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Are you over-propped?

Dec 29, 2016
Robin Urquhart’s Dufour 35 Monark being hauled out allowed easy access to the propeller.

Robin Urquhart’s Dufour 35 Monark being hauled out allowed easy access to the propeller.

Before heading out on a long multi-year trip, you might want to consider taking a good look at your propeller. Overpitched and oversized propellers (over prop) are common problems that often go undiagnosed. Unresolved, this issue can significantly reduce the performance and longevity of your engine. It can also cause untold frustration as you try to figure out why your engine is overheating.

The issue affects boats both old and new. Boats that come straight from the yard are often configured with propellers suited to clean hulls, empty tanks and only one person on board. As soon as the tanks are filled and extra weight has been added for comfort and long-distance cruising, the propeller and engine are likely no longer well matched. The same is true for an older boat, as was the case for us.

We purchased our 1979 Dufour 35 three years ago in the frigid waters of the Pacific Northwest. The previous owner had told us that the Kubota engine, adapted for marine use, would overheat above 1,800 rpm in the summer. As relative neophytes to the world of marine diesels, we didn’t think much of it, especially as we could easily cruise at 6 knots under power. We hauled out for a three-month stint in the yard in preparation for a multi-year trip to Australia. We added a water tank, solar arch and panels, and a literal ton of other equipment and supplies. The boatyard owner, upon seeing our boat, remarked on the large size of our propeller. I’m compensating, I joked. The warning signs were all there, but we were unheeding. Then we headed south…

As the water temperature increased, so did our engine temperature. We found that we could only keep the temperature below 200° F by reducing the engine’s rpm. So we dropped down to 1,700 by the time we reached Eureka, Calif. Morro Bay saw 1,600 rpm and Santa Barbara 1,500 rpm. By the time we reached San Diego, we were struggling along at 4 knots and 1300 rpm and realized we had a major issue.

The two most important parameters of a prop are diameter and the pitch of the blades.

Troubleshooting your propeller
An oversized propeller is not one of the most common culprits for engine overheating, but neither is it completely unheard of. It usually falls into the category of engine overload. Engine overload refers to the disparity between available horsepower and the load put on the engine. It can be caused by auxiliary equipment (watermakers, freezers, hot water heaters, etc.) stealing horsepower, a rope wrapped around the propeller, a significantly fouled bottom, adverse conditions, too much weight, or … an oversized propeller.

It is relatively easy to work through the engine overload checklist: Turn off the auxiliary equipment, examine the propeller and the bottom for fouling, wait for calmer conditions or unload some of the weight. It’s not quite as easy to put a new propeller on and see if it makes a difference, but there are a few ways of checking the suitability of your propeller without having to take it out of the water.

The first step is understanding how a propeller works. There are many features of a propeller, but the most important to engine loading are diameter and pitch, both of which are measured in inches.

The diameter of the propeller is twice the distance from the middle of the hub to the furthest extent of one of the propeller blades. Another way to think about diameter is to imagine a circle around the propeller such that the ends of all the blades are touching it. A line that divides the circle in half is the diameter.

The pitch is the distance the propeller ideally moves through the water in one revolution. A common analogy to describe pitch is the turning of a wood screw. A propeller moves through water similarly to how a screw moves through wood. Imagine the screw turning one full revolution; the distance into the wood it travels is its pitch. A rough rule of thumb for a displacement hull is that every inch of change in propeller pitch will consequently amount to a change of 200 rpm.

Pitch is the distance the prop moves through the water in one revolution.

When we use the term over-propped or oversized propeller, we are talking about the diameter or pitch being too large for the engine horsepower-to-load ratio. Often both are the case.

Warning signs
There are a few warning signs that may indicate an oversized and/or overpitched propeller. 

Cruising speed at low rpm. The first and most indicative evidence of an oversized propeller is the ability to reach cruising speed at an engine rpm well below the optimal range. Modern diesel engines are designed to run at 80 to 90 percent of their specified maximum rpm. For example, we have an engine with a maximum rated rpm of 3,200, which means we would ideally reach cruising speed somewhere between 2,560 and 2,880 rpm. Cruising speed is based on the theoretical maximum stable hull speed and, for displacement hulls, is based on the LWL (length waterline) of the boat. We have a 35-foot sailboat with a 27-foot LWL. Our theoretical maximum hull speed is 6.96 knots, as governed by the relationship below:

Maximum stable hull speed = 1.34 x √LWL

In the interest of fuel economy, we like to cruise at 6 knots as there is a decaying curve relationship between speed and fuel economy. Our decision isn’t based on hard math, just on anecdotal information that suggested a 15 percent reduction in speed from the maximum could result in almost half the expended fuel. You can play around with your own boat to find your own tolerance.

Monark’s new prop, above, is a 14x9, replacing the original 17x13.

