Propellor for pedal powered catamaran
Hi. I am about to start building, or rather, bodging together, a pedal powered boat.
I have a cheap, old dart 18 cat, and the plan is to sling two longitudinal timber beams under the cross members,protruding just forward of the front crossbeam, and just rear of the aft, and bolt a ply deck to these longitudinal beams between the crossbeams, in place of the tired old trampoline.
I have two factory second Seacycle drive units, which will be attached to the front of the longitudinal beams, so the Seacycle drive units will be just forward of the front cross member, and two 'Trice' recumbent bicycle mesh seats will be bolted to the ply deck, over the beams, with adjustable positions for leg length. This should allow extremely easy construction, and put the point of propulsion not far from where the centre of effort of the rig was, and the centre of gravity of the riders and passengers (up to 4 persons total?) reasonably central.
The Seacycle units are rather like skinny outboards, with pedals at the top. The units are geared at 1:6, and I understand, from other threads on this forum, and from other sources, that a cadence of around 60 rpm, rather more relaxed than the road cyclists 90ish on the flat, is suitable for the higher resistance of powering a prop in water. This would give a cruising rpm of 360 at the prop.
The figure of 125-150w, approx 1/5 hp, is usually quoted for a reasonably fit cyclist.
I am seeking opinions on two areas. (at the moment!)
1) I have both 12" and 16" props from seacycle, which are in molded plastic, fairly long and thin (blade width about 1.5") and appear to have a concave trailing surface, with a relatively high pitch. I have used a commercial model rental seacycle twice, on both occasions rather worn and ill maintained, and I was not over impressed with the performance. I am considering building these 3 blade laminated ply propellors, designed specifically for human power by Phillip Thiel. This design looks purposeful, and is clearly well considered, but differs from the 'two long thin blade' which seacycle supply, and as discussed elsewhere on these forums. Why two such different propellor designs? Any thoughts on whether I am likely to get an improved performance? - I appreciate that without actual data on the seacycle props, only 'finger in the air' views are possible - I can't find actual data on the props on their website.
2) I am intending to use, at least initially, the Dart rudder and tiller set up, in its original position at the very back of the hull, but I am slightly concerned that the now propellor driven Dart would respond better with the rudders closer behind the props, but obviously the rudders need to be fairly close to the stern too. Any thoughts on this would be appreciated.
I would add that I'm no sort of boaty professional!
Many thanks for any thoughts. And apologies to any offended Dart purists - it really is an old one, and the pedal stuff will demount for sailing, I promise!
Last edited by Tiny Turnip : 03-31-2008 at 02:04 PM. Reason: forgot a point
Don't be alarmed - someone with answers will be along soon. In the meantime, there was a great, often on-topic thread about a high-performance human-powered boat here: Human Powered Boat .
As I recall, at least one person in that thread suggested using fat blades such as those Thiel uses, but the final result on that project was two thin blades. The resident expert on hpbs here also uses skinny blades on his boats, as do most of the serious hpb racers, it appears.
Not being an engineer, I'm not sure why - it must have something to do with drag on a larger surface area. Seems to me, though, that slow boats requiring a lot of torque use fat blades (barges, tugs). Seems to me that a slowly rotating fat blade will provide more torque than a fast-spinning skinny blade. Seems to me that hpbs could benefit from pushing more water with each rotation. Seems to me that we pedal-boat afficionados more closely resemble a harbor tug than a Cessna aeroplane. Seems to me that the high pitch of that Thiel prop is just the ticket.
Because my eyes glaze over when I see equations and other manifestions of Higher Mathematics, I've decided to conduct my own field trials. I've been buying three-blade R/C airplane props (skinny) and two-, three-, and four-blade boat props (fat) on eBay. In fact, just today I received the hanging scale I intend to use to measure torque from different props at different RPMs. Of course, I can still adjust prop RPM on my project with gearing. You're kind of screwed in that element.
The Tinhorn Field Trials are, unfortunately, still a ways off. I'll be keeping a close eye on this thread.
Seems to me that the guys racing hpbs (I've been watching videos) are maxxing out pedal RPM as they attempt to keep the foils flying. What's with that? A cadence of 60 ought to be redline, no? - not cruising speed. Spinning the pedals as fast as possible at max boat speed seems silly - in the videos I watched one can observe the jerkiness of power input. I'm thinking they either need to gear up or use a back-and-forth pedal motion instead of roundy-roundy.
(Seems to me that your rudders would provide more influence the farther away they are from the pivot point, but I'm not an engineer.)
High aspect blades like you see on airplanes equate to efficiency providing they match the load. Boat propellers are usually limited in diameter for strength and draft considerations but these are not normally a constraint on pedal boats.
I would not bother making the timber prop. It is not the ideal shape. You would do better with two pieces of twisted flat steel welded on a hub. Some flat bar with welder and grinder can make a prop that will get efficiency around 85%. This is better than what most boat props get.
The sea cycle should be reasonable but they may not be ideal. If you have the hull shape and total displacement I can work out how the combination will perform.
