Propeller size

Why a higher gear ratio? Then you need an even higher pitch and that will decrease open water efficiency, since the diameter is limited.

25" seems OK for 8.5 knots, but too small, if the speed increases. But there are too many unknowns. We still haven't heard the recommended rpm (or rpm of maximum power), we don't know the wake fraction, the hull resistance etc.

 

I agree with your analysis.
Your biggest problem is the power/weight ratio. Having in mind the current performance of the boat, it looks pretty clear that 110 HP can't get you much further from where you are now, even with the optimum prop. At this point, I believe that you can hope to arrive to 9.0 kts at best, not much more. But the question is - is the effort worth such a small gain? Perhaps the best advice was given to you by Pericles in that other thread of yours - if you want a faster boat then sell this one and buy a boat which was designed for higher speeds.
In any case, at least now you have a nice swimming or diving platform, which is not that bad for your activity.

 

I think that my gear ratio 2.91 is even to big. In any case for now I plan to change only propeller pitch. Question is how much, what is the maximum pitch for this engine. Max rpm is 2200.

 

You said you got 2200 rpm on idle. That is typically not the recommended max rpm. The governor lets typically the rpm to get 100-300 rpm higher on idle than the recommended max loaded by gradually limiting the fuel injection thus not supplying the full power.

 

I realy don't have that information. I only checked RPM idle and load.

 

Don't you know the engine model? How do you even know it's power?

 

Engin is Perkins license made in Serbia, Rakovica IMR S46, 6 cylinders, 1995.

 

Like this one: http://www.imr-rakovica.com/englishbre/imr-motordms46i.html ?

 

I'm with Joakim. Due to the complete absence of hull data and current fuel consumption vs speed and rpm, a 10% increase in pitch seems too small a change IMO. You may find that there is nearly no visceral change to the boat's performance. No sound change, no speed change, no temp changes - nothing you can feel. I'd go with a 20% pitch increase because this you will be able to feel. It may be too much, but it guarantees you will get some new information.

I looked at the link to the engine Mfg and figured 1950 RPM and about 95 hp as a design point, but that was for the industrial version. I didn't see curves specifically for the marine version. If there are issues with engine cooling with your installation, this could reduce the power you can run to well below the rated performance of the industrial engine. That would often be handled by under-propping the boat. So don't be surprised if you end up needing a larger heat exchanger as part of the bargain.

 

Under the turbocharged version there are the specs for this version as well: http://www.imr-rakovica.com/englishbre/imr-motors46t.html

And the charts are swapped: http://www.imr-rakovica.com/englishbre/imr-motormS46TGrafbig.htm

77 kW at 2250 rpm. So you should get clearly more than 2200 rpm without load. Have you checked that your tachometer is accurate? I have no experience about this engine, but smaller marine diesels rated for 3600 rpm maximum power reach about 3900 rpm on idle. So I guess you should get about 2400 rpm.

 

What you think about possible speed with new gear with ratio 1:2?

 

With the current propeller 1:2 is way too heavy. Something like 1:2.6 would be appropriate and it would give a slightly better open water efficiency than increasing the pitch with current gear ratio.

1:2.4 would be still slightly better, but then you would need 19-21" pitch depending on the speed you are getting.

At 1:2 you would need 15-16" pitch and get worse efficiency than any above.

 

Since I am starting to get confused by the multitude of info and opinions coming from the folks who participate to this thread, let me try to gather some facts and numbers, from the info we have had till now.

