Propellor selection – pitch, diameter, and number of blades?

Hi, We are currently sailing in the shallow (less than thirty feet) milky green waters of Phang Nga Bay. While the Bay is not terribly big, it has many shear-sided mountains that rise vertically out of the water. These limestone mountains are small in their diameter as compared to their height. Most of the islands in the bay are uninhabited, offering secluded anchorages under soaring cliffs fringed with jungle. Many of the islands have caves in which we take Therapy into to explore. Most of the caves open up to a small lake with shear sided walls, which are opened to the blue sky. Sometimes the cave is long giving one an aerie feeling as one paddles your way using a flashlight whose beacon is eaten up quickly by the cool blackness. Watch your head as the stalactites may reach down from the ceiling and touch you. Do not worry about the moaning of the water as your wake laps the limestone walls, and the quite talking you here is only the bats that you are disturbing along your way. #018, 2 Extreme is going to spend the year sailing between Thailand and north Malaysia. During the year, we will haul the boat, at that time I would like to change her propeller. Currently we are using a three-bladed fixed propeller that is powered by a Perkins 4-108 and a Hurth gearbox. I would like to know what the proper pitch, diameter, and number of blades for the most economical operations should be. I would appreciate the information, if someone in the club could help me out. Thank You, Henry and Mattie, (S/V #018, 2 Extreme), in Thailand waters.

a. Unfortunately I do not know the exact size of the propeller you need. To find the right prop, you also need the reduction of your gearbox. You should start with you present propeller, and determine the most important factor: the engine MUST be able to turn at its maximum RPM or no less than 100 RPM than the maximum RPM as seen in the engine manual. This would be the most efficient propeller and you should expect it to change as the engine wears and tears. If the propeller is too small you are spending more fuel for a given distance and if it is too big, you will burn your engine. The end result is obtained by trial and errors. Usually, I” of pitch will give or remove 200 RPM and 1″ of diameter will add or remove 400 RPM. This is not an exact science. Change the pitch when possible before changing the diameter. If you try to rev up and the engine does not reach its maximum RPM and black smoke comes out at the exhaust, calculate the number of RPM missing, divide by 200 and have your prop’s pitch adjusted by the number of inches you obtained. Hope this will help and have a wonderful day. Marius Corbin

b. A very important piece of information is needed. What is the reduction in your Hurth Gear? It varies widely, and is quite often different in forward and reverse. The propeller people want to know: 1. Gear reduction in forward, 2. Shaft horse power, and 3. Weight of the boat. I think much can be learned from the propeller you have. Since you want to change it, I’m assuming you’re having a problem, and want to change the pitch and / or diameter. You said you want to achieve the most economical operation. I interpret that as meaning you want to reduce engine RPMs and still travel at the same speed. With a fixed propeller, it’s tricky, and could well involve trial and error. Too much propeller (Pitch & diameter) will raise exhaust temperatures and result in engine problems. You can also experience engine lugging (No power) from a stand-still. They make all kinds of fancy (and expensive) non-fixed propellers to deal with these problems. Any decent prop shop can re-pitch your propeller, but be careful. If you are already at the best possible match between your propeller and the rest of your drive train, you could run into trouble. Perhaps knowing what I did will help you. My Corbin has a Pathfinder (gotta love that) rated at 42 horse power. Behind the engine is a Hurth Gear with 2.7 to 1 in forward and 1.5 to 1 in reverse. My propeller was a fixed 3 blade, 18 inch diameter / 12 inch pitch. This was not a good match — I needed more prop. I went for a 19 inch 3 blade Max-Prop (nice piece of change) because I also wanted a feathering propeller. In those days you had to haul the boat and take the prop apart to change the pitch. Today, all the feathering props allow external pitch adjustment — in the water. Max-Prop identifies blade pitch in degrees rather than inches. You get a conversion chart to get started. Through trial and error over several years I arrived at what I feel is a very good match for my boat. My prop is now set at 24 degrees of pitch, which converts to 15.8 inches. Don’t be lured into thinking that a 19 inch fixed propeller with 15.8 inches of pitch would, therefore, be appropriate. It would not. Max-Prop blades are flat. What they lack in shape, they make up for with pitch. I would be over-propped with a fixed prop of those dimensions. If you’re inclined to buy a new propeller, there’s a number of feathering propellers available today with a variety of features, and most are for substantially less money than the MaxProp. Feathering propellers allow you to sail faster and motor more efficiently. It worked for me. Hope I’ve helped. Best from Bob Cox — o the Corbin “#070, Dorisea”

