Engines, props, and boatspeed

[Bobc]

As some of you know, I'm re-powering the boat this year.

Having almost settled on the engine, my thoughts turn to props.

My current prop gives a certain amount of drive and boat speed at a specified prop speed (so let's say at a prop speed of 1000rpm, the boat goes at 7kts.).

Assuming that the engine has enough power, my assumption is that 1,000 rpm on the prop would give 7kts with the same prop, whatever size of engine was driving it.

Is my assumption correct, or is there more to it than this?

Also, my understanding is that torque affects accelleration and stopping power, but has little bearing on the above.

 

[lw395]

Unless the drag changes, (e.g. motoring into headwind, weed on hull, whatever), turning the prop at 1000rpm will generate the same thrust and the boat will go at the same speed.

The torque needed to turn the prop at that speed dictates the power.

Your understanding of torque is utterly wrong.


Torque is a twisting force.

Think of a simpler linear force.
If you lift a weight with a force of 1N, lift it one metre you've done 1 Joule of work.
If you do 1J of work every second, that's 1watt.
So Power = force times speed.

Rotationally, power= torque times rotational speed
(times a constant in the unit we normally use!)

If the engine does not have more torque than equates to the power needed to turn the prop at any given speed, the engine cannot speed up.

 

[Bobc]

Ok, thanks for that. Very helpful.

The engine I'm looking to fit is a lower-revving engine with maximum torque at about 1800rpm, whereas the current engine is higher-revving and produces the same torque at 2200rpm. The current engine's cruising revs (at prop) is about 1200rpm, but the new one will be about 1,000rpm. Both engines produce a similar amount of power and torque at cruising revs.

So I assume that to get the same boat speed at 1,000 shaft rpm with the new engine as I currently get at 1200 shaft rpm, I will need to maybe go up a bit in prop pitch. Is that right?

 

[Caer Urfa]

Is this any use to you

https://www.vicprop.com/calculator.htm

 

[Bobc]

Is this any use to you

https://www.vicprop.com/calculator.htm

Thanks yes, looked at that.

I'm just trying to get my head around the principles, because everything I read give different answers, as do the engine and prop manufacturers.

 

[Stemar]

Ok, thanks for that. Very helpful.

The engine I'm looking to fit is a lower-revving engine with maximum torque at about 1800rpm, whereas the current engine is higher-revving and produces the same torque at 2200rpm. The current engine's cruising revs (at prop) is about 1200rpm, but the new one will be about 1,000rpm. Both engines produce a similar amount of power and torque at cruising revs.

So I assume that to get the same boat speed at 1,000 shaft rpm with the new engine as I currently get at 1200 shaft rpm, I will need to maybe go up a bit in prop pitch. Is that right?

Yes, or increase the diameter, which may not be possible.

Plug the numbers into a prop calculator like this one: https://www.vicprop.com/displacement_size.php to see how much. While the physics is straightforward, boats are complicated things so sizing a prop is as much art as science, so it might be worth trying a few more, there are loads out there and go for an average, but I doubt you'd notice if it was a little bit out. I've heard it said that being slightly overpropped has advantages as it gives more relaxed cruising at the expense of not being able to reach peak revs, so losing a bit of flat out speed.

 

[Bobc]

The torque needed to turn the prop at that speed dictates the power.

If the engine does not have more torque than equates to the power needed to turn the prop at any given speed, the engine cannot speed up.

So, what you're saying is that torque is ultimately more important than hp because it is the torsional force than pushes the water past the prop blades? If this is the case, why do engine manufacturers rate their engines in hp rather than max torque?

 

[Bobc]

: https://www.vicprop.com/displacement_size.php

That calculator says that with my current setup, the prop should be 16.5 x 9.6

My current prop is 19 x 15 and it is under-size.

What am I to make of that? This is my problem. I don't feel that I can trust any of this information.

 

[Minchsailor]

You need to consider gearbox ratio. See what gearboxes are available for your engine and what the possible ratios are, then plug the numbers through Propcalc.

 

[Bobc]

You need to consider gearbox ratio. See what gearboxes are available for your engine and what the possible ratios are, then plug the numbers through Propcalc.

I'm specifically using propshaft rpm here so not to bring gearbox ratios into the equation.

 

[Bobc]

Yes, or increase the diameter, which may not be possible.

How does increasing the prop diameter increase your boat speed for a set rpm?

Surely it is the pitch which does this. If a 15" pitch means that the boat moves 15" forward per rotation, then to increase the speed (i.e. distance per rotation) surely you would increase the pitch.

I assume that increasing the pitch will require more torque to overcome the load of the additional water it is displacing. What does increasing the diameter do? I am assuming that you increase diameter to get physically bigger blades to reduce slippage, yes?

