There is no more complex subject in yacht and ship design than propeller engineering. It is hard enough that the US Navy has spent billions of dollars on the subject. Yachts are even more difficult as they have to operate in a wider range of conditions relative to their size.

We have extensive experience with sailing designs and feathering, folding, and controllable pitch props used thereon. We’ve also been able to do real world testing with several of our sailing designs to establish baseline data. All of this is covered in detail in Offshore Cruising Encyclopedia , so we won’t elaborate here.

Powerboat props are easier to dial in – at least in theory – than those for sailboats. Where the loads vary substantially with a sailboat, from motoring in light air, to motorsailing, power boats have only the extra drag of waves and wind with which to contend.

Still, there are a host of issues to consider. As we are presently reevaluating the props on Wind Horse (FPB 83), we thought you might like to share in the reasoning.

To begin with, engines come with different ratings. Our 4045TFM Deeres are rated at 150HP at 2600 RPM with peak torque at 1800 RPM, in M4 configuration. M4 is the light duty rating, with typically no more than one hour out of 12 at full load. At the other end of the spectrum us the commercial M1 rating. This is 105HP at 2300 RPM and peak torque at 1500RPM. This is the continuous duty full load rating. There are M3 and M2 ratings between these as well.

To get the engine builder to warranty the engine you have to demonstrate the ability to reach rated RPM underway (which we do). Wind Horse will run at 13.4 to 14+ knots at wide open throttle depending on payload, turning over 2550 to 2600 RPM. However, we cruise at 11 knots at 1900 RPM. At this RPM the engines can give us 135HP and we are using just half the available HP(we have a good handle on this based on our fuel burn over man thousands of miles). This allows a huge margin of power for adverse conditions and has the engine operating where maximum torque is available yet in smooth water we require just a third of that torque. All of which leads to a very long lived engine. In effect we operate even lower than the M1 (commercial) rating most of the time.

So far so good.

With 4400 hours on the engines we are no longer worried about warranties, but we are concerned with longevity. The four cylinder engine is smooth and quiet at 11 knots/1900RPM. But it is even smoother and quieter at 1600RPM. This exercise is to see if we can drop our cruise speed a bit, over prop (reduce the top RPM the engine can reach), and still have the power to battle headseas and strong winds.

Without burdening you with excessive detail, here are some of the factors which must be considered in choosing the right prop:

• Engine rating, point of max torque, HP and rated RPM.
• Transmission reduction ratio.
• Losses due to shaft bearings, transmissions, and power take off for hydraulics and alternators.
• “Sweet spot” for the prop in terms of noise, vibration, and cruising speed.
• Structure resonant frequency in case it is harmonic with prop frequency (bad!).
• Shaft angle relative to buttock line on the hull.
• Water flow forward and aft of the prop.
• Boundary layer thickness on the hull and wake fraction.
• Propeller blade type.
• Number of blades.
• Propeller load and percentage of cavitation as loading increases.
• Blade area ratio (the percentage of the circle swept by the blades).
• Propeller diameter and pitch.
• Prop tip clearance from hull.
• Damage tolerance for blade configuration chosen.
• Harmonics of blade count relative to obstructions ahead and behind.

We’ve been working with Ed Hagemen on the repropping of Wind Horse . He has forgotten more than most know about props, and it is good to get his input and compare drag and performance analysis.

Our goal is two fold. First, as stated previously we’d like to reduce our cruise RPM to 1600, and are willing to give up a half a knot of speed to do it, dropping us down to 10.5. Second, we are looking to see if we can pick up efficiency.

Ed’s initial comments follow in italics:

First of all, from a propeller point of view, what is being proposed here appears to me to be rather prosaic, at least for the most part. Even with only 3 blades, (Individually they would be about the same blade width as your present 4 bladed wheel) cavitation is not an issue, even at wide open throttle. Theoretically, if one uses the same Blade Thickness Fraction (BTF), then blade strength is not compromised either.

Operating at the same boat speeds as before, but with a reduced RPM, an optimization scenario would demand a larger diameter. This leads to the recommendation that 28 inches be used in lieu of the existing 26″. Tip clearance then suffers by an inch.

