Jump to content

waterline length and hull speed


JeffM

Recommended Posts

Just curious: I'm familiar with the idea that a longer waterline gives you a higher displacement hull speed because it allows the hull to span the more widely-spaced waves generated by a faster-moving hull. But it seems to me that relating waterline length to hull speed this way is an oversimplification: doesn't the effectiveness of the waterline length depend on the bouyancy of the ends of the hull? For example, could a hull with a very narrow bow with little floatation really go as fast as a hull with a more bouyant bow? To take a more extreme example, the Bolger Micro has a waterline length of (I think) 15 feet, but its watertight envelope is only about 9 feet long since bow and stern are just free-flooding anchor and motor wells. Anybody know much about this?

Link to comment
Share on other sites


Yes, the waterline-to-speed ratio is an oversimplification. There are various things that will cheat the rule like changing the shape of the hull or the length to beam ratio. A narrow boat will be able to exceed the hull speed given by that rule-of-thumb.

That's about as far as my expertise goes on this one...

Link to comment
Share on other sites

Each hull type falls into one of just a few categories, each requiring an application specific speed/length multiplier. 1.35 times the sq. root of the LWL will provide you an approximate displacement speed on low power hull shapes. Increases in power, changes in shape and configuration, may add or detract from the multiplier. This particular qualifier will be typical of most non-planning modern sailboats. Christopher Columbus sailed his fleet, in ships with much less satisfying speed/length ability.

Link to comment
Share on other sites

I agree that the waterline/speed formula is a generalization for common hull types. I'm not familiar with the design that you describe but it sounds like a risky design except for very specific conditions. And generalized formulae may not apply.

The loss of bouyancy in the ends would mean that there is much greater immersion and therefore wetted surface drag. It also means that there would be little resistance to hobby-horsing in a seaway. Doesn't sound like a desireable design except maybe in flat calm conditions for a specific purpose.

I've never been a fan of Bolger's designs simply because he seems to throw out conventional wisdom without giving it much thought. It is good to challenge conventional wisdom but wise to maintain a healthy respect for it at the same time.

Link to comment
Share on other sites

Fine ends in a boat can be faster, but also could have little to do with it's speed potential. A narrow entry can't help an other wise draggy hull much. A crafts' ends being fine can be of assistance (sometimes), if it works in harmony with the other elements of the design, that have a bearing on the speed/length qualifier we use for some hull shapes, in that formula.

Bolger's engineering and design skills, rank him certainly as one of the best, in the last half of the 20th century. When you analyze one of his designs, be that a box dinghy, large fishing vessel or full rigged 1800's era ship, you find he has found the route to get the desired task completed, often quite cleverly, using a minimum of pieces or contrivance. His "box" boats prove, what has long been known in sharpie and other small sailing boats, that the more upright the sides (less flare or flam), the faster she'll go. The waterlines stay reasonable and balanced, not distorted and bulbous as they do when a heavy flared topside, starts leaning against the wave train. This "trick" which makes some children only a mother could love, also permits the lee surge bow wave to be considerably smaller, hence less pressure to round and less to drag along the side of the leeward hull. At first look his box shapes don't look to particularly efficient. On closer inspection you'll find his waterlines and runs are sweet and cheating the rules a bit, maybe with very light construction, or length/width dramatics, effective appendages and their locations, etc. Most of his small boat designs incorporate several features to make it sail faster then it looks. The dent in the wake behind a boat can be included in certain hull configurations, when factoring speed potential.

Pitching motions (hobby horsing) are usually more a mass centers environment, then end shape issues, unless there is much overhang (over 15% LWL), which by it's nature would draw out the ends a touch.

You can easy beat the sq. root LWL rule with a few design devices, but you gott'a know what they are and where to put them.

Link to comment
Share on other sites

Thanks, Paul. That's easily the nicest thing said about Bolger in my hearing. I don't know much more about boat design than I learned from Graham and from Ted Brewer, but I find many of Bolger's "Boats with an Open Mind" to be refreshingly weird. With all the designs he has in the water, I figure he has to be pretty good. And the world would be a less interesting place if all boats looked like, well, boats!

Link to comment
Share on other sites

Personally, Bolger's box boats are about the ugliest things afloat, if you ask me, but they all seem to perform better then most in their class. This is frankly, brilliant design work, though a little shape to the topsides couldn't hurt and would make these boats much more appealing. Bolger has taken "keeping it simple" to a high level in most of these boats and they show clever and innovative ways to use common methods, materials, techniques, within the ability of the average person to complete the project. This is the whole point isn't it? You can have lots of fancy designs that can't be built or only at well equipped yards, but it takes quite a lot of skill to engineer down to the most simplified form and still meet the structural requirements of the equation.

Link to comment
Share on other sites

I second that. Bolger has designed some of the ugliest boats afloat. I suppose that I can understand that Bolger is an invotative and expert engineer, I cannot understand why anone would but a great deal of effort into building an ugly boat. I can see the opposite quit easilly. A good looking boat will sell even if it is not that good a sailboat

Link to comment
Share on other sites

They sold plenty of Hugo's before the company went belly up and they were butt ugly too. The key is cheap to build, extremely simple to construct and operate, plus they out perform expectations and others in their class. If I built one, I'd have to add some flair to the topside planks and rake in the ends, for no better reason then to pretty 'er up a bit. These modifications would probably cost some performance potential though.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.