Low Length/Displacement Hulls

[ChrissyT]

I've been researching this hull form for a project, and am interested to know more about them.

They are claimed to be efficient hulls by reducing wave making drag, and they are running well beyond their displacement speed, which normally would be incredibly inefficient?

Also, (pardon my ignorance) I understand travelling past displacement speed does not mean it will plane, but are such vessels operating in displacement mode above their displacement speed, albeit efficiently?

So How do they reduce wave making drag?

Would it not be more efficient just to develop a semi-displacment hull or am i missing the point?

Are the hull forms designed to provide as little lift as possible to avoid trying to plane?

There are a few examples here; http://www.nigelirens.com/ldl/

Cheers

Chris

[bernd1972]

That´s the concept behind the commuter yachts of the 1920 era. Lightweight and a long stretched hull with soft and harmonic waterlines and a center of displacement aft of 55-60% of the waterline length leaving the stern wave well behind the vessel. One trick about it is that the Froude formula concerning hull speed becomes less relevant for a L/B ration higher that 6 and is no longer the limiting part in the wave resistance.

[Ad Hoc]

Chris

Look at the Series 64 hulls. These are good example. Also look at the stuff done by Prof Molland et al at Soton Uni.

The higher the L/D ratio the better the hull form. Some other pointers are here:
http://www.boatdesign.net/forums/boat-design/wave-piercers-marketing-myth-design-ingenuity-30296-2.html#post318800
and
http://www.boatdesign.net/forums/boat-design/dare-say-no-30261-9.html#post318285

The reduced residuary resistance is simply by being "long", it is as simple as that. To explain further requires more understanding of hydrodynamics which may yield more questions than answers, depending upon your understanding on the subject.

[bernd1972]

It´s quite a nice concept, however the space insider is quite limited for the given length and they tend to roll a "bit more" in a beam sea.

[Squidly-Diddly]

There is a US Navy tri-"steath" ship that uses this principal and makes almost no bow wave or much wake at high speed.

http://toolmonger.com/2010/01/15/its-just-cool-littoral-combat-ship/


I'm planning on designing a motor-sailor based on S-Boat hull.


The WW2 German S-Boat(E-boat) was itself developed from an advanced power yacht.

Seems to be where the "50 knot barrier", and minimum hull length for nearing such speeds intersect.

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.

I'm planning on traditional 3 masted junk rig, with lee boards, and as much traditional junk styling as practical.

Instead of the S-Boat's 3 props, I'm drawing twin props(not making a living dodging bullets) mostly to give efficient power with shallow draft. Shallow draft and the fact most motor sailors do more motoring is also reason for lee boards.

I'm thinking of twin 12-71 Detroits, but with a single 6-71 able to drive both props via belts(or chains?) so the 12-71s wouldn't be bogging and could be shut off when less than max power is called for.

http://www.prinzeugen.com/Plans.htm


Note how the engines are far foreword on the S-Boat and Nigel boat. They were even farther forward on the yacht(without a couple of torpedoes and heavy gun up front).
http://www.collectrussia.com/sBoot/earlycrosssection.jpg

S-Boat not a “wave pierce” but said to have good rough water sea keeping as well as decent fuel economy.

[Pericles]

Don't forget to factor in the Lürssen Effect, even though you are considering only 2 props.

http://en.wikipedia.org/wiki/Oheka_II#cite_note-7

[DCockey]

Do you mean "Low Displacement / Length" or "High Length / Displacement"?

"Low Length/Displacement" is short and heavy.

[bernd1972]

The S-boats were developed by Otto Lürssen based on his experienced with commuter yachts for US customers, some people refer to Oheka II as the predecessor of the S-boat design.
Ah yes, the Lürssen effekt story, the aft edges of the rudders pointing somewhat outside to keep the water abaft flat and to keep the bow low. The S-Boats had an adjustable angel between the outer rudders that went to 15°. I´d stay with a fixed angle of 3-5° if you want to play with that.
Not worth the hassle to build it adjustable.
The key is a good hull. I built a commuter style motorboat of 65ft. following that forced-displacement concept with a L/B ratio (at WL) of 5.5 and a displacement of 18t. Could have been slightly narrower but I wanted some decent accomodations. :D
However, the result was a boat capable of 18kts from only 330 horses. I guess that´s not too bad.
As I said, it tends to roll a little in a bem sea. It´s strictly a boat for harbour hopping/coastal cruising.

