Nigelpickin
Well-Known Member
sit back, enjoy your drink, let the masseuse take care of yourself
Ahh a thread with a happy ending
Hull shape how aware are you ?
- Thread starter Portofino
- Start date
Ahh a thread with a happy ending
gordmac
Well-Known Member
How would different hull profiles be at anchor if it was a bit rough? Sailboats have big heavy dangly bits which presumably help a lot but how about motor boat hull shapes?
MapisM
Well-Known Member
Well, in principle D hulls have a longer roll period, with a smoother motion that P hulls, so even if the max roll angle for comparable hull sizes/conditions might be a bit lower for the latter, due to their flatter bottom and hard chines, normally the rolling motion is a bit less annoying on a D hull. SD hulls are somewhere in between.
That said, unless you've got zero speed stabilizers (and you're willing to keep the genset on to run them), ANY mobo is bound to be less comfy than a sailboat, in a rough anchorage.
Bouba
Well-Known Member
You can use flopper stoppers as a cheap zero speed stabilizers and no need for a genny. Flopper stoppers would look silly on a planing boat but would suit the looks of a full displacement trawler style boat very well.
MapisM
Well-Known Member
Agreed. I just didn't mention them because they are pretty far from common...
Portofino
Well-Known Member
Motor boats ---
Well the better handling ones up wind by vertue of the hull form will handle a rough anchorage better .
On two levels - static stability and dynamic
Static stability 1st
Weather cocking roll is not really an issue ,it's pitching about .Flatter bottomed "tea tray " hulls ,the water /wave will get under the flat aft sections and lift it more .bob it up n down more .
A deeper V less so , but only a bit less as the hull is NOT planning ,so basically more static forces -certainly not dynamic .
Harder Chine,s wider chines @ the edges and various other flatter lifting strakes more or less 90 degrees to the surface will "resist " roll .Also the deeper V will act more of a keel and reduce yawning about more so than a flatter tea tray hull .so,s keeping the sharper end more into the oncoming waves ,contributing to less pitchy ride .
If the engines are set low and in the middle ,that will help lower CoG in reducing the motions .
Re Yacht keel - anything that lowers the CoG like no upper flybriges or hard tops will help reduce rolling (ever see a high masted sailboat rock about once it's gets rocking ? Despite its keel?)
Basicslly a knife will be better than a tea tray especially a top heavey tea tray @ anchor weather cocked to incoming waves .
If the wind veeres s a bit as it often does in a blowey anchorage and the waves hit at say 30 degrees on the bow 1/4 then a knife like finer entry will slap less than a bulbous blown out maxed out bow for accomodation reasons plastic fantastic floating appartment .The drinks will be spilled on the crock skinned dinette table
as its tossed about like a cork So as you can see its now time to think about bugging out ?
Well now its time for the dynamic side of the hull form to take over .The second level I was referring to .
Skipper of the better riding "knife " had more options and is more relaxed than the "tea tray " skipper ,who,s getting stressed out nicely tidying up all the spilt lockers and the contents of a accidentally open fridge ,a big American fridge btw
The skipper of the Knife will just slice through once into the fairway ,get it up on the plane flatten off the ride -nice n smooth ,he will not be hindered by wave /wind direction ,more than happy to take it head on ,if that's the easiest direction to find a nice sheltered bay .
Blown up tea tray boat ,top heavey too ,--well he, will start to beat up wind to the nice bay (as seen on his no dought massive screen on his latest top specced plotter ) but due to constant spray on the fly ,bobbing up and down ,pitching -cos he can,t plane he will turn down wind in search of some where else , but 1st he will put the crew and boat through hell .
He will try to plane upwind ,but the crash bang wollup will be two much for the boats locker content .If he does manage 16-18 knots it will be a miserable patio door shaking ride .All those nice colour co ordiating cushions are now on the saloon floor .
It's far too rocky on the "tea tray " to stay @ that anchorage and cook the prawns
this time !He,s having problems with his Geny ( its too choppy to climb down the ER steps ) so can,t run the stabs ( 220v ) -sods law has taken over !
The reality of the situation is a million miles away from the glossy brochure pic,s and cheesy smiles of the blue blazer wearing sales man at the boat show when he purchased it .
