K section belt power ratings for 24v alternator

Nick Newington

Further to my note about converting from V belts to serpentine system on the Amel 54 and others.

I went on the Optibelt site and had a good look at the rating tables and the engineering section.

Very broadly, for 180 degree wrap and belt tension of 10 kg per rib and a small pulley of 63mm. There is a correction for less wrap but I did not do the calculation as it is minor for 160 degree wrap.

At 2000 rpm each rib can support 0.7kw, but it goes up with RPM so at 2300 it is about 0.8kw and at 6000 more like 2kw.

At 2000 rpm the alternator only just kicks in but 8 ribs could support 5.6 kw

At  2300 rpm it would be 6.4kw 

Seeing at the Leece-Neville maximum output is 175A at 24v which is 4.2kw  the the K section 8 rib is over engineered but in my book that is no bad thing.

I guess there is a small efficiency loss but I think that is only about 1%

I am delighted just in case I want to upgrade to an even more powerful alternator in the future. Maybe if I were to go Lithium for example.

The other thing is that Amel provided idler pulleys, top and bottom on my boat. The idlers are about 50mm diameter. The Optibelt site clearly states that if idlers are used they should be double the diameter of the small pulley or larger. They also state that if possible they should be only on the slack side and that they cause belt life to be shorter. To be fair the belt deflection was minor.  And I think they are talking about an idler with a lot of wrap. See photo.

Given the over engineering of the K section 8 rib belt I will probably do away with both the idlers.

My conclusion is that Amel got the engineering of the 24v 175A alternator all wrong on the early 54’s. It has taken me three years to figure this out, primarily because I assumed that Amel would have got it right. Especially as there were no problems with earlier Amels that had the same alternator. Some bright spark cocked up the pulley engineering.


S/Y Amelia Aml 54-019

Leros Gr.