Nevertheless, with the 12-71 engines you would have some more weight (and power for that matter) anyway so it might be considerbly more steady depending on size.
Forget the additional 6-71 and think of T-gearboxes connecting the shafts so you could go with only one engine instead. That would be buildable. The belt drive story seems very questionable to me.
Next thing is: Why on earth do you want to have sails on that? The stability would not allow for a rigg powerful enough to seriously sail that boat or you would end up with a hull that´s not really suiteable for that fast forced-displacement story.

[Squidly-Diddly]

It seems to be one of those things quoted in every blurb about E-boats, but only that. I see the two smaller rudders here, http://www.collectrussia.com/sBoot/s38planbig.jpg



I get the feeling this Lürssen Effect isn't something that can be just 'plugged into' any design without some real calculating and testing.

Is it an actual AIR pocket from the atmosphere where air is being sucked from the surface or is it cavitation and water vapor?

Any info on exactly what is going on and how fast was the boat going for this to take effect?

[bernd1972]

The Lürsen-effekt spreads the thrust from the props and causes some cavitation which kept the boats more even and therefor reduced drag. It showed efficient above 30kts, icreasing the topspeed from 38 to 43kts with same hull, displacement and engine configuration. Don´t bother with that adjustble rudder stuff before the hull proves it could give an advantage.

[Squidly-Diddly]

but I believe the Maltese Falcon has a ballasted keel and does have a centerboard.

I was going to add a full length shallow keel(for battery bank) but mostly rely on aggressive ballasting in side tanks of FreshWater, Fuel and SeaWater. I'd bulge the S-Boat hull slightly on the sides mostly above the water line(hopefully keeping the hull's higher speed running surface the same) to accommodate this.

I'm also thinking of weighting the tips of the lee-boards, and having them able to not just pivot aft, but also pivot out away from the hull, thus one would act as lee-board, and the other as counter-balance(and nice over-the-water platform and boarding plank when not sailing).

I'm a big believer in recent improvements in CAD and CAM allowing us to design mechanical systems that weren't practical just a few years ago.

Length overall : 88 m / 289.1 ft Displacement : 1,240 t Beam : 12.6 m /42.2 ft Speed at max power : 19.5kt Draught : 6.0 m / 19.7 ft Guest Cabins : 6 with dagger board : 11 m / 36 ft Crew Cabins : 8 Air Draught : 58.2 m / 191 ft

[bernd1972]

That´s a pretty big thing to build. Now with that length and the intended speed you don´t have to think about Lürssen-Effect vodoo.

[Squidly-Diddly]

I was just quoting the Maltese Falcon as an example of a power hull of that beam/length ratio successfully adapted to motor sailor.


I was thinking of using the S-Boat hull as a starting point because it is about the right size at....



S-Boat figures...

Displacement: 100 tons (max)
78.9 tons (standard) Length: 32.76 m Beam: 5.06 m Draught: 1.47 m

Also a very proven (and today free) hull design. It also seemed to have been able to take a lot of added weight/power with same hull design.

The wise men on this forum say the cost of the hull construction is a very small % of total boat cost, which is why I would start with the best faired rounded steel hull.

[ChrissyT]

Thanks guys! Brilliant responses.

Could someone explain more as to why it reduces wave making drag? Is it purely that the drawn out hull doesn't create as much wave making drag?

Regards

Chris

[Squidly-Diddly]

and the forward wave's trough cancels out the wave being created by the bow at the waterline.

That is why bow bulbs are tricky to design and only work best at particular speed and draft.

[bernd1972]

Thanks guys! Brilliant responses.

Could someone explain more as to why it reduces wave making drag? Is it purely that the drawn out hull doesn't create as much wave making drag?

Regards

Chris

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)

[Ad Hoc]

Could someone explain more as to why it reduces wave making drag?


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.

[Willallison]

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..

[Squidly-Diddly]

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 http://www.boatdesign.net/forums/images/buttons/viewpost.gif (http://www.boatdesign.net/forums/boat-design/low-length-displacement-hulls-37159-post449667.html#post449667)
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..

[Willallison]

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...

[Pericles]

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

[ChrissyT]

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

[bernd1972]

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.

[Ad Hoc]

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?


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



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?


As for wave pattern, see here:

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

Trust this helps.

[Pericles]

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/boatImages.aspx?boatID=11073&iNum=1&backtoboat=%2Fboats_for_sale%2F11073%2F%3Fsearchid%3D2895505%26page%3D1%26preview%3D%26logview%3Dno

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

[FAST FRED]

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

[tom28571]

Chris
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.
.

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.

[bernd1972]

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.

[Willallison]

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.

[tom28571]

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.

