# Alternative take on hull design.

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##### Moderator
I had a nice, clear intro. After losing my text twice, I'll go straight to the discussion. I have read several designs texts on seaplane hull design, also Orion's one.

For a really wide amphibian flying boat (hull width, 8-10'), fairly slow (45 kts stall), would one of the following work?

SWATH:

The difference in buoyancy if you submerge it a bit further or a bit less (waves) is minimal. So very low wave resistance and much less sensitive to heavy pitching when the water isn't perfectly calm. At first sight, the hydrodynamic drag is making this a very bad idea. But doing the math I get a displacement/drag ratio of just over 4 @ 60 kts, not bad and not that much lower compared to a conventional seaplane. In fact, contrary to a traditional hull, the drag grows by the square of the velocity, such that during most of your T/O the drag could be much lower as a traditional hull design, resulting in a shorter take/off?

A different idea; if we move back from the SWATH a bit to something closer to a catamaran. Vertical outer sides and the inner side shaped like an ellipse. The M80 Stiletto is a good example, but then with only one "arch":

It has a weight/resistance ratio of around 4 @ 50 kts. Since the submerged parts are almost vertical, downwards pull when trying to lift off should be minimal right?

Last idea
, with a very wide hull, your draft is minimal, 2-4" or so. Would it be feasible to place fairly large sponsons a bit deeper than the fuselage itself? A tiny step on the fuselage could prevent spray from sucking you deeper in, while the sponsons function during most of the take-off as the steps on a normal hull design, but without the aerodynamic drag (they work as non-stalled short wings in flight).

#### Himat

##### Well-Known Member
I have been thinkering a lot on seaplane hulls myself, but at modelling size.

As for your proposals, SWAT. A few catches here.
First is the scaling, two tubes supporting a light plane of say 1000kg MTOW and 600kg empty get very slim, even more so the supports to keep the drag down. As bouyancy differ little with different drafts, a reason that the SWAT hull get it's performance, the difference between loaded and unladed draft will be substantial. Next, I have read somewhere that a submarine hull have less drag than displacement mode surface ship with similar displacement. As long as it is fully submerged, on the surface on the other hand it's much worse. Transition from displacement mode to flying and back can be a challenge.

The catamaran, yes but why not make both hulls symmetrical about their centreline?
Unsymmetricalhulls and a list or different draft, and you might explore unwanted sideways lift from the hull, causing the airplane to turn of course.

The very wide hull, yes but only on dead calm water. Any waves and you get a bad pounding.
There was a bizjet project "Nauticat" along this idea with a ordinary wide hull and part of the sponsons retractable.

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#### orion

##### Well-Known Member
Himat is correct - submerged to hull-borne transitions are very difficult to model accurately and are accompanied usually by a very high drag rise. Some years back I did work dealing with a transition of a submerged hydrofoil to a high speed planing surface. In this transition you get hit twice: The first hit is a dramatic loss of lift on the submerged foil as it nears the surface. The loss is as high as 50%. This then is accompanied by a drag rise as the foil starts to cavitate and you start creating all sorts of flows and eddies as you near the transition point. Unless you have gobs of power to spare, the craft just ends up dropping off the foil, burying its nose into the water. The porpoising action of this can be quite damaging to the craft.

And therein is the trick, regardless of the hull type. It's one reason that conventional planing hulls continue to be the design point, even accompanied by the drag penalties they have.

The long thin hull idea has also been tied, also combined with a longitudinal series of steps, sort of like a series of planing skis. This is not bad but the very sharp deadrise is relatively inefficient and has to be landed perfectly straight - any cross attitude will most likely flip the craft over. Which ten is also the issue with most of the concepts proposed above. It's therefore not only the landing and takeo-off itself that's critical, it's also the secondary considerations like skewed landings, floating debris, hull draft (can you beach it?), etc.

