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Author Topic: 3 or 4 bladed prop, that is the question.  (Read 1715 times)

Popeye

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3 or 4 bladed prop, that is the question.
« on: August 28, 2019, 11:03:47 pm »

What has to be taken into consideration  when deciding what type  of  prop  to use......or doesn't  it really  matter?
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malcolmfrary

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Re: 3 or 4 bladed prop, that is the question.
« Reply #1 on: September 03, 2019, 09:43:21 am »

Start off with the type of boat.
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tsenecal

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Re: 3 or 4 bladed prop, that is the question.
« Reply #2 on: September 03, 2019, 10:42:30 pm »

If it is a scale model of a real boat, what type of prop does the real boat have....
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cdnfurball

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Re: 3 or 4 bladed prop, that is the question.
« Reply #3 on: September 07, 2019, 01:11:16 am »

Sorry, but here's more questions for you to consider......
- size of prop
- pitch of the prop
- number of turns of the motor
And as you've asked, number of blades on the prop


Questions for a scale speed for a scale boat

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Hurric123

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Re: 3 or 4 bladed prop, that is the question.
« Reply #4 on: September 07, 2019, 03:53:40 am »

Hi full size boats sometimes will fit 3 or 4 blade props simply because there is not enough room to fit a large enough diameter 2 blade to match to the power requirements.
With full size there are a lot of factors involved in chooising  props. Likewise on a yacht they will often only fit a 2 blade so they can park it behind the skeg to reduce drag when sailing. This can be a achieved by putting a mark in the shaft so they can line up the prop and shaft from the inside. I am sure many of you know all this but it's interesting stuff.
Regards
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phillnjack3

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Re: 3 or 4 bladed prop, that is the question.
« Reply #5 on: May 22, 2021, 06:16:42 pm »

like to know who told you that about prop blades v pitch.


here is just 4 of my props , ranging 13 inch to 23 inch pitch, 23 is smallest diameter.
can get thesesame diameter upto 28 inch pitch. 4 blade wont make it get any more pitch.
you can actually get more pitch from a 2 blade than a 4 blade of same diameter..


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John W E

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Re: 3 or 4 bladed prop, that is the question.
« Reply #6 on: May 22, 2021, 06:33:44 pm »

 %)  Many moons ago I was told by an 'old' fitter that some tugs which were diesel powered had 4 blade props with coarse pitches and some steam tugs had 3 blade props - with a finer pitch and steam trawlers also had 3 blade props

john
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phillnjack3

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Re: 3 or 4 bladed prop, that is the question.
« Reply #7 on: May 22, 2021, 09:54:11 pm »

props can be so    strange.
 the very latest design of a propeller actually is a remake of a proper back in the 1800's
its called the SHARROW.
 its a loop propeller and many huge ships of the 1800's had these and got forgotten about. now after 100 yyear the patent has come back to life and lots of peopel have these.
 ver yexpensive and who do you get to repair one ?
(1) Sharrow Engineering Propeller (2019-) Product Review Video - By BoatTEST.com - YouTube
 this is not a gimmick its the real thing and works pretty good,
« Last Edit: May 23, 2021, 12:02:43 am by Martin [Admin] »
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GG

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Re: 3 or 4 bladed prop, that is the question.
« Reply #8 on: May 23, 2021, 12:33:10 pm »

Well, it's too windy to test the latest model, so here's something that might go someway to answering the original question. It ought to get you in the right "ball-park" with an electrically powered scale type model or at least stop you being way off the mark.


1) Establish the speed you want the model to travel at.


2) Determine the free running (unloaded) speed (RPM) of the motor you plan to use at the Voltage you intend to apply.


3) When working in the model, the motor's RPM will be lower than this, I aim for 75-80% of the free running speed (usually a good combination of efficiency with a power reserve to cope with weeds and stuff).


4) Dividing the models speed (I usually use inches/sec) by this motor speed (revs/sec) will give a value of the propellers pitch that would match this model speed. But....!


5) This assumes there is no "slip" between the rotating propeller and the water flowing through it.  Alas, this means the the propeller would generate no thrust as it fails to accelerate any water rearwards so the pitch has to be greater than this value.


6) The amount of "slip" will vary depending on the model, motor and propeller combination but 20% might be a good starting point.


7) So, multiply the pitch obtained in 4) by 125% and try a propeller with a pitch around this value.