Screeching into your slip. A larger, more aggressive propeller will propel the boat forward faster at all rpm levels, so a good indication that you may be over-propped is that you approach the dock too quickly. The slowest we could make our boat travel in gear was still more than 2 knots. We usually had to dance between forward and neutral when coming into our slip to avoid scaring our neighbors with the equivalent of an e-brake skid into the slip. We preferred a side tie whenever we could find one.

Overheating/black smoke at cruising rpm, and seasonal overheating. When a boat is over-propped it means that the engine is not working at its optimal torque range. The engine is designed to work most efficiently at a specific rpm range and it is within that range that the maximum load should occur. If the engine is maximally loaded at a lower rpm, then as you increase to cruising rpm the engine will be overloaded and will likely start to overheat and/or produce black smoke. When an engine is already maxed out, the thermostat is wide open and the reduced efficiency of other systems may translate to heat in the engine. As water temperature increases due to summer heat, the engine temperature will increase too much and necessitate a reduction in rpm. 

How to check your propeller
If you are experiencing high engine temperatures and have ruled out the more likely culprits, or if you are experiencing some of the warning signs mentioned above, you might want to think about checking your propeller.

Prop markings indicating diameter and pitch.

Propeller calculator: The easiest check of your propeller is to plug the specifications of your boat, engine and desired cruising speed into an online propeller calculator — for example, there is a good online calculator at The specifics you will need to know about your boat and engine are:

  • ​Length waterline (LWL).
  • Beam at waterline.
  • Hull draft excluding the keel.
  • Vessel weight (don’t use the factory specs, but try to figure out how much weight you have with loaded fuel and water tanks, dinghy, generator, people and any other large additions to the boat since it came from the factory).
  • Engine maximum rated horsepower and rpm. 
  • Gear ratio (it will be listed on your gearbox as a number like 1.82:1, which means the gearbox is gearing down the revolutions between the engine and the propeller shaft, so the propeller turns one complete revolution for every 1.82 engine revolutions; this is extremely important to get right, so don’t guess).
  • Number of shaft bearings between the gearbox and the propeller (usually this refers to cutlass bearings).
  • Desired maximum boat speed (some people choose hull speed, but for better fuel efficiency consider choosing something in the range of 10 to 20 percent less than the maximum).

The calculator will spit out the optimal diameter and pitch for your unique situation. The diameter and pitch of your propeller will usually be stamped on the side and will read something like “14X9X3 LHS.” The first number represents diameter, the second represents pitch and the third indicates the number of blades, which is often omitted due to its stating the obvious. The LHS refers to the direction of revolution and whether it is a right-hand screw (RHS), which turns clockwise, or a left-hand screw (LHS), which turns counter clockwise. If the numbers from the calculator differ substantially from the specifications of your propeller, it is a good indication that you are over-propped. 

What you can do about it
If you have determined that your propeller is too large, there are a number of things that can be done about it. Using the information derived from the propeller calculator and preferably talking to an expert in the field, your options include modifying the existing propeller, buying/modifying a used propeller or buying a new propeller.

Overheating can be an indicator of an over-propped situation. 

Modification of the existing propeller is usually only possible when the change isn’t too great. We looked into modifying our grossly oversized 17-by-13-inch propeller into a 14-by-9-inch. The annealing required to change the pitch four inches, risking cracking the blades. The other problem was that the blade thickness was designed for a 17-inch diameter and the blades would be too thick and hence inefficient for a 14-inch diameter. Grinding them down would take time and be very costly. In the end, the time and cost negated any benefit from using our existing propeller.

Another option is to buy a used propeller that is already the right diameter and pitch, or is not far off. This is the route we ended up taking. We bought online a 14-by-10-inch propeller from Proper Pitch LLC (, and they repitched it down an inch as part of the purchase price. The bore was one-eighth of an inch too small for our propeller shaft, so a machinist was required to bore it out. In the end, the total cost was $300 (USD), and required us to haul out of the water to remove the propeller shaft for fitting.

Buying new is the simplest option, as a reputable propeller manufacturer will take all the data of your boat, engine and desired speed and fabricate a propeller to match. There are other factors that can be taken into account, such as rake, blade size, progressive pitch, cupping and a host of other characteristics that, when optimized, will improve efficiency. Of course this is also the most expensive option, but may be the only one if a suitable used replacement can’t be located.

Sailors rarely think about the size of their propeller, but it is an important component of your boat’s drive system. Overpitched and oversized propellers can cause an engine to work harder and hotter than it should and not allow it to reach optimal cruising rpm, resulting in poor performance and decreased engine life. However, checking the propeller sizing is a relatively easy process and solutions don’t have to break the bank.

Robin Urquhart and his partner, Fiona, set sail in September 2015 on a multi-year trip to Australia, aboard their Dufour 35, Monark. Robin holds a master’s in Building Science engineering and is constantly fascinated by all the things that break on the boat. For more on DIY projects, misadventures and tips for life on a boat, visit their blog at

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