New Props for 1977 F30 Express

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Svend
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New Props for 1977 F30 Express

Post by Svend »

New Props for 1977 F30 Express...

I am curious if I can get better fuel usage (lower) by changing props. My main speed when out boating is 5-10 mph. I hardly am wot or going fast.
I think my boat has the original props for my two 318's

Any ideas suggestions for a prop size, number of blades, etc for my cruising style?

Thanks

Svend
1977 Trojan F30 Express, With Dual 318's with Electronic Ignition
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Re: New Props for 1977 F30 Express

Post by Svend »

Some interesting info on Props from my research over many many sites...

PROPS!!

I would say it is very possible to increase mileage at displacements speeds by adjusting the prop pitch away from one chosen for maximum planing speed.
However I think your example may not be accounting for rpm change with load. "700-RPM, through 1.85:1 gears, with a 17-pitch propeller" you calculate 5.25 mph with a 21" prop, but this assumes rpm will remain constant at 700rpm with the same throttle setting.

I would think with the 21" prop load would be greater on the engine and with the same throttle setting rpm should go down by some amount or the throttle must be increased by some amount to maintain 700rpm, this must be accounted for.
That is, an engines rpm is dictated not just by the throttle position but also by load, and consequently fuel economy is not just related to rpm, but instead by throttle position and net speed attained.
So it is possible that a higher or lower pitch prop may be better for fuel economy at displacement speeds than your current 17", and only experimentation will be able to determine which one allows your engine to become most thermodynamically efficient.
Engines usually have an ideal rpm range where they are most efficient, and Etec's look to be most efficient at very low RPM's because of their ability to lean out below 2200rpm.
If you look at Trawlers such as Nordhavn, they attain nearly 5mpg at 7mph, using a single small 165hp diesel running in it's ideal rpm range around 1300rpm, this in a 40+ foot boat weighing 50,000lbs. They use very large 30" 4-5 bladed props with shallower pitches than we are discussing here in order to reduce slip as much as possible because it is a greater factor than drag at those speeds.

GOOD STUFF

The effects of diameter will be discussed later. However, it’s worth planting this seed now: Diameter is the single most important factor governing propeller performance. For vessels in the displacement-speed to roughly 35-knot range, the larger the propeller’s diameter is, the more efficiently it will perform. The limitation here is, of course, how much room is available under the boat to swing the wheel. Often, a compromise must be struck, trading diameter for the other of pitch, blade area, or shaft speed.

Pitch, on the other hand, is somewhat more mysterious and is a frequently misunderstood characteristic of propeller anatomy. Contrary to popular belief, propeller pitch is related only indirectly to the angle of the individual blades. In actuality, the pitch is the theoretical distance a propeller would travel forward in one revolution if, for instance, it were traveling through a highly viscous medium, such as grease or soft wood (in the latter example, think of the threads of a screw spinning their way into wood). Therefore, a propeller whose pitch is 24 inches travels forward, pushing the attached boat along with it, 2 feet with each revolution, in theory. In reality, however, because water is not like grease or wood, the propeller slips, and as a result, it does not push itself and the boat this distance. The difference between the actual distance the propeller travels and the theoretical pitch is called, appropriately, slip, which may be as great as 25 to 40 percent of the propeller’s travel for a displacement vessel (meaning the vessel advances 60 to 75 percent of the pitch distance with each revolution). Slip is less pronounced for planing and faster vessels.

Twin-screw vessels, on the other hand, utilize counter-rotating propellers, which means they turn in opposite directions. Counter-rotating, twin-screw installations should always be arranged so the tops of the props turn outboard, or away from each other. That is, the starboard prop is right-handed, and the port prop is left-handed. If they are reversed, excessive centerline turbulence will be created, and consequently, the boat’s handling characteristics will be detrimentally affected. If both engine/gear combinations were right- or left-handed, the boat would suffer from considerable leeway, crabbing its way through the water and making course-keeping a challenge, indeed.

The alternative to the addition of blades is an increase in the propeller’s rpm. Wait a minute, Steve-you said slower is better and smaller props are bad, right? Well, slower is better, but in some cases, given a certain combination of engine, gear reduction, hull form and aperture, increasing the rpm may be the only viable option. More propeller rpm equates to a smaller prop with the aforementioned caveat of reduced thrust, maneuverability, and ability to punch through head seas. Given the choice, it’s almost always better to go first with a larger-diameter prop, then additional blades, then larger blades, and, finally, higher shaft rpm/smaller wheel combination.

There is one final option that may be available when sufficient propeller diameter cannot be accommodated: trading pitch for diameter. Although it’s not an equal swap, it’s a bit like trading in a hardcover book for the same title in paperback; it can be used, provided the propeller repair shop knows its business. The rule of thumb is, for every inch of diameter that must be sacrificed, 2 inches of pitch must be added. This is a less-than-perfect approach, however, because there is a practical limit to how much pitch can be added. At some point, to use the aircraft-wing analogy once again, the propeller’s blade pitch, or angle of attack, will become so steep that it will fail to generate lift, an occurrence referred to as stalling. In my opinion, trading diameter for pitch is less than ideal, but if it’s absolutely necessary, it should be undertaken only as a small, incremental adjustment or for fine-tuning engine rpm. There’s simply no substitute for blade surface area.