#### Jay Kempf

##### Well-Known Member
Himat is correct - submerged to hull-borne transitions are very difficult to model accurately and are accompanied usually by a very high drag rise. Some years back I did work dealing with a transition of a submerged hydrofoil to a high speed planing surface. In this transition you get hit twice: The first hit is a dramatic loss of lift on the submerged foil as it nears the surface. The loss is as high as 50%. This then is accompanied by a drag rise as the foil starts to cavitate and you start creating all sorts of flows and eddies as you near the transition point. Unless you have gobs of power to spare, the craft just ends up dropping off the foil, burying its nose into the water. The porpoising action of this can be quite damaging to the craft.

And therein is the trick, regardless of the hull type. It's one reason that conventional planing hulls continue to be the design point, even accompanied by the drag penalties they have.

The long thin hull idea has also been tied, also combined with a longitudinal series of steps, sort of like a series of planing skis. This is not bad but the very sharp deadrise is relatively inefficient and has to be landed perfectly straight - any cross attitude will most likely flip the craft over. Which ten is also the issue with most of the concepts proposed above. It's therefore not only the landing and takeo-off itself that's critical, it's also the secondary considerations like skewed landings, floating debris, hull draft (can you beach it?), etc.
I am guessing that there is a depth that the "submarine" can be below the surface where it maintains a pretty laminar drag bucket. The same boundary surface running at the surface of the fluid would have dramatic wake turbulence and the associated drag rise. So both bringing the submarine up through the surface (takeoff) and penetrating it back through the surface (landing) would be a draggy event and precarious in waves where the surface waves would mess with the overall shape of the wake. Sounds dicey to me. Seems hydrofoils have a lot of promise as a way to reduce the drag at the surface of the fluid. A three point landing on some skis that are at a relatively high angle of attack would be stable and using them as hydrofoils taking off until the wing brings them out of the fluid is the way to go. And that has been done recently.

#### Himat

##### Well-Known Member
I am guessing that there is a depth that the "submarine" can be below the surface where it maintains a pretty laminar drag bucket. The same boundary surface running at the surface of the fluid would have dramatic wake turbulence and the associated drag rise. So both bringing the submarine up through the surface (takeoff) and penetrating it back through the surface (landing) would be a draggy event and precarious in waves where the surface waves would mess with the overall shape of the wake. Sounds dicey to me. Seems hydrofoils have a lot of promise as a way to reduce the drag at the surface of the fluid. A three point landing on some skis that are at a relatively high angle of attack would be stable and using them as hydrofoils taking off until the wing brings them out of the fluid is the way to go. And that has been done recently.
If you just transition the surface at a sharp angle the wake drag could be mimimized. The side effects might be less pleasant:gig:. Didn't both USA and USSR research the flying submarine once?

More seriously, skis or advanced ordinary hulls are probably the way ahead. The Warrior is one example.

##### Moderator
Next, I have read somewhere that a submarine hull have less drag than displacement mode surface ship with similar displacement. As long as it is fully submerged, on the surface on the other hand it's much worse.
Funny that you mention it. This video in fact was the very inspiration to this line of thought:

And indeed, submarine's are 30-50% faster under water, which would mean 2 to 3 times (!) less drag submerged. That led me to believe that something like this (SWATH) might work, especially in long, narrow "torpedo's". In the end, for a 2 ton design, we're talking about 1X1X30 ft or so. But given the replies above, I guess that the dynamic transitional problems alone can prevent it this from being feasible, let alone the more practical issues brought up. Exit SWATH thus...

The catamaran, yes but why not make both hulls symmetrical about their centreline?
Unsymmetrical hulls and a list or different draft, and you might explore unwanted sideways lift from the hull, causing the airplane to turn of course.
Didn't think of the asymmetrical landing condition.

But the reasoning behind it is fairly straightforward. A normal float or hull has a lot of sideways spray, both increasing drag, ingesting the prop and blocking your sight. Having vertical sides, while the inner side works pretty much like a normal float could avoid all the spray problems. You also have some air-cushioning effect and it apparently works fairly well in the M80.

