Gearbox ratio

[pat777]

I'm trying to increase my understanding of the seagull gearbox. My knowledge is fairly basic on the subject (It has something to do with turning the propeller doesn't it!(no exaggeration)). So I see references to different gear ratios and obviously I can see huge variations in size between different gearboxes on different models but what do the numbers mean? Were there any other gearbox ratios other than the ones listed below?

Also what would be the consequences of not matching the correct gearbox to the model if making up a "bitsagull"? I guess propeller size is also an important consideration. I notice some of my propellers are marked with which model they suit, however others have no markings so does this mean they are suitable for more than one model?

Cheers Pat

FORTY MINUS
FORTY FEATHERWEIGHT
MODEL 45 -----------------------------------------------GEAR RATIO - 10/21


40 MODEL
FORTY PLUS MODEL ------------------------------------GEAR RATIO – 10/35


CENTURY 100
CENTURY PLUS -----------------------------------------GEAR RATIO – 10/35


SILVER CENTURY
SILVER CENTURY PLUS
80, 90, & 110---------------------------------------GEAR RATIO – 12/48


QB SERIES
KINGFISHER
SEAGULL 6-----------------------------------------------GEAR RATIO – 3.1 TO 1 FNR

[Charles uk]

All 102's except plus versions (4:1) were 2.5 : 1, as were the clutched 40 models, the Century model 75 & the Curlew.

The model 90, 110 & both inboard models had the FNR 3.1 : 1.

[1charan]

A 10:21 ratio means:
If you turn 21 full turns on the flywheel, the propellor turns 10 times. So the propellor has a slower turning speed than the flywheel (and almost everything else in the Seagull).

As far as propellors go, it is much more complicated. I do not have a definitive answer, there are too many variables like:
- engine speed
- gearbox ratio
- propellor size/number of blades/pitch/shape of blades
- boat size
- boat shape underwater (changes with how many people are onboard, and even if they sit in the front or the rear!)
- desired speed
- choppy water or quiet lake
- downwind or upwind

It could well be that a different gearbox and a different propellor (different than standard) give a much better (or worse) performance, depending on your boat.
Not all propellors fit all gearboxes. Try a barge pusher on a featherweight! You would have to cut away a part of the gearbox to make it fit.

Experiment! (and tell us what you found).

[pat777]

Thanks CharlesUK for the additional model gear ratios that my source of information had left out. Good to have the complete list for my reference and understanding.

FORTY MINUS
FORTY FEATHERWEIGHT
MODEL 45 -----------------------------------------------GEAR RATIO - 10/21


40 MODEL
FORTY PLUS MODEL ------------------------------------GEAR RATIO – 10/35


CENTURY 100
CENTURY PLUS -----------------------------------------GEAR RATIO – 10/35


SILVER CENTURY
SILVER CENTURY PLUS
80, 90, & 110-------------------------------------------GEAR RATIO – 12/48


QB SERIES
KINGFISHER
SEAGULL 6-----------------------------------------------GEAR RATIO – 3.1/1 FNR

Forty model with clutch
Century 75
Curlew
102--------------------------------------------------------GEAR RATIO - 2.5/1

Model 102 plus--------------------------------------------GEAR RATIO - 4/1

Model 90
Model 110-------------------------------------------------GEAR RATIO - 3.1/1 FNR
(inboard)

[pat777]

A 10:21 ratio means:
If you turn 21 full turns on the flywheel, the propellor turns 10 times. So the propellor has a slower turning speed than the flywheel (and almost everything else in the Seagull).

As far as propellors go, it is much more complicated. I do not have a definitive answer, there are too many variables like:
- engine speed
- gearbox ratio
- propellor size/number of blades/pitch/shape of blades
- boat size
- boat shape underwater (changes with how many people are onboard, and even if they sit in the front or the rear!)
- desired speed
- choppy water or quiet lake
- downwind or upwind

It could well be that a different gearbox and a different propellor (different than standard) give a much better (or worse) performance, depending on your boat.
Not all propellors fit all gearboxes. Try a barge pusher on a featherweight! You would have to cut away a part of the gearbox to make it fit.

Experiment! (and tell us what you found).

Thanks 1charan. That's a very good explanation of the gearbox ratio, very easy to understand (who would of thought it!) As for the other variables, I guess it will call for a more experimental practical approach in order to establish the best set up. I can't help feeling that any efforts in this regard could be in vain as surely the outboard designers at British Seagull would have matched the correct props and gearboxes with the different models.

I was more curious as to whether or not I could cause damage to occur to outboards by making up bitsagulls out of the "wrong" matched parts. I seem to recall in a previous thread about how having a smaller propeller could lead to the outboard over revving.

