The Steamboating Forum

Hi All

Thanks for the welcome and advice from my earlier post. Just shoe horned a hull into the workshop following a 350 mile trailer pull from Falmouth. Long day!

The hull I believe is a 15ft 6in Drascombe Dabber Launch. Only a few Dabbers made as launch version. Originally fitted with a Watermota Shrimp inboard, I believe. The hull came with a separate stern tube and propeller. Will the Stuart 5a be up to the task? The prop is 13inch diameter, 2 blades. What is the best way to measure its pitch?

Nick C

Please show some pix of the new hull.

The best way is to look on the hub of the propeller, Hopefully the diameter and pitch, and rotation direction are stamped on the hub. You might see " 13 x 11 RH " which means 13 inch diameter, 11 inch pitch, Right Hand Rotation (boat moves forward when the prop,viewed from behind the boat, turns clockwise).

If the stamping is not available, you can measure pitch from a measure of the propeller blade tilt angle (ALPHA) at a fixed radius (R) from the shaft center. The pitch is equal to 2 x PI x R x TANGENT (ALPHA)

For common launch measures:

ANGLE DIA PITCH
10 12 6.6
11 12 7.3
12 12 8.0
13 12 8.7
14 12 9.4
15 12 10.1
16 12 10.8
17 12 11.5
18 12 12.2
19 12 13.0
20 12 13.7
21 12 14.5
22 12 15.2
23 12 16.0
24 12 16.8
25 12 17.6
26 12 18.4
10 16 8.9
11 16 9.8
12 16 10.7
13 16 11.6
14 16 12.5
15 16 13.5
16 16 14.4
17 16 15.4
18 16 16.3
19 16 17.3
20 16 18.3
21 16 19.3
22 16 20.3
23 16 21.3
24 16 22.4
25 16 23.4
26 16 24.5

For example, say you go to 6 inch radius on your prop (corresponding to 12 inch diameter in the table listed above),and measure 20 degrees prop blade angle (tilt away from the shaft axis), then the table indicates 13.7 inch pitch. Note that the diameter in the table above is NOT the propeller actual diameter, but is the diameter where you take the angular measurement

wsmcycle

Pictures here I hope.

[img][IMG]http://i38.photobucket.com/albums/e121/ ... atdwfi.jpg[/img][/img]

Fredrosse

Thanks for the pitch measurement details, I will measure the prop next time I am at the workshop.

Nick C

Please give feedback on pictures, not sure if they are displaying correctly.

WOW that is a nice hull.
I have a stuart 6A in a 20 foot boat. It is OK not overly powerful but fun.

Fred, thanks for the formula for pitch. ive long pondered this and never put together the fact that the pitch is the same at all diameters and therefore the complex prop shape.

Nice find for the hull. What are you doing for a boiler? You'll probably need something light with that hull.

-CB

Not sure if the photo is showing for everybody. Some more feedback would be useful.

Have now measured prop angle. So from table above the prop is 13" dia. x 10" pitch, RH. The DAR is approx. 0.2.

Any comments on its suitability with a Stuart 5a (2.25" x 2" stroke single cylinder) in a 15ft displacement hull?

Nick C

My guess is you'll want a bit more pitch to keep her rpms reasonable. I ran a 2" bore x 2.5" stroke engine for 15+ years with a 13 x 17 prop; this worked quite well in a 19' boat. There's nothing wrong with trying it out and seeing how it does; small props like this are readily available used and you can match things up once she's running. If you tend to run heavy a lot, less pitch helps.

- Bart

I have a 14 ft O'day Javelin sailboat hull, which I converted to electric, cut out the centerboard well, fiberglassed in a PVC propeller shaft casing. 3/4 horsepower is the rating of the motor, although I suspect it runs at something less than 3/4 horsepower, with 400 RPM on the prop, turning a 14 x 17 three blade RH propeller. That seems about correct for the 5A steam engine.

Beautiful little hull you have found.

I'm sure that a 5A will push it around reasonably well. You won't pull any water skiers, but presumably that wasn't on the agenda anyway. You might want to take a look at a couple of articles in "Steamboats and Modern Steam Launches" (re-print to be had from Elliott Bay books) about how to modify a 5A so that it won't wear prematurely under relatively heavy load.

Even tiny horsepower will move a boat around pretty well if you aren't fighting too much tide, wind, or current. I have a 20' steel hull launch that weighs nearly 2 tons wet, and it was usable, if slow, with a very worn out 3x4 engine running on 50 psi at 250 rpm. At that point, it would have been making less horsepower than your 5A in properly tuned up condition.

Get a little more pitch on the prop, keep your boiler low and light, and I think you'll end up with a well balanced power plant and hull.

Keep us posted, in any case!

Happy Steaming,
Scott