UK-Cherub Class

Get Your Heart Racing


Making a Spinnaker Pole

At some point in your Cherub sailing life you will probably need a new spinnaker pole either because you break one or you just need a new one to support your upgrade plans. Obviously this will have to fit inside the support tube in the bow or snout of your boat and yet retract into the boat possibly past the mast to meet the class rules. If you are replacing a broken pole you only have to copy the existing dimensions. If you are building a new boat or modifying an older one you will have to spend a bit of time working out how long and how tapered it needs to be. This article is based on advice in an email from Gavin Simms who has made 4 poles in recent years, some modifications have later been added by Roland Trim.

Starting from Scratch

If you are building a new boat or modifying an existing one you have a bit of measuring and design work to do. It is no point making a shiny new pole and finding it won’t fit your boat or jams during every drop. The variables you have to play with will depend on how early you are in your design/build or how much modification you intend to do.

Firstly the pole has to retract into the boat and satisfy the following rules.

4.2.3 Bowsprit - The bowsprit, if fitted, shall be retractable to within 4.3m of the transom.

The outer end of the bowsprit shall be solid or capped. No sail other than a spinnaker may be

set from the bowsprit.

The first thing to look at is how far back in the boat the pole needs to retract this will determine your problems.

If when retracted the inboard end of the pole is still in front of the mast then no problems, if it is between the mast and the centreboard you could either bring the pole to one side of the mast or stump or under a mast bridge. If it comes back behind the centreboard a mast bridge has to be quite large. If the pole needs to retract to one side of the mast the pole will need to be tapered to give slack in the snout when retracted.

Critical Dimensions

As mentioned above there are some basic measurements which will affect how your pole retracts into the boat not to mention how strong it will be.

  • Distance from mast to outboard end of pole. (This is determined by your spinnaker luff length, the boat’s mast position and mast rake.)
  • Length of pole outside the boat.
  • Length of support tube.
  • Inside diameter of support tube.
  • Distance between inboard end of support tube and mast.
  • Wall thickness of pole
  • Diameter of inboard end of pole
  • Diameter of outboard end of pole

Decisions on Dimensions

If we take a hypothetical boat with some simple numbers we can assume that the boat has a bow at 3.7m from the transom and the end of the snout is 4m from the transom. The mast is 2m from the transom and leading edge of the centreboard in 1.7m from the transom. The pole while extended must reach 6m from the transom. (Note these numbers are simplified to make the maths easier a boat may or may not work with the bits in these positions.)

With these basic numbers in place I can work out the required length of the pole. This is governed by the position of the aft end of the bow tube. I am going to put this at 3.6m from the transom. The pole length is therefore 2.4m. Meaning that when fully retracted the aft end of the pole is 1.9m from the transom. In this case we can either go for a bridge or bring the pole to one side of the mast.

To bring the pole to one side we need to check the taper is adequate. The greater the taper of the pole the more slop there will be when the pole is retracted, however an overly small tip will be hard to build and thread with the string, correspondingly a large aft end will take up a lot of space in the boat.

Things that affect deflection of the retracted pole are.

  • Length of Bow tube; longer tube, the smaller the possible deflection.
  • Taper of the pole; the larger the taper the larger the possible deflection
  • Position of Mast; mast set further back require poles with less taper to achieve the same deflection.

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Spreadsheet

A spreadsheet was developed to work out the required dimensions for the pole and snout for one boat assuming a non tapered bow tube. This should be treated with care as the assumptions made about the geometry may not be the same as for this boat, however the pole made using this fits the boat with a few mm to spare.

Loads

Steady loads

The pole is under a bending load from the luff of the kite. This is difficult to determine but very roughly should not exceed the weight of the boat. (Note that helm and crew will be hanging off the back end) in strong winds. Add a factor of safety of 4 to account for most shock loads and capsizes.

Shock loads

Most poles appear to break when the boat is pitch-poled. Gav recalls most of his poles breaking in a big pitch-pole. This appears to be irrespective of how strong the pole has been made. Consider the force of the boat and crew (230kg) slowing from 20 knots to zero knots in a couple of seconds and the forces involved. It is worth noting that making a very strong and stiff pole will transfer the loads of the pitch pole on to the support tube and hull possibly breaking the next weakest area. It is therefore much better to have a pole with some flexibility that can absorb some of the impact so the inboard end of the pole doesn’t see such a sudden high instantaneous load.

In all circumstances the highest load will be at the inboard and outboard end of the support tube. The pole will have to be thicker at these points to maintain the shape and hence avoid buckling.

How the laminate works

When bent, the material under the highest stress is on the outer surface of the pole. One side of the pole is under tension and the other is under compression. Therefore the outer layers of the lay up must be able to take these stresses or transfer them to the layers beneath. The bonds between each layer must be good and the pole should ideally be made in one session. Biaxial cloths have been suggested to be good at transferring stresses to the layers beneath and many yacht spars are made incorporating several biaxial cloths within the mainly unidirectional laminate. Cherubists commonly use plain weave cloths as the first and last layers with uni layers in between. Plain weave is used in other parts of a cherub so the choice is often made for cost and availability of materials. recent thinking suggests a second biaxial or plain weave layer on the inside of the pole at the point it exits the snout. This should help to stop the pole buckling at the point of the maximum bending load.

Once you have designed your pole to fit your boat, things get more interesting.

