Visit our Popular Forums

GTom · 23-09-2019, 03:42

I was wondering, if a sailboat could make any use of a pricier class B+ AIS, with 5W transmitting power?

Antenna being on the mast, behind a splitter there are two things: more losses, than on a motorboat and more radio horizon to make use of. Both point on possible advantage of more transmitting power. However, speed is a fraction of mobo's, update frequency is not an issue.

Sailmonkey · 23-09-2019, 04:26

We have a “B+”, not for the update frequency (we’re not fast enough to change the frequency) rather for the 5 watts of power and the sotdma protocol.

Tdma works, but sotdma is better.

**tkeithlu** · 23-09-2019, 05:06

I don't see it if the cost (it, antenna installation, etc.) is a factor. You're a sailboat. You do less than ten knots, and a target is not likely to be doing more than 20. You can divert from a collision course very quickly, while the target maintains course and speed. Your closing speeds are too slow to have a 12 mile range on your screen be a problem.

Of coursde, if the target is US or Norwiegen Navy, not even being a 100,000 ton tanker will save you....

GTom · 23-09-2019, 06:17

Quote:

Originally Posted by tkeithlu

I don't see it if the cost (it, antenna installation, etc.) is a factor. You're a sailboat. You do less than ten knots, and a target is not likely to be doing more than 20. You can divert from a collision course very quickly, while the target maintains course and speed. Your closing speeds are too slow to have a 12 mile range on your screen be a problem.

Of coursde, if the target is US or Norwiegen Navy, not even being a 100,000 ton tanker will save you....

Transmission range is interesting for me, 5W gives more chance for satellite-AIS and certainly more time for sleepy containership-crew to detect us... However, if 10miles were feasible with normal B class, then I'd not bother.

crankysailor · 23-09-2019, 06:19

Hmm am I missing something or SOTDMA "for everybody" is an idea which like communism, makes sense in principle but is a disaster in practice? The ships from which everyone wants a position update as frequently as possible (the big fast guys) would find it increasingly hard to find an available time slot to send?.. I.e. it is a good thing that the big ships with the class A transponders can send a lot more often, no? or is this somehow still guaranteed, I.e. class A SOTDMA has higher priority still than B+? if so then the only point as a sailboat would really just be the higher xmit power methinks.

[EDIT]

Ok I googled this..

"Class B+ uses the same SOTDMA technology as Class A and therefore has the same priority when it comes to reserving a time slot"

But it seems that devices should be well behave and send less updates if the boat speed is slow...still...It seems to me like a way for a misbehaving device to cause problems at a busy port...

**Dockhead** · 23-09-2019, 06:58

Quote:

Originally Posted by GTom

I was wondering, if a sailboat could make any use of a pricier class B+ AIS, with 5W transmitting power?

Antenna being on the mast, behind a splitter there are two things: more losses, than on a motorboat and more radio horizon to make use of. Both point on possible advantage of more transmitting power. However, speed is a fraction of mobo's, update frequency is not an issue.

The biggest advantage of B+ is SOTDMA. In busy waters, Class B signals often get lost. You are much more likely to be seen in busy waters with B+, and I will be changing my AIS to this during this winter for this reason alone.

More transmit power is always good, can never hurt, but why are you using a splitter? There are at least three or four reasons why you want a separate antenna and feedline for AIS.

Lastly, I wouldn't agree that update frequency is not an issue. Remember that you are reporting not only your position, but your speed and COG. If you are maneuvering, it may be a serious problem if the ship doesn't see this for 30 seconds or even for minutes, in case some of your transmissions don't get through.

In my opinion, B+ is really important to have for just about anyone who does much sailing in waters where ships are encountered.

**StuM** · 23-09-2019, 07:06

Quote:

Originally Posted by crankysailor

Hmm am I missing something or SOTDMA "for everybody" is an idea which like communism, makes sense in principle but is a disaster in practice? The ships from which everyone wants a position update as frequently as possible (the big fast guys) would find it increasingly hard to find an available time slot to send?.. I.e. it is a good thing that the big ships with the class A transponders can send a lot more often, no? or is this somehow still guaranteed, I.e. class A SOTDMA has higher priority still than B+? if so then the only point as a sailboat would really just be the higher xmit power methinks.

[EDIT]

Ok I googled this..

"Class B+ uses the same SOTDMA technology as Class A and therefore has the same priority when it comes to reserving a time slot"

But it seems that devices should be well behave and send less updates if the boat speed is slow...still...It seems to me like a way for a misbehaving device to cause problems at a busy port...

A sailboat will only be grabbing a timeslot every 30 seconds, not every 5 seconds.

And SOTDMA "allows up to 4500 ships to work within close proximity of one another, automatically giving priority based on distance apart, i.e. as the number of vessels increases, the ones furthest away do not get a time slot" (How that works in practice, I have no idea. The "ones furthest away" from what do not get a time lot?)

**tkeithlu** · 23-09-2019, 07:39

We're missing something in this learned debate. You need to see, and know to avoid, far more than you need to be seen. Whether it's a container ship, a tanker, of the Norweign Navy, you're not going to be asking them to avoid you. You're going to be looking at the screen, seeing the call sign, calling it on the radio, and saying "Captain, if you will maintain course and speed, I will divert to starboard, and we will pass port to port."

Like your tag line, GTom.

