# High Thrust prop ?



## 75R20 (Jun 20, 2008)

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?
Thanks
Kary
S/V Mariah
#49080


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## davidpm (Oct 22, 2007)

75R20 said:


> 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?
> Thanks
> Kary
> S/V Mariah
> #49080


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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## sailingdog (Mar 19, 2006)

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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## 75R20 (Jun 20, 2008)

sailingdog said:


> 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.
Thanks
Kary
S/V Mariah
#49080


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## tommays (Sep 9, 2008)

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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## klem (Oct 16, 2009)

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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## 75R20 (Jun 20, 2008)

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.
Kary
S/V Mariah
#49080


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## sailingdog (Mar 19, 2006)

Often wider in diameter, but also often has more blades and more surface area overall as well.



75R20 said:


> 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.
> Thanks
> Kary
> S/V Mariah
> #49080


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## sailingdog (Mar 19, 2006)

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.


75R20 said:


> 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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## 75R20 (Jun 20, 2008)

sailingdog said:


> 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.
Kary
S/V Mariah
#49080


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## tommays (Sep 9, 2008)

I would say based on MY use the 4hp motors do marginal without a HT prop to the point IMHP it would be a decideing factor in what motor i bought 

With the HT prop they run circles around the standard and dont have the making OIL problems that using to big a prop cause on 4st motors


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## 75R20 (Jun 20, 2008)

tommays said:


> I would say based on MY use the 4hp motors do marginal without a HT prop to the point IMHP it would be a decideing factor in what motor i bought
> 
> With the HT prop they run circles around the standard and dont have the making OIL problems that using to big a prop cause on 4st motors


Help me out here. The smallest outboard that I could find a HT prop for is the Honda 8hp and most are for the 9.9 hp class of motors. Could you provide me with a link to the HT props that you have used on a 4HP motor.

Also what exactly is the OIL problems that are caused by using too big a prop on a 4stroke motor ?
Thanks
Kary
S/V Mariah
#49080


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## tommays (Sep 9, 2008)

4Stroke "Making Oil" by Willy B. Wright
I get a few customers each year with high oil levels in their 4-stroke outboards and it's really tough to explain why sometimes. So I prepared this to present to customers with the problem and I thought it would make a good FAQ.

Q: Why does my 4-stroke outboard oil level keep rising? I never add any. Shouldn't it be going down?
A: This is referred to as "Making oil". Here it is in a nutshell. Water vapor enters the crankcase of all motors from the atmosphere, and as a by-product of combustion. In other motors, the oil gets rather hot and any water vapor that may condense will steam-off and exit the crankcase breather. 4-stroke outboard motor oil doesn't get nearly as hot, so the water just keeps on collecting.

Q: Is it always water?
A: No. Tiny amounts of raw fuel also leak into the crankcase on the compression stroke, potentially diluting the oil and raising the level on a cool-running motor. Or there could be a fuel system leak. Typically the odor of the latter situation is fairly obvious, but not necessarily. Lack of a gassy smell shouldn't preclude checking the possibility of fuel system leaks. Over choking and frequent flooding will also cause fuel to get into the oil.

Q: Where does the water come from?
A: Three sources. Asmosphere, combustion and fuel. There's always moisture in the air, especially near bodies of water. Some condenses out naturally and collects in the crankcase of a sitting motor. Some comes in right along with the air as it runs. The body of water you're operating on produces a lot of atmospheric water vapor, especially a few feet above the water line ... where the powerhead is. H20 is one of the compounds that forms when gasoline is burned, along with CO, CO2, NOX, etc. Some leaks into the crankcase right along with the other contaminants and mixes into the oil. Alcohol combustion makes mostly CO2 and H2O, so alcohol-blended fuels tend to produce more water vapor than straight gasoline.

Q: Wouldn't my oil turn milky if water was in there?
A: Eventually it will. But oil has some capacity to retain water and it eventually reaches a threshold where it starts to cloud.

Q: Why doesn't the oil get hot enough to steam it off?
A: It can. If the motor is running hard enough to plane the boat, water that otherwise cools the oil isn't splashing on the sump that holds the oil. But boats that are run at non-planing speeds may not heat the oil up very much at all. The water the boat is running in cools the oil which is in the sump beneath the powerhead. That water can keep the oil pretty cool just by splashing against the outside of the aluminum sump.Motors that are mostly operated at sub-planing speeds seem to be especially susceptible to making oil. Many times the complaint is that a kicker used for trolling has the problem, but the main motor doesn't. That's because the main motor is used to get to the fishing spot fast to troll slow with the kicker. One is hot, the other is not.

