Boat vibrating when free spinning

Very few people sailing more than 20-30 miles will let a prop spin freely. Try it and you’ll work out why.
Fuel efficiency and drag are not the highest points on the list for cruising sailors, so feathering often wins because it’s the best cruising prop. Folding wins for racers for different reasons.
 
Laminar Flow said:
Having been outed by Tranona as a complete inconnu on propellers and probably everything else boaty, I decided to got back to the source.

It turns out that the German claims as to resistance reduction by letting the prop mill were waaay to conservative. Surprising that, who would have thought that Germans engineers would be so conservative, really?
According to "Principles of Yacht Design", written by a couple of Swedes this time and arguably the contemporary bible of yacht design, the reduction in drag by letting the prop mill is to the order of 75%.
For anyone further interested, they offer pertinent mathematical formulas to calculate prop drag at any desired speed. To note: the constant for a fixed blade locked prop is 1.2 for one left to spin it's 0.3, i.e. 25%

The book is available in English, in case anyone not able to understand Swedish or German needs to verify the above.

To put that into context, if your fixed & locked prop were to reduce your boat speed by 0.6 kn (as per my previous, "strange calculation"), then letting the prop spin would only reduce your speed by a mere 0.15 kn. To be sure and as the book clearly states, this is in relationship to not having any prop at all.

It certainly demonstrates the desirability of letting the prop spin freely. No doubt, it also further puts into the question the financial aspect of the matter, at least for the non racer. Of course this information was published by a couple of Swedes; clearly high on Aquavit.

Over time and on numerous occasions, I have repeatedly pointed out certain non resistance related benefits of folding/feathering props, notably their contribution to improving weatherhelm and steering control. So much to my "fixed" mind.

On prop efficiency and fuel consumption: Some people seem to have a hard time thinking outside of the box. Yachts are simply another form of water craft and only differentiate themselves from the rest by the fact that they are quite useless for most things other than sheer pleasure. Beyond that they share the same physics and yes, means of propulsion. Not every advancement in propeller design was done for Golden Hinds or even your average power yacht, but much of what was done elsewhere has found it's way into the realm of the less useful.
Fuel efficiency may not be a top priority to the average sailor, however, Voile magazine sure made a point about it in it's prop test, listing range per litre at various RPM and litres per hour averages for each particular prop. As you see, my statements are not as far-fetched as you like to make out.
Bruton came out ahead in this evaluation, alas it also has among the highest tow resistance. But, hey, Voile did a test on that too ...
Click to expand...
Despite all that waffle you still ignore the data that suggests otherwise. Perhaps it is your over reliance on research that is only marginally related to small yachts. You do not seem to have any direct experience of propellers in use unlike many on here who can tell you of direct experience. Have you ever owned a saildrive boat for example? you don't seem to know much about them.

Not sure why you keep on about prop design and fuel consumption on commercial vessels when this thread is about changes in drag of different propellers in different modes where it is clear that a folding or feathering prop when folded /feathered does not add any drag to the hull whereas a fixed blade does whether left spinning or locked.

The OP asked a specific question and has received some informed suggestions about a solution from people who have experienced the same problem, supported by third party data and detailed explanations of why the suggested solutions have other benefits.
 
lustyd said:
Very few people sailing more than 20-30 miles will let a prop spin freely. Try it and you’ll work out why.
Fuel efficiency and drag are not the highest points on the list for cruising sailors, so feathering often wins because it’s the best cruising prop. Folding wins for racers for different reasons.
Click to expand...
Folding propellers are not just for racing, although racers were the first to adopt them. However the early ones were very poor for motoring and gained a reputation for that. However over the last 30 years or so there have been significant developments and as the YM tests show, the best are comparable with both fixed and feathering propellers from a motoring point of view. They dominate the marker including for cruising boats that have the space to fit them. They are generally cheaper than feathering, less complex and need less maintenance. Making a feathering propeller work is more of a challenge, both mechanically and in blade design, but there are advantages, particularly in being able to pitch differently in forward and reverse to cope with gearboxes like Yanmar and TMC which have different reduction ratios on some models.

