Jet or Propellor driven

[Andy StonemanParticipant @andystoneman15177]

Hi recently having a chat along the waterfront about boating in general, which would be the fastest propelled boat, Given the same amount of power what would be driven the fastest, One driven by a conventional three bladed prop or a Jet driven boat ?I would be interested to hear anyones opinion or experience of both methods of propulsion

Andy

[Andy StonemanParticipant @andystoneman15177]

[Malcolm FraryParticipant @malcolmfrary95515]

Purely empirical, how many jet drive racers are there where they are allowed in class in the real world? As opposed to regular prop drive in classes where both are permitted.

Obviously a jet drive is faster than an exposed prop if the blades get whacked off due to hitting submerged obstructions.

[BanjomanParticipant @banjoman]

Given that, in full-size practice, the water speed record has been held mainly (exclusively?) by jet-powered vessels for 50 years and more, I think one may assume that in absolute terms, jet drive is faster than screw propeller drive.

Whether this holds true also when scaled down to model size, I do not know, but it wouldn't surprise me if such were the case.

However, the specific issue here as posed by Andy seems to be which would be faster with everything else being equal, which as far as I am able to understand things (which is not very far, I hasten to add) would boil the matter down to a question of propulsive efficiency (**LINK**) i.e. what percentage of the energy put in that is turned into useful energy.

Again, I would not be very surprised if it turned out that a jet drive would beat a screw propeller drive under those circumstances, but if it is so, I do not know why, nor would I be able to offer any explanations. Hopefully, better minds than mine will come to the rescue here!

Mattias

Edited By Banjoman on 22/08/2016 10:35:11

Edited By Banjoman on 22/08/2016 10:36:31

[ashley needhamParticipant @ashleyneedham69188]

Andy.

I think that we are assuming you mean a ducted fan setup when you say jet? A pure jet boat would of course be much faster if you could stop it flying, and this is the issue.

Our experience of ducted fan drives at Bushy is that you need a very special boat to achieve any sort of decent speed. Airflow into the fan is a big issue, and also he fact that the thrust line is above water as opposed to below (or ON water) for a prop. Going very fast is much easier with a normal prop drive.

I would imagine that a prop driven boat would be quicker for the same motor power unless you spent considerable time and effort into designing a special hull for the EDF.

Consider just for arguments sake, my HMS Midge 1890`s torpedo boat. This has a speed 400 engine in it and is surprisingly quick. If I were to put an EFD on it of similar power, it would pootle along like a tug towing a supertanker. One of our chaps built a hull especially to put an EDF into, an EDF of the sort that would hoover up all the ducks on the pond and it was horribly slow, It would have been horribly quick on a conventional drive.

BUT as I said, with just the right hull…

Ashley

[Ray Wood 3Participant @raywood3]

Hi All

I built and had published Wetjet in MB back in 2007 a EDF Womotec fan & brushed motor with nicads, It looked fast but never got up on the plane, the tunnel hull had 2nr sponsons but the surface tension slowed it down, but now with brushless motors and Lipo's which are lighter it would make a difference. I always thought a twice size version with a gas turbine would do the business, by the boss wouldn't like the £2.5k it would cost ! and where would you run the beast ? Coniston I guess

Regards Ray

[Dave MilbournParticipant @davemilbourn48782]

Before we go any further, Andy, could you clarify whether you mean a waterjet (which sucks in water via an impeller and blows it out of a nozzle at the back) or a conventional turbojet operating in air e.g. Bluebird K7?

I'm reminded of a story I was recently told about the great Renato 'Sonny' Levi, designer of Surfury and other rather quick full-size boats. He was approached for advice by some mechanic-types who'd been commissioned by the (very rich) owner of a Riva speed boat to remove its props and replace them with waterjet drives to improve the top speed. Sonny just sucked on his fag, smiled, shook his head and said nothing. After the job had been done the boat wasn't even as quick as it had been with conventional props, and Sonny gave a knowing smile. "Your calculations didn't allow for the extra tonnes of water you're carrying inside the jet units" he replied. The point was also made to me by the storyteller that the clearance between the impeller and the jet tube IN A FULL-SIZE UNIT has to be around 10 thousandths of an inch. Scaling that down to model size and you're into aerospace machining tolerances which, of course, isn't feasible for the price anyone would pay. Is there any wonder why, in model form, waterjets aren't as powerful as they theoretically should be.

Another mental exercise to try is imagining a multi-blade impeller rotating, completely submerged in a very restricted tube full of water. Lots of effort is required to overcome the viscous drag of all that water. Then imagine a two-bladed prop working in open water but fully submerged under the boat. Not as much effort is needed. Now picture a surface-drive prop with one blade out of the water at any one time. Even less effort. The scenario is, as you state, using the same power unit. If waterjets were as powerful as surface-drive props then all of the absolute model speed records would be held by jet-drives. They're not. QED?

Turbojets and ducted fans working in air is a whole different can of worms. I shall now leave the floor open to Dr Thomason and his magical hydrodynamic computer programs!

Dave M

[Alan Fisher 1Participant @alanfisher1]

Horses for courses surely?

Most, if not all offshore power boats use props

If jets were faster, would they not be used instead? or would that put them outside the class?

[Andy StonemanParticipant @andystoneman15177]

I was talking about a water jet propulsion, sorry to have some confusion. I guess theres no real agreed answer to my question, Personally In my thinking a jet is a jet, water or otherwise, So the jet I think has it for me.Thanks to all for your comments

Andy

[Dave MilbournParticipant @davemilbourn48782]

Personally In my thinking a jet is a jet, water or otherwise, So the jet I think has it for me.Thanks to all for your comments

Well – there's no arguing with that, is there? Is anyone game for "the meaning of life"?

