Question on Instrumentation

[Dave and Kathy]

Kathy and I are in the process of restoring Grayce Avery, our 'new to us' 1984 CD 28. Most of the work is in the category of cosmetic and deferred maintenance and I am pleased with our progress. Although it's a bit in the future, I've begun thinking about instrumentation. Grayce came to us with a mid-80s Raytheon radar unit, obsolete Loran unit and separate depth sounder and knot meter (Datamarine, probably also mid'80s). We plan to go with a combined radar/GPS multi-function display unit. The current depth sounder transducer is mounted in a faired mount on the port side. I'd like to replace the transducer with a new one in the same mount to support the new radar/GPS.

I have not seen many posts on the question of knot meters. Ours is the typical paddle wheel style. Here's what I'm wondering: the knot meter measures speed through the water, the GPS measures speed over the bottom (and, of course, provides position). Are both necessary? What are the downsides of relying only on the GPS-provided information? If the knot meter is no longer used, can the paddle wheel be deactivated and left in place, or should it be removed followed by filling in the hole?

Would very much appreciate member input. Over the years, we've learned a lot from these posts!

[Klem]

Personally, I prefer to just have the GPS. Our paddlewheel will jam up within a month or two of launching and I never bother to clear it again for the rest of the season. If you really trusted the knotmeter, you could do some interesting comparisons to figure out set and drift but very few are calibrated that well and the calibration typically changes relatively to heel angle and a few other factors. The additional information provided by the knotmeter simply is not worth it for the type of sailing that I do.

[Paul D.]

I'm old school and believe in at least some separation of duties aboard. The depth sounder I consider a serious navigation instrument and thus finally replaced the 30 year old Signet that I had repaired with tin foil and duck tape over the years with a Raymarine ST 60 last year. Reads to 400' and is not displayed through a main screen in case of breakage. Of course, it is still electronic so it can go and I have to prepare to do without but keeping it separate was important to me.

I value the Speedo less so, but when I replaced the sounder I also went with an ST 60 speed to look right on either side of the compass. Sometimes I dead reckon around the Islands instead of using the GPS.

So my thought is to have a reliable sounder at the very least.

[Shinok]

We are replacing our knotmeter transducer, but I haven't decided on whether or not to install the display.

Part of me wonders if having it in place isn't a good idea, though. The GPS will tell you your track across the ground, but it won't provide much information as far as the speed of the water flowing under the boat. I'd wonder if this information is useful for measuring currents and/or for determining when you are about to lose rudder authority, similar to how an airspeed indicator in an airplane works.

Thoughts?

[Klem]

We are replacing our knotmeter transducer, but I haven't decided on whether or not to install the display.

Part of me wonders if having it in place isn't a good idea, though. The GPS will tell you your track across the ground, but it won't provide much information as far as the speed of the water flowing under the boat. I'd wonder if this information is useful for measuring currents and/or for determining when you are about to lose rudder authority, similar to how an airspeed indicator in an airplane works.

Thoughts?

Shinok,

In theory, you can do a good job of estimating things like the current by looking at the difference of your water speed, your ground speed, your heading and a correction for leeway. However, very few installations provide an accurate enough water speed in my experience. The GPS is incredibly accurate but your total error is determined by the least accurate measuring device so you end up with a poor error band. Also, to truly figure out the current, you need to do a bit of geometry and I don't know many people who would bother with that. The GPS tell you what really matters which is where you are moving relative to fixed objects but the water speed will help with things like tweaking your boat to go faster.

[Dean Abramson]

I like having a functioning knot meter, because that lets me know how well I am sailing my boat. If we have, say, 10-15 knots of wind, and the boat's speed is only 4.5, I know I need to do something different, even if the GPS says we are going 5.5, because we can have strong currents here.

I also find it interesting to (sort of) gauge the speed of a tidal current. Way down east once, we were sailing through the water at about 5 (good for the wind we had then), while going 7 over the bottom.

Yes, my paddle wheel gets jammed sometimes. Sometimes I can free it by motoring full-speed in reverse. On the 25D, I could free it with my foot if I hung off the toe rail. Can't remember if I've tried that on the 31; I doubt I can reach it that way. I have freed it by diving down; it usually doesn't take much effort. Up here, I prefer a wetsuit for this job, but I have at least once (I swear) done it without.

A knot meter is not the most important thing on a boat, but I personally have always liked having one. I just like knowing the boat's actual speed through the water.

Dean

[boogie]

One point to mention.
The wind instrument (ST 60 Raymarine) takes the info from the speedo to calculate the TRUE wind. not from the GPS. Can not be changed in the model year I have.
Maybe it is different in the new'er models? I would check this issue before I make my final decision.

[bottomscraper]

One point to mention.
The wind instrument (ST 60 Raymarine) takes the info from the speedo to calculate the TRUE wind. not from the GPS. Can not be changed in the model year I have.
Maybe it is different in the new'er models? I would check this issue before I make my final decision.

I would hope newer instruments would be smarter than that but yes we have the same problem with our Raymarine ST60, I can't fathom what the "Engineers" at Raymarine were thinking when they did that.

Our speedo is intermittent at best. It usually is good for a few weeks at the beginning of the season but requires cleaning on a regular basis. It's not located in a place that allows easy removal from inside the boat for cleaning so it requires going over the side in the icy Maine waters.