[Adler]

Dear All,

Regarding the prime stages of this thread and the following posts and according to the base rules that Ad-Hoc described, please see the Lurssen's 1953's generation that was superseded the II War's S-boot.

The "Lurssen's effect" stern-design was then modified and also the Engine room's Installation Coordinates moved to the bow. The propellers number increased to four (4) and the power at 14.600 HP total -(Maybach engines edition) and the sides Propellers' shafts were installed related to Center Line, by angle.
LOA: 42.6m
Beam: 7.10 m
Draught: 1.90 m
Max.Tonage: 190 t
Speed: 39 knots
Escape Speed: 43 Knots

I think that there all a lot of subjects to be discussed on that.....

[Ad Hoc]

please see the Lurssen's 1953's generation that was superseded the II War's S-boot.
.....

Alder

That hull has an L/D (length displacment) ratio of 7

The videos, in my other post, showing several high speed catamarans, their L/D ratios are 8-9.

So, that Lurssen's L/D is not as high as the cats, but still good. The effects of increasing L/D can be seen again here in the first graph.
http://www.boatdesign.net/forums/boat-design/dare-say-no-30261-9.html#post318285

[FAST FRED]

"and the power at 14.600 HP total"

divide that by 16 (great for the era) and the fuel burn is over 900 GPH!

What a cruiser!

[Pericles]

FF,

What a warship! :)

[Pericles]

Latest edition of ProBoat contains an article by Reuel B. Parker about his design of Commuter Powerboat, so amuse yourselves carefully, whilst I read, mark, learn & inwardly digest. I'll be back.

[u4ea32]

Perry, this is a good list of beautiful boats. Thanks for sharing!

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/boatImages.aspx?boatID=11073&iNum=1&backtoboat=%2Fboats_for_sale%2F11073%2F%3Fsearchid%3D2895505%26page%3D1%26preview%3D%26logview%3Dno

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

[Adler]

Dear FAST FRED,

Lets as say that this hull has a fuel cost per hour at 22 Kts speed, around of 485 Lt and at 40 kts speed, reaches a quantity of 2830 Lt. (Specific Fuel Consumption around 195ml/hp.h).
Regarding the service range see please the attached diagram.
I think, not bad comparing to the most of the known type of cruisers/yachts with similar performances.

[Adler]

Dear FAST FRED,

I forgot to note that according to Lurssen the Fuel's tanks Capacity is 30.000Lt.

[Pericles]

On the River Thames at Hurley is the yard of Peter Freebody & Co. The Download Library contains real gems and the beauty of the watercraft described will delight the eye and tickle the fancy.

http://www.peterfreebody.com/about_us.php

Start from the bottom up; the restoration of "Herring Gull".

[bernd1972]

Perry, thanks for sharing these links. The Gelcye tenders (Herring Gull, Mavourneen,...) are indeed an exceptional beautiful and efficient example of the forced-displacement concept.
Really sad that Peter Freebody passed away last year. He was one of the few people who are on earth to teach us the difference between just working for money and being motivated by passion.

[bernd1972]

Here´s some fotos of the boat I did after this concept.

[tom28571]

I must have been asleep for a while but have never heard the term forced-displacement before. Where did it come from and who coined it? The meaning is kind of intuitive but is it the same as semi-displacement or semi-planing?

[Pericles]

Good question Tom. Try this on Wikipedia. http://en.wikipedia.org/wiki/Forced_displacement :D:D:D:D

Back to practical subjects.

Probably the first boat to express the principle must have been Turbinia. http://en.wikipedia.org/wiki/Turbinia

Torpedo boat destroyers. http://en.wikipedia.org/wiki/Destroyer

Fastest of them all. http://en.wikipedia.org/wiki/Le_Fantasque_%281935%29

I think the horsepower requirements are greater for forced displacement than for planing (as witnessed by your Bluejackets) for a given hull weight and a stipulated speed above conventional waterline calculations. But, catamarans are the exceptions that prove the rule. C'mon Lottery, All I want is £6 million......

http://www.youtube.com/watch?v=vdqXoIhg0dA

[bernd1972]

Well, concerning the term "forced-displacement": I googled it and was surprised that I´m practically almost the only one using that term. I read it several times in pretty old articles (around 1925 to 1940) about the commuter yachts of the 1920´s and their typical hull characteritics. The term "forced-displacement-hull" was used for fast lightweight displacement boats that exceeded what was considered a nature-given hullspeed without climbing up the bow wave but istead keeping the bow wave extremely small by extremely stretched lines and therefor without producing excessive wave resistance.
I allways considered it a common term, never realized that I might be more or less the only one using it today.

