Low Length/Displacement Hulls

Sharp and stretched lines simply mean that the water only has to go a rather short way to get around the hull and is pushed aside slower than with full lines at same speed, less energy needed for acceleration of the water as the hull slips through. That´s why you get rather little waves, so less wave resistance. (Pretty rough simplification)

 

Ok, I’ll try and explain this simply and in simple to understand terms. This is not meant to be patronising, but there are many issues behind everything I describe which requires an understanding of the science/mechanics behind it, hence the simplification.

Any ‘body’ (a hull in this case), that moves through the water will create a wave. It does so because of the pressure field around the body. The energy of the wave comes directly from the body itself. If you also consider a body that is submerged that is moving, this experiences drag, owing to the waves it also creates on the surface. But the deeper and deeper the body goes, the less waves it produces and thus reduces drag.

From this we derive an equation, this defines how a gravity wave (as it is called) moves in deep water. The speed at which the wave moves is given by

V^2 = (g.L)/2.pi

V = velocity and L = length.

We can non-dimensionalise this expression and we get what is commonly called the Froude number, Fn.

Fn = V/sq.rt(g.L)

Thus the Fn is directly proportional to the length of the body, or in this case a hull. So, the wave making resistance, or another way of saying it, the waves created by the hull (since without the hull there is no wave) is directly related to the speed and its length.

So, simplistically, for a given speed, to reduce the wave making resistance, you increase the length. This reduces the Fn (Froude number). The lower the Fn the lower the wave making resistance.

In simple numbers:

If you have a boat that is 10m long and travelling at 10 knots what is the Fn..?

Fn = (10x0.5148)/sqrt(9.81x10) = 0.52

If the boat is now 20m long but still travelling at 10 knots, what is the Fn..?

Fn = (10x0.5148)/sqrt(9.81x20) = 0.37

If the boat is now 30m long but still travelling at 10 knots, what is the Fn..?

Fn = (10x0.5148)/sqrt(9.81x30) = 0.30

So, you can see that increasing the length reduces the Fn.

Again, simplistically, the lower the Fn, the lower the wave making resistance.

Conclusion, long thin hulls are better than short fat ones.

So why long and thin??...well if we keep the displacement the same, the only variables are the length and beam (draft is minor in this context). So if you make the hull longer, at same displacement the beam is less.

This takes us into another field and Q & As which is beyond the original question.

Trust this helps.

 

Apologies for taking the discussion back a step, but I'd never heard this before... And I have to say that I was rather surprised... she certainly doesn't look like a conventional motoryacht under the water..

 

Maltese Falcon built on foreclosed hull?

I just remembered hearing that somewhere, but can't find anything to back it up now.

There are two MF boats now, the one owned by the computer guy and sold, and the new one by same boat yard.

I'd be interested in 'being built' photos or even the under the water line plans of either.

Seems info on under the waterline seems harder to come by than other info on these high end boats.

Any photos or "study plans" of these boats full hulls?


Quote:
Originally Posted by Squidly-Diddly
The "Maltese Falcon" motor sailor was built on a pure power hull that was foreclosed on by the boat yard, and it is supposed to sail pretty good.

Apologies for taking the discussion back a step, but I'd never heard this before... And I have to say that I was rather surprised... she certainly doesn't look like a conventional motoryacht under the water..

 

Well.. you might just get hold of the lines plan in about a hundred years if you're lucky and if, in time, it is considered to be a classic.. &/or if the designers chooses to publish a book of his work like so many of the well known's have..S&S, Uffa Fox, Levi etc .
But you can understand why, having spent a lot of time, effort and money on developing the shapes for these boats, that owners and designers are reluctant to share them for free....
Try a google images serach... you may turn up an in-build shot or two...

 

Bernd1972,

Back in 2005 you introduced MY "Marlin" at http://www.boote-forum.de/showthread.php?t=18229.

I found that long thread about a month ago via http://www.craftacraft.com/great_wooden_home_build and was captivated with the lines of that beautiful boat.

The German text hardly mattered, because the images spoke for themselves, but perhaps you would post some of the images here for the benefit of us all, as the underwater lines of that hull epitomise the subject of this thread and also, would you translate some of your comments please.

Thanks,

Perry

 

Adhoc thank you for taking the time to write such a detailed reply thats a great help.

I understand the Froude system. So the aim is for the operating speed to be operating within a wave system that is a single wave pattern? Without exceeding hull speed?

Because of the nature of a slender hull form/bow it is possible to exceed hull speed more efficiently than a broader hull would?

What wave pattern is created when operating above hull speed, as many of the LDL hulls do operate, some of the designs are operating at a SL ratio of 3.1?


Chris

 

Hi Perry, I´m surprised you found that. I don´t mind posting some pics an details here. Just allow me some time to chose them, theyre going to follw within the yext days. On page 12 of tht presentation on that german forum I did a little description about the history of commuter launches, fast displacemt/forced displacement boats. I´ll post a trnslation here within the next days, promised.

