A question and a possible word of warning with regard to fitting a "sail drive" prop on an outboard for use on one of the smaller Swallow Yachts.
On my BC20 I have a Tohatsu 6hp fitted with a "sail drive" i.e. "high thrust" propeller. I have difficulty in getting the motor to run at a rate which will drive Seatern at less than a knot without the motor stalling. Thus when approaching a jetty or pontoon I have to keep going in and out of gear which is quite difficult. Indeed I've even fitted a "gear lever extension" to help me do it. The guy who services the motor is adamant that my Tohatsu is running properly. I'm therefore wondering about swapping back to the standard propeller which came with the engine and which, like Andy and Graham, I've kept as a spare. I note that Graham says that his BR20 is driven "at near maximum hull speed at remarkably low revs" ...Graham do you also have trouble going slowly???
My reasoning is that, as I understand it, the sail drive propeller produces more thrust because the blades are angled such for a single rotation more water is sent backwards. This produces more linear force to push the boat along - useful for a ballasted yacht with mast etc. adding to windage - hence "sail drive". However such a propeller requires more torque to turn it ( that's what torque is - a measure of turning force). But as Rob has said, petrol engines produce most torque at mid to high (but not maximum revs). So to go at slow speeds, by fitting a sail drive prop, I'm asking my Tohatsu to produce more torque at low revs than it would need to do if working with a standard propeller - and sometimes it stalls.
If you have a bigger Swallow Yacht like a BC23 or BC26 then you need more power to propel it so perhaps, even at slow speeds through the water, your 6hp outboard is working at higher revs and a better position on the engine's torque v power curve. My BC20 is smaller and lighter (being wood/epoxy and despite the fridge etc.) and has less windage, so a normal prop which runs at higher revs to provide the same thrust and hence maintain the same boat speed, might be a better choice. But I do note Andy's comment about the sail drive being better for reversing out of cul-de-sacs! I really need to swap my props over and see what happens!
Also, am I right in thinking that the 4/5/6 hp motors are all the same but tuned in different ways? If so it's even possible that a lower hp motor with a sail drive prop might perform better if the tuning results in more torque at lower revs. There may also be subtle differences between the outwardly identical Mariner, Mercury, and Tohatsu motors in this regard, but I can't find any information. We need a torque v power curve for the motors and a torque v thrust curve for the propellers!
If I'm right about the sail drive prop and 6hp motor perhaps not being the best choice for a 20' Swallow Yacht, there's still a catch 22! If you are buying a new motor and might possibly find you want a sail-drive prop in the future, you can negotiate a much better deal by buying the two together at the outset!
Peter
