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[Catalina]

Hello all.

Can anyone point to an on-line method to manually calculate wave making resistance for models or full size vessels. There is an abundance of computer programmes but a google search does not throw up any simple mathematical method.

[Bryan Young]

Probably because there is'nt one available to the likes of you and me. You are obviously interested in the subject, so why not try contacting any of the universities that have naval architecture courses or BAE who no doubt have test tanks for the purpose.

[Colin Bishop]

Google "Froude's Law" for the basic equations. Obviously a lot will depend upon the hull form. It's a pretty complicated subject!

[Catalina]

Thanks Bryan.
Seems like a good idea to persue when the model hull is constructed. Would have preferred to complete the design detail earlier, although making reasonable estimates should not require radical hull modification after tank testing.

Thanks Colin.
Froude modelling enables maximum speed to be established beyond which power requirements become unacceptably high and uneconomic but does not allow any finite calculation of wave making resistance at any particular speed. US Navy notes refer to a wave making coefficient C subscript W which should be a factor in a resistance equation somewhere, but have been unable to find it as yet. Will keep searching.

Thanks again to you both for your responses.

[Bryan Young]

Thanks Bryan.
Seems like a good idea to persue when the model hull is constructed. Would have preferred to complete the design detail earlier, although making reasonable estimates should not require radical hull modification after tank testing.

Thanks Colin.
Froude modelling enables maximum speed to be established beyond which power requirements become unacceptably high and uneconomic but does not allow any finite calculation of wave making resistance at any particular speed. US Navy notes refer to a wave making coefficient C subscript W which should be a factor in a resistance equation somewhere, but have been unable to find it as yet. Will keep searching.

Thanks again to you both for your responses.

Dear heavens. You have totally lost me with this lot. How far do you wish to take this notion? If you build a hull (at model scale) and then drive it at scale speed, you should find that the bow wave, wave flow down the hull sides and the exit should be about right.
I only have to look at the wave forms around model tugs (in particular) to see that the wave forming is as near right as you could imagine.
To get "it" absolutely correct you will have to float the model in a liquid that has an SG of less than fresh water. (Even paraffin will be too thick).
This problem always arises in films where the little water droplets are the size of boulders....and move at interesting speeds.

[Catalina]

Thanks again Bryan for your response.

I'm building a small ship (length =1.5m, beam= 0.25 m) to meet particular tasks. It is not a scale model, although it's size rates it as a model. Consequently no full size ship dimensions are available for the usual scale modelling techniques. I have no difficulty in determing required power consumption for this model, except for wave making resistance. It would be satisfying to calculate this resistance and make the power analysis complete. Of course, an educated assessment would probably be suitable, with predicted performance compared with actual performance on testing the vessel.

[Shipmate60]

Catialina,
One problem associated with operating model boats is that the environment that they operate in is not scale.
Models are usually overpowered to deal with this, as when you scale up the wind speed we regularly sale in hurricane conditions, especially on a gusty day.

Bob

[Ghost in the shell]

Thanks Bryan.
Seems like a good idea to persue when the model hull is constructed. Would have preferred to complete the design detail earlier, although making reasonable estimates should not require radical hull modification after tank testing.

Thanks Colin.
Froude modelling enables maximum speed to be established beyond which power requirements become unacceptably high and uneconomic but does not allow any finite calculation of wave making resistance at any particular speed. US Navy notes refer to a wave making coefficient C subscript W which should be a factor in a resistance equation somewhere, but have been unable to find it as yet. Will keep searching.

Thanks again to you both for your responses.

Dear heavens. You have totally lost me with this lot. How far do you wish to take this notion? If you build a hull (at model scale) and then drive it at scale speed, you should find that the bow wave, wave flow down the hull sides and the exit should be about right.
I only have to look at the wave forms around model tugs (in particular) to see that the wave forming is as near right as you could imagine.
To get "it" absolutely correct you will have to float the model in a liquid that has an SG of less than fresh water. (Even paraffin will be too thick).
This problem always arises in films where the little water droplets are the size of boulders....and move at interesting speeds.

And then you will have intersting ballasting problems as the weight of the model will probably not be scale and the model ballasted to sail in say liquid parafin, set to the waterline may not be as stable as an identical model ballasted for sailing in water

[Catalina]

Thank you all for your responses. It's good to have your comment.

When the craft is completed and on water I'll post some details and photos.

Stay well all.

[BarryM]

Maybe this will help? Model Boats ran a series of articles in 1988 (throw nothing out!) entitled "Engineering Working Model Boats". This covered Model Scale, Model Displacement, Stability, Screw & Rudder selection, estimating the correct model speed, motor revs and motor power. The author freely admitted that his calculations did not exactly mirror full-scale practice and involved a certain amount of assumption but, nevertheless, it is a very interesting and informative series for both a beginner and perhaps some more experienced modellers. The maths is no worse than a pocket calculator can manage.

If anybody wants a copy, please let me know. I don't think I would be encroaching on copyright as this would only be a mag extract from way back.

Barry M