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Rake and Bend
To begin, we should define the difference between fore-and-aft tune and transverse tune. Fore-and-aft tune refers to rake and mast bend. Transverse or lateral tune refers to setting the mast up straight sideways and setting up the uppers to minimize lean.
Let’s begin with rake, which is determined by headstay length. Rake affects helm by moving the center of effort of the sails relative to the center of lateral resistance (keel). A longer headstay gives more rake which gives you more weather helm. A starting point for arriving at the correct rake is to measure the designed rake of the sailplan. A typical 40’ boat would have about 15-18” of rake. To calculate rake, hang a plumb bob from the main halyard and measure from the aft side of the mast along the cabin top to the plumb bob. This should be done with the backstay tensioned at about 60%. The actual amount of rake you end up with may vary depending on the normal conditions you sail in and may be a compromise between what is optimum in light air vs. heavy air.
The second aspect of fore-and-aft tune is mast bend. A certain amount of mast bend is desirable. Mast bend is determined by the relationship between the masthead position, deck partners, and mast step. If we have decided on the proper rake, then the masthead position is fixed and we have the deck partners and mast butt positions to adjust to induce mast bend. By either moving the mast forward in the deck collar or moving the mast butt aft in the step, we can induce some bend into the rig. Another factor that can affect mast bend is the angle at which the butt of the mast is cut off. If the mast is resting on the forward or aft face, the resulting moment will have a major effect on bend. We normally radius the butt of the mast so that the spar will bear near the center axis of the section, thereby minimizing the bending moment regardless of the angle of the spar to the step.
Other factors that control mast bend are double lower shrouds, babystays, and inner forestays. Double lower shrouds can be tuned to increase or limit mast bend. Babystays are typically used on boats with single in-line lowers and pull the rig forward down low in the same way as forward lowers. Inner forestays with a staysail can put a large bending moment in the spar and are usually opposed with running backstays or aft intermediates.
Spreader sweep is also a big factor in mast bend, but this factor is a design feature of the spar system that is not an owner variable since most spreaders are fixed rigidly to the spar. Aft-swept spreaders will facilitate some mast bend and in-line spreaders will restrict mast bend.
So, what does all this talk of mast bend mean and why is it important? A certain amount of forward bend is healthy, making the spar more stable and less likely to pump in a breeze. Most mainsails require a certain amount of mast bend to set properly and as the breeze increases the combination of more backstay tension and more bend will flatten the main. This will keep the boat standing more upright and ease the helm.
Another consideration here is headstay sag. Controlling the amount of headstay sag with an adjustable backstay device will allow you to optimize the shape of the genoa for a range of wind strengths. With an adjustable backstay, particularly hydraulic types, it is extremely important to establish a maximum backstay load and some lower reference points. A good limit is 30-40 percent of the breaking strength of the backstay wire or rod. This allows some margin of error in the system in the case of shock loading.