I have had both types on cruising boats and would prefer folding, particularly on smaller power saildrives where there really is no additional benefit of a feathering type unless maybe if you plan a lot of motorsailing. If however you have an older type boat where a folder will not fit there are significant advantages in a feathering propeller as I explained earlier.
 
I’ve used folding too and personally prefer the ability to reverse with a feathering prop. Folders certainly have their place but for extended cruising I’ll go feather every time. Either way, I’d never want a fixed prop on a cruising boat!
 
My Bruntons two blade feathering prop has been excellent, has done exactly what it was supposed to do for more than 25 years. It does rotate when sailing though, making unwanted noise, and needs to be locked with the gear in reverse.

Seems very efficient: over the period of ownership we have averaged 1.4 litres of fuel per hour, Sadler 34 (actually almost 35 ft, well loaded.)
 
A few things to unpack here.

lustyd said:
Very few people sailing more than 20-30 miles will let a prop spin freely. Try it and you’ll work out why.
Click to expand...
Whether a prop can be left to spin, depends on the gearbox and should be checked with the manufacturer before you attempt it.
That said, I have and we let the prop spin. We regularly make passages over 200 miles and have to date done some 20.000 miles on this boat and gear box. Our shaft runs smoothly and without undue noise, banging or vibrations. On another boat, I let the prop spin for over 7000 miles across the Pacific and back.

The idea that something could be dragged through the water with zero resistance is ludicrous. If that were the case, we would be travelling to New York by submarine, rather than by air.

So lets see what a prop manufacturer has to say about their product.
A fixed pitch prop has a very narrow range of efficiency. This makes the whole concept of "motorsailing" a rather ineffective means of propulsion, unless you have a variable pitch prop. For this reason I was quite interested in the possibility of a Brunton. I no longer am. Instead, I substantially increased the SA and now enjoy rather good light air performance on out fat, little tub instead.

These are the claims made by Brunton and can be easily verified on their site:
A Brunton prop will improve speed by up to 0.8 kn over a fixed and locked standard prop, while reducing prop resistance by 85%.

Firstly, without any reference to relative speed or overall and relative resistance, the 0.8 kn statement of reduction is absolutely worthless.
Secondly, the fact that this prop only reduces resistance by 85% shows that the claims of zero resistance are nonsense. It rather supports the calculations by the German sources I quoted and that show that the speed gains for feathering props are not all that significant.

If we presume that the data and calculation methods published by Larsson/Eliasson are correct and I have no reason to doubt this, then the drag reduction of this particular prop over a free spinning one is a mere 10% (75% versus 85%). When applied to a resistance value in correlation to overall resistance at a determined relative speed, as I did in a previous post in this thread, the factual and actual speed gain over a spinning prop would be in the order of 10% of 0.6 kn or 0.06 kn.

This kind of performance margin is easily made up by a skilled sailor such as Flaming or by one of the gentlemen of his acquaintance, for example.

This does by no means detract from the other and self-pitching benefits of a Brunton prop.

I have nothing against feathering or folding props, but I like to know the real terms on return of investment, when I rob the kid's college or my retirement funds.

And, thanks for the gaslighting.
 
Laminar Flow said:
A few things to unpack here.


Whether a prop can be left to spin, depends on the gearbox and should be checked with the manufacturer before you attempt it.
That said, I have and we let the prop spin. We regularly make passages over 200 miles and have to date done some 20.000 miles on this boat and gear box. Our shaft runs smoothly and without undue noise, banging or vibrations. On another boat, I let the prop spin for over 7000 miles across the Pacific and back.

The idea that something could be dragged through the water with zero resistance is ludicrous. If that were the case, we would be travelling to New York by submarine, rather than by air.

