2 or 3 blade prop

[slawosz]

Hi,
as I am planning some cruising ( from Essex to Cornwall), I am thinking about changing my current propeller. Its a bit tired, edge is not in perfect condition. Now, I am wondering if I should get 2 blade or 3 blade.
As for 3 blades, I see such advantages:
- less vibration
- better maneuverability in marina - something I would love to have. Marinas I visit (Bradwell, Felixstowe etc) are cramped and busy, especially when I visit during summer. Better handling would make a difference
- more power, especially when motoring against heavy seas
- it will be first time going cruising further, so more engine power will be reassuring
Disadvanteges:
- more drag - I think - 0.2 - 0.3 knots in calm conditions? - but here I am either sailing locally, or I have to hit 4 knots from passage plan so will use engine anyway
- I do racing ocassionally, but I am rubbish, and I maybe can do 2-3 local races a year?

From above its clear that I am inclined towards 3 blade, but I would like to know what other think. I don't really consider folding prop, as they are not as good as traditional ones and cost 3 times more. Plus, not much of them fit to tiny, 3/4 shaft.

And current prop:

 

[NormanS]

Many years ago, I built a Folkboat, a boat about the size of yours. I fitted a two bladed propeller, and marked the shaft coupling, so that I could always "park" it in line with the stern post, to cut down drag. Well, it may have, but in spite of having 7hp, I found that I had too little power to motor against the weather. I changed to a similar sized three bladed propeller, and it made a world of difference. Yes, there might have been more drag, but I had no way of measuring it, so it didn't matter.

 

[Stemar]

I had a two blader on my last boat then swapped it for a three, as it was getting tatty and starting to dezincify. No noticeable difference under power ahead, but the two blader was already quite big. However, going astern became far more predictable and manageable. Under sail, I might have lost a bit, but she was an old Snapdragon, so who knows?

If the budget will stretch, on anything but a heavy motorsailer, I'd go for a folding/feathering job. It's likely to give you a significant speed advantage under almost all sailing conditions. A mate had a Kiwi prop that worked well, but there are plenty of others

 

[Refueler]

One of the major considerations is actually what is your engine rated for ?

A 3 blader does not automatically give large increase in thrust - it depends on RPM and blade design.

I suggest you check what prop is recc'd for your engine + expected use.

 

[Tranona]

Hi,
as I am planning some cruising ( from Essex to Cornwall), I am thinking about changing my current propeller. Its a bit tired, edge is not in perfect condition. Now, I am wondering if I should get 2 blade or 3 blade.
As for 3 blades, I see such advantages:
- less vibration
- better maneuverability in marina - something I would love to have. Marinas I visit (Bradwell, Felixstowe etc) are cramped and busy, especially when I visit during summer. Better handling would make a difference
- more power, especially when motoring against heavy seas
- it will be first time going cruising further, so more engine power will be reassuring
Disadvanteges:
- more drag - I think - 0.2 - 0.3 knots in calm conditions? - but here I am either sailing locally, or I have to hit 4 knots from passage plan so will use engine anyway
- I do racing ocassionally, but I am rubbish, and I maybe can do 2-3 local races a year?

From above its clear that I am inclined towards 3 blade, but I would like to know what other think. I don't really consider folding prop, as they are not as good as traditional ones and cost 3 times more. Plus, not much of them fit to tiny, 3/4 shaft.

And current prop:

View attachment 150348

First thing - a folding prop will do wonders for your boat. Far better than that horrible thing you have now - but as you say 3 times the price of a fixed blade.

3 blade is a good if you are happy to live with the small increase in drag. Feed your boat and engine/box details into here Vicprop - Prop calculator for Displacement and semi-displacement hulls and you will get a good idea of size. However if you buy a new prop from somebody like Sillette give your boat details and they will recommend the correct size.

 

[slawosz]

@Tranona I don't think Gori is doing 3-blade 13-inch props - which I need. And Kiwi Prop doesn't have a 3/4 inch shaft. I wonder how much more drag I will have. But again, after 4 years of boat ownership, the sad truth is there - If I need to get there, I need to use the engine anyway. And I think it's wiser to have a more powerful prop than better sailing abilities. But I am looking for your input here. Oh, and Gori won't give me the performance as 3 blade prop I am afraid (maybe I am wrong?).

@Refueler I am in contact with T.Norris Marine and they will advise me based on photos with the dimensions I send them.

 

[AntarcticPilot]

@Tranona I don't think Gori is doing 3-blade 13-inch props - which I need. And Kiwi Prop doesn't have a 3/4 inch shaft. I wonder how much more drag I will have. But again, after 4 years of boat ownership, the sad truth is there - If I need to get there, I need to use the engine anyway. And I think it's wiser to have a more powerful prop than better sailing abilities. But I am looking for your input here. Oh, and Gori won't give me the performance as 3 blade prop I am afraid (maybe I am wrong?).

@Refueler I am in contact with T.Norris Marine and they will advise me based on photos with the dimensions I send them.

I have a Gori two blade folding propeller and I've never felt there was any loss of thrust. I can easily reach hull speed at about 2/3 throttle; wide open she just digs the stern in. That's on a Moody 31 with a 28HP engine.

 

[Buck Turgidson]

Your engine creates power, your prop converts that power into thrust. Increasing the number of blades in itself is a little too simplistic a thought . Better seek advice on your engine/hull combination to find the optimum in terms of diameter, pitch and number of blades. the Vicprop calculator is a good start.

 

[slawosz]

I have a Gori two blade folding propeller and I've never felt there was any loss of thrust. I can easily reach hull speed at about 2/3 throttle; wide open she just digs the stern in. That's on a Moody 31 with a 28HP engine.

