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Prop calc to calculate speed with pitch


casper ghost

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3 minutes ago, casper ghost said:

As I'm looking into getting a prop re-pitched or finding one, I wondered if there was a clever program to calculate the speed difference between different pitches at the same revs.. just for interest..

Thanks

Casp'

Engine speed is easier to work on, as it ignores all the variables that affect boat speed (wind, current, trim etc etc)

 

A very rough rule of thumb is that an inch of pitch changes engine revs by 200 rpm assuming a modern engine and a 2:1 box.  If you are adding an inch of pitch, whatever speed you currently do at 1200 rpm you will do at 1000 rpm instead.

 

It's only accurate within certain limits, as you start getting strange effects with slip  if you push it too far.

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My setup is somewhat unusual. The gearbox is remote so a short propshaft between engine and box then another propshaft to prop. All the uj's use up a bit of power. The engine is also unusual, being a single cylinder Ruston and Hornsby 1vsh, at the mo it is limited to 1000 rpm and only 7.5hp, but once I change the governor weights it'll be 11 hp at 1500 rpm.  It's quite slow. Lol

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Just now, casper ghost said:

My setup is somewhat unusual. The gearbox is remote so a short propshaft between engine and box then another propshaft to prop. All the uj's use up a bit of power. The engine is also unusual, being a single cylinder Ruston and Hornsby 1vsh, at the mo it is limited to 1000 rpm and only 7.5hp, but once I change the governor weights it'll be 11 hp at 1500 rpm.  It's quite slow. Lol

Ok, it won't work for you then!  I did specify a modern engine & box for the rule of thumb :D

 

I think you might struggle with any of the online prop calcs with that setup.  My beta runs between about 700 rpm and about 2000 rpm between tickover (~1.5 mph) and pushing a current on a river ( ~5 mph in still deep water) , so see if you can make sense of that compared to your numbers - what revs do you do between tickover and pushing hard?

 

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Not sure what rpm tickover is, but it's slow. Even at the current governed max of 1000 rpm the engine sounds slow as, with only 1 cylinder, you can still hear the individual pop as it fires. 

Have found a prop calc with takes account of uj's and loss of power through gearbox, you have to enter boat measurements. It predicts, using higher max rpm, a max speed of 5.13 knots for my boat with an 18 X 16 prop. I currently have an 18 X 15.. the program doesn't allow me to put my prop in to see what the max speed is then though.

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10 hours ago, casper ghost said:

Not sure what rpm tickover is, but it's slow. Even at the current governed max of 1000 rpm the engine sounds slow as, with only 1 cylinder, you can still hear the individual pop as it fires. 

Have found a prop calc with takes account of uj's and loss of power through gearbox, you have to enter boat measurements. It predicts, using higher max rpm, a max speed of 5.13 knots for my boat with an 18 X 16 prop. I currently have an 18 X 15.. the program doesn't allow me to put my prop in to see what the max speed is then though.

Iirc, the effect of an increase of an inch of pitch equates roughly to half an inch of diameter.  So for example your prop might be about half way between an 18 x 16 and a 17 x 16 - I don't know whether that might help extrapolate between sizes that you can input into the prop calc you're using?

 

Where's Dyslandia when we need him? :)

 

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14 hours ago, casper ghost said:

Not sure what rpm tickover is, but it's slow. Even at the current governed max of 1000 rpm the engine sounds slow as, with only 1 cylinder, you can still hear the individual pop as it fires. 

Have found a prop calc with takes account of uj's and loss of power through gearbox, you have to enter boat measurements. It predicts, using higher max rpm, a max speed of 5.13 knots for my boat with an 18 X 16 prop. I currently have an 18 X 15.. the program doesn't allow me to put my prop in to see what the max speed is then though.

So you haven't changed the governed maximum engine speed yet? 

Why not make the change and then measure the maximum boat speed with the existing prop, measuring by GPS?

Only if the boat easily reaches the new rev limit will there be any scope for an increase in pitch.

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3 hours ago, Sea Dog said:

Iirc, the effect of an increase of an inch of pitch equates roughly to half an inch of diameter.  So for example your prop might be about half way between an 18 x 16 and a 17 x 16 - I don't know whether that might help extrapolate between sizes that you can input into the prop calc you're using?

