Oar design for the St Ayles Skiff
Topher Dawson, Treasurer of SCRA and builder with “Ulla” syndicate and Ullapool High School writes:
Since the launch of the St Ayles Skiff project on late 2009, many groups have become involved and at the time of writing there are 11 boats complete, with another 17 in build and more on the way. Some groups have two boats, one even is starting the third!
8 or 9 groups have a season’s experience and this document is an attempt to pull together what is known about the different oars which have been made or bought for the Skiff, and peoples’ experience with them.
The hull is one design but the oars are fairly open. The build rules say
6.1 Oars shall be made of timber.
6.2 Oar blades should not be “spooned” or of the “chopper” style.
6.3 Blades to be symmetrical about a horizontal plane through the
shaft axis, i.e. the top of the blade should be a reflection of the
Oar length and clashes:
The plans show 12 ft oars but these have proved to cause clashes between the oar handle and the back of the next rower, for rowers with long reach. Rowers with short reach, like some women, may have no problem.
The clashing issue depends on the length of oar inside the boat, known as the inboard. This depends on two factors, overall oar length and also what is known as gearing. Strong rowers in a fast boat or with the wind behind them, can push the oars out so they have less inboard and more outboard, which is equivalent to top gear in a car. Weaker rowers in a slow boat going against the wind find they need to pull the oars in so there is more inboard and less outboard, which is equivalent to a lower gear in a car.
The rowing world measures this “gearing” as the outboard divided by inboard, ignoring the fact that the effective ends of the oar, where the hands act on the handle and the water acts on the blade, are a bit in from the ends of the oar. As long as we all use the same simplification we can compare oars.
Currently all the St Ayles groups use a gearing of between 2.8 and 3.2, mostly centred on 3. This means that a quarter of the oar is inboard, and three quarters outboard. Information from groups shows that oars with inboards between 33 inches and 45 inches clash some of the time, depending on reach, rowing position and timing. At a gearing of 3 this means oars between 11ft and 15ft long cause clashes.
Basically when the rowers are in or near a straight line on the centreline, they clash. There are two known ways to avoid this. One is to use long oars at or over 15ft, which stagger the rowers away from their kabes (rowlocks) and so out of the way of the next rower’s handle. Coigach have won many races this way. They have two shorter oars at bow and stroke, about 12ft long. The other option is to make short oars, 11ft or less. These result in the rowers staggered towards the kabe. Port Seton have done this with great success with 10ft oars, and we in Ulla have copied them and developed the idea.
Standard oars you can buy:
Port Seton started with standard Plastimo pine dinghy oars, which have round section shafts and flat blades. These are readily available up to 10ft long for about £37 each. Port Seton screwed stainless eyes to them, which fitted over stainless thole pins, and had a lot of success with them. They were light and easy to row at a fast stroke rate. Ultimately the screws weakened them at the point of maximum stress and they tended to break. [Metal Fittings for pin and ring are not permitted under the measurement rules].
Other oars available are Admiralty pattern, round section shaft with a long straight blade. They cost about £180 each for a 12ft spruce one, (Collars) but you can get them up to 17ft if you want. Suttons also make hollow spruce oars with ash backs, and make most of the Cornish gig oars. These are between 11 and 13ft long, and cost about £260 each. They are spoon blades which are not allowed in the Skiff, but I am sure Suttons would make you flat blades if asked nicely. See http://www.suttonblades.co.uk/frameset/mainframset1.htm
Many of us have made our own oars, some from the plans at 12ft, but these clash. Ulla’s oars started at 13ft 2 inches, heavily made from Douglas fir, but they clashed. So we went to very long ones, 17ft, by gluing bookshelf wood on to the ends, with the idea of cutting them down till they worked. Even at 15ft they were heavy and slow. Then we made 11ft oars from cheap B&Q wood (£1.50 each) and 3mm ply blades. They were light (6 pounds) and easy to row with, and we plan to make better ones of the same design.
Oar timber is usually softwood, but hardwood lifeboat oars have been tried. Sitka spruce is the lightest and most expensive timber. Douglas fir is harder and heavier, available in the long straight clear lengths we need. Ash is sometimes used for oar backs to resist the compression on the forward side, or for whole oars in heavy boats. Plywood can be used for blades, and we (Ulla) have used 3mm ply with good success. Hollow shafts used to be standard in racing oars before carbon fibre became the norm.
