Seaplane hull design developments

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Himat

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There have been a lot of talk about that there have been very little development in seaplane hull design later years. And with later years that is later than the 1940’ies. I must confess that I too have expressed this opinion. But as I have studied publications about seaplanes and hull design I do think I must rephrase the statement that there has been very little development in seaplane hull design. I think it is more that the development and progress there have been is little known or not recognized.

If I start with why the progress is not recognised there are several causes. One is that it’s one the large aircraft company flying boat’s most of the development have taken place and been implemented. And of these there have been only a handful developed and produced in some numbers the last fifty years. Then, of those that are more numerous at least in new designs, the amateur built light airplane flying boat a lot have been based on what was known to work in 1945. Another factor is that before we got internet, not much knowledge was public. Not even pictures of those seaplanes produced in numbers, and often nothing about one of prototypes.

There have also been talking about thinking outside the box. Then what box? As I see it, lack of knowledge of what have been done, why what work and the understanding of the physics is the most constraining box. A lot of “new” ideas are maybe not that new, many have been tried before and even found to work.

If we put the baseline at the 1940’ies three designs come to mind. The PBY Catalina, Dornier Do 24 and Short Sunderland. All are examples of the short after body flying boat hull. That is the hull aft of the step is rather short. Apart from the short after body they were not that similar. The Dornier have both a transversal and a longitudinal step, later marks of the Sunderland had a faired step. This is where development often seems to have stalled.

Then, for those that have not been put off by the long rant, some examples where I see that progress have been made.

During the 1940’ies and 1950’ies research showed that longer narrower hulls did perform better. Examples where this knowledge was implemented are the Martin P5M Marlin and Convair P6M Seamaster, both with a pointed step The after body hull now go all the way to the tail. The Canadair Cl215 and Lake Buccaneer also have the long after body, but with straight transversal steps. The later Shin Meiwa PS1 follows these and got effective spray suppression on the forward hull. Last developed of large flying boats are the Beriev A40, Be200. These have again long narrow hulls, now with a dual chine. Compared to the pre world war two designs; spray, hydrodynamic and aerodynamic drag is reduced. There are no longer any problems to get the plane up and planing either, even if better engines also contribute to this.

Another Beriev worth mentioning is the Be103. The “float wing” is not common, but seen before on the Coot amateur built airplane and 1970’ies Equator prototype. What I have not seen on recent seaplane is the step placement ahead of the airplane centre of gravity. It looks like Beriev have made a well function hull with these features.

Last those in development or at the prototype stage. All have been discussed on this forum.

The Centaurus, have a longitudinal step and a narrow hull without a pronounced hump speed. The developers say the hull does not plane, but I think that is down to how you look at it. To me it look more like it have a much more progressive increase in hydrodynamic lift, not a sharp increase like older hulls that get “on the step”.

The Lisa Acoya with a hydrofoil/ski undercarriage. I am not quite sure if it is either or, or part of both. Apart from pure research prototypes such a design has not been tried since the Piaggo P7 racing seaplane.

The Sea Era, a stepped hull design where the hull have got the outline of a lifting body.

The Privateer, a project where Bill Husa, known as Orion on this forum was involved. The step is forward of the airplane centre of gravity and according to pictures and the patent papers part of the forward hull is flat with no deadrise. No great leap forward, but another example that there are different ways to design a seaplane hull and gain benefits from that. Most interesting is that Orion did not follow the guidelines for hull design he published in a short article on this forum.

And with this long post, I have not included any links. On the other side, Google search of this site is in the upper right corner.
 

nerobro

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I'm a big fan of hydroplane lifted boats. The military did trials of hydrofoils for their airplanes to extend their rough water capabiltiy. It worked..... But it means "more equipment".
 

Himat

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And another example of research that has been done, prototypes built and tested. New knowledge gained, part of it documented in reports now dusting down in some archive. With the work and findings not public known. I have seen some videos and reports from this work, do you know if there is more public available?
 

Himat

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Twin hull or single hull?
I am not sure there is a definite answer, but at light airplane sizes I would say probably. Twin hulls bring a weight penalty, but so do sponsons or tip floats used for lateral stability too. Practical issues might be what tip the scale in favour of either in each case. To solve lateral static stability when afloat and make both docking and beaching of the seaplane practical are equal difficult as to make a hydrodynamic and aerodynamic good hull.

I have got around to read the appendix on seaplane design in Gunnarsons book, I’ll see if I get around to write down some considerations based on that for review and discussion on this forum.
 

harrisonaero

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Watch for some real innovation in seaplane design this year at Oshkosh- Burt Rutan's Skigull should shake things up. Especially the way it deals with the Achilles heel of float flying- rough water.

(all of this is published online- google for more info)
 

Himat

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Trimaran hull the article says.
Rutan must have looked at my proposal. (Se avatar).:gig:
Or great minds think alike.:ponder:

More serious, there are reasons to make a light seaplane a trimaran. It could be with front outriggers and a central hull or reversed. Both benefit from less spray drag and both stick free trim and stick fixed during the takeoff run can be designed to stay within a reasonable small range. Static stability is easier to achieve too.
 

