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Fish, I think the following is going imply that some people just shouldn't have engines but...

The original prop is a three bladed fixed sailor type. I haven't looked at it properly in terms of size and pitch, but the paperwork that came with the boat refers to it as a 14 x 7.

Figuring I was under propped (no smoke, max revs, no go) I went to Briski, gave all the particulars relating to boat and machinery and asked him to build me a prop to suit. I don't know what he supplied and I can't find the paperwork.

With the new prop I was hopelessly over propped so took it back for modification. This has been done and it is now a 14 x 8.

I don't know until I put the two props side by side but the new 14 inch looks a bit bigger than the old 14 inch one. Will know on Monday.

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so banging the numbers you've given for hp (29), rpm (3600) + desired top speed (6) into the prop calc spreadsheet above

 

plus the displacement of a loaded compass h28 (12000lb) and water line length (22.8')

 

spits out a recommended 3 bladed prop of 14 x 8

 

with an alternative of a 13 x 10 (for if there are space issues etc with a 14' diameter prop)

 

interesting the calcs on the prop pitch page

 

6 - Speed in knots required from Power Reqd page

1800 - Max prop shaft rpm from Torque & SHP page    

1440 - 80% of max prop shaft rpm                

608 - desired speed expressed as feet per minute.                

0.42 - desired speed divided by max prop shaft rpm to give prop feet per minute.                 5.07 - Theoretical required prop pitch in inches.                

50.42% - Estimated prop slip at required top speed.                

8 - Required prop pitch for top speed.      

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Chris Just make sure you have adequate tip clearance between propeller blade tips and the hull or rudder. An old rule of thumb was clearance minimum 12 % of the propeller diameter. 14 inch blade that equates to approximately 1 3/4" or just over 46 mm. 

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Yep. All good there.

Although I was told that adequate tip clearance was important on boats with flat aft sections, on yachts with very acute aft sections, not so much.

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Yep. All good there.

Although I was told that adequate tip clearance was important on boats with flat aft sections, on yachts with very acute aft sections, not so much.

Chris no mater what shape the hull is tip clearance is still important we did a displacement yacht once where the two bladed prop clearance  was slightly under  10% with this boat only the rudder was causing the clearance issue. we were getting bad cavitation  at anything over  2500rpm engine revolutions (3200rpm max rpm, 2:1 deduction) so effectively the boat was only able to use about  60% of the engines power. What does enable you to decrease clearance is decreasing shaft revs but certainly still giving yourself at least 10% clearance. The owner of this boat eventually went to the expense of a three blade feathering prop to increase bite but  with a diameter more suitable for the aperture size. The three blade prop was smoother and the boat lost much of the harmonic vibrations but most of all the boat could cruise at 7.5 knot at 2800rpm where before 2,500 gave just under 6 knots

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Chris no mater what shape the hull is tip clearance is still important we did a displacement yacht once where the two bladed prop clearance  was slightly under  10% with this boat only the rudder was causing the clearance issue. we were getting bad cavitation  at anything over  2500rpm engine revolutions (3200rpm max rpm, 2:1 deduction) so effectively the boat was only able to use about  60% of the engines power. What does enable you to decrease clearance is decreasing shaft revs but certainly still giving yourself at least 10% clearance. The owner of this boat eventually went to the expense of a three blade feathering prop to increase bite but  with a diameter more suitable for the aperture size. The three blade prop was smoother and the boat lost much of the harmonic vibrations but most of all the boat could cruise at 7.5 knot at 2800rpm where before 2,500 gave just under 6 knots

So how do you tell if you have cavitation, esp above a certain rpm, does if have a particular sound or symptoms?

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Cavitation  has a very distinctive sound and this along with vibration will give you a good indication . After 12 months of use the two bladed prop showed signs of errosion or pitting  from the blade tips down the trailing edge. changing to the three blade prop increased tip clearance by the reduction in diameter, no further problems were had.

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Muzza, awesome that we have a (you) prop expert here.

