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