Saving Old Seagulls

I posted some while ago about balancing props which I found were all hugely out. There wasn't much interest, well there wouldn't be from the slow yacht pushers more interested in MPG.

However should the 102cc's be used at higher revs, they will vibrate your bum off. And waste hp and fuel.

I read the crankshaft creates much vibration in horizontal movement although during vertical motion the crank weight can be balanced for the piston assembly.

Is it a waste of time balancing a two stroke single cylinder? I thought with the large influx of Antipodian engineers we might be taught . . .

Mind you, pistn hasn't got his running yet -

There is of course the matter of the flywheel balance weight....

I think you might find that all the later flywheels are almost balanced perfectly, well, to within a few grammes.

There wasn't a Seagull ever built, that was designed to rev over 4500, except perhaps the QUB series, that during it's development Seagull had a hard time getting it's revs down below 5000.

A good example is the Curlew, 64cc, 25 : 1, sealed, clutched gearbox
& the model 75, 102cc, 25 : 1, sealed, clutched gearbox both with the same 3 blade weed free prop.
The Curlew will spin the prop faster on the same hull using less fuel, whatever is said about the QUB range, there is no doubt that a looked after one will give performance & economy that will embarass many of the larger capacity more modern 2 stroke outboards.

Sorry slid off on a tangent there.

There are numerous examples still running well at 60 years or more old, so a little vibration doesn't kill them.
You only have a problem with vibration, when it shakes the transom off your boat!

Well I do run an outfit called Noosa Prop Shop but we specialise in stainless props for 50 HP plus ...have much experience in cutting down props from say 15 in to 13 1/2 and changing from chopper blades to more modern style just using a ruler angle grinder and felt pen ....we have never had problems with balance ..some motors running 250 HP.

Personally I think the flywheel is much lighter on a seagull than on many small 1 and 2 cyl motors perhaps half the weight of a 5hp suzuke 2 cyl

The one sided ignition core must cause some inbalance .

Have had mine running for some time in a tank without the prop and vibration was not an issue. If it vibrates with the prop fitted then I think its time to tweak one blade and see how the vibration changes...surely the people who race these things dont have vibration problems from the prop ???? But do they use a bow tie ...?? I doubt it .

Dynamic balance must be more of an issue than static at the low 1200 rpm prop revs we have ...I have had problems on some outboards where the prop was not running true due to not being tightened enough or the gearbox bearings loose

Exactly the same way I do it.

But I do equalise all the blade area's, leading & trailing edge heights, including cup angles.

But I never bother with static balance on the prop, at a maximum of 3500 prop revs on 2 blades, 7.25" diameter, I don't think it's worthwile.

It's the powerhead that vibrates, I run a 170 on a 5R leg & after a couple of hours the ally Amal 420 carb slide has chewed up it's locating groove on the steel locating pin.
I'm thinking of making a new slide from bronze with tighter tolerances so there is less room for it to wiggle about.

Pistonbroke, I'm sure, will agree that a prop working properly can give a speed increase of up to 5% with no increase of engine revs, for only a couple of hours effort' with basic tools.

Perhaps we should ask him to post a beginner's guide to cleaning up Seagull props, I know there are at least 100 Seagull racers over the Tasman on both halves of New Zealand that would love a few pointers.

Well I now appreciate where you are coming from with experience of commissioning larger marine engines, but we are talking solely about a small 100 cc single cylinder engine with horizontal crank throwout and no counter balance weight.
I have had no reason to have any practical experience with two strokes, the only 2-stroke cars through my hands were Wartburg and Saab and they ran perfectly. Certainly magnetos were only of acedemic interest until these damned Seagulls flew in. Even a Godiva fire pump I have I converted to coil.
So what the hell a one sided ignition core is, I'm not sure I need to know, but if its the heavy steel magnets I think you will find the flywheel well balanced at BS. Trust me pistn, running in a tank is not the same as bolting it to a lightish transom.

