Drag and speed prediction of rowboats

[Dolfiman]

I have developed a spreadsheet application dedicated to the drag estimation and the speed prediction of rowboats, developed with OpenOffice Calc and open source, here attached.

This application includes a drag estimation with 3 components :

• residuary drag, based on Delft series (parent models N°1 and N°25), and formulation as reported by Larsson and Eliasson in « Principles of Yacht Design » 2nd edition 2000
• friction drag, based on ITTC57 formulation for the friction coefficient (and L = 0,7 Lwl for the Reynolds number, as also recommended by the above authors).
• aerodynamical drag, with an average Cx= 0,5 applied to both the boat (frontal area estimated like ~ bow freeboard x beam) and the rowers (input the estimated area of rower(s) back(s) )
  

The data to input includes, for the the rowboat with its loading (rowers + their roaming eqipment if any) :
Lwl : waterline length
Bwl : waterline beam
Tc : hull draft
Disp. : boat displacement
Cp : prismatic coefficient
Sw : wetted area
LCB : Longitudinal center of buoyancy (in % of Lwl, from aft Lawl point)
Sa hull : hull frontal area (which can be estimated as bow freeboard x beam)
Srower : rower(s) frontal area
Net power : effective propulsion power (as equal to drag x speed)

The application includes also a speed prediction (on flat sea, but with various head wind forces) through the input of a net power. Question is what power should be introduced to represent the rower effort ? >> I think that for an average rower and an effort of one hour or more, a value of 40 Watt can be a valuable order of magnitude (and so 80 W for 2 rowers). Whatever one may think of this assumption (and you can input any another value), such a value is useful for comparing different designs, or different loads, or different number of rowers, in terms of speed and not just in terms of resistance.

The application also proposes 2 sets of input data (named Hull A and Hull B) in order that you can compared directly, through the curves automatical output, either a same rowboat with 2 loadings or 2 rowboat designs with the same loading.

3 examples are proposed :
** Adirondack Guideboat Ghost
** Monument River Wherry
** Race Drakeboat 18
, thanks to the data provided by @flo-mo in the woodenboat forum.
Fast rowboat inspired by Monument River Wherry and the Drake Rowboats - Page 2 (woodenboat.com)

In the figure here under, it is the speed comparison (boat speed in Knots versus head wind speed in knots, assumed flat sea) of the Adirondack Guideboat Ghost at either 125 kg / one rower 40 W (Blue curve) or at 225 kg / 2 rowers 80 W (Red curve).

By hoping this can be helpful for your rowbats projects

 

[wet feet]

I might not need to use such a potentially useful tool for quite a while,but I would like to express my thanks for all the work it represents.

 

[BlueBell]

Cool.
Well done Dolfiman.

My only other comment would be 40 watts per rower seems a bit low to me.
But it would depend, primarily, on the level of effort.
40W is pretty casual and could last all day.
Whereas 60 - 80W could be good for an hour.
While 100 - 200W might be a full out sprint.
This is fixed seat rowing, right?

EDIT: As you say, output is adjustable/variable on your spreadsheet.

 

[Dolfiman]

Thanks for your likes and kind comments.
Yes , 40 W is supposed to be a conservative value for a rowing with a fixed seat during several hours. I got this order of magnitude from a report about a rowing raid event in Sweden waters, where sufficient data on distance, time, light wind and quite no sea state conditions, type of rowboats, mass participation i.e. in good shape but not specifically racing champions (I don't found again this report in my database ..., too bad). The idea is just to have a value of reference for boat designs comparison.
I agree that 60-80 W can be considered for an hour effort (and/or with a sliding seat when compatible) , anyway you can test that or any other value with the application : you will see that the result in terms of extra speed depends of the increase of the drag slope. Practically, if you are already at around Froude 0,35 with 40 W, it is not very productive to force to 80 W, you will not have a lot of extra speed.