Himat

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Recently other threads on flying boat hull design strayed into discussing stepped hull design, and the reasons for using a stepped hull on a flying boat/seaplane.

What was not asked was why does most flying boats have stepped hulls?

After all, the flying inflatable boat, a trike on a Zodiac does just fine without a stepped hull.
FIB_Side.jpg

What benefit does the stepped hull actually give?

What I can think of is this:
  • A stepped hull make it possible to let the airplane rotate for takeoff. The trike in the picture don’t have to, the wing is instead rotated for takeoff.
  • With a stepped hull the hull size changes with speed when on the surface, minimizing hydrodynamic drag.
  • A stepped hull make it reasonable easy to arrange the buoyancy for floating level when static with great stability and at the same time place the “transom” close to the aircraft centre of gravity. This I think gives favourable dynamics if hit by a wave when at speed on water.
  • For a large transport aircraft, the stepped hull makes it possible to design a fuselage with large internal volume and good hydrodynamic properties. Here is a picture of a NACA design that failed on internal volume. Otherwise it performed well in the towing tank and wind tunnel.

NACA_RM_L9D15.jpg

Are there other considerations that I haven’t thought of?
 
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tunnels

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hitting waves and rough water at speed the bow shape is vitally important only needs to cut and part the water quickly so don't want strakes until the water has parted then small to start with increasing in size as they go aft and this is what so many people don't do and think its just spray rail when In fact if Its wide enough can lift and greatly reduce the wetted surface
 

Kingfisher

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Trimaran hull the article says.
Rutan must have looked at my proposal. (Se avatar).:gig:

Or mine!:ban:!
Actually, i could only find these ominous video screenshots shown in another thread on this forum and in an EAA document of Rutan's concept that showed a mix of Dornier S-Ray and Centaur. Is there newer stuff out there?
 

Kingfisher

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T

During the 1940’ies and 1950’ies research showed that longer narrower hulls did perform better. Examples where this knowledge was implemented are the Martin P5M Marlin and Convair P6M Seamaster, both with a pointed step The after body hull now go all the way to the tail. The Canadair Cl215 and Lake Buccaneer also have the long after body, but with straight transversal steps. The later Shin Meiwa PS1 follows these and got effective spray suppression on the forward hull. Last developed of large flying boats are the Beriev A40, Be200. These have again long narrow hulls, now with a dual chine. Compared to the pre world war two designs; spray, hydrodynamic and aerodynamic drag is reduced. There are no longer any problems to get the plane up and planing either, even if better engines also contribute to this.
The Blohm&Voss BV222 and BV238 also follows the narrow long hull design (designed before and during the war, I may add ;)):
[video=youtube;flDgvOA7k9o]https://www.youtube.com/watch?v=flDgvOA7k9o[/video]
images.png
 
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tunnels

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why is there such a interest in sea planes and THE STEP!!and seaplanes!! who is Making a sea plane?? steps i can sort of understand some interest !and hull shape from the 19?? !! We have moved on a little since those days I hope an there has be some pretty clever innovation's Hull designs , There's book I read loooong time ago called "DHOWS TO DELTS" Kingfisher and Himit would find some useful information in there for sure .was written in the era everyone seems to get there information from when NOAH was drying his boots after the rain I believe !!!
Its a big thick book so a library would be the most likely place to find I
 

Kingfisher

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I found this reference of seaplane design, which has excellent, easy to understand pictures and explanations. For example, I now understand what "hull speed" is. It also addresses some LSA limits and CFR Part 23 rules pertaining to seaplane design.

http://booksite.elsevier.com/9780123973085/content/APP-C3-DESIGN_OF_SEAPLANES.pdf

Just realised this had been discussed on this forum here...
https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/17295-big-thick-ga-airplane-design-book-3.html
 
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Himat

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I have got the book and downloaded the appendixes.
The C3: Design of seaplanes appendix I have printed out.
 

Himat

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Single hull, twin hull, sponsons, tip flaoats[HR][/HR]
Gudmundsson gives a good introduction to how to analyze a seaplane design, but not that much on the considerations on the strong and weak points on different designs. One of the first things to think about is static stability when afloat, and sideways stability is most difficult. The seaplane hull is usually rather narrow, the weight high up and the wing is easily lifted on one side by the wind. Refer to Gudmundsson for calculation of metacentre height, here are some sketches of different solution to roll stability when afloat.


SeaplaneWingFloats.jpg
Tip floats.
+ Reasonable aerodynamic drag.
+ Ease of design and construction.
- Point load on wing.
- Can make the airplane water loop.
- Interfere with docking


SeaplaneFloatWing.jpg
Float wing.
+ No extra parts.
+ Very stable.
- Interfere with docking.
- Low wing is vulnerable to damage when beaching.
- Difficult to combine with flaps.
- Imposes huge loads on the wing if hit by a wave.