1) Engine: 110 HP (82 kW) @ 2200 RPM
2) Boat: LWL = 12 m , BWL =3.5 m , Displ. = 12 t.
3) Propeller: Diam = 25.6" (0.65 m), Pitch = 22.8" (0.58 m), P/D = 0.89, Z = 4
4) Gear ratio: 1:2.97 .
5) Current max speed: 8.5 kt (4.37 m/s).
6) Target speed: 10 kts.​

So, assuming that the engine RPM reading is correct, the shaft RPM is:

Nsh = 2200/2.9 = 759 rpm (12.6 rps)​

Assuming a wake factor w=0.15, the water speed at the prop is:

Vp = (1-w) V = 7.23 kt = 3.72 m/s​

and the advance ratio is:

J = Vp / ( Nsh D ) = 0.454​

We don't know what is the type and E.A.R. of the existing prop, but let's use Kt-J and Kq-J charts for Wageningen B 4.70 (Z=4, E.A.R.=0.7) prop, attached below.
For J=0.45 and P/D=0.9, we get:

Kt = 0.24, Kq = 0.034
Thrust: T = Kt rho Nsh^2 D^4 = 6970 N
Torque: Q = Kq rho Nsh^2 D^5 = 642 Nm
Efficiency: Eff = [ J / (2 Pi) ] Kt / Kq = 0.51​

The current point of operation is indicated with red lines in the two charts.

How much shaft power does the prop currently absorb?
Shaft power is given by the formula

Wsh = 2 Pi Q Nsh = 50.8 kW (68 HP)​

How much power can the engine give? Assuming a mechanical transmission efficiency of 0.95, it is given by:

Wsh,max = 0.95*82 kW = 78 kW (104 HP)​

That's an important point: the engine currently gives only 68 HP, out of 104 HP (possibly even somewhat less, due to engine age) which is available at the propeller shaft. Most probably we are seeing the RPM-limiter at work here.


Now that we know the value of propeller thrust (T=6970 N) at the speed of 8.5 kts, by assuming a thrust deduction factor of 0.1 we get the towing resistance of the hull and the relative towing power:

Rt = (1-t) T = 6270 N
Wt = T V = 27.4 kW (37 HP)​

By calculating the drag through Holtrop-Mennen method on a hull with similar shape, dimensions and displacement, corrected by a coefficient which creates the best fit for the thrust at the 8.5 kts speed, I have extrapolated the resistance curve to a nearby speed of 9.5 kts, obtaining:

V' = 9.5 kt (4.89 m/s)
Rt' = 7900 N
Wt' = 38.5 kW​

which can again be transformed into propeller's open-water characteristics:

Vp' = (1-w) V' = 8.1 kt (4.15 m/s)
T' = Rt' / (1-t) = 8780 N
J' = Vp' / (Nsh D) = 0.507​

The new Kt is

Kt' = T' / (rho Nsh^2 D^4) = 0.30​

and from the charts:

P'/D = 1.07
P' = 0.70 m (27.5")
Eff' = 0.49
Kq' = 0.05
Q' = Kq rho Nsh^2 D^5 = 944 Nm​

The shaft power with this new prop is

Wsh' = 2 Pi Nsh Q' = 75 kW (100 HP)​

and it is very close to the max. available.
The new point of operation is indicated with blue lines in the two charts.

Conclusion:

Assuming that the input data (both measured and estimated ones) are correct, by changing the pitch of the propeller to 27.5" (approximately - depending on the propeller model), while keeping the same diameter and gear ratio should get you very close to the point in which the engine can give the maximum power. It should increase the max speed of the boat to 9.5 kt, and no more than that.


It is pretty late here in this moment, so if someone desires to spend some time to double-check the above data and calcs, he is really welcome.

Cheers

 

Ooooops, looks like the pdf files attached to the previous reply were correctly visualized only with the Foxit PDF Reader.

I have updated the attachments, these ones should be ok for any PDF reader.

 

I used a Wageningen B based calculation software with a bit different assumptions (0.1 wake fraction, 0.85 expanded area ratio) and the results were about the same. Then I used 8 kN thrust for all the changed cases

Current situation (8.5 knots, 758 rpm):
Thrust: 6.6 kN
Eff: 0.519

29" pitch at 10 knots:
722 rpm
T: 8 kN
Eff: 0.525

Then with current propeller, but modified gear ratio at 10 knots:
855 rpm
T: 8 kN
Eff: 0.539

Then with 1:2 gear ratio and 16" pitch at 10 knots:
1120 rpm
T: 8 kN
Eff: 0.504