LOA vs. LWL: A design question

[adjo]

OP asks why design the stern to sit up out of the water/

A transom that is at water level when static will drag significantly in the stern wave when sailing at close to hull speed (except when surfing of course!).

[MicHughV]

I can speak from experience on this...my first boat had relatively long overhangs...both bow and stern...it was around 39' LOA on a LWL of about 29'. It didn't really matter if the bow or stern was slightly out of trim...I had to be surfing down a wave before a stern wave would approach the very stern of my boat...it was a nice sailing boat, but clearly not as fast as.... for instance:
My last boat, a Beneteau, has your typical flat planing hull. It has an LOA of around 43', but the LWL is a whopping 39'. The edge of the transom is a scant few inches above the waterline and it's quite wide and it sailed really well. Then came the davits, dinghy, solar panels, outboard engine, etc...it pushed the stern down, so it now touches the water at rest. I've seen other Beneteau's that have the stern 2-3-4" below the water. It still sails well, but clearly the stern is dragging in the water now, which is detrimental to speed.

It's hard to find a happy medium in a small boat. The designer/builder tries to squeeze as much useable space on the interior. Add the 1,000 lbs of junk, personal gear, food, etc, on a boat, and the original waterline disappears rather quickly.
So you know you have a longer LWL, but also more weight, more boat in the water, etc...

It's almost impossible to have it all on a small boat. Some compromises have to made.
You, as the buyer, need to decide.....speed or comfort....the LWL vs. LOA dilemma will resolve itself based on what you want out of the boat.
I've said it before, and I'll say it again, get what you like and like what you get.

[MartinR]

Basically, if we except racing rules changing the shape of the boat, there is a good reason. A short waterline keeps the wet area small, in light winds, when the boat sails upright. At slow speeds, skin friction is the most important factor determining speed.

As the wind increases, so does the heel of the boat, and the effectual waterline increases, giving more speed potential.

This is sound design, but works best for fairly narrow hulls.

This is very pronounced for example in the square meter classes, which heel a lot. Small wet area when upright, long waterline when heeled.

There are other advantages and disadvantages with overhangs, some of them mentioned by other posters.

[MicHughV]

Yes, you are correct. For a long time handicap ratings ruled boat design....you could have (a) but not (b)....ie, you can't expect to have speed on a long LWL, but fat hull...but in the same breath, don't expect load capacity on a narrow slender hull, with long overhangs. No question, a deep keel gives you better windward ability, but who wants an 8-9' draft. A compromise must be made, etc, etc, etc...

Probably the reason, why there are 1,000's of different model sailboats to pick from. Each model hones in on a particular trait or need..somewhere in this mix, is the boat for you, or in this case, the OP.

Though an interesting thread, LWL and LOA should not be the only dictums for picking a boat. I've found that displacement criteria gives a better impression of what a boat is all about, rather than LWL or LOA.

I think the LWL is a misleading number. It rarely represent a fully laden cruising boat.

[waterman46]

Originally Posted by CassidyNZ

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.

The slamming is mainly due to having flatish or just gently rounded hull bottom forward, not the actual amount of overhang at the bow. Boats with "modified" or full fin keels will slam less since the V shape at the bow is continued aft, in one form or another, all the way to the keel.

It happens that those designs with plumb bows also seem to have rather flat hull bottoms forward. Probably minimizes wetted area for less resistance and works best downwind, where most races go.

You could design a boat with totally plumb bow above the waterline and still have a good V running aft to the keel. It would slam less with the V, but the main problem with a plumb bow is that the boat becomes a submarine going to weather in a seaway.

[MartinR]

Modern cruising boats do not have planing hulls, even if they superficially look this way. There is too much volume in the hull for this. And too much weight. They surf down waves, just like any other boat, though, but are still limited by hull speed under normal conditions.

The wide low sterns are not designed to facilitate planing, but to create more room. And if you look at different production boats, you will also find that the underwater section of the stern is shaped very different. From rather narrow and deep to rather flat and wide. Above water they look more or less the same.

Not sure what is good or bad. Just different. Boat design is a compromise between many factors. Moderate overhangs are time proven and beautiful, which is by far the most important criteria for choosing a boat.

[Panope]

Not sure if it has already been mentioned, but another reason for some bow overhang is to keep the anchor from striking the hull during raising or lowering.

If an extended bow roller (prod) is fitted, then LOA will be increased, and we are right back where we started: a boat with "overhang" - even if it technically has a plumb bow.

Steve

[wingssail]

Originally Posted by waterman46

The slamming is mainly due to having flatish or just gently rounded hull bottom forward, not the actual amount of overhang at the bow. Boats with "modified" or full fin keels will slam less since the V shape at the bow is continued aft, in one form or another, all the way to the keel.

It happens that those designs with plumb bows also seem to have rather flat hull bottoms forward. Probably minimizes wetted area for less resistance and works best downwind, where most races go.

You could design a boat with totally plumb bow above the waterline and still have a good V running aft to the keel. It would slam less with the V, but the main problem with a plumb bow is that the boat becomes a submarine going to weather in a seaway.

