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Old 14-01-2021, 13:02   #1
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LOA vs. LWL: A design question

Hi all - relative newbie here, so forgive if this is a dumb question!

I'm looking at boats from the 80's, esp Morgan 38x designs.What is the point of designing a hull with a 38 foot LOA with a 30 foot waterline? Or to put it another way why not maximize the waterline to obtain a faster hull speed? Because really, who wouldn't rather go a little bit faster? (within reason)

Why design the stern to sit up out of the water, sacrificing waterline length, and also reducing interior volume? I imagine the answer has something to do with the motion of the boat in a seaway and/or heavy weather. But it seems to me that the overall shape of the hull is more relevant to this issue.

Also, can anyone recommend a book that discusses these elements of hull design?

Best to all,

Steve
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Old 14-01-2021, 13:14   #2
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Re: LOA vs. LWL: A design question

Here is a good article:

https://www.oceannavigator.com/shapi...offshore-hull/

The overhangs: In many older boats the overhangs were unduly influenced by rating rules. For example, J/Boats had extremely long overhangs because waterline length was limited to 90 feet. The largest J/Boat, Ranger, was 135 overall, giving overhangs half the length of the waterline. (Designers use the overhang divided by the overall length to give a non-dimensional length of overhang. In this case 45/135 = 33.3%). The International Rule led to long overhangs as seen on 12-meter boats. These boats have a waterline around 45 feet and an overall length of 65 feet (30% overhang ratio). When one of these boats moved at speed, the bow and stern waves were displaced toward the ends of the boat, increasing waterline length and ultimately increasing speed.

Boats that race in the BOC Race (now known as the Around Alone), on the other hand, have no overhangs and tend to be very fine forward and fat aft. The International Measurement System (IMS) rule has also led to shorter overhangs, less than 10% in many cases. Is this trend a good one? For an ocean voyager, I think not. While short overhangs give a longer waterline length, they also provide very little reserve buoyancy above the waterline. A voyaging boat often has heavy weights forward (anchors, anchor chain, windlass) and aft (dinghy, outboard, propane tanks). Contrary to current trends, I believe that a voyaging boat should have moderate overhangs. The length of overhang that I like to see is between 18 and 24%. This will give the bow and stern some reserve buoyancy to help stop the bow submerging in heavy seas, and the boat from pitching in a seaway.
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Old 14-01-2021, 13:20   #3
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Re: LOA vs. LWL: A design question

Rating rules for racing incentivized shorter waterlines, but the longer the waterline length the faster the hull speed. So you design a boat with a short LWL that becomes a longer LWL as soon as the boat heels over from the wind.

These long overhangs are aesthetically pleasing to the eye and are a feature of classic sailboats. As fewer and fewer boats were raced the overhangs got shorter and shorter until we wound up with modern boats and plumb bows and cutoff sterns.
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Old 14-01-2021, 13:35   #4
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Re: LOA vs. LWL: A design question

Disclaimer: I’m not an expert.

I suspect most of the designs with large boat overhangs and stern counters are older, drawn when aesthetics were more important than they are today. Most modern design have plumb(ish) bows and a flat wide stern, mostly with sugerscoop transoms that enhance water line length and thus hull speed whilst improving interior volume and, as I understand it, better performance at heel angles.

Personally I’m not a fan of many of the slab-sided designs built today in pursuit of performance. Firstly I find them ever so slightly utilitarian and frankly, quite ugly. And as far back as the 2009 Beneteau Oceanis series (I’m talking specifically of the Oceanis 50 that I delivered Tahiti to Auckland), in any form of lumpy sea whilst sailing on the wind, the flat section in the front 1/3 of the boat slammed very badly and the course had to be adjusted to almost a beam reach to stop it, in doing so sacrificing quite a few CMG miles.

And to counter comments that “it’s just you”, the logbook of the boat which had annotations from two other delivery crews who undertook other legs of the long delivery, also frequently commented on the bone-shaking slamming of the boat whenever it was sailed on any form of beat.

My old boat (see attached image) has a shortish bow overhang and a short stern counter and sails on the wind in any sea without any sign of slamming. Maybe this is a reason for the apparently “inefficient” design.

I’m sure a lot of others will weigh in with more intelligent responses.
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Old 14-01-2021, 14:00   #5
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Re: LOA vs. LWL: A design question

Quote:
Originally Posted by shmorrell View Post
Hi all - relative newbie here, so forgive if this is a dumb question!



I'm looking at boats from the 80's, esp Morgan 38x designs.What is the point of designing a hull with a 38 foot LOA with a 30 foot waterline? Or to put it another way why not maximize the waterline to obtain a faster hull speed? Because really, who wouldn't rather go a little bit faster? (within reason)



Why design the stern to sit up out of the water, sacrificing waterline length, and also reducing interior volume? I imagine the answer has something to do with the motion of the boat in a seaway and/or heavy weather. But it seems to me that the overall shape of the hull is more relevant to this issue.



Also, can anyone recommend a book that discusses these elements of hull design?



Best to all,



Steve

A flare bow gives a somewhat drier boat so that accounts for some of the bow overhang.

A little overhang at the stern means the stern wave will rise up to the stern when underway and the waterline length the water sees is a little longer and there is less of a cutoff turbulent wake too which is higher drag. Also in very light air the overhangs minimize wetted surface area which is more important at low speeds than waterline length.

Really extreme overhangs like the J-, R- or Q-class boats of the early 1900s are related to giant gaping holes in the handicapping rules of the time.

