Propeller Math Reality Check

[SailorHarry]

Hello gang,

My steamboat project is starting to get rolling. The engine/boiler have been shipped from India, I have plans, and I believe I have found a welder to help with the hull. It seems like a good time to start looking for the next big piece of the puzzle: the propeller. A while ago I found myself a copy of Dave Gerr's Propeller Handbook and tonight I spent a couple hours running through the math. The results seem reasonable but I thought I'd put it to the group and see if the math matches what other boats are running.

First, the particulars:

Length Waterline 20' 6"
Beam Waterline 5' 1"
Hull Draft 13.5"
Displacement 3360 lbs (1.5 long tons)

The designer recommends an engine of 6-8 IHP and a 16x25" propeller. My engine will be 4" x 80mm, which using standard (conservative) calculations should yield 8 HP. For the purposes of my calculations I used 8 SHP, since I am an optimist when it comes to such things. I picked 6.5 knots as the design speed at the engine's 500 RPMs. I expect that most of the time I will be running slower than that to save fuel, but it seems reasonable to pick the propeller for the maximum design speed and slow it down from there. This equates to a speed to length ratio of 1.45, which seems reasonable based on pounds/horsepower calculations.

Using the Bp-Delta method the ideal propeller size came out to 24x17. The largest propeller that still leaves reasonable clearance is 19". Recalculating for that yields a prop size of 19x27. Using the classic slip method the ideal propeller size came out to 23x23. I found no method for recalculating to a smaller diameter and at this point my brain was finished with maths and I called it quits.

So, does 19x27 sound like a reasonable propeller for this boat? Or should I just chuck all the fancy maths and just use the designer recommended 16x25? Thoughts?

Peace,
Harry

[artemis]

Dave's book (as he will be the first to admit) is aimed at the internal combustion crowd. People like Weston Farmer (and Dave as he mentioned to me awhile back) will simply say to use the largest diameter wheel you can fit for the best steam efficiency ("Westy" told me, many years ago, that a minimum clearance between the horn timber and the tip of the blade should be not less than 2"). A square wheel is considered the ultimate (but very seldom achieved). Try to keep the engine rpm's as low as possible to lessen engine wear and ensure a "smooth" ride. I far prefer to enjoy "fun per mile" rather than "miles per hour".

[farmerden]

you failed to mention whether you will be using a three or four blade prop -that will also make a difference

[artemis]

you failed to mention whether you will be using a three or four blade prop -that will also make a difference

According to Weston Farmer and most others in the steam boat power train design crowd, a three blade is considered best as there is only one blade covering the edge of the propeller strut or horn timber, thus eliminating the vibration that occurs when two blades simultaneously pass over this part of the keel structure. A really good discussion of propeller selection is contained in Chapter 16, pg. 136 et.seq., From My Old Boat Shop by Weston Farmer. The only reason to go to 4 or more blades is to absorb more power from the propeller without having to overly increase the pitch - five blades works better yet as it meets the requirements of only one blade passing the strut/horn timber at a time.

[DetroiTug]

The prop on the tug is 20X20. The front edge of the blades only clear the strut by about 5/8". It hasn't cause any problems that I've noticed, and too the strut is only 3/8" thick, that may be why I'm getting away with it. I did it that way to help cut the weeds when it picks them up. These small lakes around here by end of summer are loaded with them.

Harry, the Horsepower of 6-8 ihp for that hull seems a bit excessive for steam. Per the PLAN formula on the Tug, I'm only seeing about 4 hp and it will run about 7 knots (downstream with a tailwind)

-Ron

[barts]

4.5 hp should be ample power for your boat, using a 3 hp per ton estimate.
At 450 rpm this means a BMEP of 50 psi, which is ok for a single running at 150 psi or so.
Gerr says this will produce 5.35 knots. Your boat is pretty heavy to go fast. Note that slowing
down to 300 rpm will produce 3 hp, which will give you 4.67 knots.

Otter is 2.5" x 2.5" and swings a 16" x 20" at 300 rpm or so. A 16 x 25 wheel is prob. close to
right.