You pays your money and the point is this hopefully make an --- INFORMED ---- choice .
Maybe hull form IS the most important factor in your "enjoyment "
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gordmac
Well-Known Member
Interesting, thanks guys(?)!
D
Deleted User YDKXO
Guest
Nah cant agree with that. Narrow deep V hulls roll like a pig in a rough anchorage. I had a Targa 48 once which was supposed to have a narrow deeper V hull and I used to avoid busy anchorages like the plague because our lunch would always end up on the deck. Much better to have a beamy tea tray as you put, albeit I agree not top heavy, it because it has far better initial resistance to rolling. In the end every type of hull rolls in a rough anchorage and the only answer is stabilisers or maybe a multihull
Portofino
Well-Known Member
For those still interested in hull form :encouragement:
Who said anything about "narrow "?
You need a wide ,Beamer and deeper V -- that's the knack
Greater keel effect --- great parking does not blow about like a crisp packet .
Weather cocks better does not veere as much inducing lateral forces to the waves and asking for rolling @ blowey anchorage .
Of course there has to be some flat sections to get lift when planning .
For lift extra wider chine flats ,and the "Tolberone " esq spray rails actually are extra wide and 90 degrees to the surface under the stern 1/2 of the hull metamorphosed into lifting strakes they also resist roll .
They may look like pieces of Tolberone at the bow --in spray rail mode ,but as they run back get wider ,flatter as they turn into lifting strakes .Thats another thing you can see that looking @ the hull form .
Tea tray flat low deadrise rear section Hulls have enough lift for the ( little Low Hp ) engines anyhow .There spray rails keep there more or less Tolberone profile from bow to stern .
Nothing's for nowt - with a wide beam AND deep V you now have more wetted area ,more drag ,more Kg,s so need far a more bigger engine Hp requirement ,which in its self adds more weight ,sinks the boat lower in the water .
Fuel burn like for like will be more ,but you will rarely need to slow down as much .
Take a look at the L and Beam here .Targa has Volvo 7.4 L 480 hp --much smaller wetted area and probably flatter mix sections ? ---- Itama MAN 15 L v8 800 Hp ---- Beamer and I bet a much deeper draft ,far greater wetted area
Fairline is the longer I think ? A 48 compared to a 46 .
http://www.theyachtmarket.com/boats_for_sale/1389112/
http://www.theyachtmarket.com/boats_for_sale/1247415/
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D
Deleted User YDKXO
Guest
You did. You referred to a 'knife' which doesnt imply fat and beamy! Yeah OK I agree a beamy boat with a deep V is probably going to roll less than a narrow boat with a deep V but the narrow boat is going to be the 'knife' through choppy seas
Portofino
Well-Known Member
-
Arh I see - knife ,tea tray anology was differentiate between cutting through ,parting the water with the minimal VERTICAl acceleration and riding over ,being lifted up with a far greater VERTICAl acceleration - which inevitably ends up ---- what goes up must come down ,where there is no water to support the hull in a trough = crash bang wollup -Slamming
It's the rate if vertical acceleration that you need to reduce to get a none slamming ride .
The debate is wether it effects the "enjoyment " and can you tell (PYB quotes 95 % of the time ) by looking @ hull form .
The fact at boat shows they skirt the hull is side issue .
You can see the hull form in yards etc .
Darwin was fascinated by the different beak shapes of Finches ,in the Galapagos islands then realised they had evolved around what they were eating by natural selection .Nobody was at the time was interested in beak form .
You can tell a lot by looking @ hull form ,but like Darwin nobody seems interested
MapisM
Well-Known Member
+1.
My objection to the main topic PF raised was that most hulls are good enough for the needs of most of us, and anyhow the only way to appreciate the differences is by trying them out there (as opposed to just looking).
Otoh, there's no denying that I'd choose a Magnum rather than any flybridge for fast cruising in rough sea, as well as I'd rather be in a pure D hull with stabs for long distance passages, or in a skiboat to go through a slalom course. Which is pretty much stating the obvious, as I said at the beginning.
