Small motorsailer difference in prop speed and power applied when under power only or power and sail

I have a question about small motorsailers(less than 25 feet LOA): If you wanted the same motorsailer(under moderate wind conditions-8-12 knots) to go a fixed speed, say 8 knots under either of 2 conditions: 1) under power only; and 2) under power and sail combined. Which scenarios is most likely: 1)The prop would be spinning at the same speed under either condition while maintaining the same speed and with the same amount of power being applied by the motor turning the prop under either condition; or 2) The prop would be spinning at the same speed under either condition while maintaining the same speed and with less power being applied using a “higher gear ratio” by the motor turning the prop under condition 2 than under condition 1; or 3)The prop under condition 2 would be spinning at a slower rate than the prop under condition 1 while maintaining the same speed; or 4)something else , entirely.

 

For a start, your dream of 8 knots for a motor sailer under 25' is just that - a dream.
A more realistic figure to aim for would be 5 knots, and if you give it a lot of welly and she digs a big hole in the water you might perhaps get up to 5.5 knots - unless you are talking about something very skinny and light, rather than a 'traditional' type of motor sailer hull.
If you want to achieve say 5 knots in 8 - 12 knots of wind your best bet would be to set the full sail area and then gradually increase the revs, and hopefully you will reach the desired speed, assuming that the wind speed stays reasonably constant.

 

Conditions are constantly varying, influenced by load, wind, and seas and direction of travel in relation to them.
Your best bet is to setup the motor for motoring, sans sails.
If your propeller is setup for sail assist only, it will be overworking the motor when lacking sail assistance.
8 knots is a tall goal, what is your designed hull speed?
A big power plant might guarantee higher speeds, but will come at the cost of much added weight and fuel, and must still be optimized to operate at ONE particular set of circumstances.

 

In general, high thrust requires high power, which requires high rpm. Therefore, if a sail is providing most of the power required for eight knot speed, (an improbable speed for a 25 foot boat) then the engine only needs to provide a small amount of power, so lower rpm. Propeller slip is higher at higher thrusts. In a motorsailer, it is possible for a propeller to operate with zero slip, but in that condition, it is also providing zero thrust.

In electric boats, propellers can operate with negative slip, and the energy of the water working on the prop is transmitted to the boat's batteries.

 

The answer is 3), but the but the value of how much slower (i.e. less RPM) is negligible, 2 0r 3 percent at the most. Unless the wind is very light or you have a "must make" speed; all motor sailing does is only slightly reduce fuel or battery consumption. This is because how a propeller provides thrust and absorbs power. See What is the max RPM any propeller can spin before it becomes useless? https://www.boatdesign.net/threads/what-is-the-max-rpm-any-propeller-can-spin-before-it-becomes-useless.65477/#post-906498
As a side note, this is one of the situations where a properly configured electric motor or a Controllable Pitch Propeller could have some usefulness.

 

First, lets assume that this is a displacement boat, and that a more reasonable constant speed target to consider would be 6 knots. We might also think of the boat having a Tohatsu 9.9 four stroke engine which would push a 25 foot motorsailer to 6 knots. 8-12 knots of wind could also push the boat to 6 knots on a beam reach.

What you said in an earlier thread provides a little better answer to the poster's question, I believe:
"In each of the quadrants there are areas where is no "thrust" as we think of it, but areas where power is actually being put into the prop (i.e. the areas where hydro-generators and wind turbines work). These are areas were extreme caution is required because you can break the props or in aircraft (such as in a steep dive) you can overspeed the engine."

Although it can be hard to overspeed many modern engines, even without electronic controls, a sailor will recognize that the engine is turning awfully fast with a medium throttle setting while sailing on a broad reach, and will reduce throttle to prevent overspeeding. Once he has reduced throttle enough to provide zero thrust (the zero slip speed) the boat will continue along at 6 knots with the engine running at the speed for zero slip at this boat speed. If the engine has a 9" pitch prop, and a 2.15 reduction, then that engine speed will be 1742 rpm, at the no thrust speed. Anything faster than this will make the boat go too fast (because our questioner asked for a constant speed).

