Replacing Water Speed Indicator

Claude L.-Auger · Feb 21, 2012

My 34 also had the old Datamarine and I replaced it with Raymarine without any problem. Same size hole from one to the other.

capjules · Feb 28, 2012

with gps being so cheap and everyone has one these days I consider speed indicators obsolete and they are very inaccurate anyway.

when mine stopped working I pulled the speed wheel, glassed the hole and
glued the cover to the indicator.

no point in spending money to fix it.

Claude L.-Auger · Feb 28, 2012

SOG vs SOW

capjules said:
with gps being so cheap and everyone has one these days I consider speed indicators obsolete and they are very inaccurate anyway.

when mine stopped working I pulled the speed wheel, glassed the hole and
glued the cover to the indicator.

no point in spending money to fix it.

Absolutely true that GPS are cheap and that speed indicators are inaccurate. However, I would not be without either one. The GPS will give you a precise, calculated, reading of your SOG but no indication of your speed over the water. So, as imprecise as they are, the speed wheel indicators do provide an idea of how fast or slow you are going and thus using both in conjonction will allow you to understand what is really going on: i.e your GPS shows a speed of 4.5 knots while your wheel indicator shows 6 kn : chances are pretty good that you are facing a current of 1.5 kn ! If you know it you might be able to do something about it and thus maximize your real speed and distance covered.

Alan · Feb 28, 2012

GPS and Knotlog give two different and important bits of information. GPS reads 'Speed over Ground' whereas Knotlog reads 'Speed through Water'. A properly placed and adjusted Knotlog will produce a more instantaneous and accurate reading that is used for proper sail trim. GPS though helpful for navigation is not as good an indicator for proper sail trim.
I would recommend having both. BTW Airmar makes a transducer that is self sealing for easy removal of the log. I always remove mine when the boat sits for more than one day. This prevents any growth from affecting the readings and it is always clean.

capjules · Feb 28, 2012

I know more or less how fast my boat should be going at any given moment considering the wind velocity, point of sail, amount of sail up and sea state.

if I'm going slower than I should be it's due to a current.

If under the conditions I have I should be going five knots and the gps says three I have a two knot current against me.

If you know your boat then you don't need the speed wheel. And also either you have steerage way or you don't. Does not matter what the instruments say, either you can make way or you can't.

And if you can't make way then you better change point of sail and go for plan B.

it's the gps that tells me if I am making way or not going against a current. and it's the unresponsive helm that tells me if I have steerage way with a following current. the speed indicator on the boat is simply going to indicate something you should already be aware of.

nick8489 · Feb 29, 2012

Claude L.-Auger said:
Absolutely true that GPS are cheap and that speed indicators are inaccurate. However, I would not be without either one. The GPS will give you a precise, calculated, reading of your SOG but no indication of your speed over the water. So, as imprecise as they are, the speed wheel indicators do provide an idea of how fast or slow you are going and thus using both in conjonction will allow you to understand what is really going on: i.e your GPS shows a speed of 4.5 knots while your wheel indicator shows 6 kn : chances are pretty good that you are facing a current of 1.5 kn ! If you know it you might be able to do something about it and thus maximize your real speed and distance covered.

On the ST60 system when you want to read true wind speed and apparent windspeed if the log indicator is not working the wind indicator can't compute the differential and you only get one wind readout for both wind speeds. So as Claude indicates if you are a serious sailor you need both GPS and log readouts.

Claude L.-Auger · Mar 1, 2012

Knowing your boat is one thing. And yes you can sail well without any instruments if you want to. The Phenicians did, then someone invented the knotmeter (with knots in a rope attached behind the boat so they could measure how fast they were going over the water). Then the watch. etc. Sailors adopted these things because they helped.

My point is simple: the more info you have at your disposal, the quicker and smarter you decision can be.

Capjules, you sail out of Miami. Merely a few miles out is the mighty Gulfstream. I have done the Ft-Lauderdale - Key West race often enough to appreciate the 2 GPS / charplotters we have on board, but also the Weather reports, the depth sounder and the thermometer included with the sounder. Yes there is a direct relationship between water temperature / depth / current. Along with the wind meter we are getting advised of the smallest change in either one of these and then "on-the-ball" much quicker than any "gut feeling" would alert us, as good as it may be and definitely allow us to take faster and smarter decisions than any boat knowledge and experience normally allow us.

The GPS gives you a calculated speed (SOG) based on triangulation and it's really great to give you a precise distance covered. Your wheel "over-the-water" speed indicator tells you instantly of any changes, down to 1/10th increments. I doubt that my knowledge of how fast my boat should be at any given time under any point of sail under any condition can allow me to be as quick.

The beauty of sailing is that anyone can do as they please and I don't think this thread will change that in the least bit. Fair winds to all.

capjules · Mar 1, 2012

Hi Claude,

I don't need to be that quick or precise, crusing sailing to me is for fun not perfection and a person would have to stare at the instrument all the time to get the benefit you describe, and that for sure is no fun.

If your point of view is from a racing standpoint then you may be correct, but for me, and I'm not a racer so what do I know, needing to be so quick to adjust to each tenth of a knot change which is happening all the time and and trying to be so precise takes all the fun out of crusing for me.

I have a color gps, depth sounder and autopilot, that's it and I have gotten everywhere I have needed to to with no worries. My style and passion is to
get the sailing done with less equipment rather than more. To me less is more,
spend less money, spend less time fixing things, less worries, more time on the water and more fun while I'm out there.

All the best,

Jules

Stu Jackson · Mar 1, 2012

capjules said:
I don't need to be that quick or precise, crusing sailing to me is for fun not perfection and a person would have to stare at the instrument all the time to get the benefit you describe, and that for sure is no fun.

My style and passion is to get the sailing done with less equipment rather than more. To me less is more, spend less money, spend less time fixing things, less worries, more time on the water and more fun while I'm out there.

I like the "less is more" - wish you worked for the govt.!

However, you don't have to "stare at the instrument all the time to get the benefit you describe" since a quick glance will tell you whether your GPS SOG and knotmeter agree or don't. If they don't (and you know how well calibrated or not (pun intended!

) you simply know you're dealing with currents.

Simple is better, sure, but more knowledge is always helpful.

Stu Jackson · Mar 1, 2012

feltensteingw said:
My '84 Hunter 34 has a very old DataMarine speed indicator and I would like to do the same thing, without having to replace the thru-hull sleeve, if possible. Does anyone know if the sleeve is standard or if there is a newer NMEA compliant product that fits the same diameter and threading?

Many thanks.

www.dmimarine.com makes transducers and repairs old Datamarine equipment.

Why not call them and ask?

Claude L.-Auger · Mar 1, 2012

Hi Jules,

Closing words of my last post was: " The beauty of sailing is that anyone can do as they please and I don't think this thread will change that in the least bit."

Sailing is a passion for me, and has become a year-long lifestyle. I race, but I also cruise extensively. Have sailed solo my 34 from Montreal to Labrador and also to the Exumas and Eleuthera. 18,000 + miles in the last 12 years).

To quote Stu: "you don't have to stare at the instruments all the time". Fact is, some of my most memorable sails were at night, sailing by the stars for hours and hours.

My whole point is that when you have the instruments, besides the safety, you also have the info available beyond the estimating that experience and knowledge allow you to do. Not to mention that your crew (if that is the case) might not have the same capacity you do.