So lets see what a prop manufacturer has to say about their product.
A fixed pitch prop has a very narrow range of efficiency. This makes the whole concept of "motorsailing" a rather ineffective means of propulsion, unless you have a variable pitch prop. For this reason I was quite interested in the possibility of a Brunton. I no longer am. Instead, I substantially increased the SA and now enjoy rather good light air performance on out fat, little tub instead.

These are the claims made by Brunton and can be easily verified on their site:
A Brunton prop will improve speed by up to 0.8 kn over a fixed and locked standard prop, while reducing prop resistance by 85%.

Firstly, without any reference to relative speed or overall and relative resistance, the 0.8 kn statement of reduction is absolutely worthless.
Secondly, the fact that this prop only reduces resistance by 85% shows that the claims of zero resistance are nonsense. It rather supports the calculations by the German sources I quoted and that show that the speed gains for feathering props are not all that significant.

If we presume that the data and calculation methods published by Larsson/Eliasson are correct and I have no reason to doubt this, then the drag reduction of this particular prop over a free spinning one is a mere 10% (75% versus 85%). When applied to a resistance value in correlation to overall resistance at a determined relative speed, as I did in a previous post in this thread, the factual and actual speed gain over a spinning prop would be in the order of 10% of 0.6 kn or 0.06 kn.

This kind of performance margin is easily made up by a skilled sailor such as Flaming or by one of the gentlemen of his acquaintance, for example.

This does by no means detract from the other and self-pitching benefits of a Brunton prop.

I have nothing against feathering or folding props, but I like to know the real terms on return of investment, when I rob the kid's college or my retirement funds.

And, thanks for the gaslighting.
Click to expand...
Once again you are being highly selective in trying to avoid the issues. Your second sentence is misleading. Nobody is claiming that these propellers offer no resistance. Please read what has been said - most in their closed form do not add any resistance to the boat. Once again see the graph in the YM article which confirms this both in quoting independent tests on saildrives and their own tests which measured the CHANGE in resistance on a towed boat with a sample of the propellers they tested..

The Bruntons claim, if a little bit clumsy is saying that feathering the blades reduces resistance from the propeller itself by 85% - seems pretty reasonable given the relative sizes of the hub and blades. The claim of increasing boats speed by 0.8 knots without any qualification is indeed questionable, and a claim not made by other manufacturers.

Whatever the improvement in boat speed under sail turns out to be it does not need the skill of a racing skipper like flaming to achieve it. It is there all the time. You may recall he said that a less than expert skipper outsailed both him and other racing skippers in identical boats because the boat did not have a propeller. In the same way ratings penalise folding propellers because of the known speed advantages over fixed blade - spinning or not.

Unsure why you ignore all this empirical evidence.
 
Last edited:
Tranona said:
Unsure why you ignore all this empirical evidence.
Click to expand...
Empirical data is based on simple observation and nothing else. Per definition, simple observation is highly subjective, biased and unreliable.

Conclusions based on such are not always repeatable nor quantifiable, are not based in logic or scientific theory and should be viewed with extreme caution.

For example: The road is wet. It must be raining. Obviously a provable observation, however the conclusion may not at all be correct. Correlation is not causation.

Flat-earth-ism, by the way, is entirely empirical in it’s belief and proof. Much of the public lore around sailing boats is as well.
 
Laminar Flow said:
Empirical data is based on simple observation and nothing else. Per definition, simple observation is highly subjective, biased and unreliable.

Conclusions based on such are not always repeatable nor quantifiable, are not based in logic or scientific theory and should be viewed with extreme caution.

For example: The road is wet. It must be raining. Obviously a provable observation, however the conclusion may not at all be correct. Correlation is not causation.

Flat-earth-ism, by the way, is entirely empirical in it’s belief and proof. Much of the public lore around sailing boats is as well.
Click to expand...
I am really pleased you have posted this as it confirms what I have suspected while reading your contributions here. You don't seem to have much understanding of the scientific process and what constitutes "good" science.

