2 years Ive tried to answer this question.

Paul A Kamen wrote a lengthy advisary on surface propulsion. Just google Paul A kamen surface propellers.

He talks of the simplicity of surface propellers simply needing to be half in and half out with the surface of the water from the transom passing through the hub of the propeller.

He further states that the water should be least disturbed to the point of not fitting skin fittings and definately not trim tabs up stream of the propeller.

Further in the report he says that one of the most common mistakes that home builders make is to place the propeller too close to the transom.

I have e mailed to the address at the bottom of his report asking why.

There are a few designs that are now coming close to the transom yet Arneson are offering shaft extensions why?

 

The only answer I've been able to determine is that the water becomes smoother and less turbulent further aft of the transom. I too am surprised that there is so little mention in available literature of why a setback of the prop from the transom is important.

 

More distance btw propeller hub and transom = less thrust angle and better efficiency. (Thrust angle is shaft angle related to basis line of the boat)
I've some results concerning piercing surface prop's in inclined flow.

More distance btw. propeller blade and transom = more prop. working in homogeneous wake = more efficiency

More distance = more manoeuvrability

On the other side more distance = more fragility and more cost..

(Sorry for my english)

 

As always, Ranchi, your English is fine.
Kamen's article, and most Arenson literature I've seen seem, to discuss a 0 degree shaft angle, i.e. a shaft angle coincident with the basis line, no deflection. The increase in manuverability is readily apparent. Do you believe the smoother water is the primary reason for the standoff distance?

Regards,

Charlie

 

No, I don't think so.

For me is the fact that less thrust angle do you have, more efficiency of the propeller.

The SDS system drive il longer than an Arneson and was designed later.....

Shorter = less cost, this is a point probably interesting....

 

If clean green solid water is important then a knife sharp trailing edge on the stern and a prop fitted say 1 foot away would give a perfect place for the prop?

8 degrees seems to be the standard for fixed surface props. Possibly because the difficulty in obtaining better isnt worth the effort.

Interesting that Seafury who also make a fixed drive specify on the web site what they call a vacuum break. This is a very sharp trailing edge. They say this is important for surface propellers.

Now taking all this into consideration and Arneson moving further away. Perhaps the answer is, it simply doesnt matter if the prop is close to the transom.

I wish Paul Kamen would have aswered my querie.

I dont see how a prop further away cause better efficiency. If the shaft ange on both is 8 degrees then pwer is being trasmitted up the shaft at that angle. Why does it matter where the propeller is on that shaft.

 

Surface propulsion design has many variables,particularly wheather you run a large or small prop,trimable and or steerable.Steerable would need a long shaft past the transom to turn (not realy desirable in fact horrible).Just a trimable S/D would not need a great distance between transom and prop and can be run there to good effect ( probably more efficient with a small prop).Motor position pushed back would mean a prop well back from the transom as a short shaft on S/D is not desirable = greater shaft angle needed. A motor midships means a prop can be close to transom.Zero shaft angle is not fesable in conventional V hulls without a dropdown gear box,even then difficult.Some say that efficiency between a zero shaft angle and 15 degree shaft angle is not measurable in practice. Small props in racing exhibited the surface riding effect for those who noticed a boat went faster while the prop was only submerged 50 percent of diameter.

 

The differences in propeller efficiency with shaft angle 0° or 15° are measurable in a cavitation tunnel (very expensive) and the results are clear...with 0° you have higher propeller efficiency.

 

I can see how a shaft angle of 0 degrees would make the prop more efficient but what about the push up effect of the shaft at 15 degrees.

Wouldnt this help to push the boat up and out of the water reducing wetted surface and drag.

I think that Armeson shaft extentions are to get a better angle of shaft simply by lengthening it. The fact that this would put it in less perfect water does'nt seem to matter.

It is becoming more apparant that the position of a surface propeller behind the boat is not critical.

I am still confused at why Paul Kamen said that putting the propeller too close behind the transom would be a failure.

 

"I dont see how a prop further away cause better efficiency. If the shaft ange on both is 8 degrees then pwer is being trasmitted up the shaft at that angle. Why does it matter where the propeller is on that shaft."....

If you have 8° fixed for 2 different prop's location..than you have to mouve the engines in a higher position...

I have istalled a propdown of FB design on a Trimax + CAT, no problems (until yet)

 

...."but what about the push up effect of the shaft at 15 degrees"....
The thrust line is above the vertical center of gravity...and you have the opposite effect, but it depends on speed. If you try to trim an Arneson at full speed down...the boat will have more wetted surface and a loss of speed.
I tried during sea trials with a 50 knots boat different positions of the Arneson.
At high speed, for 15m boat and 20 t displacement the neutral position was the best at speeds over to 35 knots. At 50 knots with the Arneson down the loss of speed was of 6 knots.

Arneson neutral = 50.5 knots
down = 44.4 knots

 

But doesnt the rake of the blades--say 8 degrees cancell the angle of the shaft?

 

Why you think so ?

 

Well,-- with a shaft of 8 degrees and the blades at 8 degrees the angle is cancelled out and the blades then push in a straight line.

 

The rake of the prop is related to the shaft angle at 90° and for this reason ..5° or 20° I don't see the effect on the shaft.