Length and Beam

[Roger Long]

While I’m sitting here drinking tea and letting the sweat from the long brisk walk I just took dry, let me throw out some thoughts on proportions prompted by the long thread on re-designing an Atkin double ender.

Because, all else being equal, increasing beam increases stability there is a natural inclination to overemphasize length to beam ratio. Try this thought experiment:

Imagine two little identical rectangular sailing punts. I think Atkin designed a boat like this. They heel a certain amount in a particular breeze and have a certain sail area to wetted surface ratio. Sail them side by side and they perform identically. If you are concerned about comfort, they each have a particular natural rolling period. Now sail one up behind the other, pull in the steering oar of the boat ahead and clamp their bow and stern transoms together. Now, you have a little schooner.

What has changed? The angle of heel? No. The sail area to wetted surface ratio? No. The rolling period? No. The length to beam ratio has been doubled but none of these critical factors has changed. Stability is primarily determined by factors better judged by looking at a boat from the bow or stern than from above.

What has changed is the waterline length. If the bow and sterns of our punts don’t have too much shape, the hull speed of our new schooner has been increased nearly 50%. The turning radius and the marina bills have also increased but those are different issues.

These principles apply to craft of complex shape more than is realized. The problems is that, when people have length to work with, they tend to fill it up and proportion freeboard, deckhouses, and sail plans to fit the length.

My father never got to build the boat I designed for him because his wife thought it was too long and skinny and would be dangerous. If I’d designed it 12 feet long, she would have thought it short and fat. At 18 feet with the same sections, it was much more stable and able to handle rough water but it looked tippy.

[Ian McColgin]

Outside the world of kayaks, a lot of folk don't get how more length for a given beam is more stability. Of course, most designers know this but, while the most important single measure of a boat's capacity is tonnage, a catboat of a mere 25' but 6T is by most measures less costly around marinas and boatyards and such than a 6 ton 35' sloop. They will both have about the same cruising stores capacity but the sloop will likely be more seaworthy and certainly faster.

[Sayla]

What has changed? The angle of heel? No. The sail area to wetted surface ratio? No. The rolling period? No. The length to beam ratio has been doubled but none of these critical factors has changed.


Perhaps the exposed profile has doubled in length, and you can receive twice the length of wave impact side on.

Sayla

[Eddiebou]

Roger, would you care to make a prediction/ evaluation of a little boat I'm building? Not my design, but a copy of an old sailing skiff.

[Roger Long]

Roger, would you care to make a prediction/ evaluation of a little boat I'm building?

Post pictures. There is a lot of knowledge and experience here besides mine.

[James McMullen]

Long and skinny is also prettier.

It's a damn shame that marina fees and haul out rates and such are based on length rather than tonnage. This has certainly got to bear some of the blame for those absurd and dreadfully fuel inefficient, fat-assed, four-story-tall power yachts infesting the marinas these days.

Look at that thing snowplowing along! Ack!

http://www.1stflash.com/files/Carver-406-Yacht-Starboard-Side.jpg

[Peerie Maa]

Long and skinny is also prettier.

It's a damn shame that marina fees and haul out rates and such are based on length rather than tonnage. This has certainly got to bear some of the blame for those absurd and dreadfully fuel inefficient, fat-assed, four-story-tall power yachts infesting the marinas these days.

Look at that thing snowplowing along! Ack!

http://www.1stflash.com/files/Carver-406-Yacht-Starboard-Side.jpg

That boat is in the wrong thread.

[James McMullen]

I think that boat is in the wrong dimension. If it were up to me, it would be cluelessly plowing through the No Wake Zones on the River Acheron, not up here amongst the innocent.

[Thorne]

The most extreme example of financial and/or legislative length limitations (say that fast five times!) that I've seen are the commercial fishing boats shown in some threads here. Nearly as deep as they are long!

Baaack to the topic. For small trailer-sailed boats the marina fees are not a limiting factor, length coming into play more for a few legislative requirements (some States require registration for all boats over 16', for example).

But materials costs and storage are more of an issue. Many older garages are tight for a 14' boat on a trailer, ditto for a lot of commercial storage sheds and other inexpensive structures (both temporary and permanent).

Other than canoes I haven't handled many small open boats over 16', so am blissfully unaware of issues with launching, turning, and rowing longer boats. But I'd sure like a chance to try! The longer boats in the local TSCA sure look fast and easy to row and sail, and several here even have schooner rigs with lugsails.

[Canoeyawl]

Speed - Another factor less talked about is the efficiency of the foils (rudder, centerboard, keel, hull, and etc) and how much better they work when moving faster.
(An amateurs analogy - Think about waterskiing... now think about waterskiing very slowly - you can't do it. With speed the water effectively gets "harder" and offers much more for the foils to work with. Any increase in speed is an increase in windward ability, often with dramatic results. Sometimes it takes only one or two knots faster and the boat may feel like it's on rails).
It was pointed out to me many years ago that windward ability is safety. Most tragedies at sea happen on the beach.

[James McMullen]

It's too bad you don't have a bigger garage, Thorne. These days I consider 18-19' to be ideal for sail & oar, with 16' being the barest minimum. Launching is actually easier, rowing is as fast or faster, sailing is much faster, seaworthiness and seakindliness are dramatically improved and quick turning is essentially a non-issue. The only place where they fall short is for car-topping and I guess storage in a garage. But except for the specialized purpose of a ship-to-shore dinghy, I would never go back. Never.

[Woxbox]

Roger -- To add one point to your thought experiment. If you stand on the rail of the single punt, your weight may push it under. But with the two boats lashed together, quite probably not. So while critical measures of stability don't change with the longer boat, it will still feel and be more secure as the crew moves about, right? (And yes, if two crew stand on the rail of the doubled boat, it's the same as one person in one boat.)

[Roger Long]

right?

You got it.

[slidercat]

I just have to point out that multihull designers take the fullest advantage of the phenomenon that Roger has pointed out. If you take his two punt schooner, split it in half lengthwise, and move the halves apart, you get even skinnier faster hulls, but also a lot more stability and sail-carrying capacity.

It's a very interesting way to look at it, isn't it?

[slidercat]

I didn't mean to kill the thread.

As one of the few, the proud, the lonely... multihull advocates on WB Forum, I feel compelled to occasionally be that voice crying in the wilderness.

[peterchech]

Length at waterline does add significantly to stability. But it seems that most sailboats are made as beamy as possible to maximize sail carrying power, rather than trying to keep the boats narrow and longer to get sail carrying power that way. At least in monohulls, I wonder if this is because a narrower boat is more difficult to plane, and planing gains more speed than an increase in length and therefore hull speed?

I think it interesting that most builders would rather increase beam rather than length, even on displacement hulls. True you pay by length at the marina, but for the small trailer-sailors and especially the cartoppers, added length seems like a much more seaworthy and comfortable feature than added beam. I think it may be because we think of a boat's size in terms of length instead of displacement.

[peterchech]

Since you're on here Ray, have you ever considered lengthening the hulls on your slider, say to 18'?

[Roger Long]

Length at waterline does add significantly to stability.

Not exactly. From some bleed over into the bowsprit thread:

However, it also increases wetted surface which means you need more sail area for the same speed in light air where length isn't a speed factor. The two punt example in that thread works because each half boat keeps the same sail plan. If you re-rigged it as a single mast craft, the mast would need to be taller exerting more leverage which means more heel in the same wind. Making a boat longer only improves stability if you can keep the rig the same weight and height, use the extra displacement to increase ballast, and use the extra length to fill out the ends to the same entrance and exit angles in the waterline. I was just trying to point out that making a boat longer does not make it "narrower" and less stable in the same way that decreasing beam does. Small changes in beam have very large effects on stability.

