Jerry Armitage is planning to replace his boat engine, but will the old propeller work? PBO expert Tony Davies has the answer
We are looking at replacing our Westerly Griffon’s 40 year old Bukh DV20 diesel engine with a somewhat newer DV24.
The DV20 has a 2.5:1 gearbox while the DV24’s is 3:1.
We were wondering whether we would have a problem using our existing stern gear – a two-blade 14in x 13in propeller on a 25mm shaft?
If there will be problems, would we be better off changing the old propeller or trying to source a 2.5:1 gearbox for the new engine?
Tony Davies replies:
As I understand it the old DV20 produced 20hp at 3,000rpm while the newer DV24 produces 22.4hp at 3,000rpm, then goes on to produce 24hp at 3,600rpm.
Moving from 2.5:1 drive to 3.0:1 generally means a larger/coarser pitch propeller as the prop is running more slowly.
So, the new engine will easily turn your old prop without any further modifications but the prop will be running more slowly meaning less speed through the water.
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However, if you used the full 3,600rpm of the new engine then you’d achieve the same performance.
In reality you’ll probably never use the maximum revs – either with the old engine or the new – but the same idea works through the rev range.
You can therefore use your old prop with the new engine and gearbox to begin with, and then replace it with a closer match at a later date.
If budget allows, when you replace your prop I’d also recommend you consider a three-blade folding-type propeller as you’ll notice a huge improvement in performance, particularly in reverse, over a two-blade, as well as less drag under sail.
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