Patto21 said..
Hi all
I have B&G stuff on my boat including a Vulcan 9 in the cabin and a Vulcan 7 on the top of the companion way. I also have 2 Triton 2 displays at the helm. The triton display show a different boat speed than the 7. It's about 1.5 knots out.
Any ideas?

Are they all displaying the same sentence? ie is one picking up GPS SOG as vs speed from a paddlewheel?

I agree with the above. I run two Zeus 3 plotters and three triton 2 displays. There are several inputs to give you a speed. SOG is calculated from the Sat input and SOW will be from the paddle wheel ( DST800 ), normally displayed as "boat Speed" (BSPD on the Triton 2).

wow way above my pay grade im just happy if the boats going forward

2bish said..
Yes I agree with Sydchris and Shaggy, likely reading two different things and the paddlwheel is often different to the gps SOG due to either current or growth on the wheel. Reminds me I must pull mine out and give it a clean, it was way off last time I was out.

Question: when under way, TWS is calculated using AWS, course and boat speed I believe. So if this is the case, is boat speed being taken from the paddlewheel or gps SOG?

That question still catches me every time, as instinctively I'd say SOG, which is incorrect, unless you are lucky enough to sail in zero current or tide. If like the rest of us mere mortals you contend with current or tide, use SoW (impeller) as the input.

The reason being is we sail in a boundary of wind and water, and it moves together. How fast the seafloor is moving under that mass is irrelevant to sailing the boat. Your TWS and TWA, VMG, polars, max speed, all of that is referenced to the boat moving through the water, not the land.

For example, you don't get to claim a new max boat speed of 18 knots because you left Darwin Harbour at the turn of a flood tide!

SoG is good for passage planning or checking your eta, as you are now referencing to fixed points on land. That's logical, as land doesn't move.
Hope that makes sense. Or it probably just helps confuse the issue even more.....

Cheers!
SB



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As well as growth on the paddle-wheel I have found that fouling on the hull forward of it can affect the accuracy. Cleaning a couple of feet forward helps (if you're not in the mood for a complete scrub). All to do with laminar vs turbulent flow I guess.