Comparing Sailboat Calculations / Ratios

[Lexi22]

I'd like to talk about monohulls from 30 to 39 ft in this thread - not buying inquiry at all, just conversing about calculations / ratios in general. I've been wondering if comparing sailboats' design ratios with other proven designs' is a good idea. If it's, then, let's say, will Contessa 35 and Ericson 38 be good ones to compare with? I've also noticed a vague trend of new monohulls becoming faster and lighter coastal cruisers...

[hlev00]

You can compare similar boats that have similar intentions but when they get too far apart the comparison doesn't provide much useful insight. When searching for my boat (eventually a Beneteau 423), I used PHRF numbers (racing handicaps derived largely from design specifications like length, beam, draft, weight, and sail area) In that case it was comparing the Beneteau with similarly sized boats from Catalina, Tartan, etc.

However using it to compare a J120 to an Island Packet 40' boat doesn't tell you that much that you wouldn't already assume. The J120 is a fast and the IP is going to be very much slower, but more comfortable offshore.

In your example, it is probably useful to use the numbers for the comparison as one dimension of how they compare. But there are a lot of other dimensions, such as quality of the build, layout etc, to consider.

Harry

[Lexi22]

Originally Posted by hlev00

However using it to compare a J120 to an Island Packet 40' boat doesn't tell you that much that you wouldn't already assume. The J120 is a fast and the IP is going to be very much slower, but more comfortable offshore.
But there are a lot of other dimensions, such as quality of the build, layout etc, to consider.

Good point. Of course, the boats have to be generally comparable. IP vs J is a good example. Still, I think we can compare IP with another full keel, and J with another racer.
No doubt, some boats might look great on paper numbers wise, but their quality could be bad.
To make such comparisions, we have to keep Harry's points in mind.

[sandy stone]

Completely agree about using ratios to compare similar boats. One minor nitpick - PHRF handicaps are not derived from design specs, but from observed performance against other boats. In this case, you might usefully compare a J-boat to an IP. The exception is handicapping a design which has never been raced before, in which case the rating committee makes an educated guess, subject to later revision.

[wingssail]

Originally Posted by Lexi22

Good point. Of course, the boats have to be generally comparable. IP vs J is a good example. Still, I think we can compare IP with another full keel, and J with another racer.
No doubt, some boats might look great on paper numbers wise, but their quality could be bad.
To make such comparisions, we have to keep Harry's points in mind.

When I look at the numbers the first things I look at (after comparing the hull shape and rig) are:

• S.A./Disp, I look for more sail area, which indicates light air performance
• Disp./Len, Overall indication of lightness
• Displacement; What is the difference in weight of two boats?
• Ballast, This contributes to stability
• Bal/Disp, Higher numbers usually mean less tender

These things tell me how a boat will sail.

What I don't look at are the subjective ratios:

• Comfort Ratio
• Capsize Screening Formula

These are meant to either support a person's preconceived idea about boats or useless projections about how it will behave in Hurricanes. I don't care and don't trust these anyhow.

[Stu Jackson]

Ted Brewer used to (and maybe still does) write for Good Old Boat magazine.


He often did boat comparisons with some of the numbers you will become familiar with. I think he also came up with a "comfort factor" number that he made up.


You might find more about it with some Google searching.


Good luck.


What is it you are trying to accomplish?

[Lexi22]

Originally Posted by Stu Jackson

What is it you are trying to accomplish?

Nothing particular. Reading about ratios and bored; wondered the opinions of the folks here.

[Pandor]

Originally Posted by wingssail

These are meant to either support a person's preconceived idea about boats or useless projections about how it will behave in Hurricanes. I don't care and don't trust these anyhow.

This is true. But i’d extend this sentiment to almost all calculations.

Lexi, if I asked you how many lbs. your car weighs, are you able to answer? Most can’t, but it doesn’t matter. You don’t shop for cars by weight just the same as you shouldn’t shop for boats by displacement. At least not before considering many other factors first.

You need to decide what your specific requirements are first and you need to get experience on lots of boats to get a good feeling what will fit those requirements best recognizing every boat is a compromise.

[TrentePieds]

Lexi:

TP is a very, very nice boat to BE in. Comes from being a "raised saloon" boat — roomy, airy and light below. Sailing-wise she's a dog. So why do I have her and not some better sailing boat that would suit me better? Because roomy, airy and light below is what MyBeloved wants :-)!

