Need to temporarily shorten a line by a few inches

[Duncan A]

8mm hemp rope has a tensile (breaking) strength of around 1100 lbs.
It is usual to allow a factor of safety of around 5 to allow for flaws in the rope or weaknesses caused by poor rigging practices etc. With soft spliced eyes you need to be very sure of your splicing and you need to be sure that any shortening method involved does not increase the stress on the rope.
Really, you should be using a stronger rope!

 

[Tortion-Stephen]

I have used an alpine butterfly in the past, great to see what people with more experience use. Always learn a lot here.

 

[HamsterJam]

Alpine butterfly or if you‘re only trying to lose a small length, a figure eight?

 

[pe2dave]

OP did say 'a few inches', so I'm with the clove hitch round a piece of dowel (repeat as necessary).

 

[Sideways]

The Alpine Butterfly knot is ideal for this. Easy to tie and doesn’t bind.

And it's an aesthetically pleasing knot in both topologies. Fun to tie too. They should teach it in school

 

[ChrisWiduWood]

Inline alpine butterfly knot, ok its probably not a very well known knot, but once learnt (its pretty simple) is a very useful knot. its used for adding a loop anywhere in a line, not needed in your case but will shorten the rope by a few inches and is capable of taking the weight. watch

is see I'm late to the party with this knot, but I'm glad too see its popularity!

 

[paulm]

It is only subject to failure if un tensioned. If tensioned it is very strong and is also use to strengthen frayed areas of rope. You can make it less subject to undoing by putting a piece of dowel through the loops at each end.

It's an inherently unsafe knot, you can make it a bit less unsafe by adding dowels as additional possible points of failure, but why would you when there are inherently better, safer and simpler alternatives available ?

 

[dizjasta]

Any knot made in a length of rope will reduce its tensile strength. Tests show that a single overhand knot can reduce strength by around 50%. It has been suggested that the use of the alpine butterfly knot can cause upto a 70% strength reduction. Maybe the safest option is a new length of rope of the required length.

 

[Sideways]

It has been suggested that the use of the alpine butterfly knot can cause upto a 70% strength reduction

But most of the references from a quick online search of rescue, arborist and similar websites put the alpine butterfly closer to 70% of the original rope strength and the figure 8 closer to 80%.
Of course it depends on the rope material and how the knot is used.
With the butterfly, you would use it to create a replacement loop which is it's stronger orientation, not to create a loop just to shorten the length between two loaded tails when it's only half as strong as the original rope.

 

[Noel John]

I have an 8mm hemp rope with a soft eye loop splice in each end I need to temporarily shorten it a few inches but any knot(s) will be subject to a load of 350 lb but will need to be readily untied

depending on the anticipated load you could either use a sheepshank or a bowline on a bight. Both are boy scout knots that have been in use fo decades.
Noel