Overproped?

[vic008]

Hi, 13hp Shibaura. Reaches 2850 in nuetral but only 2100 in gear. Dirty transom. Clean bottom. 31' full keel yacht. Gets up to 6kn okay. Heavy on fuel. Prop 12×14 (?) So prop is wrong. Any suggestions? Thanks

 

[Tranona]

Should be nearer 12*9 and check the rated revs. i would expect in the range of 3200-3600 in neutral.

Unless boat is very light, that is way underpowered and amazed you can get 6 knots. 31' long keel probably displaces over 4 tons and needs nearer 20hp to get that sort of speed.

 

[coopec]

Vic

Why don't you use one of the online propeller size calculators?

There's quite a few of them but I don't know which is best. Maybe try a couple of them to see if the results correlate?

Calculators Relating to Propeller Selection
https://www.vicprop.com/calculator.htm

PROPELLER CALCULATOR
Prop Calculator
Boat Propeller Calculator
http://rbbi.com/folders/prop/propcalc.htm

https://duckduckgo.com/?q=boat+propeller+calculator&t=hb&ia=web

 

[vic008]

They all recomend a larger prop. But surely my prop is already too large and engine not powerful enough to turn prop faster. Need bigger engine or small prop. Wouldnt a smaller prop allow my engine to rev higher? Something I'm missing here.

 

[Old Harry]

They all recomend a larger prop. But surely my prop is already too large and engine not powerful enough to turn prop faster. Need bigger engine or small prop. Wouldnt a smaller prop allow my engine to rev higher? Something I'm missing here.

always swing the largest dia the clearance will allow & adjust the rpm via the pitch.So the dia might well be correct but over pitched

 

[Tranona]

They all recomend a larger prop. But surely my prop is already too large and engine not powerful enough to turn prop faster. Need bigger engine or small prop. Wouldnt a smaller prop allow my engine to rev higher? Something I'm missing here.

Size is a combination of diameter and pitch. The normal rules is to use the largest diameter you can fit in the space and adjust the pitch to suit. The reduction gearing you have affects it as the diameter you can swing will depend on the shaft speed so the greater the reduction the lower the shaft speed and the larger the diameter you can swing (subject to clearance). For example I have a heavy (3 tons+) 26' long keel boat with a Nanni 14, a 2.65:1 reduction and a 15" feathering prop with a flat pitch. The common prop for the engine with a 2:1 reduction is a 12" with a pitch of 9" or so which would be suitable for a smaller or lighter boat.

So, back to your boat. Can you clarify the actual engine and box you have? I am not aware of the engine you quote being available under that name, so is it a Volvo MD 2010 which is a Perkins/ Shibaura base engine, and what is the reduction ratio of the box. As I noted earlier the figures you quoted do not add up. These engines rev to 3600 to produce the HP you quote so 2100 is seriously overloading the engine, which a 12*14 prop would do, but you should still be able to rev to 3600 in neutral. As I said if your boat is the typical heavy displacement 31' there is no way you can achieve 6 knots at 2100 revs as the engine is only producing around 8 or 9hp at those revs and I would expect a speed of less than 4 knots in flat water.

To achieve hull speed you need a minimum of 20hp, and 25 would probably be the optimum size. You could then swing a 14 or 15" prop if you have room in the aperture with a standard 2:1 reduction.