You definition of empirical is partial and you seem to have chosen this part of it to trash its importance - just as you have been selective in your choice of material to support your arguments earlier. The full and accepted definition is

"information collected through observation, experience or experimentation as opposed to being based on theory or conjecture. It is OBJECTIVE data that can be tested and verified through direct evidence and serves as the foundation for scientific inquiry to support or refute hypotheses"

In my working life I could almost recite that in my sleep as it was the foundation of the Research Methods I taught to Masters and Doctoral students. Carrying out research based on that principle results in greater knowledge and informs decisions.

Your contributions fail to follow this process. You rely heavily on a hypothesis derived from research that is not directly relevant with your reference to hull resistance on big ships. I have not bothered to read that as there is much more relevant research available elsewhere. I am familiar with the Delft Systematic Series methodology for calculating resistance on yacht hulls and the impact of resistance on hull speed. It is clear from this that increases in resistance reduces hull speed. Adding a propeller to a hull increases resistance, both when locked or spinning.

We now have our hypothesis - in two parts. First does a folding or feathering propeller add the same amount of resistance to the hull as a fixed blade, and second if the additional resistance is less does this result in increased boat speed.

The first part is relatively easy to test by towing as explained in the YM test. Not sure there is any dispute about that. Fixed blade propellers cause additional drag to the hull which increases with speed in just the same way as hull resistance. Folding and feathering propellers do not in any measurable way, although some designs are marginally worse than others and is independent of speed.

Now the tricky bit - how do we test out this hypothesis? The first thing to sort out is the relationship between the drag of the hull and the drag of the propeller. Hulls come in many different forms which in turn have different drag properties, mainly to do with wetted surface area. So a deep bodied full length keel boat will have more resistance than a flat bottomed fin keel hull. Bigger boats have in general more resistance than smaller. Propeller drag as a proportion of total drag therefore varies as does impact on speed.

All these variables make it very difficult to design any sort of meaningful controlled experiment. The ideal would be a series of test runs of identical boats fitted with different propellers at a range of speeds. Very expensive and time consuming and not aware of anybody having tried this.

So we have to look for other evidence and I have already suggested some. Ratings for racing are a good proxy for for assessing difference in speed for different equipment on a boat as they are tested by comparing times round a racecourse. The penalty for a feathering propeller confirms boats with it are "faster". It would also be possible to carry out spot checks on race boats because of the sophisticated instrumentation they have. flaming's experience of speed loss from a partially open propeller suggests a significant measurable impact.

Other good evidence as I have already shown comes from user experience, particularly passage logs from long distance sailors. Users often report their own experience on forums such as this and again the results are positive, but with varying values for speed improvements depending the boat and conditions. Not surprisingly the greater improvements come from modern low resistance hulls where the drag proportion from the propeller is higher.

Over the last 30 years the popularity of these propellers has grown and it is an endorsement of the claims made. I wandered around the club yard in the sunshine. There are about 60 boats out. 15 have saildrives and all bar one (the Bavaria I showed earlier) are fitted with folding or feathering propellers. Of the 40 or so shaft drive boats that still had their propellers on 12 had non fixed blade mainly Featherstreams. Boats varied from Cornish Shrimper to heavyweight long keelers like Tradewind and Challenger 35.

I would suggest that this level of acceptance would not happen if the products did not do what they claim

As I understand from your postings your case rests on your reading of theory that the reduction in drag that these propellers offer is not enough to give the speed increases claimed or reported. Maybe you could provide some robust empirical evidence (using the correct definition!) to support your position.
 
I have to say, that I am rather enjoying this discourse.

