Frequently Asked Questions - Propeller - FAQs

Q: You website says my engine will run at higher RPM with the Indigo Prop. Isn't that harder on the engine?

A: No. In fact it is actually better on the A4. It is just like driving a car up hill in it top gear. It will most likely do it but the engine is working very hard or "lugging" in order to generate the horseppwer needed to move the car up hill. The car engine worls less hard if you downshift to a lower gear. Think of it this way. It takes about 12 HP to move a typical Catalina 30 along at 6 knots on a calm day. The A4 can generate that 12 HP at a speed as low as about 1400 RPM. However, the engine is working very hard at that speed and is very near "wide open throttle". This mean that the explosions in the cylinders are quite powerful which puts a lot of extra pressure on the pistons, rings and bearings. If you could generate that same 12 HP at 1800 or 2000 RPM, the explosions in the cylinders are significantly less powerful thus putting less load on pistons, rings, and bearings. Sure components are moving faster at 2000 RPM but the forces on wearing components are greatly reduced.

Q: Will higher RPMs affect my Oil Pressure?

A: Absolutely and for the better. As the A4 turns faster, there is slightly more oil used by the bearings. At the same time though, the oil pump gears are turning faster as well. The increase in oil pump output more that offsets the increased usage by the bearings and your oil pressure actually increases in most cases. In a similar manner, cooling water flow increases as well as alternator output thanks to the increase in engine RPM.

Q: What are the little winglets on the propeller tips? Do they really do something or are they just a gimmick?

A: The winglets are there to reduce noise. On most sailboats there is not sufficient clearance between the tips of the propeller blades and the hull. Naval Architects typically want to see a clearance equal to 15-20% of the propeller diameter. As the RPM of a propeller increased, vortices are shed from the tips of the blades due to difference in pressure on the forward and aft faces of the propeller. If there is insufficient clearances, these vortices slam against the hull (it sounds like a chain rattling on the hull). The winglets divert the vortices aft, in effect increasing the blade to hull clearance and reducing or eliminating the noise. Additionally, the winglets produce a little more thrust.

Q: I have heard that 2 blade propellers are more efficient that 3 blade. Will my fuel consumption go down with the Indigo Prop?

A: Most likely not. Operating data taken on a Tartan 34 with an A4 and the Indigo Prop yielded fuel consumption of .75 gal/hr with the A4 running at 1800 RPM and the boat making 5.8kts. At wide open throttle of 2400 RPM and boat speed of 6.5kts, fuel consumption jumps to 1.61 gal/hr. Conversely, at 1600 RPM and boat speed of 5.35kts, fuel consumption dropped to .63 gal/hr. I believe these numbers are very much in line with data that would be generated by two blade propellers.

Q: You claim the Indigo Prop operates with less vibration. Is it just balanced better or what?

A: Yes and more. Each prop is individually balanced on precision rollers to establish excellent mechanical balance. However, a bigger factor in vibration is consistency and accuracy of pitch from blade to blade. We use a precision gauge to measure the pitch on each blade and adjust the blade accordingly to insure the pitch is the same on all blades. If the pitch varies, the forces generated by each blade are unbalanced which leads to undue vibration. A final factor affecting vibration is the fact that the Indigo Prop has an odd number of blades. If you consider what is going on with a 2 blade prop you find that during each full revolution of the prop, both blades are in the "disturbed water" behind the keel at the same time, twice per revolution. This creates two strong impulses to the shaft and boat which is felt as vibration. With an odd number of blades, in the Indigo case 3, only one blade is in the the "disturbed water" at any point in time. The result is 6 much smaller impulses to the shaft and boat per revolution and that equates to less vibration. Of course when you have a strut involved, this phenomenon does not occur. However, a strut normally implies a spade rudder and the prop wash from a 3 blade prop is"smoother" over the rudder than with a two blade as the flow of water is more uniform.

Q: Can I install the Indigo Prop just like my current prop or is there something different about it?

A: Yes, it can be installed just like any other prop. It simply mounts on the tapered area on the shaft with a key and one or two nuts and a cotter pin. A shaft zinc is then attached to the shaft.

Q: Can I install the Indigo Prop with the boat in the water or do I have to haul it?

A: It can certainly be installed with the boat in the water. There are dive services around most marinas for hull cleaning and propeller installation. Using the services of a diver is normally less expensive than hauling the boat.

Q: What is this "No Zinc" concept? All propeller shafts need a zinc don't they?

A: Yes and No. There is a lot of confusion regarding the Indigo Prop and my recommendation about zincs. There is nothing in the world wrong with installing a zinc with the Indigo Prop, in fact it is the preferred method of installation. Having said that, with the Indigo Prop, you have the option of installing it without a shaft zinc for the purpose of preventing marine growth from fouling the prop. By leaving the zinc off, the prop then becomes the sacrificial component in the galvanic couple formed by the prop and the stainless steel shaft and the prop will not get any marine growth on it. What makes things different with the Indigo Prop is the bronze alloy used. It is Silicon Bronze and as such has no zinc in it. For that reason, it will sacrifice itself much more slowly than a traditional Manganese bronze alloy prop which has about 30% zinc in it. The real trick to managing the amount of prop deterioration is to limit the amount of exposed stainless steel. My current recommendation is to only have about 1" of shaft exposed. If there is more than 1" of shaft available, it should be covered with an insulating paint (like Glyptal 1201) or a layer of fiberglass. If you are OK with the amount of marine growth that you currently get on your prop, by all means continue to install a shaft zinc if you install an Indigo Prop.

