Power Driven 'Freewheeling Prop'

Ok, the reversing mini g'box is the simple solution to counter rotating props.
It will need 'neutral' or a built in clutch.

Bolt pulleys to the output shafts of both main gearboxes.
Bolt pulleys to 'both sides' of the new 1/1 mini reversing g'box

One prop shaft will permanently drive the mini g'box with v-belt or chain.
The other prop shaft will temporarily drive through 'neutral' or 'clutch' of the mini g'box controlled from the helm.

Can you see Any problems with this simple set up?

 

Given that the engineering is sorted, how much improvement will there be by driving the freewheeling prop: Will a twin engined boat be able to match the economy of the identical single screw boat of the same length/displ.?

 

This is just not at all true for marine propellers. Helicopter rotor blades have an area of just a few percent of the disk area while marine propellers cover 25-100%. Drag and thrust of a rotating propeller are related to disk area and locked propeller to blade area.

There are many measurements of drag of a freewheeling and locked fixed marine propeller and the drag of a freewheeling one has always been less (as long as it rotates).

When a helicopter makes an emergency landing without an engine, it uses forward motion to keep the rotor rotating at sufficient speed, just like an autogyro. Freewheeling rotor acts like a wing of an aeroplane. Parachuting directly down is not very helpfull.

 

Just to contrast the GB 42 with a more modern take on a trawler theme here's the specs of the Swift trawler 44'


Loa 45'
Beam 13'9"
Draft 3'4"
Displ 11tons.
Twin 300hp Volvo.

1500rev....8kts....13.5 ltr.......2.8mpg (uk gals)
2200........10.........38...........1.28
2600........14........58............1.12
3600........24........117...........0.95

Wow!!!

 

In order to reach better mpg, you need to go slower! There is a huge difference between 8.7 and 8 knots not even to mention 6 knots with such a boat and LWL around 12 m.

If your speed is 8.7 knots and the diesels take 59 l/h, they producing (together) about 300 hp power and the thrust is in the order of 30 kN.

Now lets compare that to the drag of one propeller. Say you have a 20" diameter and quite large EAR. Then the drag of a locked propeller could be up to 2 kN and freewheeling up to 1 kN. So you could reduce the drag of this boat only by a few percent by rotating or removing the other propeller. Even if your propeller diameter would be 30", the drag would still be just up to 4.5/2.3 kN so up to 15% of the total.

Going slower reduces the drag dramatically. The drag of the other propeller depends on V^2 and the drag of the hull has a much higher exponent. Thus at much lower speeds you may need to worry about the other propeller as well.

 

Aside: the whole drag thing is not linear is the reason I'm going to use a CVT for any paddlewheel boat I build. Especially if steam keeps whispering to me (though for a small enough engine hit or miss Los is attractive). It is simply horribly inefficient to have the same short gear ratio at low speeds as needed for "high speeds". This in addition to the inclusion of a CVT make it easier to get the "gear ratios right" in the first place (rather than go through some iteration of swapping out gears) since I'll only have to get within a given range and not spot on for some particular cruising speed.

 

If you find it hard to locate a CVT transmission it might be worth considering the Hewland straight cut racing gearboxes; they are available second hand and you can get one with a selection of lots of different gear sets: it only takes 10 minutes to change the gears!

 

The problem is getting an accurate mpg consumption graph for a hull; magazines will always try to make the boat look good if the manufacturer is advertising with them.

Apparently the Dashew hulls work on perpetual motion, they would like to make you believe!

Anyone like to suggest a 42' hull with single engine to compare to the GB42 or ST 44?

 

Many transmissions will burn out by freewheeling. They need water circulation for cooling and the hydraulic pump for lubrication; neither of which are available with the engine off.

 

Some of the zf and Prm gearboxes are listed as ok for freewheeling.
What about the old velvet Drives?

only problem is what sort of g'boxes is an old 1980's boat going to come with?

 

CVTs (that don't go in cars or hypothetical fast boats with lots of power) are actually very simple.

My Shopsmith uses one to achieve a 1:7.4 speed range from a constant velocity motor. Just a belt run between two adjustable width pulleys made of flat-ish cones with a common adjustment screw (for by-wire control just one axis to program). To handle more power the best solution is to have more pulley sets run in parallel. More machining to build, though one could choose to term it eye candy instead, but the belts are a lot less expensive and the set up should be (home built) more reliable than trying the same thing with one much wider belt.

 

Makes good sense. I was thinking along the lines of constantly variable Cvt gearbox used by cars; but of course you would be happy to adjust the pulleys by hand.

 

I'm not familiar with Dashew, but 59 l/h for a 16 tonne 45' boat at 8.7 knots is VERY far from optimal. Any 16 tonne 45'-60' sailboat would easily do 8.7 knots at 1/3 of that. I just calculated the drag of Swan 65 (35 tonne!) at 8.7 knots. It is just 5.6 kN (smooth hull, calm water) so more than 5 times less than GB 42 (based on consumption) despite more than double the displacement. A Swan 65 would have 30 kN drag at around 11.7 knots. At 6 knots Swan 65 has only 1.6 kN drag and thus needs less than 20 hp and fuel consumption would be just 3-4 l/h.

So a hull made to be efficient at hull speed and below can have very good mpg. GB 42 and similar are far from that, but even they will be quite economical at slow enough speed. I have no data for them, but here are measured fuel consumption from a Finnish magazine for Linssen Grand Sturdy 33.9 AC (LOA 10.35 m, 8.3 tonne, 75 hp Volvo Penta):
Knots l/h
2.8 1.2
3.4 1.6
3.9 2.1
4.4 2.6
5.0 3.4
5.5 4.2
5.8 5.3
6.3 6.8
6.6 8.8
7.1 11.3
7.6 16.7

7.6 is close to hull speed and is thus equal to 8.7 for GB 42. Slowing down 28% (7.6->5.5 = 8.7->6.3) reduces consumption by 75% thus giving almost three times better mpg.

 

Maybe the GB 42 was quoted at 8.7kts because 1500revs is the minimum recommended constant use speed for the engines?

1ltr/nm at 6 kts is very believable for a 10 mtr boat, and I think a lot of trawlers like the island gypsy 32, Chb 32, Mainship 34 and maybe even the GB 32 sedan could just about do those numbers!

As you say the line on a graph for displ. Speeds is very steep indeed.

 

My 11 m 6 tonne sailboat takes 2.2-2.5 l/h at 6-6.5 knots so I feel 1 ltr/nm is huge for 10 m boat at 6 knots.

Even a Bavaria 38 Ht (11.8 m 9.1 tonne) with 2*300 hp takes just 4.1 l/h at 5.7 knots or 7.3 l/h at 6.9 knots (131 l/h at 36.0 knots) although it is a prismatic hull made purely for planing speeds.