Prop Duct/antifouling paint

[Hermit Scott Catalina30]

I was wondering if anyone has heard of adding on a duct to the prop to increase it's efficiency. I know that an air fan is up to 40% more efficient with a duct around it. I don't know if it is that efficeint in water.
Also another thing about the prop I was wondering about is; with out anti fouling paint stuff grows on a boat at a suprising rate, I heard that the paint doesn't stick to the metal props very well. Does any one have a problem with stuff growing on the prop more than any where else? What are some fixes for this? Does any one powder coat them and then paint them with anti fouling?

 

[jimq26]

prop anti fouling is well covered in the archives here.

As for your "prop duct" idea - no comment. I will leave that to the prop experts to advise.

 

[sailaway78]

Prop Duct

I have no idea whether or not it would work, and if so how well. But it seems that if it were a good viable idea, someone would be making and selling them. Lots of info on painting props in the archives. Best way to keep the prop clean of growth, is use the boat a lot.

 

[Tim R.]

Maybe

I have seen a shroud that you can buy for an outboard that claims something silly like 15% increase. I have never used one but it seems plausible.

There are ways to coat props with AF paint. You just need to apply the correct primer first. I think a lot of sailors clean them when they clean the hull. That is what I do. My prop(Flex o Fold) has a very high copper content and all I ever get is a little slime. I clean it every 2-3 months.

 

[tommays]

Tugboat

You will see it sometimes on Tugboats that really need a LOT of thrust but that is about it



Tommays

 

[Ross]

Hermit, At the speeds our boats operate the blade area ratio is the

primary factor in propellor thrust. The blade area ration compares the actual blade area to the area of a complete disk of the same diameter.

 

[Don12364]

Duct

Air fans are usually stationary and do not normally move through the air with the fan. The duct will cause additional drag.
Aircraft would use ducts on their propellers if they improved efficiency.
Some do as in a turbofan engine - but that is another story altogether.

 

[Manny]

Prop Duct

Scott, I've often wondered about putting a shroud around the prop myself. Probably not much benefit in forward but I'd guess it would help reducing/eliminating prop walk in reverse. Other possible benefits could be less chance of fouling the prop or harming aquatic life?
As far a painting a prop, definitely check the archives. Unless the proper precautions are followed, some antifouling paints can react with the prop, shaft, or strut and cause damage.

Manny

 

[Hermit Scott Catalina30]

prop

I think the basic premise about a duct is it keep water from getting thrown out tangentialy from the propeller, and it redirects the water that would get thrown off tangentially to rear thrust. I remember my dad had a prop 'cupped' when I was a kid. I think that is what that tried to achive.
As far as the extra drag is concerned, it would be negligible in light of any extra thrust.
I am concerned about hurting dolphins with the prop. I hope they are smart enough to stay away from it. They just seem to love boats and people.
I think a duct would benefit but it may be negligible or somewhat of a wash with the extra drag Donalex mentioned.
Just because no one is doing something is no reason not to. My business is inventions, I am used to trying things others have tried and failed at or things people haven't thought of yet. But with this, they already have ducts and some boats have them, so I would guess they are beneficial in some applications, I just don't know what. I am trying to think of everything I can to get more hp to the prop. My eventual fix will be a 25 horse refit. But I completely rebuilt this 12 horse for $1,000. And I am sure I will be able to sell it for twice that when I find a bigger engine.
The water jet idea appears good at first but as you increase jet volocity you decrease the jet area. So you have more pounds per square inch but less square inches. Again it probably works well in a very light hydroplaning boat but not in a submerged vehicle like a sailboat. A 15% increase would justify building a duct to me.

 

[Hermit Scott Catalina30]

shroud

Tim R. where did you see that shroud for the outboard?

 

[Maine Sail]

Trust me...

Quote: "I am concerned about hurting dolphins with the prop. I hope they are smart enough to stay away from it. They just seem to love boats and people."


Don't worry about it. You could actually try for years and years and never succeed. Unless one is sick there is nothing to worry about. I've seen the swim right in front of our bow on a 55 foot sport fishing boat with twin 1000hp motors doing 30+ knots and still we've never even come close to hitting one...

 

[Ross]

Hermit, You should look at the horse power/speed

cutves for moving a displacement hull. The 12 hp engine will push you at 5 kts or a little better but it will take 20-25 to get you to 7 kts. It doesn't matter how you configure the prop, lower prop rpm is more efficient turning a big prop than high rpm turning a smaller prop.
50 percent slip is generally accepted as normal for a sailboat prop. This translates to approximately 1 knot at 1000 shaft rpm for each 1.5 inches of pitch.

 

[Paul Brinkley]

Some shrouds I have read about were intended

to keep an outboard propeller and a rudder from becoming too closely acquainted. Somewhere I saved some info. If I can find it, I'll post it later. -Paul

 

[kloudie1]

Kort Tube

I think you are thinking of a Kort Tube.. Its function is to keep the propeller tip vortices from developing. It produces a lot more static thrust than an open prop but that advantage disappears as boat speed increases, and at higher speeds, its benefit is overcome by the additional drag it produces.. It is best used with a purpose fitted propeller (custom square ended blades at close clearance to the ID of the tube) and would probably be useful on sailboats because I think that the benefits would not have disappeared at the typical low hull speeds. They are common on large tugs and pushboats where low speed thrust is important. Page 12 of the attached link shows this on a tug model.

 

[Ian4781]

Prop shroud

I hope I am on the right track here.....try the CHANDLERY section on this site,then go to PROPULSION,then on to POWER THRUSTER ....I would love to hear if anyone has tried them.

 

[Hermit Scott Catalina30]

prop

Ross what do you mean by 'slip'?

From what I understand the best benefit about having a bigger engine is more thrust at lowspeed like 2 knots or so. It looks like you don't go much faster with a 25 horse compared to a 12 horse but when there is a strong head wind the 25 horse will continue to push through where the 12 will not provide a big enough thrust???

Square tip props make sense inside a tube. The more gap that is left between the prop and tube allows for more turbulent flow. Ususally in aerodynamic flow two objects close together is just as good as two object touching because the flow will still be smooth and not turbulent, as in a head sail and the main. If I have to make the prop too close I will probably be buying a new prop shortly after my experiment. lol
Now that I see those kort tubes on the tug and the model boat, it makes me want to try it. Looking at the power thruster it seems that even a partial enclosure will give you the more thrust aft affect.