Information wanted about the performance of small engines
|Peter Cuthbert 1||28/05/2021 11:26:09|
18 forum posts
As an intellectual exercise for the first Lock Down I posted on the Steamboating Forum the question of whether the Fairlight 406 (13ft) canoe [https://www.stanleysmallcraft.co/canoe-designs] would make a 'pocket' steam launch. The design is, in my opinion, very attractive and a number of posters picked up the thread and shared their experiences of steam canoes.
As we approach the (possible) end of Lock Downs I have gone back to the design and, with the aid of Dave Gerr's Propeller Handbook, begun to put numbers into the idea. I now have an A4 sheet of measurements and calculations that have led to the suggestion that the canoe would need 2.56 HP at the propeller. By comparison the old rule of thumb suggests that only 0.45HP would be required to reach hull speed. Using the PLAN/33000 formula a 'square' engine of 1.6" at 300 RPM would yield 2.93 BHP.
My main area of ignorance for this project is about the performance of small engines. What sort of speeds can they sustain under load, what designs would be recommended, what mods would be recommended for extended use, and so on.
A friend has suggested that the ST Twin launch engine would be ideal, but I am open to ideas. Given my age I would also probably be looking to acquire a built model if this intellectual exercise was to be realised.
I anybody wants a copy of my sheet of data on the design, just send me a PM.
21307 forum posts
Stuarts don't give HP figures for the Twin or compound but suggest the Twin is suitable for Radio control and the Compound suitable for hulls of 1.4m to 1.8m long so unlikely to be usful for your needs. They quote the Cygnet as 1.5HP and the Swan as 3HP both on 100psi steam at 800rpm. Or the 5A at 0.63HP at 1000rpm on 60psi. These may give you something to go on and I assume that is power at the Crank.
Not sure if you know the sizes of Stuart engines but the 5A is a slightly over square 2.25" bore x 2" stroke so quite different you your hoped for output at 300rpm and obviously puts steam consumption up a lot if you are going to get your required figures..
Edited By JasonB on 28/05/2021 12:28:53
|Peter Cuthbert 1||28/05/2021 21:42:05|
18 forum posts
Many thanks for the response. With a bit of a background in steam launches I am aware that some folks use the 5A in 'full size' launches but at way lower revolutions. It strikes me that 800 RPM would seem like having an ICE engine when the whole idea of steam was quietness...
Running the ST 5A spec through PLAN/33000 given a much higher BHP than Stuarts quote which is a surprise. It is also quoted as 13.5kg whereas I was thinking about 8kg would be allowable with 12kg for the boiler. I have a design for the latter, but as you can see I am at a loss for engines.
Anybody else out there with a bright idea?
|Nigel Graham 2||28/05/2021 22:31:03|
|1666 forum posts|
Ah... PLAN/33000 is indeed the theoretical power developed in the cylinder, but I take it you were careful to use the Mean Effective Pressure acting on the piston, not the boiler pressure? This is the mean of both admission-pressure and the expansion.
It is also lowered by multiplying the result above by a cautionary "Diagram Factor" that is highly variable, depending on clearance volume, compression (as the exhaust closes), superheat etc, but my references suggest around 0.8 a fair compromise.
Power at the crankshaft flange on a marine engine is usually also trimmed a bit by the condenser pumps, driven directly from the motion.
These may well explain why Stuart's figure is lower than you expected for the same engine.
|John Olsen||28/05/2021 22:50:18|
|1189 forum posts|
From what I have seen, your 1.6" bore would probably be enough. Having said that, if it was me I would probably go for a 2" bore and stroke. You can notch it up to improve cruising economy. You won't need a lot of power, the Leak compound in Dancer (thirty feet long, 2.4 tonnes) is maybe capable of 10 bhp at best, and that will drive her to about 8 knots.
Calculations of hull speed are highly empirical, the maximum for any given hull will depend on the hull form and total displacement as well as the length. A canoe shaped hull is likely to have a slightly higher speed than a bluffer shape.
Most of the Stuart engines are designed more for a scale appearance than for hard work. Increasing the journal sizes would be worthwhile for boat use. The 5A would be a good choice.
|Niels Abildgaard||29/05/2021 08:26:43|
|406 forum posts|
If two people can padle a 4 meter long canoe to hull speed ,then 300 watts sounds more than sufficient.
The IMLEC results can point to cylinder size.
Lets have some engineering.
Two persons ,one on each bank of a canal with a Y wire and a fish Weigth and we are in buisness as naval architechts.
What is the maximum screw diameter You can accomodate?
|Niels Abildgaard||29/05/2021 08:50:59|
|406 forum posts|
Found some numbers for a Hobie mirage Evolution 4m electric canoe:
1.2m/sec takes 40W from battery
1.6m/sec takes 80W and
2,5m/sec draws 400 W
More than 200 Watt on shaft is Bling
250 mm prop 12 revolutions per second as a first gues.
