Engage overdrive…

by Adrian on January 31, 2015

We’re heading west for the first time in 3 months as we cross from Antigu to St Kitts. It’s a calm day here in the Caribbean with just 5 knots of wind behind us so we’ve engaged the iron sail and are motoring along steadily at 7 knots.

PropIn the world of modern folding propellers, I’m really pleased with the choice we made when fitting out our boat. Having experienced them on a few delivery yachts I upgraded the standard factory Volvo folding prop with an 18″ Gori folding propellor.

The Gori props look and behave much like any other folding prop – the blades pivot around to form a sleek linear shape when not in use to reduce drag when sailing and they spin out to act as a normal propellor when drive is engaged.

The one slight difference is that the pitch (the angle the blades cut into the water) on the Gori blades differs very slightly when in forwards and reverse.

This means when you are in smooth seas motorsailing or just motoring along, if the propellor spins in the reverse pitch configuration you can do the same speed but with 10% less revs.

This doesn’t sound much, but we’re currently motoring along at 7 – 7.2 knots boat speed at 1680 revs. To achieve the same speed with the prop in normal forwards configuration we’d have to be running the engine at 1900-2000 revs.

This equates to quite a fuel saving over long distances, so financially makes sense as well as reducing the load on the engine.

They call this “overdrive”. It’s quite simple and there’s no additional gearing, mechanicals or switches, you just set the blades in the going backwards configuration by motoring backwards for a few seconds to get a bit of way on, then change (reasonably quickly) from astern to forwards. That’s it. You can hear the change in tone as the propellor engages and is held in the backwards setup by the water pressure whilst it spins up into forward gear.

Gori Folding Prop

Overdrive mode is a more efficient in calm seas and gives the same speed at less revs, but the pitch of the blades isn’t as good at cutting through the water in heavy swell, so you don’t use it when punching through seas.

Our top speed on a calm day is 9.6 knots at 2600 rpm in normal prop mode.

In overdrive the maximum revs we can get to is 2200 at 8.1 knots.

The prop last time it was out of the water. We covered it with a waxy prop-guard compound as an anti-foul solution to stop barnacles etc forming on the metal. This works a bit, but it is not perfect.Prop with antifoul

There are no disadvantages that we can see. There are anodes we have to replace as with any prop and astern drive is strong and engages quickly. I even bought and fitted a serrated edge type rope cutter for it from a company called AB Marine in the US. As with all rope cutters, you never really know if they do any good, but you hope it may help one day.

Prop with serrated edge rope cutter installed:
Propwith rope cutter

Details of the Shaft Shark

AB Marine is the North American & Caribbean importer and distributor for Shaft Shark line cutter, Gori folding propellers and stocks a full inventory of product, parts & zincs in house.

The Shaft Shark line, rope and weed cutter is a serrated edge circular blade that clamps onto propeller shaft and saildrive systems to keep your propeller free from rope, weeds & debris.

The SHAFT SHARK Saildrive propeller split 2-piece models are available for Saildrive propeller hubs, with no saildrive, saildrive zinc or saildrive propeller hub modifications necessary.

The SHAFT SHARK is easily installed; no drilling, tapping or dismantling, the propeller does not need to be removed to install the 2-piece split cutter. Made out of 316L S.S, maintenance is zero, no moving parts (cutters/bearings/zincs/stops) to maintain or replace annually. Easily re-sharpened if necessary.

SHAFT SHARK line cutters are also available in sizes that allow straight forward installation on Shafts up to 6.50 inch diameter, including metric sizes, with 2-piece split models to suit shafts up to 3.25 + inches diameter (7″ cutter OD) for easy/simple installation, without removing the propeller – even underwater by a diver. Custom diameters & tapers can be machined into the bore to suit shafting

From the information you supplied – to suit a Gori 3-blade Saildrive propeller, you will need a Model SS696 split 2-piece unit.

The List Price is $813.00, as an export customer we can offer you a cost of $492.00.

Shaft Shark – Instructions and Guide

{ 0 comments… add one now }

Leave a Comment

Previous post:

Next post: