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Other Propeller Options

Folding or Feathering Props (Pricy)


Max-Prop self feathering prop

If you can control the pitch at the push of a button then the next best thing for a sailboat is to fold or feather the props blades so the don't slow the boat down when it's under sail power. Max-Prop; www.max-prop.com manufactures and sells a self feathering props in a range of blade configurations. I'd love to have one, but our boat would require a 35", 3 blade propeller and the cost is $15,500.   And being a workboat I doubt it would be very long before I was needing to replace a blade or two due to damage caused by operator error.

 

Affordable Fixed Pitch Props  (Better)

Buying a Prop

If you live on the coast this will be a piece of cake, but if you live in Tulsa, Oklahoma you will likely have to shop by email.

Brent Swain's advise "Get a prop within 4 inches of the pitch you want, preferably closer. Any more and if you try to re-pitch a prop more than 4 inches the blade breaks off."

To test a used prop is to rest it on the hub and then tap the end of each blade.  It should produce a clear ringing.  If it is dull and short lived then pass it up.

CraigsList turned up more than a few props down toward New Orleans, so they are out there. Check: www.propellerplace.com

A new 36" bronze prop is about $3,600. 

Another option is a new stainless steel prop from Kahlenberg propeller that is intended for use on work boats.  We plan to frequently work in shallow waters and close to obstacles so a stronger prop might be worth the extra money. The price for
a 3 blade 304 stainless steel propeller 32" in diameter with a 27" pitch, left hand, bored for a 2.5" shaft is $3,995; F.O.B. Two Rivers, Wi. Contact: Steve Kahlenberg, Kahlenberg Bros. Co., Ph: 920-793-4507, www.kahlenberg.com

Kevin Morin builds boats up in Alaska and he highly recommended stainless steel wheels; "My examples were fishing commercially, justification for these wheels was their ability to run in gravel bars of Bristol Bay and keep running without damage."

Building a Steel Propeller from Scratch ( Yeah Baby - Field Repairable )

I came across a bit below in the Wylo-II group from Lex Hodgkinson.

"Hi Passepatu has had a mild steel propeller welded to a mild steel shaft for over 14 years now, I welded up the prop from 5mm (.1967" 3/16=0.1875)  plate scraps and welded this directly on to the shaft. This system has been pretty much trouble free
with only two cases of mild pitting on the blades when I allowed the small anode on the prop boss to get eaten away. This was remedied with an electric weld build up of the pitted areas followed by grinding, all done in Situ. (on the hard!) The system has a small anode on the prop boss, a small one on the rudder and a similar one on the hull close to the prop shaft area. I cast these myself, they are roughly 1" dia by one inch long and last around a year.

The propeller is painted with chlorinated rubber paint (as is the whole boat) and antifouled with the same paint as the hull (ablating, not the best but I found "hard" antifouling to last even less) The shaft bearing is "Thordon" or similar, some sort of fibre, and has never been replaced, the inboard seal is a "dripless" fibre ring that bears on a bronze plate, which I replaced with stainless steel. Works ok but the grease filled shaft leaks grease inboard, just a nuisance. No water leakage at all, just cobwebs in our bilge! We have no inboard bearing, all thrust being taken by the Hurth gearbox.

We have a dry exhaust and therefore no cooling water to exit at this seal and cool it, (I think that was the cause of the bronze plate wearing too fast) The whole thing works very very well, cost begger all and seems to incur no problems, I would be far more nervous with a thousand dollars or more tied up in a machined stainless shaft with bronze prop. I have a grease gun piped into the prop shaft tube and give it a squirt now an then.

Incidentally it is easy to alter the pitch if you feel so inclined, by either belting it with a hammer or twisting the blades with a large wrench The prop is two blade 17x11 approx and the motor a 35hp Nannidiesel. It will max out at around 2500 rpm (max design rpm 3000) Hope this helps, DONT let the "experts" say you cant do this in mild steel, it works and can be repaired or replaced easily and cheaply, Incidentall we have been careful not to have anything but mild steel underwater. Lex"

Lex is using a 1.8 to 1 Hurth gearbox, so the 3000 engine RPM is 1666 at the prop.

