Gut feeling on boat sizes.

[a64pilot]

I don’t know how big the crew was to be honest, I would have thought more than a dozen, even after you subtract the Capt, Navigator, cook and others that didn’t actually sail the boat.
No, I’d expect the sail area to displacement to be close, however the displacement goes up significantly as a function of Boat length, at some point the Boat won’t be able to maintain the sail area to displacement.
What I’m saying is that a 200’ Boat may have 50 times the displacement of a 20’ Boat, not 10 times, so to maintain that ratio, you have to have 50 times as much sail on a boat that is only 10 times longer.
No idea of actual numbers of course, just through those out there.

Is there a way to determine sail area by vessel length, not displacement?

[Adelie]

Originally Posted by hpeer

. . .
The wage tank tests favor the bigger boat. My gut is if I had a survival storm I would want the big boat.
. . .

Wave tank tests favor larger boats in one particular area, capsize resistance in the event that a breaking wave is encountered, more so when the wave is on the beam.

In this case a larger boat decreases the odds of an already low odds event.

Some steps can be taken to improve the situation for the smaller vessel, namely increasing the roll moment of inertia of the boat. A beefier mast, or a taller mast or more and larger rigging would help or combinations of all of these. Yes, this is counter intuitive, it would somewhat decrease the static righting moment and even the AVS (Angle of Vanishing Stability) but the results are clear, greater roll moment of inertia (and pitch) increases capsize resistance in breaking waves. The point being that a capsize is a dynamic event mostly resisted by inertia not static stability.

In the larger scheme of things I would rather roll the dice on a boat that is all easier for me to deal with during heavy weather in spite of equipment failures than to deal with the larger boat that will have a capsize advantage in really extreme weather.

[Adelie]

Originally Posted by dannc

People should read this book if interested in seaworthiness. I don't know of a better book, and if someone else does, please post the title. Dashew has written about seaworthiness over the years, and he should be read, but Marchaj's book is the best I have read on the subject.


Marchaj does not argue that small, light boats are more seaworthy but the opposite.


The list of boats sunk or abandoned during the Fastnet race in 79 would not indicate that small is more sea worthy.


Later,
Dan

Marchaj's did not argue for bigger, he argued against boats designed to a racing rule in general and the IOR rule in particular. He laid a whole history of racing rules leading to fast but not very safe boat designs. Most recently the IOR encouraged some very poor tradeoffs, undersized rigging, spars and fins, and a preference for live ballast and form stability over fixed ballast and ultimate stability.

In the case of the 1979 Fastnet abandoning of boats had more to do with the psychology of the folk aboard than the actual immanent risk of staying aboard. A number of boat that were assumed to be on the verge of foundering were found after the storm abated.

[atmartin]

Originally Posted by a64pilot

Is there a way to determine sail area by vessel length, not displacement?

I suppose we could just divide SA by LOA. Here's some examples:

Cal 20

LOA: 20 ft
Sail Area: 195 sq ft
SA/LOA: 9.5 *We made this ratio up

Island Packet 38

LOA: 38 ft
Sail Area: 870 sq ft
SA/LOA: 22.9

Tenacious

LOA: 213 ft
Sail Area: 13,100 sq ft
SA/LOA: 61.5

Maltese Falcon

LOA: 289 ft
Sail Area: 25,791 sq ft
SA/LOA: 89.2

Of course you have to pile on more sail per foot the longer you get! However, it doesn't scale as exponentially as you'd expect. If we made your IP38 289 feet long, her beam would be 96', her draft 38'. The Maltese Falcon is probably a bad example because it has some kind of board I think, but it's beam is 40' and draft 20'. Your IP38 scaled to 289' would displace about 4.5x more than The Maltese Falcon! Of course, that's just a quick and dirty estimate.

Point being, larger boats also have much smaller beams than our boats and proportionally less draft, so SA doesn't scale as viciously as it otherwise might. Your cube involves length, beam and draft and they don't all grow at the same rate.

It seems to me that they didn't make sailing ships bigger in ye older dayz due to limitations in strength of materials (and length of materials, trees only get so tall) and fabrication (no resin, no welding). As soon as iron and steel came on the scene, sailing ships got a heck of a lot bigger. If for some reason cargo ships had to start sailing again, I'm pretty sure technology would allow them to be even bigger than in the olden days.