Bowsprits and boomkins or booms that overhang the stern significantly are ways to keep the large area necessary for light air performance spread out low for less heeling moment. Today, vessels motor in very light air. Historically, the kept ghosting along with large sailplans. In craft too large to sweep, light air ghosting ability could be a survival issue in places where deep water goes right up to hazards. Poorer rigging materials and less efficient inside ballast were also factors favoring low and spread out sail plans.

[peterchech]

However, it also increases wetted surface which means you need more sail area for the same speed in light air where length isn't a speed factor.

Interesting, but doesn't widening beam increase wetted area as well (generally)?

[ahp]

Other things being equal, stability or righting moment, increases as the first power of length and the third power of width. Take a given design and scale it up in all directions and the stability increases as the forth power of the scaling factor. Scale up the sail plan and the overturning moment increases as the third power. Bigger boats are faster.

[Mad Scientist]

Dave Gerr had an article about efficient powerboats in WB #206. It was interesting to re-read it after following this thread.

When I read that article for the first time, I thought that a catamaran might be the easiest way to maximize efficiency. Glad to see that cats were mentioned here.

One of the many things I learned from that article was the idea that a narrow boat could be 'stiff', assuming that the designer made the reqired stability calculations early in the design process.

Tom

[slidercat]

Since you're on here Ray, have you ever considered lengthening the hulls on your slider, say to 18'?

Peter, I haven't. That's primarily because limited to a fixed beam of 8.5 feet, sail-carrying capacity, resistance to sideways capsize would not increase much, but weight and wetted surface would, so I think the boat would be either slower (with the same size rig), or more dangerous (with a larger rig.) One of the ways that I feel ballasted monohulls are superior to most catamarans is that the former can be sailed with a little less constant attention. With a boat that can come back from a knockdown, you can give more attention to things other than sailing, while sailing-- scenery, cooking, that sort of thing. Anyway, that's why Slider has a very modest rig.

That said, I'm about to start construction on a new cat. This one will be 20' by 12', but will fold for trailering and will have small cabins. The rig is from a salvaged Nacra 5.2, which will give the new boat 220 sq. ft of sail, compared to Slider's 140. I expect the new boat to be a lot faster than Slider, but, I hope, no scarier. The model I've done of the folding mechanism looks a little scary-- Slider's masthead is about 18 feet above the water, and the new boat's is almost 32 feet. But the numbers look okay to me.


Interesting, but doesn't widening beam increase wetted area as well (generally)?

Yes, but as a function of displacement, it decreases. The shape with the least wetted surface for its displacement is a hemisphere. This comparison was brought home to me last summer at the Florida 120. In the heavy air of the second day, to windward, I went like a rocket past Gary Blankenship's Frolic 2, a light displacement monohull. On the last day, in very light air, every time the wind went especially light, he gained on me.

[Allison]

Not exactly. From some bleed over into the bowsprit thread:

However, it also increases wetted surface which means you need more sail area for the same speed in light air where length isn't a speed factor. The two punt example in that thread works because each half boat keeps the same sail plan. If you re-rigged it as a single mast craft, the mast would need to be taller exerting more leverage which means more heel in the same wind. Making a boat longer only improves stability if you can keep the rig the same weight and height, use the extra displacement to increase ballast, and use the extra length to fill out the ends to the same entrance and exit angles in the waterline. I was just trying to point out that making a boat longer does not make it "narrower" and less stable in the same way that decreasing beam does. Small changes in beam have very large effects on stability.

Bowsprits and boomkins or booms that overhang the stern significantly are ways to keep the large area necessary for light air performance spread out low for less heeling moment. Today, vessels motor in very light air. Historically, the kept ghosting along with large sailplans. In craft too large to sweep, light air ghosting ability could be a survival issue in places where deep water goes right up to hazards. Poorer rigging materials and less efficient inside ballast were also factors favoring low and spread out sail plans.

An argt. in support of schooners!!
Now there can be no arguing with that!:D:D