Propellor Size, OM636

[elenya]

Good Day All

We are going to be replacing the propellor on our Saga 40 as its really old and well past its best. The current prop came with the boat and is a 18x12, with an inch taken off the pitch but not marked on the prop.

I wondered what prop other vessels with a Mercedes OM636 were running, and via what gear reduction.

The present prop is two bladed and only allows us to make 2300 rpm, against a rated 3200 rpm. Engine pulls full revs out of gear, but sounds like it wants to take off!! We get no black, or any colour smoke from the engine at any revs under load.

I am think of going for a three blade folding prop but thought I would ask other owners of the OM636 what they are running.

many thanks for any help.

have a good new year, its nice and sunny in New Zealand, and oh so warm.

regards Jim D

 

[Bajansailor]

It sounds like you are 'over-propped' as you are still 900 rpm less than the theoretical maximum.
It might be worthwhile coming down a bit on diameter and pitch, and going for a 3 bladed propeller rather than 2 - there should be less vibration (all else being equal) with 3 blades, although there will be increased drag on a non folding propeller.
We have a 3 blade fixed propeller on a 6 tonne 35' long keel sloop with a 41 hp (maximum!) Volvo 2003T engine, and it seems to work well.
A folding propeller might cost a lot more than one that is fixed, never mind that the drag will be less - however I dont think that this issue re drag should necessarily be a major disadvantage on a heavy long keel yacht.

 

[damo]

I've got a Warrior 35, and the OM636 drives a 16x14 3-blade fixed prop via a Hurth gear box.
I get about 3600rpm with the gear box disengaged, and about 2600 in gear.

This is a pretty standard setup on this boat, although I know other owners have 17" fixed, or folding props, but I don't know if that is with the OM636. (Variations are pretty restricted because of the size of the prop aperture)