an outboard question

The noise comes from the engine, and a change of propeller will do very little. The 13x19 is the good size; normal speed at the right RPM. Unhappily for the noise, there is little to do as the noise comes from conception but you have some cheap tricks.
Your problem is probably resonances and mechanical noises from the engine passing through the cover. It's 90% of the perceptible noise for the passengers made by an outboard.
First look the joint of the cover, a cover not closing perfectly tight makes a lot of noise as the mechanical noises can get out by a very small hole (have you checked the rubbers of the holes for the commands and cables?) or non tight rubber joint.
Is the insonorisation liner in good shape? A new liner inside the cover absorbs a lot of vibrations and diminishes drastically the noise. Sometimes a lead heavy liner makes miracles. Special rubber paint for insonorisation often works.
Adjust and tight everything inside, a slightly loose vibrating air filter makes lots of noise when the air intake is resonating (are the restrictors (small tubes) in place? a "liberated air filter is horribly noisy at high RPM).
Valves can make a terrible noise if not perfectly adjusted.
A tired impeller in the water pump makes also lots of decibels, it has a terrible high frequency. A slightly out of timing engine makes noise. The cam chain is particularly noisy when slashing and not perfectly adjusted. I would have a look at the tensor if it's in good shape. Ball bearings can be very noisy at the alternator. All that can add decibels.
Finally the only thing to do is to make a revision of the engine and the liners of the cover. After that...it's simply that the engine is noisy from birth.

 

and if you had a 70hp inboard what would the prop diameter be to do your max speed which is how many knots?
when the full throttle operating range is 4500 or less max you might be able to call it a commercial engine
I cant see any Tohatsu engine specifically made for commercial use, do you have a link?

a v4 omc from the 50's and 60's were a far better commercial engine ( fuel consumption aside) than anything made today
Just look at the old OMC 40hp that army's around the world used, there is no engine today that can do that job

 

Diameter propeller for what boat? size? displacement? type of hull? If it's the oyster barge the max speed needed is around 4-5 knots. More is totally unnecessary and very costly. If using a diesel a 50 HP is largely enough, but it's more interesting to have a wide range of RPM (1000 to 5500) for the maneuvers, with a good capacity of acceleration, going against the very strong tide currents and the ordinary 15 mph wind with directional orientation of the propeller for tight maneuvers at very low speed. The currents and wind explain the use of a 70 HP and not a smaller engine. You can move a 5 tons sail boat at 4-5 knots with a 5 HP if the sea is flat, no currents and no wind...but with a 5 knots current plus a 15 knots wind and 2 feet waves 30 HP are far better...In France for fishing boats in Brittany and North Sea we put 3 times the power needed for flat sea/no wind, for some working boats in all weather it's until 5 to 7 times. Trying to get through the Raz de Sein with a 35 knots wind in the nose added to a 7 knots current and a 30 feet swell need plenty of power, and it's a very common situation in these waters, so it's better to plan carefully the hours and coefficient of the tides and to have a big cavalry, that saves from a lot of misery...
The conditions of use of the oyster boats explains why the oyster farmers in Brittany use only outboards for these very peculiar boats. Also you can let the boat which is totally flat lying on the mud at low tide, or push it with a pole in a few inches of water.
If you use a inboard you'll have to enter in the complications of a vertically adjustable shaft and propeller, plus a lifting big rudder able to cope with tight maneuvers at 0.3 knots...
The size of the propellers depends on a lot a factors; number et type of blades, admissible pressure, etc... A first sight I have no idea, I would guess (and it's only a guess) that that for such a boat, making exception of the oyster boat special needs, an efficient propeller for a 70 hp inboard engine would be around 15 to 18 inches with 3 or 4 cupped blades. Something very classical as for a displacement fishing boat going around 7 knots and able to cope with wind and tide currents not too hard.

The basic line for fishermen by Tohatsu are the old 2S carbureted engines http://tohatsu.com.au/outboardengines/2stroke, the same technics as used by Yamaha on the Enduro line.
These engines until the 90HP were primarily made for Japanese fishermen in the 1970, for example the good "ole" 70 HP sold by millions in Japan and some other countries in Asia and Europe from 40 to 90 HP 3 cylinders.
http://tohatsu.com.au/content/m70c
These engines have torque from 800 RPM to 5500. You have to be careful as they are almost impossible to stall and would rather break the rubber hub.
Excellent cooling system, good quality and engineering, very simple to maintain and repair, plenty of spare parts at decent prices. Drawbacks now; mixing your gas, emissions and consumption.
During the 80ties they made them a bit fancier for the yachting market but basically they have remained the same work horses. I'm not sure that these engines can be sold in countries with stringent emissions laws.
The 50HP 4 strokes is also a strong boy and an acquaintance of mine uses it for commercial work. He has used also 2 75HP MD75C2 with direct injection and he is satisfied since 2006 by the reliability and the consumption. The maintenance is not so simple.
In the States and Japan the Tohatsu brand name is NISSAN.

 

thanks for that
yes carby 2 strokes are gone from Europe and the USA for several years

 

According to a former OMC engineer, assume that about 10% is lost in the gearcase. So a 1975-88 Johnson 75 is worth about 68 shp, a 1991 Johnson 70 (shp) pulls about 78 hp at the crank.

 

All outboard hp ratings these days are at the prop shaft.

 

In terms of absolute sound pressure I admit there isn't much difference, but at 3300 rpm the frequency is almost 20% lower.
The engine is 6 years old and well maintained; I have not been able to locate any loose parts. Yet there is a whining noise that irritates me, a Suzuki or Honda of the same size is much more quiet.
When this season is over I will take the boat out, remove the plastic cover from the top of the engine and try to locate the origin of the noise.

 

Yes a lower frequency is more supportable, it means also that something is whining and getting louder when the RPM go higher. Sometimes a very high frequency noise cannot be heard however it's perceived as disagreeable as it's making decibels by its energy. The sound does not seem to change but the total decibels is higher.
That's relatively common with the alternator in cars as it spins far faster than the engine. You hear nothing particular but the noise of the car becomes unbearable after X RPM. Often it's a ball bearing which is beginning to die.
I do not remember if on the Yamaha FJ100 the alternator spins directly or has a belt thus has its own ball bearings.
Unhappily the sound proofing of the Yamahas is not the best, and on the small engines they are gone cheap. You can improve the sound proofing of the cover, (one heavy leaded rubber directly glued on the plastic plus foams of different densities) your engine is 6 years old and lining is getting old.