There is a simple reason why in rough weather you get a lower rate of knots than clear weather. If you take note, in rough weather the boat is tossed arouond quiet a bit. Watch the propellers, in rough weather on stock they can lift out of the water and the screws will race. This means you loose forward momentum, and will increase full consumption. It can be noted if you also watch the telegraph speed, it will bounce between your set speed, and a few knots less for each time the screws lift clear of the water.

It's not the headwind/tailwind that affects your speed, it's the sea state. Rough seas mean you are effectively sailing uphill and downhill, but the game measurement of "knots" is in nautical miles per hour as the crow flies. Thus, you're slower in rough seas.

I have yet to see the sea state effect my speed.

Maybe I just never noticed, but typically Ill set for a speed (9.5 knots) in lieu of ahead foward etc.

I do see the uhhh 'spedometer' bounce back and forth alot so I guess the net foward speed is reduced a bit. But again Ive never really noticed it increasing fuel consumption.

Hmmm, that's odd... I've watched the screws for sure, but they just rotate really quickly no matter what.. perhaps you have to have a special viewing apparatus to enable you to see that they are 'racing' ? How much faster do they rotate if they are out of the water compared to when they are in the water? What RPM are they turning at in the water during a normal sea and a rough sea?

Unless you have results regarding the performance of the boat in various sea states, you don't have a theory at all.

Sorry its not theory its fact. Simply by observation can you see the forward speed of the boat getting reduced in heavy sees as the screws lift clear of the water. If the screws are not in the water, they are not providing forward motion. Reduced forward motion @ a set RPM speed compared to flat sea automatically means increased fuel consumption. This is because the distance traveled is higher in rough seas than calm, its basic physics and is included in the game (as per the speed change noted above). If the sea state played no part in speed change there would be no change in range or fuel consumption. This is the main reason I use the diverate mod, as it tends to keep the screws in the water allowing for better forward speed in rough conditions.

And racing screws describes ANY prop that is out of the water, not its particular rational speed.

I do realise I am a new member here but I do have a background in shipping and sailing. I have also physically used ship mounted Radar in RL and sailed on a number of vessels since childhood. Plus there is a rather large Nuclear Submarine base not 30 miles away from me I can see from my living room window across the river and we also have a shipyard in the middle of town. To not know of sailing here is practically unheard of.

Example;
I travel 11.5 miles in an hour @ 10 Knots in calm seas
I travel 11.5 miles in an hour, 15 minutes @ 10 knots in rough seas
Therefor Rough seas = longer travelling time which = more fuel consumption.

To show this, the game has your forward speed change in a heavy sea. If your not maintaining the same forward speed 100% of the time and are loosing speed at intervals you are going to take longer to travel the same distance.

Do you understand now?

I don't know how it "happens" in the game engine but "lower then set" speeds in high seas are there alright. If you use TC you can easily see that you're actual speed is lower a significant fracture of your traveling time. So I just assumed that your fuel consumption was based on your "set speed" (equivalent to setting the engines to a certain RPM level) with your actual speed dropping due to high seas.

I also tried setting a lower speed (the equivalent to shifting to lower gear) but I can't say if it made a great difference or not. This requires a experimentation/verification procedure (small test scenarios) via the scenario builder (with wich I'm not familiar with,yet...). Would be a nice and useful project though.

Lexandro, I'm sorry if my earlier post sounded a little harsh. That wasn't my intention. If 'racing' means purely lifted from the water, then in real life that would explain things, but not in game, where things are often set up strangely.

You get slightly reduced speeds in rough weather even if the screws don't come out of the water. I believe what you are seeing is not the speed dropping when your screws coming out of the water, but when another point gets deeper. When the screws are out, ie when they are in a trough, there is also another part of the boat that is under a peak of water. I think it's the increased depth on that particular part (possibly conning tower) that affects the speed.

I also did some testing on the RPM of the engines, to see if that was modelled - e.g. increased RPM for the same speed in different weather. Rather bizarrely, the RPM of the engines is HIGHER in calm seas than it is for rough seas, at any given speed or telegraph setting. So, RPM is not a factor in fuel consumption. I believe that speed is modified by the depth of one point (conning tower?) and the fuel consumption, as Diopos says, is simply throttle setting multiplied by time.

Using stock, I got hardly any noticable difference in speed in rough weather, probably about half a knot as it flipped between target speed and one knot below. Mods that change wave height will probably cause more noticable speed drops. The speed reduction is linked linearly to the depth.