What's the available options Serb? Going from 11.5 inch diameter to 12 inches seems no big deal, but the pitch going from 8 inches to 15 is a huge jump. I would suggest telling Austral what happened with this prop, then swap it for something with the same diameter, but far less pitch, perhaps 8 inches as before, or 10 inches.
Some of the problem is that there are so many variables - boat weight, hull shapes, engine size, etc. that finding comparisons are not easy, so it is some trial and error.

My outboard motor came with a 3-bladed 8inch pitch, but I spent another $200 to get a 4-blade 5inch pitch propeller of the same diameter. Most outboard propellers are for speed boats, but I have a 3 500kg yacht, so the pitch needed to be much smaller.

Serb1980 said..
I just don't understand why is so hard to find the right bloody dimensions for this propeller. Even people in Austral can not fix it. The old one was 11.5x8 the new one is 12x15 and is over pitched is there anybody in this world that csn tell me what size prop I need to buy to get close to 6knots of speed. The old prop was pushing 4knots but was broken, the new one will break the engine and goes 2 knots.. something in the middle? 12x10? I just don't bealive in this...so much time and money spend and nobody can get it right...
I was hoping that at least one person on this forum will have similar boat with the same engine to suggest the prop side but it look like it just not the case.....

Looks like you now realise the prop is over-pitched. The reason is you kept insisting on a high boat speed selection, but your engine/boat combo is not big enough to get there. Pitch is like gearing in a car, and on a boat you only have one gear. You asked for top gear when your "car" needs 3rd to get up the hill.

UncleBob said..

Serb1980 said..
I just don't understand why is so hard to find the right bloody dimensions for this propeller. Even people in Austral can not fix it. The old one was 11.5x8 the new one is 12x15 and is over pitched is there anybody in this world that csn tell me what size prop I need to buy to get close to 6knots of speed. The old prop was pushing 4knots but was broken, the new one will break the engine and goes 2 knots.. something in the middle? 12x10? I just don't bealive in this...so much time and money spend and nobody can get it right...
I was hoping that at least one person on this forum will have similar boat with the same engine to suggest the prop side but it look like it just not the case.....

In one of your previous threads I suggested Porters at parramatta, I stand by that suggestion.

+1 Porters are great with this sort of thing

8" is a bit too little. Propeller only responded to half throttle after that engine would scream but no response. Austral is blaming me because I told them the wrong gearbox ratio (,3:1 instead 2:1). I apologised for misinformation. I got the wrong info from yanmar. Now Austral is repitching the prop to 12x11 so 4" down and that will probably give some more revs to the engine. I was a bit suspicious when they told me that pitch is 16 but they will make 15. I knew that 8", is too little but 15 is almost double..

I will let you know On Thursday how 12x11 will perform. So far the old one(11.5x 8) 4 knots in good shape, 12x15 3 knots but chocking the engine, will see 12x11....

Keep in touch

I am new to this forum as you can see that this is my firs post. I have had to join in as I do have some relative information which may help Serbs propeller problem.
My current yacht is a Tasman 26, some 40 years old in good condition, which recently had a new Yanmar 1GM10 (9hp) diesel installed after a traumatic runaway diesel episode with the original engine,
In addition to the new engine the original propeller was replaced due to a cracked hub. The old prop was a 2 blader, 13inches in diameter, pitch unknown as any numbers/letters stamped on the hub were long gone.
The replacement prop was made by Austral Propeller Company, again a 2 blader with the following specifications:
26foot OAL/2.5T/Monohull yacht with Yanmar1GM10 9bhp @ 3600 RPM & 2.62:1 Gearbox.
Prop details:
13"dia x 9"pitch RH 2B fixed yacht-AB2. Prop stamped as: 13DR2YH
Bore details: Will depend on propeller shaft details.
Whilst these propeller details refer to a 2blade prop and not the three bladed version that Serb is using, the diameters are not vastly different, the point of issue here being the prop pitch's. I don't believe there should be a big difference in pitch between the 2blade and 3blade propellers, keeping in mind the differences in engine RPM (at 3600 WOT) and gearbox ratio (2.62:1), compared to these details for Serbs engine and gearbox combination. These will determine the speed of rotation of the prop and in combination with pitch will determine the boats speed. The trick is to match the Tasman's hull speed (6.14kts) to the wide open throttle (WOT) engine speed with a correctly pitched prop. Not an easy task as there are lots of other variables to consider.
BTW its worth considering a 2 bladed prop as they can be locked in gear in line with the keel and strut to minimise drag when sailing.
Hope some of this will help.

