nordic_ranger
Well-Known Member
How do you size a propeller and tell what hand it is.
Propeller size.
- Thread starter nordic_ranger
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VicS
Well-Known Member
Someone on here has a prop size calculator but I don't seem to have it bookmarked.
You can tell what hand it is by standing behind it and looking at the direction it will rotate when going forwards. If that's clock wise it's right handed, if it's counterclockwise it's left handed.
Or by comparing the angle of the blades witha screw thread.
Or did you mean how do you determine the size of a prop. Measure the diameter but not so easy to measure the pitch.
usually stamped on somewhere (Diameter X pitch)
You can tell what hand it is by standing behind it and looking at the direction it will rotate when going forwards. If that's clock wise it's right handed, if it's counterclockwise it's left handed.
Or by comparing the angle of the blades witha screw thread.
Or did you mean how do you determine the size of a prop. Measure the diameter but not so easy to measure the pitch.
usually stamped on somewhere (Diameter X pitch)
davidwf
Well-Known Member
Good tip to find hand of prop is to to place the prop on the floor engine side down and try to stand on the blades only one foot will be able to, so if right hand foot can stand on it its a right handed prop.
If you try it you will see what I mean.
If you try it you will see what I mean.
VicS
Well-Known Member
[ QUOTE ]
if right hand foot can stand on it its a right handed prop.
[/ QUOTE ] And if the blade breaks off it's dezincified!
if right hand foot can stand on it its a right handed prop.
[/ QUOTE ] And if the blade breaks off it's dezincified!
nordic_ranger
Well-Known Member
This one has neither pitch or diameter marked on it. I have cleaned it well and there are no markings so I was wondering how they could be calculated. It is off my boat and I am at present changing from a Bukh 10 to a Bukh 20 engine so I'm hoping that it would do. It is a sound prop with a good ring to it when struck.
VicS
Well-Known Member
Given that the Bukh 20 has twice the power output of the Bukh 10 you will need a prop of a much larger diameter. (To give twice the area I guess, so that means about 1,4 times the diameter.) Since the speed range (RPM) of the two engines is the same you will need a prop with the same pitch as the original.
If you use the existing prop, assuming it is correctly sized for the Buhk 10, you will find that the engine max rpm will be reached far before full throttle and it will be developing much less than its full potential power output. If you open the throttle the power of the engine will not be utilised in pushing the boat forward simply because the prop will not be big enough to transfer that power to thrust. That will result in the prop cavitating.
But don't listen to me if someone who is a real expert on props comes along.
If you use the existing prop, assuming it is correctly sized for the Buhk 10, you will find that the engine max rpm will be reached far before full throttle and it will be developing much less than its full potential power output. If you open the throttle the power of the engine will not be utilised in pushing the boat forward simply because the prop will not be big enough to transfer that power to thrust. That will result in the prop cavitating.
But don't listen to me if someone who is a real expert on props comes along.
VicS
Well-Known Member
On further reflection I imagine that you want the boat to go faster, although it wont go faster than the speed determined by the waterline length. If that is the case and it will be at the same engine revs that you use at present then the prop pitch will have to be a bit more than your present pitch.
It is all an area in which you need to take proper advice and fit the size prop recommended for the Buhk 20 in your boat.
It is all an area in which you need to take proper advice and fit the size prop recommended for the Buhk 20 in your boat.
srp
Well-Known Member
About 18 - 24 months ago someone posted a method of calculating the pitch of an existing prop by taking a few measurements - I have tried and failed to locate it. All I remember was that it involved drawing a line across the prop blade at its' widest point. I also remember trying it out on my (then new) prop and getting a sensible answer. It's out there somewhere.....
VicS
Well-Known Member
[ QUOTE ]
It's out there somewhere.....
[/ QUOTE ] I remember it too but it'll be difficult to find. I think the size of the present prop is irrelevant. It won't be suitable and the only sound advice is to get the correct prop for the new engine.
Someone on here has a calculator or spreadsheet for determining the correct prop.
Edit: I think I heve found the calculator On this website
It's out there somewhere.....
[/ QUOTE ] I remember it too but it'll be difficult to find. I think the size of the present prop is irrelevant. It won't be suitable and the only sound advice is to get the correct prop for the new engine.
Someone on here has a calculator or spreadsheet for determining the correct prop.
Edit: I think I heve found the calculator On this website
nordic_ranger
Well-Known Member
Thanks for your replies, I don't want the boat to go faster under normal conditions. As you rightly say the speed is governed by the hull size. What I want is to have more power so that in difficult conditions I can motor home faster. With my present set up ( Bukh 10 )heading into an on coming sea I can be slowed down to about 2/3 knots so an extra couple of knots would be just great.
Eeyore
Well-Known Member
It's easy to tell what hand your prop is without knowing the direction of rotation or resorting to standing on it. Looking from astern, if the edge of the blade furthest away from you is on the right, it's a right hand prop and rotates clockwise when going forward. Obviously the reverse applies if the furthest edge is on your left. Determining the size required and the pitch is more complicated and not an exact science, and one which the 'experts' get wrong too. Engine hp, ratio of gearbox, number of bearings between engine and prop, waterline length, weight of boat, usage of boat, clearance from top of prop to hull of boat are just a few of the measurements needed. Every one inch increase in pitch size slows the engine considerably and does not give a corresponding increase in distance travelled with each rotation of the prop because of slippage. The 'experts' told me I needed to increase the pitch by 5 inches when I re-propped my little sailboat a few weeks ago....that would have all but stopped the engine!! Googling the info you want provides enormous amounts of interesting bumf. Good luck.
