Prop recommendation for Defender 110

[willk]

Some of you will be aware of my recent adventures: http://www.rib.net/forum/f16/land-ro...tml#post694692

All good fun and games but I have a serious question for the Prop Polishers - can RIBnet come up with a better prop for this puppy? (see photos below)

The "vessicle" is powered by a very standard engine - I think it's a 300tdi so maybe making 100-125hp? I'm not sure if it's still turbo charged. Actually, I need to get more info about the current set up - prop size, shaft fitting, HP etc, but we can think about it anyway. The current setup makes a top speed of 3.5kts. We'd like to increase that by a knot - it would help immensely!
Some facts: The vessicle sits partly in the water and steers with it's front wheels and/or a tiller. The prop is JUST below the water and there is a crude anti ventilation plate fitted to the bodywork. The drivers tend to run it in fourth gear at 1500rpm. At those revs it makes 3.5kts and if pushed harder will begin to overheat with no noticeable increase in speed. I understand that she is aircooled as per a standard landy - not seawater cooled. I'd say we could look at a radiator spray fitting to resolve that! The current prop is from some sort or canal barge or cruiser - or so they think.
The total rig must weight 2.5 tonnes loaded. It is a heavy displacement load. One catch is just that - the prop can't be deep enough to catch when driving on land. Anyway - your thoughts please?

[John Kennett]

The basic equation for maximum hull speed (in knots) before your vessicle starts climbing the hump is 1.34 x the square root of the waterline length (in feet).

Your waterline length is about 5m, say 16ft. So your theoretical top whack is 5.36 knots. Given the fact that you are dragging four wheels through the water, you're unlikely to get close to that.

From the bottom pic it does look like a lot of prop effort goes into making froth. Even if it was better directed that elusive extra knot may be hard to find, but that's where I'd start looking. Could they put some sort of nozzle around the prop?

[Rokraider]

We have a couple of amphibious Land Rovers in our collection. The original was the APGP Wader built on a 109 series IIA chassis using Avon tubes that were inflated using the exhaust gas.. That had a propeller mounted on the rear propshaft , just ahead of the rear diff. The wheels would be driving as well and the steering is done by the front wheels. We swam that a few times and it struggled to do better than 2 knots. It was intended to just be able to get across rivers and wasn't very good at that even. Land Rover also built a hard hulled amphib, it was a prototype for the Australian Army to evaluate (see pic) It never got beyond prototype and spent most of its life at Eastnor castle as a fishing vehicle for the Major to use on his lake.

We also have a Discovery that has been fitted with tubes and a hydraulically powered stern drive on the rear. That seems to work quite well and we took it to a Land Rover launch in Geneva (I think) where it was trundling around the lake. Land rover also built a 90 Amphib that was used at Cowes week some years ago when they were the main Sponsor.
All the time you have axles hanging under the hull and tyres , it is going to struggle. If you look at the Amphibs of WWII such as the Jeep and DUKW, they were designed as a hull that wrapped around the axles improving the waterflow. The DUKWs were very successful and a few crossed the channel for the D Day celebrations. The Jeep was ok as well.
I would have thought a sterndrive running off the PTO on the transfercase would be fairly efficient, but it is going to struggle to push it through the water at any speed with all that drag.
The Land Rovers had all their cavities such as chassis, bumpers and other nooks and crannies filled with expanding cellular foam, to help with buoyancy.

[willk]

Interesting info Boss. I reckon they can pull a steady 3.2kts on dead water. Even 0.5 of a knot would make a huge difference over distance. I believe someone else had suggested a nozzle previously - so maybe one for them to look at - they're very mechanically minded (read Biffer on speed).

The machine has to remain road legal so everything has to be easily removable.

Rokraider - it may not be relevant, but do you recall what gear and rev you guys would run in/at?

[Rokraider]

I will ask my brother who has used it in anger. The APGP only had a 2.25 petrol engine, so wouldn't be making anywhere near as much power as a TDi.
The one in the picture has a 2.6 petrol engine, makes reasonable torque. I will check it out.
What propulsion setup do you have on the Defender?

[willk]

Originally Posted by Rokraider

I will ask my brother who has used it in anger. The APGP only had a 2.25 petrol engine, so wouldn't be making anywhere near as much power as a TDi.
What propulsion setup do you have on the Defender?

They have a shaft and prop running from the back PTO. I think 4th gear is "straight through"? I should stress that I'm not part of the team - I just did a bit of support boat stuff for them...

I'm beginning to think that power isn't the issue here - maybe more the aeration issue and drag. Still, can't hurt to consider any possible improvements. I think they're planning something bigger

[John Kennett]

Unimog? That would be cool. And reassuringly expensive of course

[willk]

Originally Posted by John Kennett

Unimog? That would be cool. And reassuringly expensive of course

Did I say bigger? I meant Further!

[Rokraider]

If they fitted a stern drive and a hydraulic motor and pump, they would have a much more efficient drive and you can trim it and tilt it right up to get the prop out of the way when not in water. You could also do away with that hideous rudder setup. A good size hydraulic tank could be positioned almost anywhere in the back to get the balance right with a heat exchanger to keep it cool. 4th gear is a direct drive and probably the best one to use. Fit an oversized turbo which will be lazy on road, but will make better power when the engine is working hard on the water.

[Siochair]

Would it be possible to fit removable grp panels underneath to smooth out the water flow? Even with cutouts for the wheels to retain steering it should have some effect, whether enough though... If you have the time and know someone with the ability you could also build in a prop tunnel to help smooth the water flow over the prop. Whilst I don't think this would make a huge difference, if done well, it should maybe give you a knot or so.

[Erin]

From my fairly restricted experience of shaft drive semi-displacement boats I think shaft speed tends to be around 1750rpm for best efficiency. I guess after that you start to induce too much prop slip and lose thrust. I wonder whether a smaller diameter 4 blade might help. It could be mounted a tad lower which would also keep it further from the surface and reduce ventilation. There is a rule of thumb somewhere about distance between top of prop and underside of hull. I'll let you know if I find it.

[Starovich]

Go the clarkson route and fit a oversized OB

[willk]

All grist for the mill, guys - keep it coming!