Engine sizing

The guys who own Maringret, another HR41, have put up a calculation on their blog at http://maringret.wordpress.com/maringret/projects-41/p41_engine/ of how they sized their new engine. It’s a great step-by-step calculation which shows an application of the chapter “Engine selection and installation” in Nigel Calder’s book Marine diesel engines (and if that doesn’t interest you, you should check out their collection of links) . Here’s how our calculations (leading us to aim for a 40hp engine) compare with those of Maringret (leading them to target 60+ hp).

Boat                                                 Maringret        Anna #1        Anna #2          Anna #3
LWL, m (ft)                                     10.4 (34.1)          10.4 (34.1)     10.4 (34.1)       10.4 (34.1)
Bow wave velocity, knots             5.84                     5.84               5.84                  5.84
SLR (theoretical)                            1.34                      1.34               1.34                 1.34
Hull speed (theor.)                        7.83                      7.83               7.83                 7.83
Target max speed, knots             ??                           6.5                 7                      7.83
Displacement, tons (long tons)   18 (18.29)            14 (14.2)       14 (14.2)         14 (14.2)
Water resistance, lbs / long ton   55                        20                  28                    55
Total resistance, lbs                        1006                  284                 398                 781
Effective hp                                       23.6                   5.5                   8.4                 18.3
Propeller efficiency, %                     50                      65                    65                  65
Required hp                                      47.2                   8.5                   13                  28.1
Adverse allowance, %                      33                      33                    33                  33
Target hp                                           63                      11                    17                   37

The calculations are a great illustration of how the energy required increases exponentially as you try to push the boat faster than the bow wave. And that most boat issues are a matter of compromise and a consequence of intended purpose.

On the Volvo Penta web page, there’s a nice little engine configurator to identify the recommended engine size depending on length of waterline, displacement and required top speed;

http://www.volvopenta.com/VOLVOPENTA/SE/SV-SE/MARINE_LEISURE_ENGINES/ENGINE_CONFIGURATOR/Pages/engine_configurator.aspx