SeaplaneSponsons.jpg
Side sponsons
+ Aerodynamic “clean”. (If the design is well executed.)
+ Does not interfere with docking.
- Difficult to optimize hydrodynamic design.
- Heavy?


SeaplaneOutriggers.jpg
Outriggers.
Same pro and con as for sponsons?


SeaplaneOutriggersB.jpg
Twin hulls.
+ Very stable.
+ Does not interfere with docking.
- Heavy
- Might interfere with downward view.

Anyone that have comments and want to elaborate?
Or am I out of step with the audience?
 
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tunnels

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Single hull, twin hull, sponsons, tip flaoats[HR][/HR]
Gudmundsson gives a good introduction to how to analyze a seaplane design, but not that much on the considerations on the strong and weak points on different designs. One of the first things to think about is static stability when afloat, and sideways stability is most difficult. The seaplane hull is usually rather narrow, the weight high up and the wing is easily lifted on one side by the wind. Refer to Gudmundsson for calculation of metacentre height, here are some sketches of different solution to roll stability when afloat.


View attachment 36791
Tip floats.
+ Reasonable aerodynamic drag.
+ Ease of design and construction.
- Point load on wing.
- Can make the airplane water loop.
- Interfere with docking


View attachment 36792
Float wing.
+ No extra parts.
+ Very stable.
- Interfere with docking.
- Low wing is vulnerable to damage when beaching.
- Difficult to combine with flaps.
- Imposes huge loads on the wing if hit by a wave.


View attachment 36793
Side sponsons
+ Aerodynamic “clean”. (If the design is well executed.)
+ Does not interfere with docking.
- Difficult to optimize hydrodynamic design.
- Heavy?


View attachment 36794
Outriggers.
Same pro and con as for sponsons?


View attachment 36795
Twin hulls.
+ Very stable.
+ Does not interfere with docking.
- Heavy
- Might interfere with downward view.

Anyone that have comments and want to elaborate?
Or am I out of step with the audience?

I am interested in the last one ,Twin hulls !! why should it be heavy ? please explain!
Why should it interfere with down ward view ?
Tell me why the one above with outriggers isn't the heaviest of them all ??
 

Kingfisher

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Single hull, twin hull, sponsons, tip flaoats[HR][/HR]
Gudmundsson gives a good introduction to how to analyze a seaplane design, but not that much on the considerations on the strong and weak points on different designs. One of the first things to think about is static stability when afloat, and sideways stability is most difficult. The seaplane hull is usually rather narrow, the weight high up and the wing is easily lifted on one side by the wind. Refer to Gudmundsson for calculation of metacentre height, here are some sketches of different solution to roll stability when afloat.


View attachment 36791
Tip floats.
+ Reasonable aerodynamic drag.
+ Ease of design and construction.
- Point load on wing.
- Can make the airplane water loop.
- Interfere with docking


View attachment 36792
Float wing.
+ No extra parts.
+ Very stable.
- Interfere with docking.
- Low wing is vulnerable to damage when beaching.
- Difficult to combine with flaps.
- Imposes huge loads on the wing if hit by a wave.


View attachment 36793
Side sponsons
+ Aerodynamic “clean”. (If the design is well executed.)
+ Does not interfere with docking.
- Difficult to optimize hydrodynamic design.
- Heavy?


View attachment 36794
Outriggers.
Same pro and con as for sponsons?


View attachment 36795
Twin hulls.
+ Very stable.
+ Does not interfere with docking.
- Heavy
- Might interfere with downward view.

Anyone that have comments and want to elaborate?
Or am I out of step with the audience?
My brain is tired, it is too late in the day:tired:. The auxiliary float (Short Empire,,,) and sponson (Dornier) is probably the most widely used. If auxiliary floats are retractable (PBY, Bv238 or Do26), it is probably aerodynamically cleaner, otherwise sponson should be better.
Also, reading about the Bv238, they designed their auxiliary floats so that they cannot accidentally be submerged, which causes extreme drag and potential damage to the float/wing/plane in rough seas.

I think to truly evaluate which is the lowest drag, one really has to crunch some numbers (as HJS said). One has to create a sample design with the same weight and wings, and come up with the submerged volumes and wetted areas for the different configurations. So many ways to make a mistake (e.g. sponsons to low, wrong AOA, who knows...).

Your other considerations make sense, too. Position of wing (high or low) could also affect docking, besides the float/hull arrangement. For wet wing like Beriev, I'd be worried about corrosion.

Gudmundsson gives calculations for single floats and double, one could probably derive advantages of one or the other. However, the clear advantage of the "Planing tail" hull from the NACA tests with regards to a lesser spike in hydrodynamic drag re-appears in this more modern book ;)! Obviously, the planing tail hull has less usable volume, and the static buoyancy and resulting attitude in the water will be quite different. However, for my twin hull this should work perfectly. Although I'm unsure whether I executed the design correctly, since my step is further back than the NACA pictures.

Gudmundsson also mentions tendency to "water loop" when the forward float/hull area/volume is too large...
 
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