Ratio and prop - options

[Philip_Oceanis 361]

Engine 29hp @ 3600rpm
Option 1: Reduction gear Ratio 2.35:1 and propeller 16X10 (3 blades)
Option 2: Reduction gear ratio 2.05:1 and propeller 16X9 (3 blades)
Option 3: Reduction gear ratio 2.05:1 and propeller 15X10 (3 blades)

Given the above 3 options which one(s) would you choose any why (and which one(s) not to select and again why)?
Thanks

 

[kloudie1]

I'd go with 2.35:1 reverse, because it will get the engine revving a little quicker and develop power quicker than the other options in reverse. My reverse is 3.16:1 and forward is 2.61:1 with a 15 X 15 prop. Your forward transmission ratio should dictate the prop pitch

 

[Philip_Oceanis 361]

I'd go with 2.35:1 reverse, because it will get the engine revving a little quicker and develop power quicker than the other options in reverse. My reverse is 3.16:1 and forward is 2.61:1 with a 15 X 15 prop. Your forward transmission ratio should dictate the prop pitch

Oops! I have just understood my mistake and edited my post. I was referring to the forward reduction gear Ratio (confused with reverse gear box; sorry it's my English terminology).
So, forget reverse, just forward reduction ratio. What would you select and why?
Thanks

Edit: don't bother about reverse ratio. Let's assume it is the same.

 

[Ross]

my forward and reverse ratios are the same. I am turning a 13x13 three blade with ten hp and a 3:1 ratio.

 

[kloudie1]

This one would be best answered by the propeller vendor's calculations and a little trial and error testing.. You can get a little feel for what is happening by looking at the amount of water theoretically pushed by the prop at a cruising rpm.. say 2900 rpm for comparison..
The 2.35 and 16X10 moves 1431.4 cubic feet of water per minute
The 2.05 and 16X9 moves 1480.5 "
The 2.05 and 15X10 moves 1442.3 "
The first and last combinations are less than 1% apart, so they are esssentially the same except for other considerations .. Generally (talking displacement hulls), a larger, slower turning prop is more efficient than smaller and faster turning one.. and more prop pitch is generally less sailing drag .. (painting with a wide brush here) because less water is deflected by the propeller's movement through the water...
Sooooo.. if ya planning on a lot of motor sailing, the top one is probably a smidge better.. otherwise, the bottom is probably a bit better under sail.. But those are gusses and subject to the imposition of the real world of boat drag and how clear of a shot the water has of getting to the prop.. The experienced propeller folks are best for this kind of question.. Good Luck !
EDIT: whichever combination is chosen, the final test is that the the engine should run to its max continuous rpm in flat water at wide open throttle. This should be measured with an accurate tachometer, usually hand held ones are best.

 

[Ron20324]

I'd think the x 9 would give you less prop walk.

 

[Philip_Oceanis 361]

This one would be best answered by the propeller vendor's calculations and a little trial and error testing.. You can get a little feel for what is happening by looking at the amount of water theoretically pushed by the prop at a cruising rpm.. say 2900 rpm for comparison..
The 2.35 and 16X10 moves 1431.4 cubic feet of water per minute
The 2.05 and 16X9 moves 1480.5 "
The 2.05 and 15X10 moves 1442.3 "
The first and last combinations are less than 1% apart, so they are esssentially the same except for other considerations .. Generally (talking displacement hulls), a larger, slower turning prop is more efficient than smaller and faster turning one.. and more prop pitch is generally less sailing drag .. (painting with a wide brush here) because less water is deflected by the propeller's movement through the water...
Sooooo.. if ya planning on a lot of motor sailing, the top one is probably a smidge better.. otherwise, the bottom is probably a bit better under sail.. But those are gusses and subject to the imposition of the real world of boat drag and how clear of a shot the water has of getting to the prop.. The experienced propeller folks are best for this kind of question.. Good Luck !
EDIT: whichever combination is chosen, the final test is that the the engine should run to its max continuous rpm in flat water at wide open throttle. This should be measured with an accurate tachometer, usually hand held ones are best.

Thank you for the answer. I am a bit confused though. Why the 16x10 is slightly better when it moves the least amount of water? I would think that the best is the 16x9 for motor sailing.

 

[alexco38]

May be interested to you:

speed = (RPM / GearRatio * 60 * pitch) / 6076.11549*12 * slippage
pitch = inches, 6076.11549 = foot per knot, slippage about 0.5 for my boat

Diameter of the prop affects slippage
So for the same diameter you can use real world measurement to calculate how it will change if you re-pitch.

 

[kloudie1]

Philip, since the other two are mostly the same, and the 16X9 is noticeably higher in water throughput, I figured someone from a shop had recommended these.. More likely two with the same answer are more correct.. and the 16X9 might be a bit over-propped with that ratio.. The first sentence of my post and the part of my post after the edit are most important !
The lowest drag while sailing would generally be the 15X10, then the 16X10, then the 16X9
Alexco is right on..