 

[lw395]

So, what you're saying is that torque is ultimately more important than hp because it is the torsional force than pushes the water past the prop blades? If this is the case, why do engine manufacturers rate their engines in hp rather than max torque?

Because torque without RPM is no use.
What you really need to know is the power vs RPM curve, or the torque vs RPM curve.
If you know one, you can derive the other.

The only engines which aren't quoted in power terms tend to be aircraft jet engines, which are quoted in pounds of thrust. Power = thrust times speed, but there's a fudge factor for turning knots x lbs into watts or HP....

 

[lw395]

How does increasing the prop diameter increase your boat speed for a set rpm?

Surely it is the pitch which does this. If a 15" pitch means that the boat moves 15" forward per rotation, then to increase the speed (i.e. distance per rotation) surely you would increase the pitch.

I assume that increasing the pitch will require more torque to overcome the load of the additional water it is displacing. What does increasing the diameter do? I am assuming that you increase diameter to get physically bigger blades to reduce slippage, yes?

All props slip.
So a 15" pitch prop might be moving the water around it say 14" relative to the prop, but the boat could be tied to the dock and the prop is just moving water around, the boat might move 10" and the water goes 4" back or any combination.
A bigger diameter prop will want to make the boat move forwards more and the water back less.
A smaller diameter prop of the same pitch will move a smaller volume of water backwards faster.
Getting the diameter right balances efficiency against flexibility.

Can someone post a link to explaining that better please?

 

[Bobc]

All props slip.
So a 15" pitch prop might be moving the water around it say 14" relative to the prop, but the boat could be tied to the dock and the prop is just moving water around, the boat might move 10" and the water goes 4" back or any combination.
A bigger diameter prop will want to make the boat move forwards more and the water back less.
A smaller diameter prop of the same pitch will move a smaller volume of water backwards faster.
Getting the diameter right balances efficiency against flexibility.

Can someone post a link to explaining that better please?

Ok, so what you're saying is basically that size of prop moves volume of water and pitch of prop defines forward motion?

This is all good. Keep it coming.

 

[Bobc]

So thinking about this a bit more overnight, based on the above comments, I assume that a big heavy boat that won't go very fast (like a Thames Barge) would have a big diameter prop with a small pitch (driven by a lor-revving engine with lots of torque), so it can shift a lot of water in order to get the bulk moving, whereas a hull that is easily driven and that wants to go fast, will have a smaller diameter (because it takes less to get it moving, but will want more pitch so that it can reach and maintain the desired speed, and would be more suited to a higher-revving engine with more hp and less torque.

Is this right?

 

[[70521]]

Personally, I'd be contacting the prop manufacturers to get their view on the matter.

 

[lw395]

Vaguely, yes.
A bigger prop more tightly couples engine revs to boat speed.
A smaller prop allows the revs to build before the hull speeds up.

A powerboat for instance, you open the throttle and the engine revs up quickly, the boat takes a while for the speed to build.

A big prop will be efficient at cruising speed, but if you run aground, it may deliver less thrust to get you off.

If I overload my tender with its little outboard, full throttle won't allow full revs, so it doesn't develop much power, despite needing more...

There are whole books on props.
Sections in few books on yacht design, e.g. Larrson and Elliason (sp?)

 

[Bobc]

Personally, I'd be contacting the prop manufacturers to get their view on the matter.

I have, but:-

1./ They have all given me different answers
2./ I don't just want to accept what I'm told without understanding why, as buying the wrong size of prop is an expensive mistake to make.

 

[Jcorstorphine]

If you want to keep present prop and run (the prop) at around 1000 rpm then you could fit a gearbox with a 1.5 to 1 ratio. There are a few out there, I have Hurth 1.5 : 1 and run engine at 1500 to get 1000 prop.

You could also download this excel workbook if you Google

Surbaud prop calculator

 

[Bobc]

If you want to keep present prop and run (the prop) at around 1000 rpm then you could fit a gearbox with a 1.5 to 1 ratio. There are a few out there, I have Hurth 1.5 : 1 and run engine at 1500 to get 1000 prop.

You could also download this excel workbook if you Google

Surbaud prop calculator

No, don't want to keep the current prop because a)it is a fixed prop, b) is is too small, c) the new engine rotates the other way.

Just found this article which is quite interesting. Similar size boat and engine and has 19 x 14 prop which is undersized (so very similar situation to mine). I am surprised to see that the props he is testing are about 22 x 17, which is a big jump up in size.

http://yaffa-cdn.s3.amazonaws.com/yaffadsp/files/dmfile/foldingprops.pdf