Q: Why is it a problem with my motor and not my neighbor's?
A: Two things really aggrivate the problem. Cool running temperature of the oil, and the abundance of moisture available that inevitably collects. The environment the boat is kept in, water temperature, the fuel you use, and the way you operate the motor have huge effects on whether or not this is going to be a problem.

Q: Wouldn't a hotter thermostat solve the problem?
A: Doesn't help much, but it helps. Problem is that you're regulating coolant temperature and not oil temperature. The oil will get a little warmer as a result, but the powerhead will be running hotter and that's probably not good. Then consider that if the sump is getting splashed by water, that pretty much cancels it out.

Q: How can I be sure of what is making the oil?
A: You can have the oil analyzed. There are many labs that test automotive oil. Outboard readings may not be consistent with what would be expected from an automobile engine, so interpreting the data could be problematic. But it sure will show what is in the oil. A quick search yielded a lot of labs offering this service through the mail for a wide range of prices. Shop around, but it could be money well invested. One site with abundant information is http://www.bobistheoilguy.com/ They also have a Forum if you have any questions.

Q: So what can I do about it?
A: Right now, the approach is to minimize the amount of contamination allowed to enter. That means keeping the motor as unexposed to atmospheric moisture as possible. Practical in some situations, impossible in others. The manufacturers are also stressing proper and agressive break-in procedures to mate surfaces better and keep leakage into the crankcase (blow-by) to a minimum. Avoid alcohol-blended fuels if you can. Finally, allow the motor to get hot enough to steam-off what water will inevitably collect in there. That'll probably mean running it hard for awhile. (If your lake has a speed limit ... Ouch!)If all of that fails, have the motor checked for gasket leakage. It could be coming from the cooling system. More frequent oil & filter changes may be necessary. But try running the motor hard for several minutes every time you go out. That's the easiest, cheapest, and most enjoyable thing to try.

Q: So you mean I HAVE to open it up and go fast once in awhile?
A: Dang shame, ain't it?

Thanks to Bill, Bo, DJ, Gary, Jim, LubeDude, Ray, Robby, Rockhopper, TheOilDoc, and of course JB for their encouragement and invaluable assistance in preparing this FAQ










Aluminum propeller that fits the Nissan/Tohatsu 4 hp, 5 hp and 6 hp outboards in both 2-stroke and 4-stroke. Also fits the Mercury 4 hp, 5 hp and 6 hp outboards in 4-stroke models. Skillfully crafted to produce maximun thrust even in the most adverse conditions. Made of marine grade aluminum for rugged durability. Big, wide blades. aka PIG PROP OR ELEPHANT EAR.

Load Recommendation: For Heavy Loads...You should have bought a larger engine and now your in trouble kind of heavy loads.

Color: BLACK
Size: Diameter 8..26 x Pitch 5.9999










Best 100 dollars i ever spent on a boat was that prop as the motor could never reach RPM with the 7" even on my really easy to move J24


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## 75R20 (Jun 20, 2008)

Thanks for posting that. Interesting read.
I doubt I'll have a problem with my 20' 1500 lb. centerboard sailboat that draws 1'11" with the board up.
Kary
S/V Mariah
#49080


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## tap (Apr 1, 2009)

tommays said:


> 4Stroke "Making Oil" by Willy B. Wright
> I get a few customers each year with high oil levels in their 4-stroke outboards and it's really tough to explain why sometimes. So I prepared this to present to customers with the problem and I thought it would make a good FAQ.
> 
> Q: Why does my 4-stroke outboard oil level keep rising? I never add any. Shouldn't it be going down?
> A: This is referred to as "Making oil". Here it is in a nutshell. Water vapor enters the crankcase of all motors from the atmosphere, and as a by-product of combustion. In other motors, the oil gets rather hot and any water vapor that may condense will steam-off and exit the crankcase breather. 4-stroke outboard motor oil doesn't get nearly as hot, so the water just keeps on collecting.


As I have discovered, at the cost of an aborted delivery and a $200 repair bill, a broken thermostat can cause the same problem at an accelerated rate.


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