How to Speed Up A Seagull??

[JERSEYMAN]

Hello All, this may be a real no-no in some peoples book but I have a problem and need to make my seagull faster when pushing my dingy!

The problem I have here in Jersey is that with a 40 ft. rise and fall of tide I find that seagulls are just not fast enough to outrun the tide hence I find myself sometimes going backwards or barely moving on full throttle, a little worring when after an hour full pelt, the jetty I was aiming for is further away then when I started, then of course the tide turns and everything speeds up drastically. I have several Seagulls, 40+, century, silver century plus and they seem more or less the same in performance.

The question I really have is, what is the best powerhead to use, what is the fastest gearbox to use, I dont mind clutch or no clutch and what would be the fastest prop to use?
I have loads of spares all pre 1978 so I could easily make up a "Fast" Seagull if that's at all possible the only problem is I don't know which configuration of engine/drive/prop to use.

I can understand that some will think it wrong to "make up" motors but I really don't want to dig out my honda, There is something quite special and satisfying using these low tech, antique looking workhorses.

Many Thanks, Rene.

[blue]

time past i fished the paternosters out of greve de lecq for years with a 102----it pushed my heavy 12 foot dinghy up to maximum displacement speed---after it was stolen i got the first modern engine in greve de lecq----a 6 hp evinrude fisherman twin-----didn t make the boat go any quicker----i would suggest you do not need to make your seagulls go faster-- just get them running well and the speed should be sufficent----regards blue

[phil]

If the boat is going hull speed already it may be difficult to "improve" a Seagull enough to make much difference. You may do better with a longer and lighter boat. A long, narrow, light, flat bottom boat with no rocker aft should give good speed but will likely sacrifice rough water ability.

Here's one a pram: http://www.atkinboatplans.com/Dinks/Chatterbox.html

Here's one on the long and narrow theme: http://www.atkinboatplans.com/Oar/Scandal.html

[Rex NZ]

Jerseyman

The output of your motor is approximated by this formula
Engine Output(BHP) = Brake Mean Effective Pressure(P) x Length of stroke(L) x Piston Cross Sectional Area(A) x Rotational Velocity(N) / Constant(C)

SO, to keep it simple we'll look at this on an empirical basis
P is largely derived from the compression ratio. You can readily increase on this a little.
L is rather fixed
A is rather fixed & given the undesirable bore/stroke ratio I'd suggest you don't want to increase this
N is worth some experimentation. Selecting a prop that has N approach 5000 will give a slight output boost
C is a complex value that is dominated by the scavenge ratio. It also factors every other variable, incl;
* induction pressure
* charge temperature
* oxygen ratio
* fuel air ratio
* fuel calorific value
* ignition timing
* & on & on & on

Now, you might have noted that C increases with N in a non-linear fashion. So you get the characteristic of HP increases with N up to a point approaching 5000rpm, then the HP declines
So, in PRACTICAL terms your HP is limited by 2x dominating factors;
* Compression ratio
* Crankcase to combustion chamber transfer cross sectional area

The compression can be increased only a little, but is limited by interference between the head & piston.
I believe the best opportunity to incr HP lies in the transfer cross sectional area. However, there's quite a bit of engineering needed to achieve this. Pays to own an engineering workshop

I have a mathematical model of the seagull engine which I use to test ideas before implementing them. It's showing the good old Seagull is an extremely difficult project.

Here are some practical steps you can take to optimise your standard motor
http://seagulloutboard.com/1_5_Tips-Tricks.html

Here's some ideas to take development to the next level
http://seagulloutboard.com/1_7_How-to-G ... Power.html

Rex

[JERSEYMAN]

What a great site, another side to the British Seagull !!

[blue]

looked at your pictures ---that 102 is exactlly like the one i had stolen-----not blaming you!!------it was found a couple of years later by divers----someone had chucked it off the pier-----regards blue