PDA

View Full Version : Pitch



Victor
09-04-2004, 09:08 AM
I'm confused. Higher propellor pitch gives you more acceleration, right? AND a better top speed. So why would you go for a lower pitch? I've read some explanations but still don't know why I would go for anything but the highest pitch I can get. As far as I can tell, you start with the highest and only go lower if it's lugging the engine.

[ 09-04-2004, 09:08 AM: Message edited by: Victor ]

John Bell
09-04-2004, 10:08 AM
Not really. Think of prop pitch like the gears in your car. Low pitches equal low gears, high pitches equal high gear. A too high pitched prop doesn't give good acceleration; just like trying to start your car from a standing start in fifth gear would not. A too low pitched prop will get the your boat out of the hole very quickly, but at the expense of over-revving the engine at top speed.

The truth is that you should put the prop on your boat that allows it to run up to the manufacturer's suggested revs at wide open throttle.

Let's use my boat for an example. If I took your advice and put a 21" pitch prop on it, it would be slower to accelerate and might top out at 3500 rpms compared the suggested maximum of 5000-5200. At wide open throttle, the motor would be lugging heavily and subject to overheating and premature wear. It would eventually fail, and maybe even catastrophically. And at lower rpms, the motor would still be lugging and at risk of damage. Fuel economy would suffer as well.

If I put a 9" pitch prop on it, the boat would accelerate very quickly but WOT rpms would be 6000-7000. Kaboom!

I've got a 14" pitch prop right now. At WOT, it runs at 5100 rpms. This means I've got the right 'gear' for my boat/motor combination. The motor is not under undue stress across it's whole rev range.

If I were to go down in horsepower, I'd likely need to go down on pitch. If I doubled my HP, I would probably have to go up in pitch. It's very important to put the right prop on your boat/motor combination. And for each boat/motor combo, there is going to be the perfect prop.

Hoe this helps.

ErikH
09-04-2004, 10:20 AM
Ergo the use of an Autoprop adjustable pitch prop on some friends who can actually afford it.

Having owned a 3 blade feathering (not folding) prop on my last boat, which was entirely adjustable, I can say taht the above poster is completely correct. I tried many pitches--all with the same prop, same boat, same engine--and only one of them 'worked' at both slow and high speeds.

Ian McColgin
09-04-2004, 10:31 AM
Look at "The Propeller Book" by David Gerr (Garr?). Pitch, diameter, RPM, HP and speed are all interconnected.

You can put 1,000 HP into a hot cigarello boat and go really fast but if you tie her to the dock and rev up you may get only a few hundred pounds of static 'bollard' pull.

Take the same 1,000 hp and put it in a tug with a nice big high pitch but slow turning prom and you can pull the end right off the dock.

Controlable pitch props work nicely if you have varying loads and if you don't need to go too fast since you can match the revs and pitch to the load. For a displacement hull, a controlable pitch prop, whether the new automatic type or the old type with a skilled (emphasis on skilled) operator can out accelerate any fixed prop.

Even on a slower planing hull. I lost track of whether these were actually put into action during Vietnam, but one summer I helped test a patrol boat model - really a 40'something bertram hull - equipped with controlable pitch v. same boat with fixed pitch. Once I knew how, I could out-accelerate from standstill to 35 kt and could 'crash stop' from 35 kt to zero much better than the fixed. It took a definate knack and I've no idea if I could have performed as well under fire. I think that the strain of such acceleration and especially of the 'crash stop' is less with a controllable pitch prop than on a conventional tranny with a fixed prop but it takes an ear on the engine and an eye on the tach.

John E Hardiman
09-04-2004, 11:43 AM
While controlable pitch props have advantages for vessels that have two or more designed operating points (such as an towboat or the aforementioned PC), it must be remembered that a CP prop still only has one efficient operating point and due to design considerations that point is less efficient than a fixed wheel(about 95%). Using a CP as a gearbox is a very expensive option and rarely justified for most vessels. A CP operating off its design point is about 70% the efficency of a properly designed fixed wheel.

notwoodbut...
06-22-2005, 02:06 PM
Pulled this thread up in my search for info on a new propeller.
We're installing a new/used Volvo MD6A in my 22' catboat. Of course, the engine wants a Left Hand propeller. The replaced engine (vintage 1973 Palmer was a Right Hand prop. With the change in RPM's etc, I'm soliciting opinions on propeller pitch etc.
Boat is 22' long, 10' beam, approx. 6000#. I'm not sure quite what RPM's we'll be running although the Volvo manual says 2450-2550 RPM max. speed.
What do you think?
Jim

mmd
06-22-2005, 02:21 PM
I think you need to visit Michigan Wheel's website (http://www.miwheel.com/) and avail yourself of their "Prop-It-Right" service. It never hurts to consult the experts. ;)

notwoodbut...
06-22-2005, 02:36 PM
Thanks for the reference.

