Bukh DV36SME

[petedg]

We have an LM30 fitted with a Bukh DV36 sail drive. The drive has a 2 blade folding prop and today I noticed increasing vibration when the engine revs were increased to 2000rpm. I’m concerned that there may be a problem with the prop and will have her lifted out next week to check. It set me thinking that if there is a problem and the prop needs replacement what would be the recommended prop for a DV36 sail drive? Is it worth staying with a folding type or would a standard non folding be more robust. I have no knowledge of how to spec a new prop and would welcome any advice.
Thanks in advance for any comments you have.
Peter

[Sula]

First of all don't panic - there is a good chance you won't need a new propellor. It may be something else (e.g. rope trapped round the propellor, folding prop not opening correctly, etc.) that can be fixed.

Propellor specification is a mix of science and art (not to mention blind prejudice and sales pitch!).

I can't imagine there is anything particular about the Bukh Saildrive or an LM that requires anything specific. The main thing is that the propellor diameter, pitch and number of blades match the available space, engine maximum power, propellor revs (considering engine max speed & gearbox ratio), and the hull type and maximum 'hull speed'.

If you do an internet search for 'propellor calculator' you'll find a range of calculators for prop size to give you a general idea of the size required as a starting point, but before buying a propellor do seek advice from a specialist propellor supplier - they.

A non-folding type will be more robust (and cheaper), but a folding (or feathering one) one should not usually be a problem. A non-folding or feathering one will reduce your speed under sail (perhaps 1/4 to 1/2 a knot). If I remember correctly this is mainly an issue at low boat speeds (i.e. in light winds) - at higher speeds it makes little difference, because other factors predominate.

Three blades will give more powerful drive, but also (if non-folding) more drag. A fixed two blade prop will (in neutral) tend to align itself behind the sail drive and keel.

Opinions vary greatly on the relative merits of folding vs. feathering vs. fixed, and between different brands of folding & feathering props, and the extent too which they enhance or reduce. You can find some good discussions on the merits and demerits of different types of propellor, and recommendations of UK prop suppliers on the YBW forums forums.ybw.com