Propeller size and pitch? Help

[barryassheton]

Hi, I have a 20' bilge keel carvel cabin boat. When we got the boat the prop shaft and prop were already there. We have since put a volvo MD11C engine in her but the recommended prop for the MD11C is a 14x12. The maximum size prop we can have is 12", so what pitch would I need to get the maximum performance out of her. The gearbox is an MSB 1.91:1 if it helps.


Thanks Baz

 

[Tranona]

Use the Propcalc programme on www.castlemarine.co.uk entering 12" as max prop diameter along with the other required data for your boat. 12" is a very small diameter prop for that size engine (and the engine rather too big!).

 

[PaulGooch]

No matter what you do, 6-7 knots is going to be flat out. The engine is about twice as powerful as the boat needs, so if you prop it right you should be able to keep the RPM down and get good economy.

 

[Latestarter1]

Hi, I have a 20' bilge keel carvel cabin boat. When we got the boat the prop shaft and prop were already there. We have since put a volvo MD11C engine in her but the recommended prop for the MD11C is a 14x12. The maximum size prop we can have is 12", so what pitch would I need to get the maximum performance out of her. The gearbox is an MSB 1.91:1 if it helps.


Thanks Baz

Baz,

I needed to make some assumptions.........

Displacement 1 1/2 tonnes

LWL 19ft

MD11A 23 hp @ 2,500 rpm

Your given gearbox rato 1.91:1

Hull speed say 6 knots with power requirement of no more than 7 hp.

Because of your very limited aperture, hope you have allowed 15% tip clearance you will need a three bladed 12X17 heavily oversquare and going with a four blade will only save you an inch of pitch.

Good luck

 

[sailorman]

Hi, I have a 20' bilge keel carvel cabin boat. When we got the boat the prop shaft and prop were already there. We have since put a volvo MD11C engine in her but the recommended prop for the MD11C is a 14x12. The maximum size prop we can have is 12", so what pitch would I need to get the maximum performance out of her. The gearbox is an MSB 1.91:1 if it helps.


Thanks Baz

14"
a rough rule of thumb for every inch less dia, add 1" of pitch

 

[barryassheton]

Many thanks everyone for advice I will try the calculator see what comes up. Failing that I will go for the 12x17 suggested by Latestarter1.

Thanks Baz

 

[Tranona]

15*11 or 14*12 would be about right. Your basic problem is that your engine/box combination is not suitable for the boat as you cannot fit a prop to absorb the power you have. The very course pitch prop suggested might work, but it will probably be a beast at low speeds. Anything over 10hp is wasted on the boat.

 

[Latestarter1]

15*11 or 14*12 would be about right. Your basic problem is that your engine/box combination is not suitable for the boat as you cannot fit a prop to absorb the power you have. The very course pitch prop suggested might work, but it will probably be a beast at low speeds. Anything over 10hp is wasted on the boat.

'The very course pitch prop suggested might work'......I use Crouch's method what did you use?? 15X12 for 1 1/2 tonnes is actually optimum, not the #'s you have stated.

Yes the nasty old low revving old thump thump iron suggested may may not suit the character of the vessel, however that is what the OP has chosen. Unlikely boat be be a beast at low speeds as prop diameter is small and will have little paddlewheel effect.

When WILL you guys learn the basics that propellers move boats NOT engines. However how much potential power engine can produce at any speed it will only produce as much as propeller askes for.

I took the trouble to draw a propeller law curve based on input data and the 12X17 will be asking engine to produce 7.3 hp @ 1,800 rpm when engine has potential to produce 11 hp which sounds sweet, in addition this is around peak torque, best point on Bsfc curve.

Oversquare is the term or maybe coarse but never course.....

 

[Tranona]

Sorry for the finger trouble - I did of course mean coarse. The sizes I got were from the Propcalc programme and not hugely different from your suggestion. Perhaps if the OP had been aware of the need to swing a 15" prop he would not have gone to all the effort and cost involved in fitting an unsuitable powerplant.

 

[Latestarter1]

Sorry for the finger trouble - I did of course mean coarse. The sizes I got were from the Propcalc programme and not hugely different from your suggestion. Perhaps if the OP had been aware of the need to swing a 15" prop he would not have gone to all the effort and cost involved in fitting an unsuitable powerplant.

Apologies patience never good in the early hours.

Prop calculators are OK however you are stuck with their slip which is always a big SWAG.

Agree over the old VP thumper, yuk.

 

[barryassheton]

Hello again all, I know this is an old thread I started but it seemed silly to start a new one for the same topic. I have still not been able to find the correct pitch propeller. I have had a suggestion on here for a 12x17 and from a company and I also had 2 other companies quote a 12x8 and a 12x14, the 12x14 is what I currently have and I get about 5 knots at 2500rpm. The propellers are a staggering amount of money and I need to be sure I get the right one. We did have the idea of an auto prop but finding them used in good condition isn't easy.

