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post #1 of 15 Old 04-18-2010 Thread Starter
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High Thrust prop ?

Am I correct in thinking that a "high thrust" prop will have less pitch then the standard prop, allowing the motor to reach a higher rpm (higher power) or is this backwards?
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post #2 of 15 Old 04-18-2010
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Quote:
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Am I correct in thinking that a "high thrust" prop will have less pitch then the standard prop, allowing the motor to reach a higher rpm (higher power) or is this backwards?
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Kary
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I'm thinking backwards!

Choosing the Right Propeller For Your Boat

Propellers are sold with a given diameter and pitch, usually expressed as Diameter X Pitch, or D X P. The diameter is usually dictated by engine and gearcase design, with few if any options offered, so changing to a propeller with a different blade pitch is the most common propeller change. A prop with a higher pitch pushes more water in each revolution than one with a lower pitch.

Given that, why wouldn't we want a very high pitch prop? There are a few reasons. First, the engine has to work harder to get that high pitch prop spinning, meaning the boat will suffer from slower pickup. Second, a propeller with a pitch that is too high for the boat/engine combination will overload the engine, preventing it from rotating within the recommended maximum RPM range at full throttle. Third, high pitch propellers tend to pull the boat sideways during slow speed maneuvers near docks more than their low pitch counterparts.
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post #3 of 15 Old 04-19-2010
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A high thrust prop is usually a wider diameter with a lower pitch. This is pretty commonly found on outboards, like the Sailmaster, which are designed for displacement sailboats.

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Quote:
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A high thrust prop is usually a wider diameter with a lower pitch. This is pretty commonly found on outboards, like the Sailmaster, which are designed for displacement sailboats.
Can you please clarify what you mean by "wider" .......is it that each blade of the prop is wider, or the diameter is larger? There is not much room in an outboard for a larger diameter prop, so I wonder. I understand going to a four blade, or perhaps even a five blade on a large outboard.
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post #5 of 15 Old 04-19-2010
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What motor do you have ?

As you will have to look motor specfic due to the different shaft sizes on all the brands they do NOT interchange

The HT prop looks compleatly different ,on outboards pretty much the same OD with blades that have maximum surface area with minium pitch to let the motor spin up to full RPM

On my motor its allmost like a waterjet drive compared to the standard prop

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post #6 of 15 Old 04-19-2010
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While not an entirely accurate representation, two extreme cases to compare are a tugboat and a raceboat. The tugboat wants high thrust at low speed so it has a prop with enormous surface area and low pitch (the shaft turns very slowly so it has much more pitch than if the shaft turned the same speed as the raceboat). The raceboat is all about top end speed so it uses a very small diameter prop with tons of pitch.

There are two factors, area and pitch. Blade area is not directly proportional to thrust for a given pitch but the two are positively correlated. Blade area can be affected by changing the diameter or the blade shape/number of blades. On an outboard, you can't increase the diameter very much so you end up with wider blades usually.

You want relatively little slip so pitch loosely determines engine rpm for a given boat speed (this is a bit of a stretch). You want to run in the powerband of the engine which is high rpm so for a high thrust low speed prop, this means taking some pitch out.

Since props do slip in the water, everything is a bit fuzzy but it is easy to understand if you assume no slip (that would take a very large diameter prop).
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Thanks one and all.
This conversation, while answering my question and confirming my thoughts, doesn't really matter much, as there is no "High Thrust" prop available for my motor.
It's a small outboard that I have just purchased for my sailboat, and comes with a 7.5" diameter prop that is available in 3 different pitches.
The motor comes with a 6.5" pitch stock, and there are 6" and 7" pitch props available. I'm sure that what I have will work fine, but wanted to understand my options and see just what might work if the stock pitch is ineffective.
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Often wider in diameter, but also often has more blades and more surface area overall as well.

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Originally Posted by 75R20 View Post
Can you please clarify what you mean by "wider" .......is it that each blade of the prop is wider, or the diameter is larger? There is not much room in an outboard for a larger diameter prop, so I wonder. I understand going to a four blade, or perhaps even a five blade on a large outboard.
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I'd point out that if you said what outboard you have it might help. There are aftermarket props also available. For instance, Pirahna Props makes composite props that are quite good for a wide variety of outboards.
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Thanks one and all.
This conversation, while answering my question and confirming my thoughts, doesn't really matter much, as there is no "High Thrust" prop available for my motor.
It's a small outboard that I have just purchased for my sailboat, and comes with a 7.5" diameter prop that is available in 3 different pitches.
The motor comes with a 6.5" pitch stock, and there are 6" and 7" pitch props available. I'm sure that what I have will work fine, but wanted to understand my options and see just what might work if the stock pitch is ineffective.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Quote:
Originally Posted by sailingdog View Post
I'd point out that if you said what outboard you have it might help. There are aftermarket props also available. For instance, Pirahna Props makes composite props that are quite good for a wide variety of outboards.
While the basis of my question did not refer to any particular motor, the engine on my boat is a Honda 7.5 and I'm going to replace it with a Suzuki 4 hp four stroke.
Pirahna props doesn't make a prop for the Suzuki or Honda that I have.
I had already looked while trying to understand this "high thrust" topic.
As I stated earlier, I doubt that I need to change props at all.
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