Edited By Niels Abildgaard on 29/05/2021 08:51:25
Edited By Niels Abildgaard on 29/05/2021 08:55:20
Edited By Niels Abildgaard on 29/05/2021 09:05:01
|133 forum posts|
From practical experience, I think that 2.6 HP would be way, more than required for a relatively slim hull:
A 2.2 HP petrol outboard would push our big, fat, inflatable dinghy with 3 people on board at more than walking pace on tick-over (this was an issue, as it didn't have a neutral!). At full throttle with one person on board, the thing could plane.
Rowing the same boat was a dismal experience - far less forward progress than a lesser effort in a wooden dinghy.
I think the ~0.5HP is much more likely. (Hull speed is ~5 kts, it becomes an exercise in rapidly diminishing returns if you are trying to exceed this.)
|Niels Abildgaard||29/05/2021 09:14:17|
|406 forum posts|
A 5 inch Netta did ca 10 rev per second ,made 200 W at an efficiency of 2%,coal in, work out, during IMLEC 2019.
In the ballpark.
With condensor and steam jacketed compound cylinders 5% should be within reach.
Edited By Niels Abildgaard on 29/05/2021 09:14:59
Edited By Niels Abildgaard on 29/05/2021 09:16:45
Edited By Niels Abildgaard on 29/05/2021 09:17:29
|Speedy Builder5||29/05/2021 09:44:08|
|2388 forum posts|
Looking at an old copy of the Steam boat Register (7th edition),
"LAUGHING WATER" LOA 19' 6", compound engine 2.5 + 4.5 x 2.5, boiler loco type 200 psi, 3 blade prop 15 x 25.
HEATHER LOA 17' Beam 2' 10" 3 cal radial 1.25 bore, boiler 3 drum water tubed, prop 3 blade 8 x 8
COUGH LOA 16', stuart turner No 1 (2x2), boiler flash steam 40psi, prop 3 blade 10x15
|Peter Cuthbert 1||29/05/2021 10:33:31|
18 forum posts
Thank you all for contributing data and ideas. For this exercise I am trying to be 'scientific' rather than taking the approach I took with my 'full size' steam launch. For that I ended up using what I could get rather than what would be the most sensible from an engineering point of view. The outcome was a rather weighty fit out.
The canoe design has the following statistics:
LOA = 4070mm, 13.35 ft.
Breadth = 980mm, 3.22 ft.
Depth = 139mm, 0.456 ft, 5.47 inches
Waterline Length = 3680mm, 12.07 ft.
Displacement (LB) = 0.2587 cuM, 258.71 kg, 0.254624 Long Tons
Block Coefficient = 0.00000052
V = 1.341 X (WL in ft)^0.5 = 4.66 knots, 5.36 MPH.
Rule of thumb power req’d = 1.75 horsepower per ton (2240 pounds) of displacement = 0.45 HP
|Peter Cuthbert 1||29/05/2021 10:35:08|
18 forum posts
Using Gerr's Propeller Handbook I have calculated:
DL Ratio = Disp Long Tns/(0.01 X WL)^3 = 144.80
SL = 8.26 / (DL)^0.311 = 1.76
Shaft Horsepower at the propeller = SHP=(LB*SL^3)/1213.0608 = 2.56 HP
Estimated Slip = 1.4/(kts)^0.57 = 0.583
Knots to feet per min = Multiply by 101.3 = 471.66 ft/min
Approx Pitch = (distance traveled per min/prop speed)*12 = 18.87 inches
Suggested Diameter = (632.7* (SHP^0.2))/RPM^0.6 = 25 inches.
I am being called for a trip out now, but will look up Cough on return. That feels intuitively in the right ball park. Please feel free to add comments and corrections.
|Peter Cuthbert 1||29/05/2021 13:00:07|
18 forum posts
Thanks Niels and Everybody
I think Niels point about the Hobie Mirage Revolution 4m Electric canoe is probably going to be the key one for this project. Dave Gerr's Propeller Handbook is excellent but oriented to large IC engines so working at the end which is always off his charts the figures may be less than reliable. Indeed I have heard from another forum that one naval architect uses his calcs and then reduces the proposed power plant size by 10%.
The Hobie measurement 2.5m/sec at 400W translates into Gerr Speak as 4.86 knots and 0.54 HP. That fits well with the hull speed calcs for the Fairlight Canoe while the suggested propeller of 250mm at 12 RPS translates to 10" at 720 RPM in Gerr Speak. The power plant in Chough on the SBA' Steamboat Register is also probably worth my further investigation. That has a 10" X 15" propeller. I think a little more work with Mr G will help me see if I can take the propeller size down from 10" by increasing the number of blades and their area.
I think that perhaps the Stuart No 1 is probably worth my investigation. I see that the Stuart website gives no data on weight or power, but the latter is easy enough to calculate. Anybody out there got a No 1 gathering dust that could be weighed?
