View Single Post
Old 12-26-2013, 08:32 AM   #40
twistedtree
Guru
 
twistedtree's Avatar
 
City: Vermont
Join Date: Jan 2013
Posts: 10,092
Quote:
Originally Posted by CaptTom View Post
What's the origin of the figure of 1.1 x hull speed that's been mentioned here?
I think it's just the max speed that a lot people target for a boat. You might use it when selecting the max power for the engine.

Quote:
You can factor in the effects of wind, waves and current, or even an inexperienced helmsman, but you need a starting point. That's NMPG.
I think NMPG is an interesting way (better way, in my opinion) to operate a boat for a passage. You know the distance, assuming you don't reroute for weather etc. And you know how much fuel you have. The big variables/unknows are wind, current, and waves, and the extent to which they will cause you to burn more or less fuel than planned.

But knowing the distance and available fuel, you can easily calculate the max allowable NMPG to reach your destination with an acceptable reserve. If you operate at or above that NMPG rate, you will get there on the available fuel. Maintaining the desired NMPG rate will mean adjusting speed to compensate for all the above variables, but I think it makes it very easy giving just one number to watch. Of course this depends on having accurate instrumentation for fuel burn and course made good, and preferable something to do the math for you, but newer chart plotter can do it, and new engines report their fuel burn very accurately without floscans and other such stuff.

Food for though.

Quote:
Technically, a better number would be gallons per 100 NM. It's sort of the same thing, but it's a more useful way to think of it. A slight change in NMPG might sound significant, while the same figure expressed as a change in Gallons per 100 NM might not. I can't explain it any better than that, but try it and see if it works better for you.
Sure, use whatever units works for you. I think the key it to have some form of damping for the numbers so you don't have to do it all in your head. But we are all probably pretty used to averaging out speed in our heads from a bouncing GPS reading..

Quote:
One more thing, back to the hull speed calculation. Someone above was suggesting that hull configuration was irrelevant. While minor hull shape changes do have only minor impacts on efficiency, rest assured that a multi-hull will throw all these calculations out the window. Hull shape does matter.
Yes, I said that. It was meant in the context of displacement trawlers.
__________________
MVTanglewood.com
twistedtree is offline   Reply With Quote