DM

[Paul TParticipant @pault84577]

My entry in The meaning of life game is:…………………………………………………………………………….doh

 

Dave

Ducted fans are an under used method of propulsion.

Edited By Paul T on 24/08/2016 17:36:25

[Dave MilbournParticipant @davemilbourn48782]

Ducted fans are an under used method of propulsion.

Probably because they're not called jets.

DM

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Dave Milbourn on 22/08/2016 12:20:30:

…..The point was also made to me by the storyteller that the clearance between the impeller and the jet tube IN A FULL-SIZE UNIT has to be around 10 thousandths of an inch. Scaling that down to model size and you're into aerospace machining tolerances which, of course, isn't feasible for the price anyone would pay………..

Dave M

You probably don't need to scale it down very much, or at all. Remember your Reynolds numbers? But that doesn't alter your general point that water jets are a sub-optimal way of getting a boat to go fast. The skin drag alone inside the tube, which probably doubles the wetted area…

[Dave MilbournParticipant @davemilbourn48782]

DG

I think the horse has bolted, m'duck – much as I would love to debate viscosity and hydrodynamics with you .

DM

[Colin BishopModerator @colinbishop34627]

Paddle wheels are actually very efficient if set up properly. The drawbacks of paddlers are that when the boat rolls you get differential thrust on each side and steering isn't brilliant with no propwash to act on the rudder.

That said, in the heyday of the excursion paddle steamers they were faster than their screw equivalents. Back in the 1930s you could do a day trip to Cherbourg from Southampton, picking up passengers from Sandown on the Isle of Wight en route. Pretty impressive really.

Colin

[Charles OatesParticipant @charlesoates31738]

Back in the day I had a Taplin Baker hydrojet with an E D Viking engine. It was set up in a modified Piranha hull. After one season I swapped the hydrojet for a conventional drive and had about a 30% increase in speed. The hydrojet was sold on. In the model world I'm certain that a hydrojet drive even if built to modern standards would not get close to a conventional system. There are too many losses in the system due to the density of water.

I wish I'd kept the T B drive though, worth quite a bit now, I sold it for a fiver!

I've just remembered that the buyer put it in a mustavago ( anyone remember that?)  It was a failure too

Chas

Edited By Charles Oates on 24/08/2016 23:44:04

[Malcolm FraryParticipant @malcolmfrary95515]

The thing with conventional stuff of pretty much any description is that it became conventional because it worked well. Probably the reason why so many types of boat use propellers.

[Andy StonemanParticipant @andystoneman15177]

Years ago, I had the opportunity drive a water jet powered boat, It had a ford 350hp motor coupled to a Hamilton Jet, When paddling along it was difficult to keep in a straight line, As soon as we had clear water I opened the throttle, I can tell you it took off like a rocket, At 47kts it was flying, I have driven launches of similar size and engine power but Propellor driven, I can tell you there was a hugely marked difference in the boats performance.

Andy

[Charles OatesParticipant @charlesoates31738]

Hi again Andy, it seems you have asked a question expecting a particular reply, so it is probably worth reading a test available on line at boating mag .com The test compares outboard, sterndrive and jet drive on the same Glastron boat. In brief, while jet drives have many fantastic features, performance isn't one of them. The jet drive had 100 more horsepower for an almost identical speed, and economy was poor. Subjective views can't beat physics and actual tests give the answers.

Chas.

Edited By Charles Oates on 25/08/2016 23:29:14

[Andy StonemanParticipant @andystoneman15177]

Hi Charles, I wasn't particularly expecting a reply either way, i was hoping that someone with modelling experience of both of the subjects may have tried out both methods of propulsion in similar situations. But I do think that very good engineering would make a difference with the jet performance as would different propellors, re three bladed two or four bladed,Even the propellor pitch as the fast boat sailors know .

I could`t find the Boating mag feature but will keep looking. It would seem there is no definitive answer to the question I asked, but thanks for your input.

Andy

[Colin BishopModerator @colinbishop34627]

I think that, as in many other areas of modelling, such as sailing for example, the act of scaling down introduces major parameter distortions compared with the full size original.

There are many fast ferries which use waterjets successfully (if expensively in fuel) but if you scale them down then the volume of water in the tube becomes disproportionately smaller to the internal tube surface area which will result in a relative increase in frictional losses. Then again the viscosity of water is the same in both cases, where if were scaled down, the 'water' would be a much thinner liquid.

So really, you are looking at a different situation altogether when searching for efficiency.

Colin

[Paul TParticipant @pault84577]

Hello Colin.

A masterful summing up of a complex subject.

As far as I am aware the only advantage that a model sized water jet has over propeller propulsion is that a water jet equipped model can operate in very shallow water.

On the subject of water viscosity here is something for the members to consider is the current military developments on introducing bubble streams to encapsulate submerged machines such as torpedos, when travelling at high speed the bubbles reduce the interface friction between hull and water allowing the machine to travel even faster.

There is even some discussion about such machines in the near future being capable of reaching hypersonic speeds (mach 5 and above) at these speeds the water surrounding the machine will vaporise and the torpedo will effectively be operating in a near vacuum.

Paul

[Tony HadleyParticipant @tonyhadley]

Earlier in the thread Charles Oates made reference to the Taplin Baker hydrojet unit and whilst rummaging the archives yesterday I came across this 1962 item from Model Maker which makes interesting reading.

Spurtster MM695 was later designed (by Vic Smeed) specifically for the hydrojet.

[john symondsParticipant @johnsymonds38545]

hello boys

I am about to build the precedent perkasa 49" torpedo boat. and I need some info on the running gear. I don't know

what motors or what size prop shafts I need. or anything else for that matter. so if somebody could help me I would be very grateful. thanks simbad

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