But now, suggesting that some types of P hulls are significantly better than others in terms of rolling behaviour AT ZERO SPEED is almost a joke - unless we are talking of hair splitting differences, that can only be measured instrumentally, if that.
Also, I completely disagree with PF ref "roll is not really an issue ,it's pitching about".
It happened to me once that the wind suddenly rotated 180° at night, turning a well sheltered anchorage in a completely exposed one.
I don't even know when that started because we were well asleep, but by the time pitching was bad enough to wake us up, the bow cabin was going up and down by one meter or so, and no, and I'm not exaggerating.
Would they have been on our beam, much less than half of those waves would have made the boat roll enough to throw us out of the bed.
Let's face it: at zero speed, any non-STAR equipped monohull rolls like a pig in anything but a reasonably flat sea, period.
And there are only two realistic solutions to such problem:
- move elsewhere (my favourite one, but just because I'm a tightwad!
)- fit STAR stabs
Anything else is just wishful thinking.
Bouba
Well-Known Member
There is forecast a big blow in Le Lavandou SoF tonight, I managed to get to the boat about five to batten down the hatches. Must have been close to 50mph and should reach 69 in the wee hours, I answered petem's thread today by saying I have four covers on my flybridge, I should have said I 'started' today with four covers. We'll see what the morning inspection brings.
What I noticed in the port was that all the motorboats were happily bobbing about. But the yachts were going crazy. They just could not keep still even the ones on pontoons couldn't keep along side, on top of that they were heeling all over the place. And of course the yachts and motorboats are all mixed in together, there's even one next to me. Luckily for me they are live aboards and when i pointed out how close we are getting they were unable to keep their yacht still but said they'd keep watch. They seem like nice people.
My point is, to the anchor question and Portofino's lovely story, 'knife' or 'teatray', I don't care. As long as it's not a yacht!
What I noticed in the port was that all the motorboats were happily bobbing about. But the yachts were going crazy. They just could not keep still even the ones on pontoons couldn't keep along side, on top of that they were heeling all over the place. And of course the yachts and motorboats are all mixed in together, there's even one next to me. Luckily for me they are live aboards and when i pointed out how close we are getting they were unable to keep their yacht still but said they'd keep watch. They seem like nice people.
My point is, to the anchor question and Portofino's lovely story, 'knife' or 'teatray', I don't care. As long as it's not a yacht!
Portofino
Well-Known Member
+1.
My objection to the main topic PF raised was that most hulls are good enough for the needs of most of us, and anyhow the only way to appreciate the differences is by trying them out there (as opposed to just looking).
Otoh, there's no denying that I'd choose a Magnum rather than any flybridge for fast cruising in rough sea, as well as I'd rather be in a pure D hull with stabs for long distance passages, or in a skiboat to go through a slalom course. Which is pretty much stating the obvious, as I said at the beginning.
But now, suggesting that some types of P hulls are significantly better than others in terms of rolling behaviour AT ZERO SPEED is almost a joke - unless we are talking of hair splitting differences, that can only be measured instrumentally, if that.
Also, I completely disagree with PF ref "roll is not really an issue ,it's pitching about".
It happened to me once that the wind suddenly rotated 180° at night, turning a well sheltered anchorage in a completely exposed one.
I don't even know when that started because we were well asleep, but by the time pitching was bad enough to wake us up, the bow cabin was going up and down by one meter or so, and no, and I'm not exaggerating.
Would they have been on our beam, much less than half of those waves would have made the boat roll enough to throw us out of the bed.
Let's face it: at zero speed, any non-STAR equipped monohull rolls like a pig in anything but a reasonably flat sea, period.
And there are only two realistic solutions to such problem:
- move elsewhere (my favourite one, but just because I'm a tightwad!
- fit STAR stabs
Anything else is just wishful thinking.
Nope to "most hulls are good enough for most of us "
See this
Taken from the "planning" para
---- "In my experience, the majority of the planing boats sold today are poorly suited to venturing offshore because their hulls are too wide for their length, too flat and full forward, and often too flat aft.-----"
http://www.soundingsonline.com/boat...how-different-hull-types-react-in-rough-water
Your description of your experiance is EXACTLY what I mean -----hull form .
