boat models and lengths

[Piers]

Out of curiosity, do you specify a minimum safe distance from points reachable by anyone onboard, for that sort of stuff?
IIRC, the arch on theSq78 is not that high, so I guess it's not easy to place the horn in a place where you are 100% sure to not blow a passing crew member's ear out...

We publish some H&S around just this: http://www.kahlenberg.co.uk/safety-advice-for-kahlenberg-horns

Some people fit them on the eyebrow in front of the fly, such as here on a Princess 50: http://www.kahlenberg.co.uk/news/42-kahlenberg-d-0a-and-m-485a-princess-50-case-study

Piers

 

[Deleted User YDKXO]

Just to mention that US boats often state a length which excludes the bathing platform and pulpit ie just the absolute hull length so US boats often look 'small' in adverts compared to Euro boats when in actual fact they may be just as big or bigger. Also don't forget that beam plays as big a part in how large a boat feels as much as length so compare beam as well as length.
In the end what matters is not what the manufacturer's stated dimensions are but how big a boat feels to you and whether it's big in the places that you want it to be. For example a boat with big cabins and small saloon may suit you but not other people. When we look at boats, my SWMBO carries a tape measure in her pocket (I'm serious) and measures the spaces that are important to us (eg saloon and master cabin) and compares those measurements with our existing boat to see exactly where we would gain or lose

 

[jfm]

Out of curiosity, do you specify a minimum safe distance from points reachable by anyone onboard, for that sort of stuff?
IIRC, the arch on theSq78 is not that high, so I guess it's not easy to place the horn in a place where you are 100% sure to not blow a passing crew member's ear out...

Yup, it couldn't go on the mast. As Piers says, I am having it fitted on the fibreglass apron/lid, in front of the flybr windscreen and above the lower helm windscreen

Colregs actually stipulate BOTH a Hz level and a minimum sound level for a boat over 20m AND a rule that it must be placed so it does not deafen the crew. Achieving BOTH those things is quite difficult

 

[jfm]

Is it? An up/down platform should always be considered in LOA, afaik.
But Piers mentioned "retractable swim platform", which is a very different thing.
Actually, there are just a very few boats with platforms sliding out from the stern (which is what "retractable" reminds me), but in some big stuff it's not unusual to see the whole stern hinged at its bottom, opening to create a huge platform. I would think that those platforms fit the "retractable" concept, but not the hi/low ones.

Yes i totally see your point. It's just that some poeple might argue that something is not fixed if it is moveable. There is some fine semantics and language. I agree with you though that in the case of up/down platform "you'd think the draftsman didn't intend to exclude" it

 

[Piers]

Yup, it couldn't go on the mast. As Piers says, I am having it fitted on the fibreglass apron/lid, in front of the flybr windscreen and above the lower helm windscreen

Colregs actually stipulate BOTH a Hz level and a minimum sound level for a boat over 20m AND a rule that it must be placed so it does not deafen the crew. Achieving BOTH those things is quite difficult

COLREGs also state that horns must point forward. Although somewhat obvious, have a look around and see just how many horns are mounted pointing down. I've even seen horns mounted pointing vertically downwards.

 

[jfm]

The horn you've specified is loud - and I really mean LOUD.

By the way, because I sail in the Med and do not know what fog is, its main function will be (a) a general boys toy ie useless but nice to have and interesting to show your mates; and (b) to join in the "boat horn applause" where all of the anchored boats blast their horns to say thank you after the many firework displays in Cote D'Azur anchorages in the summer.

I'm very much looking forward to it Piers and thanks to you and Toby for the help in specifying the right hardware

 

[Piers]

By the way, because I sail in the Med and do not know what fog is, its main function will be (a) a general boys toy ie useless but nice to have and interesting to show your mates; and (b) to join in the "boat horn applause" where all of the anchored boats blast their horns to say thank you after the many firework displays in Cote D'Azur anchorages in the summer.

I'm very much looking forward to it Piers and thanks to you and Toby for the help in specifying the right hardware

Thanks, jfm. Enjoy, as they say...

 

[MapisM]

COLREGs also state that horns must point forward. Although somewhat obvious, have a look around and see just how many horns are mounted pointing down. I've even seen horns mounted pointing vertically downwards.

Actually, I never really understood the logic of pointing horns HORIZONTALLY forward, particularly in head wind conditions.
I don't think that sound propagation would actually be better if compared to horns pointing forward at say a 30° down angle.
But you surely could tell us more about that...?
I agree that vertically downwards doesn't make a lot of sense, anyway.

 

[MapisM]

a general boys toy

Tut, tut.
Do you mean like firing expired parachure flares at night, while anchored somewhere?

 

[Piers]

Actually, I never really understood the logic of pointing horns HORIZONTALLY forward, particularly in head wind conditions. I don't think that sound propagation would actually be better if compared to horns pointing forward at say a 30° down angle. But you surely could tell us more about that...?

I'm told that horizontal projection really does achieve the maximum fwd distance, and that adjusting the horn up or down, even a bit, reduces the distance. There's physics to back it up somewhere....

I agree that vertically downwards doesn't make a lot of sense, anyway.

Agreed. The main reason for lowering a trumpet at all is to prevent or at least reduce any sea water ingress from corroding the horn's diaphragm. That, of course, depends on whether your horn has a diaphragm that can corrode....

But to have them pointing vertically downwards is just crazy. How about the first to spot which manufacturer does this wins a virtual beer?