Preparation

All successful laminating jobs depend on good preparation. Once you have started you have to keep going to avoid the first layer curing before you have finished laminating. Make sure that all materials are available, cut to the right size and that tools are to hand. It doesn’t hurt to have spare mixing pots, sticks and brushes, gloves, in fact anything that might be needed. I would even turn off my phone to avoid any disruptive calls.

The Mandrel

If your pole has to be a certain diameter and taper you may need to make or purchase a bespoke mandrel. If you can find a suitable mandrel you may decide to design your pole around this.

In the past tapered windsurfer masts have been used as mandrels. Gavin Sims has a tapered aluminium mandrel that is 2“ wide at the base and a bit less than an inch at the top. His poles have been 2.1m long so the mandrel will be a bit longer. If you need a longer pole (Some are nearer 2.8m long you will need to find another mandrel or some how extend the tapered one at the outboard end.

Paul Croote found himself in this situation and made the mandrel for ‘Cheese Before Bedtime’ by laminating timber around a steel tube. He then turned the timber down on a lathe to produce the taper. The inboard end was about 80mm in diameter so quite different from Gav’s, however Paul did have a full workshop at his disposal. More info about this process can be found on Cheese Before Bedtime own page.

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The outside diameter of the pole is critical. Too big and it won’t fit in the boat. When choosing your mandrel ensure that you have accounted for the thickness of the laminate and any protective layers between the mandrel and the pole. (ie. if using the candle wax release method).

A tapered pole should be made over length and trimmed once fitted to the boat as the exact position where you get a tight fit with the boat will depend on the thickness of the laminate and any protective layers.

Preparing the Mandrel

Once you have found or made a mandrel for your pole you have several options to make it slippery enough to stop the pole from sticking to it and to get the finished article off.

  1. If the mandrel is smooth and non porous and you want to reuse it, coat it with several coats of mould release wax.
  2. Alternatively and more low risk is to coat metal mandrels with a thick layer of candle wax. This is melted out once the pole has cured allowing the mandrel to fall out. There is more on this on the Building Carbon Masts page
  3. If you are less confident with the surface finish of your mandrel or it is made from a more porous material, I would recommend an alternative. Cover the mandrel neatly with parcel tape and wax with mould release wax. Then wrap the mandrel with a layer of Mylar film (drafting film) and cover with more parcel tape. Avoid sticking the parcel tape to the mandrel itself. Don’t substitute the Mylar for cling film because true to its name it will cling to the mandrel.

The Lay-up

Starting from inside going out

  • 1 x 200g plain (or biaxial) carbon
  • 4 x 300g unidirectional
  • 1 x 200g plain (or biaxial) carbon

With a tapered pole you need to taper the width of each section of cloth to maintain an even laminate thickness along the length. Twisting the unidirectional cloths slightly along the length of the pole in opposite directions for each layer will also help increase the strength in torsion.

Of course you may not want an even laminate if what you want is the lightest possible pole. As the loads on the pole are higher at the inboard end particularly where the pole enters the boat it may be worth tapering the laminate even more to get a decrease in laminate thickness along the length of the pole. If this is done then the outer extremity of the pole should be strengthened to give it enough strength to cope with the point loads from the tack line.

As soon as possible after laminating the laminate should be wrapped in heat shrink tape being careful to keep the side with release agent against the laminate the heat shrink tape should overlap by at least half the width of the tape, the more the better. Once fully wrapped and with the ends secured the shrink tape should be gently heated with a paint stripping heat gun. This will consolidate the laminate and squeeze any excess resin out, sometimes at high pressure so safety glasses are a must, an eyeful of hot epoxy is not fun.

Alternatives to heat shrink are wrapping the laminate in strips of peel ply or glass tape.

Vacuum bagging has also been used in recent poles, but is considered by some to be difficult to avoid creases. For this instead of heat shrink, a tight overlapping helical layer of peel ply is added, then appropriate breather and it is all placed in a plastic bag and the air sucked out of it. The vaccum consolidates the layup. If using this method the home build advice is to leave any hoop fibres in the layup for a second hit (i.e. inner hoops and uni first, layer of weave over the top added as a separate hit). Thick vac bags appear to also result in creases, use of very thin plastic (decorators dust sheet) is reccomended by some for spars. The mast and pole on several of the recent boats (all spars on EJ) were made using this method.

Getting it off the Mandrel

Be prepared - it is hihgly unlikely that the pole will simply slide off the mandrel. You generate alot of friction just consolidating the layup and the laminate shrinks slightly on curing. There are may diffferent methods but the following may help you think of some more:
1) Rotating the pole against the mandrel often means you are only stressing a small area of bond. So twisting can help.
2) When pulling off a mandrel you are likely to release all of the bond at the same time, bits can go flying at this point as the energy in the ropes is released accelerating freed objects.
3) If pulling be careful of damaging things. Tow bars can bend and you will want that to tow the boat with. A non vehicle technique using two trees and a “spanish windlass” may be less fun, but can generate higher loading with no risk to your chassis or your clutch.

On the other hand, for some using a truck is part of the fun:

Finishing

Using the above techniques and spec you can create a stiff and tough pole. But if you are after a better finish you should leave off the final weave in your laminate. Once the laminate has cured you can fair out any bumps or hollows to get a smooth base for the final layer of weave. This should either be done with a UV resistant epoxy or top coated with a UV resistant lacquer. ASCII

Further Reading


tech/spinnaker_pole.txt · Last modified: 2013/06/25 15:55 (external edit)