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Author Topic: Is there a mechanism to change the angle of the propshaft while boat is moving?  (Read 2247 times)

RipSlider

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Hello all.


I have a feeling that this will lead to a confusing post, as I can't currently draw a picture to explain it. if so, please let me know and I will post some tomorrow.


Background:

Many model boats have their motors in the middle of the hull, and the prop comes out of the back. The prop creates forwards motion, which acts on the area where the prop tube is glued through the hull, and partially on the motor itself, which passes these forces via the motor mount and into the hull.


The angle of the prop shaft has an effect on the forwards motion. If the tube/prop comes straight out of the back of the hull, then 100% of the force is used as forward motion ( it's not really, but good enough for now ).

If the prop tube faced downwards, then 100% of the force would be used to lift the hull out of the water, and 0% on forwards motion.

If the prop tube emerges at 45 degrees, then some of the force will create forward motion, and some will lift the hull.


Question.

The model I am kinda/sorta thinking about might be a bit front heavy. Also, it might not deal well with chop. So, my thinking is to mount the motor as far forwards in the hull as possible - right at the bow.

If I then angle the prop at say 30 degrees, there will be plenty of forwards force, but also some upward element, which would create dymanic bouyancy - the bow would be lifted by the some function of the motors force.


All well and good, but in a perfect world, there would be a way of altering the pitch of the prop with speed. When the hull starts to plane, and the imparted wake is supporting the body, then there is less need for the power train to be lifting the bow, and therefore, it would be good if there was some way to alter this angle so that there was more forward force delivered - the powertrain would tilt and move the prop closer to parrallel with the water surface.

This is a long way of asking: Is this possible? Have models which have previously been built ever had the ability to alter the angle of the power train, and so shift the upwards and forwards forces acting on the hull?

If so, does anyone have any details? i would be interested to see how everything was kept water proof.


many thanks

Steve
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J.beazley

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Not sure if this is of any help but her goes

http://206.206.85.209/xtdoc/catalogue.aspx?ProductType=64drive_gearbox&store=mhzusa

Designed for IC race types but im sure could be adjust over to electric drivetrains.

Jay
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SteamboatPhil

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I think you might be over complicating things a little. If you think the front of the boat will be too heavy, the last thing you want to do is stick the engine up there as well.
To get forward motion the shaft needs to be parallel to the bottom of the hull, this you can do by using a flexi shaft, or a skeg. Props on the full size powerboats are fixed and not variable, however they do have the ability to alter the angle of the prop in the water and have trip tabs to help keep the boat flat. I think you will find that you will not loose that much efficiency if you have a shallow angle drive train, and a well balanced boat (just look at the OMRA boats to give you an idea of the speed they are getting with fixed skegs)
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tobyker

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The famous "Dispro"  (DISappearing PROpellor) motor launch as built for use on small lakes in the US had a UJ in the shaft outboard of the hull so that you could hoist the prop into a tunnel when you ran her up the beach for your picnic. I dobn't think you were meant to run the engine with the prop hoisted, though. Planing boats with your problem have adjustable transom flaps, but they are usually used to get the nose down. You could always try a horizontal rudder (elevator?) mounted on a skeg below the prop.
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furball

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Quote
This is a long way of asking: Is this possible? Have models which have previously been built ever had the ability to alter the angle of the power train, and so shift the upwards and forwards forces acting on the hull?

You get the same effect with trim tabs don't you? Much easier than moving the drivetrain.

Lance
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dreadnought72

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And don't forget that the downwards thrust of a slightly downwards pointing shaft is low -

Tilt horizontal vertical
0      1.00      0.00   
5      1.00      0.00   
10     0.98      0.02   
15     0.97      0.03   
20     0.94      0.06   
25     0.91      0.09   
30     0.87      0.13 

Tilt is degrees off horizontal, horizontal is "useful thrust", vertical is "lift".
Andy
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dreadnought72

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Well that didn't work.

Martin???!!

Horizontal thrust is cos(angle_off_horizontal) where 1 = max.
Vertical thrust is 1-horizontal thrust. Virtually forgettable up to 30 degrees.

Andy
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johno 52-11

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One way I can think of doing what you describe is to build an outboard leg and mount it in a bar in a tube which is glued into the hull. The tube has a slot in the top and bottom and the bar has a hole through which the vertical leg of the outboard leg is passed. If the bar was a tight fit and greased it should keep the water out but still allow the bar to rotate in the tube thus altering the angle of your prop. Here’s a simple diagram of what I mean. The only problem with this is you would need to be a skilled engineer to build it.

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Martin [Admin]

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Well that didn't work.
Martin???!!
No, I can't get tables to work on SMF either!   :-X
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Bunkerbarge

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An interesting point to take into consideration here is that on a large cruise ship the shafts are angled downwards as well as inwards and the effect is that the bow actually sits lower the faster the ship goes. 

When you think about it if the boat was sat still in the water and you applied an upwards force to the stern of the model then not surprisingly the bow will sit lower as it rotates around the centre of bouyancy.  So the component of force that you are considering to lift the boat could well actually have the opposite effect and this, combined with putting the motor up in the bow, could be quite hazardous.
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Martin [Admin]

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http://206.206.85.209/xtdoc/catalogue.aspx?ProductType=64drive_gearbox&store=mhzusa
Designed for IC race types but im sure could be adjust over to electric drivetrains.
Jay

Wow! nice stuff on there Jay.... cheap!.... NOT!  :o
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