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:
Wave-piercers - marketing myth or design ingenuity?
and
Dare to Say No

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...l-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).


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.
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.