Seagull Racing

Discussion in 'Multihulls' started by rosbullterrier, Jan 1, 2009.

  1. rosbullterrier
    Joined: Feb 2005
    Posts: 83
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    This missive will be beneath the contempt of most of you, I'm sure.
    You may remember the usage of those smelly little british anacronistic outboards before the Japanese came along to set the world right.

    Well, I have joined the perverse band of those who punish themselves by actually racing each other in small boats powered by these noisy inefficient little buggers.

    Not that I am in the exalted sphere of the Bermudan and New Zealand racers who not only enjoy warm blue waters but also tune their boats to plane at up to 28mph - no, we have a ridiculous Christmas race in Cornwall (UK) which even has a Fowey HarbourMaster speed limit!

    Anyway, there are several classes of permitted boat. The over 12 foot class is easy to win with a long slim skiff. So I want to compete with a 12 foot catamaran.

    I tried with this at Christmas and did very badly. Wrong engine, wrong propeller, possibly wrongly positioned on the boat.
    With my box of on-board spanners I found that adjusting the engine up, it ventilated badly - down and it churned up the mud until the channel at the top of the estuary gradually deepened towards the sea. There are two laps, so we return back up to the start and back again. (this year's was the 21st annual race)

    Afterwards I thought (after advice from your redoubtable columns) the single Seagull's prop, mounted centrally behind a constructed transom was too subject to ventilation and an engine would be required behind each sponson - I read 80 to 90% of Cats are powered as such.

    However after reading a piece on fairing the small horsepower outboard to prevent ventilation, ( http://www.duckworksmagazine.com/07/howto/fairing/index.htm ) - I thought, hang on, the Seagull drive legs are not in the water; the gearbox pinch bolt is at the waterline and only the gearbox will be in the water, whatever is above it ( or in front of it, if behind a sponson )
    Complete engine location redesign with one behind each sponson, or should central
    propulsion be adequate (possibly even two central engines!) ??
    The engine used was fixed to the transom, with steering by rudders at rear of sponsons. (this is a small Hobie style sailing catamaran)

    Now if your eyes are glazing over, my reason for trying to raise technical inspiration for such perverse end is because pure endless experimentation is not feasible and also I don't want to try to re invent the wheel. Beside the fact that I am trying to limit this strand in my life to a minor hobby and not allow its obsession overcome all else - there is also the quest to find the most efficient propeller for an engine which the manufacturer could not give an honest horspower figure and produced a wide range of the most esoteric prop designs with unstated reasoning. ( there also is no official WOT figure)

    I regularly contribute and submit questions to the 'Save our Seagulls' forum. However their technical prowess is mostly limited to 'how to make them work' or the 'collection of magneto numbers and their polishing'. Oh dear - I hope none of them read this . . .
     
    Last edited: Jan 1, 2009
  2. Guest625101138

    Guest625101138 Previous Member

    THe seagull outboards are not intended to go very fast. The leg does not have any significant anti ventilation plate. My first approach would be to increase the size of the plate. It needs to be nicely faired so it does not create its one bubble and be set deep enough to prevent. I would try about 6" wide and 8" long to ensue affective air cut-off. The leading edge needs to be about 1" ahead of the front of the existing leg.

    Some powered catamarans have a central pod just in front of the outboard that is almost as deep as the hulls and has a sloping front face. They are typically about 8" wide and shield the fat part of the outboard leg from water flow. If you are regularly immersing the muffler on the Seagull and it creates a large wake then a pod in front might be the solution.

    The most efficient prop will be the largest diameter that can be fitted but it needs to be pitched correctly to allow the motor to rev out and the blade area needs to be the right size to get the best performance. Generally it is easiest to just try different ones unless you have a all the necessary data on motor output and hull drag. Your boat will be easily driven so smaller blade area is likely to be best. A good prop can make a big difference. I would be looking around for two bladed props that have about the right pitch and are a close fit on maximum diameter.

    Rick W.
     
  3. rosbullterrier
    Joined: Feb 2005
    Posts: 83
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    Thank you Rick for very useful information.

    The gearbox used is the '102' which although is the most streamlined with a 12:30 ratio has the anti ventilation characteristics of a club foot - wherever the engine/s end up fitted, a plate as you describe, sandwiched under the water pump housing directly above the prop would work well.
    But, do you mean this flat horizontal plate - when you say 'faired' I think of an upright shape shielding the gearbox. I'm just being slow . .

    A shielding pod in front of the gearbox sounds a real cure. Would you mean it to be a long streamlined shape; how far forward of the gearbox?