It's a round bilge D speed boat with knack all resistance to roll ,like a deep V ,hard chine flats -extended fwd. spray rsils that morph into lifting strakes the hull L adding more resistance ,
More "tea tray " it pitched 1m or more cos more wave got under it rarther than went round it -lifting it .
That why you need stabs a kinda self fulfilling phrophecy @ anchor and running .
Like Bouba --- tonight observing some boats roll more than others .you can plainly ( no pun intended ) see @ anchor some boats of similar kind ,lets use Mike F ,s Targa 48 and a Itama 46 which is shorter btw rolls less .
It's down to hull form in those P boats .
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jfm
Well-Known Member
+1. Much of PF's analysis defies the laws of physics. The idea that spray rails reduce at-anchor roll is pure comedy (as is the idea that the flat surfaces on spray rails cause lift to make the boat plane, but let's not go there, again. It is like saying a tender imparts less downward gravity force on its chocks if it has spray rails, rather than a rail-less V hull, which is self evidently wrong. Spray rails are to reduce drag by shedding spray, period).+1.
My objection to the main topic PF raised was that most hulls are good enough for the needs of most of us, and anyhow the only way to appreciate the differences is by trying them out there (as opposed to just looking).
Otoh, there's no denying that I'd choose a Magnum rather than any flybridge for fast cruising in rough sea, as well as I'd rather be in a pure D hull with stabs for long distance passages, or in a skiboat to go through a slalom course. Which is pretty much stating the obvious, as I said at the beginning.
But now, suggesting that some types of P hulls are significantly better than others in terms of rolling behaviour AT ZERO SPEED is almost a joke - unless we are talking of hair splitting differences, that can only be measured instrumentally, if that.
Also, I completely disagree with PF ref "roll is not really an issue ,it's pitching about".
It happened to me once that the wind suddenly rotated 180° at night, turning a well sheltered anchorage in a completely exposed one.
I don't even know when that started because we were well asleep, but by the time pitching was bad enough to wake us up, the bow cabin was going up and down by one meter or so, and no, and I'm not exaggerating.
Would they have been on our beam, much less than half of those waves would have made the boat roll enough to throw us out of the bed.
Let's face it: at zero speed, any non-STAR equipped monohull rolls like a pig in anything but a reasonably flat sea, period.
And there are only two realistic solutions to such problem:
- move elsewhere (my favourite one, but just because I'm a tightwad!
- fit STAR stabs
Anything else is just wishful thinking.
Anyway, fwiw, I agree all of what mapism writes above.
MapisM
Well-Known Member
Porto, I have a funny feeling that I'd better agree to disagree, rather than debating further...
But it's a cold winter night, and TV schedule is rubbish as usually, so here we are.
First of all, I couldn't find in the last (and Nth...) article that you linked anything that wasn't discussed here before in some occasion - and most often than not, done to death.
In fact, I don't think anyone here in the asylum would argue against the concept that a deep-V P hull is better suited to keep going fast in conditions that might force other ("flatter") P boats to slow down.
But the point is, so what?
If you hope to convince not only myself, but also Deleted User, MYAG, JFM, BartW, Hurricane (to name but a few) that they should have choosen an Itama because their boats, according to that article, "are poorly suited to venturing offshore", well, I neither want nor can speak on their behalf, but I can only wish you good luck. 
And ref. rolling at anchor, as I understood your previous point was that deep-V hulls PITCH less than flatter hulls - something on which I might agree in principle, but it's irrelevant because pitching is almost never the problem, at anchor.
Otoh, you are now saying that they ROLL less, which is, quite simply, not true.
Depending on the conditions (wave heigth, length, period, etc.), of course you can have a bigger hull rolling more than even a small tender, but again, so what?
One day an Itama 46 might roll less than a F630, but rest assured that another day it'll be the opposite. And I know on which of them I'd rather live aboard, anyway.
If stopping the rolling motion of a monohull were as simple as designing some rails, nobody would have invented STAR stabs.
Also, ref. the D hulls behaviour, you've got it completely wrong.
First of all, for any given length, just half of the keel of a D hull gives more roll resistance than all the rails/chines/whatever you can think of in any P hull.
