# Resistance: Surface Running vs Sumberged 'SWATH' and Wave Making Impacts

Discussion in 'Hydrodynamics and Aerodynamics' started by zstine, Apr 14, 2022.

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### zstineSenior Member

I'm interested in comparing the resistance (as in which requires the least power) of a submerged torpedo to a surface running hull operating at 10 to 15 knots, the length is 50ft max, and given that each hull must support the same weight (10 ton), neglecting the resistance of struts. The submerged form could be shorter if that lowers total resistance by minimizing surface area (I avoided speak of Froude number as the two can have different Froude numbers at a given speed since the torpedo may be shorter). I assume the max L/D on the surface running hull is most efficient, but max L/D has more effect on increasing surface area of a submerged torpedo and so a shorter, wider torpedo may have less drag in the speed range given. See figures

aside: From my research, it seems that wave resistance is generally understood as an Empirical Formula. It includes the energy transfer to generate surface waves (pressure waves manifested on the free surface), as well as the interaction of those waves with each other (adding and subtracting) and their interaction with the hull (squat/gravitational effect). It seems as though the big hump is associated with the increase of potential energy (mass * g * height) of a planning hull vs displacement hull, where the former is physically higher than the latter. Though in the numerous papers I've read on wave resistance, I've never actually heard any author claim the hump at the displacement-planning transition is in part the energy associated with increasing the potential energy of the vessel, just words like "climbing up the bow wave"... I digress.

Since a sufficiently submerged torpedo would not have free surface wave action is there "wave resistance" at all? Certainly there's a submerged pressure wave from the frontal area of the hull form, but is this not what is predicted in viscous drag equations? Do these pressure waves follow the same wave length principle as surface waves (below) and cause humps and hollows in the resistance as the bow and stern pressure waves interact? Apparently not based on the viscous drag equations.. I would conclude there is no "drag hump" from wave resistance in a submerged torpedo, correct?

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### philSweetSenior Member

This may sound weird at first, but the thing you need to keep constant when making the comparison is the stability margin of the two vessels. Wave making resistance gets worse when stability is increased. So figure out what the cg of the two vessels will be, and set a GM value that is comparable for the two vessels. It's real easy to make a completely unstable boat efficient. So the density of that ten tons is very important. Ten tons of marshmallows vs tens tons of tungsten will change the answer.

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### zstineSenior Member

Given that the Hab spaces are identical and the 2 demi hulls are close to identical, the torpedo is 100% displacement and the surface running hull is 110-120% displacement, then it is safe to say the difference in weight of the 2 demi-hulls would be negligible. And we can assume both vessels have identical C.G. several feet above the water line. The GM gets interesting. In this concept, the vessel only runs on one demi-hull while under way and uses active steering to balance the boat on the hull, like a bicycle. So the boat is completely unstable without active control. During slow speeds the habitability space becomes waterborne and is then statically stable. So how do you determine which hull is optimal?

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Optimal - For what parameter?

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### zstineSenior Member

minimizing drag at 10knts and again at 15knts... given the 50 max length as per post 1. This would be just before and in the range of the drag hump. But the submerged torpedo should not exhibit the displacement-planing transition hump. Also, I find the vessel SLICE interesting in that they shortened hull length to increase Froude above the hump and so the optimal L/D may be shorter for a fully submerged demi-hull. I'm assuming max L/D is best for surface running.

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You can't minimise a single variable in 2 different operating conditions. By definition it is no longer 'minimal' if it already minimal in another condition!

That's the 'theory' but in practice, not so. The length-displacement ratio has a far greater effect, which has increased.

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### zstineSenior Member

I wasn't suggesting the same hull form would be optimal at both 10knt and 15knt speeds. I'm curious whether the optimal hull type (surface or submerged) will change between the pre-hump (10kn) and in the hump range.

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### Robert BieglerSenior Member

That sounds like the HYSWAS (Hyodrofoil Small Waterplane Area Ship) concept, and this talk mentions 50+ academic papers written on the topic:

Perhaps that keyword finds you a paper with the answer.

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### zstineSenior Member

The 'submerged torpedo' concept is similar to a HYSWAS, but water plane is as close to zero as structurally possible. The surface running concept that I want to compare it to, is not HYSWAS of course. I'm not confident I can get realistic results trying to do hand calculations with an estimated Cd. I've looked at some HYSWAS papers, generally, they are fast, much much higher than Fn 0.6. So the data is not relevant. I'm guessing there's some software people have available that would make this easy and more accurate.

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