"The prop was built using a simple steel jig to compare the blade shapes to ensure they were the same and at right angles to each other, pitch was checked at two or three locations with more attention paid to the outer 1/4 and the tips. Balance was checked simply by mounting on centers and grinding to achieve no bias. The blades with some reinforcement at the roots were mounted into slots cut into a 2"dia hub which was drilled to 1 1/4" to accept the shaft. The hub is welded to the shaft only at the aft end."  --Lex.

Our prop is 36", not 17" so we got a bit more force involved in order to pitch the blades.  They make special anvil called a "pitch block" for hammering a prop into shape, but that's for smaller speed boat props.  However a pattern would be really helpful when bending the blades.

Propeller Design

Rotation

Most engines are left hand rotation.  Meaning that if you look at the engine's  fly wheel from the back it will rotate counter clockwise, or the top of the flywheel will be moving toward the left. Also most inboard props are left hand meaning they push the boat forward when they spin counter clockwise as seen from the back end.

Prop Sizing

A good place to start is with some of the manufacturers sites like Michigan Wheel: www.miwheel.com/propellers/prop-it-right/

Max allowable propeller diameter 34" - 36"  Need 10% clearance between the blade tip and bottom to reduce noise.
Vessel Max & cruising boat speeds 12 knts max 10 knts cruise in 15 knt wind
Displacement, length & LWL of the vessel 40 ton, 20 ton cargo, 74ft, 28ft
Shaft length:  20 ft ??

Using a Cummins C8.3 210 hp, 2400 max rpm,  446 lbft @ 1300 rpm 
The following calculations are from Boat Mechanical Systems Handbook
DAR=0.5 for 3 bladed standard prop, 0.7 for 4 blades.
Min Prop Diameter Inches as a function of blade area or DAR = sqrt( ( 125 * hp)  / (DAR * max kts * sqrt( max kts) ) )
sqrt( ( 125 * 210) / ( 0.5 * 11 * sqrt(11) ) ) = 37.9 inches for 3 blade
sqrt( ( 125 * 210) / ( 0.7 * 11 * sqrt(11) ) ) = 32 inches for 4 blade
Ballpark shaft diameter is 1/14 of the prop diameter; or 37.9 / 14 = 2.7 in for 3 blade and  32 / 14 = 2.3 in for 4 blade.
Shaft Dia In = cube root ( (321000 * ( .96 * hp ) * SF ) / ( 316 Stainless Steel Yield * ( rpm / reduction ) ) )
SF= Safety Factor
Shaft Dia In = cube root ( (321000 * ( .96 * 210 ) * 4 ) / ( 40000 * (2400 / 3 ) ) ) = 2 inches

Using an Allison 545 automatic transmission in 1st gear with reduction of 3.45:1
Engine 2,500 RPM = 725 Shaft RPM
Engine 1,600 RPM = 464 Shaft RPM

Using a Marine Tranmission with a 4:1 reduction ratio
Engine 2,500 RPM = 625 Shaft RPM
Engine 1,600 RPM = 400 Shaft RPM

From Dave Gerr's book "Boat Mechanical Systems Handbook"

Minimum Blade Area & Diameter for Displacement Hulls
= sqrt((125 * hp) / (Disk Area Ratio .5 for a 3 blade std prop * kts * sqrt(kts)))
= sqrt( (125 * 200) / (.5 * 11 * sqrt(11) ) = 37.0 inches

Prop Tip to Bottom of  Hull = .2 * prop diameter 37.0 =  7.5 inches
Between Prop and Rudder = .15 * prop diameter 37.0 =  5.75 inches
Between Prop and Skeg = .3  * prop diameter 37.0 = 11.25 inches

Rough Shaft Diameter = 1/15 * prop diameter 37.0 = 2.5 inches

Shaft Dia = cubrt( (321,000 * 200 hp * .96 [shaft hp] * 4 Safety Factor) / ( 20,000 psi yield strength 304 stainless * 725 rpm))
= cube root ( (321000 * (200 * .96) * 4) / (20000 * 725) ) = 2.57 inches

An 36" bronze prop from deepblueyachtsupply.com is $3,610 (2011) and it requires a 2.75" shaft.
A 2.75" x 10ft shaft in Aqualoy 17 from deepblueyachtsupply.com is $1,700.  (2011)

Shaft Bearing Spacing if feet = 4.6 * sqrt( Shaft Dia )
= 4.6 * sqrt( 2.5 ) = 7.27 feet
= 4.6 * sqrt( 2.75 ) = 7.62 feet