AlphaCentauri said..
I am new to this forum as you can see that this is my firs post. I have had to join in as I do have some relative information which may help Serbs propeller problem.
My current yacht is a Tasman 26, some 40 years old in good condition, which recently had a new Yanmar 1GM10 (9hp) diesel installed after a traumatic runaway diesel episode with the original engine,
In addition to the new engine the original propeller was replaced due to a cracked hub. The old prop was a 2 blader, 13inches in diameter, pitch unknown as any numbers/letters stamped on the hub were long gone.
The replacement prop was made by Austral Propeller Company, again a 2 blader with the following specifications:
26foot OAL/2.5T/Monohull yacht with Yanmar1GM10 9bhp @ 3600 RPM & 2.62:1 Gearbox.
Prop details:
13"dia x 9"pitch RH 2B fixed yacht-AB2. Prop stamped as: 13DR2YH
Bore details: Will depend on propeller shaft details.
Whilst these propeller details refer to a 2blade prop and not the three bladed version that Serb is using, the diameters are not vastly different, the point of issue here being the prop pitch's. I don't believe there should be a big difference in pitch between the 2blade and 3blade propellers, keeping in mind the differences in engine RPM (at 3600 WOT) and gearbox ratio (2.62:1), compared to these details for Serbs engine and gearbox combination. These will determine the speed of rotation of the prop and in combination with pitch will determine the boats speed. The trick is to match the Tasman's hull speed (6.14kts) to the wide open throttle (WOT) engine speed with a correctly pitched prop. Not an easy task as there are lots of other variables to consider.
BTW its worth considering a 2 bladed prop as they can be locked in gear in line with the keel and strut to minimise drag when sailing.
Hope some of this will help.

Thanks buddy,
I got the same answer from Austral yesterday. As soon we understood that gearbox ratio is 2:1 Stephan from Austral sad he will reduce diameter to 11 and compensate with bigger pitch 11 instead of 9 because the propeller they got was originally 12x13 and they can only go 2 inches in pitch up or down.... I am not trying to overpitch to get my coffee quicker, I am trying to get max of the engine without killing it... potters sad the same thing for left rotation 12x13 prop casting and than we will change the pitch but due to incorrect gear ration they recommend 13x13.

Let's see how 11x11 will perform That is the minimum they can go on this casting.. the old broken prop was 11.5x 8 and was reving like crazy, with a little bit of luck this might get close to full RPM to achieve that bloody 6 knots in the morning having my coffee. The problem is that in the morning there is no wind otherwise I would sail around...
Will let you know how I go. Can you achieve max speed of 6 knots in calm water?
Thank you
Cheers

Serb

Serb,
The reason I can fit a 13" prop is because of underwater differences between the two boats in the way the prop and and shaft are mounted. My understanding is your prop and shaft are mounted on a strut some distance out from the keel and stern bearing. The length of this strut will constrain the clearance to the hull.
The arrangement on my Tasman has the prop mounted right back at the rear of the keel where the prop shaft exits the keel through a bulge containing the stern bearing. The clearance here is some 10 to 11 inches prop shaft to hull, plenty of room to accommodate the 13" prop. A photo would explain this more clearly.
Can't tell you if the new motor and prop will achieve 6 knots, as ill health and lockdowns have prevented me from testing. The boat is moored at Port Stephens and I live in Sydney, so getting to the boat for an extended period has been a bit of a problem.
It will be interesting to see how the 11"x11" prop will go. You said your gearbox reduction ratio was 2.0:1 but what was the max engine RPM you were running to achieve hull speed. I am sure the 11" pitch will be an improvement over the 15" pitched prop as that was way overpitched. Remember that if you don't get everything right in the max engine revs, gearbox ratio, prop pitch train it will be difficult to get near the desired hull speed.
So let us know how the 11"x11" prop goes and good luck.
BTW has your hull been antifouled ?

Cheers........................ AC

Omg.....
Have a look at this.. enough seaweed to feed 50 people

Woow,
That is so cool. I cannot touch the boat it's still wet from chemicals..

Cool

Thanks

6 knots
Yesssssss

Hooray! Happy sailing Serb. Who needs to meditate in a cave to achieve profound acceptance of the uncontrollable? Just buy a yacht!

Tank is a kind offer, Serb.

Hope you can enjoy our coffee now.

ary

Sunday morning, it shouldn't be more than 10 min in calm water to get to opera house and enjoy my coffee...13km/h is actually crazy...been told by many not to dream about 5 knots but flying 7 it's just unreal...

Sooooooo, after all the argy-bargy and profound wisdom we all put in, what about that famous bloody cup of coffee???????