Aluminium Hull Shafting

[Boa-Tr]

Hi, we are building 20 m aluminium hull motoryacht, and I need to understand how the sterntube detail should be. Is there any detail that we should check at first. Let me give some detail.
This is a motoryacht with 19-20 knots top speed and 12-14 knots cruising. There will be 2x435 hp engines installed.
We will use a v-drive and the length of the shaft (from coupling to P-bracket) is around 2.5 meters. This length is short and we think that there is no need for mid support through the sterntube. The sterntube will only be used for seawater seperation to the hull with a seal.
What is your recommendation for this. Aluminium sterntube with a seal or is there any innovative new solution? Do you have bad/good experiences with duplex stainless or 316L steel shaft and aluminium hull?

 

[steve123]

What is the shaft diameter you are proposing to use ?

 

[steve123]

Also what is engine max rpm ?

 

[Boa-Tr]

Also what is engine max rpm ?

Sorry for the delay. The engine max 3500 rpm
Shaft diameter is 50 mm at the coupling of reduction gear and 60 mm rest of the length.

 

[JSL]

Your naval architect should provide this info. If not, hire another one

 

[steve123]

The general rule for bearing spacing is more than 20 x shaft diameter and less than 40 times shaft diameter....you do need a second bearing.

 

[JSL]

no mention of the reduction ratio but working from your 50mm shaft I would guess your max prop size is about 700mm (28") diameter. That's fairly small props (small blade area) for a 20m yacht - unless it is very light. You might want to check this out.
All the data needed is not available but from what I see, the entire installation (shaft, prop, etc) appears a bit undersized: From my experience, more appropriate for a 12m boat