Propeller

Controllable Pitch Propeller

The benefits of controllable pitch propellers (CPP) specifically for a sailing/cursing/workboat.

* The props speed vs. power can be adjusted according to varying cargo loads.
* Drag can be reduced or increased to meet the sailing conditions.
* The pitch can be adjusted match meet the required rpm of a generator that is being powered from the drive shaft.
* No reversing gear is required.
* Prop walk can be reduced by reducing the pitch.
* With a reversing gear and prop you can prop walk the boat in either direction.
* Individual blades can be replaced.

Typically, the drop in propeller efficiency owing to the larger boss (prop hub) size of a controllable pitch propeller is about 2 percent. However, if a controllable-pitch propeller is well designed and correctly operated, it can result in fuel savings of up to 15 percent compared with a fixed-pitch propeller operating in a nozzle. --www.fao.org

The big down side to a controllable pitch prop is the cost. Hundested; www.hundestedpropeller.dk, is renown for some of the best controllable pitch props so got a quote from McGowan Marine Inc. 1-508 990-1114  stevegow@aol.com, on a new system for a 70ft motor sailor is $66,913 plus shipping. Needless to say I am no longer looking a new Hundested drive systems. 
 

Affordable Fixed Pitch Props

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. This happens to work out just fine when using a differential as a gear box provided the engine is mounted on the port side so that the right axel turns the prop shaft.

Prop Sizing

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 an Eaton Rear End Differential  gives a 3.25/1 reduction.  Read more about that here: So Here Is What I'm Thinking About
Mounting the engine to the differential abeam (transverse) on the port side
Using a John Deere 6068SFM M1; 119 SAE hp ( 88.7 kW) @ 2,132 RPM. Read more about that here: John Deere
Using a fully loaded boat @ 40 tons, or a partially loaded boat with good towing ability.
The recommended prop is a 3 blade, left hand, 32 inch, 27 pitch

The calculations below are from www.boatdiesel.com  but the calculator is limited unless you buy a membership.

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.

I located Nap via CraigsList who has more than a few props down toward New Orleans.  Nap has a new prop that someone did not come and get. It is a 3 blade, left hand, 33" dia. x 33" pitch, bronze prop with a 2.5" hub,  for $1,100.  And for $1,250 we can have it cut down and re-pitched to 32 x 27 - 2.5".

Stainless Steel Propeller

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 2.4" 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

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 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 propshaft area. I cast these myself, they are roughly 1" dia by one inch long and last around a year.

The propellor 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 therefor 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.

Vibrators

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?

Prop Resources

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

Examples

76 Centreboard ketch, 200 hp at 2200 RPM; Cruising speed 10 knots Twin disc MG 507 gearbox driving a Hundested variable pitch VP 4 FRQA propeller with a drip free type shaft seal.  

85' Ketch, Volvo Penta 180hp diesel (1982) Gearbox: low torque with twin disc transmission. Propeller: Hundested variable pitch propeller.

56 Motorsailer, 173-horsepower L1066T.2 Lugger marine diesel engine and Twin Disc transmission connected to the Hundested propeller.

65-foot, twin keel sailing vessel, Gardner 8LXB with a Twin Disc MG 509 reversing and reduction gear. This is a 13.93 liter in line 8 cylinder direct injection engine which develops 176 BHP at 1500 rpm. Idle is set at 450, and cruising rpm is 750 to 1000. Sterngear is a Hundested VP4-FROA Variable Pitch propeller system. Pitch control is from the pilot house.