And how about the reverse performance and prop walk?

 

[Tranona]

Suggest if you are going down the folding prop route you look at a FlexoFold 2 blade which can be bored to 3/4" for your shaft. You would not want a 3 blade folder or featherer for your boat, although I did have one made for a 1GM for my Eventide, but that was 3.2 tonnes long keeler so very different from your boat.

You have just enough potential power to reach hull speed of just under 6 knots and if you have the common 2.2:1 reduction a 12*8 2 blade or 11*8 3 blade should achieve that. Your prop looks like a 11, a bit small for a 2 blade - worth double checking. A 2 blade Flexofold will perform better than a fixed 2 blade and probably as good as a fixed 3 blade.. If you plan to keep the boat for some time and do a lot of sailing then a folder will be a good investment as it will give you better sailing performance and shorter passage times. This far more noticeable when you do longer passages when the extra speed of 0.3-0.5 knots will show up - 1 hour off a typical cross channel passage for example.

 

[Bobc]

In your shoes, I would go for a 2 blade Featherstream or Maxprop.

 

[Daydream believer]

A 3 blade will give less vibration. However, you do not say what size engine you have. From the picture you seem to indicate a 12 inch prop. That suggests a 10HP engine. (When new) So you are probably only getting 7HP of thrust at the blades. Hardly worth going for 3 blades.
I expect your motoring speed is only 5 kts in calm conditions & 3 in choppier ones.
As for reversing, Bradwell etc are easy to get in & out of & a modern folding 2 blade is really only a perceived problem. A modern one would be more efficient. The gain would be a .3-.5 kt increase in speed . That is not a lot ie 60 miles at 4.5kts =13.3 hrs & at 4.9 kts is 12.25 hours. I would still want that 1.1 hours up my sleeve
However, there are other ways of gaining .4 kt boat speed.

 

[AntarcticPilot]

And how about the reverse performance and prop walk?

Both fine, but I have nothing to compare against. No reason reverse performance should be any different to forward; the blades are pretty much symmetrical.

Later!
Thinking about it. a two-blade propeller should have a larger diameter than an equivalent 3-blade one, so I would expect the prop-walk to be somewhat greater. But as far as I'm concerned, it is what it is, and you learn to use it. Prop-walk is not necessarily a bad thing. On my boat, the rudder easily overcomes it going forward when the prop wash passes the rudder, but in reverse, it can overpower the rudder until the boat is moving fast enough to get water flowing over the rudder. I generally find that a short burst at high power to get her moving, then neutral while steering does the trick.

 

[slawosz]

A 3 blade will give less vibration. However, you do not say what size engine you have.

The engine is 1GM10, the gear ratio is 2.21, and the current prop is 12 inch.

 

[penfold]

Even a modern 2 blade folding will be streets ahead of that knackered old paint stirrer; there was a fad for cropped blade tips in the 70s and 80s, prop technology has moved on enormously aside from the wear your prop has incurred. I sailed on a few times on an Impala with the same engine and a relatively new Gori folding prop and it performed very well ahead and astern.

 

[Tranona]

The engine is 1GM10, the gear ratio is 2.21, and the current prop is 12 inch.

The pitch should be stamped on the boss. My guess is 8". Can you get maximum revs (3600)? Worth doing a revs/speed test in flat water at 200 rpm increments from, say 2400 rpm to see what sort of speeds you are getting. Always best to know what you are starting from before you make any changes. Ideally you should be able to make 5 knots at around 3000 rpm and just under 6 at 3600 rpm.

 

[RivalRedwing]

A chat with Darglow may be helpful to you (they are nice people). A feathering or folding prop is the way to go if you have the beer tokens.

 

[Praxinoscope]

Another very helpful propeller company is Castle Marine in Caernarfon.

 

[slawosz]

I got several quotes. 2 blade Gori is not the end of the world. But I am a bit concerned. I heard about a few instances of losing the propeller (in theory it could be blamed on poor installation). Also, the fact it is not always opening properly (read here in the forum), especially in reverse, is discouraging. On the plus side, I won't catch any pots when sailing I understand? I am a bit fixated on 3 blade, as it might give me bit of a pull agains heavy weather, but I presume above 25 knots (where I would prefer to be home anyway), even 3 blades won't help with my current engine.

So I guess, 3 blade prop would be help when going against wind 15-22 knots, below and above doesn't make much difference. I might have lower fuel consumptions, but with the such a small engine, we are talking pennies.

As for reversing, Bradwell etc are easy to get in & out of & a modern folding 2 blade is really only a perceived problem. A modern one would be more efficient

I think my 'fear' against maneuvering in the marina, and Bradwell in particular, is lack of practice. I was struggling last year, during busy summer weekends, but the fact was - it was my first time driving boat with inboard. I haven't hit anything, but it was lot of stress. This year I will go there and practice - ideally near fishing boats, as they would not mind if I touch them...
Before I had outboard in the well, which was much better for manoeuvers - the engine was much closer to the center.

 

[Daydream believer]

When going into a marina one has 2 main factors to consider. Wind & prop wash. Make sure you understand how your boat reacts to propwash in forward & reverse & how quick it responds to throttle.
Then when approaching the berth think what the wind will do to one's direction
In most marinas, current is not a major factor. Just be aware of it, if up the chuff, when entering the berth. Rig fenders both sides with a couple near the bow. Might be an idea not to have the anchor in the bow roller for the first attempts. They can do some serious damage.
When I bought my boat I went out in the river & practiced nosing up to a mooring buoy a few times first, just to get the feel of rudder & throttle etc. I also reversed up to it as well. That gave me an idea of rudder response.