Q: If there is no zinc on the shaft, won't my thru-hulls be eaten away by galvanic action?

A: Most likely not. The only things that are protected by the shaft zinc are those pieces electrically connected to the shaft. Galvanic activity occurs when you have two dissimilar metals electrically connected to one another in the presence of an electrolyte (in this case salt water). On a boat, that is normally a bronze propeller on a stainless steel shaft. Unless other thru-hull fittings are electrically connected (bonded) to the shaft or engine or negative battery terminal, they are not protected by the shaft zinc. Most boats do not have the thru-hulls bonded. Therefore, if the shaft zinc is omitted, the only piece that is affected is the propeller.

Q: Will the Indigo Prop have more drag than a 2 blade prop?

A: Yes and no. Compared to a typical 12 x 8 Sailor 2 blade prop, the Indigo Prop has about 9% more frontal area. Compared to a 12 x 7 CDI or Perfect Pitch prop, the Indigo Prop has about 11% less frontal area. Therefore, the Indigo Prop will have slightly more drag than the typical 2 blade prop and slightly less than the CDI or Perfect Pitch.

Q: I can "hide" my 2 blade prop behind the keel. Since I cannot fully "hide" a 3 blade prop, won't it have more drag?

A: There are some boats such at the Alberg 30 and the Pearson Triton that have a very wide "deadwood" (the keel stops abruptly and has a flat area about 3.5" wide on the aft end of the keel). On boats with this configuration, you can indeed mark your shaft inside the boat and thus align the prop to "hide" it out of the water flow in this deadwood area. If you are diligent enough to perform this alignment operation each time you stop motoring, the 3 blade Indigo Prop will have more drag (although I cannot quantify how much additonal drag that might be). For those boats with a nicely faired in trailing edge on the keel (like the Tartan 34) and those boats with the prop positioned with a strut (like the Catalina 30), there is no opportunity to "hide" the prop and thus no significant difference.

Q: Will the Indigo Prop give me better astern thrust?

A: Yes. You are able to obtain more RPM astern (about 2900) with the Indigo Prop and therefore develop more power. You have to remember 3 things about reverse on the A4 1) There is a built in speed reduction in the reversing gear of 1.3:1. This means that for the same shaft RPM that you get going forward you will have to have about 1/3 more engine RPM astern. 2) The A4 reversing gear is quite noisy. It is OK to go up to 3000 engine RPM for short periods as needed 3) There is no "detent" in reverse like there is in forward. You have to physically hold the reversing lever in the reverse direction to make sure the clutch does not slip.

Q: I have a lot of prop walk in reverse. Will the Indigo Prop help this situation?

A: Most likely. Prop walk is caused by the difference in apparent pitch on a propeller blade as is goes through its 360 degrees of rotation. This difference in pitch causes asymmetrical reverse thrust which will cause the boat to typicaly "walk" off to the Port side. If the prop shaft was perfectly horizontal, prop walk would not be an issue at all. Unfortunately, that is not the case with A4 installations on sailboats. The prop shaft is at some angle below horizontal. The greater this angle, the worse the prop walk. Once you get to a certain angle (about 11 degrees), no propeller will help eliminate prop walk. After reviewing the design of many boat with A4s, it has has been found that there are three other factors (other than shaft angle) which can make prop walk more severe: 1. Keel form. Fin keels forms allow a boat to turn much more quickly than a full keel form. Accordingly, the prop walk forces will make a fin keel boat turn much more easily than a full keel boat. 2. Prop to centerline of lateral resistance (keel fore and aft centerline) distance. The greater the distance the prop is from the centerline of the keel, the greater the "moment arm" for the prop walk forces. The greater the "moment arm", the more easily a boat will turn for a given amount of prop walk force. Research has shown that this distance is all over the map on the boats in question. 3. Fore and aft trim. The more a boat is trimmed down by the stern from the design waterline, the greater the effective shaft angle and the greater the prop walk forces. If you put 600 pounds of passengers in the cockpit of a 27 foot boat, the boat will be trimmed down by the stern more so than a larger boat would be (Having a couple of folks go forward while manuevering will most likely help your prop walk issue). Additionally, storing a lot of "stuff" in the aft end of a boat will cause it to be trimmed down by the stern. From my experience, two boats that seem to be in the extreme prop walk category are the Cal 29 and Islander 28. The Cal 29 has a design shaft angle of 15 degrees and the Islander 28 has a prop to keel centerline distance that is much greater that similar boats. A means of judging your shaft angle is the amount of oil you can recover when you change oil. If you get about 3 1/2 qts, you shaft angle is OK. There are two propeller factors affect prop walk. The first is diameter. The smaller the diameter, the less prop walk. The second is the Projected Area Ratio (PAR). The Disc Area is the area of the circle which is generated as the blade tips rotate. The Projected Area is area of the propeller if it was projected like a shadow on a flat surface (sometimes called frontal area). PAR is the Projected Area divided by the Disc area. The closer this ratio is to 1.0, the less the prop walk. The Indigo Prop has a PAR of about .35 and most 2 blade props have a PAR of about .2 to .25.



Return to Propeller Page
| Home | Ignition | Oil Filter | Fresh Water Cooling | Propeller | Tensioner |