Tag Archives: Vetus M4.17

Selecting a new engine

On a previous boat, a HR35 Rasmus, we replaced the original Volvo Penta MD21 with a VolvoPenta D2-40. On paper, this was a change to a weaker engine, but we were very satisfied after three years and 200 hours of running time; the physically smaller engine made access to service points easier and the fuel consumption decreased by almost 50%.

So, considering that we are planning to make a rather long trip in the summer of 2010 with our HR41, we are thinking of replacing its 32 year old Volvo Penta MD21. We have been running the numbers in Volvo Penta’s motor guide, checked the power calculations in Nigel Calder’s book “Marine Diesel Engines” and recalled some rules of thumb:

  • 1.5 hp per thousand pounds of displacement, ie 44 hp
  • 1 hp per foot of length, ie 41 hp

This has made us aim for a 40 hp engine and we have decided to go for a Vetus M4.17, which has a power curve not very different form the original MD21.


The MD21 was originally marketed as being 65hp and even 75hp sometimes. According to Volvo Penta’s own product specification, the MD21 did produce 61 hp (on the prop shaft) at 4,500 rpm, but this was only permitted for shorter time periods. For continuous use however, the maximum permitted rpm was 3,000 – at which the engine produced 45 hp at the prop shaft. The engine data is available in the data sheet from Volvo Penta: md21a