I input my info as best as I could. Site says about 10-12 days for a response. In the meantime, I'm open to suggestions.
Oh, did I mention, I'd like to stick with a two bladed prop. It hides nicely in the slot while sailing.

thanks

[ 06-22-2005, 02:40 PM: Message edited by: notwoodbut... ]

John B
06-22-2005, 04:55 PM
The other data you need to calculate the prop is the gearbox ratio and an estimated ,realistic ,target speed. You've owned the boat with another engine in it so you'll know the that point where it all gets a bit hard.You know what I mean eh.. the boat will easily run up to a certain speed,then there's a strain to get the next half knot .that first point is what you'll cruise at most. About 5 knots I'd guess ?

ssor
06-22-2005, 04:58 PM
I have a 9 hp diesel 3:1 reduction gear and a 13x15 three blade on my Islander sloop. Get hull speed at less than WOT but can reach 3200rpm at WOT. The shop tells me that if I had aperture room for a 15x10 three blade it would be even better. It all depends on how fast you can drive the boat as to how you prop it.

Venchka
06-22-2005, 05:20 PM
Originally posted by notwoodbut...:
...Oh, did I mention, I'd like to stick with a two bladed prop. It hides nicely in the slot while sailing.

thanksDave Gerr had some articles in Boatbuilder magazine a few years ago. He said that more blades helped a lot of stock boats that left the factory "underpowered." The fault lay in small 2 blade props. He also mentioned a variable pitch prop.

Something like this, as I recall.

Variable Pitch Prop (http://www.sailnet.com/collections/articles/index.cfm?articleid=matthe423)

Wayne
In the Swamp. :D

Meerkat
06-22-2005, 05:35 PM
Is a feathering prop a CP prob by definition?

I always thought that a feathering prop on a sailboat made sense, but not if it costs that much (per Mr. Hardiman) in efficiency.

ssor
06-22-2005, 06:46 PM
If you are concerned about the extra drag from a three blade prop, then calculate the difference in blade area and find a tin can with this cross section and tow it and see how badly it affects your speed.

notwoodbut...
06-22-2005, 09:03 PM
As usual, thanks for all the quick responses.
I must confess, I'm not a mechanic by any means. Besides, all these numbers sound suspiciously like math.
I have seen a few calculators and I actually contacted Michigan Wheel. They got right back to me which was a surprise. I've got to go back to their site with some more info later to fine tune the calculator.

John- you're right, the boat will only go so fast. Maybe 6-point-something. After all, she's a catboat. My old engine was really overpowering for the boat. I'm quite confident that this engine will be fine with the right prop.

As far as reduction gears etc., the book I have shows a "reduction reverse gear MS, red. 1.91:1." Is this the reduction gear we're talking about?

Wayne-those folding and variable pitch props are sweet but the cost! After all, I'm going with a used engine to save money. Some of those props probably cost as much as the machine.

ssor-I'm not overly concerned about drag. Most of these boats use a big old three blade prop. However, even though I'm in a cruiser, I still have a little racer in me. I don't think I'm losing much in thrust with the 2 vs the 3. If I found a good used three, you watch how fast I'd try it out.(by the way, she's got a 7/8" prop shaft if there are any three's out there).

Again, thanks to all
Jim

ssor
06-22-2005, 09:47 PM
Bietzpadlin came with a 13x7 two blade and when I shifted into gear the engine didn't change speed but with the three blade 13x15 I can tell when the transmission is in gear. Check with the old marinas for servicable props in the size you want. Many will work with you and there are bushings and adapter keys to permit the use of props with oversize bores.

Venchka
06-23-2005, 12:22 AM
Originally posted by notwoodbut...:
...I don't think I'm losing much in thrust with the 2 vs the 3.
JimYou are actually. A lot. I won't bore you with numbers. For all of their faults, Seagull outboards got one thing very right: large surface area 3, 4 and 5 blade propellers. When they ran right, they could move mountains.

Wayne
In the Swamp. :D

John E Hardiman
06-24-2005, 11:34 AM
Originally posted by Meerkat:
Is a feathering prop a CP prob by definition?
Depends Meer, some "feathering" props are not really CP. They use centrifigal force to open/rotate the blades to a fixed pitch when the shaft reaches a certain (normally low) RPM. Not really the same as a CP prop that the operator is able to adjust the pitch independent of RPM.

As an aside, IMHO the best prop for a inboard sailboat is an on centerline 2 bladed low speed with high pitch and moderate BAR, and a fixed shaft stop so the blade is locked vertically inline with the keel/rudder wake.