In spite of that, D hulls do tend to roll more if you measure rolling only in terms of angle (due to CoG, submerged volumes, and several other factors) compared to a P hull, but the motion is more bearable anyhow - just re-read what I wrote in post #63.
Secondly, most D hulls, mine included, couldn't be more far from the "tea tray" analogy. Their entry point at the bow is razor sharp in comparison to ANY P hull, no matter how deep-V they are.
Trust me, you don't want to experience the feeling of a D hull cutting through a head sea, because if having a non-slamming boat is your priority, you wouldn't consider your Itama acceptable anymore. Just think of how disappointing you found the difference between your Itama and the Rivale, and double that - at least!
In fact, if you think that I would have rather been on a P hull that night, when in spite of a strong pitching the motion was smooth enough to keep sleeping, well, think again!
Not to mention that on a P boat (and much more so if plastic, of course), one foot high waves on the bow, which are nowhere near enough to make the boat pitch, can already wake you up just for the slapping noise. But that's another chapter altogether...
But it's a cold winter night, and TV schedule is rubbish as usually, so here we are.
First of all, I couldn't find in the last (and Nth...) article that you linked anything that wasn't discussed here before in some occasion - and most often than not, done to death.
In fact, I don't think anyone here in the asylum would argue against the concept that a deep-V P hull is better suited to keep going fast in conditions that might force other ("flatter") P boats to slow down.
But the point is, so what?
If you hope to convince not only myself, but also Deleted User, MYAG, JFM, BartW, Hurricane (to name but a few) that they should have choosen an Itama because their boats, according to that article, "are poorly suited to venturing offshore", well, I neither want nor can speak on their behalf, but I can only wish you good luck.
And ref. rolling at anchor, as I understood your previous point was that deep-V hulls PITCH less than flatter hulls - something on which I might agree in principle, but it's irrelevant because pitching is almost never the problem, at anchor.
Otoh, you are now saying that they ROLL less, which is, quite simply, not true.
Depending on the conditions (wave heigth, length, period, etc.), of course you can have a bigger hull rolling more than even a small tender, but again, so what?
One day an Itama 46 might roll less than a F630, but rest assured that another day it'll be the opposite. And I know on which of them I'd rather live aboard, anyway.
If stopping the rolling motion of a monohull were as simple as designing some rails, nobody would have invented STAR stabs.
Also, ref. the D hulls behaviour, you've got it completely wrong.
First of all, for any given length, just half of the keel of a D hull gives more roll resistance than all the rails/chines/whatever you can think of in any P hull.
In spite of that, D hulls do tend to roll more if you measure rolling only in terms of angle (due to CoG, submerged volumes, and several other factors) compared to a P hull, but the motion is more bearable anyhow - just re-read what I wrote in post #63.
Secondly, most D hulls, mine included, couldn't be more far from the "tea tray" analogy. Their entry point at the bow is razor sharp in comparison to ANY P hull, no matter how deep-V they are.
Trust me, you don't want to experience the feeling of a D hull cutting through a head sea, because if having a non-slamming boat is your priority, you wouldn't consider your Itama acceptable anymore. Just think of how disappointing you found the difference between your Itama and the Rivale, and double that - at least!
In fact, if you think that I would have rather been on a P hull that night, when in spite of a strong pitching the motion was smooth enough to keep sleeping, well, think again!
Not to mention that on a P boat (and much more so if plastic, of course), one foot high waves on the bow, which are nowhere near enough to make the boat pitch, can already wake you up just for the slapping noise. But that's another chapter altogether...
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Portofino
Well-Known Member
jfm
Well-Known Member
On what do you base that statement? It's just your hunch PF; it isn't actually true. If your clinker or smooth dinghies are is sitting at anchor and then a wave comes, the last thing you want is grip on the sea, because that will make you roll in line with the wave. You'd rather have slippy. In any case, the drag/grip effect of all the chines/rails at these water velocities is immeasurable except with instrumentation
As for the rest, you are lost in the fantasy world where you think that a deep vee hull needs lift strips for lift, because in your world a sloping surface doesn't provide enough lift. That defies physics, though I agree it is an oft-quoted concept. If it were true the addition of spray rails to a V hull would cause the boat to sit higher in the water at rest, which obviously is not what happens.