From the CCA era on the overhangs would be adjusted modestly to suit a rule but mostly they are there for other reasons
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Old 14-01-2021, 14:33   #6
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Re: LOA vs. LWL: A design question

if you delve deeper.....you'll see that long overhangs increased the LWL when heeled over, giving a theoretical faster speed..
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Old 14-01-2021, 14:43   #7
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Re: LOA vs. LWL: A design question

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Originally Posted by MicHughV View Post
if you delve deeper.....you'll see that long overhangs increased the LWL when heeled over, giving a theoretical faster speed..
That is a faster speed than when it is flat, but not compared with a boat that has a longer static LWl. The only advantage is in its racing rating or for better reserve buoyancy.
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Old 14-01-2021, 15:28   #8
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Re: LOA vs. LWL: A design question

What is the make of your old boat?
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Old 14-01-2021, 15:31   #9
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Re: LOA vs. LWL: A design question

Thanks to all of you for the informative replies. Very helpful.
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Old 14-01-2021, 15:51   #10
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Re: LOA vs. LWL: A design question

Shmorrell for a lot of designers I am sure the LWL is the last thing on their mind. That's what makes boat designing an art and so interesting. Much like admiring a woman, not everyone wants a tall, angular broad arse Sheila. Some of us enjoy a few curves within a shorter length and are happy to sacrifice a bone jarring ride for a bit more comfort.
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Old 14-01-2021, 16:07   #11
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Re: LOA vs. LWL: A design question

I quit sailboat racing some years ago, because the sailboat handicap system is a tangled morass of numbers, that can be manipulated in any number of ways.

Sailboats are typically slow.....so a boat with a 36' LWL will have a " theoretical" hull speed of about 8.04 knots...a boat with a 38' LWL will have a "theoretical" hull speed of about 8.26 knots, but it's clear that a tub with a 36'LWL will not have the same speed potential as a slicked out race boat with a 36' LWL.

A good sailor can make a tub go faster than a poor sailor in his racing sled.

A boat's handicap value is really only relative when racing, and it is not unusual to see two identical boats have different ratings...depending on age of sails and many other variables.

To the OP, forget about the rating system, LWL vs. LOA, etc..outside racing, it has little value.

Get the boat you like, and like the boat you get. It's that simple !!
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Old 14-01-2021, 16:08   #12
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Re: LOA vs. LWL: A design question

Racing rules used LWL of a non-heeling hull and if you could increase the LWL when you heeled, staying in the same class, then the boat could (hopefully) go faster and (hopefully) win.
This made me go check out the PHRF rating for my boat again. I really should race her, I'd clean up!
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Old 14-01-2021, 17:14   #13
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Re: LOA vs. LWL: A design question

I raced one time with a "pro" onboard. While I think of myself as a good sailor, this guy was something else. He adjusted something here, tweaked something there, etc. and the next thing I knew, we were miles and miles ahead. Won my first race with him onboard. After the race, there was a lot of talk about some shenanigans going on with my boat, as I had never won before....not only won on handicap, but first over the line, but there was no shenanigans, just an " expert" on the boat.

A handicap rating will only get you so far...I'd rather have a skilled sailor onboard than a good handicap rating.
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Old 14-01-2021, 17:57   #14
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Re: LOA vs. LWL: A design question

Actually yacht designs have evolved gradually. It is only the exceptional designer who deviated far from existing boats so changes, such as longer waterlines, shorter overhangs, and keel shapes; all came along slowly. We cannot attribute every change to a racing rule (though some certainly were). Racing rules followed the designs as much as designs followed the racing rules.

If you ask why the Morgan 38 had a shorter waterline compared to it's overall length than modern boats do, it's because that's what boats looked like back then. They were evolutions of their predecessors.

Designers shortened the overhangs and got longer waterlines over time because it resulted in swifter boats (and looked more modern). They didn't change all at once. One designer made his overhang a little shorter and the next did it some more.

Personally I'm kinda partial to the 1979 version.
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Old 14-01-2021, 18:14   #15
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Re: LOA vs. LWL: A design question

Quote:
Originally Posted by MicHughV View Post
I quit sailboat racing some years ago, because the sailboat handicap system is a tangled morass of numbers, that can be manipulated in any number of ways.

Sailboats are typically slow.....so a boat with a 36' LWL will have a " theoretical" hull speed of about 8.04 knots...a boat with a 38' LWL will have a "theoretical" hull speed of about 8.26 knots, but it's clear that a tub with a 36'LWL will not have the same speed potential as a slicked out race boat with a 36' LWL.

A good sailor can make a tub go faster than a poor sailor in his racing sled.

A boat's handicap value is really only relative when racing, and it is not unusual to see two identical boats have different ratings...depending on age of sails and many other variables.

To the OP, forget about the rating system, LWL vs. LOA, etc..outside racing, it has little value.

Get the boat you like, and like the boat you get. It's that simple !!
MicHughV, a lot of what you said is true, especially about the value of a good sailor on board.

But you are a bit harsh on handicapping systems. Many people are when they don't understand why some other guy's boat has a better rating.

All handicapping systems are sincere efforts by smart people to determine how fast a particular boat can go and then to give it a rating number which represents that speed potential. Most handicapping systems are pretty good.

Intentional manipulation is often attempted but rarely achieved.

Mostly if a skipper complains about the rating system it's when he can't sail his boat to its rating. That is probably not because of the complex rating system (although sometines it can be).

Even of you are not interested in racing, the boat's rating is valuable information because it tells you how fast it is, which is of interest to some people.
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