GTom · 23-09-2019, 13:13

Quote:

Originally Posted by tkeithlu

We're missing something in this learned debate. You need to see, and know to avoid, far more than you need to be seen. Whether it's a container ship, a tanker, of the Norweign Navy, you're not going to be asking them to avoid you. You're going to be looking at the screen, seeing the call sign, calling it on the radio, and saying "Captain, if you will maintain course and speed, I will divert to starboard, and we will pass port to port."

Like your tag line, GTom.

Very right. I might be too chicken in a small boat, I'd say 90% of the cases I do the avoiding, not trying to muscle a container ship, which may don't even see me. Meaning, the receiver function is just about 90% more important.

Quote:

Originally Posted by Dockhead

The biggest advantage of B+ is SOTDMA. In busy waters, Class B signals often get lost. You are much more likely to be seen in busy waters with B+, and I will be changing my AIS to this during this winter for this reason alone.
More transmit power is always good, can never hurt, but why are you using a splitter? There are at least three or four reasons why you want a separate antenna and feedline for AIS.
Lastly, I wouldn't agree that update frequency is not an issue. Remember that you are reporting not only your position, but your speed and COG. If you are maneuvering, it may be a serious problem if the ship doesn't see this for 30 seconds or even for minutes, in case some of your transmissions don't get through.
In my opinion, B+ is really important to have for just about anyone who does much sailing in waters where ships are encountered.

Thanks for the valuable input! Right, I think I'll give a try without splitter - but need to find a place where to put the antenna to give a decent range (horizon) without disturbing the VHF. Not buying a ketch...

**Dockhead** · 23-09-2019, 15:06

Quote:

Originally Posted by tkeithlu

We're missing something in this learned debate. You need to see, and know to avoid, far more than you need to be seen. Whether it's a container ship, a tanker, of the Norweign Navy, you're not going to be asking them to avoid you. You're going to be looking at the screen, seeing the call sign, calling it on the radio, and saying "Captain, if you will maintain course and speed, I will divert to starboard, and we will pass port to port."

Like your tag line, GTom.

And if the ship is making 20 knots, and you 5?

I will have to disagree with this.

Speed is power to change the crossing and resolve a risk of collision, and we often have little of it, making it sometimes a matter of life and death being visible to larger vessels.

Many recreational sailors don't understand the distance reference frames of large vessels in open water. The crew of large vessels are trained to make decisions at about 10 miles, where many of us are hardly aware of other traffic. If you are supposed to stand on, then you need to stand on for a while and give the other vessel a chance to make his move.

If by three or four miles you are still on a collision course, and he was supposed to give way, it most likely means he didn't see you, or has other traffic preventing him from making an effective maneuver, then you need to make your own move, and calling him to let him know that you have taken up the active role is good advice. Not much later than that if there is a big difference in speed. If there is really a 4:1 speed difference, then by one mile out in many crossings there is not much you can do -- you are more or less a sitting duck. The crossing by that time is entirely in the hands of the faster vessel.

In any case, the more visible you are, the better. It reduces surprises and miscalculations and makes you a lot safer. Few radar reflectors are worth anything. Effectively communicating your position, speed and course and indeed presence by effectively broadcasting AIS could save your life.

Captain Anthony · 23-09-2019, 16:07

OK, Sailors, I'm in the process of a refit for my newly acquired Tartan 37 for east coast and Island sailing. I have a blank canvas, what AIS do I install? Class A or B system? I have read very good arguments for both, here.. This winter I will be stepping the mast and will be able to install anything that is needed.

What are the advantages of AIS in a VHF radio? Or, what are the disadvantages of AIS in a VHF radio? Can anyone offer a link to an in depth tutorial about the pro and/or cons of AIS?

Do you all have AIS at the helm or only at the Nav station?
Thank you, Captain Anthony

GTom · 23-09-2019, 16:20

I see absolutely no point of getting a Class A system for a moderate size sailing boat. If you feel the need of any advantage over Class B, B+/SOTDMA will get you there:
- enough power to be picked up by satellites(!) <-however, this is not hopeless with simple class B either.
- more frequent transmissions
- <half price compared to Class A (still more than Class B).

Telemark43 · 23-09-2019, 17:01

Quote:

Originally Posted by tkeithlu

I don't see it if the cost (it, antenna installation, etc.) is a factor. You're a sailboat. You do less than ten knots, and a target is not likely to be doing more than 20. You can divert from a collision course very quickly, while the target maintains course and speed. Your closing speeds are too slow to have a 12 mile range on your screen be a problem.

Of coursde, if the target is US or Norwiegen Navy, not even being a 100,000 ton tanker will save you....

My current mast is 61 feet high, and I routinely see AIS offhoreyat thirty miles. Sailboats with mast rigged antennas really get great AIS vision

Topspin · 23-09-2019, 17:54

The biggest factor is quality and height of the antenna, so an antenna in the mast top is what we should be looking at. If you don´t talk a lot on VHF, a splitter will provide access between AIS-transceiver and antenna all the time. There might be a small loss inside the splitter, but I would deem that insignificant. So no need for me to think about a second antenna.

If you look at the market, you will find, that a class B+ AIS is just a little bit more expensive than an older class B device, so I will surely buy a class B+ just because of the higher power. So you will buy a little bit of more security by just a little bit of extra money.

Regards,

-Richard (VK4WRS)