You could also think about it as taking the Privateer, well-known to some here:

And cutting the floats vertically in half, replacing their axis of symmetry by a vertical wall. Then place a rectangular fuselage over them, it's walls parallel with the outer (now vertical) side of the floats.

I'm not certain about the vertical spacing though, how much clearance you would need between the bottom of the fuselage and the water. I guess waves play a big role in that question, but would a spacing of 2' or so from float bottom to fuselage bottom be sufficient for practical operation?

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#### BDD

##### Well-Known Member
A submerged hull will have a significant drag increase when it breaks the surface. The streamlined shape of a modern nuclear submarine hull has less drag when fully submerged. With planing hulls, once on the plane, there is not much drag increase with speed and the drag drops to zero as you lift off which isn't a problem. Planing and displacement hulls are also much more stable than submerged hulls. The foils at the sides of the SWATH hull pictured above are probably for lateral stability and to provide heeling moments as needed. It would really be needed for a sailboat. The submerged hulls themselves don't add stability and the surface piercing foils wouldn't add much of a righting moment (it would be far less than a planing or displacement hull would which displace greater volumes). The submerged foils also wouldn't contribute much to stability until some minimum speed is reached.

Submerged hulls don't seem very workable for a seaplane. For a boat they would work if stability is handled and the drag of the piercing foils minimized.

I think I must have been typing while Autoreply was posting above.

#### BDD

##### Well-Known Member
Aren't the Privateer sponsons acting basically like tunnel hull hydropanes? I guess if the tunnel were wider it would provide more lift. These probably provide some tunnel lift and mostly provide a place for the water to go without spraying to the sides.

#### orion

##### Well-Known Member
No, there is no appreciable tunnel effect in the Privateer - in order to achieve the proper conditions the tunnel has to be a lot longer and be properly shaped.

#### Himat

##### Well-Known Member
Designing a well perfoming seaplane hull must be one of the more difficult tasks in aircraft/ ship design.

One observation I have done looking into this is the different placement of the CG on a V-bottom speedboat and on the step-forward part of a seaplan hull (or float). The CG is (and for different reasons have to be) more or less above the step on a conventional seaplane hull, on the speadboat the CG is more like 1/3 of the waterline lenght forward of the transom. The speedboat CG location is determined to planning at max speed. A seaplane without an afterbody and a CG like the speedboat would then maybe have a better planning hull, but rotation for takeoff might be impossible. The effect at a to hard arrival could be "interesting" too.

The longer the hull, the longer waves it will stradle, making it work in higher waves. This make the lenght/width less than optimal and exessive structure(weight) have to be carried making more drag in the air.

One idea I have been thinking of is a hull more like the shovelhead (round nose) hydros. At rest and at low speeds all of the hull will be in the water and at moderate speed planning at all of the bottom surface. Gaining speed it would be planing on just the forward sponsoons with the rest of the hull out of the water. With the sponsoons transom close to the aircraft CG the aircraft should then be free to rotate for take of. And no, it's not really my idea either, just a possible execution of the aft planning hull. (Search the NACA archives for planing tail hull.) And in principle quite similar to Autoreply's idea #3 as I interpret him, but with a narrower hull.

Maybe one day I get around to try it on an RC modell.

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#### Himat

##### Well-Known Member
But the reasoning behind it is fairly straightforward. A normal float or hull has a lot of sideways spray, both increasing drag, ingesting the prop and blocking your sight. Having vertical sides, while the inner side works pretty much like a normal float could avoid all the spray problems. You also have some air-cushioning effect and it apparently works fairly well in the M80.

