New prop for Iron Chief

[DetroiTug]

I found a new prop for the tug. I'm currently running a 20 X 20 3 blade. It seems to slip too much on take off and the engine seems to turn too fast at higher speeds. I've always felt it need a bit more pitch and a little better bite at RPM's. I'm going to try this one in the pic. It's a 4 blade 21 X 23. I tried a 19 X 25 and it just seemed like it was too much at high RPM's/hullspeed and slipped at low RPM's. The price was right - brand new for 375 on Ebay. For some odd reason, the same prop is now 550? I know I won't know until I try it, but what do you guys think?

I'm going to increase the size of the rudder to help with steering backing up. It is steerable now, but only moving quickly backwards. Otherwise it just propwashes to port.



-Ron

[S. Weaver]

The diameter, pitch and area should slow you down. Give it a whirl - out of all the performance mods one can do, props are probably the cheapest. (Saints preserve us - I'm starting to sound like Lopez! )

[farmerden]

I currently am running a 19X28 and figure the slippage at 37% So I did all the math ,read Weston Farmer's book and came up with 21X24 four blade.I've been talking about switching for years now but the "Honey Do List" is overflowing and have never got to it!! Sometimes I envy you single guys -you seem to have more time! Heck you built the whole boat and I can't even change the prop!!! So I think you are right on the money with that prop. Den

[Albert]

Hi Ron,

The proof of the pudding is in the eating... However, compaired to the current wheel (20 x 20 3 blade) the new one (21 x 23 4 blade) seems to me to be a "quantum leap". It may correspond to 23 x 23 3 blade. In finding the suitable wheel, the hull performance surely plays a not negligible role. This new wheel will for sure slow down the engine speed. Without doing any calculations or consulting the respective wheel diagrams, I have the feeling that the new wheel would slow down the engine more than the 15% pitch increase (20 to 23"). If at the end the engine does not turn more than 80% of the usual current speed, would the boat reach the desired speed?? Or only at the cost of a higher mean steam pressure?

Hope you are satisfied with the results.

Albert

[DetroiTug]

Thanks for the input on this. Albert, how do you think the 21 X 23 4-blade would compare to a 19 X 25 3-blade?

Last time I had her out, we anchored off the ramp to get out of the way and build steam - there was no dock at the ramp. The anchor got hung up in something and we had a hard time retrieving it. The engine just raced up as the steam dropped trying to pull it free. We eventually got it free, but only after we lunged at it. The engine never really labored. In this video below on Eagle River in Wisconsin, at hull speed, this seems as though the engine is turning too fast. It's one of those rare occasions where I could hold steam, really clean flues and really dry oak wood, and started out with lots of water in the boiler and didn't have to add any. Normally at this speed and RPM it will only run for short durations. Then I'm back 30 psi. My thinking is, if I can slow the engine down some, this will use less steam? Per the PLAN formula, the higher pressure will offset the lower number of turns?





-Ron

[Albert]

Hi Ron,

Judging by the Elliot Bay boat (Isabelle?) running parallel to yours, your speed may be in the range of 5 knots, the engine running at some 400 RPM (?). I see you have a twin engine of unknown dimensions to me.

Let me play a little bit around with the Weston Farmer diagram. At 400 RPM, your current 20" wheel would absorb 4 HP which for a weight of 2 tons (?) looks like the right figure. In order to absorb these 4 HP, a 19" wheel would have to be turned at about 450 RPM. At a boat speed of 5 knots, this would mean an unacceptable slip.

Coming back to the 21 x 23 wheel: to absorb the 4 HP, the wheel (corresponding to 23" 3 blade) would have to be turned at 320 RPM... It might indeed work. Lower RPM at a higher mean pressure which is more efficient...

Happy testing

Albert

[DetroiTug]

Albert,

The engine is 3 + 3 X 4 inches

The hull is roughly 4500 pounds.

Thanks for your help on this, Ron

FarmerDen wrote: "Sometimes I envy you single guys -you seem to have more time!"

When I was married every discussion about steamboating and the like, ended with "we need a new couch"

[fredrosse]

A few years ago a friend hooked up our home computer so that whenever you touched the controls, the film clip from the movie "Jaws", (where Roy Scheider is throwing bait overboard, turns around and is startled by the face of the great white shark, then goes to the cabin and says to Robert Shaw "You're gonna need a bigger boat." This greeted every session on the computer.

Wife Katharine was annoyed countless times by this little film clip, then one day she saw the light and blurted out.... So, now I understand, you are thinking of a bigger boat!!

Low engineer humor

[barts]

Old rule of thumb - a inch of prop diameter is worth three inches of pitch... but we're often limited by the hull aperture,

Otter was much happier swinging a 16 x 20 than a 13 x 17 ...

- Bart

[Albert]

Ron wrote:

Last time I had her out, we anchored off the ramp to get out of the way and build steam - there was no dock at the ramp. The anchor got hung up in something and we had a hard time retrieving it. The engine just raced up as the steam dropped trying to pull it free. We eventually got it free, but only after we lunged at it. The engine never really labored.

I had a similar situation two years ago. The engine raced up but was of no help. The reason for this is the huge increase in the angle of attack at the single blades. Eventually the water flow stalls. The Elliot Bay 20x30 wheel is very prone to it, especially when reversing, because of the extremely narrow blades. On the other hand, however, the wheel is very efficient - low drag of the single blades - if mounted on a light-running boat.

This issue speaks for your 21x23 4 blade wheel as a "work horse". The blades are very wide, resulting in a stall-resistant wheel (although of slightly lower efficiency).

Albert