Depending on special details of the hull shape concerning even distribution (a soft curve when shown as a graph) of change of displaced volume and a L/B higher than 6 a good forced-displacement hull will allways be superiour to a planing boat in terms of economy up to 1,6x the speed resulting of conventional waterline calculations (don´t want to upset some fellows by using the term hull speed :D)

[tom28571]

Glad I asked guys. I liked the link to the Fantastique Pericles. I served on a destroyer for 2 1/2 years that fit this definition although we did not press the speed very often. At half power, we ran at S/L 1.5 during aircraft launch operations and could do 1.87 or so in a pinch at full power. Not so fast as either Fantastique or some of the WW2 Japanese destroyers though. I can accept that a forced displacement hull would be more fuel efficient than a planing boat if they were both restricted to the same displacement but not otherwise. Too many variables for any direct comparison anyway.

In all my reading, which has been considerable, I never before ran across the forced-displacement term so the intuitive guess of its meaning was wrong. There aught to be a better term for the phenomena since the practice is widespread, especially in multihulls.

[tom28571]

Photo of hybrid planing/canoe body powerboat test model. L/B of canoe keel is about 14:1 carrying 70% of displacement. This fits in there somewhere.

[DCockey]

Dave Gerr has an article about efficient powerboats in the latest "The Masthead: The Journal of the Westlawn Institute of Marine Technology". http://www.westlawn.edu/news/WestlawnMasthead17_Mar.11.pdf
He includes his own formula for maximum hull speed/length ratio which is based on the D/L ratio and is never less than 1.34.

[Pericles]

What a coincidence. My issue of The Masthead was waiting for me this morning and I planned to read it at my leisure later today. I think I'll read it sooner than later.

Regards,

P

[Pericles]

Tom,

Your attachment looks very intriguing. I guess you are about to undertake another water towing series of tests, as you described at your website for the original Bluejacket? One term used in the UK is "Swim". See Hadar Narrow boat.

The other attachment is one I cannot attribute. It is not mine, but I had saved it quite a while ago, to assist & remind me in learning about hull forms. I hope the designer will identify themselves here. Might it be PAR? This hull form has been discussed at length in this forum. Remember these brilliant threads.

http://www.boatdesign.net/forums/powerboats/help-economical-semi-planing-designs-16746.html

http://www.boatdesign.net/forums/powerboats/displacement-glider-powerkeel-etc-12512.html

http://www.boatdesign.net/forums/boat-design/displacement-speed-question-27879.html

From acorns, oaks might grow:!::?:

http://5magazine.wordpress.com/2010/09/08/superyacht-ra-by-adam-voorhees-design/

Regards,

P

[Willallison]

P - I do believe that was one of mine from quite a few years ago... an early iteration of the hullform that I coined the name "monomaran" for. It is essentially the same hullshape as Malcom Tennant developed to avoid squatting in displacement catamarans, and is also similar the one that Tom is now working on. Indeed there is another variant faetured in the same edition of the Masthead, which was my entry into the recent Proboat design comp... though that one (Sliver) was a rather more extreme development of the idea - really more of a stabilised monohull.

Tom - Glad to see you're moving ahead with your work on the Glider... I wish I could find a little time to do the same! My own version (of this type) is similar, albeit with the canoe body faired more into the chine up forward in an effort to reduce slamming. It'll be very interesting to hear the results of your testing....

A number of others must have been watching also, for a I have recently come across a number of craft with similar underbodies - the most recent being the new Azimut Magellano 50. A less extreme example of the idea... but surely a reasonable example of proof of concept!

[Pericles]

Will,

Thank you very much for putting me right. Monomaran is a great name. Those ProBoat competitions are very thought provoking.

Mike Storer wrote about a similar hull form some time ago. Scroll down.

http://www.storerboatplans.com/Solarboat/solarboat.html

Regards,

P.

[Willallison]

Scoop them all up, stick 'em in a bag, give it a shake and what drops out? A boat with a box keel...
Proof that there is rarely really anything truly new in boat design...;)

[Pericles]

EMF posted this at the forum, but there were no replies.

http://www.boatdesign.net/forums/attachments/boat-design/4280d1129182433-first-attempt-draft.jpg

[Pericles]

Will,

Good grief, your design just appeared again, as I Googled "displacement glider".