All of this is a sideline because some posters seem to be suggesting that boats over 30-something feet can't be built as robustly as their tiny counterparts and, frankly, that kind of thinking blows my mind.

[Terra Nova]

Originally Posted by a64pilot

...Is there a way to determine sail area by vessel length, not displacement?

There is no need to do this. It is one of the reasons we have naval architects.

[Dan GB]

Originally Posted by atmartin

I suppose we could just divide SA by LOA. Here's some examples:

Cal 20

LOA: 20 ft
Sail Area: 195 sq ft
SA/LOA: 9.5 *We made this ratio up [emoji3]

Island Packet 38

LOA: 38 ft
Sail Area: 870 sq ft
SA/LOA: 22.9

Tenacious

LOA: 213 ft
Sail Area: 13,100 sq ft
SA/LOA: 61.5

Maltese Falcon

LOA: 289 ft
Sail Area: 25,791 sq ft
SA/LOA: 89.2

Of course you have to pile on more sail per foot the longer you get! However, it doesn't scale as exponentially as you'd expect. If we made your IP38 289 feet long, her beam would be 96', her draft 38'. The Maltese Falcon is probably a bad example because it has some kind of board I think, but it's beam is 40' and draft 20'. Your IP38 scaled to 289' would displace about 4.5x more than The Maltese Falcon! Of course, that's just a quick and dirty estimate.

Point being, larger boats also have much smaller beams than our boats and proportionally less draft, so SA doesn't scale as viciously as it otherwise might. Your cube involves length, beam and draft and they don't all grow at the same rate.

It seems to me that they didn't make sailing ships bigger in ye older dayz due to limitations in strength of materials (and length of materials, trees only get so tall) and fabrication (no resin, no welding). As soon as iron and steel came on the scene, sailing ships got a heck of a lot bigger. If for some reason cargo ships had to start sailing again, I'm pretty sure technology would allow them to be even bigger than in the olden days.

All of this is a sideline because some posters seem to be suggesting that boats over 30-something feet can't be built as robustly as their tiny counterparts and, frankly, that kind of thinking blows my mind.

Not arguing for any particular point of view and I am certainly not a naval architect, but synchronicitously (not sure that is a word) I read today in a magazine I subscribe to something like: double the length of boat, means four times the sail area, means eight times the forces.

I am sure it is way too simplistic, and not true in all size ranges, but from a common sense perspective illustrates the space, at least generally.

[GILow]

I think that’s a pretty reasonable set of figures.

I’ve always worked on the rule of thumb that doubling the boat length gives you 8 times more boat.

Certainly feels like it when it comes to maintenance. [emoji6]

[NormanMartin]

A place I go to help with my analysis of different designs is: Sail Calculator Pro v3.54 - 3200+ boats

I compare a known to an unknown design with his calculator. Another site is:PHRF New England - Handicapping - Base Handicaps

If you can get a copy of "Cruising Sailboat Kinetics" by Danny Greene, get it. A good book for the discussion of what works and why.

[hpeer]

I know folks are determined to measure boats by length. I still think displacement is a better, more descriptive measure of boat size. Not the only factor to be looked at, but one of the first.

We have 2 steel sailboats:
33’ is 8 tons
44’ is 20 tons.

Length comparison gives 44/33 or 30% larger.
Displacement comparison gives 20/8 or 150% larger. A much more accurate comparison.

A Catalina 309 (33’) has a advertised displacement of 5 tons. Doing the same comparison as above in length it is comperable to my small boat. In displacement (8/5) my boat is 60% “bigger.”

Now let’s take a storm at sea, which boat would fare better, which would be more comfortable? My GUT says I’d rather have my steelie in a nasty, sustained blow.

[Mike OReilly]

Originally Posted by hpeer

I know folks are determined to measure boats by length. I still think displacement is a better, more descriptive measure of boat size. Not the only factor to be looked at, but one of the first.

We have 2 steel sailboats:
33’ is 8 tons
44’ is 20 tons.

Length comparison gives 44/33 or 30% larger.
Displacement comparison gives 20/8 or 150% larger. A much more accurate comparison.

I think you’re onto something. My previous boat was 34-foot, 6 tons. My current boat is 37-foot (36’ 9” to be exact), 15 tons. There is no question that my current boat is far more seaworthy than my first boat — not just a factor of three feet. I do think size makes a difference, but perhaps more as a proxy for displacement.

