Knots, hitches and splices in High Modulus Polyethylene (HMPE) rope will reduce its strength, possibly by up to 80% of the maximum breaking strain.
HMPE rope such as Spectra® and Dyneema® has a high tensile strength for its weight and good resistance to chemicals and Ultra Violet light – all of which make it great for boating but there is a drawback, HMPE does not tolerate tight bends which reduce the strength of the rope. Knots, hitches and splices, although unavoidable, reduce HMPE strength more than other type of rope, and a simple overhand knot which produces a very tight curve typically reduces it by as much as 80%.
There is no easy answer to this. If you have purchased 10mm HMPE rope with a breaking strain of 5000kg then you might be surprised if it fails under a load of 1750kg at the point where you have tied a knot or bent the line around a fixing point. The advantages of buying something that is lightweight and strong might simply not be up to the task you bought it for once you have put a knot in it.
The industry uses a formula to express the tightness of the curve called the D/d ratio based on the diameter of the fixing around which the rope is bent (D) and the rope diameter (d). Typically a D/d ratio in the region of at least 8:1 is required to maintain 100% of its strength.
If you are going to buy HMPE for sheets, halyards and control lines then it is inevitable that you will need to fasten it to something such as a shackle. It is worth seeking advice from a rigger to discuss your intentions and remember when buying shackles etc. that at a D/d of 2:1 the breaking strength is typically 65% of the maximum. At a D/d of 1:1 the breaking strain reduces to 50% of maximum.
Either use oversized rope to achieve the strength required or increase the diameter
of the fixing point to which it is to be attached. Finally, splices should be used for HMPE in preference to knots.