Two-Blade vs Three-Blade Prop

[Bash]

...came new with a fixed two-blade prop that had a serious vibration problem at certain RPMs. The dealer switched me to a fixed three-blade prop and that eliminated the vibrations. After a year I upgraded to a three-blade folding prop (flex-o-fold) that improved light-air sailing performance considerably.

[CarlF]

While I agree that a little over propping seems to help - with no obvious sign of engine strain - Yanmar has been known to refuse warranty coverage due to over propping. They will ask about your prop and whether you can reach full rated RPM.

Not a problem if you are beyond warranty (except, of course, if they are right that it hurts the engines).

Carl

[GordMay]

Originally Posted by defjef

... How do you measure the pitch of a prop?
As you can see I need a good "Propellors for Dummies". ...

Manual Procedure for Propeller Pitch Measurement
➥ Manual Procedure for Propeller Pitch Measurement

Or use a Pitch Gauge:
➥ Gauging Propellers
➥ Pro Prop Pitch Gauge

The Propeller handbook ~ by Dave Gerr
The Complete Reference for Choosing, Installing and Understanding Boat Propellers
➥ The Propeller Handbook: The Complete ... - Google Book Search

[Northeaster]

Highseas -

I had my fixed 3 blade prop machined this spring, from a 15x10 to a 14x9. Last year my old diesel would smoke hard (black smoke) after about 2200 and doing about 6kts. I didn't really try to push through that, as boat speed didn't increase, and the engine didn't sound good. the boat would reach about 3600 in neutral. It's likely been like that for 10+ years - perhaps since it was repowed even before that. It's my 3rd summer with the boat.

After losing an inch of diameter, and an inch of pitch, I can now do 6.5 kts at about 3200-3400 (guage may be off a bit), and the engine sounds great. Smokes a tiny bit (seems more blue), but much less than it smoked last year at 2200 rpm. And, I can do about 5 kts into a 20 kt headwind. The blue smoke may be a bit of oil passing the rings, or something else, but what do you expect in a 25 year old engine. I will keep an eye on oil comsumption.

I would recommend taking off some pitch and possibly diameter, if you have all the symptoms of being overpropped. The engine sounds much happier now, and I am able to use more HP. At 2200 I only had about 8hp or so, and at 3200 would have closer to rated 13hp. You should be able to reach or almost reach rated rpms in gear.

[captain58sailin]

Good Job Gord,
Now if there were only a way to measure prop pitch hmmmmm...

[Sandero]

I am still in the dark on this. I am not going to remove my prop for the purposes of measuring it. By the way the blades are not shaped as many are that I see.

Having said the above there is no probem with powering the boat from the 18" Ø 2 blade fixed with a 36hp engine. The shaft is enclosed in a log and the prop goes vertical when not motoring. The I put in reverse and it seems to have little drag.

I can get up to hull speed no prob with a clean bottom at 2100 or so RPM and pushing it to 3000 only squats the stern.

No trouble in reverse though it's a reverse so it backi to starboard and that's the side I like to dock to as I come in bow and then hard reverse stops it and brings the stern in.

I can't see messing with something mechanical that I can work on as it's submerged. YUCK. If it ain't fixed don't break it.

[wolfenzee]

I have a Vetus m4.14 diesel, 33hp (at 3000rpm) (Mitsubishi engine) it reaches the top of it's torque curve at 2100rpm, I have a 16x9 three bladed prop and was wondering about the advantage of a two blade with a deeper pitch, but threads on this topic have led me to learn alot about my engine. My hull spped is 6.75kts at 2000rpm I am doing 6.5kts using less than 1/2gph. With all the figures it appears as though I have about the best possible configuration...if it ain't broke don't fix it.

[wolfenzee]

As far as over propping is concerned a mechanic told me if you cannot reach your max rpm, then you are over propped, The ideal pitch is to be able to run your boat it most efficient fuel usage at just under hull speed. That is at the top of the engine's torque curve, you are getting the most amount of power out of the engine using the least amount of fuel (for the power produced) at the fastest possible speed. That is why engine manufacturers publish the most effiecient fuel usage in terms of g/KW/h in the case of my engine it is at 2000rpm, just under the top of the torque curve and coincidentally at 6.5kts (just under hull speed)

[blewett_john]

I have a Searunner 37 trimaran with a Volvo Model 2003 28 hp diesel. There is currently a 19" x 11" solid 2-blade prop. I can get my hands on a 18" x 12" feathering 3-blade prop.

