Prop sizing & effect of revs

[Jegs]

Good evening,

Some time ago our prop , 22x14, was damaged & replaced by a 21x15. I have now had the original repaired & wonder whether it is worth refitting it.
The people who supplied the replacement said that the 1" on the pitch approximately equated to the loss of 1" on the diameter; is this correct or is the 22 [orig. spec.] better than the 21?

Another question: the gearbox has a ratio of 2.63:1, would using a lower ratio, e.g. 2:1 produce a performance difference because it would increase, I believe, the prop rpm by app. 30%?

I thank you.


John G

 

[sailorman]

as a rule of thumb
reduce the dia by 1 inch & add an inch to the pitch will give the same results.
its better to spin the larger dia

 

[Tranona]

In general for best performance under motor you fit the largest diameter prop you can consistent with the amount of space you have to swing it. That also means for displacement speeds you need the slowest prop speed which means a large reduction ratio. If you changed to a 2:1 reduction you would have to significantly reduce the diameter of the prop to run at the higher shaft speed. As already suggested "size" is a combination of diameter and pitch and the trade off is roughly 1:1.

Small, fast turning props are commonly used because there is not enough room for a large diameter or for lower drag when sailing. A 2:1 reduction is common with engines running up to as much as 3600, giving shaft speeds up to 1800, but a 2.6 or so reduces shaft speed to around 1400. As an example on my boat with 14hp @ 3600 and a 2:1 box I could swing a 13" prop. However as it is a heavy long keel boat I want as big a diameter as I can so I have a 2.6:1 reduction and a 15" feathering prop.

 

[cmedsailor]

Another question: the gearbox has a ratio of 2.63:1, would using a lower ratio, e.g. 2:1 produce a performance difference because it would increase, I believe, the prop rpm by app. 30%?

I thank you.


John G

If you change the ratio and keep the same prop you will be overprop. I changed from 2,35:1 to 2,05:1 and with the same 2 blade 16x11 prop max rpm was dropped by 500 rpm (from 3450 to 2950). I eventually changed to a 3 blade prop 16x9 prop which again is overprop (3050 max instead of 3200-3600 recommended by Volvo) but since i can now run at 2200 with same speed as previous 2500 and slightly better fuel consumption , can achieve hull speed at 2700 and at full 3050 there is not any black smoke (but you can smell fuel though) i will keep it as it is. I run the engine now at 2200-2400 and occassionally at 2700.

 

[Jegs]

If you changed to a 2:1 reduction you would have to significantly reduce the diameter of the prop to run at the higher shaft speed.

Tranona,

Thank you for your reply. A silly question, I am sure, but could you explain why it is necessary to reduce the diameter of the prop if the revs are increased as proposed?


TIA,

John G

 

[Tranona]

Because the bigger diameter prop will not allow the engine to rev to produce the power. apart from knowing that in theory, I found out the hard way by forgetting to specify a 2.6:1 box and getting a standard 2:1. Did not check before I installed and only got 2600rpm instead of the 3400 I was expecting with a 15" prop. Fortunately supplier changed the box for me.

Don't see why you would want to increase shaft speed anyway. It is the propeller that moves the boat, so you start by finding out the size of the most efficient prop to achieve the sped and then work back to the amount of power to turn it. Larger diameter props are more efficient so you choose the largest to fit the space and then work back to the power required to turn it. Sometimes, of course you can't fit the largest prop, or you want to reduce drag under sail so you may well choose a sub optimal prop and change the reduction ratio to increase the shaft speed.

Another example. Volvo saildrives of the older type had 2.47:1 reduction to give a shaft speed of around 1400 with the engines running at 3600. The later engines run to 3200 so reduction ratio was changed to 2.19:1 to give the same shaft speed and therefore use the same prop. On one of my boats I fitted a new saildrive to the old engine, meaning the shaft speed went up to 1600 so I fitted a different prop from standard.

If you want to play around with different combinations suggest you put your boat's data into the Propcalc programme on www.castlemarine.co.uk