Suppose the winds dies, entirely. Then the sailor will advance the throttle to provide all the required thrust. Depending upon how clean the bottom is, and all the other factors affecting resistance, a good guess would be that 4000 - 4500 rpm would be required for 6 knots, on something like a Hunter 25 or Catalina 25.

At other wind speeds and points of sail, engine speed will vary from 1742 (no thrust required to maintain 6 knots) to 5500 (directly into into a headwind, sails flogging).

That difference -- 1742 vs 5500 -- is much greater than 2 or 3 percent.

 

Your argument is too simplistic and not in agreement with how propellers or sails work. At issue here is how the proportions of sailing thrust (i.e. relationship between speed & course over ground and apparent wind) and the propellers thrust ( i.e. relationship between J, n and thrust deduction) combine to give total thrust. Even is the best case (hard on the wind) it is not linear because running the propeller causes additional drag on the hull while reducing sail thrust. I doubt you will see anything like a 70% reduction in rpm; granted that 2-3% may only be seen for DDW situations.

 

I checked your profile and see that you are retired and own a Catalina 22. Therefore you are very well-equipped to test your theory, which sadly, is thoroughly incorrect. I'd suggest Dave Gerr's Propeller Handbook to you and the OP.

Propeller slip is something of a misnomer, because the propeller is not slipping in a meaningful way (although the water around it undergoes shear, which could be thought of as slip). Nevertheless, it is a convenient concept, and one that is helpful for a discussion like this one, in which the OP seems new to the topic. (I assume this based upon his quoting such improbable speeds for a tiny motorsailer, along with his not simply having observed the relationships between thrust and motor speed in a small yacht. If he had, he would not be asking the question. )

As an old codger, like me, you no doubt remember taffrail logs. These operated at near zero slip, for the reason that the non- linearities that show up at higher blade loadings would make the device useful at only one speed through the water. A freewheeling prop on a small boat can be used as an impossibly crude taffrail log, or even knot meter. (A great many small boat sailors have listened to the speed of the prop in the water and have gained a sense for how fast the boat is going. If the prop is really humming along, thoughts can turn to buying an electric motor and generating some electricity. More often, the noise and vibration of the prop spinning causes the sailor to remember to pull the motor up out of the water, if he has an outboard. ) The boat's propeller, in this case, is operating with negative slip, and significant slip because the propeller is poorly adapted to being driven by the water.

Here is a thought experiment: Let's assume you have a 5 hp motor on your Catalina. Assume it has a 7" pitch prop. Go out on a relatively calm day. (There are loads of those in the PNW.) Leave the sails down, and motor at 5 knots. You will find that about 3500 rpm is required -- at least that is what you should be imagining. Now try 7 knots. You will find that full throttle and 5500 rpm is required... and even then, you may not reach 7 knots. Getting to 7 knots means you are trying to exceed hull speed, and even a small increment above hull speed requires a lot more power than hull speed does. (5 knots, a knot below hull speed, requires quite a bit less power, because we are operating in the steep part of the resistance curve.)

Now, how much slip is occurring in each case? Here's a calculator which anyone reading this can use:
continuousWave Propeller Calculator in KNOTS https://continuouswave.com/calculators/propCalcKnots.php

3500 rpm, 2.15 reduction, 7" pitch, and 5 knots yields 46.7% slip. That is a large slip number.

5500 rpm, 2.15 reduction, 7" pitch and 5 knots yields 66.1% slip. That is an even larger number.

(Put this same engine on an 8 foot hydroplane with a light driver, and you might get 10 percent slip with a speed of 13.3 knots, a pretty solid plane for an 8' boat.)

Try this real world test: (Anyone with a Catalina can verify these numbers... I'm not trying to trick you. ) Sail your Catalina on a beam reach on a nice sailing day. Unless your Catalina is a very strange one, you will be able to do 5 knots on a beam reach on a nice sailing day, even with the motor down and in neutral. If you can't get it going 5 knots, you fail the seamanship part of this test.

