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Old 03-11-2014, 16:20   #1
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Speed sensor in each hull

It's being suggested that I should install a speed sensor in each hull, Due to pressure differences for each tack.
I am Using a B and G 5000 system in a cruising cat.
Any one have any input on this.
Thanks
Paul
Freydis 49 "Cheorae"
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Old 03-11-2014, 16:39   #2
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Re: Speed sensor in each hull

It's a cruising cat. Yes it's possible one hull or the other will come out momentarily in extreme conditions and big waves. Not enough to matter, IMHO. The only thing a cruising boat needs a speed sensor for is to derive true wind and speed through water. These aren't critical measurements requiring accuracy, except to racers. (Again IMHO).

Compare boat speed to SOG and you can know whether the current is helping or hindering your progress. Other than that "nice to know" stuff, GPS speed and course are all that matter to most cruisers. I've had my speed transducer plug pulled for the last year because I don't care that much and it tends to foul quickly. GPS and apparent wind are all I really care much about.
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Old 03-11-2014, 16:42   #3
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Re: Speed sensor in each hull

I doubt it would make any practical difference.
I did install a second depthsounder in the other hull on the cat I used to be on after going through the Dismal Swamp Canal and meeting oncoming traffic.
Even that wasn't really necessary, but we did use it occasionally after we put it in.

Forgot to mention - If I recall correctly we couldn't operate both depth sounders at the same time because they cancelled each other out. That problem was fixed by purchasing a different make of d/s which apparently had a different frequency.
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Old 03-11-2014, 16:47   #4
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Re: Speed sensor in each hull

On second thought, redundancy is nice, as transducers tend to be failure-prone. If it's a new boat under construction, have them install the wiring. Much easier to do before the hull liners are in. Otherwise I'd wait to see if you need it.
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Old 03-11-2014, 17:59   #5
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Re: Speed sensor in each hull

I would think that each hull is going pretty much the same speed except when turning and it's hard to imagine why this would make a difference. Both will end up at the same place at the same time.
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Old 04-11-2014, 05:39   #6
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Re: Speed sensor in each hull

Hey Paul, where is your boat now? I didn't see it in Shelter Bay, but maybe you moved it back into the storage yard.

Nice electronics you put on it! If you are not racing, I can't see why two sensors would be necessary. We don't see any noticeable difference on tacks, and our speed transducer is mounted on the inside of a hull along a keel instead of in the middle of the hull in front of the keel where it should be. On the other hand, if you install two triducers like the DST800 - one in each hull, you will have both speed and depth in each hull.

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Old 04-11-2014, 05:58   #7
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Re: Speed sensor in each hull

The B &G supplier seems to think there is a pressure differential between the windward and leeward keels which will cause a speed error. It's not about the flying a hull. We don't do that. The current transducer looses depth when it's the Winnard heel but it's a little to close to the bow in my opinion.

Cheorae is at just cats in ft Lauderdale. We are waiting on a new mast and boom from Lorima France. I ordered the B and g 5000 system at the Annapolis show.
Also having lithiums and solar installed . Hope fully she will be finished by January
Regards
Paul
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Old 04-11-2014, 06:09   #8
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Re: Speed sensor in each hull

You need to change your boat location in your settings! While it is technically true that there is a pressure differential, I can't see how that matters in practice at all. Any speed error would have to be so slight as to make no real difference to a cruising boat. After all, does it really matter if you read 10.1kts on one tack and 10.3 on the other? The AP will use delta speed values in calculating its response, so the actual number doesn't matter much.

The only thing I can think of is that your 5000 system allows your AP to use polars and laylines. There, an accurate SOW on both tacks would be useful - but again, really only if racing.

If you get two transducers, are you going to go through the trouble of continually calibrating them? This isn't an easy task, but is required if you wish to take real advantage of having two. Otherwise, they are just redundant systems...

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Old 04-11-2014, 06:15   #9
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Re: Speed sensor in each hull

old Chinese saying....man with two wrist watches never knows correct time.............
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Old 04-11-2014, 06:31   #10
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Re: Speed sensor in each hull

Quote:
Originally Posted by atoll View Post
old Chinese saying...man with two wrist watches never knows correct time...
I like that!
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Old 04-11-2014, 06:40   #11
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Re: Speed sensor in each hull

Sounds like the B&G supplier wants to make a bigger sale.

I'd be willing to bet the tiniest amount of scum buildup will have far more effect than any "pressure differential".

Similar to another comment, we cruise and have had the sensor out...pretty much since we bought the boat. I can estimate current pretty well by knowing how fast the boat is for a typical throttle settings, so there isn't a lot of benefit over just using the GPS speed.
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Old 04-11-2014, 17:34   #12
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Re: Speed sensor in each hull

That is not my experience at all. And on two different cats.

Before I put two sounders in the first cat I was told they would interfere w/ each other if they had the same frequency. Was not the case with two Datamarines. On current boat, we have two Raymarine sounders and they are happy together.

I'm guessing that the very short time for the signal to return to the transducer is so much less than the interval of sending the signal that the transducer knows which return is relevant. But hey, I'm a CE that needs to see something before I really believe it.

And I find two sounders invaluable in shallow water. Two keels on a boat that you can't heel can be a real problem. Stereo sounders telling you which way to go offsets that a bit.

Quote:
Originally Posted by FSMike View Post
I doubt it would make any practical difference.
I did install a second depthsounder in the other hull on the cat I used to be on after going through the Dismal Swamp Canal and meeting oncoming traffic.
Even that wasn't really necessary, but we did use it occasionally after we put it in.

Forgot to mention - If I recall correctly we couldn't operate both depth sounders at the same time because they cancelled each other out. That problem was fixed by purchasing a different make of d/s which apparently had a different frequency.
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