3 Bladed Prop vs 2 Bladed

[Stone]

I thank you all for the great discussion. I don't think my budget will allow for a folding prop at this time. I've got a heavy ( 15000 lb.) 32' Gulf that I sail in the Gulf of Maine with swift currents and tides and hope a solid 3 blade prop over the 2 bladed I now have might better suit my conditions here.

[Pete7]

Originally Posted by Stone

I thank you all for the great discussion. I don't think my budget will allow for a folding prop at this time.

You and me both. The extra free 0.5 knots under sail would be great but the £1000 price tag for the flex-o-fold 2 blade is just too much unfortunately.

Pete

[Celestialsailor]

Originally Posted by rossad

I think a 2 bladed prop is everything as good as a 3 bladed prob going forward but if there is serious resistance 50 plus knots a 3 bladed would have more grunt once on full rev's. The most important aspect is getting the prop size and pitch to your boat and engine.

Let me also point out that with a 2 blade, at some point, both blades are hidden behind the aperture. This is why a 2 blade produces more of a thumping vibration through the hull. A 3 blade always has 2 of the 3 blades exposed to the water flow.
Sizing is important. Depending on the actual final drive ratio, it needs to be noted to keep the blade a minimum of 1 1/2" away from the hull. More if you can afford it. Also, a note to the wise. Make damn sure that when the rudder is hard over either side that the blades don't whack the rudder. Don't ask me how I know this.

Originally Posted by osirissail

Generally the choice between 2 blade and 3 blade props has to do with hull shape and aperture where the prop resides.
Based on engine power and transmission you select a 2 blade prop to fit. However, if the planned 2 blade is too large (diameter) for the hull shape and/or if you have a keel hung rudder with aperture, you switch to a 3 blade that will have shorter (smaller diameter) blades. Also your prop selection need to be about 25% of radius away from any hull surface above and below to reduce cavitation.
Another consideration is to chose a prop with enough blade diameter to "reach" out from the shadow of the keel in front of it to get clear flow water to bite into. All in all, there are a lot of compromises in the prop size / type selection.

Actually diameter is not a huge factor as to having to switch to a 3 bade. Pitch is changed after the maximum diameter of the wheel is established. Also keep in mind that the more the pitch the less drag it has. this is why I scoff at all the numbers of how slow a non- folding prop is. If the angle of the blade is 10 degrees lets say (they're usually measured by inch/screw through water) and compared to one that was 45 degrees. The 10 degree blade would be more of a barn door through the water than the 45. It is folding prop companies that usually come up with these numbers.

Originally Posted by skipmac

Damn expensive is right. Not yet on the top of my list for where to put more of my vast quantities of disposable income.

Performance. Say you get an extra 1/4 kt which I think is in the ball park (real world experience from others very welcome on this subject). That would be an extra 6 miles/day. In a week long passage that's an extra 42 miles which is starting to sound almost worth while. If you got an extra 1/2 kt thats 84 miles/week.

I think at slower boat speeds the savings is more significant than when close to hull speed. This is when I would care most about the savings. Again, real world experience would be interesting.

However I have another motive for considering a folder; no prop spinning under sail. Transmission manual says its not a problem but there will be wear on the cutlass, the stuffing box and my nerves. The boat came with a shaft lock which is not working but could be repaired but it adds more junk it a very tight spot on my drive train due to the V-drive configuration.

Now Skip...stop that. If you go with a folding then never troll for fish, it will cause drag. Never pull into a beautiful anchorage while on your way to a destination. You can't afford the time. And please, please, no extras like beer, wine, and unnecessary food. It adds weight and slows the boat down.

[Heart of Gold]

I think the Campbell 3 blade will cost you around 700. If you have the older 3gmf from the 80s, the transmission is supposed to be in neutral to let the prop rotate.

[rossad]

If you go to the trouble of installing an engine and fuel tanks and all the needed attachments ... filters etc and put a shaft through the hull and the decide to put a (She will be right) prop on you have just undone all the work for a a special purpose called propolsion. Meaning the most important aspect to going forward is correct size and pitch fitted in relation to your size engine max revs and hull design.

[Heart of Gold]

My friend and I had the 3 blade campbell installed at the same time 3 years ago by NorthbyWest. He has a cs36 weighing in at around 14,500. He loves the prop and is very pleased that he can back up now with control. I have a hunter 34, and of course at 11,500, it is no problem at all hitting hull speed at around 3000 rpm(7.2knots). I usually travel at 6 knots under power, 2500rpm. If you make sure that it is sized correctly, you will also see this performance. Cheers

[colemj]

Originally Posted by Celestialsailor

I scoff at all the numbers of how slow a non- folding prop is.

We have folding props. They sometimes do not fold right away and need to have the gear put in reverse to fold them.

When not folded, we lose almost a full knot of boatspeed - So ~0.5kt per prop.

Mark

Edited to note that these are 3-blade folders.

[Cheechako]

alot more control in marinas etc with the 3 blade. The folding props I've been exposed to are terrible (all 2 blade) a feathering prop like the Max prop is amazing....

[markpierce]

Whatever, but the propeller needs to be matched to the engine to avoid over/under-propping.

Unless you do a lot of motoring, it's hard to see the need to change props. From my observation, however, that's common.

[builder dan]

unless your a balls out racer the extra drag wont be noticeable,change over for sure and if its no good you can always change back.If the 3 blader has too much pitch your motor will struggle and smoke if it has too little pitch it will just be a paint stirrer.only one way to find out.Good luck.

[NZGrant]

Now I know that this is not quite in line with the thread but I do have a question. Folding or Feathering?

[stevensuf]

In my experience folding props are worth a good 15-25nm per day on long runs on a 40ft plus boat, the smaller the boat the less effect you will see (less drag from the prop). they are worth 2 days on an ocean crossing. On the other hand they require more maintenance and those barnacles can cause them not to open and create lots of vibration.

[Zanshin]

On my previous boat I had 3-bladed folding Gori propellor. It worked fine and I was very happy with it, I used the "overdrive" mode extensively when motorsailing or motoring in light winds and waves or downwind. The "overdrive" mode is pretty simple, you put the engine in reverse and the Gori switches to the reverse pitch, which is angled higher than the normal pitch. Once the boat is actually moving backwards you go from reverse idle to forward idle, this reverses the direction of the prop but because the bat is moving backwards the propellor doesn't fold and retains the reverse pitch. With this 'overdrive' setting the RPMs needed for a given speed are much, much lower than in normal mode. The downside is that it is now easy to put a lot of strain on the engine and even overheat it since one is effectively overpropped.

On my current boat I have a 3-bladed folding Flex-o-fold installed and that is working like a charm as well. A picture of my boat with the propellor (taken at the factory while Zanshin was being built) follows:



I notice the difference when I get on a 2-prop boat and compare reactions, in addition neither of the last two boats had any prop-walk at all; I'm not sure if it is due in part to the hull shape but certainly the 'bite' that the 3-blades give in reverse is noticeable.