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Z.Burn · 24-06-2021, 13:11

Hello Everyone,

I have been lurking around various cruising forums for a while now trying to learn as much as possible and there is one thing I am still having trouble understanding. When comparing the LOA with the LWL/DWL and BMX what is a good or dare I say proper ratio? Obviously like much of sailing this must be partially driven by personal preference but I also imagine the science of sailing being one so specific lends itself to have formulas to get within a ballpark of what are considered acceptable dimensions.

As note this boat will serve as my home while on a solo expedition to study marine wildlife through The Sea of Cortez and around South America, hopefully in the future also on more expeditions to Antarctica and The Med.

What would you consider to be the ideal dimensions of a water sailing">blue water sailing vessel between 25'-40' and how does changing these dimensions effect performance?

**tkeithlu** · 24-06-2021, 13:22

Welcome to the forum, Z.Burn.

I don't think that our question has an answer. It's not that thre is not a heavy dose of science in hull design, or that popular shapes have not changed across the time I've followed boats. The cruising boat of today is rather different from the cruiser of 1960 or so. But, every design is a compromise between speed, seaworthiness, comfort, ease of handling, and a bunch more. Dimensions are not all of it by any means. Way back, we had rudders that were protected by full keels, but interior space sucked, speed was not much, and handling was very heavy work. Just look at the difference between the hard-assed racing boat of today versus a family cruiser.

So, what do you want? You can choose a boat according to the strengths you want versus the characteristics you will compromise. Dimensions will change accordingly.

**TrentePieds** · 24-06-2021, 14:25

Wellcome Z.burn

You ask: "What would you consider to be the ideal dimensions of a water sailing">blue water sailing vessel between 25'-40' and how does changing these dimensions effect performance?"

This question, as you ask it, is NOT amenable to a simple answer. There are a million answers to it :-).

All boats in your stated size range are capable of going "blue water", which, by the way, is merely a noxious marketing term disregarded by anyone who knows anything about boat design and seafaring at all. It is NOT the boat that takes skipper and crew safely across oceans. It IS the skipper - I say again: THE SKIPPER - that takes boat and crew safely across oceans.

Now, start here: The theoretical maximum speed ("hull speed")of a keel boat, full or fin keel, is 1.35 x LWL^(1/2), so a boat 25Ft LWL is 1.35 x 5 = a shade under 7 knots. The hull speed of a 40 footer with a LWL of, say, 36Ft is 1.35 x 36^(1/2) = a shade over 8 knots. But these are theoretical figures and have little to do with seaworthiness. Just with straight line speed on "flat" water.

Typically a 25Ft boat has a displacement (i.e. it weighs) 2 tons. Typically a 40 Ft boat has a displacement of 10 tons. Thus the bigger boat has more inertia and is able to "punch through" seas (waves) that would stop the smaller boat in its tracks.

Now imagine that you draw the shape of a boat's water PLANE on a sheet of paper. Not the water LINE, but the water PLANE. Then you circumscribe that figure with a rectangle as long as the LWL and a wide as the B(max). I assume that you meant B(max) when you wrote "BMX" . Now calculate the area of the rectangle and measure the area of the water plane using, f.ex a polar planimeter (or you can do it using Simpson's Rule). If the A(wp) is less than .6 of the A(r) you have a boat with a fine hull (although there is really more to that definition than just this ratio). If your A(wp) is more than .7 of A(r) you have a "fat" boat. Fine boats are easily driven and therefore require less sail than "fat" boats which tend to be "dogs" requiring much sail.

There is much, much more to this discussion than it would be sensible to pursue in a post on a forum.

Get you self off to a good start by getting this book: Francis S. Kinney, Skene's Elements of Yacht Design. It's a classic and was for many years the text for introductory courses Yacht Design. When you have understood the material in this book well and truly, come back and let us find an answer to you questions.

Remember that boats are designed for the particular people who have to sail them, and for the particular waters they have to sail in. There is NO SUCH THING as a yachting equivalent of a RAM 350!

All the factory built boats you see on the market are compromises between design characteristics that make for good sailers and design characteristics that make for good apartments. Therefore all them are ADEQUATE, but none of them are particularly desirable for the man who knows HOW he sails and WHERE he sails.

Most factory boats are marketed as cruiser/racers. They are compromises that neither race well nor cruise well. It's up to you to know how you learn so you can choose a boat that is the least unsatisfactory for what you intend to do with it.

All the best

TrentePieds

LittleWing77 · 24-06-2021, 14:30

Well that was easy. Question posed and answered.

Kudos to Keith and Trente

This thread can now be closed.

LittleWing77

**Adelie** · 24-06-2021, 14:54

LOA 36’
Beam 10’
LWL 30-32’
Plumb bow, overhanging stern
5’ draft
3200lb ballast in a bulb
Displacement 9500lb
30” side decks
SA/D 20
2’ bowsprit
Cutter
Fin keel
Spade rudder

**Jim Cate** · 24-06-2021, 15:44

The critical dimension is that the boat fit comfortably in your bank account...