 

Where's Dyslandia when we need him? :)

 

An inch of pitch is roughly equivalent to half an inch of diameter for a "square" prop (e.g. 18 x 18) because absorbed power at a given rpm goes up proportional to [pitch*swept area] and area is proportional to diameter squared.

 

So for an undersquare prop (most out there) an inch of pitch is equivalent to more than half an inch of diameter -- for an 18 x 12 an inch of pitch equals 2/3 of an inch of diameter.

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1 hour ago, Murflynn said:

So you haven't changed the governed maximum engine speed yet? 

Why not make the change and then measure the maximum boat speed with the existing prop, measuring by GPS?

Only if the boat easily reaches the new rev limit will there be any scope for an increase in pitch.

Not done the weights yet. I have an identical engine in my shed, so i'm going to do a test run on that first to see how easy it is. I have run the engine in gear and pulled the governor arm by hand so it revs much higher than its governed 1000 rpm. When I did this it really through out some wash from the prop compared to at 1000 rpm, but, you are right, a test would be a good idea. The real test, for me, is going up the Llangollen by the moss. 

1 hour ago, IanD said:

An inch of pitch is roughly equivalent to half an inch of diameter for a "square" prop (e.g. 18 x 18) because absorbed power at a given rpm goes up proportional to [pitch*swept area] and area is proportional to diameter squared.

 

So for an undersquare prop (most out there) an inch of pitch is equivalent to more than half an inch of diameter -- for an 18 x 12 an inch of pitch equals 2/3 of an inch of diameter.

So for an 18 x 15 an increase in pitch to 16 would equal an increase in diameter of ?

 

I had an 18 x 19 on before and that was too big for this engine, it did go but when I put it in gear it shook everything...

Casp'

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7 hours ago, casper ghost said:

So for an 18 x 15 an increase in pitch to 16 would equal an increase in diameter of ?

Roughly equivalent to an 18.5" by the rule of thumb I offered (which I did say was roughly), but perhaps it's more like 18.67" using IanD's "under square" adjustment.  Whichever, it'll be like a bit bigger prop, but not an inch bigger one!  :) 

 

Perhaps more important than what diameter equivalent you're looking at is working out what actual pitch you choose now.  You know exactly what it was like at 18 x 19 and what it's like now at 18 x 15, and you're not exactly inexperienced Casp, so you're pretty well placed to estimate how far up the scale between 15 and 19 you think might be the sweet spot. 

 

I'm no expert and my earlier post was to float the idea of using extrapolation in the calculator you were using, but if you want my two pennorth I'd guess 18 x 18 would be too close to the 18 x 19 that didn't work well; obviously 17 would be the mid point but, if you think that will still be  a bit much, 18 x 16 will be like a little bit bigger prop than you have now whilst being plenty less than the over-propped 18 x 19 you had before. IanD might be able to offer something more concrete perhaps?

 

Incidentally, your existing 18 x 15 should be "tweakable" by a specialist to 16" or maybe even 17" pitch so you may not actually need a new prop.

 

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1 hour ago, Sea Dog said:

 

Incidentally, your existing 18 x 15 should be "tweakable" by a specialist to 16" or maybe even 17" pitch

I was thinking of getting the 18 X 19 re-pitched that way I can swap for he props at one time, but they can only really re-pitched that down to an 18 X 17. If I could find a good cheap 18 X 16 then I would try that first..

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Why not "tweek" the governor/revs with the fitted prop & check the performance before prop swapping  you then have a baseline /start point to adjust to get a prop/revs/performance that you are happy with or the best you can achieve with your boat/engine set up you might find with the reves increased the fitted prop is an OK size

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1 hour ago, X Alan W said:

Why not "tweek" the governor/revs with the fitted prop & check the performance before prop swapping  you then have a baseline /start point. you might find with the revs increased the fitted prop is an OK size

I will be doing the governor first, so I will find that out. I was just interested in whether there is a prop calc to give examples, only really for interest at the mo, couldn't swap a prop until the spring anyway.

