Do I sense a note of frustration, Frankie? You beat me to the punch.
Yipster, it always comes down to cost versus performance. That is why you don’t hear much about contrarotating propellers because they are higher performance, higher cost, higher maintenance with reduced reliability. Below you’ll see a chart of propeller efficiency for various systems and although it is graduated at the top for ships it applies equally well for boats and small craft.
The vertical axis is efficiency and the horizontal axis is the propeller loading coefficient Bp which is Prop RPM x Power^0.5/Speed^2.5. Power is the power in horsepower delivered to the propeller and speed is a little less than boat speed, reduce it by 1 knot to keep things simple.
So if we took a boat running at 15 knots, 1800 rpm and 100 hp Bp is 24.5, or 25 knots, 1500 rpm and 200 hp is 7.5, making a twin screw instead of single screw multiply Bp by 0.7 assuming no change in speed, revs and total hp.
In this range you can see the typical recreation boat propeller – Gawn. It is less efficient than the B series but cheaper. However, if you check the luxury yachts and commercial vessels the B series is very popular or propellers having the same general characteristics.
The best are the contraprops, about 5.5% better than a Gawn but a B type is about 3.5% better. I have read many times about gains of 15 or 16% for contraprops on ships but I take a pinch of salt.
You must be careful when talking about percentage improvements. Use the cube law when comparing power and speed, in other words power is a function of speed ^3 and expressing things is percentages is the best way. It is accurate enough to use one third of the power increase for the speed provided the power increase is less than about 12% or 15%, beyond that use a calculator
So Gawn to contra would give only 1.8% more speed.
If you really want improved propeller performance you should investigate the “manually operated” controlled pitch propellers. When the boat is moving slowly for prolonged periods you need something with a fine pitch whereas you need coarse pitch for speed. I have no idea about their cost but it makes as much sense as contraprops.
There is a cross-river ferry service where I live and the single screw wheel effect was awful when it was being manoeuvred. Until another skipper came on the scene, he did everything wrong and the docking was smooth! If you fight the wheel effect you’ll lose, every time, work with Mother Nature’s laws and she’ll smile at you.
Michael
Yipster, it always comes down to cost versus performance. That is why you don’t hear much about contrarotating propellers because they are higher performance, higher cost, higher maintenance with reduced reliability. Below you’ll see a chart of propeller efficiency for various systems and although it is graduated at the top for ships it applies equally well for boats and small craft.
The vertical axis is efficiency and the horizontal axis is the propeller loading coefficient Bp which is Prop RPM x Power^0.5/Speed^2.5. Power is the power in horsepower delivered to the propeller and speed is a little less than boat speed, reduce it by 1 knot to keep things simple.
So if we took a boat running at 15 knots, 1800 rpm and 100 hp Bp is 24.5, or 25 knots, 1500 rpm and 200 hp is 7.5, making a twin screw instead of single screw multiply Bp by 0.7 assuming no change in speed, revs and total hp.
In this range you can see the typical recreation boat propeller – Gawn. It is less efficient than the B series but cheaper. However, if you check the luxury yachts and commercial vessels the B series is very popular or propellers having the same general characteristics.
The best are the contraprops, about 5.5% better than a Gawn but a B type is about 3.5% better. I have read many times about gains of 15 or 16% for contraprops on ships but I take a pinch of salt.
You must be careful when talking about percentage improvements. Use the cube law when comparing power and speed, in other words power is a function of speed ^3 and expressing things is percentages is the best way. It is accurate enough to use one third of the power increase for the speed provided the power increase is less than about 12% or 15%, beyond that use a calculator
So Gawn to contra would give only 1.8% more speed.If you really want improved propeller performance you should investigate the “manually operated” controlled pitch propellers. When the boat is moving slowly for prolonged periods you need something with a fine pitch whereas you need coarse pitch for speed. I have no idea about their cost but it makes as much sense as contraprops.
There is a cross-river ferry service where I live and the single screw wheel effect was awful when it was being manoeuvred. Until another skipper came on the scene, he did everything wrong and the docking was smooth! If you fight the wheel effect you’ll lose, every time, work with Mother Nature’s laws and she’ll smile at you.
Michael
yipster
thanks Brian, and compliments on your site! fisherman did not list one but who knows, i'll reply!
