Depth of Prop in Water

[cyberbadger]

Soon I will be installing the propeller and it's shaft below Nyitra's Deck. I "guarantee" that Nyitra will be sitting with 19" diameter pontoons half submerged or more.

So let's just assume that water line is exactly at 1/2 way down the pontoons and that the hull floats levelly.

The prop I have is 18" in diameter.

If I have the center of the prop positioned at the bottom of the hull, I will have 0.5" of water covering the prop.

Is this enough? It doesn't seem enough, but I would like to keep the shallow draft of the pontoons as much as practical.

Where should it be for reasonable performance?

-CB

[Bob Cleek]

The catch-22 of steamer props! The wheel diameter is supposed to be 10% of the waterline length. That's a rather large prop by high-speed IC standards. A forty foot steamer would need a prop about four feet in diameter if the books are followed. The diameter can sometimes be lessened by the design of the prop... more blade area and the like. The clearance between the tips of the prop and the hull and skeg, if any, is supposed to be 20% of the wheel diameter. A 48" wheel would require nine or ten inches above and below the wheel tips, if the "rules" are followed. At least, that's how I understand them.

As for depth, though, at least with slow-turning steam-driven wheels, it seems not to matter so much. I don't know if there is any "rule of thumb" like there is for prop size relative to the vessel or blade clearance, but in practice I've seen steamships which regularly will have a fair amount of their blade tips out of the water when they are light. This causes them to kick up a lot of water and, I presume, to sacrifice a bit of power, but they keep moving nonetheless. You also must keep in mind that if you are powering through any sort of chop, or taking a wake across the stern, this "dry prop" area will, momentarily at least, be increased. While that will correspondingly decrease efficiency, as the steam wheel is going to be turning relatively slowly, you won't get the "runaway prop" effect one does when a high speed prop "bites air."

Take a look at this video at about 21:46 to see what the Liberty ship, "Jeremiah O'Brien," looks like under power and above her load line. Note the mess her "dry" prop makes. https://www.youtube.com/watch?v=wiFe_KpVmnk

Odds are one of the real "steam engineers" in here can give you a better answer than this, but I thought I'd give it a try.

[cyberbadger]

Bob,

Well Nyitra has a 24 ft Hull Length. Nyitra's first prop is 18 inch diameter x 24 inch pitch. See photo below for blade geometry.

Bob.1)
10% of 24 feet is 28.8 inches. (28.8 inches == 2.4 feet)

That is quite large.

I only have an 18" prop for now, this is fixed for the first sea test.

Bob.2)

Ignoring The first result, but following the 20% rule as you mention is confusing, 3.6 inches are required above and below the propellor. (I am assuming wheel diameter == prop diameter)

The calculation result is 3.6" inches. I don't quite get what that means in my case with Nyitra...

Nyitra is a Tri 19 inch diameter pontoon boat with the center pontoon being only 20ft and alligned at the bow to the other pontoons. So there is 4 feet in the stern underneath the deck after the center pontoon. There is 4 feet 8 inches (56 inches == 4 feet and 8 inches) between the port and starboard pontoons at the closest.

The imaginary 1/2 way down the pontoons waterline is also almost the same that was the water line in her former 9.9/15HP 2-stroke life with different deck accouterments. Exactly 9 inches above the bottom of the pontoons which are again 19 inches in diameter - so 0.5 inches from the imaginary 1/2 way line.

The Prop Shaft is two feet in length The prop has a 4" long taper - so the Prop can be no closer then 2 feet from the Center Pontoon. But not more then 3 feet 8 inches from the center pontoon or it will protrude past the past the ends of the port and starboard pontoons - my rule:).

Confusing it a little the Deck ends before the sternline.

The Center pontoon trailer bunk can bet cut/modified if need be. It's already supported in three places.

