6:1 keel lift modification

By Mark Weeks, P27/147 Mymo, March 2016

1. Background

The Parker 27 original fit was a 2:1 lift through the keel box cover. Several have been modified to a 4:1 lift by adding a second opposite hand sheave opposite the original.

Mymo (No. 147) was thus configured but still required winching to raise the keel. With a spray hood that when up precluded a full turn of a winch handle, lifting the keel was an arduous task at the best of times.

My intent was to make it possible to raise the keel by hand (at least to half keel depth which I require for maximising my mud berth access) without the use of a winch, in a similar method to that devised by Chris Turner for the Super Seal. Experimentation revealed that an 8:1 was an easy pull and a 6:1 would probably be OK. On this basis I had the keel removed and began work.

2. Strip down.

Picture 1 below shows the keel with the original bits removed and lying on the keel. Note that the original 2:1 keel sheave is all metal and the additional one is a cobbled together Tufnol sheave. The items on right are the forward keel roller resin block for two rollers (nearer to camera) and forward rubbing block

Picture 2 shows the The keel with the sheaves removed. Note the red line on the keel. This was derived by balancing each end of the keel on 35mm dowelling and adjusting them until the keel balanced. This produced a centre of gravity line down the length of the keel. Note that the original sheave (rust stain slot) is roughly on the CG.

Having removed the keel and retrieved it to my garage (I weighed it on bathroom scales and it came out at 120Kg) with some effort, I procured three secondhand Tufnol blocks which were 60mm I/D, 70mm O/D and 10mm bore.

I sketched up the design shown in Figure 1 and had it made at a local engineering firm. For reference the final component was not as drawn and contained the following features:-

  • The TIG welding at the inner plates was only for about 10mm at each end and on one side only due to access.
  • The plate thickness used was 5mm not 3mm. This proved to be a better solution as you can get 60mm 316 stainless strip.
  • The bracing plates between the centre pulley and the two outer pulleys were not full width and of 3mm 316 stainless.
  • The upper plate was scalloped post manufacture to give a better lead for the rope. Photo 3 shows the assembly prior to fitting and scalloping.
  • The central bar was not threaded through the las plate but was made approximately 2mm longer than the assembly at each end and one end was peened to allow assembly from one end only.

3.Triple Keel Sheave Block Assembly

Picture 4 and 5 show the marked out are to be removed and the initial cutout. Note that the block of wood in which the original sheaves were mounted is completely removed laterally and additionally some of the steel plate is also removed.

The approximate limit of the steel plate was determined using a stud locator see photo 5 below.

Note the differing thickness of the glass from each side of the keel. (zoom in to look at the edge of the glass next to the pencil - you can see the 6mm glass skin plus the infill; note little infill apparent on the other skin)

At this point I weighed the removed steel (40mm thick) and the triple sheave assembly; the steel was 30g heavier. So including the glass removed and the two original sheaves I have probably lost ½Kg from the keel i.e. 0.5%. I deemed this acceptable.

4. Rebuilding keel and cover.

The cover rebuild was fairly straight forward. The original sheave was left in place, the additionally added sheave was moved its length forward and a Barton upright sheave (No. 3) added where the original rope termination was.

Moving the additional sheave forward involved adding reinforcement to the cover as the original cover has a brass sheet insert through the centre section where the sheaves are placed. This was not long enough to accommodate the movement of the additional sheave forward so I bent up a piece of aluminium tooling plate (3mm thick) that I had to hand and glassed this in place.

The moved sheave positions were then marked off, drilled and tapped for assembly. From advice given by Chris Nichols I ensured that the rope slots through the cover were sufficiently long to not allow rubbing and additional friction.

Picture 7 below shows a trial fit up using 8mm rope for alignment purposes prior to repairing the hole to take the assembly. (Centre sheave aligned with original sheave position)

7.Trial fit

Once happy with that alignment could be achieved the hole was repaired ready for fitting using CRP paste (Polyester resin system) to fill the main deviations, subsequently rubbing back to get a fair surface. The four sides were then glassed using epoxy resin with some chopped strand mat I had lying about to get close to the final shape, followed by 100mm glass tape which allowed for it to be folded over the edges.

The thickness of epoxy glass added allowed for slots to be made in the two end surfaces to accommodate the protruding ends of the spindle (slots cut from one edge only to centre) and to allow for the recesses necessary for the retaining straps.

Once glassed and faired back a final fit was carried out which showed a small gap all round. After a final triail fit. The retaining straps were drilled off (note the aft two use self tapping screws and the forward two required to be drilled and tapped throught he glass and into the steel for 4mm x 25mm CSK machine screws).

For final fitment and to get a full purchase on the upper plate 3 layers of tape were weted out and applied to the upper surface to fill and gaps when the assemble was located and screwed in place. All surfaces of the assembly that would touch the hole sides were heavily waxed so that the epoxy would not stick and the units could be removed in future if necessary.

The retaining strap attachments to the assembly and the retaining strap forward fixings to the keel were assembled with locktite thread lock. The slots into which the protruding spindle ends located together with the retaining strap lug recesses and the self tapping screws were all filled with a polysulphide sealant to allow for removal at a later date.

Lastly all gaps between the sides of the assembly and the hole were filled with epoxy resin by damming from below and drizzling epoxy in to the gaps from the top until full. The surface surrounding the assembly was faired for the rope leads and generally sanded before painting (see Picture 9 for final appearance)

5. Completion

Once the keel had been replaced and rigged the load to raise was checked. Approximately an 80lb (37Kg) pull is required to lift the keel in air with the rope passing a closed jammer. Hopefully the initial pull will be a little less in water. The keel was initially rigged with 8mm rope but this was a little hard on the hands so the lift was reverted to 10mm as in Picture 10.

6. Conclusions

As the original 4:1 lift was not possible to use by hand the 6:1 has made a difference, although in the process of constructing the 6:1, better sheaves have been used, therefore a well oiled 4:1 may still be pull-able. No grease has been used to aid the running of the keel sliders. I may try this latterly.

The total cost including the lifting in and out of the keel was about £375 not including my labour.

Cost breakdown

  • Keel sheaves: £3
  • Rebuild of original upright block: £50
  • Keel assembly manufacture and materials: £160
  • Epoxy and glass (SP junior pack): £25
  • Replacement keel rollers: £10
  • Additional Barton upright block (secondhand): £7
  • New Lift Rope (15m of 10mm B on B): £20
  • Keel lift in and out: £100
  • Abrasives, Filler , Paint etc. (already had): £0

The only downside is that you now have an additional 8ft of rope to pull in to raise the keel, but as it is possible to gain 3ft at each pull this is not unduly noticeable.