And always remember that when you hoist the iron genny, your tacking angle reduces to zero.

But look at this: She's stated by published figures to be 9,000lbs displacement. She is 25 feet LWL. That gives her a D/L of 256 which is bad enuff. But I carry a ton (literally) of assorted clobber, so that brings her laden displacement to 11,000lbs and therefore her LADEN D/L to 313.

She is stated in published figures to have a sail area of 375 SqFt which, at a disp. of 9K lbs, gives her an SA/D of 13.9 which is wimpy enuff. At her operating displ. of 11K the SA/D is 12.1

So with that sort of variation twixt what the designer sez and what reality sez, just how useful are these sorts of calculations? You will find that in the size range of boats that you've been contemplating, the variation twixt "as designed" and "as operated" is always great. So beware of spurious accuracy!

Now, last night in the waters of the estuary of the mighty, muddy Fraser River it was gusting over 40 knots and blowing a steady 27 - so sez the met station there stationed - so had I been there in TP, which thank the Lord I was not, Ma'am, an SA/D of 12.1 woulda been too much, and being roller reefed fore and main she might well have become utterly intractable. However, on a lovely August afternoon when there is no better place to be, I have to go hunting for even so much as a cat's paw of wind, and an SA/D of 26 would be a delight. But to achieve such an SA/D I would need 700 SqFt of sail!

How do you hang 700 SqFt of rags on a 30 foot hull :-)?

Here is a nice calculator that will save you getting out your iphone. It'll even spare you some pain-inducing thinking:

https://nsc.ca/web2/library/tools/ke...cs-calculator/

But be careful with it — it's canaydian :-)

TP

[clockwork orange]

Keep in mind that many of the hydrostatics you may find on various online websites are quite inaccurate and may be misleading, this is because most of these numbers are not published so someone has estimated them based on what they can glean from what is out there. Another thing is that even if they were to be somewhat accurate they are assuming that the boat is floating on its designed waterline. Once you start adding all of the items you have been led to believe you need to go cruising and the waterline is raised these numbers are no longer valid.

[Lexi22]

Originally Posted by Pandor

Lexi, if I asked you how many lbs. your car weighs, are you able to answer? Most can’t, but it doesn’t matter. You don’t shop for cars by weight just the same as you shouldn’t shop for boats by displacement.

I can answer that right away, ~3000 lbs... Actually, one of the first things I checked was the power and weight ratio buying the car. Anyhow, as I said, I am not shopping for a boat now, so not too worried about such a decision at the moment.
The numbers don't mean anything, but I sense a vague correlation among them with good designs - needless to say considering what a certain sailboat was designed for... Of course, Compac 27 and let's say Dehler 29 were created for different concerns in mind; they won't be a great couple to compare. Despite their fundemantal differences, still comparing Compac 27 with IP 26 on the other hand might make sense.
A Compac 27 might look beefy from outsite to novice eyes, but just by reading her actual numbers, I can close my eyes and visualize how it could behave in certain situations. Here, these numbers help in the name of giving me a general idea, before considering thousands of other factors...

[Lexi22]

Originally Posted by TrentePieds

So with that sort of variation twixt what the designer sez and what reality sez, just how useful are these sorts of calculations? You will find that in the size range of boats that you've been contemplating, the variation twixt "as designed" and "as operated" is always great. So beware of spurious accuracy!

Yes...

[Lexi22]

Originally Posted by clockwork orange

Once you start adding all of the items you have been led to believe you need to go cruising and the waterline is raised these numbers are no longer valid.

Man, I should've made a long list of requirements!? I agree with you.

[Mike OReilly]

Thanks for the sail stats calculator TP. Another one to add to the list of things I can play with when bored .

I like this database for comparing boats: Sail Calculator Pro v3.54 - 3200+ boats

As I said in the other thread, these stat comparisons are not to be taken too seriously. A sailboat is always more than the sum of her stats.

That said, it can be a useful way of comparing similar types of boats to each other. Personally, I find it most useful to compare unknown boats to one I am intimately aware of. That allows me to better judge the differences indicated in the stats.

[johnn33]

Hallo Lexi22 I hope this website will be of interest to you

https://ircrating.org/
John