shaft

[dsw]

how much shaft should be showing from the back of the boss to the end of the stern tube ?

 

[eyehavit]

My rule of thumb for conventional drive shafts is preferably:

(1) Not more than shaft diameter x 1
(2) Not less than amount of forward axial movement + 2mm

 

[Tranona]

Thats only a rule of thumb. There is often more. The "norm" is about 15mm to allow water to flow in and out of the tube. Less may cause problems with cutless not getting enough water. If you fit a rope cutter the gap has to be around 40mm on a 25mm shaft - well above the 1 x shaft diameter "rule" and causes no problems.

Is your question because you have a large or small amount showing?

 

[eyehavit]

Are you asking me or dsw?

 

[Bilgediver]

(1) Not more than shaft diameter x 1
(2) Not less than amount of forward axial movement + 2mm

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I would suggest in some circumstances these do not work out as the best solution.

There needs to be adequate gap between the propeller and cutless bearing for a free flow of water to flood and circulate in the bearing and I feel 2mm does not meet this requirement. In fact one only has to do an eyball survey around a marina to see that most folk agree with me and it seems that a normal gap is around 2/3D on a 1" shaft .

If the propeller sits inside a cut out then this also may be a determining factor

 

[dsw]

i have no shaft showing at all but there is plenty of adjustment so to let the shaft slide back and the shaft is 48mm and the prop is 27x16 stern tube is flush with the steel hull.

 

[Tranona]

Following the previous posts, if you have a conventional water lubricated cutless bearing then you need a gap to let the water out - 15/20mm. You may on the other hand have an oil or grease filled tube with a seal on the outboard end in which case you do not need a gap.

 

[eyehavit]

"You may on the other hand have an oil or grease filled tube with a seal on the outboard end in which case you do not need a gap. "

As long as there is zero axial movement, assuming his engine is solid mounted as opposed to elastomer mounts.

Elastomer mounted engines under full load can move as much as 9mm.

 

[eyehavit]

To any "Rule of thumb" there will always be exceptions dependant on individual circumstances. That is why I said "Preferably".
I dislike grossly extended shafts as they place undue stress on the aftmost prop shaft bearing.

 

[Tranona]

Will be interested in DSW reply. Unlikely that the engine will be solidly mounted. More likely to be flexibly mounted but with flexible coupling and thrust bearing such as an Aquadrive or similar. This allows the shaft to be effectively fixed with no fore and aft or axial movement. An oil filled tube with lip seals such as used on ships can then be used. Have seen this kind of arrangement in Dutch steel boats.

 

[chrishelen]

How is your stern tube lubricated?

 

[dsw]

Yes my engine is flex ably mounted with a R&D coupling on the gearbox then another R&D coupling in-between two split shaft couplings where the main shaft then goes through a pillow block bearing then into the stuffing box /gland .and the stern tube is water cooled

 

[Tranona]

You say water cooled, but does it have water lubricated cutless type bearings? If so then there needs to be somewhere for the water to come out of the tube because cutless bearings rely on a continuous flow of water.

 

[TigaWave]

4mm is fine as a minimum for water exit, we set 3 as a minimum in cutless and stern tube bearing installations. But it sounded like your thrust loads are taken on the engine mounts, in which case add another 9.00mm for foreward movement = 12.00mm min
If the pillow block also has thrust bearings then 4-5mm as a minimum is fine.