Every time your boat is lifted out for it's annual (or how ever long) Hull work, the prop needs to be carefully inspected. Just as a Pilot would inspect his prop and leading edges of wings before taking off. Three things should always be looked for. Pink patches in the Bronze. Pitting due to cavitation erosion. This can be anywhere on the props surface, but edges are the most prone. And damage to the edge after the prop has hit something, needs to be addressed. Cavitation erosion is caused by turbulence that creates the shearing of the water and this produces a bubble that is in fact a vacuum. These bubbles can be created and destroyed at the speed of sound and create a lot of energy.
A mistake made by some is to "sharpen" the edges of prop blades when they clean the prop up. That is not how a prop is designed to work and can cause cavitation and one other funny thing, "Prop Singing". The leading edge needs to be blunt and rounded. Think of the leading edge of other parts of your boat, like Rudder and Keel. They are not sharp for a reason. The trailing edge is where "Singing" if produced. Its a constant vibration that makes the prop ring. The trailing edge is usually thinner than the Leading, but still a rounded edge. Not sharp.
Now I am sure that Muzza would be able to tell us that the edges have a specific shape that the guys like him would put on the prop when they are repairing one. My comment above is just in relation to the times I see someone cleaning and dressing the edges of their prop and think it needs to be a knife edge to cut through the water.

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Where does prop area come into all this?. If I have two 14 x 8 three bladers, but the blade areas as percentage of a circle of each prop are different, what effect does this have? Shouldn't this also be part of the sizing calculation?

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blade ratio area

 

presumably there is a standard blade area ratio for normal speed, displacement drive props operating in water

 

as even the props on the titanic

titanic_propellors.gif

 

look to have the same basic ratio as the tiny plastic props on AA battery boat toys

 

xmodel-boat-propeller-lindberg-trawler.j

 

sure special purpose props running at the surface or inside ducting start to look different but the h28 probably fits quite comfortably between the above extremes

 

and once the viscosity of the medium changes dramatically, ie air, so does the basic prop shape 

 

lots here

 

https://www.vicprop.com/propeller101.htm

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Blade area provides more "bite" and therefore thrust at lower shaft speeds - but is not so efficient at high speeds. This is why Race props are normally two bladed, and why some towing vessels use 5 blades or more.

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Wheels I don't touch  the edges of blades unless there is signs of  nicks or burrs from hitting submerged objects such as minor debris. I don't have the specialist gear needed to maintain balance I also feel the same holds true for changing the shape of leading edges, for any damage more serious I would send the blade into Henleys or one of the other manufacturers who can make a proper repair. I can remember back in the 70's seeing a friend of my dads screw up (pardon the pun but I couldn't resist it) a good prop on a 34ft launch by hack sawing 3/4 inch off the blade tips to reduce diameter. The vibration just became intolerable over 1800 engine rpm but the owner of the boat was convinced his modification wasn't the problem. I can remember in power boat racing back in the 70's there was a few self professed experts that were cupping the blades with nothing more than a ball pein hammer  to get a perceived speed advantage, of course this was done by eye with no accurate way of measuring the results and often the results had less than the desired effect.

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we have kiwi prop and last monday off park pt,incoming tide sw 25+ at times 16hp yanmar 3/4 throttle  at times 0.5kt forward,normally in flat water would of got 5.2 on clean bum.So is it the prop not biting in a sea way? first time in a sea way for me in this yacht.

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Harry with yachts a lot comes into play. Often a propeller is just thrown on and a she'll be right added into the equation. Launch owner obviously spend a lot more time refining things, or should. Displacement vessels such as the H28 for example are known for hobby horsing (pitching) and with the propeller a long way aft on the waterline this rapid vertical movement in waves robs the propeller of efficiency. Propellers in apertures are often  compromises and sometimes not good ones at that. Often the diameter is less than ideal typically the fiberglass H28 as first built 40 years ago  had a 10hp engine installed so aperture was designed around this so adding additional horsepower often has little benefit as it can't effectively be used. I am assuming that you are using a two blade prop, to get better performance under motor it may be worth considering a three blade and if drag under sail is a concern go three blade feathering. Getting off the subject for a moment a well designed aperture should only be installed into the hull of the vessel and not cut into the rudder especially on long  keel boats other wise much of the turning moment is lost, both under power and under sail. One thing I might add which is a personal observation is that the propellers such as the Briski two blade flip flops use a lot of power to push those fat blades through the water. We did a prop change on a townson 34 from two blade folding to two blade feathering both 16 x 10 blades, both had similar  blade area. The feathering prop actually enabled the engine to pick up the 300 rpm it was down with the folder and boat speed went up about half a knot.