7476 forum posts
+1 for Niel's approach.
The problem with applying the 'scientific' approach directly is there are so many unknowns at the moment, for example:
Niel's suggests towing the canoe with ropes attached to weight scales. With a stopwatch, the actual power needed to pull a representatively loaded canoe at several speeds can be accurately determined with his technique. It gives the minimum Shaft Horse Power, or output watts, the motor must deliver for each speed. Minimum because the shaft bearings will add friction and the propeller won't be 100% efficient. Most of the output will be wasted if the propeller is run at the wrong speed, too near the surface, or is obstructed by the hull.
I couldn't find any power output data for Stuart motors on the web. Surprising, because I'd expect it to be well known after all these years. PLAN/33000 is far from accurate for real engines; so I'd hook up a Stuart to a dynamometer and measure it. Motors deliver maximum power at a particular RPM, which is likely to be closer to flat-out than chugging along. Measure the motors actual RPM and match the propeller to it with a gearbox as necessary. The size of the propeller is limited by the displacement of the canoe unless it's mounted outboard on a long boom and small propellers must be spun faster than large diameter propellers.
The power output of the motor is throttled by pipework size, insulation effectiveness, and boiler capacity. Bigger the better! For other than short runs, I suspect the size of the boiler and fuel supply that can be carried will limit what can be done.
How about a steam-electric hybrid? Lithium Ion batteries are relatively light and could be hidden away at the back and used to drive the propeller at adjustable speeds with an electric motor. However, on open display at the front a non-critical working steam engine, brass boiler, stack of coal and authentic smell. The steam engine efficiently drives a generator at constant speed to top up the batteries, while the electric motor sorts out propeller rpm. Range and speed is determined by whatever is stored in the batteries plus whatever can be generated by steam. If necessary, Lead Acid batteries could be used to ballast the canoe for stability.
21307 forum posts
HP of the No1 given here though at lower psi plus a good history of the engine.
|noel shelley||29/05/2021 20:02:01|
|711 forum posts|
Hi Pete, I have a No 1 with reversing gear sitting here, I will try to weigh it tomorrow. Best wishes Noel.
|Peter Cuthbert 1||29/05/2021 21:29:25|
18 forum posts
Hi Noel and Model Engineers
Many thanks for that kind offer. In the context of weights this is what I had been thinking. I had calculated that the designed displacement was just short of 260 kg. Information from various sources suggested that a boiler of 12 kg was possible with perhaps 8 kg for the engine and maybe the same for the propeller and drive shaft assembly. That would leave around 230 kg for the weight of the boat and the passengers. In the specifications for the design it is suggested that the hull should be around 45 kg completed. That would leave 187kg for passengers and their kit. Taking 60kg as the typical weight of a person, or 120 kg for two, the possibility of the Fairlight as a ‘pocket steam launch’ did not seem too ridiculous.
Thanks Jason for the link about the No 1. That was fascinating and the power output is useful to know though I would hate to think what the engine looks like at 1500 RPM.
Thanks too to Dave for your observations. I am certainly with you there, but I had proposed the steam electric hybrid idea elsewhere and did not receive overwhelming support. "Direct motor drive would have several efficiency penalties, if driving a typical trolling motor, and they use very inefficient propellers, propeller efficiency drops from around 60% efficient (for typical appropriate applications) to around 40% (or excessive speed, small props with pitch to diameter ratios far less than unity). That implies that you need 0.675 horsepower at the prop shaft.
I also like the idea of using the canal, friends and scales to get actual data. However, this project is still at the 'play with the numbers' stage and, apart from the boat plans, not printed out yet. Sadly nothing to float just yet.
Thanks to everybody for contributing.
|John Olsen||29/05/2021 23:14:13|
|1189 forum posts|
While I would agree that the No 1 is about the right size, it has the single sided crosshead guide. This is not as robust as the bored crosshead design on the No. 5. Also the main and big end journals are bit on the small side for continuous work. This applies to the 5 to some extent too. I would suggest making a new crankshaft with the maximum diameter that the brasses can be made to accommodate.
The prop size is compromise, you want it large and slow turning for efficiency and small to permit low draft. Can't have your cake and eat it too!
Electric transmission makes sense when we have to match an engine to a highly variable load, but we already have a prop, which acts a bit like a torque converter, and a steam engine is also very flexible, so we don't need it.
21307 forum posts
Regarding rpm it's worth remembering that when these engines were first produced they were often used for electrical generating and battery charging where the highspeed was desirable rather than low down grunt and robustness.
The cygnet was the preferred marine engine around this size and used a lot of the 5A parts and could be had with condenser, pumps etc or as a non condensing engine.
Edited By JasonB on 30/05/2021 07:05:19
|540 forum posts|
Not sure where you are in the world but 60Kg is too low for Europe. See https://www.worlddata.info/average-bodyheight.php
For the UK male=86.7 female=72.7
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