Jim

LittleWing77 · 24-06-2021, 16:19

Quote:

Originally Posted by Jim Cate

The critical dimension is that the boat fit comfortably in your bank account...

Jim

You're the bee's knees, Jim.

If you weren't already happily married to the lovely and talented Ann, I would be lining up!

LittleWing77

Rohan · 24-06-2021, 17:16

Quote:

Originally Posted by Z.Burn

When comparing the LOA with the LWL/DWL and BMX what is a good or dare I say proper ratio?

You have some good answers here, but I'll add another one.
Proper ratio for what ? Speed? Storage space? Comfort? Ability to handle heavy seas?

If speed is the top consideration, I would recommend a trimaran actually. If it's storage, then a wide monohull. If it's comfort, I would recommend a heavy displacement full keel boat, etc. Obviously you want all of the above, so you will have to compromise on something.

You say you want to study marine life - then I think speed is not the most important factor, if you are going to be on the water awhile you want a boat with enough amenities for yourself and that is strong enough to carry all the equipment you need. Beyond that, it also matters what latitude you are working in. For tropical/warm areas, I would lean toward a catamaran, for cold areas, a monohull. You say you plan to go around South America, Antartica, the Med? That's pretty ambitious and beyond my experience level. But I would assume you are going to have crew with you - you need a boat large enough for them all , and if money is no object, I would buy one with an aluminum hull. There aren't a lot of those to choose from, so of you go that route, your LWL ratio and all else would depend on what is available. Even if you are alone, I would go with the higher end of your 25-40 range. 25 is a little small for that much exploring, but people have done it.

Greatest Lakes · 24-06-2021, 19:16

There is no perfect boat. Everything about or on a boat is a compromise in one way or another. Jim was closest to correct mentioning cost. Personally, I think the best boats are the ones where everything works like it should.

As far as LOA/LWL is concerned, a water line closer to the LOA is a faster boat, but likely more wet, as the hull shape is not going to permit overhangs which deflect water as well as increase comfort by dampening motions. Additional trade off of the plumb bow, wide stern seen on most modern boats, aside from how water seems to slap the hull, is often the keel attachment. Most will have fin keels that focus a lot of stress where they are mounted to the hull. Not always a problem, but the forums have plenty of cases of people fixing damage from grounding that damage the hull. They react to windage a bit more and the bilge often can't contain much water, not that they usually have to.

Older boats with canoe sterns would deflect following seas and were favored for this ability for a long time. The more pointed bows, and some sterns, LOA a good bit longer than LWL, tended to keep the deck dryer, and softened the motions at sea. The hull shapes also tended to be more favorable to strong keel attachments that were more integral to the hull, rather than a bolted on slab. Deeper bilges, so you could get a handle on small leaks without damaging the floorboards.

The modern hulls are mostly designed for interior space and ease of manufacture. They know most boats spend most of their time at the dock, so that's what they build for. Older boats tended to still assume you were actually going sailing, so they were designed to be comfortable underway.

That's my 2 cents. Use what you will...

capn ken · 24-06-2021, 19:18

opinions as too lpg sniffers on my cal -29 .

Greatest Lakes · 24-06-2021, 19:48

Quote:

Originally Posted by capn ken

opinions as too lpg sniffers on my cal -29 .

Xintex is the Cadillac, Seachoice is what I used to have on the Hunter 37. But that doesn't really go with the thread.

capn ken · 24-06-2021, 21:46

thanks for the help .I cant seem to figure out how to post ?

**Don C L** · 24-06-2021, 22:01

You say 25 - 40'. I just spent the day reorganizing my little 29 footer and there still isn't enough room for all the stuff! I would say, and this is from someone who has had lots of fun on a 24' and now a 29', for what you are planning, start at 30' at least.

And as someone mentioned, speed is mainly a function of waterline usually, (unless you have a boat that can surf.) Anyway this site has lots of fun boats to look through: https://bluewaterboats.org/
Olin Stephens, my favorite designer, believed that a boat with a displacement to length ratio of around 300 was the best for offshore; not too light, not too heavy. Personally I think a sail area to displacement ratio of 15 or below is going to be too low on power. I'd also be sure to chose a hull that doesn't mind going upwind pretty well, and one that performs well in light air too.

Z.Burn · 25-06-2021, 10:19

Thank you everyone, many of you touched on both some theoretical aspects about speed I was having trouble with and practical knowledge that I have yet to hear expressed. It's actually a relief to read the reassurance that it really is just more about an individuals personal preference and where you are willing to compromise to balance between safety, comfort and speed.

If price was not an issue though I probably would buy a brand new Reinke S11. Most of the boats I have been looking at fall into the 35' range and I am especially partial to the Colvic Victor 34 but finding a DS boat that is a bit of a project but doesn't need extensive structural repair has been proving difficult in the US so I'm just trying to stay as open minded as possible and just keep learning so when I do finally get my boat my plans and the continuous life forward on the it won't seem quite so ambitious.