A simple propeller question

[CDK]

Can anybody tell me what the hub diameter is for the average 15" to 17" 3-blade bronze prop ? Is there a standard for hub dimensions or a fixed relation to the prop diameter? Even the extensive Michigan catalog does not mention this at all.

[jehardiman]

For standard B, Ka, and Kd props, the hub is 0.167 D. In general, ship/workboat sized hubs are sized to be stronger than the shafts even after the press on. On smaller wheels, the hubs are usually sized to ensure no damage to the hub when the shear key goes. Older wheels vary depending on how they attached to the shaft (keyed, pinned, pressed, etc.).

[CDK]

Simple question, simple answer. Thanks very much!

[jehardiman]

Originally Posted by CDK

Simple question, simple answer. Thanks very much!

Now you've embarassed me.

That hub diameter is at the line of the blade face only.

On the back end (fwd side of the hub) the hub is a little larger, on the face side (the aft side of the hub) it is a little smaller, so it has a taper to it. That taper generally follows the specified shaft taper, normally 1:12 for shafts over 6", less for smaller shafts.

[CDK]

No need for embarassement. I wanted to know the hub dia at the blade face. The prop shaft will rotate in a tube with the same dia as the hub, so the waterflow will be as undisturbed as possible.

[redtech]

CDK,
what would seem to be a simple question may not always have simple answer. lets ask some question to make this simpler. horse power? prop shaft diameter? drive type? prop material? thru-exhaust? over the hub exhaust? this information i think will help answer your question.

[CDK]

Hello Redtech,
Here are the answers: 2 diesel engines, 70-80 HP each, 25mm prop shafts, approx 2 ft long, down angle 5 degrees, encased in oil-filled stainless steel tubes. A tunnel drive, like Saro, but incorporated in the bottom, not bolted on. Bronze props, probably 15", larger if necessary, one turning left, the other right if there is any advantage, both turning right if it makes no difference. Four small rudders alongside the props. Exhausts are thru-transom, just above the waterline now.
This may all sound a bit weird but has to do with Berkeley jets that will be removed after two years of service. They leave gaping holes in bottom and transom that I will turn into half circular tunnels.

[Çemberci]

We are finding Propeller Hub Diameter according to Shaft Diameter.
See Attached File.
Oktay Çemberci
www.cembercidenizcilik.com
İstanbul/Turkey

[CDK]

The decision point for the props is getting nearer. I've made a mold for the tunnels that has a width of 16,5" and 2 ft. lenght. Allowing 1,5" clearance, the prop diameter must be 15". But I cannot decide what pitch to use.
The original boat was powered by two 140HP Mercs, had 30 kn. top speed and could steadily plane at 22 kn with 3200 rpm. At the time they were also available with twin 120 HP Volvo Penta's, which had approx. the same performance but took a bit longer to climb out, so most owners added trim flaps.
I've added 1 ft waterline lenght, there is no drag from 2 Alpha-One stern legs and nearly 500 lbs less weight. But I only have two 75 HP diesels.....Is 150 HP enough to obtain planing speed? Don't ask me about weight or displacement, I am not in a position to find that out.
It is my (well informed) estimate that the torque of the VW turbodiesels is about the same as that of the Mercs at 3000 rpm, the diesels just don't rev as high as the gasoline engines, hence the difference in output..
Could I set my speed target at say 24 kn. or would that be too high?
I've read the discussion about comparing a 50 HP outboard and a diesel powered Levi drive in 25 ft Asian fishing boats, but I have no idea about the construction. They surely do not have two cabins, a bathroom and a kitchen to carry around.
And what sort of props do I need?
My starting point was that I would make surface drives, but when I look at it now, I doubt there will be anything but water in the tunnels. They are fully submerged while at rest and will come up in plane, where they may draw in some air from the sides if I leave gaps there.

[Frosty]

I like your idea but your venturing on dangerous ground there. I wish you well. You may get success and you might well, but i'le guarantee you will need to play with a lot of props there.

Your idea sounds ok but how long are the tunnels, they will need to start as far forward as possible.

Im not a naval architect or anything but the hub of a prop depends on the stress of the root of the blade. Im not too sure why you are concerning yourself with this.

[Çemberci]

Dear CDK
Some Hull parameters we need to calculate hull resistance and also Tunnel effect..
Hull waterline Lwl
Hull Chine Lenght Lpx ....
Hull Chine Beam Bpx
Deadrise Mid ship
Projected Chine Square
Center of Gravity
.............
Engine Horsepower
Engine Rpm
Reduction Ratio
.....................
Tunnel Dimension

Oktay Çemberci

[CDK]

Attached are sketches of the boat's transom and hull, everything measured in millimeters. These are not the shipbuilder's drawings, but adaptations from earlier sketches I made while the boat was out of the water.
Center of gravity was at approx. 1/3 boat length with sterndrives, it will have shifted a fraction now because there is no stern load anymore.
Engines are 2x 75 HP at 4000 rpm, gearbox reduction 2:1.
Tunnel width is 425 mm, length 620 mm plus a faired part from the original hull on both sides of the stern tube.

(From Frosty "Im not a naval architect or anything but the hub of a prop depends on the stress of the root of the blade. Im not too sure why you are concerning yourself with this."
It seemed like a good idea to make the tube the same dia. as the hub to reduce turbulence....

[Çemberci]

Dear Mister
I find these sizes for your boat.
Hull Speed 24 knot
Hull weight 2.8 ton

Diameter = 14.00 inç
Pitch = 17.33 inç
Dar = % 59.05
3 Bladed
OR
Diameter = 13.00 inç
Pitch = 18.50 inç
Dar = % 50.00 .. Michigan Dyna
3 Bladed

Oktay Çemberci

[CDK]

Hello Çemberci,

Many thanks for the excellent service.

Just to satisfy my curiosity: I made the tunnels 16,5" so they can accommodate 15" props, yet you suggest 13 or 14 inches. Is there a philosophy behind that?
Would you recommend left and right hand rotation or will the difference not be noticable?

Regards,
Cornelis Koger

[Çemberci]

Dear Mister
Left and Right propeller is recommended.Three weeks later some one
is coming to New York/USA.If you like to order our propeller.Here is
Our Price for ManganBronze 13 inç ***211 Euro .14 inç ***290 Euro
each one
OKtay Çemberci
İstanbul/Turkey