Baz

 

[Jcorstorphine]

Props

Here are a few to look at Got to be a left hand prop for MDxx

This one is 13 x 13 LH ( might just fit if you cut your clearance)

http://www.ebay.co.uk/itm/mastercra...tEquipment_Accessories_SM&hash=item337f68d01d

This one is 12 x 15 but is for 1 1/8" shaft so you need to check

http://www.steeldevelopments.net/re...e-propellers-10-11-12-diameter-selection.html

 

[barryassheton]

Here are a few to look at Got to be a left hand prop for MDxx

This one is 13 x 13 LH ( might just fit if you cut your clearance)

http://www.ebay.co.uk/itm/mastercra...tEquipment_Accessories_SM&hash=item337f68d01d

This one is 12 x 15 but is for 1 1/8" shaft so you need to check

http://www.steeldevelopments.net/re...e-propellers-10-11-12-diameter-selection.html

Thanks but I need a RH drive the max diameter the boat will take is 12" and the shaft size is 25mm. Before spending large amounts of money I need to be 100% sure I am buying the right part.

Thanks Baz

 

[Jcorstorphine]

Are you sure about the RH. The volvo MD engines runs clockwise (looking from the front of the engine) and the output of the gearbox is also clockwise (looking from the front of the engine engine) unlike most other engines where the reduction box changes the output to anticlockwise. see this link http://www.ybw.com/forums/showthrea...enta-2003-prop-rotation&p=2828315#post2828315

 

[Tranona]

You are getting maximum revs and nearly maximum hull speed so whatever you do you are unlikely to get much faster. You could try an inch more pitch by having the existing prop repitched by a prop specialist. This might give you the extra half knot or so to get up to hull speed.

 

[barryassheton]

Are you sure about the RH. The volvo MD engines runs clockwise (looking from the front of the engine) and the output of the gearbox is also clockwise (looking from the front of the engine engine) unlike most other engines where the reduction box changes the output to anticlockwise. see this link http://www.ybw.com/forums/showthrea...enta-2003-prop-rotation&p=2828315#post2828315

That made me think a little. Lol you are right we do need a LH rotation prop. We were using the one that was in the boat when we got it. Thanks

You are getting maximum revs and nearly maximum hull speed so whatever you do you are unlikely to get much faster. You could try an inch more pitch by having the existing prop repitched by a prop specialist. This might give you the extra half knot or so to get up to hull speed.

This makes alot more sense than the information given by various companies. So I guess I now need a RH 12x14-17 Prop.

Thanks everyone this has been made much clearer.

Baz

 

[robertclan4]

Propellers is operating at high rotational speed it is basically used work on boats when we got a boat the prop and propeller were already there.
_____________
boat propellers

 

[laurie65]

14"That is about right im working on a new type of prop that is 15 inch die x 3 blades and with a 12 hp at 1800 into a 2 to 1 box the revs are 849 and it has 13 inch of pitch the boats own prop as 12inch of pitch and touring at 874 revs and is 16 inch x3blades about both the same BAR, but the bollard pulls was 15 inch =220 kilos 16 inch =180 kilos.... on the test in a closed dock

 

[davey]

The last time that I tried to use Castle Marine's Propcal I had a problem. The program is written for 16 bit computers! Now I guess that Windows 95, Windows 98 and Windows Xp might be OK if they have Host Mode but Windows 7 most definitely won't be. Allegedly if one pays through the nose for Windows Professional (A £200 upgrade the last time I looked!) the software patches to run sixteen bit software on a 32 bit machine will be there. Two hundred pounds is a lot of money so I discovered a better way!

There is a Canadian company, Victoria Propellers and they have a calculator named Vicprop. This runs quite happily on 32 bit and 64 bit Windows 7 so my advice is to hang on to your £200 as you will probably have a far greater need for it than B G does!

As to marine diesel engines, the important thing is not to be over-propped as this will result in black smoke and excessive pressures within the engine. The engine may also get clogged-up with carbon and need a decoke. Barnacles on the propeller will also cause black smoke. Good luck!

 

[davey]

You are getting maximum revs and nearly maximum hull speed so whatever you do you are unlikely to get much faster. You could try an inch more pitch by having the existing prop repitched by a prop specialist. This might give you the extra half knot or so to get up to hull speed.

Hmm Hull Speed. This is not a precise thing unlike the "sound barrier" with aircraft. If the hull is slender the rules don't work properly. The Turbinia and the boats used in rowing races bend the rules somewhat.