    I have a couple of early Seagull two blade propellers which are raked back, presumably to add diameter. I have tried one and it produced a lot of froth - the two methods above should prevent this.
    If this prop worked well after these mods, would the next step be to alter the pitch (if neccessary) to create peak revs at wide open throttle? I believe these engines peak at about 3800 to 4000. Does running at peak revs mean top speed is achieved?

    The pitch of Seagull props is not stated, I shall study the Gerr Prop book to calculate this.
    I'm not sure if we have the expertise in the UK for prop reshaping.
     
  4. Guest625101138

    Guest625101138 Previous Member

    By fairing of the plate I was intending that it should have some thickness so it will tolerate some variation in angle to the water flow if it is tilted slightly or as the boat pitches. If it is just a flat plate then it could have a tendency to create its own bubble at the leading edge and feed air into the prop. The plate can be very thin at the rear.

    A thin foil section of about 8% thickness would probably work well. It needs a nice rounded leading edge. Ideally it will sit just below the surface at speed with everything trimmed correctly. It does not matter if the flow separates over the top of the plate, which is likely.

    A flat plate with a few degrees of upward tilt forward might also do the job. Its function is to feed unaerated water to the prop.

    This link shows a modern leg:
    http://www.picture-newsletter.com/boatexh/propeller-fj3q.jpg
    The plate is quite thin but it will be meant for operation right behind the transom so it is receiving aligned flow. In your case you do not have a transom to control the flow unless you build a pod in front of the the outboard. There are two other forward fins above the plate but I think they will be intended to prevent water spraying upwards from the thicker leading edge above the plate particularly if the outboard is mounted a bit low.

    If you can weld with some proficiency it is not hard to make a propeller that would be designed specifically for your application.

    You want the motor to be operating at its peak power at your top speed. This is usually just below the maximum rpm. Unless you have motor data this can only be done with trial and error.

    Rick W
     
  5. rosbullterrier
    Joined: Feb 2005
    Posts: 83
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    Rick, this is the propeller I mentioned. It fits between the pump housing to skeg quite snugly. What do you think of the pitch?

    [​IMG]
    [​IMG]
    The plate would fit between the gearbox top and the water pump casting
    [​IMG]
    After thinking about your advice that a faired pod would be useful to aid water flow past the gearbox fitted centrally in the catamaran, then the importance of having the gearbox behind a divided water flow means the best place would be behind the sponson.
    If I fit an additional central pod it would increase drag, and with one engine per side the extra one would give a bit more push. We are allowed up to 10 engines so long as they all are working at the end!

    Sorry, I now competely understand the anti ventilation plate now.

    Yes I am proficient at welding - but making a complete propeller?! I can chose between various shapes and styles of prop and modify then experiment, I suppose I'm just looking for advice as to where to start.
    Large diameter for efficiency, smaller with large pitch and high revs for speed?
    I am told by those who know that the standard Seagull revs at about 3800 to 4000 although the tuned 5R can achieve 6000rpm.
    Of course those real Seagull racers have very flimsy planing hulls and are using about a 2:1 reduction. (and I'll bet the warm blue water in Bermuda and New Zealand helps - I'm sitting in front of a cold desk, shivering, thinking of Antipodian pictures and the brown mud of our estuary) mind, we've very few sharks . . .
    Thanks Andy
     
  6. Guest625101138

    Guest625101138 Previous Member

    Two motors will give you more than one of course so that is a better option. The stern of the Cat hull may not produce the best flow so it might need some faring blocks added. Have you a photo of the hull transom?

    It is a bit of effort to work out the best prop but we can start going through it.

    First thing is to decide on the two motors or not.

    Next is a total weight estimate and some pictures of the hulls.

    Then you have to make some estimate on the HP of the motor. I guess we take 3800rpm as the best speed. Also need the gearbox ratio.

    This gives a starting point for a prop design. You need to know the maximum diameter that will fit under the exhaust.

    I still think you will need a prop antiventilation plate but if it is behind a bluff transom then it only needs to be flat plate.

    Rick W
     
  7. CTMD
    Joined: Dec 2007
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    Location: Melbourne, Aus

    CTMD Naval Architect

    Can you post a picture of the boat, particularly the transoms? It might help us help you.
     
  8. rosbullterrier
    Joined: Feb 2005
    Posts: 83
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    Thank you Rick. I was bringing it back here on Saturday. Unfortunately just as it was hitched I realised the trailerboard hadn't been brought and it would be dark in 10 minutes. No registration plate is bad enough, but no lights is too naughty.

    So I hope you won't have completely lost interest before next week when I go down again to get it!

    I really would prefer to use one engine per hull to give as much advantage as possible. Yes I shall still fit anti ventilation plates as you describe.
    The intention is to win the team prize with three boats entered, each in a different class.
    At present my 18'6"skiff can win fairly easily, although I'm sure there will be contenders soon enough.
    However the catamaran is probably the most difficult boat to propel competitively (apart from skips/bathtubs etc) which is why I intend to try it.

