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Jet or Propellor driven

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Andy Stoneman21/08/2016 21:27:44
128 forum posts
12 photos

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

Malcolm Frary22/08/2016 10:08:00
892 forum posts

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.

Banjoman22/08/2016 10:34:22
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1142 forum posts
2414 photos

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 needham22/08/2016 11:34:13
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6573 forum posts
144 photos

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 222/08/2016 11:55:10
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1980 forum posts
706 photos

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 Milbourn22/08/2016 12:20:30
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3999 forum posts
282 photos

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 122/08/2016 18:29:40
7 forum posts

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 Stoneman22/08/2016 22:58:30
128 forum posts
12 photos

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 Milbourn22/08/2016 23:13:56
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3999 forum posts
282 photos

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 T24/08/2016 17:32:58
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7140 forum posts
1204 photos
2 articles

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

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