 

[jfm]

I've never done that have I? [trying to remember if I have 'fessed to something years ago...]

 

[jfm]

Does the wind matter much? The sound waves travel at 680mph so a 50mph (say) headwind is a 7% reduction in the range of the horn. I suppose it depends on how much you care about 7%.

 

[Piers]

Does the wind matter much? The sound waves travel at 680mph so a 50mph (say) headwind is a 7% reduction in the range of the horn. I suppose it depends on how much you care about 7%.

Yes it does. I'll try and draw the effect and attach it.

 

[MapisM]

I've never done that have I? [trying to remember if I have 'fessed to something years ago...]

Nope, no worries, you didn't - not as I recall, anyway.
I was just making another example of using as a "boys toy" something which is not supposed to...

 

[MapisM]

How about the first to spot which manufacturer does this wins a virtual beer?

That's a difficult one.
I did see a few horns installed that way, but I never noticed any correlation between that and a specific builder.
C'mon, name and shame!

 

[Piers]

Does the wind matter much? The sound waves travel at 680mph so a 50mph (say) headwind is a 7% reduction in the range of the horn. I suppose it depends on how much you care about 7%.

Hi jfm,

Not being able to attach a drawing, I've found suitable words from researching sound propegation on the interweb. Here goes,

"It’s a common observation that sounds can be heard better downwind of the source than upwind, or that 'the wind carries the sound.' A true statement, but why?

"Like water in a stream or air moving over an aerofoil, wind is retarded at the surface of the earth (or sea) in an equivalent boundary layer effect, whilst it increases in speed aloft. Whatever the speed difference, this is called wind shear.

"A sound wavefront propagating with the wind will therefore have its top inclined forward whilst it slows at the surface, having the effect of returning it to the surface. Conversely, a wavefront propagating against the wind will be deflected upwards and away from the surface.

"The effect can be marked even in light winds, but becomes more obvious the stronger the wind. The result is that audible distance is greater downwind and shorter upwind.

Does this answer the qn?

Piers

 

[jfm]

Hi jfm,

Not being able to attach a drawing, I've found suitable words from researching sound propegation on the interweb. Here goes,

"It’s a common observation that sounds can be heard better downwind of the source than upwind, or that 'the wind carries the sound.' A true statement, but why?

"Like water in a stream or air moving over an aerofoil, wind is retarded at the surface of the earth (or sea) in an equivalent boundary layer effect, whilst it increases in speed aloft. Whatever the speed difference, this is called wind shear.

"A sound wavefront propagating with the wind will therefore have its top inclined forward whilst it slows at the surface, having the effect of returning it to the surface. Conversely, a wavefront propagating against the wind will be deflected upwards and away from the surface.

"The effect can be marked even in light winds, but becomes more obvious the stronger the wind. The result is that audible distance is greater downwind and shorter upwind.

Does this answer the qn?

Piers

No, I'm afraid not . It is indeed true that there is a boundary layer effect when a fluid (air/wind in this case) moves close to a fixed surface (eg the sea). It is therefore true in a headwind that the soundwaves in the bottom part of the "sound cone", close to the sea's surface, will propogate faster/further that the ones at the top of the cone that are fighting more headwind and therefore walking the wrong way agianst a moving conveyor belt. Therefore, the cone of sound sort of gets deflected upwards

BUT BUT, I would say that the effect here is incredibly insignificant. The boundary layer thickness for air moving at any windspeed velocity you care to think of is incredibly small. Boundary layer thickness is generally defined as the distance from the surface (the sea, in this case) where the moving fluid reaches 99% of its free flow velocity. For air travelling at say 50mph (20m/sec) the boundary layer is less than 10mm thick. Now you can increase that for various factors including the rough surface of the sea caused by waves, but you still have you have a totally insignificant boundary layer

In contrast the simple "running against a moving conveyor belt" effect i described makes a 7% reduction in range of the horn and is much more significant than the boundary layer effect.

Happy to be corrected if Kahlenberg's sound engineers think differently!

 

[MapisM]

In contrast the simple "running against a moving conveyor belt" effect i described makes a 7% reduction in range of the horn and is much more significant than the boundary layer effect.

Mmm... Intriguing point, this one.
I would tend to agree with all your considerations, but surely the experience suggests that the difference in sound propagation when up or downwind is much higher. Even just hearing someone speaking from the bow to the cockpit, is very different depending on which way the wind blows.
I'm only thinking aloud of course, but based on gut feelings I would think that there must be some other factor which we are now missing.
A difference of a few % points couldn't be so evident to the ears.

 

[jfm]

Mmm... Intriguing point, this one.
I would tend to agree with all your considerations, but surely the experience suggests that the difference in sound propagation when up or downwind is much higher. Even just hearing someone speaking from the bow to the cockpit, is very different depending on which way the wind blows.
I'm only thinking aloud of course, but based on gut feelings I would think that there must be some other factor which we are now missing.
A difference of a few % points couldn't be so evident to the ears.

Hmm yes i agree your intuitive conclusion. Not sure why. A lot of the relationships here are parabolic or at least non linear, so depending where you are on the curve you can have a big change in y for a small change in x. That's not a full explanation i know...

BartW may well be the best person to explain all this but I bet he is cruising on his superyacht somewhere in the sun. He has become such a playboy

 

[rafiki_]

Could there be another influence like other noises being garnered raising the ambient background noise? At home, we are 3 miles from the motorway, and very occasionally, with the wind in the right direcion, can we hear traffic.