Oars have to resist bending stress, the highest being at the kabe. For this reason oars are usually thickest there. Most working fishing boats had rectangular section oars and did not feather (rotate the blade parallel to the water on the return stroke) them. The blades are long and narrow, in Irish currachs they are just part of the shaft, 2 inches wide. Some people think narrow blades have less windage than unfeathered broad blades. The long narrow blades do have the advantage that they are less critical about how deeply immersed they are, which is good in waves. In the St Ayles Skiff it is not yet clear whether feathering the blade is worthwhile. It does reduce windage when the oar is out of the water but requires more effort from the rower’s wrists to feather.
Rectangular cross section oars are in fact more efficient in resisting bending stresses, compared with circular or oval cross-sections, because more of the timber is at the extreme front and back of the oar, where the stress is. This means they can be made lighter for the same strength, and are obviously easier to make. Sliding seat racing oars have hollow tubular round section carbon or glass shafts, with the section at the pivot being rounded with flats to locate the oar at the feathered and the upright position.
Longer oars have more bending stress and tend to be heavier than short oars. Balance in an oar means that with the weight of the rower’s relaxed forearms and hands resting on the handles, the oar is balanced. In a longer oar, lead may need to be added to the handle to achieve this. The greater weight means that there is more inertia to change direction at each end of the stroke, so longer oars tend to lead to a slower stroke rate. This does not mean the boat is slower, as Coigach have showed.
Oars bend a little when the force is applied, or a lot depending on how flexible they are. Carbon oars are very stiff, but some of us like oars which flex a little. Oars near to breaking tend to feel very “soft” in bending, due to invisible compression failures on the forward face. The degree of whippiness desirable in an oar is very individual to the rower, and in a wooden oar can be increased by carefully planing wood off the shaft. Making the oar wide in the horizontal direction makes it stiffer and also stronger. This is more efficient than adding wood to the vertical thickness.
The plans show kabes which are the traditional Fair Isle/Shetland/Faroes/Scandinavian form of rowlock. They are rectangular section wooden thole pins in slots in the gunwale, and the oars are held against them by lanyards called humlibaunds. Many of the groups tried this system but could not get it to work, finding it prone to friction and slackness. Coigach have persisted with it. Several groups used stainless steel thole pins, either with stainless rings or holes in the actual oars. These pins were finally banned in October 2010, and metal crutches or rowlocks are also banned.
North Berwick has adopted the Cornish gig system of two round tapered thole pins, a hardwood pin forward of the oar and a softwood one aft of it, to break in emergencies. They report it to be friction free but it still has a “clonk” due to the fact that the pins have to be further apart than the width of the oar to allow it to rotate.
Following the banning of stainless pins, Ulla is going for hardwood round section thole pins (actually broom handles) with wooden blocks fitted over them with a bracket supporting the oar. See http://ullcoastrow.wordpress.com/2010/10/31/skiffology-will-out/
These are wooden versions of the racing swivels which sliding seat boats use, and have light lanyards holding the oar on, which in an impact will let the oar go.
Some traditional systems allow the oar to be freely slid in and out, which allows quick gear changes and lets the rower pull the oar quickly out of the way of an obstacle. Others lock the oar at a particular point on its length, such as the thole pin and eye system, and some Faeroese boats which have notches for the humliband to locate the oar lengthwise. These systems have the merit that tired or inexperienced rowers do not have to think about holding the oar in or out. St Ayles groups are divided on this point.
The 12’8″ length of the original set of Anstruther’s oars was dictated by the length of the very fine grained Douglas Fir that fell off the back of a passing Fife Council outside my workshop. These worked well with the Anstruther women’s crews with the 37″ room between the tafts (thwarts) on the prototype skiff Chris o’Kanaird. The men rowing the skiff tended to have problems knocking the backs of the person in front, so, with a new set for St Ayles, the length was increased to 14ft, with the rowers sitting offset from the centreline. The situation was helped with the 39½” room between the tafts, and in practice, even with very tall rowers, there has not been any further problem with with clashes.
The basic plan of what we are using is shown in Anster Oars.