Prop Diameter vs Pitch

[Mav58]

Obtaining the right size prop for my 45' wooden ketch has been problematic with ebay delivering the wrong hand or incorrect pitch more than once. So my learned friends out there, who can help me. According to the prop calculators my ideal prop is 26 x 20 pitch in 3 blade. The original prop was 26 x 20 2 blade but badly corroded and, as I dont race anymore, 3 blade is good. Just how far down the diameter scale and up the pitch scale can one go? My prop calculator goes to 24 x 24 but will a 23 x 25 pitch work?

[NCboatrx]

I think that you are going about this the wrong way. If you want the correct propeller for your boat, I suggest that instead of trying to buy a cheap one on ebay, that you contact a reputable propeller shop and let them work out what prop you need. Having someone who knows what he is doing, because it is his business, is often cheaper than trying to reinvent the wheel yourself.
The prop company will want the following information;
weight of the vessel
waterline
HP of the engine and max RPM
transmission ratio.

My company works out propeller calculations for our customers but always recommends that you let the prop company make the final recommendation. This way you only buy one prop that works instead of the hassel of a few incorrect ones.

[GordMay]

Greetings and welcome aboard the CF, Mav58.

[S/V Antares]

Welcome, It is also important to know how much clearance you have in the aperature or to the hull if you have a strut.

I used General Propellor in Bradenton FL. for my new 3 blade and they were spot on. Got almost 1/2 knot and much lower vibration.

[Stumble]

I would highly recommend a folding or feathering prop. Better under power and sail.

If not that, then call a reputable dealer and go with thier recomendations. Cheap props from questionable suppliers can be made with all sorts of weird alloys. Wich for cost purposes usually means injecting lots of zinc. It also makes them disolve in salt water.

[Boracay]

Without knowing what sort of boat/engine/gearbox/existing propeller you have and what sort of conditions you're planning to use the boat under it's hard to make suggestions.

The prop calculators that I tried were set up for a very wide range of boats and motors and did not give acceptable results for a heavy cruising yacht.

I found with Boracay (44', 15 tonne, 85hp at 1500rpm John Deere, ZF63 2:1, 8 kts max) that she was overpropped with a 22x15 (max 2200rpm clean, dia x pitch approx!) and had the prop repitched and cut down to 20.5 x 13.5 and then got 2250 rpm. The prop shop opined that I should have a little more diameter and rather less pitch.

However when I had Propspeed applied the max rpm came up to 2500 leaving me a happy little vegemite

happy little Vegemite

.

[westwinds]

Is the engine original? What's the make and model. On a calm day with no tidal influence, do a run in both directions and note RPM. Use a knot meter and GPS readings, but GPS only is OK for a calm day and no tide. What is the top RPM and speed for the boat? What is the maximum rated horse power for this engine and at what RPM? Also, what is the reduction ratio of the transmission? Since this is a boat with an engine already installed where real data are available, propeller calculations are easier than trying to determine what is best from making assumptions the propeller calculation software uses. Also note if the bottom has been recently cleaned.

[westwinds]

Propellers has a two blade 26X20 with a 2 3/4 inch shaft for $245. The prop has a right turn direction.

[Mav58]

Thankyou all so far. I will try to answer most questions. Boat is 39' WL 48000lbs displacement full keel. Engine is a Ford 80hp @2600rpm with 2.9:1 Velvet Drive which I know is not desirable and gives a slow prop speed although achieving 8 kts is easy. Original prop was 26x18 2 blade but is badly corroded hence my decision for 3 blade. Rotation is LH. I have input to a couple of prop calculators with similar results. Clearance to tip appears ok at 2-3". More suggestions appreciated.

[westwinds]

Originally Posted by Mav58

Obtaining the right size prop for my 45' wooden ketch has been problematic with ebay delivering the wrong hand or incorrect pitch more than once. So my learned friends out there, who can help me. According to the prop calculators my ideal prop is 26 x 20 pitch in 3 blade. The original prop was 26 x 20 2 blade but badly corroded and, as I dont race anymore, 3 blade is good. Just how far down the diameter scale and up the pitch scale can one go? My prop calculator goes to 24 x 24 but will a 23 x 25 pitch work?

Rules of thumb from Dave Gerr’s Propeller Handbook.

An increase of one inch in diameter decreases the pitch by two or three inches, and of course one inch decrese in diameter increases the pitch by two or three inches. Be careful, this is not exact.

Every two inch increase in pitch will decrease RPM by 450.

You need a tip clearance of at least 15% of the propeller diameter for acceptable noise.

The skeg or strut holding the propeller shaft should have at least 30% clearance between it and the propeller for good efficiency and low noise.

The shaft diameter should be one-fourteenth the propeller diameter. Some adjustments might be needed for the type of metal the shaft is made from.

[westwinds]

At an engine RPM of 2600 and a 2.9 gear ratio the propeller maximum speed is 900 RPM so tip clearance can be 10% minimum or 2.6 inches for your prop.

[Mav58]

Thanks westwind so my clearance is OK. Back to the original question on how far can I push the diameter/ pitch relationship

[NCboatrx]

It all depends on the type of propeller. My results are as follows:
26"x20" three blade with 50% BAR or
24"x19" three blade with 65% BAR.
Either one will give you your 8 knots.

[westwinds]

The larger the diameter and the slower the propeller speed, the more efficient it is. Since the original pitch is 18 inches, that is what should be used. The Blade Area Ratio (BAR) should not be any less than that for the original two bladed propeller because of problems with cavitation. Cavitation is where the propeller is so highly loaded that the water is actually thrown away from the propeller blades with such force that water vapor forms and then the water collapses back onto the blade with such force that it erodes the surface, sort of like hammer blows. You can find pictures of this on the Internet and you will see that it is to be avoided. It’s also a problem with pump impellers. There are several definitions of BAR. The one usually used it the Developed Area Ratio (DAR), which is the area of the blade outline if the blade could be untwisted. That is like detaching a blade from the propeller hub and squashing the blade all-flat. You can measure the DAR by taking a piece of paper and forming it to the contour of the blade, marking around the edge and then flattening the paper out and measuring the area in square inches. DAR is important because it is used in a formula with horsepower to determine if cavitation is likely to occur. If I understand the propeller formulas, and I am not sure that I do at this point in my study of it, you do not want to have a DAR on a three blade propeller that calculates out with a loading higher than found on your current two bladed propeller. At a certain diameter the three bladed propeller will be too small to prevent cavitation. I will see if I can calculate the minimum diameter, but remember the smaller the diameter, the less efficient the propeller so you will probably will want to go with the largest three blade you can find up to 26 inches. I may eventually ask for a DAR on your two blade propeller, but not sure yet.

[westwinds]

Would you know what the speed in knots is at maximum RPM? The speed needs to be accurate to a tenth of a knot or better and for a propeller in good condition. It would make the calculations accurate for pitch and diameter, no need to make assumptions with the equations