Thanks again for the responses,

Cheers Pat

[1charan]

Thanks CharlesUK for the additional model gear ratios that my source of information had left out. Good to have the complete list for my reference and understanding.

FORTY MINUS
FORTY FEATHERWEIGHT
MODEL 45 -----------------------------------------------GEAR RATIO - 10/21


40 MODEL
FORTY PLUS MODEL ------------------------------------GEAR RATIO – 10/35


CENTURY 100
CENTURY PLUS -----------------------------------------GEAR RATIO – 10/35


SILVER CENTURY
SILVER CENTURY PLUS
80, 90, & 110-------------------------------------------GEAR RATIO – 12/48


QB SERIES
KINGFISHER
SEAGULL 6-----------------------------------------------GEAR RATIO – 3.1/1 FNR

Forty model with clutch
Century 75
Curlew
102--------------------------------------------------------GEAR RATIO - 2.5/1

Model 102 plus--------------------------------------------GEAR RATIO - 4/1

Model 90
Model 110-------------------------------------------------GEAR RATIO - 3.1/1 FNR
(inboard)

Hello Pat,

In the gear ratio's you sometimes put the higher number first, and sometimes the lower number first.
Model 102 plus ---- 4/1 means 4 revolutions of the flywheel gives 1 revolution of the propeller.
Featherweight ---- 10/21 means 2.1 revolutions of the flywheel gives 1 revolution of the propeller.
Since the gearbox is bringing the revolutions down, I would probably put the higher number first.
As far as I know there is little chance for confusion, I don't think there are outboards with an overdrive.

Charan

[geofflena]

Silver century is 35 - same as century. Only the Silver Century plus is the large 48 box.
( well thats what it says in the parts book ! )
edit
also the Century Plus is 12/48
Geoff

[Charles uk]

Charan
You've still got an error in your capitalised list the model 90 & 110 were fitted by Seagull with the FNR gear box which has a ratio of 3.1 : 1.
It might be easier for the readers if you edited your list to correct it rather than posted it again.

Gear ratios are denoted with a colon, i.e. 4 : 1, where as the number of teeth before you have simplified the fraction is written 48/12.

Several racing outboards were fitted with overdrive lower units.

[1charan]

Hi Charles UK,

It was Pat's list, not mine. But thanks for the corrections, I hope it makes the list better.

Charan

[pat777]

Sorry I've not been ignoring this, I just did not notice the additional posts until browsing this morning. Thanks 1charan for further clarifying the orientation of the numbers with regards to the gearbox ratios. I have decided to list them with the number of turns of the flywheel first and the number of turns of the propeller second to make it easier for newbies like myself to understand.

Thanks to geofflena for the updated corrections, I have altered the list accordingly.

Thanks CharlesUK for the corrections regarding the colon, I have edited the list accordingly. This is the webpage where I procured the information in the original list.

http://www.classicseagullscornwall.com/ ... fications/

This list had the forward slash as opposed to the colon and as I was just copying the information, I laid it out in the same manner. Also the list on the other website has omitted some of the gearbox ratios and also has some errors. Hopefully now the list below is correct. Unfortunately the only way I have of correcting the list is to post it again as I am no longer able to edit my earlier posts due to the time passed since posting. (I think you are only able to edit posts for 24 hours?)

If there are any more corrections or omissions I'll be glad to edit or update the list accordingly. Also if any of the moderators are still able to edit my posts then please delete any of the incorrect information as you please.

Cheers Pat

UPDATED LIST OF SEAGULL OUTBOARD GEARBOX RATIOS

FORTY MINUS
FORTY FEATHERWEIGHT
MODEL 45 ------------------------------------------------GEAR RATIO - 21 : 10


40 MODEL
FORTY PLUS MODEL ------------------------------------GEAR RATIO – 35 : 10


CENTURY 100
SILVER CENTURY-----------------------------------------GEAR RATIO – 35 : 10


CENTURY PLUS
SILVER CENTURY PLUS
Model 80--------------------------------------------------GEAR RATIO – 48 : 12


QB SERIES
KINGFISHER
OSPREY
SEAGULL 5 & 6 (grey coloured)------------------------GEAR RATIO – 3.1 : 1 (FNR)


SEAGULL 5 & 5R (light blue, Yamaha leg)------------GEAR RATIO - 2.03 : 1 (FNR)


FORTY MODEL WITH CLUTCH
CENTURY 75
CURLEW
MODEL 102-----------------------------------------------GEAR RATIO - 2.5 : 1


MODEL 102 PLUS----------------------------------------GEAR RATIO - 4 : 1


MODEL 90
MODEL 110
SD110 INBOARD
SD160 INBOARD-----------------------------------------GEAR RATIO - 3.1 : 1 (FNR)