Elessar
Well-Known Member
you only have to worry about paid crew, it's illegal to expose them to high levels of noise. AFAIK it's quite legal to deafen passengers.
Boat in build pics (2013 Fairline Squadron 78)
- Thread starter jfm
- Start date
you only have to worry about paid crew, it's illegal to expose them to high levels of noise. AFAIK it's quite legal to deafen passengers.
jfm
Well-Known Member
I don't get your physics rafiki about cancelling forces. What cancelling forces?, other than the rudders of course, which = drag.
You have 2 crankshafts spinning on a longitudinal axis in the boat, both same direction. Then the boat pitches in a head sea, ie rotates as you view the boat side-on. As a matter of physics, the boat must then precess unless something else (rudders) applies an opposite turning moment, ie it will rotate as you view it from above, ie turn left/right. There can be no doubt about the physics of that and I was only asking whether anyone knew the magnitude - seems to me pretty small and not worth worrying about but these racing guys are pretty hot about saving 1 second per hour in a Formula 1 kind of way
Piers
Well-Known Member
you only have to worry about paid crew, it's illegal to expose them to high levels of noise. AFAIK it's quite legal to deafen passengers.
We publish a Safety Advice doc on our website which you may find interesting. Note that the COLREGS state, “A whistle shall be placed as high as practicable on a vessel, in order to reduce interception of the emitted sound by obstructions and also to minimize hearing damage risk to personnel.”
However, have a watch of this 'Horn battle between the QE2 and the Queen Victoria' which was to be the last time these great ships were together before the QE2 was retired. The effect on the passengers says it all!
http://www.youtube.com/watch?v=e5wh098yEwE
Piers
rafiki_
Well-Known Member
I don't get your physics rafiki about cancelling forces. What cancelling forces?, other than the rudders of course, which = drag.
You have 2 crankshafts spinning on a longitudinal axis in the boat, both same direction. Then the boat pitches in a head sea, ie rotates as you view the boat side-on. As a matter of physics, the boat must then precess unless something else (rudders) applies an opposite turning moment, ie it will rotate as you view it from above, ie turn left/right. There can be no doubt about the physics of that and I was only asking whether anyone knew the magnitude - seems to me pretty small and not worth worrying about but these racing guys are pretty hot about saving 1 second per hour in a Formula 1 kind of way
Hmmmmm, I gotta think about this some more. I've shut the grey cells down for the Christmas vacation
JFM, as the engines are effectively bolted down (assume for now bolted solid to the hull) any precession torque will apply around the relative centre of moment of the hull, the keel line. My assumption is that while not perfectly balanced, with 2 engines, the forces will cancel each other out. I recognise this is not a perfect maths argument, but I think like you, the resultant forces are going to be so relatively small to be inconsequential. Happy to hear a pure mathematics argument otherwise though.
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benjenbav
Well-Known Member
you only have to worry about paid crew, it's illegal to expose them to high levels of noise. AFAIK it's quite legal to deafen passengers.
I 'preciate that that was a joke but fer chrissakes don't anyone toot yer hooters at any passengers or pupils who have even heard a rumour about the possibility of human rights laws or that thought is one which might just need to be revised.
benjenbav
Well-Known Member
I don't get your physics rafiki about cancelling forces. What cancelling forces?, other than the rudders of course, which = drag.
You have 2 crankshafts spinning on a longitudinal axis in the boat, both same direction. Then the boat pitches in a head sea, ie rotates as you view the boat side-on. As a matter of physics, the boat must then precess unless something else (rudders) applies an opposite turning moment, ie it will rotate as you view it from above, ie turn left/right. There can be no doubt about the physics of that and I was only asking whether anyone knew the magnitude - seems to me pretty small and not worth worrying about but these racing guys are pretty hot about saving 1 second per hour in a Formula 1 kind of way
Maybe set up the ballast with a bias to the opposite side so as to drive flat at full chat?
MapisM
Well-Known Member
Blimey J, here goes my concern about technical thread drifts!
I thought to have heard and read a lot about speedboats optimization (the weirdest thing probably being the removal of a bolt frm a surface transmission giving a 2kts improvement - and it was no less than Buzzi chief mechanic to tell me about it, go figure...), but the gyro stabilizing effect of a rotating crankshaft probably beats them all.
Actually, I can't see any valid reason why it shouldn't exist, in principle, but I suppose it must be worth less than half a iota, not only for the good reasons you already stated, but also because the effect should be well kown (sort of) if it would be practically meaningful.
