distance from log to prop

[emnick]

I would like to re install our rope cutter but have since added a folding prop. To get the clearance for the folding prop and the Ambassador I need to space the shaft by 10mm. It will protrude from the cutlass more. Will this then make the prop too far from the log / cutlass? should it not be more than the dia of the shaft (30mm) I am on that now.

Thanks for any help

 

[sailorman]

dont forget allowance for the prop pushing the engine mounts forward

 

[emnick]

That would make distance shorter? my problem is I think it might be too much distance and perhaps induce some vibration? Appreciate the advice I had forgotten that.

 

[earlybird]

I don't think there's a problem. I have a Spurs cutter between a Flex-o -Fold prop and the cutlass bearing, with an extra ~4mm gap to allow for engine movement in forward gear. Shaft is 1".
Works fine.
Edit; my Beta mounts are fairly stiff, so 4mm clearance is possibly on the low side for other engines.

 

[sailorman]

That would make distance shorter? my problem is I think it might be too much distance and perhaps induce some vibration? Appreciate the advice I had forgotten that.

And needs to be allowed for

 

[vyv_cox]

From a rotordynamics viewpoint a rule of thumb is that the clearance should not exceed the shaft diameter. In many cases it does, especially if the shaft is short and fat, but where it is long and slender there is a risk of resonance and high vibration.

 

[NormanS]

I don't know how much space is required for a rope cutter, although I have one, but so do lots of other people, so it's obviously not a problem.

 

[Len Ingalls]

Also should be an allowance of additional 4mm or so for water to lube the cutlass.

Cheers/Len

 

[vyv_cox]

I don't know how much space is required for a rope cutter, although I have one, but so do lots of other people, so it's obviously not a problem.

The required gap for a 30 mm shaft is 36 mm for a Stripper. Based on the OPs photo his gap looks close to 2 x shaft diameter.

 

[emnick]

I guess I should have used a more up to date photo. When the Bruntons auto prop is fitted the gap is reduced...but still not enough to get the cutters in, hence adding a 10mm spacer. I guess I could get the spacer turned down some? Vyv thanks for the input, do you think I will cause problems then ? perhaps I should re post when all fitted but its a bit of a pain to fit the spacer especially as I used loc-tite stud lock on the bolts last time.

 

[vyv_cox]

I guess I should have used a more up to date photo. When the Bruntons auto prop is fitted the gap is reduced...but still not enough to get the cutters in, hence adding a 10mm spacer. I guess I could get the spacer turned down some? Vyv thanks for the input, do you think I will cause problems then ? perhaps I should re post when all fitted but its a bit of a pain to fit the spacer especially as I used loc-tite stud lock on the bolts last time.

I can't really guess for you as I don't know your details. My shaft is 1", 4 ft long on a P-bracket and Aquadrive. The prop is a Bruntons, therefore quite heavy. (In rotordynamics this is a poor situation - long slender shaft with poorly supported heavy overhang, perfect situation for stage 1 critical resonance) You can see my previous arrangement here http://coxengineering.sharepoint.com/Pages/Pbracket.aspx but the Stripper has now gone, replaced by a Prop protector with unchanged gap, as suggested by Ambassador, 36 mm. When motoring in following seas my shaft deflects at certain revs, ~2000 rpm and the shaft clatters against the stern tube. The drive is quite noisy at the same revs and many loose items in the boat resonate. We also get clashing of the shaft and tube when manoeuvring, alternating between forward and astern. I suspect that reducing the overhang would eliminate this but it's a bit of a job to do and I have not got around to it.