In my experience "slamming" is not due to the flat sections of the bottom forward it is the slab sides. While sailing upwind the boat is heeled over and the flat sides of the boat are what is striking the water and causing the slamming. When boats delaminate due to slamming it is the sides which have the problem not the bottom. V bottom sections do little to prevent this.

When a boat slams while motoring into the waves it will be the bottom and a ddep v shape helps aleviate that.

[MicHughV]

My first boat had a steel hull with hard chines.....it doesn't get much flatter than that, but it had long overhangs. I don't recall that boat ever slamming into waves. On a heel, the side of the boat presented a V-shape, ie, the upper and lower plates meeting at the first chine.
The Beneteau has your wide flat stern, but to be effective, speedwise, it needs to be out of the water.

I never thought I'd see the day when I would like or agree to the wide "open" transom look favored by Beneteau, but I'm a convert.

My only beef with flat bottomed boats ala Beneteau, is that there is no bilges per se. The cabin sole is about 9-12" above the hull. The bilge, such as that is is, is a shoebox size indentation in the top of the keel. When heeled over, any water inside the boat runs to the low side of the boat, exactly where you don't want it...you'll get several 100 gallons of water inside, before the bilge switch will activate, but won't pump much, as the bilge hose is inside the "shoebox"..and the water is slopping around on the low side.

Gimme a nice old fashioned full keel Hans Christian any day for seaworthiness.

[wingssail]

Originally Posted by MartinR

Modern cruising boats do not have planing hulls, even if they superficially look this way. There is too much volume in the hull for this. And too much weight. They surf down waves, just like any other boat, though, but are still limited by hull speed under normal conditions.

The wide low sterns are not designed to facilitate planing, but to create more room. And if you look at different production boats, you will also find that the underwater section of the stern is shaped very different. From rather narrow and deep to rather flat and wide. Above water they look more or less the same.

Not sure what is good or bad. Just different. Boat design is a compromise between many factors. Moderate overhangs are time proven and beautiful, which is by far the most important criteria for choosing a boat.

On the biggest racing boats the wide low sterns ARE designed to facilitate planing, there is no need for room on "Commanche" for example.

Beautiful is in the eye of the beholder and moderate overhangs are only "time proven" because they have been around for a while. In 20 years today's designs will be "time proven".

Photos:
In the first photo below the boat with the black sails, same length as the boat with the white sails, has 5 feet more LWL and minimum overhangs (and wide flat sterns) rarely planes, but is faster on nearly all points of sail.

2nd photo, I like the looks of these boats

[wingssail]

Originally Posted by MicHughV

...
My only beef with flat bottomed boats ala Beneteau, is that there is no bilges per se. The cabin sole is about 9-12" above the hull. The bilge, such as that is is, is a shoebox size indentation in the top of the keel. When heeled over, any water inside the boat runs to the low side of the boat, exactly where you don't want it...you'll get several 100 gallons of water inside, before the bilge switch will activate, but won't pump much, as the bilge hose is inside the "shoebox"..and the water is slopping around on the low side...

Yes, this is a problem.

Our boat has no "bilge" (even though the hull is not flatbottomed). The hull is only 6" below the cabin sole. 2.5 gallons of water seems like a flood and slops over the sole when heeled. The bilge pump can only pump water when the boat is perfectly upright, and then it only takes it down to about 1/2".

Creative solution: A Whale Gulper bilge pump connected to a long hose. You stick the hose wherever the water has collected and suck it dry. Works all the way to the bow and stern. Our cabin sole has openings which come up to reveal the various places water can collect.

[jtsailjt]

I understand modern, wide, sterns with little to no overhang but I don't understand plumb bows at all. If you look at the boats in the photos just above, both have very long anchor rollers to effectively lengthen them enough so the anchor doesn't bang against the bow when being retrieved and I think they look quite ungainly. So, what I don't understand is, why not take the same boat and also add a little flare to the bow so it's a bit wider at deck level (more working space on deck) and also extend the length of the bow up at deck level a couple of feet. so everything could remain exactly the same from about a foot above the waterline down and sailing performance would be unaffected except in big waves you'd add a small amount of reserve buoyancy near deck level, but you'd have more room at the bow on deck and the anchor roller wouldn't have to be cantilevered so far out in front of the boat. I can't see any disadvantage to this and think it would look a lot better along with the practical aspects I mentioned. So, except for stylistic reasons, why have plumb bows become popular on boats that aren't pure racers?

[MicHughV]

I've been boating now for almost 40 years...I've seen a lot of different boats...
My dream boat would be to take a little from this boat, a little from that...a piece from this one, a piece from that one...and put it all together somehow....and still make it look nice...that would be the perfect boat for me, but likely, nobody else..off course, a custom built boat would be cost prohibitive...which nixes that idea..

[Don C L]

I think if you're the one forced to sleep in the bow, your idea of a perfect hull design will be VERY different from someone who gets the quarter berth all the time!

[MartinR]

Originally Posted by wingssail

On the biggest racing boats the wide low sterns ARE designed to facilitate planing, there is no need for room on "Commanche" for example.

Yes, but I think we are talking cruising boats here. Modern racing boats are indeed made for planing.