    I've just realised we have some pics saved from the recent race:

    With the spare engine fitted. The more suitable 170, prepared for 6 months, decided to become temperamental. But it shows the transoms. The two engines will replace the rudders
    [​IMG]
    Look at the wave in front of that tube
    [​IMG]
    Not everyone is a serious competitor
    [​IMG]
    Not all the engines neccessarily finish
    [​IMG]

    When I get the cat back I shall weigh, measure and take other pics. I shall also list the other facts and figures together. The gearbox is 12:30
    Thank you, Andy
     
  9. Guest625101138

    Guest625101138 Previous Member

    You will need a couple of small fairing blocks at the stern of each hull to flatten and level the flow to the outboard. The outboard will need antiventialation plates that sit near the line of the newly formed transom.

    I expect with two motors on this cat you will do quite well. Look at how much water the outboard leg is pushing now. You should be able to get this much cleaner with a nice transom and the leg sitting directly behind.

    The rudder will need to go of course so you will need to steer the outboards.

    Rick W
     
  10. CTMD
    Joined: Dec 2007
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    Location: Melbourne, Aus

    CTMD Naval Architect

    Rick has nailed most of it (as he usually does)..

    Remove the rudders and link the outboards as per the rudders for steering.

    Replace the "pod" with a more traditional tramp as its no longer needed to support the outboard.
     
  11. keith66
    Joined: Sep 2007
    Posts: 346
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    Location: Essex UK

    keith66 Senior Member

    We used to have a Seagull race at our yacht club, it died out as the engines got fewer. It was huge fun, I repitched a two blade prop by annealing the blades & twisting them with a pair of stillsons, To anneal alloy paint a stripe of soft soap on the aluminium & heat with torch, when the soap turns brown you have reached the right temperature then quench in water, repeat as necessary.
    Has anyone thought about using or tuning a British Anzani, or are they disallowed? they share some common parts but are far more tunable!
    We found large water pistols were de rigeur as fired into the oppositions carburretor it tended to slow them down a bit!
     
  12. rosbullterrier
    Joined: Feb 2005
    Posts: 83
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    Sounds excellent Keith. Seagulls and their spares are plentiful and are becoming very popular. Join in on the SOS forum.

    I agree Anzani, Archimedes and such are much better engines. Seagulls are nasty smelly noisy heavy and irritating icons of British engineering. Which is why so many (I am told) use them here to push their slow displacement barges. And the even more perverse buggers race them . . .

    And unfortunately they do become very collectable - there is often an Ebay frenzy

    The raked two blade prop seems to be favoured as efficient but they are a bit scarce now (late 1940's) and I would be scared to bend one. I've got just two. I'd rather find the best diameter/pitch for the boat and engine and match up a Japanese without a through prop exhaust.

    The forum members are worldwide but mostly from your area in the UK. They keep saying they would like a race venue to happen nearer there. Here, half way down in Cornwall it was started by yachties 21 years ago betting a bottle of whiskey who would get back first. It's strictly for the original two legged Seagull.

    You will gather its not as technically serious as the Bermuda and New Zealand races. One of the SOS forum moderators has spent a shedload race building an engine and presently holds the world speed record. No he doesn't race in this country.

    He's just practising here. I believe the hull is one of those Stealth models which can't be picked up on the radar -
    [​IMG]
     
  13. rosbullterrier
    Joined: Feb 2005
    Posts: 83
    Likes: 2, Points: 8, Legacy Rep: 21
    Location: Cornwall, UK

    rosbullterrier Junior Member

    Chris you say -
    "Replace the "pod" with a more traditional tramp as its no longer needed to support the outboard"
    - I think you mean the original trampoline. Yes, good idea, save a lot of weight although we would have to sit well forward with those engines further back

    And next year - no mast, no dragging a cloud of balloons . . .
     
    Last edited: Jan 5, 2009
  14. Guest625101138

    Guest625101138 Previous Member

    Working on 3HP per engine and a 12ft cat displacing 250kg I estimate you could get around 13kts with a good set up.

    I think this would be a reasonable basis for a prop design. Say 2kW per engine at 6.5m/s. Prop speed of 1500rpm and will say a 300mm diameter will fit.

    Attached shows the geometry of the blade required for an E193 foil.

    Rick W
     

    Attached Files:


  15. Frosty

    Frosty Previous Member

    Hello I think I saw someone cheating there. One boat had more than one and even had 2 Anzanis on each end.

    I like it, typicaly British ,--what!

    But poor ole engines, they do all that work all thier misserable life then just when they should be in a home or museum the have to do what a younger outboard should do.

    I will report this to the RSPCO.
     
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