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Old 02-06-2009, 20:17   #1
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Log Speed vs GPS [SOG] Speed?

Three of us were having a debate the other day about whether or not you need to retain a log speed readout when you have GPS speed available.

All of us have cats with integrated Raymarine nav/log/wind systems. The log can be replaced in its thru-hull fitting by a blanking plug which [it transpires] we all use when moored for an extended period.

However, one of our group doesn't put the log in the water even when extended cruising offshore. He says it's unnecessary and having done likewise when just sailing the bay and inshore, I'm inclined to agree with him. Note that we're not racing, just cruising, so increased water speed with sail trim is not a major part of our day.

So, any thoughts on leaving the log in the locker and just using GPS SOG?

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Old 02-06-2009, 20:33   #2
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Always nice to know if the current is with you or against you - an important factor sailing aroung here. A 1 knot current with you for 12 hours can make a lot of difference for example!
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Old 02-06-2009, 20:43   #3
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Speed through the water is not speed over ground. They are not any way related even if they can be the same. Doing five knots over ground against a 3 knot current means you are doing 8 knots through the water. Without the knot meter you wouldn't know. In cross currents it makes a huge difference. Computing the set and drift will yield the shortest course. When the cross current speed reaches the through the water speed you could be in a situation where you can't get there from here due to the current if it is ignored or unknown. A boat in a cross current is still moving at the same speed and direction as the current. You can also be moving in a second direction too but the net net is both combined. Nothing can change that unless you know it. Your engine or sail driving on a given heading won't yield the desired course or track. The knot meter would indicate the current condition at least in terms of a net variation. You could then compensate the course heading to end up where you desire.
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Old 02-06-2009, 20:59   #4
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Here in Puget Sound and especially in the North parts where current can exceed 6 knots during a big tide change, it's important to find back eddies. Having both SOG and STW lets us know when we've got the most advantageous course. Reading the water is equally important.

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Old 02-06-2009, 21:03   #5
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Smile

Thanks guys for the messages so far!

Certainly the lack of water speed takes away some of the vector parameters into the Raymarine [or other integrated plotting system.] However, in our part of the world, the East Coast current is the major factor offshore and it's reasonably benign if you hug the coast and/or take in the mapping available on the web. We don't have huge tide range and currents until you get up to the Whitsunday area of the Great Barrier Reef, hence the question "Can we do without?"

However, the valuable input above does make for the "No" case for extended cruising offshore!

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Old 02-06-2009, 21:56   #6
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Just another example of what has been said. A couple of weeks ago in light wind I was heading north in Moreton Bay. Speed through the water was 3 kn but the GPS said I was going backwards. A look at Cleveland to port and it was going north as were the trap buoys in the area. If I didn't have the chart plotter I wouldn't have known and could have gone backwards a fair distance.
Helps to have both.
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Old 03-06-2009, 00:35   #7
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I'm not sure the East Coast Current is that benign Sandy = it fair rips past some headlands all up the coast at times:
East Australian Current - Wikipedia, the free encyclopedia

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Old 03-06-2009, 00:37   #8
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If the plotter or GPS should fail, it's always nice to have a log, a good idea of the current and a set of charts. This will let you fall back to dead reckoning. I always keep track of distance sailed on the log along with position updates in the chart, so if the plotter should fail I'll know exactly where I am and it will be just like sailing pre GPS again.

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Old 03-06-2009, 04:06   #9
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I recently replaced all of my instruments following a lightening strike. I decided to leave out the paddlewheel and fit 2 depth units (one is a fishfinder). I think Depth is vital and redundancy is important.
The B&G system I fitted will calculate true wing from SOG unlike the Raymarine sytems.
Here in the Mediteranian there is little tide and with a satellite differential fix the speed information and derived calculations like true wind are much more acurate than I could acive with a paddlewheel which was forever getting fouled.
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Old 03-06-2009, 04:26   #10
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My 2 cents worth: a log is not a necessity to a casual navigator but a very useful tool to the cautious, careful navigator.
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Old 03-06-2009, 08:47   #11
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I'm currently poking about the Fraser River delta, and it's my first experiences of a tidal estuary. In some places the current oscillates 180°, in others the tide deflects the river flow in one or another direction and the flow can vary immensely.

The log is regularly mystifying me. Slowly I'm learning how to tease out the current, STW, SOG and how they affect my course steered. It's a challenge, and I hope it will improve my navigation skills for future coastal and offshore journeys.
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Old 03-06-2009, 09:35   #12
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All you really need to know is your progress. Personally I wouldnt install a knotmeter if I didnt have one. If you know your boat, you know how fast it should be going in certain conditons. If the GPS says your going faster... you're in the right current! If it says you are not making progress... you're in the wrong place! Just my two cents worth....
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Old 03-06-2009, 11:07   #13
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Then there is the performance issue. GPS updates slowly and the lag makes it difficult to sail the boat efficiently. The log makes it much easier to see what effect sail trim and heading are having on your boat speed. Using both is the best way to sail efficiently.
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Old 03-06-2009, 12:49   #14
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Quote:
Originally Posted by Sail Warehouse View Post
Then there is the performance issue. GPS updates slowly and the lag makes it difficult to sail the boat efficiently. The log makes it much easier to see what effect sail trim and heading are having on your boat speed. Using both is the best way to sail efficiently.
I though this would be an issue. The Gps only updates at 1Hz. The B&G padlewheel log updates at 4Hz. The B&G instruments will provide a SOG to 1/100 of a knot which is helfull in picking up speed changed due to sail trim alterations. So far I have been very happy with the SOG compared with the previous Raymarine padlewheel. Raymarine don't specify an update rate for their padlewheel, but my practical experience would suggest the update rate is no quicker than the GPS at 1Hz.
I have noticed the B&G wind (at 4HZ) is much quicker to repond than the old Raymarine wind, so maybe a B&G padlewheel sysem would also be much better.
I agree the ideal is both (providing you can swith back to SOG easily if the paddlewheel becomes fouled), but I have not missed the old Raymarine padlewheel and I have a spare depth instrument without cutting any extra holes.
The bottom line is for negliable cost I could add paddlewheel speed (but loose the fishfinder) I dont wont be doing this, at least while I am in mainly non tidal waters and the GPS sytem is functioning.
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Old 03-06-2009, 14:05   #15
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When your boat moves up and down over ocean swell or through wind waves of any size, you need an average speed anyway to make any real sense of course or sail changes. Your paddle wheel is spinning faster or slower as your boat goes up the wave front and down the back of the wave. As a cruiser, you are concerned with what happened with your speed to your destination or to your plan to get to that destination. ie: Take note of your average speed, crack the genny off a little, take note of your average speed etc... In fact, some of the functions on your GPS may be more useful in determining the real effect of what you did....as opposed to the perceived short term effect.... JMHO
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