Prop slip calculation.

[burgundyben]

While idly musing about some prop pitch work a chum of mine is looking at on one of his boats, I wondered about the way pitch relates to revs/speed on Playtime.

Here's what I came up with.

Pitch 22"= 558.6mm =0.5586mtr

Max revs =2600rpm

2600 * 0.5568 * 60 / 1000 = 86.7 km/hour.

86.7km/hr * 0.54 = 46.8 knots theoretical speed at 2600 rpm

But I only get 38.6 knots.

38.6/46.8 = 82.5%

So I am losing 17.5% to slip.

Correct?

 

[Beamishken]

Have you taken the reduction gear ratio into account? I'm assuming 2600 is engine rpm or is that output shaft rpm?

 

[burgundyben]

Gearboxes are 1:1, no reduction.

 

[tico]

Gut feel says that's about right.
To reduce slip I think you need to increase the driving surface area of the props, but that has the disadvantage of more loading on the engine thus making it harder to pull away cf. a car with too high 1st gear.

 

[jfm]

Ben, gut feeling says that is bang on. About 17% slip is typical for a fast boat with normalish props. Sure you can get get less by increasing prop area (with other consequences) and you can get into hi tech props etc and improve things for the same area, but your 17% is a typical value. Indeed, when I read the heading on your post I thought you might be asking "what prop slip % should I assume if sizing/choosing pitch for props?" and if you had asked that I'd have said "Assume 17%"

 

[MapisM]

Correct?

Yup. I just threw your numbers in my props spreadsheet, and I actually got 17.99%, but I guess it's just a matter of roundings.

As an aside, actually I would expect - generally speaking - a slightly lower slip from a 40kts boat.
I mean, 18% is nothing to worry about, but 14/15% is normally achievable with no special equipment.
My last speedboat had a 13% slip, for instance.
Ok, faster speed, single outboard, so not directly comparable. But she actually had a stock Merc Bravo outboard and a Merc 4 blade s/s prop, also stock...

 

[burgundyben]

Yup. I just threw your numbers in my props spreadsheet, and I actually got 17.99%, but I guess it's just a matter of roundings.

As an aside, actually I would expect - generally speaking - a slightly lower slip from a 40kts boat.
I mean, 18% is nothing to worry about, but 14/15% is normally achievable with no special equipment.
My last speedboat had a 13% slip, for instance.
Ok, faster speed, single outboard, so not directly comparable. But she actually had a stock Merc Bravo outboard and a Merc 4 blade s/s prop, also stock...

I guess you have to consider that its a deep V hull for 40 knots.

The best bit is that my maths was right!

 

[Latestarter1]

Barnacle Bill and I chewed over the numbers for Huntsman 28 with Perkins Sabre 225's and slip came out at 19%, ran your numbers which gave 37 knots and 17.9% slip.

Remember propellers do not work WITHOUT slip.

 

[MapisM]

ran your numbers which gave 37 knots and 17.9% slip.

Actually, if a 22" prop spinning at 2600 rpm pushes the boat to 37 kts, its slip is 21.39%, in my books.
The 17.99% I mentioned before was based on 38.6 kts, as per OP.
Not that we are talking of critical differences from a practical standpoint, but I can't understand the maths behind your 17.9%....

 

[MapisM]

I guess you have to consider that its a deep V hull for 40 knots.

I suspect that the slip has more to see with the fact that she's on shafts rather than outdrives, outboards or surface transmissions, as most speedboats are.
But as I said, I wouldn't be worried one bit, in your boots.

 

[jfm]

I suspect that the slip has more to see with the fact that she's on shafts rather than outdrives, outboards or surface transmissions, as most speedboats are.
But as I said, I wouldn't be worried one bit, in your boots.

It's worth remembering that a highish slip value isn't universally a bad thing Ben. You should not be aiming for 0% - it's not a clutch! It is absolutely not the case that 14% slip is "better" than 17% or whatever. The slip value is merely the angle of attack, if you think of the prop blade as a wing or a waterski. which is the correct way to think of it. An aeroplane designed for take off on a short runway with a large payload will have a large wing AOTBE so that it can get off the ground, and if it then flies at Mach .9 the wing will be quite big, so it will have a smaller AofA at that speed (putting things very simplistically). Another aeroplane built to a different spec might have a larger AofA. Neither one is right or wrong, and as Latestarter said there MUST be some slip ie there must be some AofA due to Newton's 3rd law. Your 17% is a perfectly good number Ben and while you could reduce it with a bigger prop area the drag effects might then work against you. In summary therefore, slip isn't "a bad thing".

 

[burgundyben]

Back up the bus...

I'm not making any changes, Playtime is absolutely flying, props are spot on.

I was musing about it as a chum has props that are miles out, I was using PT as a vehicle for checking out my maths.

 

[Trundlebug]

Would the slip % be different for a displacement boat?

If so, what sort of typical value would expect? I'm about to get my props repitched and balanced this winter, after removing them last weekend.

I need to find out a couple more values before I can send them in though, exact LWL and gearbox ratio (displacement I'll have to take a guess at).

I too was musing about what they might do, as engine max rpm is nominally 3500, I've never managed above 2900 but top speed is achieved at around 2100.
Yes, it might seem a bit overpropped but for quiet economical cruising that suits me fine, I'm not sure I really want the engine thrashing around at 3000rpm to do 7.5 knots...

Engines have plenty of power & torque to maintain any revs in any conditions currently so there doesn't seem to be a problem.
Just curious!

 

[burgundyben]

Would the slip % be different for a displacement boat?

If so, what sort of typical value would expect? I'm about to get my props repitched and balanced this winter, after removing them last weekend.

I need to find out a couple more values before I can send them in though, exact LWL and gearbox ratio (displacement I'll have to take a guess at).

I too was musing about what they might do, as engine max rpm is nominally 3500, I've never managed above 2900 but top speed is achieved at around 2100.
Yes, it might seem a bit overpropped but for quiet economical cruising that suits me fine, I'm not sure I really want the engine thrashing around at 3000rpm to do 7.5 knots...

Engines have plenty of power & torque to maintain any revs in any conditions currently so there doesn't seem to be a problem.
Just curious!

Trundlebugger, I couldn't comment on your specific install, but I'm fairly sure that over propping to gain fuel economy is fundamentally wrong and likely to cause issues like higher EGT than it should be at high load. A lightly loaded diesel is a happy diesel (must credit that expression to LS1). Not hitting max revs is a worry and should be sorted.