Prop-er job

Fol­low­ing an en­gine change on a Colvic Wat­son, con­strained revs in­di­cate the need for the yard to fit a smaller prop

Practical Boat Owner - - View from the Boatyard -

We re­cently car­ried out an en­gine change on a Colvic Wat­son. All went well: how­ever, the orig­i­nal huge prop had now be­come a lit­tle too huge when un­der power. the North Sea vis­i­bly in­creased speed through the Dover Straits, and the new en­gine would only rev to around 1,500rpm.

So, we needed to re­duce the size of the prop and pos­si­bly in­crease the pitch. On the pos­i­tive side, re­duc­ing the size would re­duce sailing drag, but there wasn’t enough in the own­ers’ bud­get to stretch for a feath­er­ing or fold­ing prop.

There’s no doubt that a big three-bladed prop causes drag, es­pe­cially one cou­pled to a hy­draulic gear­box that can­not be left to free­wheel. A smaller prop would cause less drag, but the dan­ger of re­duc­ing the prop size on this de­sign is that so much of the prop is hid­den in the shadow of the keel. We could have kept the prop the same size and just in­creased the pitch, but the own­ers like to sail, so im­prov­ing ef­fi­ciency by re­duc­ing drag was the best op­tion – and, given the bud­get con­straints, a smaller prop was the way to go.

The task was to im­prove the flow round the back of the keel and give the smaller prop half a chance: at the same time, the flow over the rud­der would be vastly im­proved. How­ever, the ini­tial plan of a short fair­ing just didn’t look or feel right to me. My rule of thumb is to look at an un­der­wa­ter ob­struc­tion and times the size by three to gain an idea of when the flow might start to get itself sorted out. So, a 5in-wide trail­ing edge would be cast­ing a shadow at least 15in aft. There’s no way I could get 15in of fair­ing, so I just worked on get­ting as much as I could.

A few more chunks of Celo­tex and a glue gun gave me some­thing to work with to get a rea­son­able shape. I shaped it by eye rather than spend­ing a lot of time mak­ing tem­plates, which kept the costs down.

Around 10in of ex­ist­ing keel was ground back to pro­vide a good key and anchor

for the fair­ing. The keel and foam was then tied to­gether with four lay­ers of 450g chopped strand mat and polyester resin. Three lay­ers of gel­coat was then ap­plied, and a fi­nal coat of gel­coat with added wax in styrene which acts as a top­coat.

While a great gel­coat fin­ish isn’t re­quired in an area soon to be cov­ered in lay­ers of primer and an­tifoul­ing, I ad­vise you to still ap­ply mul­ti­ple thin lay­ers of gel­coat rather than slap­ping on a thick coat. The issue is that when mix­ing you will in­evitably in­tro­duce air into the mix. Ap­ply­ing the gel­coat in a thick layer will trap these tiny air bub­bles, which can make the gel­coat por­ous – and if you cut back to get a re­ally shiny fin­ish you will get lots of tiny holes that are im­pos­si­ble to fill. Cut­ting back more just ex­poses more holes, and so on.

Elim­i­nat­ing the air

Gel­coat will not cure in air: it gels and stays sticky. When ap­plied to a mould and then glassed over, the air is elim­i­nated and the gel­coat will set hard. So, when ap­ply­ing on top of glass without a mould we need to elim­i­nate the air some­how. To do this we add wax in styrene in roughly the same quan­tity as you add the cat­a­lyst.

Dur­ing the cur­ing process the wax comes to the sur­face, ef­fec­tively seal­ing the gel­coat from the air and al­low­ing the gel­coat to set hard. It should be noted that if you need to add any ad­di­tional lay­ers of gel­coat you will have to re­move this wax by key­ing well with wet-and-dry sand­pa­per or sim­i­lar.

Once com­pleted, our sea trial proved we had cho­sen the right size of prop as the boat’s top speed is un­changed, but the en­gine now revs to 2,500rpm. The big­gest change was at slow speeds, where the slow han­dling has been transformed for the bet­ter.

The fair­ing has been a huge suc­cess. The owner says: ‘There’s re­duced noise and vi­bra­tion, but what’s amaz­ing is the ef­fect it and the new prop have had on fuel con­sump­tion. It’s ex­tra­or­di­nary: 30% less, and we’re mon­i­tor­ing it closely. Also, the prop now spins from 2 knots through the wa­ter rather than 4; a spin­ning prop has way less drag, so we sail faster too!’

Next time she’s out of the wa­ter I need to con­vince the own­ers that the barn door they use for steer­ing needs some match­ing hy­dro­dy­namic help...

and grew...

The di­rect shadow caused by the trail­ing edge of the keel over the old, huge prop. The new smaller prop would strug­gle

The trail­ing edge of the Colvic Wat­son’s keel is not the most hy­dro­dy­nam­i­cal­lyfriendly de­sign

I de­cided that it sim­ply wasn’t enough, so the fair­ing grew...

The ini­tial plan was for a small fair­ing us­ing a foam core

With a coat of In­ter­na­tional Pri­mo­con and then a cou­ple of coats of Sea­jet an­tifoul­ing, she’s ready for the wa­ter

The fi­nal shape, with the prop tem­po­rar­ily fit­ted to check clear­ance, ready for the gel­coat

The fair­ing after three coats of gel­coat and one of top­coat or flow­coat gel­coat (es­sen­tially gel­coat with added wax)

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