log location

[30boat]

I am replacing the original sumlog in my westerly with an electronic one(Nasa).
The previous log was sited a litlle forward of the exit point of the propshaft just aft of the engine.This is an unusual location for a log on a sail boat but it would be convenient for me to use the already existing hole for the new transducer.Does know of any reasons for not doing so?

Thanks

 

[Shanty]

Sounds like the transducer will be sitting just aft of the keel(s), and thus will be in pretty turbulent water. It probably won't be very accurate in that location. It would be better sited just forward of the keel, so that it sees a clean laminar flow (or as near as you are going to get), and thus has a chance to give a reliable indication of speed.

 

[MikeKopman]

If it's just fwd of the prop shaft does this mean it's just aft of the keel?
Could be all sorts of turbulence there... if i remember correctly the recomended place is close to the centreline, about 1/3 of the way aft on the waterline.
Mike

 

[Rob_Webb]

On all my previous Westerlies, the sumlog impeller has been located halfway between the prop shaft case and the port keel. And I never had a problem with the reliability or accuracy. I always presumed that if fitted from new, someone as competent as Westerly would have considered this and not done it unless proven!

However, my latest boat uses a (Nasa) paddlewheel impeller which is located on the centreline and slightly in front of the fin keel, which presumably gives it very clean water flow. It could be that these electronic devices need smoother water than the sumlog and must therefore be sited in such a place.

To be honest, I would take more advice before proceeding. The temptation is to try the original site although I don't think the sumlog mounting hole would be a good fit for the Nasa impeller, so you might not be saving yourself work in this respect (other than cabling routing). And the risk is that the Nasa gives unreliable readings here, that even the wide calibration facility cannot overcome - requiring you to re-site it forward anyway.

Sorry not to be conclusive!

Rob.