Otoh, there is another effect of engine rotation which is well known, particularly on small single screw speedboats, and it's the torque effect which makes the boat list to stbd/port when accelerating/decelerating quickly - as it often happens with fast boats, when jumping waves.
But the gyro effect at constant RPM...?!?
That's another kettle of fish, and even if I can't claim that it isn't there in theory, I'd be very surprised if anyone would tell that it has ever been considered - let alone addressed with some kind of counter-measure.
MapisM
Well-Known Member
By chance, aren't you thinking about the torque effect I just mentioned?
As I understand, that's not what jfm has in mind.
But I might well be the one who misunderstood him...
rafiki_
Well-Known Member
By chance, aren't you thinking about the torque effect I just mentioned?
As I understand, that's not what jfm has in mind.
But I might well be the one who misunderstood him...
Iirc precession torque is 90 degrees to the rotational torque. We get it on cars when tyres are not sufficiently round, and with most tyre sizes, the first order is at 55 to 60 mph. Usually people pick up the 1.5 order harmonic at 75 - 80 mph, as this is a cruising speed, whereas the lower one is more transient.
Actually, I'm not sure I'm helping much here
Piers
Well-Known Member
Iirc precession torque is 90 degrees to the rotational torque. We get it on cars when tyres are not sufficiently round, and with most tyre sizes, the first order is at 55 to 60 mph. Usually people pick up the 1.5 order harmonic at 75 - 80 mph, as this is a cruising speed, whereas the lower one is more transient.
Actually, I'm not sure I'm helping much here
Hi guys,
I'm following this torque discussion somewhat bemused at the physics and not understanding much about it at all - but I'm really enjoying reading it!
Once there's agreement, can you place a statement on the thread so that my poor brain can understand what's been said?
Thank you - and have a great New Year.
Piers
rafiki_
Well-Known Member
Piers, simplistically if you use your thumb, forefinger and middle finger as the z, y and x axises this will show you the direction of the various torque vectors when something rotates. Iirc your middle finger is the precession vector, but it is a long time since I worked on any of this, without delegating to someone with more patience than me
Cookee
Well-Known Member
The way I have always looked at prop rotation is that "turning in" (R/H on the port side) results in more stern lift (where all of the weight is) and more speed but not so good on handling due to the paddle wheel effect pushing the water down. Conversely turning out is more stable but slower. This is a general rule although we now turn outwards as it seems to be faster on our boat over the range of conditions experienced offshore.
The boat I was talking about now runs in the US as Caveman #131 with Geico sponsorship (Website It's a Skater 39' monohull and runs at over 130mph - it was called "HoneyParty" whilst it raced in the UK, it was built and rigged in the US overseen by Mark Pascoe, long time friend of the owner and the same as the Pascoe RIB brand where he has returned to. As I said above somewhere, it's a slim boat and whilst it needs all of the stability it can get I doubt the difference is very measurable although I am prepared to be corrected on that as I have no direct experience of the handling of the boat.
jfm
Well-Known Member
Rafiki, I think you're mixed up on gyro forces. Your mention of two engines bolted to the hull "cancelling out" very much misses the point. The along-the-keel-line torque you are referring to is NOT precession. It is the action=reaction (Newton's first law) force that MapisM refers to 4 posts above. As the crankshafts/flywheels accelerate (not rotate, but accelerate, whether +ve or -ve) clockwise there is an anticlockwise torque on the hull, and vv. Having two engines both spinning say CCW creates this force as the throttleman does his thing, and having two engines counter rotational eliminates it. But this is not precession; this is merely Newton's first law. ( BTW, I'm talking about engines being counter rotational, not merely prop shafts).Hmmmmm, I gotta think about this some more. I've shut the grey cells down for the Christmas vacation
JFM, as the engines are effectively bolted down (assume for now bolted solid to the hull) any precession torque will apply around the relative centre of moment of the hull, the keel line. My assumption is that while not perfectly balanced, with 2 engines, the forces will cancel each other out. I recognise this is not a perfect maths argument, but I think like you, the resultant forces are going to be so relatively small to be inconsequential. Happy to hear a pure mathematics argument otherwise though.