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16hp yanmar 3/4 throttle  at times 0.5kt forward,normally in flat water would of got 5.2 on clean bum.So is it the prop not biting in a sea way?

A hard one to say, but I would think that 16Hp was not enough to push the boat against the Blow and Tide. The prop may have been working best it could, but remember, with slip, which lets just say is 30% which is good for a Sailboat, then effectively you are getting 70% of max engine Hp at 3/4 throttle, so that is about 11Hp, then less 30% due to slip and you are working with something more like 7 to 8Hp. Now would a 7Hp outboard be the kind of thing you would hang off the back and get the required Boat speed in normal conditions.

 

If I have two 14 x 8 three bladers, but the blade areas as percentage of a circle of each prop are different, what effect does this have? Shouldn't this also be part of the sizing calculation?

Yes, that is why on line prop calcs should not be relied upon. the kinds of software the experts use are not free or even cheap online products. There are so many variables. I note that gear ratio was not in the Calculation supplied above by eric. That may have been entered, but it doesn't show. So the result would be wildly inaccurate.

     There are two main categories of props. Sail and the other Power. Within both are many many designs. The Powerboat side is huge in number. Every man and his dog prop designer has their own idea. Not quite so many for Sail, but still dozens of designs.

Then the difference within the Sailboat side, is that Sailboat props often try to work in a world of compromise. Power boat side is simple. Get power to water as efficiently as possible. Sail props are about getting power to the water with very much smaller engines and the least amount of drag when the engine is not running and the boat is under sail. There are two main categories in Sail. Fixed and Movable. Self feathering, self pitching, auto pitching, mechanical pitching, mechanical feathering and blah blah blah. Mechanically controlled props, which are very expensive and are large, so often found on larger vessels, are often the best re efficiency, because the maker can put a lot of work into the best blade shape. Self feathering and pitching are often the worst, because the Blade shape is also about how the prop works the blade to make it do what it has to do. And being a prop that is there to fold up when not in use, their is as much work put into the folding and trailing folded as there is in propulsion.

A Prop is a rotary wing. The prop itself does not push the boat through the water, it in facts lifts itself just like a wing, but because the water moves, the water is thrust out the back and the result of the Law of physics of every action has an equal and opposite reaction is what makes the boat move forward. So the blade is shaped to act like a wing, not some "pushing" device The design of that wing is based on what the prop is designed to do and the speed it is likely to spin at in the water and how the water moves past the mechanical parts of the prop as efficiently as possible and ther perameters goe on and on.

Surface area increases efficiency(less slip) but takes more effort to spin it due to water friction. Also the faster you turn a prop, the greater the water resistance and thus slip Decreases. A slower turning sailboat prop could be as inefficient as much as 50%. 30% is good. 10% can be had in some of the bigger mechanically altered props. A highly tuned fast spinning race prop can be as efficient as having much less than 10% slip.

 

 

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gearbox ratio? 

 

2nd page of the spreadsheet

 

29 - Engine Horsepower    

3600 -  Engine R.P.M. (max)    

42 - Engine Torque ft/lb            

28 - SHP - Shaft Horsepower at gearbox output.    

3 - Enter number of bearings between gearbox output and propeller.    

2.00 - Enter Gearbox reduction ratio. Eg. 1.5            

4.50% - Percentage power loss due to shaft bearings.            

28 - Shaft Horsepower at propeller.    

1800 - Propeller RPM    

82 - Propeller Torque ft/lb            

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cav 26 3 blade kiwi feathering and no I would not like a o/b hanging off the stern because when hobby horsing prop would pop out of water . to get o/b lower enough,long shaft, head would not high enough above water for my liking,what would be worst,a o/b under water or doing 0.5 with inboard??

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