Is this a good swap?

Also, is a feathering prop left hand (LH) and right hand (RH), or is it either specifically LH or RH? The prop I am looking at is in a consignment shiop and is not marked well. It does not look like any of the the feathering props I have seen online. The pitch is not adjustable, the blades have tabs that hit stops. The blades rotate by the shaft turning an inner hub.

Thanks,

[Rakuflames]

Originally Posted by Bash

...came new with a fixed two-blade prop that had a serious vibration problem at certain RPMs. The dealer switched me to a fixed three-blade prop and that eliminated the vibrations. After a year I upgraded to a three-blade folding prop (flex-o-fold) that improved light-air sailing performance considerably.

Check whatever holds that baby on to your prop periodically. I have a friend whose three-blade folding prop worked its way loose. He turned on the engine and it spun into the mud behind his boat. He searched and searched but never found it.

I have a "Perfect Pitch" prop, very light weight (and inexpensive), and with no current it can push my 31' boat at 6 1/2 knots. I don't race, so I have no need for a folding prop (my friend lives to race). If someone is looking for a new prop, I'm going to put in another plug for Perfect Pitch. The company has no sense of how to promote and market or I think it would be better known and more popular.

[wolfenzee]

I just figured people usually don't get the prop size/pitch right so was looking into options. In doing so I decided to learn about my engine, according to factory specs the most efficient rpm for my engine (in terms of grams of fuel used/KW of power) is 2000rpm at this rpm I am doing 6 1/2 kt (1/4 of a kt under theoretical hull speed). 25gal of diesel will take me over 400nm at 6.5kt. Prop is 16x9 prop, low pitch (called a cruising prop) gives a smoother ride and less walk. I don't race, but boat is fast under sail anyway, can't argue with what I've got.

[Davey1000]

Originally Posted by GordMay

Considerations On Choosing A Sailboat Propeller
by Cliff Friesen
West by North Enterprises - Choosing a Propeller


"The Propeller Handbook" ~ by Dave Gerr
The Propeller Handbook: The Complete ... - Google Book Search

I live in England and as to the book I wasn't particularly impressed. I see people commenting about two blade vs three blade but I don't see anyone talking about Disc Area Ratio. How can anyone compare two blade vs three blade fairly if the DARs are not the same?

There used to be an excellent computer program "Propcalc" by Castle Marine of the UK. Unfortunately its a sixteen bit program and unless one still has a Windows 98 or Windows XP machine it won't run. It WILL NOT RUN on Windows 7.

The best free propeller calculator that I have seen so far is "Vicprop". It is very good indeed. As to horsepower calculations there is a simple rule of thumb for a modest size of displacement hull. "Minimum considered horsepower should be 3 to 4 HP per ton." (Supertankers can manage with a lot less as they are more efficient) There is also the fact that no one yet makes a diesel engine that produces a million horsepower! 100,000 HP is the most that one can get.

Watch out for barnacles as they can cause black smoke.

[barnakiel]

Fat 3 blade gives more kick. Slim 2 blade easier to hide in the turbulence behind the keel.

All other things equal, I would go for a fathering prop.

b.

[Davey1000]

The feathering props are definitely the best for sailing but a fat wallet is required! Darglow of Poole used to make folding props. Unfortunately in the old days left handed props were popular but most modern engines need a right hand prop. Like most things in life there is no best, there is just the best compromise!

In England it is unwise to spend a lot of money on a propeller. The reason is that one can surmise that there are sub-aqua thieves who presumably use an air-wrench powered from the aqualung cylinders.

What is known for sure is that a Plymouth sailor found that he had no drive in ahead. He also discovered that there was no drive in astern. Next check was that the shaft was rotating (it was). It was then discovered that the propeller had vanished. How can this happen? The boat was OK when it was last used. This happened on the River Tamar which is patrolled by River Police who guard the nearby dockyard. Incredible but true!

[colemj]

Originally Posted by Davey1000

I live in England and as to the book I wasn't particularly impressed. I see people commenting about two blade vs three blade but I don't see anyone talking about Disc Area Ratio. How can anyone compare two blade vs three blade fairly if the DARs are not the same?

Some props, like the Campbell, have narrow shaped and cupped blades that provide more lift and thrust than regular wide flat blades. DAR's are less important when these are in consideration.

Mark