|Andy Stoneman||21/08/2016 21:27:44|
|128 forum posts|
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
|Malcolm Frary||22/08/2016 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.
1142 forum posts
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!
Edited By Banjoman on 22/08/2016 10:35:11
Edited By Banjoman on 22/08/2016 10:36:31
|ashley needham||22/08/2016 11:34:13|
6573 forum posts
|Ray Wood 2||22/08/2016 11:55:10|
1980 forum posts
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
|Dave Milbourn||22/08/2016 12:20:30|
3999 forum posts
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!
|Alan Fisher 1||22/08/2016 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 Stoneman||22/08/2016 22:58:30|
|128 forum posts|
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
|Dave Milbourn||22/08/2016 23:13:56|
3999 forum posts
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"?
|Paul T||24/08/2016 17:32:58|
7140 forum posts
My entry in The meaning of life game is:........................................................................................doh
Ducted fans are an under used method of propulsion.
Edited By Paul T on 24/08/2016 17:36:25
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