Paddle-wheel vs GPS knot meters

[TintedChrome]

In an effort to close off through-hulls on my boat and reduce them down to just one, I've been looking at gps-based knot meters like the Velocitek ProStart.. I understand this gives me SOG, as opposed to boat speed relative to the water which can be a significant difference depending on currents, etc.

The question is, other than seeing what effect trimming has on boat speed relative to the water and getting a true indication of hull speed, is there a significant advantage to a paddle-wheel setup over GPS for a guy like me that's not going to be racing in anything other than the local beer-cans?

[SVCarolena]

We've been relying on GPS alone for a few years and although a knot meter would be nice, we don't really miss it. That said, currents in our neck of the woods generally run 1 knot or less. I figure it would be more useful for racing, as you mention, or in areas with significant current. The only reason we don't use our knot meeter is that the travel lift broke one of the impeller fingers and becuase it is an old unit we've been unable to find a replacement. Thus far, I've found may other uses for the money and agrivation of installing a new meter.

[Omatako]

The direct comparison of GPS SOG and the traditional boat speed indicator reflects current/tidal stream and this can be of significant value in areas where there are strong currents and in particular, counter currents. The difference can seriously influence trip times and of race results.

[jrd22]

Also useful for DR navigation. We have strong currents and I'm always using the knotmeter to find the strongest counter currents.

[jackdale]

+1 what John said

In integrated systems such as the Raymarine E series, the data from the paddle, GPS, wind instruments are valuable as the computer calculates set and drift, etc..

Also the paddle provides a faster indication of the effects of changes in sail trim. The GPS provides an average speed of a series of fixes.

[TintedChrome]

Cool.. thanks for the info guys! Looks like both is the way to go..

[tdw]

Our log died a year or so back and I've not got round to replacing it. I find using the GPS is quite adequate but we are talking about plodding around offshore and not racing.

As Andre so correctly states, the GPS indicated speed is not as instantaly reliable as a log.

whoops...not Andre, Jack.

[brak]

actually, how "instant" is your SOG as provided by GPS depends on the type of your GPS device, display system and settings. some will be a lot more instant than others.

at the same time standard paddlewheel STW (speed through water) devices are also not instant - rather they usually have dampening (often not configurable). You can test that with boat out of the water by rotating the speed wheel manually (or if water is warm enough and wheel is accessible - put a mask on and dive to it). Either way, you are likely to find that as you spin the wheel up quickly, speed as shown by the instrument ramps up quite slowly, and goes down just as slowly - over a course of a few seconds at least. This is likely done on purpose, because otherwise as your boat goes up and down waves speed may vary a lot - too much to be useful really.

That said, speed through water is very useful for set/drift calculation (of course only provided your speed log is well calibrated).

[jackdale]

Brak - good points. I am used to knotmeters that respond quickly.

Most knotmeters can be pulled while the boat is in the water. They have a plastic plug that can be inserted. Newer ones have a baffle that slows the flow of water while inserting the plug.

[pdqaltair]

A former racer, I no longer have any use for the paddle wheel knot meter. Too much maintenance. In fact, it works perfectly well but I have had the plug in for over 2 years.

It would be nice to see the true wind function, but not nearly nice enough to bother. I believe, except for racers and gear nuts, they are (nearly) as obsolete as a sextant; IF the difference between the GPS heading and the compass heading and the difference between the boats GPS speed and visual speed through the water aren't obvious to you, well, spend the money and cleaning time if you must.