Propellor size

actionoptics

actionoptics

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Just had to replace my Bukh 24 hp and the book says it should give max output at 3600 rpm. I can only get up to 2800 rpm flat out with the boat ( an LM 30 ) moving at 7.2 knots and trying to climb her own bow wave. Does this suggest the 16 inch diameter 2.75 inch pitch prop is incorrect?
 
16 x 2.75???? Can't be. The engine would be screaming and barely moving the boat. My Bukh 20 (basically the same engine) is noticeably overpropped at 16x14 and gives around 6.3kts on a 22ft waterline (26ft boat)

If you are unable to reach full revs then the prop is too large. However if the boat is already making big waves at full throttle as mine is, then the extra power gained by a smaller prop will actually make little difference. You might get 7.3 to 7.5kts, but at cost of noticeably higher fuel consumption.

If top speed really is important Then try a 14" prop with probaby around 10 - 12" pitch.

Try to find out what pitch you have now - its probably between 10 and 14 inches for the speed you are getting. Use the same pitch, but reduce the diameter. I will shortly be putting a 14 x 13 on mine to see what happens - mainly because I got hold of one cheap! But I have a feeling I might actually regret it - more noise, more fuel, and more waves to irritate people with!
 
More info required (LWL, displacement, gear ratio etc) but it seems to indicate you have too much pitch - PropCalc comes up with 16 x 11 for a three blader on 'guessed' hull figures at 85% of 3600, but at 14 x 10 inch for 3600 rpm. Your result seems to indicate too much pitch, but your 2.75 figure is confusing - was this 12.75 inch pitch?

It would seem that re-pitching existing prop to 16 x 9 would give you closer to max revs.
BTW is the propellor clean, and is the bottom clean also?
 
you really need to consider what prop you need in adverse conditions. Any boat should be able to top hull speed on a flat calm day, but to punch into waves you may need to pitch down slightly and up surface area (or diamater as this does the same thing) to keep cavitaion down.

Underwater profile has a big effect - I believe that long keeler props generally do not get fed very well with water, so tend to cavitate when the hull is moving slowly and the prop turning at a hight rate. With a good sharp nose this may not happen too often - but a stubby bow and a short waterline may make this difficult.

really you need to start with the right theoretical prop for the boat, and then apply your experience of your hull to decide whether to go outside of this.

It does sound as though your prop is too powerful for your engine. Either the engine is not producing full power (injectors good etc.?) or you need to pitch down a bit.
 
LM30 LWL is 26ft
Beam 10ft
Draught exl keel unknown - I used 2 ft
Displacement 11020 lbs
Bhp 24, max rpm 3600
Gearbox ratio unknown I used 1.5:1
Recommended prop was 11 x 7. and the above and a 1 in shaft would give a bollard pull of 479 lbs and hull speed a shade under 7 kts
This was using Propkings spreadsheet.

Using Propcalc, I also get a prop of 11 x 7 and a hull speed of 6.73 kts.


My earlier 16 x 6 was wrong.
 
11 x 7 seems very small for a 24hp.

Did you apply the gearbox ratio the correct way - ie a reduction rather than an increase?

Doesn't look at all right. Mine is a 13.5 hp with a 12 x 9 as the recommended prop.

You do need to find out the gearbox reduction if you are to get this right.
 
as I wrote, this was using a 1.5:1 ratio, using a 2:1 the prop requirement changes to a 13 x 7 and would give a bollard pull of 538 lbs. These figures would be for a standard bronze 3 bladed prop.

2 bladed prop would be 14 x 7
 
This is a very useful thread.

I have just taken my boat out (Virgo Voyager, LWL 19ft; 13.5hp BETA) to try and solve a suspected overpitched prop.

I can only get 2400 rpm at full throtle, rather than the 3600 I should get. I need to increase engine revs under load to maintain efficient engine cooling.

Anticipating the need for a new prop, and being new to this game, can anyone recommend a prop supplier?

Thank you

H
 
Just to clarify..............