Concerning the gossip that "Maltese Falcon" was build from a motoryacht hull, well Mr. Squidly Diddly, forget that.

 

Chris

The aim is to have a low Fn as possible….and/or low wave making resistance. High speed displacement hulls do this, like the Series 64.

I never use the term “hull speed”. This is a somewhat misnomer and coined in the days before modern engines and hull forms really. It just basically means a “traditional hull form” had a theoretical maximum speed it could achieve, owing to the hydrodynamic effects that occurred with increasing speed around a hull shape with a low L/B ratio and full lines.

There are endless examples of displacement hulls, with high L/B ratio, ie long and slender that easily exceeds what you are referring to as “hull speed”. More correctly the main prismatic hump which occurs at a Fn around 0.5. The prismatic hump occurs when the hull length and wave generated by the hull are of equal length.

The long slender hull forms, exhibited very little trim and have a high L/D ratio (length displacement ratio)…as such they “push” through the water rather than ride ‘on top’, when the Fn is around 0.8-1.2. The hull is not trying to plan at all, quite the opposite. (Again, detailed explanation goes into a lot of hydrodynamics and fluid mechanics).

Here is one example:

http://www.youtube.com/watch?v=zCYF0gU7N-k&feature=related

and another:

http://www.youtube.com/watch?v=DF4Ih0u6yQ4&feature=related

As for wave pattern, see here:

http://www.boatdesign.net/forums/boat-design/wave-pattern-question-28667.html#post290938

Trust this helps.

 

Thanks Bernd1972,

Raised foredeck boats are so much better looking than trunk cabin boats IMO. The Sterling Atlantic 43 is a modern interpretation.

http://www.theyachtmarket.com/boatI.../?searchid=2895505&page=1&preview=&logview=no

Then there are Lake Union Dreamboats.

http://pacificmotorboat.com/dreamboats2

Stan Laurel owned a boat designed by Willard Van Brunt.

http://www.idamay.org/IdaMayintheMedia.html

Classic British motor cruisers.

http://www.adls.org.uk/t1/content/reda-now-janthea-0

http://en.wikipedia.org/wiki/File:Sunday_4_April,_Ramsgate,_Dunkirk_Little_ship_Sundowner.JPG

http://www.adls.org.uk/t1/content/bluebird-chelsea-previously-blue-bird

http://www.adls.org.uk/t1/boats

Sam Devlin built Czarinna with a large berth and storage area in the fo'c'sle, accessed through a hinged hatch. Later, he modified her by raising the foredeck.

http://www.devlinboat.com/gallery/main.php?g2_itemId=2378

Regards,

Perry

 

The real advantage of long hulls is at lower speeds the advantages stay.

Not that many owners delight in 50-200GPH fuel bills , yet the ones that are willing to slow a bit (but still faster than the 3-1 fat boats) can enjoy the savings.

FF

 

This should be in the first paragraph of any text on hull design. The term "hull speed" has prevented more understanding of hull dynamics than any other factor.

 

I agree that the term "hull speed" is misleading. Basically it just discribes the situation of a full displacement boat typically with some form of round stern or lines that in another way follow the ideas of old Mr Archer. That boat is trapped between bow wave and stern wave. Known thing.
And you´re right the we know today that there are in fact several boats that get over these limits by different ways.
All faster hulls have one thing in common: they are not prevented from getting faster by a huge wave that builds up at the stern. The limiting problem of traditional full displacement hull does not really apply for them.
One trick with the lightweight narrow forced-displacement boats is that they run away from the waves the hull produces because the lines are done in a way that the water pushed aside is mainly accelerated in one main direction (more or less away from the hull, perhaps with some changes), it can flow back when the boat is somewhere else.
Full bodied displacement boats accelerate the water away from the hull first then they accelerate it in the other direction. Therefor we find this speed-limiting wave system with these.

 

I think that most with even a passing knowledge of hydrodynamics would recognise that the term "displacement hull speed" is a bit of a misnomer... just as semi-displacement, planing and a host of other terms that have been coined to loosely define the various states and aspects of boat behaviour. And I agree with the sentiments expressed, but the obvious question is how else are you going to describe these things to the jo-blo boat-buying public who really have no interest in understanding anything other than how much is their boat going to cost, how much fuel will it use, and how fast will it go...?
Sure, there are ratio's and formulae... but I doubt any would expect our friend Jo to carry even the most basic of them around in his/her head.

 

I suppose it is ok to feed the total layman buyer with some terms that may make it easier for him to think he knows what is going on. Trouble is, if he ever wants to know the real physics behind his boat and why it does what it does, it is first necessary to get rid of some of the gospel that he thought he knew. While many who have been exposed to some hydrodynamics theory may understand the limitations of "hull speed" as it predicts boat performance, it still finds its way into this forum on boat design with regularity. Almost none of the books that discuss hull speed offer any discussion on its limitation to a certain type of hull form.