Reliable Small sailboat knot meter and depth finder

[ROCKYIV]

I’m really new at posting questions and asking for guidance so here goes. I have a MORGAN 28 that I’d like to install some basic electronics like a knot meter as well as a depth finder. I’m open to your suggestions for the next couple weeks since at the end of July I’m gonna pull the boat and have a much needed ‘bottom job’ done an Dev wanted to take advantage of the timing w/ the boat already being out of the water. Suggestions that won’t break-the-bank will be much appreciated.
ROCKYIV.

[TrentePieds]

Electronics are a fine hobby within a hobby but far from necessary for successful coastal cruising.

As I have said many times, spurious accuracy is the bane of the cruising man.

Your boat is 28Ft. LOA, so call it 25 foot on the waterline. Your "Hull Speed" (theoretical maximum speed through the water ["STW"]) is therefore 1.35 (25^1/2) = 6¾ knots. When she's "in the grove", on the wind, she'll be doing 5½ knots. You don't need a knot meter to know that with ADEQUATE accuracy.

Off the wind is a little trickier, cos the wind may not drive her more than, say, 2 knots. 2 Knots is 12,000 feet in an hour, near enuff, so in a minute you go 12,000/60 - 200 feet which is close enuff to 30 feet a second, i.e. just a little more than a boat length in you case. You crumble a wad of toilet paper and chuck it over the bow. Then you count elephants: "One elephant, two elephant,...". When the paper passes your transom the counting stops. In your boat it'll only take one elephant for the boat to pass the paper. That's two knots. How much more accuracy do you want? Let alone need?

You could also make a chip log. Most stores that sell marine knick-knacks will sell you a "bosun's glass". It's like an old-fashioned hourglass, but it runs out in 3 minutes. That's just fine, 'cos your hull speed is all the speed a guano drougher like Padua would make for days at a time, and a chip log is how she woulda measured her speed.

As for depth, make a lead line. The lead on the lead line has a divot in the bottom where you can put your Wrigley's chewing gum. Used, of course. When the lead comes up, the goop on the bottom of it tells you what the bottom is made of. Mine is marked to compensate for my draft. If I don't have a proper lead, and old wrench will do. When the first knot on the leadline is even with my upper lifeline, I have five feet under the keel. We'll talk about compensating for serious tides, if you like. No knots from the lead as far as that knot, but from there on, a knot every two feet till I have 30 feet (five fathoms) under me. Anchoring a 30-footer in more than 30 feet of water, is often done, but it's a pain. You may not have room for more scope than that in your cable locker!

So back to the navigation: Being on passage is utterly boring in benign weather, so what is a moderately intelligent man to do? Well, I bring the retrofit GPS from my car with me to sea. Every fifteen minutes I read the coordinates and plot them on my - ahem -paper chart. Just for something to do. Using pencil so I don't wreck the chart. Now I have the best damn fix available to me. Even in fog. And in the quarter hour since the last fix I've gone no more than a nautical mile and a half. So if I should need the Coasties, my coordinates are still fresh in my mind so I can tell 'em in proper seamanlike manner just where I am.

Like I said: Electronic instruments are, for the vast majority of coastwise cruising men, a hobby within a hobby. For navigation, all you need is your i-phone so you can google for the coordinates. And the paper charts of course. Here, in this 'ere colony, we are REQUIRED to carry paper charts even if we have all the gizmos :-)!

No need to make life complicated and maintenance more costly than it needs to be by installing stuff you just don't need :-)!

All the best :-)

TrentePieds

[Nekton73]

The cheapest speed/depth combo that I know of is from NASA (marine):


https://www.nasamarine.com/product/clipper-duet-system/


I had the combo on my boat when purchased and it functioned quite well until the displays were stolen, at which point I removed both through hulls, glassed in the holes, and went with this depth sounder only:


https://hawkeyeelectronics.com/collections/boat-mount


I mounted the transducer inside the hull on epoxy, no through hull, works just fine.


I currently rely on gps sog readout for speed. There are drawbacks to this but I accept them and deal accordingly.

