DIY tunnel drive

[tom kane]

If you are going to run a propeller close to the water surface you should expect that you are going to run your prop in surface piercing mode if you want a good speed and will have to design your drive accordingly.Tunnel drive and jets are surface drives.Hickman got good speed from his design which would have been better with modern surface-piercing props.The tunnel in the picture was used in race boats (60 MPH +) and did not like turning.You can not expect water to change direction abruptly what ever the design is unless it is in a cylinder which would increase drag.Hickmans drive would have been effected by interference from the side rudders when turning the same as a tunnel does when turning.The tunnel in the picture has half a venturi built into the top half which did improve the drive.

[Ad Hoc]

baeckmo

"....And CDK, your project is a stimulating challenge; it contains ingredients from various disciplines..."

Indeed, fully concur and and "interesting" problem to solve, especially with more than one liners!

I agree a single leg bracket/strut is best, for all said reasons. Establishing some basic loads from an out of balance prop, by losing one blade, this is a relative simple load case to ascertain the strength requirements; I've used this load case many times on single P-bracket arrangements with success all to Class, and/or as using a Class based type as you've noted.

I agree, the 15 degree would be better - I was just noting (for ref) that w/j's use the 20~22.5 angle to minimise space requirements and hence didn't really want to add too much potential costs to the solution, poor bloke is probably spending heaps of money.

"...but I was afraid the welds between stern tube and flange would not hold..." Yes, this detail is critical and requires more than just a cursory glance. The whole structural arrangement in terms of load paths and localised stiffness is far more important. Not to mention the detailing of the joint and alloy grade and temper. You need to ensure you can get a full pen weld.

Frosty
Look at the previous posts by me and beackmo, ie one liners wont solve this, it is like any design issue, it is iterative.

[tom kane]

Enclosed picture is a S/P prop on an (American) displacement hull. Surface drive replaced two Volva inboard-outboard drives which gave low speed. One S/P drive gave big improvement in speed and economy.

[Frosty]

Thats not a surface prop Tom. If it was the blades would be concave looking from the back, cupped.

Thats more of an equipoise.

[tom kane]

Acording to PAUL KAMEN N.A. What is a surface-piercing propeller? Simply stated,a surface-piercing propeller (or surface propeller) is a propeller that is positioned so that when the vessel is underway the waterline passes right through the propeller hub.This is usually accomplished by extending the shaft out through the transom of the vessel,and locating the propeller some distance aft of the transom in the relatively flat water surface that flows out from the transom bottom edge.etc.....My comment.The design of a surface-piercing propeller is difficult to define.Any propeller can work in surface-piercing mode.You could say that Hickmans propellers are not surface-piercing propellers.

[Frosty]

Thats a description of its duty not of what the prop is.

Read further down paragraph starts with "What distinguishes a surface propeller from an underwater design".

I would have thought you knew that Tom.

Here it is,--- http://www.well.com/~pk/SPAprofboat.html

[tom kane]

Nearly all succesful designs have moderate to heavy trailing edge cupping..what would you call Hickmans props as in the photo.They seemed to be successful props for his purpose.

[Frosty]

Its an equipoise propeller,---Very successful and popular 40 years ago. Cupping is something you can do after and generally tames a torquey engine or its what the prop man would do if you took it back to him and said your RPM was high.

[tom kane]

Thanks Frosty,I will have to asume that the American surface propulsion company who built this drive did not know what they were doing when they discribed the prop as being suitable for their surface drive propulsion unit.It is strange that the magazine editor did not know.I suppose at a quick glance it has been missed.

[Frosty]

The propeller in the posted picture does not cover the criteria of your surface drive guru Paul Kamen does it?

Now if you have the right picture or not I don't know or do I know what that is.

You described the picture as a surface prop on a displacement boat.????

I have no idea where you are going or what you are talking about

[baeckmo]

What you see in Tom's photo is a transcavitating propeller, probably Newton-Rader series, which is the section to select when there is a risk for air ingress into the flow. With varying degrees of cup it also serves fine in surface piercing operation at moderate speeds, where pure cleavers are overkill. Arneson drives will be found with similar props from Rolla. Vosper Thornycroft have used very similar ones on their smaller SES ships (HM series). Correct cavity size was ensured by introducing ventilating air through the bearing struts.

For those who might wish to go beyond speculation, test data under sp conditions will be found in "Fast-91, Methodical Series Test Results" reported by Rose and Kruppa. Further info on the VT versions of Newton-Rader props at various working depths is found in "Schiff und Hafen", 5/1973: "Modellversuche mit Schiffspropellern an der Wasseroberfläche" by H Brandt.

Ventilated propeller operation is not restricted to the situation with the hub at surface level, but can be applied even with fully submerged propellers, depending on the specific design goal.

There´s a additional paper by Brandt / Maksoud reffering to the farfield effect. 3D test were done at Ad Hocs main playground.
see:
http://www.fsopt.com/downloads/1995_5-1bericht.pdf
in German, but I guess Cornelis will understand that.

Regards
Richard

[CDK]

Originally Posted by tom kane

If you are going to run a propeller close to the water surface you should expect that you are going to run your prop in surface piercing mode if you want a good speed and will have to design your drive accordingly.Tunnel drive and jets are surface drives.Hickman got good speed from his design which would have been better with modern surface-piercing props.The tunnel in the picture was used in race boats (60 MPH +) and did not like turning.You can not expect water to change direction abruptly what ever the design is unless it is in a cylinder which would increase drag.Hickmans drive would have been effected by interference from the side rudders when turning the same as a tunnel does when turning.The tunnel in the picture has half a venturi built into the top half which did improve the drive.

That tunnel resembles mine, but I used a longer tube so the prop is located near the end. When I first presented my design with an oil filled stern tube I met a lot of friction on the forum. Now I see that I am not the only one on this planet, it feels good!

[CDK]

Originally Posted by apex1

There´s a additional paper by Brandt / Maksoud reffering to the farfield effect. 3D test were done at Ad Hocs main playground.
see:
http://www.fsopt.com/downloads/1995_5-1bericht.pdf
in German, but I guess Cornelis will understand that.

Regards
Richard

Yes Apex1, I do understand the language, but I need a bit of time staring at the math. Haven't seen diff's and integrals for quite some time, but I'll catch up.

Originally Posted by CDK

Yes Apex1, I do understand the language, but I need a bit of time staring at the math. Haven't seen diff's and integrals for quite some time, but I'll catch up.

It´s not that important Cornelis, Being focussed on Bäckmo´s calc.s is leading to a solution (at least we all hope that). Though sometimes it´s nice to know the theory behind the mystic machines.

Richard