VMG computer for best time to waypoint decisions

[jbinbi]

I have B&G instruments, but not sure how to get info I seek below. Polars are not loaded into them. (don't think you can load them if you have them)

Here is the problem: You are sailing say at 5kts 50 deg TWA. You have a waypoint set at this. You want to know if cracking off 10 deg will help. I know from trig that tan 10 deg is 17% more distance I need to make up to my waypoint (assuming I maintain that heading). So I need a speed of 6kts at 60 deg to decide to do this. I try this, and if no good, go back.

If I had polars loaded in, my instruments should be able to tell me, based on wind speed/wind direction and my destination, it should be able to continuously figure out the best course to get me to my waypoint in the shortest amount of time.

I would think that since my instruments know TWS/TWA/STW/SOG somehow they could record this as I sail and actually make a set of polars for my boat! Then it should be able to use this info to give me best course to waypoint based on the environment.

Pretty sure my instruments can't do any of this? Is there a program that can? I know opencpn can load polars, I don't think it will suggest a course even if you are inputting WS etc.

I know big time racers have this info, no way is the helmsman/tactician eyeballing this. Is it custom SW? Thanks.

[Pandor]

You really need to get the polars for your boat if you want to explore this. From the polar plot, you will see that at any specific true wind speed, there are two specific points on each curve. The two points mark the true wind angles that produce the best VMG for upwind (beating) and downwind (gibing). Anything in between and you will always maximize Velocity Made Course (VMC) by heading directly for your waypoint. So far, no VMG computer is required if you know the upwind/downwind maxima for curves within your range of TWS.

Your instruments most likely can tell you VMG, or at least VMC. But you can’t really use instrumentation VMG to steer. Over the short term, VMG can be very misleading because it doesn’t tell the whole story. For a short period of time it’s easy to increase VMG by sailing higher upwind or lower downwind because boat speed drops off slowly due to the boat’s inertia. As the dynamics stabilize, the boat speed will decrease so much that VMG will be less over the longer term. VMG will be bad and cumulative loss over the long term will be substantial.

[jbinbi]

Thanks. I would have expected that today's instruments could record data and kind of create a polar set. And for a boat I had, I did have a paper pic of the manufact's polars. But that wasn't convenient to view on the chart plotter.

[Pandor]

You are correct. That capability exists, at least for well calibrated racing boats. For example, the s/w program Expedition allows you to record all instruments and use your data to tweak your polars or build multiple sets for offshore or inshore, lumpy or flat seas. But your instruments have to be pretty well calibrated for it to be even a little useful.

Generally, for cruisers, I think most would get more out of laminating a polar table and taping it to the cockpit. Then take notes while on various TWS/TWA combinations and adjust by hand as necessary.