homer
Well-Known Member
Is it possible to straighten a very slightly bent prop shaft ? And, if so, what is the best way to do it? Any experiences?
bent prop shaft
- Thread starter homer
- Start date
vyv_cox
Well-Known Member
Yes, it is possible. The procedure is to apply heat on the inside of the bent point, tapping occasionally with a light hammer to release the stresses. Specialists who do this with big shafts take 20 years to learn how to do it. In your case, you don't have a lot to lose, so go for it.
I have seen an expert straighten a reciprocating compressor shaft 5 metres long, weighing 7 tonnes, using a propane torch and a half-pound hammer.
I have seen an expert straighten a reciprocating compressor shaft 5 metres long, weighing 7 tonnes, using a propane torch and a half-pound hammer.
PuffTheMagicDragon
Well-Known Member
It can be straightened, BUT ......
You need 2 good Vee-Blocks, a dial indicator (sometimes called 'a clock gauge'), a hydraulic jack fixed to a frame so that it can push down, a very solid workbench and lots of patience!
Support the shaft on the vee-blocks, place gauge against it and slowly rotate the shaft while watching the indicator needle (the part that goes round). Mark the spot that has the highest deflection with a felt pen. Secure jack in position as close to the gauge as posssible and push the shaft down at the point where there is the mark. Push until the deflection is close to what it was before - but on the other side. Leave everything as is for a couple of minutes. Release pressure from jack and check / rotate the shaft once more, noting any progress. Repeat as necessary...Repeat as necessary...Repeat as necessary...Repeat as necessary...Repeat as necessary......!
That's as far as the theory goes.....!
NOTE that if the unsupported length of shaft between its bearings on your boat is close to the limit (approx 40x the diameter) it will behave as a 'slender' and any inaccuracy will result in whipping as your revs go up, rapidly getting things back to square one.
Since you ask how it's done I imagine that you that you are not an engineering person (no offence meant) and therefore it would be unlikely that you would have the necessary bits.
While it is not rocket science, it is not quite so easy unless done in a well-equipped workshop; more so since you have never done it before.
I would therefore suggest that, relistically, your best course would be to get a quotation from an engineering workshop as well as the cost of a replacement shaft and then decide which is more cost-effective. Me, I'd just make myself a new shaft, though I do have the equipment and the background. Sorry!
(I have assumed all along that you cannot claim from insurance: you did not say what bent the shaft).
Whichever way you go, good luck!
You need 2 good Vee-Blocks, a dial indicator (sometimes called 'a clock gauge'), a hydraulic jack fixed to a frame so that it can push down, a very solid workbench and lots of patience!
Support the shaft on the vee-blocks, place gauge against it and slowly rotate the shaft while watching the indicator needle (the part that goes round). Mark the spot that has the highest deflection with a felt pen. Secure jack in position as close to the gauge as posssible and push the shaft down at the point where there is the mark. Push until the deflection is close to what it was before - but on the other side. Leave everything as is for a couple of minutes. Release pressure from jack and check / rotate the shaft once more, noting any progress. Repeat as necessary...Repeat as necessary...Repeat as necessary...Repeat as necessary...Repeat as necessary......!
That's as far as the theory goes.....!
NOTE that if the unsupported length of shaft between its bearings on your boat is close to the limit (approx 40x the diameter) it will behave as a 'slender' and any inaccuracy will result in whipping as your revs go up, rapidly getting things back to square one.
Since you ask how it's done I imagine that you that you are not an engineering person (no offence meant) and therefore it would be unlikely that you would have the necessary bits.
While it is not rocket science, it is not quite so easy unless done in a well-equipped workshop; more so since you have never done it before.
I would therefore suggest that, relistically, your best course would be to get a quotation from an engineering workshop as well as the cost of a replacement shaft and then decide which is more cost-effective. Me, I'd just make myself a new shaft, though I do have the equipment and the background. Sorry!
(I have assumed all along that you cannot claim from insurance: you did not say what bent the shaft).
Whichever way you go, good luck!
Superflid
Well-Known Member
You could always take it to any decent engineering workshop and let them sort it out.
steverow
Well-Known Member
It is possible, but my expereince with my bent Starboard shaft two years ago did not go too well...went back to the`"straighteners" twice but it still vibrated badly.
I ended up sending the old shaft to Lancing marine who made a new shaft to the pattern.
If I had done this striaght away I would have saved myself about £150
Steve.
I ended up sending the old shaft to Lancing marine who made a new shaft to the pattern.
If I had done this striaght away I would have saved myself about £150
Steve.
sailorman
Well-Known Member
new shaft at £200 > 250
its a no brainer
its a no brainer
haffiman37
New Member
You may manage to bend the shaft straight by using the suggested jacks etc, however if You do not take the 'stress' out of it with heat treatment it will most probably go almost back again!
sailorman
Well-Known Member
all the trouble to remove -refit -might not work /forums/images/graemlins/shocked.gif
all for £250 job done first time /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif
all for £250 job done first time /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif
PuffTheMagicDragon
Well-Known Member
[ QUOTE ]
all the trouble to remove -refit -might not work /forums/images/graemlins/shocked.gif
all for £250 job done first time /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif
[/ QUOTE ]
That, precisely, was the main point of my contribution!
The simple fact that something can be done does not imply that it ought to be. People sometimes do not appreciate the distinction between what is technically feasible , and what is economically viable .
all the trouble to remove -refit -might not work /forums/images/graemlins/shocked.gif
all for £250 job done first time /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif
[/ QUOTE ]
That, precisely, was the main point of my contribution!
The simple fact that something can be done does not imply that it ought to be. People sometimes do not appreciate the distinction between what is technically feasible , and what is economically viable .
homer
Well-Known Member
Many thanks for all contributions!
supermalc
Well-Known Member
First of all, why and where is it bent. If the rudder has hit something, or the boat has rested on the prop, bending the shaft, then yes I would suggest having it straightened. But if it has a slight bend over much, or all of it's length, then it suggests to me the bend has been caused by stress, which is inherent in the metal, so the bend can return later. A new shaft would be the safe option.
It also depends on the boats age, use etc. and how much your lift will cost.
I straightened the original bronze shaft with quite a bend near the prop. I did it on the riverbank, using the crudest of methods. I have a portapower, a hydraulic jack, as used by car bodyshops, and firemen to free trapped people etc. Using this as a makeshift press, and with only a screwdriver held on a spade stuck in the ground, I managed to get it true to within a few thou. Before I did it the end of the shaft drew an imaginary circle, afterwards you could not detect any visible movement.
It also depends on the boats age, use etc. and how much your lift will cost.
I straightened the original bronze shaft with quite a bend near the prop. I did it on the riverbank, using the crudest of methods. I have a portapower, a hydraulic jack, as used by car bodyshops, and firemen to free trapped people etc. Using this as a makeshift press, and with only a screwdriver held on a spade stuck in the ground, I managed to get it true to within a few thou. Before I did it the end of the shaft drew an imaginary circle, afterwards you could not detect any visible movement.
