Prop Drives and pitches

[YoungMind]

Hello all,

My name is Josh and I work for a Wholesale distributor of marine parts (not in competition with iboats, and will make sure not to mention any business specifics in respect to iboats). I do not currently own a boat myself but have come to this forum multiple times to find answers for my customers and find many of the people here to be very informative and straight forward with their answers.

My questions today are;

What are the benefits of running multiple drives(as in 2 sterndrives or a 2 engine inboard with 2 prop shafts)

Is fewer blades always better?

What difference in performance comes from the entire range of Diameter for a motor( ex. 135HP Mercury has recommended Dia range 14" - 15" in my book)

And my last question is the stumper for me: How do you know what size props work together on a Duoprop or Bravo 3 setup? I haven't been able to figure out how you know what size to put on the back of the shaft to compliment the front or if it really matters.

I can't wait to see some of the input from this. Thank you all in advance

Josh :wave:

 

[alldodge]

Re: Prop Drives and pitches

Hello all,

My name is Josh and I work for a Wholesale distributor of marine parts (not in competition with iboats, and will make sure not to mention any business specifics in respect to iboats). I do not currently own a boat myself but have come to this forum multiple times to find answers for my customers and find many of the people here to be very informative and straight forward with their answers.

My questions today are;

What are the benefits of running multiple drives(as in 2 sterndrives or a 2 engine inboard with 2 prop shafts)

Dual power plants allows for better handling during docking maneuvers. If you have trouble with one engine you can usually limp back home so long as the weather is not to bad. Boats are designed to handle with the power installed at manufacture. If the boat looses one engine there is not enough power to get the boat up on plane any longer.

Take two boats of equal size and weight, then add 2 engines to one and 1 engine to the other. The single engine boat needs less HP to run at speed over the 2 engine because of the extra weight and drag of engine and drive.

Is fewer blades always better?

The less blades the better due to drag but the need for more blades is needed for balance and power transfer. The newer 4 bladed B3 designs indicate they come out of the hole better, but Merc says limited to a max of 350HP for Gas engines. The lower HP rating IMO tells me this will help under powered boats so they will still perform and can increase fuel ecomany.

What difference in performance comes from the entire range of Diameter for a motor( ex. 135HP Mercury has recommended Dia range 14" - 15" in my book)

Don't really understand this one unless your talking about the diameter of the prop. The diameter of the prop is one factor in the type of drive being used, weight of boat and hull design.

And my last question is the stumper for me: How do you know what size props work together on a Duoprop or Bravo 3 setup? I haven't been able to figure out how you know what size to put on the back of the shaft to compliment the front or if it really matters.

I can't wait to see some of the input from this. Thank you all in advance

Josh :wave:

The props on both B3 and DP are both the same pitch, the difference is their diameters. The front prop can be 16" and the rear would be 15". The diameters change as the prop pitch increases.

And :welcome: to iboats Josh

 

[YoungMind]

Re: Prop Drives and pitches

To clarify the Diameter question. Would there be any significant change in performance when using the different widths as long as the changed are not drastic(I have come to find that some consider 1" change in diameter to be drastic). I understand the difference that pitch can make for performance, does diameter matter as much as pitch or is it more of a set amount based on the pitch used and the application?

 

[alldodge]

Re: Prop Drives and pitches

To clarify the Diameter question. Would there be any significant change in performance when using the different widths as long as the changed are not drastic(I have come to find that some consider 1" change in diameter to be drastic). I understand the difference that pitch can make for performance, does diameter matter as much as pitch or is it more of a set amount based on the pitch used and the application?

This could be long winded and I'm not the expert, but pitch and diameter all effect performance. Diameter has a greater impact on reducing slip then pitch. Balancing the slip of the prop is how performance is achieved. It would probably be better to read through the below link which should help

Learning how propellers work.

 

[jestor68]

Re: Prop Drives and pitches

If you go online and google "diameter versus pitch in boat props", you'll find several articles written by the experts.

Read thru several of them and be enlightened(or confused). :faint2: