outboard leg length

The leg length required on my boat is 15" to 16"(which I believe is a standard shaft)
I am looking around for another outboard.(used)
The availability seems to be for mainly short shaft or long shaft engines.
I realise a short shaft would not be suitable.
I am wondering if a "long shaft" would be ok to use.
I can't alter the transom height because of the steering setup.
The boat is a 14ft f/glass boat,which I use for sea fishing.
Speed is not an essential factor.
Would the extra couple of inches make much difference?

 

15" is a short shaft, a long shaft is 20"

 

Thanks O,that certainly clears it up.
It's been a bit confusing,different places and manufacturers seem to have different interpretations of short,standard and long leg lengths,and then there is the extra long leg for sailing boats.
Looks like a short shaft will be the one to look for.
Before I get one,I will put a tape on it to make sure.
There is also a bit of confusion where the measurement is taken,I personally measure from the anti cavitation plate to the underside of the transom brackets(where they sit on the transom)
Some people measure from the centre of the prop.Obviously,checking is the answer.

 

http://yamahaoutboards.com/outboards/Portable/specifications

You are correct, the distance from the ANTI VENTILATION plate to the underside of the transom brackets

 

Over here "standard" is not a term used to describe length, and the prop shaft is never used in the measurement.

AV plate to underside of transom bracket is where it's measured.

Not all boat builders or outboard manufacturers use exactly 15", 20", 25" or 30" as the length though, but it's close enough.

 

I know at least some "short" shafts were in the 16-17" range from bracket to cav plate. Reason I suspect is that lots of small boats had a centreline shallow skeg, and it is better to be out of the downstream effects of it, by going a little deeper.

 

My boat has a V hull at the bow,and a very shallow V (almost flat) at the stern.
The measurement from the lowest part of the hull to the top of the transom is 15" to 16"
My present engine measures 16"

 

I checked the length on a 9.9 Evinrude S/Shaft, it is 17" from the barcket to the cav plate, there is a splash plate 3'' above that.

 

Hi Mr E,thanks for checking,
my 18hp Evinrude is 16",my 5hp Yamaha is 16",the 3.3 yamaha I sold recently was 15".
My 6hp evinrude is 16".
It looks like taking the measurement before I buy will ensure I get what I want.
Like you said earlier,a couple of inches longer than the 16" will probably be ok.
What I was wondering about was,if the engine is deeper in the water,does it effect the exhaust back pressure.
I tried the 5hp yamaha I have on a scissor bracket for the spare engine,it was a bit low in the water.
Normally it is a good starter (2nd pull from cold) it wouldn't start until I lifted it up in the water and cleared the excess fuel that had built up.It behaved perfectly once started.
I always fire the engines up before I leave home on a trailer trip,just to make sure,the yamaha started no problem.(I have a tank on wheels)
I have a couple of small British Seagull engines that I have brought back to perfection,there is a 1/8"
drilling in the leg to relieve the exhaust back pressure,if you run them deeper than the relief hole,the back pressure chokes the engine and slows it down.I do realise evolution has left them behind,but
I still wonder about the exhaust back pressure.

 

Not a concern at all. When the boat is underway, you basically have a low pressure area behind the prop hub, that is easily filled by exhaust gases. Idle speed rpm of an outboard is slightly less in the water than out of it, out of gear.

 

Well,you live and learn.I can see how that works.
It has probably answered another problem I had.
The prop on the 18hp was a 9.25"X11" I've changed it for a 9"X9"(not tried it yet)
The boat was creating a huge wide wake,but not getting on the plane.
I suspect because it wasn't cutting a wake trough the build up of pressure behind the prop wasn't allowing it to reach it's revving potential.Hence no difference in speed from half throttle to full throttle.
I have obtained a tacho to check the revs when I try the new prop.
From a few opinions on here,the boat should plane quite easily.
Very interesting subject don't you think?

 

If I understand what you're trying to say, then no, it has nothing to do with it not getting on plane.

 

Hello again O,so what is your conclusion?
This is what I have to work with.
The boat has a nice planing hull,and is not in the least heavy.
The engine runs well.
The anti cav.plate is at exactly the required height.
From half throttle to full throttle there is very little change,and the revs don't increase much.
Looking at the prop over the stern at max power,there is no V cavity in the wake,but it throws a huge wake up the river banks.
I got the feeling that with more attainable revs,the boat would plane.
I have no doubt if I fit a larger engine it would solve the problem,but this would niggle at me forever.
Be nice to know the answer.

 

Your motor has a backpressure relief port on the back of the mid-section so that at low RPM's exhaust can escape there and not create problems during low speed operation, once moving at all most exhaust comes out near the prop though.

As to why it won't plane, every boat and motor combo is different, so while we have given you possible reasons, we don't have direct knowledge of what's happening, only your perception of what's going on, which may or may not be accurate.

You say the motor runs well, we don't actually know if that's true, it may only be running on one cylinder, the spark advance may not be working, it may have a bad coil, the carb butterfly may not be opening all the way, the carb may be dirty, timing may be off, etc.

The boat may be much heavier than you think, as I said, I had a similar size boat that wouldn’t plane with the same motor. Some hull designs plane easily with very little power, others, even ones that look similar, may need much more HP to get on plane. You may weigh 25 stone and sitting in the back of the boat, etc.

 

Like you say,it's trial and error and my interpretation.
One thing I have wondered about is if the butterfly is fully opening.
I have put new coils and leads on the engine,the plugs are new and the points and condensors are like new.The timing is factory set.The spark on both plugs is very strong.
The next test trip might work out ok with the new prop.
Have a nice Christmas. "V"