[Brian2]

Hello,

I removed the paddlewheel for our knotlog and have the dummy plug installed. Instead I rely on the GPS and a Knotstick when needed, which I tie to a stanchion. A large plastic washer trails behind the boat, and a plunger inside a tube indicates speed thru the water.

We have separate radar and chartplotter on our 28. I like the idea of multi function, but not the idea of having both units down due to one malfunctioning display.

best regards - Brian

[frankott3]

Like Dean I find the knotmeter helpful feedback when sailing in strong currents. I replaced the original non-functioning datamarine with a raymarine plastic thru-hull because it also fit a two inch hole. Raymarine part E 26008. This transducer has clamshell doors inside the tube so you can remove it to clean the paddle wheel without a major inrush of seawater. For the depth I removed the fairing block and transducer and installed a P-79 shoot thru transducer after glassing in the hole. I used the P-40 Bi-data display. I'm completely satisfied with the Raymarine products.

[Dean Abramson]

Rich and Jacob,

I am trying to get my head around what you guys are talking about. (I have no windmeter, so this is academic for me.)

I think you are saying:
The wind sensor says, "I feel 15 knots. And it is 20 degrees right of the boat's heading, which is (say) 90 degrees." (Wind-gauge and compass data.)
Then the knotmeter says, "We are going 5 knots through the water."
So the computer does the trigonometry (or whatever applies here) and says, "Okay, that means that the real wind is actually 50 degrees right of our heading, 140M, and it's blowing 11 knots. (Or something. I don't know the real answer.) At least, in the World According To Raymarine.

But you guys are saying that the boat speed (thru the water) is irrelevant? That what you really need is the SOG? If that IS what you are saying, I think I agree with you and cannot fathom how Raymarine could get that so wrong.

Because (to use an extreme example) if the boat is going 3 knots directly against a 3-knot current, the knotmeter would say, "We are going 3 knots," but in reality the boat would be stationary in relation to both the ground and the wind. Right? Therefore, the apparent wind would be exactly the same (speed and direction) as the true wind. Right? So having the knotmeter's input would actually distort the results. Right? The computer needs the GPS to say "We are, in fact, not going anywhere. That wind you feel is exactly where, and as strong, as it seems."

Is THAT what you guys are saying? Because if (and it's a big if) I get this, the knotmeter (boat) speed is irrelevant to calculating true wind, and having it so that a lack of knotmeter data prevents you from getting True Wind data is indeed unbelievable!

It seems to me that having the wind gauge data, the compass data, and the GPS data is all the computer would ever need to do the True Wind math.

Am I getting this? Do I have too much time on my hands? Is it not yet spring?

Dean

[Dean Abramson]

Dave and Kathy,

Congrats on the new boat! Keep us posted on the progress. Great boat, the 28.

Dean

[bottomscraper]

Is THAT what you guys are saying? Because if (and it's a big if) I get this, the knotmeter (boat) speed is irrelevant to calculating true wind, and having it so that a lack of knotmeter data prevents you from getting True Wind data is indeed unbelievable!

It seems to me that having the wind gauge data, the compass data, and the GPS data is all the computer would ever need to do the True Wind math.

Am I getting this? Do I have too much time on my hands? Is it not yet spring?

Yup that's it in a nutshell. If the GPS data wasn't avaialable on the Seatalk network they might have an excuse but it is avaialable and in all cases would be preferable for calculating true wind. We know that Seatalk has the GPS data because the ST60 Speed Display can show either GPS or speed impeller data!

The ST60 Wind Display can display either true wind or apparent wind. Apparent wind doesn't require boat speed to calculate so that is correct even with a non-functional speed transducer. I can't imagine it would have been more than a handful of lines of code to allow the user the ability to select which data to use for true wind calculation.

[boogie]

Dean
You are right.
The right way for Raymarine would be to take the info from the GPS to calculate the TRUE wind and if GPS does not exist than to take the info from the Speedo. In my old (ST 60) it takes the Speedo info only. May be it was changed in the newer systems.
Like everything in life and especially in the marine life you cannot go blind based on info you get from an Instrument. You have to know the methods of gathering the info and the limits for every part of the system. In many cases the Speedo is good enough to correct the wind and in some cases it is completely wrong. The instruments are tools to help us know what’s around but the best tools are our eyes and ears and nose.
I clean and paint the Speedo with the black Transducer paint and I had success from the time I have started doing it.

[Dean Abramson]

That is incredible. What were they thinking?

I was trying to think, "Is there any reason one would want knotmeter (Boat Speed) data for calculating anything you'd want (other then a boat-speed reading itself.) Can't think of any.

I've also been trying to get my brain around how if the boat is crabbed a little (pointing higher than its track), would that come into play at all? My preliminary conclusion is that that is irrelevant also. But... The computer knows the boat's heading, and it knows the track. It seems to me that the math should also compensate for any difference between heading and track. But in any event, the track data would not be usable unless the boat has been going in a straight line for a little while. For that reason, maybe the calculation just ignores the diff between heading and track, so that the result is real-time. (Close enough for government work, as they say.)

Having said that, I still cannot see how knowing Boat Speed would help. But maybe it has something to do with this...?

D