Question on speed/power/range curve in Voyaging Under Power

[na2020]

Hi,

I hope this is the right section for this quesiton; if not please let me know and I'll delete/repost appropriately.

I just started reading Voyaging Under Power, 4th Edition. I was having trouble interpreting the speed/power/range curve on page 52 and correlating it with the narrative on page 53. The narrative says

"
For this particular vessel the range is:

2,800 miles at 1.1 S/L
2,200 miles at 1.2

"

I can't match those numbers when I read the chart. For 1.1 S/L, I start at 1.1 on the S/L curve, go up to the range curve and then left to the range y-axis and read a range well above 3000. At 1.2 S/L, I go up to the Range curve and then across and read around 2500.

I realize there are no grid lines, but the first reading of 1.1 S/L is so far off from what he says in the book.

Am I not using the chart correctly?

This is a great book so far, and I want to make sure I understand how to read these charts correctly.

thanks for any comments.

-- N

[ranger58sb]

I think you're referring to Figure 6-1, yes? (I have the softcopy version, so page numbers vary depending on selected type font/size.)

If so...

Look at 1.2 on the horsepower curve, then look due left from that to the range chart. Looks like about 2800.

Look at 1.1 (approx, midway between 1.0 and 1.2) on the horsepower curve, then look due left to the range chart. Look like about 2000.

In each, the reading would be at an intersection of a straight horizontal line from either 1.1 or 1.2 on the horsepower curve to where that horizontal line intersect the range curve (due right) or the range chart (due left).

Much of this is related to the maximum theoretical displacement speed -- 1.34 * SQRT(LWL). Beebe's bottom line is that running at somewhere between 1.0 to 1.2 versus 1.34 can be significantly less expensive and can offer significantly more range.

IOW, 1.34 is about the max you can do without completely wasting any additional fuel expenditure, but 1.0-1.2 will be more better.

You may find additional useful discussion on trawlerforum.com (sister site).

-Chris

[na2020]

Thanks very much! Yes, I was talking about figure 6-1. I was making things more difficult than necessary.

I appreciate the help, and will also take a look at trawlerforum.

This has been a great site as well. I've learned a lot just browsing the discussions and seeing other questions/answers.

Thank you very much for the help, I appreciate it.

-- N

[GordMay]

Greetings and welcome aboard the CF, na.

[na2020]

Thanks - great to be aboard.

[na2020]

Originally Posted by ranger42c

Look at 1.2 on the horsepower curve, then look due left from that to the range chart. Looks like about 2800.

Look at 1.1 (approx, midway between 1.0 and 1.2) on the horsepower curve, then look due left to the range chart. Look like about 2000.

In each, the reading would be at an intersection of a straight horizontal line from either 1.1 or 1.2 on the horsepower curve to where that horizontal line intersect the range curve (due right) or the range chart (due left).

I'm looking at this again today, and upon closer investigation, what you're saying is the opposite of what is said in the book. The book states the higher range at 1.1, whereas what you're stating shows the higher range at 1.2

I thought the higher range should be at 1.1. This is why I thought the chart would be read by going from the horsepower curve straight up to the range curve then to the left along a horizontal line (effectively mapping the horsepower number onto the range curve to do the range lookup).

The points about running at 1.1 or 1.0 being more efficient than 1.34 make perfect sense. But I can't get the range numbers to align with that.

[ranger58sb]

Sorry, I didn't mean to imply 1.0 or 1.1 would be better than 1.2, always, or in that specific example.

Given that resultant speed -- which is also dependent on hull length -- plays into it, and that in turn brings distance and time into play... I think the actual outcome would be situational. And in that example situation, I think he decided to illustrate 1.2 as providing the better outcome.

-Chris

[FAST FRED]

Only sail boats attempt to cruise near hull speed , power boats will seldom spend double the fuel for the difference between 1,34 and 1.15.

Remember there is only so much volume in any boat hull.

So fuel for 2,000 to 4,000 miles and water and stores for months will make a far smaller interior .
The ride will also be different with 200G on board from 1,000 Gallons on board.

[na2020]

I'm just finding his chart confusing. If I read it horizontally, it implies a faster speed increases rang. That doesn't sound right, isn't the way I'd read a chart with a second curve on it, nor does it match the narrative. But if I read it by mapping the speed onto the range curve, I can't match his numbers, though the relationship in this specific example of decreasing speed --> increasing range is held.

I also realize that decreasing speed doesn't always mean increased range, as I believe he shows in later examples. This is really more of an issue in reading the chart correctly. The charts are made to be a big deal, but don't have good examples to confirm how they're ready. Even presenting it on a grid would have helped. Maybe I'll consult on this for the 5th edition ha ha.

Regardless, this book is a great read. Really enjoying it, and appreciating the tips on this thread and elsewhere.

Hope everyone is safe and well, and getting to enjoy being out on the water if possible.

-- N

[Singularity]

On the topic of real-world mpg/range, I'd suggest giving the following thread a read:
https://www.cruisersforum.com/forums...se-208057.html

[a64pilot]

Unless your in current and or winds, lower speeds will result in longer range. Particularly on a displacement vessel.
The way current or winds can get you is your going 4 kts through the water into a 2 kt current, so you have 2 kts ground speed, increase speed through the water to 6 kts and you double your ground speed. the increase to 6 kts may not double fuel burn, but it doubled speed over the ground.