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