I have attached a couple photos of props that are optimised for their application on pedal boats. One was welded using stainless flatbar and then ground to profile. The other was milled from aluminium bar stock. Performance is similar. These give you an idea of the ideal shape for low drag hulls and spinning at 4X cadence.
If you want to take some time to learn about propellers then Google Javaprop and have a play. You need to set the options to suit water and know a bit about your hulls but as a start you could use your 360rpm, 150W at 2.2m/s (5mph). Select the E193 profile with Re# at 100,000. You need to get diameter up to the 16" mark to get over 80% with this shape but you can do better with other profiles. See attached screen image with these settings.
Tinhorn, Rick Willoughby, thankyou very much for your detailed responses.
I did read the global challenger HPB thread from end to end before I posted, (should have been working!) and found much useful information in there. I do find it difficult to detach myself from the Theil prop - it looks so purposeful, powerful, and well, beautiful, but I totally acknowledge Rick's authority on the matter, backed up by the bulk of the contemporary thinking on HPB props I have subsequently uncovered, centered on Warren Beauchamps HPB section of www.recumbents.com. Including some lovely prop fabrication material from you, Mr. Rick Willoughby! Very clear, and practical, easy to follow instuctions, can I thankyou for posting them, and congratulate you on their clarity to a layman! The only weak link is my own metal shop skills - I shall have to look carefully at the section on accurate jigging for making the weld between blades and hub - thats the bit I find scariest. My welding has never been very precise, but I may well be tempted to have a go in due course. My instinct is to get the boat on the water asap, and play with the 16" seacycle props initially, taking the Macready quickndirty approach to learning.
I was also interested to see the HPB props offered by Free Enterprise, but I imagine they are not cheap.
I have had a little play with Javaprop, and need to spend much longer with it, and try printing out to get the unwrapped prop blade plans. What a great, simple-ish tool though.
Anyway, Rick, I've taken some pictures of the Seacycle 16" props, and the Dart hulls. Forgive my naiveity, but I wasn't sure exactly what you meant by hull 'shape.' I have uploaded a simple plan and elevation of the hulls, but I couldn't find any proper Dart lines drawings; I will attempt a description.
The hulls are approximately 17 feet long at waterline. the 'keel' deepens gradually to the skeg point approximately 4 feet from the stern, where it forms a slight 'skeg' and then cuts back, running increasingly softly to meet the stern at the waterline. At the deepest point, the skeg, it is approximately 12inches below waterline. The plan at waterline runs smoothly and very gradually from a fine, pointy bow, to a maximum width of perhaps 11.5 inches, some 4 feet forward of the skeg. It reduces to perhaps 6 inches at the transom. The section is a deep, sharp vee at the bow, becoming a slightly bulbous vee toward the skeg. Aft of the skeg, there is a smooth transition from the bulbous vee to a shallow arc.
The hulls weigh approximately 40kgs each, plus two crew @ approximately 85 kgs each, plus say another 100 kgs for beams, ply deck and some gear. Say 350kg ish, plus or minus, displacement?
On the pictures of the seacycle props, I've marked a blade with tape in three positions, to highlight the section and pitch. From the hub, the tape is at 40mm, 100mm, 165mm. The corresponding pitch angles to the axis of the shaft are 30deg, 45deg, and 65 deg. approximately.
The seacycle units rotate the props anticlockwise when viewed from the stern, and the props and hence the spinner trail the drive unit.
Fastest cruising speed would be my priority, but I do see this project as a versatile boat; fishing and lobster potting, swimming platform, camping aboard (the deck will take a two person pup tent erected.) and sprinting:Burning off sailing boats on Windermere with my keen cyclist crew!
leading edge to the left.
leading edge to the right. concave underfoil surface, resembling MA409 profile from Javaprop.
'underview' of hulls in my dusty workshop
hull from skeg to transom in elevation.
I am reassured by your thoughts on the rudder position, Tinhorn, and by most of the other examples I have seen with this sort of configuration. If I did move the rudders forwards, towards the drive unit position, then turning the rudders would surely start to move the boat sideways, and diminish the turning moment? Do you have any threads/ web pages on your projects btw? I have found Ricks, and I shall go looking in detail after I've made this post. Many thanks.
I'm fascinated with human-powered boats and want to build both a prop-powered "speedster" and a two-person sternwheeler that look like "real" boats. Heh - that means no cats, although last fall I bought a cheap inflatable catamaran so my sweetie and I can at least get onto the water.
Last edited by tinhorn : 04-03-2008 at 01:58 AM. Reason: typo
You have provided some very detailed information and plenty for me to do some work with. I will not get time till the weekend to work on it but will give feedback then.
The hulls are nice. Try to keep weight down as mush as possible. With 17ft hulls you should not expect better than around 6mph with two people but I will give some numbers on this. In a real full effort sprint you might tough 10mph but it would be only for a few seconds unless both riders are extremely fit.
You sjould try the seacycle props. I will do some predictions and then see how close I am. I should be able to give cmparitive results with the best possible prop.
