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Old 15-09-2017, 09:13   #16
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Re: Do you need a dedicated speed instrument

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Originally Posted by CatNewBee View Post
I assume you have had some sailing classes some day...

If your electronics fail, you still will have a compass, a log (speedometer) and a wind vane. With paper charts you can estimate your location using calculations of youur last position, the course sailed and the speed.

So it is important for redundancy and is good seamanship. You can of course use a long line with some knots on it, throw it on the bow into the water and measure the time while it passes along your boat - so you can calculate the speed from m/s or ft/s in kn.
The chances of a speed through water instrument + associated display electronics of failing are probably 45x higher than the chance of all GPS device failing (multiple old phones, multiple GPS navigators that people have on board).

Most people do without a speed through water instrument unless in a high current environment. Also, you can always just reverse course for a few seconds to calculate the current in a particular spot if you are so concerned about that.

Now, if you are tracking multiple ship targets on your MARPA radar, there is some benefit to be able to sync the image to either the GPS speed (ground stabilization) or the speed through water (sea stabilization). But by the time you get this equipment on board the cost of the log instrument will seem trivial.
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Old 15-09-2017, 19:24   #17
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Re: Do you need a dedicated speed instrument

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Originally Posted by alctel View Post
I'd say the strong currents make one MORE useful. Improves the reading for true wind speed, helps work out current direction/speed and helps with sail trim tweaking
I rely on the log. Adding to Alctel:

1. Say you're on a close hauled tack trying to make it around a headland.

Comparing SOG with log speed through water gives understanding of the current direction/speed. Good to assess whether the current is pushing you into the headland or away from the headland.

Assessments of current set on charts or cruising guides may not be detailed or might change locally of with the conditions of the day.

2. Also, a trip plan, or TR (Trip Report) to a Marine safety authority will require an ETA at destination, which requires an understanding of current, and whether the current is behaving as predicted. Our waters are affected by 5 to 7 knots of tide at times. Getting the current wrong and not realising it can add many hours to an 8 hour trip, meaning arrival after dark or getting caught in the next bad weather front.

3. Speed through water is a better assessment of how well you are sailing. SOG isn't, because of current assistance.
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Old 15-09-2017, 20:05   #18
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Re: Do you need a dedicated speed instrument

If you want your speed for navigation, then the GPS is sufficient.
But if you want to tweak and go faster, the speed instrument is crucial.

After many years of sailing, I can accurately see my speed simply by looking over the side. Anyone can do it. So perhaps stop relying on gizmos and just see how fast you are going...with your eyes.

FWIW, I've rarely had a working speed instrument on any of the boats I have sailed. And since your speed is constantly changing, you have to wonder just how useful it is anyway.

After you've sailed your boat for a while, I'm sure you'll have a pretty good idea just how fast it goes on most points of sail, for most wind and sea conditions. How much cost and effort are you willing to commit to know your speed to the last decimal place?
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Old 15-09-2017, 21:04   #19
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Re: Do you need a dedicated speed instrument

No, you don't need one. GPS SOG plus a modicium of commonsense works perfectly well. Most speed through the water sender rapidly foul up and are rarely accurate, esp at low speeds. One less hole in the boat is good. If you really feel like you must have speed through the water something simple like the knotstick is a much better way to go in my opinion.
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Old 15-09-2017, 23:22   #20
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Re: Do you need a dedicated speed instrument

If you want accurate true wind speed and true wind angle then you will want a speed log. If you don't really care about the accuracy of TWS and TWA then it doesn't matter.

As for those advocating knot sticks, really? If you have already got one or more GPS on board than what possible benefit is there to manually measuring boat speed?
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Old 20-09-2017, 14:30   #21
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Re: Do you need a dedicated speed instrument

All of the above answers are correct. Let me add that if you are a racer or are trying to push your boat, then a knot meter is pretty darned vital. Looking at the water can tell you a lot, but it can't tell you if you've dropped from 6.7 knots to 6.5 knots. And this is exactly the information you need to keep boat performance at it's best. I find that I sail by the knot meter most of the time, even when I'm not racing. I'm always trying to squeeze every tenth out of the boat and constantly asking why I lost a tenth here or gained a tenth there. Even when I'm running with autopilot I press the buttons to head up or head down 2 degrees at a time based on what the knot meter is doing.

So as others have said, the answer to your question depends on what type of sailing you are doing.
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