2 years Ive tried to answer this question.

[Frosty]

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?

[charmc]

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.

[RANCHI OTTO]

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)

[charmc]

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.



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

[RANCHI OTTO]

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....

[Frosty]

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.

[tom kane]

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.

[RANCHI OTTO]

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.

[Frosty]

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.

[RANCHI OTTO]

"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)

[RANCHI OTTO]

...."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

[Frosty]

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

[RANCHI OTTO]

Why you think so ?

[Frosty]

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.

[RANCHI OTTO]

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.

[Frosty]

This photo was taken from Shafts thru the transom thread and was posted by tom kane. I have always remembered this photo because of the closness of the prop to the transom. I do not know if it was succesfull or not maybe Tom could shed some light on it.

[RANCHI OTTO]

NICE! Never seen this picture, very interesting, the transom angle is 0°..and the propeller have traditional profile and works 50% in water...I'm curious....

What are the 2 things on the side? Do you have an idea?

[Syed]

I think the shaft length and angle has got something to do with the form of wave behind the boat.

[RANCHI OTTO]

..but the form of the wave behind the boat is related to speed...

[Syed]

I had a feeling that its wave length is a function of the boat length and its amplitude is a function of the boat speed.

[RANCHI OTTO]

At speeds over 40 knots the behind wake is near to flat...only spays due to the surface device skegs and propellers...

In the picture you don't see well the wake after the transom, but is near to flat. Both the boats are running at 50 knots.

If you have shafts on line + rudders + brackets + strut palms + bossings and speed of 30 knots abt. than you can observe the nice wave behind.

[RANCHI OTTO]

Syed..are you on board? I've found the picture in your gallery...

Where is the wave?

[Syed]

So you have found out that I am only a tiny boater. This is Usman, my son in the picture. I am making a shallow water propulsion system for this boat, therefore, I am interested to learn about this subject.

[RANCHI OTTO]

Thre are not tiny boater but only boater...!

Where is the boat in a lake? No waves at all...

If I'm not to curious, which kind of shallow water propulsion are you developping?

[Syed]

It is a river near Lahore with the name Ravi, the water depth is very uneven after every few minutes the outboard tilts. There are a lot of small heaps under water (I don't know the technical term for such things). I am working to have some thing like 'Gator tail outboard' if I find that it is mandatory to have a bit longer drive shaft. I shall be pleased to have any advice on this issue or any alternate solutions from all respected forumites.
There were waves, I shall find another picture showing some waves.

[RANCHI OTTO]

Thanks for the info, Syed!

[Syed]

You are welcome, RANCHI OTTO.

[charmc]

Seafury Marine's surface piercing drives seem to use shorter shafts than others. Their SF22, rated for up to 650 HP, has a standoff distance of only 14 inches/388 mm, and a shaft angle of 6 deg. They use a rudder behind the prop to steer. Leads one to wonder if the standoff distance is not as critical as Paul Kamen believes, provided there is a clean edge on the transom to leave water less disturbed, as jack frost suggested.

[tom kane]

A S/P (or any) propeller with rake will give a different thrust angle run in surface piercing mode.And still have an effect of pulling the rear of the boat down with an angled shaft.My trimmable drive when not locked down will stay at the lowest angle because the downward pull is greater in the decending blade than the accending blade and takes some pull to lift it up even with full power.It is surprising that just about any prop can be run in S/P mode.I/O`s and outboards are the only ones that realy can achieve a zero shaft angle and many of those often run with considerabe shaft angle because of boat plane angle or the need to trim for some reason.

[tom kane]

This photo was taken from Shafts thru the transom thread and was posted by tom kane. I have always remembered this photo because of the closness of the prop to the transom. I do not know if it was succesfull or not maybe Tom could shed some light on it.

Most of Hickman`s projects were very sussessful especially for that time.The boat shown ran close to 60 MPH.good for heavy machinery.

[tom kane]

NICE! Never seen this picture, very interesting, the transom angle is 0°..and the propeller have traditional profile and works 50% in water...I'm curious....

What are the 2 things on the side? Do you have an idea?

They are for steering,side rudders,not much steering ability there.

[RANCHI OTTO]

Thanks Tom for the info....spectacular boat!

[Frosty]

Seafury Marine's surface piercing drives has a standoff distance of only 14 inches/388 mm,

Really? , that is interesting indeed and this thread is definately going the way I was hoping.

Its just niggling that when a Surface drive guru like Paul Kamen says some thing and you cant ask why.

I would really like to know what he meant.

I am hauling out next week and I am definately going to fit a "vacuum break to the trailing edge of the hulls.

I need 3000 RPM for 12KTS and 3400 for 17KTS and only 3800 for 24KTS

(Or there abouts)

The dryer the props get the more they bite, wierd stuff isnt it?

I am hoping that the vacuum break will dry them earlier and reduce my cruising speed RPM.

[tom kane]

So you have found out that I am only a tiny boater. This is Usman, my son in the picture. I am making a shallow water propulsion system for this boat, therefore, I am interested to learn about this subject.

Have a look at a new thread in propulsion..A simple jet for a small boat.

[FAST FRED]

Looking at the Sea Sled it would seem the "rudders" are more spoilers than rudders , unless they are mounted further out from the hull than the photo seems to show.

AS the units seem to be well along side the hull, perhaps the drag when going straight was lessened , compared to standard rudders?

FF

[Syed]

Have a look at a new thread in propulsion..A simple jet for a small boat.

Thank you, tom kane !

[Janamon]

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.

Seafury require a vacuum break because of it's 45 degree transom. If the vacuum beak wasn't there the water would follow up the transom, effectively burying the prop..

[Frosty]

Janamon, Yes that absalutley correct, but no.

I recenty hauled out my cat and extended the hulls by 2 inches making a knife sharp edge on the planing section of the hulls transom. I did this because I thought that maybe I did not have the props out high enough, and that the props would be burried.

It made no difference what so ever.

I had experimented for hours with models of transoms and a hose pipe and it is easy to sea a big difference in the wake of hulls with the slightest roundness of the transom.

Handing the helm to my girl freind I left the bridge to go down and look at the props. There was no difference at all. Not even a difference in the rooster tali wich is only say 10 feet and about 4 feet high at 18KTS.

[mydauphin]

Surface drives require air in prop. If you put prop too close to transom then prop is going to be fully submerge at anything above idle speed. The heavier the boat the worse the problem. You have to move prop behind wake, after the hump or at least move prop higher. With Arneson you have to trim up to get over hump then trim down when you get to speed. I put surface drive on a shallow draft boat- no problem but when I put it in heavy boat - I couldnt get prop into air. I am looking for ways to ventilate prop like powervent using exhaust. Like I said the draft/weight of boat has a lot to do with it.