Using Propcalc

Input Data
Hull Type: 1
LWL in Ft.: 26
Displacement Lbs: 11020
Engine Type: 3 Inboard
Total BHP: 24
Max RPM: 3600
Gearbox Ratio: 1.5:1

Calculation Results
Shaft Horsepower: 21
Shaft RPM: 2400
Speed/Length Ratio: 1.32
Max Speed: 6.73 kts
Slip Ratio: 0.47
Pitch Ratio: 0.66

Recommended Prop Dia: 10.90 in
Recommended Prop Pitch: 7.15 in

Choose the nearest prop to the above = 11 x 7

This was for a standard 3 bladed bronze prop there are other prop/pitch alternatives and if you would like me to run those, I can and not using Propcalc, I could give a lot more information. Changing the gearbox ratio is just one variable in the calculation, but indeed an important one. /forums/images/graemlins/smile.gif
 
Thanks Old Harry. You are right to question my measurement of pitch. I measured from the front to the back of one blade - I now see that pitch is the distance the propellor will move forward in one revolution - is it simply three times the one blade measurement? I'm not after extra speed but would like to be able to use the full 24 hp when conditions required.
 
Rick, brand new propellor and just anti-fouled the hull and keels. The engine revs to 4000 without load so I am trying to get the use of the full 24 hp when punching a tide/wind. I got the pitch wrong so I will now try to get the correct measurement.
 
Ideal info is, Displacement, LWL, Beam, Hull Draught with out keel and other bits, Engine HP, Gearbox Ratio, and Max Prop Size and to be really clever, shaft dia, and bearing spacing ie. Stern tube to P bracket.

Try Steel Developments for your prop.

Incidentaly, I think I still have a bronxe 11 x 8 LH lying around
 
Which Propcalc are you using?

I have run it through an XLS spreadsheet version which has come up with similar numbers to you (although I am confused as to its comment on surface area - it seems to be using 33% and no way of changing it - should be more like 50% or 55%)

I have also run it though a different program and it gives the same as yours for the 1.5, but for 2:1 it is giving 13 x 10 (based on 55% surface area).

As I have said mine is a 13.5 2:1 with a shorter waterline - the recommended was 12 x 9 but I put on a 13 x 8 to improve heavy condition performance. It is a little over propped (I get 3,200 rpm out of 3,600) but I would be surprised if a 24hp would not need something a tadge bigger.
 
If you are using an Excel sheet, then it is probably PropKings which is what I prefer to use, as Propcalc is not as comprehensive, but a lot simpler.

Propcalc uses a DAR of 55%, but Propkings gave me 33% also.
 
No - for finding out the pitch of yours already you either need to find the marking on it (if it has one) or look at the blade where it is widest and measure the ratio of the circumference at this point of that that has metal to that of fresh air. You then need to multiply up the ' back side to front side' of the blades (ie miltiplied by 3 in the case of a 3 bladed prop)

Don't know how else to describe it if this is not inteligable!
 
I would guess you will buy a 55%? In which case PropKings (can't find the name on it but I am sure you are right) can't give the right answer unless you can change this. /forums/images/graemlins/confused.gif
 
It's a bit strange that both programs come up with a different %, but the same prop sizes. /forums/images/graemlins/blush.gif

The PropKings spreadsheet used to be availble via the Surfbaud website, but I think they may have removed that now.
 
More spooky - it does for the 1.5:1 but not for the 2:1.

have closed it now, but don't remember an option for changing slip or keel profile.

Looking at the Bukh website I cannot find the actual gear ratio. It has 2 options for reverse (unless one of these is supposed to be forwards?). Really this ratio is fairly critical in answering the original question.

I also note that the difference in 3000rpm and 3600rpm is only about 1.6hp, so achieving an rpm somewhere in this area is not going to be too far short of capacity. If the goal is to improve punch into seas then losing rpm with a prop woth a bigger surface area (or diameter) is probably the only way anyway.
 
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