Next, take the sails down and motor at 5 knots. You will find that you perhaps require 3500 rpm to do so. This will cause about 47% slip. That is a high number, but typical of a small motor pushing a relatively heavy boat. (A planing runabout of the same weight would use perhaps 60 hp to go much faster, and with much lower slip numbers.) If you can't get your Catalina to go 5 knots under motor, you fail the motoring seamanship part of the test.

Next, put the sails back up. Sail at five knots, motor still down, in neutral and idling. Now put the motor in gear. Idle thrust will slow the boat barely enough to perceive, so adjust the throttle to produce 5 knots. You will find that just a tiny tiny bit of throttle above idle is required. This small amount of power is compensating for the drag of the propeller water-milling, and trying to speed up the engine. The speed at which the prop is delivering zero power is the zero slip speed, 1865 rpm. Just a little above idle. To cancel the drag inherent in the vagaries of blade twist not being optimum for every speed, etc, means that you might need 2000-2200 to truly achieve the no drag, no thrust condition.

Suppose you are unable to adjust your throttle carefully, and claim, to support your unsupportable argument, that some other incorrect figure like 10% less rpm is required (having realized that your 2-3 percent makes absolutely no sense.) That would mean 3150 rpm is required for zero thrust. However if you use the calculator, or do the math yourself, you will see that such an rpm causes 40.8% slip, a figure consistent with relatively high forward thrust. One cannot add substantial thrust to a boat going 5.0 knots without it speeding up.

Where the OP's question has some utility is with electric boats, where the sailing boat's ability to drive the prop at negative slip can be used to charge the batteries. Doing so, of course, must slow the boat, (as compared to its non-regen speed) unless laws of physics are to be ignored. In such boats, one can watch the ammeter, and see the transition points from forward thrust to zero thrust to regen. Props are now available that are a good compromise between forward thrust and regen.

So understanding how propellers function is useful. Suggesting that combining power of motor and sail has negligible effect on motor speed will not help the OP gain that understanding.

 

Yes I agree that understanding how propellers work is useful. But I doubt you or the OP wish to expend the time or money to get the college degree and professional license to fully understand the physics of the issue. Unfortunately, or not, I have. That is why I say you spend 90% of the time and money to get the last 10% of the answer. You do not appear to be willing to spend the time to properly analyze the issue. I will reiterate that your analysis is simplistic and does not consider all hull forms, all directions of sail, all wind conditions, it does not integrate sail thrust to prop thrust, and does not (just like the slip calculator you referenced) consider wake fraction and thrust deduction. While Gerr's handbook is suitable to allow an amateur to grasp the fundamentals of propeller design where most shortcomings can be fixed by throwing Hp at the problem, it will not suffice for detailed propeller thrust computation, especially where multiple propellers and/or sails are in play. Thought experiments are meaningless to "prove" anything (otherwise Winston Churchill is a carrot); show me the correct physical model and the math.
FWIW, if you have sailed a Catalina 22 then you know that in anything other than a DFC, the best place for the engine is between the V-berths. This is so great an advantage that it is specifically outlawed under the class sailing rules. Perhaps, as you suggested, the OP not motorsail, but learn to sail better.

 

I'm glad to see that you are maintaining your sense of humor.

I have several fluid dynamics and aerodynamics friends who have doctorates from prestigious universities, and I have never heard one of them claim to fully understand the physics involved in even something as simple as an airplane wing in flight. I remember my old high school physics book still had the "equal transit time" explanation, which as you know, is not even close to being correct.

However, I spent good part of my career teaching airline pilots, and many of them were probably better pilots than I was, and many of them thought that the equal transit time theory was applicable. And believing that didn't matter at all regarding their ability to fly the plane. If all I wanted to do was impress them with my ability to use arcane terms or run a CFD program, I might have challenged them to explain how a Pitts Special with symmetrical wing sections could possibly fly. But there would be no point in doing so. My job was to train them more quickly and effectively than my competitors, and wasting their time with stuff they do not need to know would be counterproductive for both me and them.