The function of a rail is to shed spray, which is critical to (a) drag in the context of fast boat performance, and (b) dryness on less performance-oriented boats.
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Portofino
Well-Known Member
Porto, I have a funny feeling that I'd better agree to disagree, rather than debating further...
But it's a cold winter night, and TV schedule is rubbish as usually, so here we are.
First of all, I couldn't find in the last (and Nth...) article that you linked anything that wasn't discussed here before in some occasion - and most often than not, done to death.
In fact, I don't think anyone here in the asylum would argue against the concept that a deep-V P hull is better suited to keep going fast in conditions that might force other ("flatter") P boats to slow down.
But the point is, so what?
If you hope to convince not only myself, but also Deleted User, MYAG, JFM, BartW, Hurricane (to name but a few) that they should have choosen an Itama because their boats, according to that article, "are poorly suited to venturing offshore", well, I neither want nor can speak on their behalf, but I can only wish you good luck.
And ref. rolling at anchor, as I understood your previous point was that deep-V hulls PITCH less than flatter hulls - something on which I might agree in principle, but it's irrelevant because pitching is almost never the problem, at anchor.
Otoh, you are now saying that they ROLL less, which is, quite simply, not true.
Depending on the conditions (wave heigth, length, period, etc.), of course you can have a bigger hull rolling more than even a small tender, but again, so what?
One day an Itama 46 might roll less than a F630, but rest assured that another day it'll be the opposite. And I know on which of them I'd rather live aboard, anyway.
If stopping the rolling motion of a monohull were as simple as designing some rails, nobody would have invented STAR stabs.
Also, ref. the D hulls behaviour, you've got it completely wrong.
First of all, for any given length, just half of the keel of a D hull gives more roll resistance than all the rails/chines/whatever you can think of in any P hull.
In spite of that, D hulls do tend to roll more if you measure rolling only in terms of angle (due to CoG, submerged volumes, and several other factors) compared to a P hull, but the motion is more bearable anyhow - just re-read what I wrote in post #63.
Secondly, most D hulls, mine included, couldn't be more far from the "tea tray" analogy. Their entry point at the bow is razor sharp in comparison to ANY P hull, no matter how deep-V they are.
Trust me, you don't want to experience the feeling of a D hull cutting through a head sea, because if having a non-slamming boat is your priority, you wouldn't consider your Itama acceptable anymore. Just think of how disappointing you found the difference between your Itama and the Rivale, and double that - at least!
In fact, if you think that I would have rather been on a P hull that night, when in spite of a strong pitching the motion was smooth enough to keep sleeping, well, think again!
Not to mention that on a P boat (and much more so if plastic, of course), one foot high waves on the bow, which are nowhere near enough to make the boat pitch, can already wake you up just for the slapping noise. But that's another chapter altogether...
It's not about comparisons it's about looking for any features on the hull ,it's form. In your size n price range that may effect the enjoyment of the boat .
If so identifying them .They a very subtle but nether the less there .
A guy just asked about hull form @ anchor if it's choppy and i said more or less those features that make it above average going up wind should and do by abservation make the hull a more pleasant place to be in a choppy anchor ,
Reduce pitch and roll ' ending up with a more stable platform .
The links seem fine well balanced interesting stuff for MoBo ers
Portofino
Well-Known Member
Clinker --- think about it - as the wave hit it side on it does indeed start to lift some of the "clinks" out as the boat rolls .
BUT many more on the other side are dipped in and the clinks resist the roll more cos more are submerged on the Lee side ans more airborne on the wave ,windward side .
No a deep V does not NEED spray rails in absolute terms to provide lift .
Buy they can as said when they run back morph ( underwater no spray ) into wider sections of flatter "tea tray "ness
Along with wider hard chines ,those surfaces will provide lift and harden the ride and increase roll resistance .
Obviousley planning dynamic lift
But are talking static at anchor .
Put your hand flat into a sink ,then @ 45 degrees .The flatter hand has more resistance .The flatter the hand to the water surface ,think belly flop dive
the more resistance .