Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[BJ]

This is a bit of a follow on from http://www.modelboatmayhem.co.uk/forum/index.php?topic=9719.0 in that there must be a relationship somewhere between the Positioning of Engine Pistons & Prop Blades?

Thinking about how to achieve the maximum speed from an engine and drive line in any boat using a surface or semi surface prop then there is surely a maximum power relationship between the position of the piston and a “normal” two-blade propeller.

I would have thought that the blade entering the water should be doing so at sometime just after the piston has begun its downstroke following the ignition of the fuel.

This should mean that the maximum amount of work from the fuel burn has gone into the drive caused by the prop blade immersed in the water.

 I would assume that if using a three bladed prop then one of the blades should be set at Top Dead Centre when the piston is also at TDC.

So the question is "Do you set your props to your engine’s piston position?"

[DickyD]

Is this a for real question or something to keep you awake at night thinking about. ?  :-\

[John W E]

hi there

to keep it plain and simple; a single cylinder engine (whether it be of I.C. or Steam) has a ruddy great big fly wheel on the end of it - this overcomes bottom end inertia.   In other words, it evens out - a lot of unevenness of the stroke of the single cylinder engine either firing on 1, 2 or 4 strokes. 

This also has the effect of cancelling out any unevenness put on the engine by the work being done either by driving the propellers or other unbalanced machinery.  Obviously, though, it is a lot more complicated than this - but, that just gives a rough outline.

If you notice, any 2-cylinder engine - the cranks are not at 180 degrees to one another - in other words when one crank is at 'top dead centre' TDC - the opposite one is not at 'bottom dead centre' BDC.   They are normally set at 45 degrees to one another - slightly more even.

This is to balance - or even out the unevenness of the firing strokes & have you noticed on smaller multi-pistoned engines the fly-wheel tends to become smaller - apart from the 'dreaded' diesel.

aye
john e
bluebird

[DickyD]

So the answer is no then John. :-\

[Martin (Admin)]

I would say yes..... but you would need a lot of money and time to proove it! 

[John W E]

Martin

Only got two words for you ..... Kinetic Energy - energy stored up in a flywheel from the initial power stroke of the piston.

aye
john

[Martin (Admin)]

I agree, but if we can come up with a plausible enough reason to look for the variance in instantaneous speed variantion through the 360^o rotation of a 2 and 4 stroke engine, I'm sure we could get a huge grant from someone to do an in depth study....... on a sunny island somewhere!  :)

[John W E]

Hey I like the idea of the sunny island, forget the rest......but, do we have Kinetic Energy to supply the drinks 

The other part of the question was to do with propeller - now then, the propeller is of a fixed resistance - and it does not have shall we say a phase of high resistance or low resistance in its rotation - it is a constant set resistance against the motion of the motor.   So, in theory, cancelling out any high or low peaks of energy generated by the power-source.

aye
john

[DickyD]

Absolutely. What ? :-\

[Casper]

Just to get back on topic  I used to have a very high powered outboard (225bhp), now this had a three bladed prop with a v6 engine. The engine was disconnected from the prop by the gear box which was not linked to cylinder firing order, in other words depending on when it put the engine in gear the relationship the firing of a particular cylinder would vary.

Opening up to full throttle from standstill produced a signinfcant amount of slip by the prop, when on the plane slip reduced and the boat accelerated to its maximum potential speed, so even if you transmitt 100% of the piston power stroke to the prop then the prop will never transmit 100% of the piston power to produce forward motion.

But as ever I could be wrong

[Martin (Admin)]

TOPIC SPLIT FROM "Triumph motorcycles, engines, TDC .... & other things that also nearly killed me! " 

http://www.modelboatmayhem.co.uk/forum/index.php?topic=11390.0

[omra85]

An interesting thought BJ -
obviously with a 2 blade SURFACE PIERCING prop, which is running with the water level exactly at the hub, one blade's action will cancel the other blade's (or equal it) ie as one enters the water the other leaves it.  All nice and straightforward 

But what about the 3 blade!  During a 180 deg rotation, 2 blades will be in the water and 1 blade out.  During the other 180 degrees there will be 1 blade in and 2 out.  This will equalise.  But what if you, as you suggest, time your engine so the the power stroke (180 deg) coincides with the 180 deg in which 2 blades are in the water?  Even given the inertia of the flywheel (which on modern engines is becoming more like a belt pulley) you would still be delivering the most power when it will be most effective.

The simplified sketch where the blades are represented as lines 1, 2 and 3 actually show very little without an explanation which would take me too long to write, but can be used to give an idea of when the blades are 'working'.  If you were to set the blades so that, when the engine is just leaving TDC, blade 1 is just entering the water ....

You can follow the track -

I shall set my boat up like this for this weekends race.  If I win, your theory will be proven 

Danny
(cue for Dicky to come in with "don't hold your breath" or some such witticism)

[DickyD]

WIN ?  Don't hold your breath. 

[BJ]

I can alter my Cherokee Chief 2 fairly easily since it it is solid shaft driven by a muff coupling and set up for surface.
The engine is a Thunder Tiger 25 and the flywheel is fairly small. Think I may have a small 3 blade prop to try as well.

Playtime next week then......if the weather is OK

[John W E]

hi there all

Danny & BJ in all fairness to make your experiments true and fair and to give you accurate results - you should first take the model as standard set up with NO alterations; fuel it up so the model is at its running weight - and - take note of the weight by weighing it.  Then, set 2 markers out onto the Lake - say 100 metres apart - and time the model between these markers - for at least 3 runs in each direction.  Then, take the average time of the 3 runs to give you your average time.

Then, do your alterations to the model - i.e. reset the piston to the propeller blade setup; then refuel the model and weigh it - then repeat the 3 runs.

This will give you the most accurate readings/observations.

The other thing I would add - both of you are saying that your flywheels on the motor are small, it is not the size or diameter of the flywheel that counts - it is actually the density or the weight of the flywheel that stores the kinetic energy.

aye
john e
bluebird

[omra85]

Hi John
Yes, absolutely correct! Or even use one of those fancy Garmin thingys!
You may notice a slight element of 'tongue in cheek' in my previous post - to be honest, if I'm racing, the LAST thing I'm going to be thinking about is getting an extra squillionth of a mph out of my boat!  Too busy trying not to hit the bouys  ::) ::)

Danny

[BJ]

The other thing I would add - both of you are saying that your flywheels on the motor are small, it is not the size or diameter of the flywheel that counts - it is actually the density or the weight of the flywheel that stores the kinetic energy.
john e
bluebird

Going back many years, I seem to remember that the inertia (I) formula is  I =mk², see http://www.efunda.com/math/areas/RadiusOfGyration.cfm on this.
Now CMB 90 series flywheels are LARGE O/D's and fairly narrow as opposed to the "normal" fairly small O/D's but a lot longer units fitted on the run of the mill marine engines. Same mass, bigger diameter, higher inertia?
Anyone know a good mathematician?