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What speed increase can one expect by saving weight ?

QUES: Salut Mike.   First, thanks for all the great info you’ve posted over the years. To get all this free c'est incroyable and should help us get better boats down the road.

My question is this… if I save say 10% in boat/build weight, is there any easy way to estimate what speed effect this might have?                                                                                           Jacques V,… France

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ANSWER: Thanks for your appreciated words Jacques, but better to share it free than lose it all in our inevitable dust ;) 

For anyone who knows me, they will know that I will enjoy this question, as I love playing with numbers;)  But to make sense of numbers we need some way to keep them in order (like scientifically-based formulae) and a stack of good data.  A precise answer would still be almost impossible due to the many variables like sinkage rates, weight distribution & form changes with sinkage etc.  But I think we can make a reasonably close stab at it 'with that stack of data'. 

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Back in the 50’s while studying in the UK I was able to access some local tank-test data and then in the 60s’ also came across a technical summary of resistance results of some 100 ships of various types and speeds that has proved useful over the years.    As we now know that ship models can simulate large ship residuary (wave) resistance by comparing their Speed/Length Ratio (SLR = Speed/Length0.5), this ship data can be a very useful source for small boat analysis, which for itself, could never justify expensive tank tests.  (I used this to help design the W17/W19 series)     Frictional resistance will vary more by surface area and roughness, but for each boat type, there is now enough data available to assign a reasonable % of the total resistance to friction, to get ‘a ball-park’ value for the TOTAL resistance at each Speed/Length0.5 ratio.   As this has interested me in the past for making quick power estimates, I am happy to share these graphics with you..

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First, there is the data from the 100+ ships noted above … a graph giving their Mean Average Resistance per Long Ton (fractionally bigger than a Metric ton), plotted against their design SLR.  This means they were (in the ‘50’s) optimized for that SLR.    Now while it's very possible today to find vessels of lower resistance per ton, it's not very important for how I will be using the data, as I am principally interested in the RATE of Change for different SL ratios .. and that will be similar, even if all the figures were to be reduced by some percentage.

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This graph is old and a page from my old ‘black book of data from the last century’, so bear with me.     

It shows an Average Mean curve of Total Resistance (in lbs) per long ton (2240 lbs) of Displacement for a very large number of vessels, plotted against their Designed Speed/Length Ratio, as noted along the base.

From the clearer original, I was able to lift off the plotted specific resistance values for 8 selected SLR’s (noted below as SLR-1) plus the higher specific resistance value for a SLR roughly 10% higher (noted as SLR-2), which effectively gives me the slope of the curve at that point.  This permitted me to ‘see’ the extra resistance per ton for a raised speed.  Based on that slope or rate, I then adjusted the value to equate to a 10% increase in displacement to find the new speed given by the adjusted Speed Length ratio, all “autocalc’d” through the magic of spreadsheet formulae.

The figures are noted in the table given below. 

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If we now plot the Speed/Length Ratio against the % Speed Increase, we can get a curve showing what might likely be the gain for a specific vessel when designed for the SLR of interest.    This plot is shown below.    It’s now easy to see the effect of the much higher wave drag as the boat reaches maximum displacement speed, and how that reduces the overall gain possible at this critical speed, to about half.  But equally visible is the gain possible once one is going at or above, a SLR of 2 (10kts for a 25 ft waterline), when the ‘efficiency returned’  can probably rise over 80% for fast vessels that either plane or can slice through the water with very low wavemaking.   

              

So we can now see with some realistic numbers what many of us have known or sensed for years .... that for boats not able to exceed their displacement speed due to their hull form, weight savings will not give back much speed gain (or inversely, a wave-speed-limited-displacement-mono can carry extra weight without too much speed penalty), but for boats designed to surpass a Speed/Length Ratio of 2, the speed gain due to weight saving can indeed be worthwhile.   Keep in mind this is based on a Total Displacement reduction, not just a boat weight reduction.

Mike Waters ….. 2022

NOTE:  As this well researched information is given out FREE, I hope I can ask readers a small favor.   If you also visit the Forums like BoatDesign.net and see something being discussed that perhaps one of my articles can shine some light on, please post for them a website reference so they can visit here,  Who know, perhaps they will like one of my boat or mast designs and bring in funds to cover another article.    Thanks guys, ,,,, we can all help each other a little here.

 

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