On tip clearance there are 2 issues:

1) The pressure pulse as a blade sweeps past the shell plate: A tip clearance of 18 to 20% is often used for modern very highly loaded high speed props. I often encounter tip speeds that are in excess of 160 feet per second at wide open throttle. Your existing cruise scenario at 1900 RPM using 26” wheels yields a mere 87 fps. If you drop to 1700 RPM and hold the 11 knot cruise speed with the larger 28” props, the tip speed drops further yet to 84 fps. The older texts recommended 10%, and that was back in the days when lightly loaded props like this one were being used. On the face of it, losing that inch of clearance won’t even come close to that old standard.

2) Sweeping the blade tip through the ship’s boundary layer: The existing prop already does sweep through; and the tip sees about a 5% velocity defect (slower than it would have otherwise been). The bigger prop reaches in another inch and encounters something like an 8% defect. I judge the difference to be small enough not to be bothersome.

In either of these 2 cases above, a propeller designed with skew will soften the disturbance by spreading it out over more time. The heavy scantlings you used give a structure with very high natural frequencies. Not only will you reduce the shaft speed, but the change from 4 blades to 3 markedly drops the exciting frequency (blade speed) even further away from any resonances. In comparison to other vessels, I really don’t see much risk here.

The purpose of the exercise is to drop engine speed with respect to any given boat speed. The strategies of a slightly larger diameter, and a smaller blade area ratio with one less blade are all targeted at not only salvaging fuel efficiency but in the end, improving it by better than 4%.

The other side of the risk lies in distressing the engine by act of over-wheeling it.

The key point Ed makes is the last – the very important issue of engine longevity.

We have long experience with overpropping in our sailing designs. It does require management by the skipper, but can lead to substantial improvement in efficiency (especially with Maxi feathering props). The general rule with an overpropped boat is to find maximum achievable RPM, and then back off by 20%. If your 3000RPM rated engine can only achieve 2600, and you run 20% below this, you should be OK with loading (but are out of warranty parameters).

To get a handle on what the overpropping scenario would do to the life or our engines we called Jim Trelstad at Hatton Marine in Seattle. Jim is a great source of real world info on this sort of thing. His first comment was to check the genset configuration against what we were trying to achieve. These same engines are rated at 67kW continuous (90HP) at 1800RPM for gensets. Lots of them have been used in the commercial fishing fleet where they expect 30,000 to 40,000 hours between overhauls.

With the revised prop we are considering our smooth water calm wind power requirements are well under this number. The real issue is how much extra drag we see when there are big seas and lots of wind on the nose.

The best indication of this is our exhaust gas temperature, a direct reflection of the fuel being burned and power being consumed. With the present props at 1900RPM and 11 knots EGT runs about 700F. The most we typically see in upwind situations is 740, with the worst recorded being 770F. This is an indication that upwind increases in drag are typically no more than 15%. A note here on EGT. The key element is not the temperature itself, this varies from engine to engine (of the same type) depending on the location of the thermocouple which measures temperature. What we are after is the change in temperature as ambient conditions vary, not the absolute number.

The engine torque data for the new props we are considering are roughly as follows (with a 20% allowance for extra drag upwind) :

1600 RPM – 10 knots – 64% available torque

1800 RPM – 11 knots – 61% available torque.

Actual speeds for smooth water are 10.5 knots at 1600 and 11 knots at 1700.

So, on this basis we have a substantial fudge factor. But what about longevity of the engines?

Coming back to the genset continuous rating of 90HP at 1800 RPM, we are at 82% of the available power in our adverse upwind scenario, leaving 20% fudge. Ed told us that when a prop for ship is designed the norm is to work on the 80% point for normal cruise. Using the genset data it appears as if the change in props makes sense and is within commercial parameters.

Where this would lead to a quick death for a conventional trawler yacht engine, we can get away with this because of the relatively low increase in loading from adverse wind and sea states which Wind Horse experiences. What we lose is the ability to run the boat upwind against high load wind/wave conditions at speeds faster than 11 knots. With the existing props we have the ability to cruise at 12 to 12.5 into big seas and/or lots of wind. Since we have never used this capability we do not mind giving it up. The theoretical gain in efficiency and quiet with a 10.5 knot cruise at 1600RPM is a good tradeoff.

We will advise next spring how this all works out in reality.

There is a great deal more on this subject in our Offshore Cruising Encyclopedia with particular attention paid to Maxi feathering and Hundested controllable pitch props (as well as folding wheels). This starts on page 746 in the section devoted to propulsion systems.