# Displacement for a racing Paddleboard, is this correct?

Discussion in 'Hydrodynamics and Aerodynamics' started by Claudio Valerio Parboni, Jul 17, 2018.

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### Claudio Valerio ParboniJunior Member

I'm designing a racing flatwater SUP for myself, I just wanted to know If my reasoning is correct.
So, I weight 85kg, and I estimate the final board to weight about 10 kg so the displacement of the board should then be 95kg right? I'm not entirely sure since racing SUPs tend to pitch slightly, rise and sink during and after the paddle stroke. should I go for a slightly lower displacement when designing it?

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### HeimfriedSenior Member

Hi Claudio,
"... go for a slightly lower displacement ..." sounds a bit strange to me. I know nothing about SUPs but a little about hydrostatics. If the mass of SUP plus the mass of the paddler is 95 kg, then the SUP will displace 95 kg of water (in calm conditions, in absence of additional forces). In your design plans: what fraction of the SUPs entire Volume will be immersed in this condition?

Last edited: Jul 18, 2018
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Claudio, Heimfried knows a lot more than just a little about hydrodynamics. He is a modest guy.

I suspect that you should design for an overall displacement of considerably more than the 95 kg that you mention. The videos that I see show that the boat/board has perhaps half to two thirds of the boat above the waterline. If that is to be taken as proper then the total displacement of the board should be somewhere between 190 kg and 285 kg.

Consider that if the boat is taking water over the bow, it will almost surely make the progress slower. .... hulls with pointy sterns will probably be easier to paddle if the objective is ease of effort and being satisfied with slower speed. For a hot rod board I would think that a broad, nearly flat, stern section would be more desirable.

Maybe we can entice some of the other members to comment about design criteria for floating objects like SUPs.

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### BlueBell. . . _ _ _ . . . _ _ _

Closer to double would be prudent.

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### SamSamSenior Member

With those numbers that would theoretically place the waterline at the top of the board.

For simplicity, say you had a (weightless) shape that was a set length and width, and was 5 centimeters thick, and it's displacement was 85 kg. If you put 85kg on it, it would be level with the surface of the water. If you wanted it to submerge only half way with 85kg on it, you would have to double the volume (thereby doubling the displacement) by doubling the thickness to 10 centimeters.
Of course as you say, you have to add the weight of the SUP itself into the picture, and as you increase the volume of it, it's weight will increase, so it becomes a little bit of a juggling act unless you can accurately predict how much more it will weigh.

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