Also, gyro forces have nothing to do with out of balance-car-tyres. Out-of-balance spinning objects also behave in accordance with Newton, and create harmonics because of the rotation. But that too is not precession
Precession is a different thing altogether. Suppose you have two crankshaft/flywheels both spinning the same way along a keel line axis in a boat. Then the bow hits a wave and rises - this means the hull rotates about an athwartships axis. Absence anything else you will then get precession - precession here means that (a) the boat will rotate about a vertical axis, in other words if you are above the boat in a helicopter you will see the boat turn left or right without the helmsman doing anything - THAT is precession; and (b) the bow will lift less than it would if the crankshafts were not spinning (being the gyro stab effect)
Alternatively you can stop the precession, eg by having a perfect helmsman or a/pilot who perfectly adjusts the rudders to eliminate it, but that creates drag (and allows the bow to lift fully with the wave because the absence of precessions means there is no gyro stabilisation anymore).
There is no doubt about the forces and effects i'm describing. All I was asking at the beginning of this discussion was whether their magnitude is so tiny that even the hard-core racing guys who care about 1 second in an hour don't even worry about it. If they did worry about it, it would be a reason, in addition to MapisM's Newton-1st-law reason, to have counter rotating engines in the ultimate race boat. As I said above, I guess the effects are too small to worry about
jimmy_the_builder
Well-Known Member
rafiki_
Well-Known Member
jfm; Also said:
JFM, I get all of this, and fully understand the physics, but am trying to guesstimate the relative forces and vectors. However, I'm still in holiday mode so brain is not fully engaged.
My point on tyres was not about balance, but the precession caused by out of round, or concentric forces. I've had to fix a few designs to reduce the sensitivity to this, as it is very difficult to build a perfect tyre. Apologies again for the Fred Drift, and if the analogy was not helpful.
jfm
Well-Known Member
Confused here. You are talking about a tyre that is out of round but not out of balance? The physical effects of that out-of-roundness seem to me to be Newtonian forces as the egg-shaped tyre (aka, to an engineer, a cam) runs on a flat road, and that has nothing whatever to do with gyroscopic precession. But am I missing some link with precession, and can you explain the link that I'm missing?JFM, I get all of this, and fully understand the physics, but am trying to guesstimate the relative forces and vectors. However, I'm still in holiday mode so brain is not fully engaged.
My point on tyres was not about balance, but the precession caused by out of round, or concentric forces. I've had to fix a few designs to reduce the sensitivity to this, as it is very difficult to build a perfect tyre. Apologies again for the Fred Drift, and if the analogy was not helpful.
In my book precession occurs by virtue merely of a spinning mass being rotated also about an axis other than its spin axis. It has nothing to do with out of round (or out of balance) effects which are generally Newtonian
rafiki_
Well-Known Member
JFM, think of a spinning top that is slightly off its axis. If the tyre is slightly out of round you get this precession effect. You will also get a Newtonian out of balance effect, but this can be balanced out. It is the precession that is harder to eliminate. However if you look at the relative masses, the heavy tyre relative to the suspension mounts, compared to the masses involved in an engine, then I don't think the precession effects will be significant in a boat. But I'm happy to learn otherwise.
Elessar
Well-Known Member
I 'preciate that that was a joke but fer chrissakes don't anyone toot yer hooters at any passengers or pupils who have even heard a rumour about the possibility of human rights laws or that thought is one which might just need to be revised.
only sort of a joke. I started an F1 car up inside once so had to do a bit of research. 350 people were to be present. I was bound to ensure the volume and duration of exposure to the paid staff was measured and controlled. To the public, all I had to do was put up a sign saying it was loud. I gave them all disposable in ears just in case.
Part of the problem was I rang the F1 team the day before and asked how loud it was. "Very" they said. "can you quantify" said I. "no we haven't built it yet......."
It was indeed very, very loud.
So perhaps Match just needs so foam earplugs as part of the brief
MapisM
Well-Known Member
Yup, I agree on the conclusion, as I already said.
But I can see why jfm has the doubt, and tbh I also don't understand what the "slightly out of round" has to see with the precession.
If you look at this video, the effect shown is there regardless of any unbalance.
And if I understand what jfm means, considering that both engines in a boat spin in the same direction as the wheel in the video, the precession of both crankshafts/flywheels should constantly try to steer the boat to port.
And the opposite with V-drives, of course.
I would agree with jfm that in theory such effect would be increased, rather than compensated, with two mills spinning both CCW (as in 99.99% of boats), even if not aligned with the keel.
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jfm
Well-Known Member
Rafiki, I'll give up please at this point. I honestly don't understand anything you're saying; not one word of it. I have no idea what "precession effect" you are referring to in your 2nd sentence
