Calculating gearbox ratio/prop size

[eddystone]

How do you go about calculating prop size for a new engine installation and almost certainly changing 2 blade fixed for 3 bladed feathering? As I understand it the aim is to achieve maximum theoretical hull speed at fast cruising rpm, but not max rpm, so what percentage of maximum revs that would be, or does one look at the torque curve for the engine. With a feathering prop, I thought pitch is not a variable, although the Featherstream does allow some manual adjustment, possibly to fine tune the installation. I also know the gap between the arc of the prop and the hull should not be less than 10% of the diameter. Beta Marine seem to supply a gearbox with either 1.5:1, 2.0:1 or 2.6:1 ratios so this must also be part of the calculation. Any other variables?

 

[Tranona]

You don't have to do anything, unless you want to for interest.

The norm these days is to have a 2:1 reduction. The 1.5 is for higher speed applications, and the 2.6 where you want to use a large diameter flat pitch prop, for example on a heavy boat with space to swing a large prop. None of this applies to your Sadler 32 as your prop size is restricted and 2:1 gives you the right shaft speed to turn a prop that fits your space. The actual size will depend on the engine size you choose (25 or 30hp) and the design of the prop. This you leave to the supplier - if you buy a Featherstream, Darglow will pitch it exactly for your boat based on their extensive experience of similar installations. Other prop suppliers will do the same.

If you are interested in doing your own calculations then suggest you look at the Propcalc programme on www.castlemarine.co.uk As you will see it needs LWL, displacement, engine power, reduction ratio, 2 or 3 blade and any diameter restriction (which you don't have). This will give you a reliable estimate, but maybe not exactly the same as the actual prop you buy.

As you say a Featherstream is adjustable but you are unlikely to need to change it, although some people (like me) have the reverse pitch set a bit coarser to improve stopping power. I did this because the boat is heavy long keel with only 14hp so runs a 2.6 reduction, 15" prop (which is much larger than the normal 13") low pitch in forward and coarser pitch in reverse. You should not need this on your boat - just given to illustrate the principles of prop sizing.

 

[eddystone]

You don't have to do anything, unless you want to for interest.

Not so much out of interest as knowing enough to see whether any flaws in reasoning or calculation of supplier. Darglow (whom seem a decent outfit) have similar installations on file with 16". Installer's first thought was 14", assuming I was having a 3 blade fixed - both could be right.

 

[Tranona]

Not so much out of interest as knowing enough to see whether any flaws in reasoning or calculation of supplier. Darglow (whom seem a decent outfit) have similar installations on file with 16". Installer's first thought was 14", assuming I was having a 3 blade fixed - both could be right.

In general terms the bigger the diameter the better, particularly in adverse conditions. The important thing is the combination of diameter and pitch. Therefore if you have a 14" diameter you will need a greater pitch to put the necessary load on the engine.

Suggest you go with Darglow's recommendation for 2 reasons. First they are then responsible for the performance you get and secondly they are arguably the most experienced propeller people for yachts in the country with getting on for 50 years' data to draw on. They have done 4 boat/propeller combinations for me and each has been spot on.

 

[JumbleDuck]

If you are interested in doing your own calculations then suggest you look at the Propcalc programme on www.castlemarine.co.uk As you will see it needs LWL, displacement, engine power, reduction ratio, 2 or 3 blade and any diameter restriction (which you don't have). This will give you a reliable estimate, but maybe not exactly the same as the actual prop you buy./QUOTE]

It also needs an antediluvian OS to work.

 

[Tranona]

If you are interested in doing your own calculations then suggest you look at the Propcalc programme on www.castlemarine.co.uk As you will see it needs LWL, displacement, engine power, reduction ratio, 2 or 3 blade and any diameter restriction (which you don't have). This will give you a reliable estimate, but maybe not exactly the same as the actual prop you buy./QUOTE]

It also needs an antediluvian OS to work.

Yes, I know but not a problem for pioneers like me!

 

[RichardS]

If you are interested in doing your own calculations then suggest you look at the Propcalc programme on www.castlemarine.co.uk As you will see it needs LWL, displacement, engine power, reduction ratio, 2 or 3 blade and any diameter restriction (which you don't have). This will give you a reliable estimate, but maybe not exactly the same as the actual prop you buy./QUOTE]

It also needs an antediluvian OS to work.

Just what are you smokin' JD? This post comes to you courtesy of Vista ..... still Bill's best I reckon.

Richard

 

[JumbleDuck]

Just what are you smokin' JD? This post comes to you courtesy of Vista ..... still Bill's best I reckon.

And 0.61% of the internet agrees with you!

https://netmarketshare.com/report.a...7-01","dateEnd":"2017-12","segments":"-1000"}

 

[RichardS]

And 0.61% of the internet agrees with you!

10 times less than are still using XP? What madness is this?

Richard

 

[Tranona]

10 times less than are still using XP? What madness is this?

Richard

I hedge my bets with XP on this desktop and Vista on my (rarely used) laptop.

Nice to be member of at least one elite!

 

[lw395]

How do you go about calculating prop size for a new engine installation and almost certainly changing 2 blade fixed for 3 bladed feathering? As I understand it the aim is to achieve maximum theoretical hull speed at fast cruising rpm, but not max rpm, so what percentage of maximum revs that would be, or does one look at the torque curve for the engine. With a feathering prop, I thought pitch is not a variable, although the Featherstream does allow some manual adjustment, possibly to fine tune the installation. I also know the gap between the arc of the prop and the hull should not be less than 10% of the diameter. Beta Marine seem to supply a gearbox with either 1.5:1, 2.0:1 or 2.6:1 ratios so this must also be part of the calculation. Any other variables?

If you want a full answer, you need a book like Dave Gerr's Propellor Handbook. And possibly something on yacht design.
Diameter and pitch are the main variables.
Choice of diameter affects the pitch.
A larger diameter will slip less, the blades operate at a smaller angle of attack to generate the same thrust. So it will have a finer pitch, and probably be more efficient.
But a smaller diameter with a slightly coarser pitch might work better when the loading changes, e.g. into a headwind, where the big prop which is optimal for flat water might be too much for the engine to turn.
It's not exactly like picking a gear ratio for a car, the prop works partly by throwing water back as well as pushing the boat forwards.
It is a game of balancing speed and efficiency in flat water against flexibility in less ideal conditions. You want to be able to use the power of the engine at 4knots into wind and chop as well as cruising fast in flat water.
Blade area and blade shape also make differences.
So there may well be a range of 'correct answers'

 

[eddystone]

If you want a full answer, you need a book like Dave Gerr's Propellor Handbook. And possibly something on yacht design.
Diameter and pitch are the main variables.
Choice of diameter affects the pitch.
A larger diameter will slip less, the blades operate at a smaller angle of attack to generate the same thrust. So it will have a finer pitch, and probably be more efficient.
But a smaller diameter with a slightly coarser pitch might work better when the loading changes, e.g. into a headwind, where the big prop which is optimal for flat water might be too much for the engine to turn.
It's not exactly like picking a gear ratio for a car, the prop works partly by throwing water back as well as pushing the boat forwards.
It is a game of balancing speed and efficiency in flat water against flexibility in less ideal conditions. You want to be able to use the power of the engine at 4knots into wind and chop as well as cruising fast in flat water.
Blade area and blade shape also make differences.
So there may well be a range of 'correct answers'

Very helpful info. - thanks