Nearly Sunk 'er

[capn_billl]

Originally Posted by PamlicoTraveler

I am obsessed with my bilge pump cycle counter. Still, on a mooring you would eventually run the battery down and the boat would sink even with it on.

Not for a few days. I use an alarm on my bilge pump so I know if it is cycling. And leave a trickle charger on to keep batteries topped off.

[Uncle Buck]

Originally Posted by nv5l

Hi Boatman:

I'm a bit leery of relying too much on GPS. What do you recommend as a backup? Assuming the worst, i.e., total electrical failure.

thanks...
don

Carry a handheld GPS/chartplotted and spare batteries. Better yet, carry two of them. You can get them for less than $300 now. I prefer hand held equipment to built on. Sure, the screen is smaller but adequate, but they are much cheaper and do not rely on the boat's systems. Easy to replace and no repair bills or time. Speed/direction over ground if what matters, not through the water - unless racing of course and then you need both.

[Dockhead]

Bit of thread drift, but speed through the water and speed over ground are completely different things and no substitute for each other. If you don't know your speed through water how in the world can you tell what you're sailing like?

In an area with any kind of currents, you need this information, even if you are not racing.

[Uncle Buck]

Through experience and observation. I have a pretty good idea of how fast I'm sailing without having a knot meter (perhaps from years of racing Lasers without any instrumentation). I've never missed my knot meter since it stopped working, and I saill in SF Bay - plenty of current here.... Tide/current lines are very easy to ready visually. If you need to know " exactly " how strong the current is, head directly into it, note speed over ground, then head on the reciprocal course and do the easy math.

Instrumentation is not a must, just a nice convenience to have.

Cheers

[lockie]

"LOL..... what I don't understand is why pull it out in the first place"...................

Well, I take the impeller out because it stops working when the wee beasties grow on it after a week or two of inaction. And even if it's still working, any growth will surely degrade the accuracy. It's left in if I'm on a trip.

It's very useful, together with the GPS, to give some idea of the current. Interestingly though, the error between them even in still water is quite significant with any growth on the hull. I noticed the correlation improved greatly after anti-fouling. I guess the growth creates a slowed-down boundary layer of water immediately against the hull.

lockie

[skipgundlach]

Originally Posted by boatman61

LOL..... what I don't understand is why pull it out in the first place....

(clip)

If you're in foul water, the impeller will foul very quickly, and be useless without either removing and cleaning, or diving to do that. Because we're in warm water, we pull it every time we're going to be somewhere not moving for more than a day or two...

L8R

Skip

[David M]

The most accurate way to determine speed through the water are with instruments that use sound, like a Doppler unit. They are very expensive though.

All of these different measurements are going to fluctuate, especially as the waves increase, until you get to the point where you are doing a mental average out of the different speeds you are reading...even for Doppler instruments as the boat accelerates and decelerates and the flow around the hull becomes uneven.

My experience with paddlewheels is that when they are not fouled they are nearly worthless and when they do get fouled they are completely worthless.

[SabreKai]

Originally Posted by gonesail

i have one of those impellers and it always worries me .. i never use it and the blank has always been screwed in. fooling with holes in the boat when you are tired is bad enough but leaving the bilge pump off is bad too IMHO. GPS is better for speed measurements.

My last Grampian had a mechanical knot meter, with the cable drive. At some point one of the fins on the drive spinner broke off, so I decided to go with a newer all electric job with the little impeller on a removable fitting, as you are describing here.

I was worried about leakage too, so when I drilled the hole into the hull, I did it close to a bulkhead in the V-berth. Once the fitting was in and bedded down I then build a well around it that came up to the level of the berth (about a foot above the water line). The cable came up the well and out a grommet hole just below the lid. All this was glassed in place, and now I can pull the impeller unit out, clean it and reinstall it in the water without getting too much water in the boat. 5 minutes with a scoop and the water is mostly removed, then mop up with a sponge and its dry again. I do this at least twice a year, at liftout and launch. Given the hull shape at that area, its too easy for the lifting slings to damage the impeller. So we pull them and then sling them.

With the original configuration of the lockers any leakage would have ended up in the bilge and eventually could flood the boat. This way, any leakage is contained in the well which will flood to the waterline. Total volume would be 8"x8"x about 12-14". The well is large enough inside so that I can remove the fittings if needed and also remove the thru-hull mounting.


Sabre

[Charlie]

I am getting ready to install my Garmin knot meter and am looking forward to being able to tell the difference in speed when I adjust the sails. You don't get that kind of accuracy with a GPS. Also you can't tell if there is a small current going with or against you.

[lockie]

Good point about sail trim - you get an instant reading of any speed change on an impeller log, whereas the GPS takes its time.

lockie

[Pete7]

Originally Posted by lockie

Good point about sail trim - you get an instant reading of any speed change on an impeller log, whereas the GPS takes its time. lockie

Surely this is down to the re-fresh rate you have the GPS set to. I haven't fiddled with out latest Garmin but an earlier one and a previous Lowrance could be changed.

Pete