It seems unlikely that the OP is still around or engaged. But there will be others who will find this thread; for their sake, I would like to clear the air -- and if nothing else, to strongly suggest that they do their own experimenting, and to not rely on either you or me. I have done this experimenting, and it indicates 1. That Gerr's book is excellent and sufficiently complete for 99% of yacht designers and boat owners, and 2. that your numbers are incorrect. Anyone who motorsails frequently (perhaps especially those driving condo cats through tacks) will agree with me that going going faster than the speed supported by wind alone requires engine power, and that the rpm difference between a small amount of additional power and a lot of additional power is large, not incredibly small as you suggest.

If I suggested that "the OP not motorsail, but learn to sail better," I apologize to him. I cannot find that suggestion in anything I posted. I am not in the habit of insulting people. He might very well be a better sailor than I am. He may also be Roger MacGregor, trolling for tedious pedantic twits like me (or you), who might claim that 8 knots is a ludicrously high speed for a small motorsailer. I owned a MacGregor 26 (by far the most popular small motorsailer in the world) for a few years when I was rebuilding a home on an island in the PNW. Fact is, it would do 12-13 knots with full water ballast and closer to 20 knots without ballast. Because I was using the boat like an SUV, hauling building materials, and not usually leisure sailing, the high speeds were often very handy. When sailing with the wind dying and a schedule to meet, adding a little power (requiring only very low rpm) could, by moving the apparent wind forward and increasing its velocity, both increase the boat's speed and also stabilize it, making things much more comfortable on my passengers than the rock and roll of slatting along.

It is vaguely fun to bicker with you, but you seem to be confused regarding making extraordinary claims and the burden of proof for supporting those claims. Your claim that engine rpm will not vary more than 2-3 percent is really very extraordinary. Your ad hominem claim of having had some education does not change that fact.

So, show me the math, adding any complexities that you feel are necessary for the OPs understanding. You might want to send the OP a copy of your CFD program, just so you are on the same page when you begin with discussion of fluid dynamics. So far, the only "math" you have presented is your conjecture that the changes in propeller speeds with changes in thrust are 2-3 percent or less. Show us the math. If you want to demonstrate to those doubters that even a very small boat can go 8 knots, then use the data from a MacGregor 19 with a Honda 40 for your modelling.

I'm sorry you have such disdain for thought experiments. I start every design project with thought experiments, and find it helpful. Most physicists find them very helpful. But if you have a disdain for thinking, then post your data from the on-the -water tests I suggested you do. They will only take ten minutes, and will give you hard, cold facts.

Incidentally, my practical experience flying aerobatics conflicts with your hand wringing in your post I quoted before. In flying a loop, one dives vertically not long after going over the top (and, in my low-powered plane, one would also have to dive steeply to gain sufficient speed to begin for the maneuver). To prevent overspeed, one simply reduces the throttle setting, and the engine slows by not 2 or 3 percent, but dramatically: 30, 40, 50 percent slower, depending upon throttle setting. There is not an almost perfectly fixed relationship between airspeed (or Vb) and propeller rpm.

You have the perfect equipment to prove your point conclusively. Sorry to suggest that you haul your motor out of your cabin, but doing my experiment will help you understand how this stuff works in the real world. You may be one of those folks with a 2 hp Honda, so you will have to adjust my figures above, regarding pitch and rpm, and getting to 5 knots may be difficult, but you can try 4 knots: the same principles apply. If you don't have a ready means for recording your rpm, just record the sound, and I can help you translate that into rpm.

Sorry to hear, BTW, that you think that your education was in some way unfortunate. Many people find education to be an excellent investment of time and money. I still take college physics courses, even after all these years, just for fun.

I await your math, and the results of your on-water experiments.

 

But as you pointed out, I'm retired, it's not my job, and I don't want to waste anyone's time.