1987 F36 Tri-Cabin - prop sizing

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Re: 1987 F36 Tri-Cabin - prop sizing

Post by Misty » Tue May 17, 2016 12:23 am

Sorry. Should have marked this to respond. I have 20x19s that were just redone and I have the sheets on. I do have vee drives. They are three blade and I like the handling a lot. I think the plunge of the shaft angle may give me more maneuverability due to the vees.

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Re: 1987 F36 Tri-Cabin - prop sizing

Post by larryeddington » Wed Sep 28, 2016 11:11 am

I finally can respond to this post as 84 tri cab was out of water. Mine has Crusader 270s and has (came?) with 20x17's and after cleaning bottom got 24 mph but only 38 to 3900 RPM. Me thinks I need 20x15 to get max performance?
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Re: 1987 F36 Tri-Cabin - prop sizing

Post by Svend » Thu Sep 29, 2016 11:56 am

I found a lot of great prop info on the net... thought i would share...


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.


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.

This should generate some good thoughts, ideas...
In my case... i am looking at SLOW cruising at 5-7 knots, so that is the goal of my ideal props

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Re: 1987 F36 Tri-Cabin - prop sizing

Post by tdogg » Mon Feb 20, 2017 6:54 pm

Thank you Svend.

I'm getting excited to get the boat back in the water this spring...Not too much longer!

Great info. I'm also interested in the slow cruising.

5-7 knots is ideal. I did realize that running a single engine at slightly higher RPM I was able to achieve the 6 knots speed. This came about by accident, as I hit a rock which dislodged my port engine. So I had to get the boat back on the remaining engine.

To my suprise, once the boat is moving above 2-3 knots, steering is quite reasonable on a single engine. I also realized that I saved a lot in fuel.

So the next trip, I'm going to alternate engines for long stretches where minimal course corrections and steering are required. Definitely more difficult to pilot the F-36 into a lock on a single engine. Lol. But I can say I did it.

Very much looking forward to next season.

- TDogg
1987 Trojan F36 Tri-Cabin

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Re: 1987 F36 Tri-Cabin - prop sizing

Post by tdogg » Mon Apr 02, 2018 10:19 am

Hi All,

keep coming back to prop sizing.

I found a couple brass props that I can get my hands on that are 22x22 4 blades.

I'm looking for more efficient hull speed, and cruising speed. I'm sure I would have to get these adjusted, but wondering the forum's thoughts on what best to go with.

my drives are 1.91x1 direct Velvet drives with Crusader 350s (270 HP)

- TDogg
1987 Trojan F36 Tri-Cabin

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Re: 1987 F36 Tri-Cabin - prop sizing

Post by prowlersfish » Mon Apr 02, 2018 11:58 am

Way too much prop . You will badly over load the engines . Too large to even get close even when reworked
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