Casp'

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2 hours ago, casper ghost said:

I will be doing the governor first, so I will find that out. I was just interested in whether there is a prop calc to give examples, only really for interest at the mo, couldn't swap a prop until the spring anyway.

Casp'

Have you tried putting your boat/engine/gearbox details into the calculator at https://www.vicprop.com/displacement_size.php ?

 

In previous discussions this always seemed to come out pretty close to recommendations from engine suppliers and other people in the know like Crowthers.

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Just looked at the governor weights on my spare engine, which is exactly the same model. On these the weights have a hole where a bolt is used to increase the weight, I had expected mine to have the bolts in, but there is just a hole, no bolts. I had thought that to increase the governed speed you would need to lighten the weights by removing the bolts..Am I wrong?

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15 minutes ago, casper ghost said:

Just looked at the governor weights on my spare engine, which is exactly the same model. On these the weights have a hole where a bolt is used to increase the weight, I had expected mine to have the bolts in, but there is just a hole, no bolts. I had thought that to increase the governed speed you would need to lighten the weights by removing the bolts..Am I wrong?

The greater the mass, the greater the centrifugal force (even though there’s no such thing as centrifugal force, simply Newton’s second law). 

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8 minutes ago, WotEver said:

The greater the mass, the greater the centrifugal force (even though there’s no such thing as centrifugal force, simply Newton’s second law). 

So, just to get that in simple terms, am I correct I. Thinking that adding the bolts would govern the engine to a lower max rpm, in which case a previous owner must have removed them..

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11 minutes ago, casper ghost said:

So, just to get that in simple terms, am I correct I. Thinking that adding the bolts would govern the engine to a lower max rpm, in which case a previous owner must have removed them..

As I understand it (experts might be along shortly!) the heavier the weights the sooner they’ll move outwards and hence the sooner they’ll limit the revs. So yes, adding the bolts will lower the max rpm. 

 

That’s just theory in my head though, so let’s wait for someone who actually knows. :)

 

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5 minutes ago, WotEver said:

 

 

That’s just theory in my head though, so let’s wait for someone who actually knows. :)

 

That ties with what I was expecting.. this engine is definitely plated as the lower rpm model, I guess a previous owner must have removed the bolts to increase revs, just lik I want to do on my boat engine..

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6 hours ago, IanD said:

Have you tried putting your boat/engine/gearbox details into the calculator at https://www.vicprop.com/displacement_size.php ?

 

In previous discussions this always seemed to come out pretty close to recommendations from engine suppliers and other people in the know like Crowthers.

Using that prop calc, it shows that the, once I have sorted the governor, the optimum speed of 5.13 knots is achieved with a 19 x 14, I currently have an 18 x 15 and 18 is the biggest diameter I can comfortably fit on.

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1 hour ago, casper ghost said:

Using that prop calc, it shows that the, once I have sorted the governor, the optimum speed of 5.13 knots is achieved with a 19 x 14, I currently have an 18 x 15 and 18 is the biggest diameter I can comfortably fit on.

You're not gonna be too far off with your present prop then Casp, are you?  As a couple of folk have suggested, give it a go, see how it copes.  If it feels too under propped to live with, then you can consider a pitch adjustment. I feel my boat is just a little under propped, and I was going to get a pitch adjustment, but the fuel consumption is absolutely fine, she does stop pretty well if not spectacularly so, and she doesn't get me shouted at by hurtling past moored boats on tick over.  Actually, there might be an advantage there as I can pass long lines of moored boats with enough revs on to keep the alternator output high enough for the long haired admiral to use electrical appliances below.  The boat might need a few extra revs to achieve a certain speed, and she will pull the red line quite easily in deeper water, but there's a compromise to find which isn't necessarily answered by the biggest prop you can swing.   As I said earlier, where's Daslandia when we need him? :)

 

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3 hours ago, WotEver said:

 

Spoke to the previous owner of my spare engine, he never touched the governor weights and he doesn't think it would rev to 1500.. if I find there are also no bolts in the governor weights on the boat engine then it'll mean there is nothing I can do about the governor, but it's a bit of work to check.. I could put the flywheel back on the spare and start it up to see what it revs up to, if it Max's out at 1000 then I'm stuck. 

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