Thrust Pounds = ( 326 * shaft hp [200 * .96] * displacement hull propeller efficiency .55 ) / ( .9 * knots 10 )
= ( 326 * 200 * .96 * .55 ) / ( .9 * 10 ) = 3,825 pounds
1,275 pounds per 16 inch blade

From Dave Gerr's "Propeller Handbook"

Torque = (5,252 * hp) / rpm
(5,252 * 200 * .96) / 2,500 = 404 pounds feet

Hull Speed = 1.34 * sqrt ( water line length )
1.34 * sqrt ( 68 ) = 11 knts

SL Ratio - Speed / Length or theoretical hull speed when the wave length is equal to the waterline length.

Kts =11 Desired Speed in knots
WL=68  Water Line
LB=80,000  Displacement in Pounds
SHP=
200 * .85 Shaft Horsepower  = 170 Hp
SL RATIO = Kts / sqrt(WL)   = 11 / sqrt(68)  =1.33
From the SL RATIO chart, an SL RATIO of 1.33 gives LB/HP (Pounds/Horsepower) of 525
80,000 / 525 = 152.3 Hp

Pitch and angle of the blade are not the same.  Consider that the root of the blade closest to the boss cuts through much less water in one revolution than does the tip of the blade.  So the tip of the blade has a much smaller angle to the shaft that the root.  If both the tip and the root are pitched the same then both would travel the same distance through the water when given one rotation.

Bad Vibrations

The frequency in Hz transmitted by the propeller to the hull is given by
where: n = number of blades N = number of rpm of the propeller shaft.
Source: www.nautica.it/superyacht/527/tecnica/vibrations.htm

So our 3 blade prop turning at 500 cruising to 719 RPM running at wide open throttle gives us an frequency from 25 to 36Hz. If we dropped to 2 blades, that would be 17 to 24Hz.

A prop also have a natural frequency which is mainly derived by the length and thickness of the blades. If you whack the blade with a hammer, it will resonate at it's "natural frequency", like a tuning fork. If the generated frequency from the RPM is close to the props natural frequency then the amplitude or strength of the vibrations will greatly increase. Like two kids on either end of a jump rope. If one shakes the rope up and down fast and the other does it slow then the rope never bounces that high. But if they get in sync with each other then the wave in the rope is magnified. Soon the rope breaks and hits both kids in the head and their parents subsequently sue the school for not training their kids on how to properly use a jump rope; but I digress.

So the idea is to separate the frequencies as much as possible. The natural frequency of the prop is a function measurement of how stiff the blade is built. A really thick, stiff blade will have a higher frequency that an thinner, floppier blade. Since a floppy blade is easier to destroy it makes since to make the blade with a higher frequency. The problem is that the bigger the blades the lower the natural frequency. We could make the frequency created by spinning the prop higher by increase the rpm, but that adversely impacts power. However a 3 blade prop is better that 2 so that is an easy choice. And since adding thickness to a blade only slightly affects it's performance. Doubling the plate at the root of the blade would likely go a long way increasing it's frequency. It won't do any harm when it smacks into a log either.

Is that it? I don't guess there is an easy way to predict the natural frequency of a blade is there?

Stuffing Box / Shaft Seal


PSS Seal

Volvo, Rubber Stuffing Box
   

PSS Dripless Shaft Seals are a popular choice but they do leak when shifting directions if the shaft has any slip in a thrust bearing, and they are also $440 for a 2" shaft.  For me the down side is that the PSS is not designed for contact with oil or grease and I may want to use a mild steel driveshaft and protect it by sealing the outboard end of the shaft log and flooding the log with oil.

Volvo Penta makes a Rubber Stuffing Box for 2" shafts with 2 3/4 sleeves which sell for $165 (2010).  They say to grease it every 200 hours so it should be fine to flood the shaft log with oil. The requirement for a 2 3/4" sleeve is a bit of a problem too as a 2 1/2" pipe has an OD of  2.88". Water resistant grease is applied to the inside of the seal by squeezing the sleeve to vent it and then applying the grease to the inside.

A Traditional Stuffing Box, drips, and we don't want the drip, especially if we flood the shaft log with oil to protect the log and steel shaft.

Prop Resources

www.boatdiesel.com
www.usedprop.com
www.a1discountprop.com
www.tacomapropeller.com