I kind of feel like ...
"OK people, the folks upstairs have come up this, and we've gotta come through. We've got to find a way to make this fit into the hole for this using nothing but that." Apollo 13(1995)




This is kind of also touching on elements from another thread regarding by malcolmd "Sizing Toothed/Synchronous Belts for prop drive" http://www.thesteamboatingforum.net/for ... f=8&t=1637

I will post pictures of the propellor shaft soon - it's at the machinists ready for pickup. It's a propellor, two unit bearings, and a timing belt pulley. 1.25" diameter shaft.

-CB

[fredrosse]

Can you post an arrangement drawing? The guidelines for a steam wheel are generally more applicable to conventional displacement launch hulls. I would think the long skinny pontoons would change the rules somewhat, and the 18 inch broad blade prop you have should be just fine. I am assuming your belt pulley will be in the water just aft of the center pontoon? Any slope to the shaft? What horsepower and engine and gear reduction are you planning?

[cyberbadger]

I am assuming your belt pulley will be in the water just aft of the center pontoon?

Yes. But I am trying to determine that positioning also - how many inches aft of the center pontoon. It will be in the water.

Any slope to the shaft?

No slope. It will be parallel to the Deck and the Pontoons.

What horsepower and engine and gear reduction are you planning?

6HP 1902 Toledo Steam Carriage engine. I was planning to start close to 1:1. There is however a slight gearing. The Toledo 60 roller chain sprocket has 26 teeth. The shaft that will be on the Deck meets it with a 20 tooth sprocket. Haven't measured that ratio beyond that. The timing belt pulley's are the same - no ratio there.

Can you post an arrangement drawing?

How about these?

Stern view:idler


Seagull view (From Above):
Errata: Center Pontoon should be a dashed line - it is not visible to a seagull from above because the Deck is covering it.


Stern view overlayed on photograph:


Propellor Shaft
1.25" 304 Stainless Steel. FYH Unit Bearings, Timing Belt Pulley. 2 feet in length


Transmission minus Toledo and Chain, deck shaft 1" 304 Stainless Steel:


-CB

[Bob Cleek]

The "rule of thumb" on prop diameter is apparently most often observed in the breach. A pontoon arrangement, as a catamaran arrangement, may well make for a much easier driven hull with less wetted surface. The prop diameter, blade area, and pitch affect the efficiency of the transmission of power from the engine to the propeller.

[cyberbadger]

More then 1 error and added new detail of proposed engine location.

Seagull View Rev A



-CB

[cyberbadger]

Ok so this is what I came up with.

Layers:

• 3/4" marine plywood (Not pictured) deck

• 2" square box tubing with 1" hole spacing (Only two cut-offs were thrown in the picture - they will be Longer)

• 1/2" 6061 T6511 Aluminum bolt plate

• 19" tall Aluminum Center portion of 2-stroke gasoline outboard filled with great stuff expanding foam, painted black

• 1/2" 6061 T6511 Aluminum bolt plate

• 2" to center of bearing/center of prop shaft/center of propeller

For a total height of 23"

And by my calculation 1.25" of the prop will be out of the water assuming imaginary waterline of 1/2 up pontoons.
(You have to remove the 3/4" Deck from the total height because the distance I have is 14.5" between water and BOTTOM of Deck.)

This is what I'm going with for now, I'll let you folks know how it turns out.



-CB

[Lopez Mike]

When my monohull is at rest with no one in the boat and tied to a dock, the top of the prop is about two inches below the surface and it thrashes away splashing water everywhere and producing poor thrust. Even with a couple of people aboard it sucks air at first when taking off. Only when the boat gets moving and the water begins to follow the rise of the hull at the stern, does the prop get a bite and start to work properly.

With no rise at the rear of your pontoon, I think you would be wise to bury your prop as deeply as you can.

[cyberbadger]

Well my calculation of "1.25" of the prop will be out of the water assuming imaginary waterline of 1/2 up pontoons" was pretty close.

It's either exactly that or slightly lower. It seems to match the old waterline before the addition of the center pontoon.



I am happy with the propeller strut, but only tests will show if it is rigid enough for the task.

-CB