But I also wonder how much difference we’re talking about anyway. If you’re considering ultimate seaworthiness (keeping the crew alive), a storm that threatens my 37-footer is also likely threatens a 55-footer. In the end, all our boats are fairly small.

[Pete7]

Originally Posted by dannc

The list of boats sunk or abandoned during the Fastnet race in 79 would not indicate that small is more sea worthy.

Dan

Ah, but the reason so many smaller 30ft yachts were in trouble during that race is the storm called a bomb struck when the smaller yachts were directly in its path. The larger 50ft yachts had mostly cleared the path and were across on the Irish coast. that isn't to say they didn't get pretty beat up but nothing compared to the smaller yachts racing.

Pete

[NormanMartin]

Post race analysis of Fastnet showed that the wide beams and often low bridge decks made the boats unseaworthy in short steep seas. Also, in that era, cockpit drains were often too small and too close to waterline.

Some remarkably small boats have made open water voyages successfully. Think Trekka and Sopranino for starters. They are rugged little craft that have sea going layouts on deck and below.

I have an Aphrodite 101 which is 33 ft LOA and 8 ft beam. What are her weak points? No bridge deck and poor cockpit drainage. She was designed for ocean racing in the North Sea and Baltic... back in 1978. Today, we'd do her completely differently. In the 60s, S&S did a similarly narrow 33 ft sloop, Spirit, that sailed across many oceans and won races. She has/had a much better deck layout. A guy from Gloucester, MA, USA just finished an RTW non stop in a Westsail 28.

A lot more than length goes into sea worthiness.

[a64pilot]

OK displacement is the “true” measure within limits of a vessels size.
However very few when asked how big is your boat reply with a displacement number, so length is the accepted figure.

Size does matter, however the bigger you get, the harder it is to maintain strength, that is why there is a limit as to how high a brick or even steel building can be built.

Very small, can be made to be very seaworthy, lifeboats for example, they can withstand storms that can tear full size ships apart, however if strength and design were equal, size wins.
The bigger you get, the harder it is to maintain the same strength, if you built a Piper Cub the size of a 747 it would just collapse under its own weight.
However in the size difference of boats we normally talk about say 30 to 60, it’s not likely all that big of a factor, it exists of course, and the 60’ has to be much more heavily built to maintain the same strength of the smaller Boat, but a quality one is of course.

So really it does seem to get back to design and construction determines ultimate “seaworthiness”, or else only a fool would get into one of the tiny little plastic orange boats big ships carry.

[atmartin]

Originally Posted by Pete7

Ah, but the reason so many smaller 30ft yachts were in trouble during that race is the storm called a bomb struck when the smaller yachts were directly in its path. The larger 50ft yachts had mostly cleared the path and were across on the Irish coast. that isn't to say they didn't get pretty beat up but nothing compared to the smaller yachts racing.

Pete

This does suggest the wisdom in faster (usually larger) boats having less chance at grief due to faster passage times.

Another possibly interesting sideline is wave power. Wikipedia gives the formula as:



Hm0 the significant wave height, Te the wave energy period, ρ the water density and g the acceleration by gravity

We don't need to crunch numbers, but note that the output is given in kW/m (power per length of wave encountered). This might be an area in which the scaling effects of larger boats (displacement growing faster than length) actually favors length. Given the same wave and assuming the fearful beam-on scenario, a 60 footer will experience merely twice the wave power as a 30 footer. Say the average 30 footer is about 8,000 lbs (it's a cruiser!) and 60 footer is 65,000 lbs. You can see a serious advantage here. Also, if that wave is going to break on your cabintop, it's much easier to make a 60-footer's top beefy without making her top heavy. This is a sometimes over-looked issue in smaller boats. For stability and performance, you don't want to over-scanting her decks and house, so you sometimes make compromises in deflection (especially in the case of doghouse tops) or strength.

Mike OReilly is probably right as far as survival seas go, we're all very small craft compared to 30' waves (if you run that formula, the energy involved is HUGE).

[a64pilot]

I agree up to where you say it’s easier to make the top of a bigger boat strong, just the reverse is true.
There is a reason that at a certain size, boats are made from metal.