A Question About Hull Characteristics

[NormanMartin]

A super discussion
Books written on the topic are interesting reads
In the end, it is all about purpose. Our Aphrodite ticked all the boxes for our intended purpose and that's why we chose her.
The question is: For the purpose of (X) what are the key hull features.
Norm

[Lee Jerry]

Originally Posted by Don C L

Btw, does anyone here know why the GZ is not included in the CSF?

Originally Posted by sandy stone

Because the CSF was created as a quick and dirty, back-of-the-envelope calculation to give a rough comparison between largely similarly shaped hulls, using only widely available, published data.

I agree 100% with this.

However, assuming he means GM, there is also another answer, which is that it sort of is included. (Maybe I'm channeling Adelie here.) Beam is the dominant factor in the waterplane inertia and WP inertia is the dominant factor in metacentric height, therefore beam is a proxy for GM in the CSF. So, if you tilt your head and squint a bit, you may see it there...

It is a wonder to me that people still consider a certain value of the CSF to be the definitive qualification for a "blue-water" boat, calculated to three significant figures, when a boat's displacement (for instance) is rarely known to much better than two significant figures.

Because the displacement is to the cube root in the equation, the effect of displacement is reduced in the final answer, meaning the significant digit "requirement" of displacement is reduced (by quite a bit). I'm not sure I'm saying that right, or how to say it better, so maybe by example:

Using the boat mentioned above, B of 12 ft, displacement of 20,000 lb:
CSF = 12 / (20000/64)^0.333 = 1.77

If displacement is 22,000 lb:
CSF = 12 / (22000/64)^0.333 = 1.71


If displacement is 18,000 lb:
CSF = 12 / (18000/64)^0.333 = 1.83

So a 10% change is displacement causes only a 3.4% change in CSF, and this (or should be) transferred to the significant issue as well. Maybe the term I'm looking for is sensitivity.

[Lee Jerry]

Originally Posted by wingssail

I don't agree that your boat is narrow (and I do agree with Bob Perry about that and most other things). My boat is only .9' beamier and is considered "beamy".

When I compare your boat to mine I see that you have more sail area for the displacement and less displacement for the length (so you are lighter and more powered up). Maybe your problem with being tippy is due to carrying too much sail for the conditions.

And lets get rid of the obsession with numbers like Capsize Screening Formula and Comfort Ratio. Deciding what is a "blue water boat" by looking at a bunch of numbers is a waste of time. By most of these kind of measures my boat is no way a blue water boat. Yet is has been happily cruising in blue water for 30+ years. It has never capsized or come near to that, but my capsize screen formula, at 1.99, at the verge of being too dangerous to even go to sea. We are not "tippy" because we reduce sail. Even though it is a classic IOR design, it IS NOT a hand full or an aerobic exercize off the wind even in a sea way. In fact the monitor windvane steers the boat in ALL conditons. We just relax and ride. Up wind we don't find that it pounds but it does make miles going to weather when our friends with more "cruising" boats are nearly stopped dead trying to motor upwind in waves.

Does our fin keel boat track well? Absolutely! Because the rudder keeps it on course.

Did IOR result in "sketchy" hulls shapes? Wierd maybe, but they do fine.

We've cruised around the world. Our boat is fast and comfortable and easy to sail. We typically cover 150-175 miles a day while sitting on our butts. We get plenty of sleep and take showers and arrive clean, well rested, and first.

This winter we took a little trip, 150 miles each way and it was windy and rough. Down wind we had breeze in the mid 20's and it was quite rough. But we were hitting 9's and 10's and saw 11knots a couple of times. The windvane steered and the half full coffee pot on the stove never spilled. Coming back we had true wind speed of 25-29 knots and waves as big as cars. The last leg was 147 miles direct upwind. We sailed (beating) the whole way in 30 hours. Our track covered 175 miles. It was rough and wet but the pounding was not terrible. When we got home we really didn't have to do anything except put our wet clothes in the laundry. Then we took off the dodger and wind vane and entered a major 4 day regatta and got second place. Tomorrow we race again and expect to win.