Yes, the more mathematically perceptive amongst you might have spotter that you could have obtained much the same value for propeller pitch by ignoring slip and using the motor's free running speed.  But this would have been to akin magic which is dangerous (a failure to understand the technique/technology that is being used).


As to the diameter and number of blades being used, this is where some experimentation can be called for.  Also, all propellers are not the same, some have thin, correctly curved sections, whilst others have thick blunt and maybe even flat sections.


This method is something of the traditional Engineers "back of an envelope" calculation but have given me successful results many times.  It did predict my latest model would be way over scale speed and, by golly, it was..!
Glynn Guest.
P.S. don't forget to allow for any gearing effects between the motor and propeller shafts. 


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phillnjack3

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Re: 3 or 4 bladed prop, that is the question.
« Reply #9 on: May 23, 2021, 05:51:16 pm »

slip of 20% would be a totally crap set up... this happens a lot with surface props.
ideal ona  sub surface is anything below 10%.. ona  outboard you can get it down to 2 to 5 %



props being different , 100% on that.
you can get 2 identical props , even if come off a cnc machine and give different results..
same happens in the real full size world
prop modifications is a wierd and wonderfull thing..  so much can be done, but do it a bit wrong and its scrap.


now as for scale props they are terrible things,  even their full size counter parts are crap.
reason they are like this is because most of those are made to give a very good reverse as well as forward thrust so they are just a compromise.


when you get into fast boat props you can forget about reverse and just get whatever it has to offer..
material also makes a huge difference too. plastic is just rubbish , nylon a bit better  brass/bronze better yet, and the top you will get is stainless steel, these are the best material you will get.. they are less prone to damage, will not bend under power like all the others and can be made to be sharp as a razor..
i even get my full size props razor sharp to give better performance, i also balance them and polish them mirror finish.
is mirror finish better than sating ? YES it lets you see if any cracks are starting.
speed wise mirror v satin there is no proven winner. 
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John W E

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Re: 3 or 4 bladed prop, that is the question.
« Reply #10 on: May 23, 2021, 06:26:28 pm »


.is mirror finish better than sating ? YES it lets you see if any cracks are starting.


Hi there, yes this is one reason, but not the main reason - the main reason is to help prevent cavitation.  In other words, tiny little bubbles that build up around the blade of the prop and implode on the blades.   If the blades are polished to a high standard the bubble has less grip on the blade.

If we go into the realms of surface piercing blades, this is why they are a weird shape.   If you want to see anti-cavitation props at extreme have a look at nuclear submarine prop design.


John

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RST

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Re: 3 or 4 bladed prop, that is the question.
« Reply #11 on: May 23, 2021, 06:48:39 pm »

Gents the OP is almost a couple of years ago and the first 3 responses seem MOST appropriate to the question.  The OP never responded to any of them so is there a point to digging up an un-answered thread which originally went nowhere then throwing all kinds of things at it in a tangent?  It's not making much sense so far...
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phillnjack3

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Re: 3 or 4 bladed prop, that is the question.
« Reply #12 on: May 23, 2021, 11:21:38 pm »

RST


the post is now continuing so whats the problem ? it is stil la post about propellers


what made you come to this very topic and moan if not interested in propellers or finding out a
bit more about them ?


would you prefer it if we made a new topic and went over the exact same stuff all over again ?






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Tug Fanatic

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Re: 3 or 4 bladed prop, that is the question.
« Reply #13 on: May 24, 2021, 10:09:46 am »

Gents the OP is almost a couple of years ago and the first 3 responses seem MOST appropriate to the question.  The OP never responded to any of them so is there a point to digging up an un-answered thread which originally went nowhere then throwing all kinds of things at it in a tangent?  It's not making much sense so far...

What is there to complain about?

Now that the original poster has gone we can have an interesting and broader discussion which I, for one, am enjoying. Propellers are largely a mystery as far as I am concerned. I just try & buy one that works.

There are so many myths & unsubstantiated rumours about props.

Keep posting,
« Last Edit: May 24, 2021, 11:29:19 am by Tug Fanatic »
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phillnjack3

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Re: 3 or 4 bladed prop, that is the question.
« Reply #14 on: May 24, 2021, 01:39:30 pm »

yep.
propellers realy are a mystery
once you find one that works good keep it and look after it.
you get people who can make good props and peopel who just make props. big difference..


if running fast boats you need leading edge sharp as possible, and the trailing edge needs to be dead flat.
curved or rolled edge on trailing edge will cause you problems.
same with back of boat. the transom at the bottom needs to be sharp...

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