12-07-2003, 04:44 PM
http://www.boatdesign.net/forums/boat-design/displacement-glider-2545-2.html

The Alsphere website appears to have disappeared.

http://www.epicos.com/Portal/Main/Home/Pages/ItemDetails.aspx?ff%2FDJnKYCF%2FUgxj8035If%2BVJak6DbhPebAlN4%2F36w%2BEfT9ZkpsLliXoDGTHVBm8h

[Pericles]

Narrow, longish, heavy, ownership in doubt, probably rolls on a wet lawn. Built just so, in order to cruise rivers without a wash, so as not to upset anglers and disturb nesting wildfowl. A fine stem to part the waters with nary a splash. A stylish buttock to leave no wake. 2.2 BMC diesel, probably frost damaged. Perhaps the antithesis of the subject of this thread. Needs a miracle.

Never look at "project" boats:!:

P.

[Adler]

Dear Pericles,

Please, for your comments.

http://www.ocea-yacht.fr/pages/okeanis.html

[Willallison]

Will,

Good grief, your design just appeared again, as I Googled "displacement glider".

12-07-2003, 04:44 PM
http://www.boatdesign.net/forums/boat-design/displacement-glider-2545-2.html

The Alsphere website appears to have disappeared.

http://www.epicos.com/Portal/Main/Home/Pages/ItemDetails.aspx?ff%2FDJnKYCF%2FUgxj8035If%2BVJak6DbhPebAlN4%2F36w%2BEfT9ZkpsLliXoDGTHVBm8h

Yikes... that was back in 2001.... how time flies...;)

[Alik]

Dave Gerr has an article about efficient powerboats in the latest "The Masthead: The Journal of the Westlawn Institute of Marine Technology". http://www.westlawn.edu/news/WestlawnMasthead17_Mar.11.pdf
He includes his own formula for maximum hull speed/length ratio which is based on the D/L ratio and is never less than 1.34.

I've read that one and it makes not too much sense to me. There are generally accepted indicators of relative speed: FnL (or just Fn) and FnV. Second one is called volumetric Froude number taking in account weight of boat and is used for high speed displacement, semi-planing and planing craft. Say, FnV<1 is displacement, FnV=1...3 is semi and FnV>3 is planing mode.

Using common terms in naval architecture, there is absolutely no need to invent SL Ratio (that itself is analog of FnL) with correction to DLR. One can just use FnV instead.

Systematic series suitable for resistance predictions for type of craft in question (de Groot, NPL, SSPA, 63, 64 and others) are using FnV as indicator of relative speed. So why do we need modified SL Ratio??

[Alik]

Will, seen Your design. Looks great!

We were also working on power-saving concept, but this is for combining of cruising mode and planing mode, for 100' motoryacht. This one:

[tom28571]

Will,
Saw your Sliver again on the Masthead as well as the earlier page on Graphite. How long have we been looking at this canoe keel concept? Wish someone would fund some boats that really exploit the idea. One reason I don't like the box keel term is that it is already in use to describe boats that don't get very far into the basic concept of the separate keel/body philosophy.

Pericles,
You have kept a good record of past ventures into the canoe keel and I will add that list to the files.

The reason my model hull has the wide chines forward that it does is that it must offer good accommodations in the forward cabin in a boat of trailerable length. A longer and softer bow shape would be easier on wave entry but I'm interested in practicality of the principle. I made the model without much softening of the shape forward to first test this shape with the objective of being able to soften it a bit later. The model mirrors the chine of the Bluejacket that it will be compared to in the tests to give a better baseline of comparative results.

Frankly I don't think slamming will be much of a problem at the intended speeds and that only moderate shaping will be needed. The boat is intended to run in level trim at all speeds (even more level than Bluejacket) and this should control slamming to a large degree. Bluejacket runs at 2 degrees trim which maxes at top speed and is all bow rise with zero squat at any speed. Even flat bottom hulls don't slam when run level in chop at moderate speed. Its all in the incident angle that the water meets the hull bottom.

The model has been sitting about for a while waiting on both me and a suitable test vehicle. Hopefully this Spring.

[Pericles]

Hello Adler,

Well, one thing, Okeanis would never fit under the bridges on the River Thames, but that is the only negative I can see.

I picture myself mooring up at the Ferry in Cookham this summer and so, the beam can only be 14 feet. :p

http://www.theferry.co.uk/

With a cruising speed of 15 knots with only 2200 installed hp, the Okeanis is a great design. Her waterline length looks close to her overall length, so the L/B ratio is about 5.6 to 1? I'm just guessing here, but from looking at images of the water passing the hull, were Okeanis 43 metres length overall, she might make perhaps another 3 knots with the same engines. The raised foredeck with that sweeping sheerline; she makes the hair on the the back of my head stand up Heaved to, Okeanis would look like she was racing away. Thank you for posting the link.