- Bart

[SL Ethel]

I am running an 18x23 three blade on my steel launch, which is 18'lwl and 6' beam on the water, and probably weighs around 3,500 wet with passengers. I have it geared up slightly to an equivalent pitch of 30", and it's enough to get me to hull speed with a 3x4 engine running around 300. To get your boat to hull speed (1.3 - not the 1.4+ you are shooting for), a 19" dia prop with a bunch of pitch on it (it's a steam launch, not a space rocket - you have plenty of room for error), will be plenty. If you really want to max it out and hit 500 rpm and as much speed as you can, you might think about a chain drive so that you can change the ratio for the cost of a sprocket or two. This may also give your the opportunity to pick up a 19" prop on ebay for $30 that has not quite enough pitch, and make it up in gearing. Saves hundreds on a new prop.

Overall, it sounds like you are planning for a small, heavy, high powered boat. Presumably it will have an unusually deep draft. I don't know if you are planning on doing any work with it, but it sounds like a good recipe for a small working tug. I can tell you that towing a disabled IC engine boat with your steamer is one of the most fun things you'll ever do.

Cheers,
Scott

[SailorHarry]

Hey gang,

Thanks for the replies. It took a few days to get the conversation rolling and by then I had quit checking in.

I agree that 8 hp is overkill but when the guy told me I could have eight for the same price as six, well, it would have been un-American to say no. I don't have to use the extra power but it will be nice to have.

[SailorHarry]

I am running an 18x23 three blade on my steel launch, which is 18'lwl and 6' beam on the water, and probably weighs around 3,500 wet with passengers. I have it geared up slightly to an equivalent pitch of 30", and it's enough to get me to hull speed with a 3x4 engine running around 300. To get your boat to hull speed (1.3 - not the 1.4+ you are shooting for), a 19" dia prop with a bunch of pitch on it (it's a steam launch, not a space rocket - you have plenty of room for error), will be plenty. If you really want to max it out and hit 500 rpm and as much speed as you can, you might think about a chain drive so that you can change the ratio for the cost of a sprocket or two. This may also give your the opportunity to pick up a 19" prop on ebay for $30 that has not quite enough pitch, and make it up in gearing. Saves hundreds on a new prop.

That's not a bad idea about using a chain drive. I'm working on how to layout the machinery right now, I'll try drawing that up. You're using one? How noisy is it?

Regarding my use of 1.45 vs the more usual 1.35 "hull speed" speed-length ratio, I've been reading a fair bit about these things. The "hull speed" concept is considered obsolete and isn't used in modern naval architecture. Of course, steam engines are obsolete too but we still use them. My point is, I was figuring a reasonable maximum speed that my oversized engine could achieve, not that I will care to stoke up a big enough fire to actually do that very much.

Overall, it sounds like you are planning for a small, heavy, high powered boat. Presumably it will have an unusually deep draft. I don't know if you are planning on doing any work with it, but it sounds like a good recipe for a small working tug. I can tell you that towing a disabled IC engine boat with your steamer is one of the most fun things you'll ever do.

At the waterline the hull will be roughly 20'6" by 5' with a draft around 25" to the bottom of the keel. The designer gives a nominal displacement of 1.5 long tons (3360 lbs). So it's not particularly big or heavy, I have just gone a little big on the power plant.

[Lopez Mike]

I hate to be the bearer of sad tidings but you will probably not get the right prop on the first try. And the right prop is probably not a concept that is calculable. The formulas will get you in the ball park but you may be surprised to find how variable your uses will be from time to time and how much they will change with experience.

I lucked out and I have a speed/rpm relationship that is not unusable and seems like a decent compromise. But if I continue to not run into the strong headwind and seas condition I have been allowing for, I will most likely try about a 10% longer pitch just to settle the busyness down. The relationship I am seeking is the speed and fuel consumption v.s. commotion ratio. Or something like that.

The only design variable that I would be careful about would be leaving room for as much diameter as possible. You may not need the room but vibration from inadequate tip clearance is difficult to rectify.

I started out my steam experiences fifty years ago with any number of wonderful rules and ideas most of which have fallen by the wayside. The real world keeps sticking its nose in.

Mike