Speed vs Engine Revs

[DBA Forum (B) - Daniel Boekel]

I did some measurements last weekend, here are the results:
Loading Image...

LWL 24m x 4,7 x 1,2
60-70 tons?
prop: 32x15 (but that doesn't fit the measured numbers, pitch must be bigger)
rpm = prop rpm, electric motor directly on propshaft
average fuel consumption about 10 ltr/hr - but that is running 3/4 throttle and towing a lot of other boats...


2014-09-09 12:10 GMT+02:00 DBA Forum (B) - Colin Stone <dbabarges-pvYRptiajiAdnm+***@public.gmane.org>:


Intrigued by Charles Fitzhardinge's comment on Sp vs Engine Revs - erpm - in a recent post, (8+kph/1200erpm) I thought that a small survey would be of interest to see what the figures are for the same 8kph.



I guess LWL, Displacement, prop size and gearbox reduction are also important and overall fuel consumption.



So for KEI:

LWL 20m (BWL 4.85 Draft 1.1) 70 tons 30x20inch 4 blade reduction 2.5:1.  800erpm 8kph. Approx 1 kph/100erpm.  Fuel 2.66ltr/hr this year.



Previous same size prop with much thicker blades was 2kph slower for same revs.



Any other stats?  Will longer LWLs have higher speeds/erpm?



Colin Stone

KEI



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[DBA Forum (B) - Charles Fitzhardinge]

I am surprised at the interest in my originally off-the-cuff comment on boat speed vs rpm! I thought perhaps I was the only nut interested in "Volts and Amps" (as I was described in my "second retirement" spent making and selling electric boats - barging is my 3rd attempt!

As my working life was spent as a consulting engineer (with only an amateur sailor's connection with things marine) I cannot help an interest in how (and why) things work, and in application "Mathematical Models" for predictions - however as an old fashioned Engineer I also retain a healthy respect for the limitations of the assumptions and simplifications on which all such models are perforce based (unfortunately the average younger engineer seems prepared to accept the computer output without concern for the assumptions made by the programmer)!

For those with similar interests I attach a sheet with a fair representation of mathematical formulae for predicting boat performance.

For those who prefer I also attach a sheet with a nomograph for predictiong boat performance which I found useful when fitting electric drives to small (up to about 10m) boats. Out of interest I have superimposed the relevant particulars for Anna Maria. Of course the advantage of a nomograph is that one only needs a straight-edge to make the prediction! A look at the first sheet will give a hint of the assumptions and therefore the limitations of the nomograph, but for a rough prediction it has done very well in my case.

Charles Fitzhardinge
De Vrouwe "
[attachment=224]BoatPerformanceFormulae.pdf[/attachment]


[attachment=225]AnnaMaria-EstimationofSpeedvsPowerwithActualMeasurementofSpeedvsEnginerpm.pdf[/attachment]

[DBA Forum (B) - Kevin Wade]

My boat has constants such as shape, weight, HP, length, width, gearing, prop, etc, etc. My observations are that, width, but mostly depth of water below me is the biggest changer of speed V's RPM equation.

[DBA Forum (B) - Charles Fitzhardinge]

As stated on the sheet with my speed vs rpm results, these tests were carried out in a pretty deep (6m - joys of the Netherlands) lake, so no restrictions on the water finding its path below and around my vessel.
It is basically obvious that if the water can only get behind your boat via a very restricted remnant channel your resistance will increase enormously and your speed over the ground will slow accordingly (your boat will still be travelling well through the water but as the water rushes past as it tries to fill the void you would otherwise leave behind a considerable adverse apparent current is generated)! You only need to watch a large laden commercial barge crawling into an empty "montant" lock to see this effect at its extreme.
When starting from scratch and trying to estimate the speed a given boat should achieve with given engine power (and appropriate gearing, propeller.etc) you can only assume water conditions are good.
Charles Fitzhardinge