Pistn mentions propeller imbalance.
I have put in several posts regarding the need to balance Seagull propellers. I have been as ascerbic as the sensitivities of the precious yacht/ barge pushers/engine polishers could tolerate, with absolutely no return interest in balancing whatever.
I have static balanced Seagull 2 blade, 3 blade, 4 blade and 5 blade taking water flow into account from where to remove the metal. They were all notably out of balance. The difference of a balanced prop to the vibration of the bum, boat hardware and thus lost hp was significant. We are not looking at Charles L racing rpm, just the standard rev range.
Pistn has unfortunately no experience of them, came in after these posts, or ignored them.

However I was looking for any wisdom or advice in helping the balance of a Seagull engine without prop. I have read that balancing such an engine is a waste of time. And no one disagrees - this must be the case then . . .

Without a contra rotating balancing shaft or multiple cylinders, there is no way to achive dynamic balance in the powerhead. It becomes a tradeoff between fore and aft vibration due to the piston movement and lateral vibration due to any attempt to balance the piston by adding weight to the side of the flywheel opposite the crank pin. Fore and aft vibration is less likely to shake the engine off the transom and with the tiller hinged in a vertical plane is less unpleasent to hold than lateral vibration.

When one weighs up the difficulties against the potential benefits, just about everyone NOT racing leaves the powerhead stock. If however you are making your own ultra light pistons and conrods, you will need to rebalance the power head.

I wonder what revs you could get a Seagull up to if you had a titanium piston and con rod half the weight of the standard ones? (might be advisable to find out how fast you can spin the flywheel without grenading it first )

for Rosbullterier
No counter balance weight ..??? eh ? whats that weight welded around the crank pin ??

Your version of static balance is to remove weight in such a way that balde areas are equalled as well as pitch...what you are doing is improving dynamic balance .

the one sided ignition core referes to the magenets passing the ignition magneto core in one position only per revolution ...on many motors the magnets pass two cores on oposite sided giving balance and often have 4 magnets at 90 deg ....the attraction of the magnet to the core gives another unbalancing component.

Charles ...I dont fully understand your horisontal and vertical arguments as when running its more a question of inertia than which way gravity is acting ,,

I have not ignored your post I just dont comment unless I have something useful to say ( ha ha )

having just built a 16 ft "wide " canoe for the seagull to cruise the noosa everglades there is little you can tell me about light transomes...

Seagull racing boats have to be built light,
Search on the web for "Anaconda" a Bermudian 35 foot race boat, that sadly died due to domestic ply disease, stong enough for the North Atlantic, could carry 6 but could be carried by 2 & still holds records due to it's wave piercing design & waterline length.

6 to 10mm seems to be current transom thickness fashion, though my latest test hull has a 6mm framed with a 2mm skinned transom.

and do we get a photo of the racing hull ??

I'm sure the fastest man in the world won't mind:

I bet you envy me the groupies, all that work wasn't waisted.

Yes it is a Seagull, it's a 170 powerhead with all the useless bits dumped, on top of a 5R leg.

I think I will call it a Seagull 175.

Wait till you see the Mk 2 version.

The guy in Bermuda who shares the record with me, is working on his 5R & talking up a storm on hull design so I might not have my name on it for too much longer.

so what is this record speed then ???

Carnt be very fast as I carnt see a seagull driving a 19 pitch prop !

Not as fast as I would like.

In New Zealand last easter 3 hours 42 mins for 88 miles, you work it out, & that's going downhill.

I can't do a mile in 2 mins in still water yet, but I'm getting closer.

I have a couple of ideas, I want to try on the blue motor, on MK2 I'm going to get scientific with the port timing, rather than cleaning them up to make them do what they are supposed to do.

A Seagull might drive a 19" pitch it depends on the gear ratio.

On a decent lightweight planeing hull you'd be suprised what a straight out of the shed century or 102 will do, after a quick once over to check that all is as designed.

thats 23.5 MPH then .....

so why not work backwards ..you want 30 mph .....so what pitch must it swing with 15 % slip at your known prop rpm ... ??

you work it out !!!