One thing that is worth doing is to do some serious engine calibration.
Get you and the other prospective engines down to a gym and find a nice recumbent trainer with power meter. Spend about 1 hour on it trying different resistance levels and cadences. Work out the preferred rpm and the sustainable power level at that cadence.
This is very useful information for designing a prop to match the engine and the boat.
I did some modelling of the cat. I have attached an image of the model and a performance curve.
The Seacycle prop should be OK. Your angle at 100mm radius seems low but the rest is about the right pitch. The blade chord is bigger than you need and could be trimmed with care if you wanted to optimise. As is might be better for one person but I have not checked this.
The performance is reasonably good. I think you will be pleased. Anything over 4kts is useful progress. So if you have a couple of young fit cyclist then you could expect around the 5.5kt mark.
The boat responds well to extra power. You should see 8kts in a real sprint.
I have attached an image of a prop that would be well suited to work at 6X cadence on the TT Cat.
The maximum chord is 40mm. The diameter is 342mm.
This shows that the 16" SeaCycle prop is likely to be bigger than necessary. However the pitch is about right so if you are unhappy with the performance you could reduce the chord to the point where you felt strength was compromised.
If you want to play around with the prop in JavaProp then the things to know are:
rpm is 360
there are 2 blades
diameter - play with this to determine the best compromise - if max chord is less than 35mm then you will not have enough blade strength unless you use a thick section
the boat drag is 88N at design speed so each prop needs to thrust with 44N
the spinner diameter can be say 25mm (0.025m)
the design speed is 2.9m/s
choose the E193 Re# 100,000 foil and adjust the AoA to get the best result
the density of saltwater is 1025
the viscosity of water can be taken as 0.0000013
You should be able to get efficiency around 85% using this foil and these conditions. The prop shown in the image would give around 87%.
oh, Rick- thanks so much for all your work! and I'm delighted that it is so reassuring about both the hulls and the prop. Under sail, I Know the Dart cats can achieve 20-25Knts, obviously a different order of power, but it adds testimonial to what you say about responding to more power. I shall Keep playing with Java prop, and get some more seacycle props to try cutting down- they're only $10 usd. I found this supplier of model aircraft props in the UK and wondered if any of their profiles might be likely to give me an improvement over the seacycle props? They have a good range. including a 16"x 16" 2 blader, also some 3 and 4 blade models.
its snowing here, preventing me from finishing repairs to my winKlebrig (a little 16 foot gaff rigged trailer cruiser) but your posts have made my weekend. The pedal cat should be well faster than the trailer cruiser!
The nice thing about adapting R/C props is their ready availability, if only they were appropriate to the task. They seem to max out their pitch at 14 to 16, and judging by Rick's illustration, his recommended prop has a pitch much greater. (I'm inserting a couple pics of a 13-pitch prop.)
Larger diameter R/C props I've seen have hollow blades. Nice that they're light, but I worry about joint strength where the two halves are bonded, and there is a pinhole in each blade for the expanding air to escape during the curing process. This would certainly have to be filled - securely - before putting the prop into water.
I'll tackle Javaprop again to see if playing with diameter and RPM will make a lower-pitch prop practical. Man, I used to think boats weren't complicated.
I have attached a screen image from the design page of JavaProp for the 150W case. You need to set the airfoils and options as noted earlier to get this output. You can then change things to see how performance changes.
If you are not familiar with 44N it converts to around 10lbf. So each unit will produce this thrust. It is not a huge amount of force. If you go for a sprint then the forces go up quite a bit. I know model plane props have been used with success. You can actually do a bit of shaping to improve their performance.
You will see from attached that the pitch is quite large. A 16" pitch means you would be spinning quite fast but it would be in the range for a trained cyclist. For cruising I design for 75rpm for me and for experienced recumbent riders I design up to 90rpm. I know one cyclist who uses short cranks and runs much higher on his recumbent bike.
If you get serious with a pedal boat then you will find it much more relaxing using "clipless" pedals with proper cycling shoes. It looks a bit strange clipping in when you get in the boat but if you are pedalling for a while it is more efficient and a lot more relaxing.
Sea-Cycle pedal power drive units
Note that the 12" prop starts out as a 16" prop which is cut down & polished. On the actual Sea-Cycle unit, the 16" props can hit/mar the pontoons when in the upright position.
Last edited by SeaCycle : 11-09-2011 at 03:32 PM. Reason: additional info
|Thread Tools||Search this Thread|
|Thread||Thread Starter||Forum||Replies||Last Post|
|Pedal Boat Design||BG_Geno||Boat Design||676||05-23-2009 10:50 PM|
|Need dual pedal crank sources||Carlazzomark||Materials||1||03-06-2008 02:36 PM|
|2 blades pedal propeller||Hisham||Props||1||01-23-2008 06:11 AM|
|MY propellor is too BIG||rich99uk||Props||1||07-24-2007 11:53 AM|
|propellor-curve?||Guest||Powerboats||0||11-06-2003 06:07 AM|