[Bowdrie]

Originally Posted by TrentePieds

You crumble a wad of toilet paper and chuck it over the bow. Then you count elephants: "One elephant, two elephant,...". When the paper passes your transom the counting stops. In your boat it'll only take one elephant for the boat to pass the paper. That's two knots. How much more accuracy do you want? Let alone need?

As for depth, make a lead line. The lead on the lead line has a divot in the bottom where you can put your Wrigley's chewing gum. Used, of course. When the lead comes up, the goop on the bottom of it tells you what the bottom is made of.

TrentePieds

Well said, I've done that too, although I used a stopwatch instead of counting elephants.
Yes, a sounding lead that you can "arm" should be standard equipment.
For the OP, surely on your charts of the area you sail in there must be a measured mile somewhere?
If so, then run it in both directions on a calmer day at various RPMs on your engine, noting/averaging the times for each run.
Now you'll have a chart of what kind of speed to expect at some known RPMs.
This can provide a useful tool at times.
Also, a well adjusted compass with deviation card is priceless.

[HeinSdL]

Originally Posted by TrentePieds

Off the wind is a little trickier, cos the wind may not drive her more than, say, 2 knots. 2 Knots is 12,000 feet in an hour, near enuff, so in a minute you go 12,000/60 - 200 feet which is close enuff to 30 feet a second, i.e. just a little more than a boat length in you case. You crumble a wad of toilet paper and chuck it over the bow. Then you count elephants: "One elephant, two elephant,...". When the paper passes your transom the counting stops. In your boat it'll only take one elephant for the boat to pass the paper. That's two knots. How much more accuracy do you want? Let alone need?

Small correction, that would be 3 feet a second. Which is lucky because doing one boat length per elephant would really be something! More like 10 elephants

[thomm225]

Instead of buying an expensive knot meter, most any hand held or mounted VHF Radio with GPS will give you Speed Over Ground (SOG) which is what most folks use these days.

Also with the GPS you will have you exact position that you can see on your paper chart or homemade chart plotter (you can download free charts from OpenCPN)

Many of us also have build homemade Chart Plotters either with a Raspberry Pi Computer (Linux OS) or an old Windows Laptop running OpenCPN.

We bring the GPS information in via NMEA 0183 or NMEA 2000 thru an adapter to our computers then to OpenCPN for our boats location and SOG.

See OpenCPN Lat, Long, and SOG display on gray Bar at bottom of screen in link below.

https://digitalyacht.net/2018/03/10/...ate-autopilot/

Video is my setup for OpenCPN where I also have AIS displayed coming in from my VHF.

VHF handheld in video is displaying SOG also. OpenCPN is telling me the speed of the approaching Tug and our CPA. GPS and AIS info is coming in from my mounted VHF and a backup GPS USB Dongle which cost $17.00

https://www.youtube.com/watch?v=-TOdSCDHb2s

[Jammer]

West Marine, Defender, etc., have cheap depth instruments. Let price be your guide. I have one called a "HawkEye DepthTrax" ($100) that works well enough. I glued the transponder to the inside of the hull. It's intended for transom mounting but on most hulls you can glue it to the inside and it will work OK, especially in shallow areas where it matters the most.


If you want something with a transducer that mounts in a hole in the hull, take a look at the Raymarine i50.

[Spot]

I would consider a Garmin Striker 4 or whatever entry level model 'fish finder' gives you basic depth and maps and GPS. I taught my Garmin CV43 to talk to a tablet with OpenCPN but I usually find myself not wanting to mess with it ( cables all over and a difficult to read tablet screen) so it stays stowed or at home. Where I sail, inland waterways, a waterproof chart and Mark 1 eyeballs keeping lookout are the best nav tools. I do have a sounding line and non-electronic compass and binoc's. The sextant also stays home...

[Bill O]

Agree in general w/most comments above.

Primary purchase would be a depth finder. Since your hull is solid glass you can mount inside (w/o drilling any holes) and shoot thru the hull. It will work just as well as a thru hull mounted, except will not get foul by growth.

For speed as suggested would use GPS. Not as accurate as a thru hull, but you aren't racing and you just need speed over ground to do rough ETA calculations.
Also w/the GPS, your "speed" could be faster than your theoretical hull speed. Then you can tell people that you sailed super fast and exceeded the physical limitations of your hull design.

Bottom line, keep it simple.