I'm not certain about the vertical spacing though, how much clearance you would need between the bottom of the fuselage and the water. I guess waves play a big role in that question, but would a spacing of 2' or so from float bottom to fuselage bottom be sufficient for practical operation?
Making the hull throw the spray inward into the tunnel in a "cat" might be a god idea to awoid said problems. ORION pointed at one problem, how to make the hull behave in a sideslip arrival. The other consideration is the impact load the spray put on the structure. If the tunnel is not to wide, some air-cushioning might be had too. Just watch out and do not get a venturi that make the plane stick to the surface like an F1 car. Actually in a 1980ies modell aircraft magazine I did read about a floatplane with negative V-bottom on the float's.)

Spacing is depending on wave height but watch out, slamming forces from the waves are large. (They got it wrong on some high speed ferries here. The tunnel "roof" got ripped out by a wave.)

#### orion

##### Well-Known Member
One very key design consideration, hulls have a very narrow range of maximum L/D characteristics. The ideal trim angle from most hulls is about four to five degrees in relation to the keel line. For this very reason seaplanes and float planes do not rotate on takeoff. A typical float plane or seaplane comes up to speed, gets on step, and then at constant attitude simply flies off the water. Yes this may be helped slightly with elevator control but mostly seaplanes use flaps for takeoff and simply fly off the water when the plane reaches proper speed. Any attempt to rotate while the hull is still supporting a significant fraction of the weight will create a dramatic drag rise and could increase the takeoff run significantly over ideal.

##### Moderator
Bill, which hull shape would you deem feasible for a very wide (say 10') fuselage for a plane that's roughly 2 tonnes? Would the pontoons/floats of the Privateer, "glued" to the fuselage work, and if so, how much clearance does the fuselage need?

Do you see any merit in the above configuration with vertical outer sides of the pontoons/floats to reduce spray (drag)?

#### orion

##### Well-Known Member
A twin sponson design of some form would probably be the best. A ten foot wide fuselage would have a significant issue dealing with cross flow, spray and surface drag, and not to mention a sizable bow wave. A high finesse ratio hull would do better with the latter and could be design reasonably well for the other factors also. But a lot of this depends on design details and the other operational factors so each hull type would have to be investigated to find an optimum. I went through probably five or six hull configurations before selecting the Privateer layout - once the twin sponson configuration was gelled, the sponson design went through probably five or six more iterations before getting what you see in the picture. The bottom line is simply that for hull and seaplane design, there seem to be no quick answers if you're looking for any optimal qualities.

#### orion

##### Well-Known Member
http://www.centaurseaplane.com/
Not SWATH, Not Cat, but sort of Wave-Piercing...?
I was trying to remember this one - it seemingly has promise (there has been some Navy work on this hull type) but has been in development for as long as I can remember and still nothing beyond pretty pictures and an RC model. Given the initial statements regarding the design (about eight or nine years ago), I would've thought it to be flying by now.

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#### Himat

##### Well-Known Member
You gotta love a company with a model and a website complete with an investor relation tab.
Yes, but they can't have done everything wrong, they got paid to develop and produce two 4m wingspan Gull UAV's.

#### Himat

##### Well-Known Member
Bill, which hull shape would you deem feasible for a very wide (say 10') fuselage for a plane that's roughly 2 tonnes? Would the pontoons/floats of the Privateer, "glued" to the fuselage work, and if so, how much clearance does the fuselage need?

Do you see any merit in the above configuration with vertical outer sides of the pontoons/floats to reduce spray (drag)?
I'm not Bill but anyway, one possible elaboration of the wide vessel/ narrow hull is the SES, Surface effect Ship. A catamaran type hull where the tunnel is closed fore and aft and a lift fan provides air to lift the vehicle out of the water. There is the weight penalty of the lift fan and skirts, but a seaplane hull is a compromise, it's a matter of selecting penalties.

#### jhausch

##### Well-Known Member
I wonder what sort of performance gain one would see in flight if there were a movable fairing at the step? I am imagining something which pivoted well aft of the step and when lowered would prevent the low pressure area at the step.

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