All of this in a IOR boat that, by the numbers, should never be considered to be a good all around sea boat or a blue water boat.

Get your head out of the numbers and do more sailing. You can find a well designed boat and it will do you well

I'm glad you like your boat.

In your boat's 30+ years of blue water cruising how many times has it been beam on to large breaking seas? I'm guessing not very often. Or even IN large breaking seas? 30 kt winds don't count.

Tracking (yaw stability) and efficient steering are two different things.

Yeah, that's why they had to invent a whole new sail to help control them downwind.

Averaging a 33 deg true wind angle would be most impressive. Most boats can't do that in 10 kt and flat water, so to do it in 25-29 kt and big seas...

And finally, it's not always possible, in fact probably rare, that you get a chance to sail a used boat model before purchase. So "the numbers" can help give you some idea as to how it performs.

Again, i'm glad you like your boat.

[Don C L]

Originally Posted by Lee Jerry

However, assuming he means GM, there is also another answer, which is that it sort of is included. (Maybe I'm channeling Adelie here.) Beam is the dominant factor in the waterplane inertia and WP inertia is the dominant factor in metacentric height, therefore beam is a proxy for GM in the CSF. So, if you tilt your head and squint a bit, you may see it there...

Actually I am referring to GZ and the righting moment generated as the boat heels. Clearly a deeper ballast confers a greater righting moment and a greater resistance to capsize, no?

Also, in my albums I have a letter from Olin Stephens in response to the original owner of my boat regarding metacentric height. He related that it didn't concern him in his designs because the ballast is so deep in sailboats:
https://www.cruisersforum.com/galler...ageuser=155384

So I always thought that metacentric height was of more concern to ships, especially cruise ships, who are concerned about the rolling motion of their hulls. I am wondering how the metacentric height would figure in here, unless we are considering the rolling characteristics of a given hull.

[Bowdrie]

I once observed an inclining experiment being conducted on a 65' schooner that the owner wished to get certified for passengers.
Lots of 50 gal barrels of water were used.
The Coast Guard determined that additional ballast was needed.
Ballast was added and the experiment was done again.
The additional ballast was enough to satisfy the CG with the stipulation that the topsails not be used while under sail.

[Don C L]

You know, in practical terms, I have sailed reaching in breaking seas, and I can understand the advantage theoretically to having more freeboard, but the weight of water that can slam the windward side can be tremendous and I am not sure I see it figured into the mix. Adding a foot or two of freeboard translates to a lot more force of water on the hull to rotate it and push it down the face of the wave. Consider a ballasted box vs. a ballasted surfboard hit by a wave beam-on. The numbers on that I do not know, I am only going by seat-of-the-pants calculations.

[Lee Jerry]

Originally Posted by Don C L

Actually I am referring to GZ and the righting moment generated as the boat heels. Clearly a deeper ballast confers a greater righting moment and a greater resistance to capsize, no?

Also, in my albums I have a letter from Olin Stephens in response to the original owner of my boat regarding metacentric height. He related that it didn't concern him in his designs because the ballast is so deep in sailboats:
https://www.cruisersforum.com/galler...ageuser=155384

So I always thought that metacentric height was of more concern to ships, especially cruise ships, who are concerned about the rolling motion of their hulls. I am wondering how the metacentric height would figure in here, unless we are considering the rolling characteristics of a given hull.

GZ is a variable, it changes as the vessel heels. That is the righting arm curve. I don't know how you would incorporate a variable into the CSF. GM is a constant (for a given loading condition, such as lightship or full load or half load). GM gives an indication of the initial stability only, from upright through a range of 10-15 deg (depending on hull form). It gives you an idea of the early shape of the RA curve but tells you nothing about it past those first 10-15 deg. Therefore, it naturally has its limits, but is useful for what it does. One of those things is roll motions, as you mention. Another could be initial sail carrying ability.

I'm not surprised that Olin Stephens didn't concern himself with metacentric height in 1963. I expect the S&S office knows the GM of every design they produce today.

[NormanMartin]

Look for a book: Cruising Sailboat Kinetics by Danny Greene
It answers all your questions and makes a good read.