Propeller Drive on shallow Draft Boat

I have a project for 17m Tour Boat with maximum draft of 0.4m and single shaft with propeller drive.
Looking at the previous boats owned by this client the single shaft has 2 propellers in a shaft tunnel in line. When the boat is static the propellers are only half submerged with an access hatch above dwl to access and change props. Apparently when the boat moves the tunnel fills up fully submerging the props, the 2 props allows smaller diameter. Max speed is only 9kts.
Has anyone come across this type of set up before or have any information that might be helpful please ?

 

Tandem propellers may be useful when diameter is limited, or when a single prop would not cope with the propulsion power required (high power on a small propeller leads to reduced thrust due to cavitation).

First probably used in the SS "Turbinia" in 1897 to blow all previous speed records away, so you might say the principle is known.....

If you look at modern Volvo Duoprop or Mercruiser Bravo III outdrives, they use the same idea, although with counterrotating propellers. In order to produce comparable thrust/Power figures, single propellers would have to be larger in diameter. For similar diameters, the counter rotating prop has better efficiency than the single shaft tandem, which in turn is better than the single propeller.

For minimum draft operation with props in tunnels, the shape of the tunnel is critical; f.i. the outlet should be submerged in order to reduce ventilation problems.

 

I seem to recall reading that tandem propellers which are not contra-rotating must be either separated by some distance or the aft prop run faster the the fore, or both. This is because, IIRC, with closely spaced tandem props (not contra-rotating) the aft one does not actually accelerate the water downstream from the forward and is caught up in its wash.

This is not unlike a similar problem faced when they tried to use tandem paddlewheels (which can, of course, not be used contra-rotating ... unless one got really silly and mounted them perpendicular to the direction of travel and redirected their water race with some high tech nozzle system ... heh, that's so convoluted maybe I should approach the US Navy or DARPA for a development grant?).

 

Thanks for your input, yes did notice an additional plate on the transom in way of shaft tunnel to stop air being sucked down.

 

Does anyone have an alternative idea to the single shaft with props in tandem ? We also want to do this as electric driven, the 0.4m maximum draft is a must.
The tandem props are what all the boats on the lakes and river in this particular location are using, but is there a better alternative ?

 

How about waterjets

 

False rumours....There is no need to run tandem props at different speeds, but the aft must have higher pitch and the inflow corrected for the outlet swirl from the fwd prop. This setup is simple and far more effective than a waterjet at low speeds, the calculation straight forward if you know propeller basics.

Now, if you consider electrical drive with a limited draft, the propulsive efficiency is critical. You should consider splitting the required Power on more shafts, so that the thrust load on each propeller is kept as low as possible. But fist, there has to be preliminary design values for displacement, hull shape, dimensions et c., before aiming at the propulsors.

As for runners with "perpendicular flow" (something like a submerged paddlewheel) that is a common configuration in fans. And if there is a "pumping application", there is Always a turbine variety; google "Banki turbine" and "Darrieus turbine".

 

Btw Steve, it would be interesting to see a couple of pics of existing boats.

 

An interesting comment about this set up being FAR more effective than a waterjet at low speeds.

Can you give a couple of calculations to support this. I would have thought that a tunnel would have allowed more pressure bleed off from the prop face as the water could move away radially from the axis of the prop and the same scenario with the "inflow corrected" device, stators.

As compared to a "closed tunnel" multiple staged axial flow water jet with tighter tip clearances, and possibly more thrust recover from the stators.

Plus of course the benefit of enhanced slow speed steering from a jet than can change the direction of the thrust. Additionally, not having transmission would reduce the initial costs.

 

Depending on the diameter of the tandem props ?

 

We also want to do this as electric driven,

Think 4-5K not the 9K now seen.

 

Small multi blade prop instead of tandem? Or pivoting trailing shaft motor (dragontail)? Seems like I remember a recent post on a huge trailing shaft propulsion controlled hydraulically- maybe could be modified for electric drive. A search on this list may turn something up.

PC

 

Ok, so let’s have an example. Say that we check what differences we get with a 100 hp engine, nominal shaft speed 2500 rpm (like a Cummins 4BT3,9), draft 0,4 m, ship speed 9 knots and a 5% wake fraction. It is possible to arrange a 22” Wageningen B series propeller in a tunnel. This would not be too far away from the needs for a 17 m vessel.
With this setup (and without detail fiddling) we arrive at a single propeller rotating 1100 rpm, and delivering ~7900 N thrust. A tandem prop might come out with something like 8300 N or better.

Now for a generic, direct driven waterjet. First we check what the market has to offer “off the shelf”. About the biggest impeller inlet diameter that would allow cavitation-free operation at 2500 rpm and 9 knot is ~350 mm in a standard configuration. Typical thrust value for such a unit with 100 hp input is ~5600 N, which in my world is far below the propeller performance. The jet needs a 140 hp engine, instead of a 100.

Going bigger in waterjet to get more thrust means going down in speed (ie use a gearbox) and at some point adding a second impeller. This means heavy stuff, increased losses, reduced performance and increased investment. The idea was to have electrical propulsion, which means you have to chase for every percent of efficiency and every kg of “non-productive” weight in order to have a shadow of a chance to make it work.

 

So the prop diameter is the key point.

 

Aye, this is one case where size matters more than "finesse".......