Wheel diameter selection is, once again, a formula based on several factors. As mentioned earlier, however, in the design stage, it’s generally accepted that larger wheels are more efficient and thus are more desirable for vessels operating under about 35 knots. If the engine’s power and shaft rpm are known (shaft rpm rather than engine rpm, as altered by the reduction gear), a chart such as the one found in Dave Gerr’s Propeller Handbook can be used to select wheel diameter. The limitation, however, is the requisite clearance required between the prop’s blade tips and the underside of the hull. Ideally, this distance should be no less than 10 percent of the propeller’s diameter for displacement vessels (more is always better) and 15 percent for higher-speed semi-displacement or planing hulls. Insufficient blade tip clearance almost always results in excessive generation of noise, often sounding like sand or gravel being blasted against the hull in the area of the propeller. Blade area must also be factored into this calculation, ensuring that the blades do not suffer from overloading, which leads to inefficiency and cavitation.
Doing the necessary homework where propeller selection is concerned will always pay dividends in the form of efficiency and smooth performance.

We had a long discussion on this over on PMM a time ago...

The reason it works with less HP is because of high torque which is available from a dead stop thru full RPM. That means full torque is delivered the moment the prop starts to turn allowing a larger prop with larger pitch, which would normally stall a combustion engine before it could ever get up to the power band.

As an example of low HP w/ High Torque: My 85ft 90ton boat was originally powered with 16hp steam power (steam engines have similar torque pattern as electric). This 16hp high torque steam engine drove the boat to 12 knots and could break 15inches of blue ice with a full head of steam.

That 20hp electric motor could probably drive my boat nicely.

Some of the advantages of DE propulshion are:
1 Slow speed operation. One can be continuiosly unded way at any attainable speed... sutch as 1 knot or slow enough to count the revolutions.
2 Manouverability. Made better be very high toqure, no gears and associated problems and more control of propeller speed.
3 Installation lattitude. Engine can be installed almost anywhere and greater lattitude for propeller location is also possible.
4 Less weight
5 Less noise and vibration. Aqua Drive, Python drive and similar drives are not needed.



Svend
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prowlersfish
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Re: New Props for 1977 F30 Express

Post by prowlersfish »

I think you really over thinking this . you are very limited to what you can do with your props , you could maybe a inch or so in diameter and drop some pitch .for maybe a little gain or not . To go to a much larger prop you will need a lot ($$$$) of changes . reduction gear trans . Then larger shafts . Then how to clear the props to the hull ? You would have to chance the shaft angle . all 3 come with a cost be sides $$$ reduction gears take some HP to turn as do larger shafts . larger shafts and related parts = more drag . increasing the shaft angle more power goes to lifting the stern and less to pushing the boat . that's wasting power .
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Re: New Props for 1977 F30 Express

Post by Svend »

hmmm... interesting...
so in 40 years, and including my goals of slow causing 90% of the time, there is no reason to look at another set of props if i hit a rock and have to get some new ones... you suggest just stay with what I have...

Thanks

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Re: New Props for 1977 F30 Express

Post by Cmount »

So not that I can give you much info on your specific application, I did just re-prop.

I purchased my boat just about two years ago. I had a set of props on her to start with gave me both great idle speed and great cruise speed. However, I am not a slow cruise guy. So while I was at cruise, I had a lot of black smoke...3208 cats. Not good for the power plants. So I dropped down to a 24 x 24 from 24*28 4 blades... Wow smoke was gone, lost two knots at idle and more than that off the top.

So what to do! I reached out to S and S props in Long Isl NY...gave them my data...and spend $$$$. They are not cheap...but I got back all the speed, both at idle and low speed and at cruise, and wa-la! No smoke or very little smoke. The WOT is almost there when I put it to the pins and the engines and boat are much happier and easier to run.

Now I have Catapult 4 blade 22*26 wheels and they are worlds away from where I was. But at dead idle, 600rpms the boat is 1.5 to 2 knots faster, with no pressure anywhere than the second set of 24*24s and matches the 24*28's . Marine gear is 2.1 ratio 320hp...

Hope that helps. it can make a difference, but may take a lot of fuel to make up the thousands you will spend. I wanted the cruise speed.
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Re: New Props for 1977 F30 Express

Post by prowlersfish »

Svend wrote:hmmm... interesting...
so in 40 years, and including my goals of slow causing 90% of the time, there is no reason to look at another set of props if i hit a rock and have to get some new ones... you suggest just stay with what I have...

Thanks

Svend
That's not exactly what I said . I was saying you are limited on what you can do a far as props with out other major changes . lets say your running 14x11 3 blades right now ( stock prop for your boat ) you could change to say a 15x10 or a 14x10 4 blade and so forth if you went to a 17" you maybe getting to close to the hull . you don't want to much prop as you will start luging the engines and may not be able to get on plane or that 10% of the time .

You can play with the props and maybe help burn less fuel but I don't think you will see a major change .

Remember this is a planning hull so it will never be as efficient at slow speeds as a displacement hull

FWIW your maximum hull speed is around 7 knots anything above that your plowing water ( wasting fuel) until on plane

One thing I do to save fuel some times is run on one , below hull speed of course
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Re: New Props for 1977 F30 Express

Post by lawyerdave71 »

My 78 F30 has its original 1978 props.

And in the bottom of my boat closet there is a brand new set of props with original Trojan price tags.

Turns out the new props are the exact same size as the props that have been on since 1978.

Since I can get the old gal up to 27 mph on the old props, re-propping is very low on my bucket list. Although she's about 6 mph under what the "Trojan" book says she can do!!

Like you say, if you are going to travel at low speeds and the props are not bent, damaged, dinged, and you are not having any vibration issues, probably better to let it ride and enjoy the ride!!!!
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1978 F30 Flybridge Express
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