c. A fixed prop is usually designed for a very narrow operating range and will not meet the requirements for all types of motoring conditions. A suggestion would be to go for a variable pitch prop such as the Brunton Auto Prop which automatically adjusts itself for any conditions encountered. Other variable pitch props require manual adjustments to get the “right” pitch and then it becomes a fixed feathering prop. We have used the Brunton Prop since its introduction in the UK in 1987 and have found it to be exactly as advertised. The Corbin now can be manoeuvred in REVERSE and has very little prop walk when docking in tight spots (our prop shaft is in-line with the hull). When motoring, one just sets the most comfortable engine rpm’s and the prop will adjust itself for the best pitch and speed with savings in fuel economy. The stopping power is phenomenal, but I have not tried the demonstration that Brunton factory showed of docking a 36ft sailboat at 6 knots with the boat stopping within its waterline length just before it reached its dock. The Corbin does stop very quickly but I don’t trust my flexible engine coupler to withstand the pull in reverse at full throttle. When sailing just leave the engine in gear, otherwise the prop will spin in reverse. The blades will feather to the flow of the water over the prop blades unlike other feathering props that feather to the prop shaft angle. The only maintenance I have had to do was to adjust the free play in the blades (each blade is independent of each other) this can be done in or out of the water. There is nothing to dismantle, to pack, or to repack. Lubricant and a general inspection will suffice. To obtain the correct size of prop you must specify your engine/gearbox combination and preferably have a dimensional drawing of your shaft/strut arrangement. I took a photograph of my strut with a scale rule attached to the shaft and enlarged the picture. My original fixed prop size was 18 ins diameter; the Brunton prop is 17ins Dia and my engine is a VW diesel (not a Pathfinder) coupled to a Hurth HBW 150 gearbox. Practical Sailor gave this prop top marks in their tests. Janis Priedkalns “#023, Simmerdim”.

d. Dear Henry & Mattie, Just for you I went outside on this sunny day at -13-degrees C to check on prop size. The hard paint on my prop conceals any engraved numbers but the diameter is 17″. I am pretty sure pitch is 12″. My engine is a Pathfinder 50 with 40 HP max. I think I could manage a larger pitch as the engine easily reaches max revs and in calm water we get about 7.5 knots. Don’t forget that prop tip clearance from the hull should be at least 15% of prop diameter, say 2.5″ minimum. It is worth checking the shaft taper as it is possible to have different tapers. SAE taper is 0.7500 in +/- 0.002 per foot (1/16″ per inch). For U.K. specifications taper may be 1″ per foot. Specify keyway also. Regards, David Salter, #050, Opportunity. We have just had #050, Opportunity away from the dock for the first time since installing the Autoprop. It was a quick motoring trial in rather lumpy conditions but seemed to perform well although it felt quite different from the fixed blade prop. We are off for a few days and will have a chance to get some real experience with the prop. David Salter (#050, Opportunity)

e. Dear Henry and Mattie. I would suggest an AUTOPROP for maximum thrust and economy. Other benefits include the ability to back up in straight lines as this type of propeller almost eliminates propwalk. Phone them at 1 800 801 8922 e mail will get you all the info you need. The propeller you buy will be tailor made to your exact configuration by them. This propeller also almost completely eliminates drag. I would suggest a V strut unless one is already fitted. Take care, keep the news coming, Jeremy Parrett ..(s/v Pelican1) I have just installed an Autoprop.It has virtually eliminated propwalk, and I can actually drive the boat astern in complete control. Amazing!! On trials we achieved a speed of 8 knots at 2600 rpm at 2200 rpm we can maintain 7 knots….amazing!! It has put a big strain on the drive train though, and I will be rebuilding the Vetus coupling and installing a new bearing in the thrust block…both are complaining! Altogether though this add on has made an amazing difference to the boat. I cant wait to see the fuel economy figures. Jeremy Parrett (s/v #101, Pelican 1 / Two Pelicans)