[MHml] Engine power

[Alan Christoffersen]

Impressive work Martin!

Could you explain a bit more about what you have done. I can follow it 
through the slide showing kNewtons versus speed - and understand that this 
is for the bare wetted hull alone. So you get to power by converting knots 
to m/sec, and multiply with kN...? Do you have any idea of what influence a 
few degrees of heel will give? Probably alot of work needed to run sets of 
figures I expect?
Do you have any idea of the size of wind drag? I saw that John Shuttleworth 
has some figures on his site, that gave an equal amount of water and air 
drag on an open bridgedeck cat at around 12 knots, it will proably be at an 
even lower speed for Afterburner given the relatively high freeboard.

I don't understand the x and y axis values in slides 9 and 10, could you 
enlighten me?

So to get Afterburner to 14 knots we see we need 17 kW or 23 hp (according 
to the curve). Afterburner does 7 knots with a 20 hP outboard - where the 
curve reads about 2.1 kW or 2.9 hP.

23/2.9 = appr. 8

So the old guesstimate of doubling speed needing 8 times more power seems 
pretty valid!

If we continue down that line, then we have a "conversion factor" of 20/2.9 
= 6.9 times from curve to real life given the inefficiencies of the 
outboard, as well as the fact that it probably only runns at around 75%. 
 Also wind drag increases with the square of velocity and not the cube like 
the immersed hulls. So to get to 14 knots we need 6.9 x 20 x 0.75 = 103 hP 
minimum!!! More like 140.

So for 20 knots the curve read 43 kW or 58 hP. Multiply this with 6.9 and 
you have 400 hP!!!!!

Conclusion: You need to get her up on a plane at around 15 knots, or else 
it is not going to be easily doable.



Alan Christoffersen

-----Oprindelig meddelelse-----
Fra:	Martin Schöön [SMTP:martin.schoon at gmail.com]
Sendt:	4. februar 2007 14:35
Til:	Informed discussion of multihull issues
Emne:	Re: [MHml] Engine power

I have done some (w)reckless number crunching. The results are
summarised in a pdf-dcoument that can be downloaded at:
http://hem.bredband.net/mschoon/Modelling_Afterburner.pdf