that the higher resistance (hence lift, if any) of rails can be translated/compared/analyzed with a linear extrapolation of the transversal component alone, recalculating the fluid speed accordingly.
Anyway, if there's one thing I'm sure of, it's that when we discuss the effect of these things on heavy and big flybridge boats, which usually cruise at 20 something knots and can barely exceed 30 at WOT, we are splitting hairs for good.
Christ, even the concept of the drag introduced by whisker spray alone, analyzed (very accurately - I fully agree on that!) in the article of the thread you linked, actually made me smile.
Just think of it: it takes no less than a couple of massive C32 to keep the sheer mass of your Sq78 going at P speed, burning a fair bit of the planet oil reserves in the process.
Let's assume it would be possible with a magic wand to eliminate whisker spray completely, how much efficiency do you think you might gain?
My bet is that it would take at least the second decimal digit to measure the percentage of improvement....
PS: the last assumption in your post #147 is 100% correct - obviously, I hasten to add! :encouragement:
Blimey J, your link puts in an entirely different perspective my belief that here in the asylum we are the masters of hair splitting, at times.
I had to re-read some parts of that article 3 times, eventually understanding 80% - if that!
But back to your comments, I think they make good sense.
In fact, my thought was more about the principle as such, on which as I understand you don't disagree, than the relevance of the effect.
If I should really try to argue against your reasoning (not that I want to, or that I think it's relevant, but just for sake of not losing the touch vs. hair splitting...
), I don't think it's correct to extrapolate only the transversal component of the water flow and compare it to a 3 or 5 knots fluid movement - which of course I agree is neither here nor there in terms of lift, because water at that speed is not hard enough, if I may steal a rough but effective expression which I heard in a Reggie Fountain video.I mean, the fluid movement relative to the hull still goes at 30 knots or whatever, and I am not convinced that the higher resistance (hence lift, if any) of rails can be translated/compared/analyzed with a linear extrapolation of the transversal component alone, recalculating the fluid speed accordingly.
Anyway, if there's one thing I'm sure of, it's that when we discuss the effect of these things on heavy and big flybridge boats, which usually cruise at 20 something knots and can barely exceed 30 at WOT, we are splitting hairs for good.
Christ, even the concept of the drag introduced by whisker spray alone, analyzed (very accurately - I fully agree on that!) in the article of the thread you linked, actually made me smile.
Just think of it: it takes no less than a couple of massive C32 to keep the sheer mass of your Sq78 going at P speed, burning a fair bit of the planet oil reserves in the process.
Let's assume it would be possible with a magic wand to eliminate whisker spray completely, how much efficiency do you think you might gain?
My bet is that it would take at least the second decimal digit to measure the percentage of improvement....
PS: the last assumption in your post #147 is 100% correct - obviously, I hasten to add! :encouragement:
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Designo
Well-Known Member
Re: As promised in another thread....
So, what is the board saying about this..? http://petestep.com/the-petestep-technology/
So, what is the board saying about this..? http://petestep.com/the-petestep-technology/
Portofino
Well-Known Member
Re: As promised in another thread....
Like it --
See my post 117 page 12 --the last couple of paragraph s
Here's the patent link --- mentions lifting amongst other things
https://www.google.com/patents/WO2014200407A1?cl=en
Amarti allready figured out the benefits one of which is LIFT of taking HIS rails at a certain position in the deep V to the transom .
These "petesteps " -- Similar function but spreading laterally as they go back (if I understand Petestep correctly ) which claims a greater or added benefit of forward thrust , less noise as well the normal benefits I've been banging on about .
That pic is on your link is very similar to change of spray pattern I get at above a certain speed
Slowish sub 30 knots butterfly wings ,like any other P boat ,curling up and dropping
Above x (say 30-40) knots in the same conditions the boat lifts right up and the spray patern move to the stern 1-2 M only and takes on a glider wing throw out ,long ,thin low shafts of water that hug the sea , 0.5 m high .
Mine are noisey though @speed. I think they are angled down a few degrees the nearer they get to the stern .