Regards,

P.

[Pericles]

Alik,

An intriguing design of bow. Ram shaped wave piercer, with very high mounted chines (would that be the right term?) to provide buoyancy and deflect water, as necessary in plunging seas. Almost, Carolina flare concept, to prevent green seas over the prow. It should cut through waves like a tailor's shears through silk fabric. Thank you for presenting it to the forum.

Nautilus, a curious fictional submarine from the film "The League of Extraordinary Gentlemen", would work very well as a wave piercer, because any swells that reached the bridge would be so large as to require the vessel to submerge. Maybe that's the way to go? ;);) Surface when the sun appears.

http://www.hlj.com/product/WAVTL-02

Regards,

P.

[Alik]

Alik,

An intriguing design of bow. Ram shaped wave piercer, with very high mounted chines (would that be the right term?) to provide buoyancy and deflect water, as necessary in plunging seas. Almost, Carolina flare concept, to prevent green seas over the prow. It should cut through waves like a tailor's shears through silk fabric. Thank you for presenting it to the forum.



We call this shape 'can opener bow'. Advantages - reduced wetness, better arrangement of anchor and mooring gear.

[Pericles]

Alik,

No Joke. That was what I thought I should have included in my last post, only we Brits call it a tin opener and I did not want to be misunderstood, ;) :):):)

[Alik]

Alik,

No Joke. That was what I thought I should have included in my last post, only we Brits call it a tin opener and I did not want to be misunderstood, ;) :):):)

This means one day we can start designing kitchen utensils :D

[Pericles]

Wok do you mean? :rolleyes:

Take a look at these culinary delights.

http://www.peterfreebody.com/downloads/Yachting_Classic_Issue27_2005.pdf

Regards,

Perry

[Adler]

Hello Adler,

Well, one thing, Okeanis would never fit under the bridges on the River Thames, but that is the only negative I can see.

I picture myself mooring up at the Ferry in Cookham this summer and so, the beam can only be 14 feet. :p

http://www.theferry.co.uk/

With a cruising speed of 15 knots with only 2200 installed hp, the Okeanis is a great design. Her waterline length looks close to her overall length, so the L/B ratio is about 5.6 to 1? I'm just guessing here, but from looking at images of the water passing the hull, were Okeanis 43 metres length overall, she might make perhaps another 3 knots with the same engines. The raised foredeck with that sweeping sheerline; she makes the hair on the the back of my head stand up Heaved to, Okeanis would look like she was racing away. Thank you for posting the link.

Regards,

P.

Dear ΠΕΡΙΚΛΗΣ,

Thank you for your notes.
I worked on speed issue and last summer the matter was solved.
Of course we didn't increased the length.
We design and built new propellers.
See please the attached file.

New Speed Performance Data:
Mean Cruising Speed: 16.7 knots
Mean Max.Speed: 18.6 Knots.
Effective Horse Power: not changed. (2200CV).
Reduction ratio: not changed.
The measuring speed method was through GPS.
When was measured on mile post std - the results were:
Mean Cruising Speed: 17.1 Knots
Mean Max.Speed: 19.0 Knots.

[Pericles]

Ah, the old propeller trick. I might have know you'd be devious. Well done.;)

Best wishes,

ΠΕΡΙΚΛΗΣ,

Addendum

How many blades on the previous propellers please?

[Adler]

Ah, the old propeller trick. I might have know you'd be devious. Well done.;)

Best wishes,

ΠΕΡΙΚΛΗΣ,

Addendum

How many blades on the previous propellers please?

Dear,

Previous propellers 5-blades Dia:1200mm
Installed propellers 6-blades Dia:1295mm

The important point is that: The commissioning sea trials were applied under light ship condition.
The last sea trials were applied in heavy ship condition at 81% of Full Load (Fuel+Water+Waste).
Please see the attached file.

Thank you,

ΜΕΓΑ ΤΟ ΤΗΣ ΘΑΛΑΣΣΗΣ ΚΡΑΤΟΣ

[Pericles]

Adler,

I can only translate using Babelfish, so in answer to ΜΕΓΑ ΤΟ ΤΗΣ ΘΑΛΑΣΣΗΣ ΚΡΑΤΟΣ I wish you ήρεμες θάλασσες

Thank you for the Sea Trials evaluation and the information about the propellers.

Kind regards,

Perry (My given name) ;)


Addendum.

Adler, I think I'm a dunce. I did not scroll down on the first link you sent me. Now I have done, I can see those propellers in Technicolor. Thanks.

