I took A89 Colchide out of the water for winter a few days ago, accompanied by East Coast Rep- Nick Phillips and Martin Bennett. They were surprised how small A89’s prop is. Martin actually called it an “egg whisk”. It will only propel A89 at about 3.5 kts.
It seems it was bought by Uncle Bernard in Switzerland as a 14 x 12, but when it was offered up to A89, it was felt that there was insufficient clearance to the hull, so one inch was cut off each blade. The picture shows the original 2 bladed folding prop behind Martin’s spare 3 bladed 12 x 7 prop, which also has the markings 1284 & 2147. We are going to try Martin’s spare prop next year.
I then spoke to our Commodore, Colin and he has a 11.5 x 11 3 bladed prop with markings 1284 & 2962, and this will propel Hiran at 7 kts. There is a very interesting website vicprop.com, where you can input various figures and find out the ideal engine HP and prop size for an Atalanta. I am interested to hear from any other owners of: Engine HP, Prop size and no of blades, max speed in still water. Many thanks.
2 thoughts on “Propellor sizes”
Hi Richard – When I swapped the 3 blade for a folding 2 on Jaunty picture of prop below (from Jaunty folder). This was a 12 inch and -as noted in the ST article – easily propelled Jaunty beyond hull speed. Have opted for a similar prop on KP. The problem is more likely lack of rpm. Possibly wear on the hydraulic pump or motor has reduced efficiency over the years, so testing these would be worthwhile before reverting to the 3 bladed option. I agree with Trevor on the downside of 3 bladed fixed props – the phrase “dragging a bucket” sums it up.
I am not sure that the three bladed propeller will make that much difference to the boat speed under power. It will probably cost you 1/2 to 1 knot under sail!
However I think the big issue is the power taken to drive the hydraulic system itself. I remember reading articles about them in the past, and meeting owners with hydraulic drives in the dim and distant past. My memory suggests that a system like yours will absorb about 10hp in just driving itself. So Colin’s 17hp driving through a gearbox and giving 7knots should be compared with 7hp (17hp minus 10 hp) through an hydraulic system giving 3.5 knots. Both seem about right.
This was why hydraulic drive was only really used with larger engines. If you have 100hp you can afford to lose 10hp without it affecting performance significantly. It was also usually associated with some other design requirement, such as getting the weight of the engine forward in a racing boat, or an awkward shaft line.
So in summary I suggest you don’t discard the old propeller! The shaft diameter is not the critical issue in fitting the three bladed prop to your shaft. What matters is the taper and the keyway. 1 in 12 is normal. If both props have the same taper, and key dimensions, and they probably have, then the three blader may just fit on without alteration. Whatever you do don’t alter the shaft in any way which prevents you reverting to the folding prop!