Like it --
See my post 117 page 12 --the last couple of paragraph s
Here's the patent link --- mentions lifting amongst other things
https://www.google.com/patents/WO2014200407A1?cl=en
Amarti allready figured out the benefits one of which is LIFT of taking HIS rails at a certain position in the deep V to the transom .
These "petesteps " -- Similar function but spreading laterally as they go back (if I understand Petestep correctly ) which claims a greater or added benefit of forward thrust , less noise as well the normal benefits I've been banging on about .
That pic is on your link is very similar to change of spray pattern I get at above a certain speed
Slowish sub 30 knots butterfly wings ,like any other P boat ,curling up and dropping
Above x (say 30-40) knots in the same conditions the boat lifts right up and the spray patern move to the stern 1-2 M only and takes on a glider wing throw out ,long ,thin low shafts of water that hug the sea , 0.5 m high .
Mine are noisey though @speed. I think they are angled down a few degrees the nearer they get to the stern .
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MapisM
Well-Known Member
Re: As promised in another thread....
A small update with a few pics that I had the opportunity to take only today.
Which are interesting because the hull is built by a yard whose reputation for big flybridge boats is right at the top of the food chain.
Interestingly, rails do NOT go all the way to the transom - which is somewhat consistent with the principle that they only make sense on fast(ish) boats.
Another thing worth mentioning, which btw gives a hint on the yard/model, is the lack of submerged exhausts - which I very much like, against all odds.
Njoy!
A small update with a few pics that I had the opportunity to take only today.
Which are interesting because the hull is built by a yard whose reputation for big flybridge boats is right at the top of the food chain.
Interestingly, rails do NOT go all the way to the transom - which is somewhat consistent with the principle that they only make sense on fast(ish) boats.
Another thing worth mentioning, which btw gives a hint on the yard/model, is the lack of submerged exhausts - which I very much like, against all odds.
Njoy!
Portofino
Well-Known Member
Re: As promised in another thread....
It's looks a bit pancake flat -- low deadrise -- gets enough lift as it is via the flattish aft sections .
Does not really need lifting strips --mid 20 ,s chugger ?
As well as noise suppression ( underwater exhausts ) they were primarily invented by Amarti -well he was the first builder to do underwater E --- he found a performance benefit of airating parts of the hull cutting drag ,along with other arguably small improvements to improve performance .
They are not round or oval on my boat they are square with a deflector plate about 40-50 cm wide , positioned v near the chine .
Gases coming out will seperate the water wetted area reducing friction --gaining a few knots ?
They may destroy lift --sinking the hull in totality in theory --but the wide calculated lifting strip lower down outside the gas stream taken to the stern --adds that lost lift back ,net result --less total drag --cos of the airated sections =more speed
I think he did not put them there without good reason --
Whole of the chine behind is gassed ,the "strip " is below in full flow of water .
It's looks a bit pancake flat -- low deadrise -- gets enough lift as it is via the flattish aft sections .
Does not really need lifting strips --mid 20 ,s chugger ?
As well as noise suppression ( underwater exhausts ) they were primarily invented by Amarti -well he was the first builder to do underwater E --- he found a performance benefit of airating parts of the hull cutting drag ,along with other arguably small improvements to improve performance .
They are not round or oval on my boat they are square with a deflector plate about 40-50 cm wide , positioned v near the chine .
Gases coming out will seperate the water wetted area reducing friction --gaining a few knots ?
They may destroy lift --sinking the hull in totality in theory --but the wide calculated lifting strip lower down outside the gas stream taken to the stern --adds that lost lift back ,net result --less total drag --cos of the airated sections =more speed
I think he did not put them there without good reason --
Whole of the chine behind is gassed ,the "strip " is below in full flow of water .
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D
Deleted User YDKXO
Guest
Re: As promised in another thread....
How to rubbish somebody's pride and joy in 2 sentences
MapisM
Well-Known Member
Re: As promised in another thread....
Porto, you already know that I highly rate Itamas, but if given a choice between ANY Itama and an SL72, well, I'll have the pancake, thank you. :encouragement:
LMAO, yeah...!How to rubbish somebody's pride and joy in 2 sentences
Porto, you already know that I highly rate Itamas, but if given a choice between ANY Itama and an SL72, well, I'll have the pancake, thank you. :encouragement:
Portofino
Well-Known Member
Re: As promised in another thread....
Hey come on guys I,am not rubbishing anybody's boat ,just describing what I see .
Thinking cars and the Nurburgring ,if a guy in a Range rover or Rolls Royce Phanton turns up and asks why he can,t lap in less than 9 mins -or you ask me why the Lamboghinini superleggera can lap in less 9 mins --I,ll tell you why and show you the different features the Lambo has ,or the other two have not -some obvious like ground clearance ,some subtle like thinner glass and Perspex in the Lambo. Compared to thicker double glazing glass in the RR --adding weight ,-great for internal sound insulation ,but not good Kg,s wise for lap times round a circuit .
Buts that's not rubbishing there pride n joy --/ they asked
MapisM -put the pics up of that boat -turned out a v nice FB boat Indeed a SL72 -yup highly desirable -agree .
He hinted it was not fast ---I kinda could see that looking at the hull form , like the RR and R Royce --nice cars .
Buy best kept off the track IMHO.
It is interesting why they have NOT gone down the underwater exhaust route -- for the sake of sound reduction /refinement ?
Wonder what other design tools they have used to "refine" exhaust noise -curious ?
SL 72 over any Itama ----- yes ordinarily but does that include the 75 -comparable size -to make it a fair choise , if you toss one of those in the mix --hmm ability to cruise @40 knots and more compred to a mid 20s slugger of the SL72 --and get this ----more or less the SAME fuel burn !
Now that's a tough one ---depends how you are gonna use it ,like the RR ,R-Royce and the Lambo superleggera
Nice props --this one has only a 20 degree deadrise
That's not rubbishing some ones pride n joy --
LMAO, yeah...!
Porto, you already know that I highly rate Itamas, but if given a choice between ANY Itama and an SL72, well, I'll have the pancake, thank you. :encouragement:
Hey come on guys I,am not rubbishing anybody's boat ,just describing what I see .
Thinking cars and the Nurburgring ,if a guy in a Range rover or Rolls Royce Phanton turns up and asks why he can,t lap in less than 9 mins -or you ask me why the Lamboghinini superleggera can lap in less 9 mins --I,ll tell you why and show you the different features the Lambo has ,or the other two have not -some obvious like ground clearance ,some subtle like thinner glass and Perspex in the Lambo. Compared to thicker double glazing glass in the RR --adding weight ,-great for internal sound insulation ,but not good Kg,s wise for lap times round a circuit .
Buts that's not rubbishing there pride n joy --/ they asked
MapisM -put the pics up of that boat -turned out a v nice FB boat Indeed a SL72 -yup highly desirable -agree .
He hinted it was not fast ---I kinda could see that looking at the hull form , like the RR and R Royce --nice cars .
Buy best kept off the track IMHO.
It is interesting why they have NOT gone down the underwater exhaust route -- for the sake of sound reduction /refinement ?
Wonder what other design tools they have used to "refine" exhaust noise -curious ?
SL 72 over any Itama ----- yes ordinarily but does that include the 75 -comparable size -to make it a fair choise , if you toss one of those in the mix --hmm ability to cruise @40 knots and more compred to a mid 20s slugger of the SL72 --and get this ----more or less the SAME fuel burn !
Now that's a tough one ---depends how you are gonna use it ,like the RR ,R-Royce and the Lambo superleggera
Nice props --this one has only a 20 degree deadrise
That's not rubbishing some ones pride n joy --
Last edited:
Bouba
Well-Known Member
Re: As promised in another thread....
How does it perform without the optional bathing platform?
Nice props --this one has only a 20 degree deadrise
That's not rubbishing some ones pride n joy --