[tom28571]

I read the article on efficient powerboats in the Masthead but am not sure why it was written. Seemed a long way round to draw some pretty obvious conclusions.

Mik Storer's Solar boat does use the same basic canoe hull shape but I'm not sure it goes fast enough to reap any benefits from it.

[Adler]

Adler,

I can only translate using Babelfish, so in answer to ΜΕΓΑ ΤΟ ΤΗΣ ΘΑΛΑΣΣΗΣ ΚΡΑΤΟΣ I wish you ήρεμες θάλασσες.

Dear Perry,
the translation by word is:

ΜΕΓΑ = GREAT

ΤΟ = THE

ΤΗΣ ΘΑΛΑΣΣΗΣ = SEA'S

ΚΡΑΤΟΣ = STATE =[ The Authority that Controls and Keeps the Law , the Treasure and the Power of Democracy;The humans' freedom.]

GREAT THE SEA'S STATE = had the meaning "A State is Great if is based on Sea's Control and keeps the Sea ways open and free".

It was the main phrase from Perikles' Speech to the Parliament of Ancient State of Athens regarding to convince them voted the extra balance which spended to enlarge and extend the number and size of Navy Ships for an effective State's defence that affected to the Social Progress.

Actually Pericles when expressed his idea had established the base and the corners of marine field.

At your disposal

[Willallison]

Will, seen Your design. Looks great!

We were also working on power-saving concept, but this is for combining of cruising mode and planing mode, for 100' motoryacht. This one:

G'day Albert... Wild looking beast - that evil looking bow is somewhat reminiscent of the VSV Mary Slim http://www.multimarine.co.uk/VSV.html

Truth be known, I ran out of time to fully refine some of the styling elements of Sliver - like the slightly odd-looking windscreen. I think the idea has merit, though other variants - like Tom's glider and my own version of the same concept - are likely to have wider appeal.

Re the Masthead article, I haven't read it yet, but I think much of the "debate" over SL vs Fnv derives from locality as much as anything else. SL seems more commonly used in the US, Fnv in european countries.
Perhaps, once they come to understand the elegance of the metric system, Americans will also start using Fnv...:P

[Alik]

Re the Masthead article, I haven't read it yet, but I think much of the "debate" over SL vs Fnv derives from locality as much as anything else. SL seems more commonly used in the US, Fnv in european countries.
Perhaps, once they come to understand the elegance of the metric system, Americans will also start using Fnv...:P

Not just that. Before inventing formulas one should fully understand the subject...

[Willallison]

Will,
Saw your Sliver again on the Masthead as well as the earlier page on Graphite. How long have we been looking at this canoe keel concept? Wish someone would fund some boats that really exploit the idea.



Well... I was rather surprised to see that the image Perry posted was from way back in 2001!

Albert...
Why not a letter to the editor of the Masthead (Norm Nudelman)... perhaps you could elicit a reply as to why Dave uses the DLR / SL as opposed to Fnv...

[DCockey]

I've read that one and it makes not too much sense to me. There are generally accepted indicators of relative speed: FnL (or just Fn) and FnV. Second one is called volumetric Froude number taking in account weight of boat and is used for high speed displacement, semi-planing and planing craft. Say, FnV<1 is displacement, FnV=1...3 is semi and FnV>3 is planing mode.

Using common terms in naval architecture, there is absolutely no need to invent SL Ratio (that itself is analog of FnL) with correction to DLR. One can just use FnV instead.

Systematic series suitable for resistance predictions for type of craft in question (de Groot, NPL, SSPA, 63, 64 and others) are using FnV as indicator of relative speed. So why do we need modified SL Ratio??


You should direct your question to Mr Gerr. My message was just to inform this group of the article in The Masthead and Mr. Gerr's formula, not agreement with the contents of the article or the formula.

[Alik]

Albert...
Why not a letter to the editor of the Masthead (Norm Nudelman)... perhaps you could elicit a reply as to why Dave uses the DLR / SL as opposed to Fnv...

I know the answer :p

If one has serious interest about resistance and power prediction read Larsson, Savitsky, Almeter, PNA, etc. Do not read amateurish writings.

[Alik]

You should direct your question to Mr Gerr. My message was just to inform this group of the article in The Masthead and Mr. Gerr's formula, not agreement with the contents of the article or the formula.

There is nothing to ask about; there is also no need to take those formulas/writings seriously. Look at attached file to see what happens with other performance prediction formulas from same source - there is no match with common methods.

