Importance of hull speed...

[CapnK]

With all 3 boats still on the table,  ::) , I was thinking about something I'd read the other day about 'faster boats making quicker passages'. And that being a safety issue/buying factor/pro vs con in favor of bigger boats.

How much faster, makes how much a difference, I thunk just now. So I did Maths. Here they is:

Distance - divided by Hull Speed of Boat equals Hours - divided by 24 equals Days - Boat type.

4000m

/7.1 = 563 / 24 = 23.4 days.   I-36

/6.24 = 641 / 24 = 26.7 days.   A30

/5.76 = 694 / 24 = 28.9 days.   Ariel

So 1.25kts less, makes for 5.5 more days in/over 4k miles.

Theorentically, of course.

Could it be argued that a heavier boat is more likely to carry speed better, being less likely to get slowed by ocean motions?

Tho' a lighter boat is more easily driven...

I always remember what 2x OSTAR/2x Vendee sailor Neal Petersen told me, that his *Average windspeed* for the *entire* Vendee route ('round the Great Capes, IOW) was 13kts. Given the extremes he experienced down South, that means it likely that pulling those high peaks from his stats would drop that average to 10 or less. IOW - light air. So is smaller better, maybe, on the Averages?

Thoughts?

:)

[Auspicious]

No. The premise is flawed.

The concept of hull speed dates back centuries at least to when most ships had pretty consistent hull forms. Even then 1.34*sqrt(LWL in ft) = kts was used to compare similar hulls in the early stages of design. As design matures, hull speed has less and less value.

In general longer means faster. If you compare geosims (same hull form, just scaled up or down) the longer one will be faster. Two different hulls the same length the narrower (less projected area in section) will be faster. Less wetted surface means faster, thus fin keels faster than full keels.

Then there is the impact of hull form on seakeeping. Here I can give an example based on personal experience. A little big for most of this audience, the Passport 40 and Valiant 40 are both drawn by the same talented naval architect. The Passport 40 has very full sections especially forward. Good for storage. Bad for seakeeping. It crashes into waves in a developed sea and slows to a crawl. Offshore you can expect passage speeds of around 3 kts. The Valiant 40 on the other hand has a fine entry forward and as it happens less longitudinal moment of inertia (less weight in the ends). Passage speeds of 6 to 7 knots. You don't predict that performance difference looking at hull speed numbers.

Naval architects look at lines and weight estimates to make predictions of performance. We cover factors like hull speed in school for historical reasons, because they play a role in conceptual stages of design, and because it helps build the skill to look at (or draw) lines and understand the implications. Consider the difference in hull form of a super tanker and an aircraft carrier.

The first person to bring up comfort ratio gets keel hauled on a dirty bottom. Look at the numbers for the Passport 40 and Valiant 40.

@CapnK - if you're still boat shopping and can get me lines for your candidates I'm happy to look at them for you.

Thus spake the naval architect. *grin*