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Old 22-11-2012, 16:53   #31
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Re: Prop Diameter vs Pitch

For more dialog and a couple of charts to do the calculations this might give you some idea.

Proper Prop Size

My experience is that a higher pitch to diameter ratio is meant for power boats.

I don't know why you would consider your 2.9-1 Velvet Drive to be inadequate. Very adequate because you want to decrease your prop shaft speed for more efficiency and it can handle a lot more power than you currently have.

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Old 22-11-2012, 20:28   #32
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Re: Prop Diameter vs Pitch

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Originally Posted by westwinds View Post
On the Dave Gerr formulas, did you use Crouch's Propeller Method, or the the Bp-d formulas? By the way, I am currious, what changes did you make with your new engine, power and weigh wise? You must have gone to more power where the engine went to full governed RPM, but you did not have enough diameter and pitch to take all torque the new engine put out?
Without taking away from the OP's thread, the repower project is discussed here:

Repowering with Perkins 4.108 -- Have some questions...

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There might be a cavitation problem. What was the original propeller diameter? Use that diameter and increase the number of blades if possible to decrease blade loading? Need to do calculations to determine blade loading.
Yes, there has been a considerable amount of cavitation, even with the higher pitch prop. I'm taking the pitch higher by a factor of four with the new propeller I purchased. If this does not put enough stress on my engine at higher RPM's, I will re-gear.
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Old 23-11-2012, 12:27   #33
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Re: Prop Diameter vs Pitch

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Rules of thumb from Dave Gerr’s Propeller Handbook.

An increase of one inch in diameter decreases the pitch by two or three inches, and of course one inch decrese in diameter increases the pitch by two or three inches. Be careful, this is not exact.

Every two inch increase in pitch will decrease RPM by 450.

You need a tip clearance of at least 15% of the propeller diameter for acceptable noise.

The skeg or strut holding the propeller shaft should have at least 30% clearance between it and the propeller for good efficiency and low noise.

The shaft diameter should be one-fourteenth the propeller diameter. Some adjustments might be needed for the type of metal the shaft is made from.
Gerr does have a conversion factor table for two and four bladed propellers from three bladed propeller although I did drop the column on efficiency adjustments:

Diameter Pitch
Two-bladed 1.05 1.01
Four-bladed 0.94 0.98

Starting with a two-bladed propeller and using the above conversions gives:

Diameter Pitch Pitch
Two-bladed 26.0 20 18
Three-bladed 24.76 19.8 17.82
Four-bladed 23.27 19.4 17.46

The 18 pitch was the first two-bladed propeller used on this boat. The 20 pitch was a later two bladed propeller. The OP states that the engine has a top RPM of 2600; however, the 80hp Ford Lehman diesel develops its peak horsepower at 2800 so the 20 pitch propeller keeps the engine from developing the maximum horsepower and apparently the 18 pitch propeller is the best match for the boat. The 20 pitch would allow 74 horsepower at the engine.

Dropping to 23.0 inch diameter on a 4 blade would bring pitch up to about 20 on this 4 blade, but an original pitch of 26X18 was used on this boat so a 23X19 seems reasonable and Ebay has a Federal Equi-Poise Bronze 4 Blade Propeller 23" x 19 p LH, 1 3/8" Bore. The bore on this is small and the propeller may not fit your propeller shaft. The four-bladed propeller is less efficient and is 95 percent of what a two-bladed would be.

You can also use the rules of thumb with the table to develop other propeller options. Without hull depth and water line beam, this is probably about as good a guess as we can get because we are starting with a propeller that gives a known result and adjusting from there, not just using formulas that do not take into consideration the dimensions of this particular sailboat.
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Old 23-11-2012, 12:46   #34
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I am in a sort of similar position except that we are far from home and what we have will likely have to take us through two few hundred mile passages to get back. All of the prop calculators seem to pitch the prop for our boat too low and the engine has to work to make hull speed (a lot). We used to cruise at 1600 rpm and for the next leg of our trip (until I can get to a real prop shop), we will have to motor at around 2100 rpm. As far as I can tell it feels like we still need about two more inches of pitch and when we get back to the USA, I will add them.

Best of luck.
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Old 23-11-2012, 14:21   #35
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Re: Prop Diameter vs Pitch

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Many thanks for your words of advice. I have contacted Deep Blue props and asked for a recommendation based on my inputs. All prop calcs so far agree with my learned forum colleagues but unfortunately my boat is in Hawaii so no chance of a loaner. My last purchase ended up being the wrong hand so I still have the option of the transmission adjustment from LH to RH.
Unless I'm mistaken I don't believe you have that option on a Velvet Drive. You can adjust the transmission LH to RH to coincide with your engine rotation but it will not change the direction of your shaft output.
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Old 23-11-2012, 14:27   #36
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Re: Prop Diameter vs Pitch