[Pericles]

Adler,

I misunderstood "Great the sea state" as "The sea state is great", meaning high or rough sea, hence I wished you "calm seas". :D:D I was lost in a sea of translation. :rolleyes:

I have a copy of "Pericles of Athens and the Birth of Democracy" by Donald Kagan, (1991) and Wikipedia offers a reasonable assessment as well. He knew that the Athenian Navy was the key to Athenian security.

Perry

[Adler]

Dear Perry,

I 'll be back tomorrow regarding the matter of this thread about the Semi -displacement definition.
I am sorry for the interruption ; my flight is departed on next 40 minutes.

Thank you for your posts.

[Pericles]

Tom #71,

Karl Stambaugh also designed a box keel, Red Wing 26, into which he installed the engine. It's an improvement over the flat bottom version. The reasons are listed on the site.

http://www.cmdboats.com/rw26boxkeel.htm?cart_id=9e330486c020f548c41f6bd7f85696b7

Regards,

Perry

[u4ea32]

There is nothing to ask about; there is also no need to take those formulas/writings seriously. Look at attached file to see what happens with other performance prediction formulas from same source - there is no match with common methods.

Interesting spreadsheet, thanks very much!

Can you give me pointers to the paper(s) or book(s) that describe the deGroot method?

Specifically, there are 5 parameters on the deGroot page. The only one I understand is "B,m" for beam in meters. What are the other parameters?
Ca?
Ke?
Z?
Kws?

Thanks!

[Alik]

There are floating comments for evey value; position mouse on red traingles and You will see them.

[tom28571]

Tom #71,

Karl Stambaugh also designed a box keel, Red Wing 26, into which he installed the engine. It's an improvement over the flat bottom version. The reasons are listed on the site.

http://www.cmdboats.com/rw26boxkeel.htm?cart_id=9e330486c020f548c41f6bd7f85696b7

Regards,

Perry

Been away a few days Perry. I did not explain the intent of my statement well enough. I was not thinking of low displacement speed advantages of a box keel as in both Mik's and Karl's boats.

[Pericles]

Tom,

The Hunter Landau 20 twin tunnel hull has the ability to run very smoothly at hull speed on rivers and plane easily elsewhere. Take a look at this images

http://www.hunterassociation.org.uk/site/album_pic.php?pic_id=21

The transom is well illustrated on this site. A fine bow to cut the water, two outer sponsons to swallow the bow wave and a flat bottomed box keel for easy planing. A clever design that seems not to have gained wider acceptance.

http://www.boatinland.co.uk/hunter-landau-20-orlando

This last photo is just to reward the eyes.

http://classicyacht.org/gallery/albums/userpics/turningpoint.jpg

P

[bernd1972]

I really like the boat in your last link, looks like a Blanchard?

[Pericles]

Bernd,

It's "Turning Point" 1928. Location: Seattle (CYA), a Lake Union Drydock Dreamboat.

http://pacificmotorboat.com/dreamboats2

With regards to "Marlin", Kiel, did you design and build her? That would be an excellent vessel to be built as a Standard Series like the Blanchards.

http://classicyacht.org/aboutclassics/images/Dreamboat_PMB_OCT1926.pdf

regards,

P

[bernd1972]

Hi Perry, yes I have to confess that the Marlin is my boat, designed by me and built with my own clumsy hands. She will be availeable for charter as soon as I got her through the inspection.

[Pericles]

You have built the most amazing and beautiful boat. You really must post more photographs and all the engineering details. The first time I viewed her on the German website, I was captivated. Well done.

Perry

[bernd1972]

Thank you Perry.
The hull is cold moulded plywood with epoxy and a generous amount of glass. Weight empty is 17.3t ,18.5t with fuel and water. 600l of diesel give her a range of 600nm @ 8.5kts. At 12 kts the fuel consumption is still no more than 1.8l per nm.
Maximum speed is about 16.5kts with 2 diesel engines of 125kW each.
The boat offers accomodation for 6 in 3 cabins, 2 bathrooms and a separate shower room + a generous and very functional galley.
So I believe the Marlin is quite the perfect boat for coastal cruising for a family and occasional guest.

[bernd1972]

Thank you Perry.
The hull is cold moulded plywood with epoxy and a generous amount of glass. Weight empty is 17.3t ,18.5t with fuel and water. 600l of diesel give her a range of 600nm @ 8.5kts. At 12 kts the fuel consumption is still no more than 1.8l per nm.
Maximum speed is about 16.5kts with 2 diesel engines of 125kW each.
The boat offers accomodation for 6 in 3 cabins, 2 bathrooms and a separate shower room + a generous and very functional galley.
So I believe the Marlin is quite the perfect boat for coastal cruising for a family and occasional guest.