I have the manual on CD for that transmission (I have the same one) and studied it thoroughly before I installed engine and trans in my boat. I can let you know where I got my CD from if you don't have the manual.
You don't want to have a faster shaft speed. Faster is less efficient for displacement hulls.
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Old 11-02-2013, 11:32   #37
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Re: Prop Diameter vs Pitch

I have a Vetus m4.14, a 4 cylinder 1415cc ( Mitsubishi K4E )
rated at 33hp max rpm 3000rpm, max torque 78Nm (57.5ft/lb) at 2100rpm, Hurth trans 1.89:1
fuel consumption is listed at 260 g/Kw-h (gph?) at 2000rpm.
My boat is 30' w/25' lwl 15,000lbs(hull speed 6.75kt)
1000rpm=4kt, 2000rpm=6.5kt, 2200rpm=7kt (stern squatted down)
I have a 16"x9"" three bladed prop, because I have room for such a big prop could I replace it with a 2 bladed prop with more pitch and achieve the same thing...I could line up the prop with the keel while under sail and cut down drag).
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Old 11-02-2013, 11:58   #38
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Re: Prop Diameter vs Pitch

Making guesses at this can get expensive. Pay a little more up front and hire a professional who can come down to your boat and have a look. Chances are he will get it right the first time. Formulas are still guesses because they can not take into account the boats resistance through the water at speed. Different hull forms have different resistances through the water for a given speed and displacement. Resistance can only be guessed at unless you do a tank test of a model of your boat.

Don't reduce your diameter, change your pitch. Less diameter is less efficient. By going up from two blades to three blades, your pitch will need to come down significantly.
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Old 11-02-2013, 13:06   #39
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Re: Prop Diameter vs Pitch

My 3 blade 16x9 pushes my boat at about hull speed at the hieght of the torque curve (2100rpm = 6.75knt), the best you can ask of a prop. I was wondering about a two bladed prop that would accomplish the same thing (that could be lined up with the keel under sail).
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Old 12-02-2013, 10:54   #40
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Re: Prop Diameter vs Pitch

Usually the reason a three blade propeller is needed, rather than two, is because of more propeller blades means more area to distribute the engine load. If the load is too high, cavitation becomes an issue. What happens is the forces on the blade are so high that the water becomes a vapor at the surface of the propeller and then the water collapses back onto the propeller with such force that it severely erodes the surface of the propeller. If you have enough room between the propeller tips and the bottom of the boat, then a two blade will work. Another option of course is a propeller that folds when not in use, or a propeller that is variable pitch. For further information, see Dave Gerr's The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers.
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Old 12-02-2013, 13:38   #41
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Re: Prop Diameter vs Pitch

The empty space for the prop is big enough for a large prop, but the empty space in my bank account would not allow for a folding prop or variable pitch...besides I like the idea of fewer moving parts
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Old 13-02-2013, 09:42   #42
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Re: Prop Diameter vs Pitch

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The empty space for the prop is big enough for a large prop, but the empty space in my bank account would not allow for a folding prop or variable pitch...besides I like the idea of fewer moving parts
How much clearance from tip of propeller to hull? Note that straight up may not be the shortest distance so move that ruller around to get the shortest distance.
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Old 13-02-2013, 10:13   #43
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Re: Prop Diameter vs Pitch

15% of blade diameter minimum. So if a 16" diameter prop for instance, about 2.5 inches.
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Old 13-02-2013, 15:31   #44
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Re: Prop Diameter vs Pitch

I have a 16" prop with a relatively flat pitch of 9" (some call it a "cruising prop, compared to a deep pitch of a "power prop"). I could switch to a 2 bladed prop with more pitch and achieve the same thing (some achieve the same thing as a larger dia by squaring off the tip). My hull speed is aprox 6.75kts, the top of the torque curve is 2100rpm, 2000rpm pushes me at 6.5kt...so the engine, trans and prop I have are just about ideal....I just wondered about achieving the same thing with a two blade....no reason to mess with a good thing, but would be nice to know
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Old 13-02-2013, 16:05   #45
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Re: Prop Diameter vs Pitch

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I have a 16" prop with a relatively flat pitch of 9" (some call it a "cruising prop, compared to a deep pitch of a "power prop"). I could switch to a 2 bladed prop with more pitch and achieve the same thing (some achieve the same thing as a larger dia by squaring off the tip). My hull speed is aprox 6.75kts, the top of the torque curve is 2100rpm, 2000rpm pushes me at 6.5kt...so the engine, trans and prop I have are just about ideal....I just wondered about achieving the same thing with a two blade....no reason to mess with a good thing, but would be nice to know
I know it is not intuitive, but a two-blade propeller would have the same pitch and would be slightly more efficient. Again, it has to do with blade area and cavitation. With a larger diameter, two blade, you should be able to get the area you need, but calculations are needed. I wonder why the marine architect did not specify a two blade if there is room for it.
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