On hull resistance:
In a simplified form, we can divide hull resistance into two parts. Frictional resistance and form resistance.
At speeds up to a relative speed of 0.8 - 0.9, the curves for both frictional resistance and form resistance are fairly linear, flat and congruent.
After this, form resistance rises exponentially and becomes by far the most dominant factor. Of course, frictional resistance continues to rise as well, but near linearly and compared to form resistance it becomes less of a factor. (I did make the point previously why this relative speed is important)

Form resistance is directly linked to displacement, or more precisely, to the length/displacement ratio. Also, surface area increases to the square, whereas volume increases to the cube, which shows that even in a heavier boat, of the same general size, frictional resistance plays a lesser role in the overall picture and particularly at relative speeds over 0.9.
The lighter the vessel, the shallower the curve for form resistance becomes and, conversely, the higher the ultimately attainable speed will be.


The detrimental effect any parasitic drag, be that from non-folding props, exposed shafts or any other underwater protrusion, is directly proportional to the hull's overall resistance, as you have already intimated (also see "Principles of Yacht Design"). This is why lighter and much lighter boats benefit from resistance reduction more and over a wider range than a heavier vessel where the relative influence diminishes towards the higher speeds.

However, up to that magical, relative speed of 0.9 (or close enough), the playing field is more level and the resistance curves more similar for both heavy and light boats and which is why I specifically chose that speed in my initial calculation.

Now, the entire premise for this discussion was the desirability or benefit of letting a fixed prop spin to reduce resistance and whether said reduction was enough to question the viability, financial or otherwise, of switching to a feathering prop.

On empirical knowledge in naval architecture.
A few years ago we were in Karlskrona, Sweden. where they have a very interesting marine museum. In spite of an arts background, I can never manage more than two hours in an art gallery without suffering from information overload. In Karlskrona, I had to be escorted out by the security at closing time. Karlskrona had been the haunt of Fredrik Chapman, a Swedish ship builder in 18th century Sweden and who is credited as having been the first naval architect. Chapman was responsible for how marine craft and their shape have come to be represented and which is still in use to this very day. He was also the first to mark centres of gravity and buoyancy in plans and to calculate ship stability. After the Wasa incident, one might presume that the Swedish authorities had a heightened interest in predetermining the stability of a given project.
What I didn't know, was that Chapman was also amoung the first to apply tank-testing to models. Lacking any particular technology to measure resistance, two models were towed on a yoke and the one drifting further back, lost the competition. In effect, this was a trial and error process of elimination and very much an empirical approach.
However, without knowledge of basic concepts, such as relative speed (Froude), form or wave making resistance (ditto), the effects of prismatic coefficient (Taylor), the information gained from such "empirical" experiments was limited and results were non-predictable, calculable and could not be scaled to full size craft. Only much later and once mathematical models had been developed, could such "empirical" information become predictive and have a formative influence on naval architecture.
(And yes, I am familiar with the Delft modelling)

Before I go on to some "robust empirical" evidence to support my position, a couple of examples from history as to why science matters in naval architecture.

From an early age on, France, unlike the British, treated naval architecture as a science. In fact, it was the French who were the first to do model testing and ages before Froude. There is little doubt that during the Napoleonic era the French built the better ships. They sailed better and faster than British designs, based on obsolete traditions. French-built back then was considered a mark of excellence.

Much fuss was made when "America" beat the cream of British yachting, racing around the Isle of Wright and thus initiated a veritable frenzy in yacht (re)design.
What is rarely mentioned, is that the much (much) smaller British yacht, Aurora (47 tons versus 100 tons), crossed the finish line a mere 18 min later. Had anything been understood about the concept of relative speed, not readily arrived at by empirical observation, it would have been recognized that Aurora was factually and relatively the faster ship. Instead, the empirical take, based on the powers of observation, including those of her Majesty, was that the British fleet was a bunch of lemons, needing flatter sails and hollow bows. What they really needed, were better SA/D ratios, something that is more difficult to achieve as ships get bigger and possibly not a concept easily discovered by simple observation. In fact, SA/D was not defined until mid 20th century.

For my much anticipated, empirical evidence, I shall relate an experience we had last August with our own boat. I am using this because I have accurate data and I have spent a good amount of time analyzing the event and it's outcome.

As many here know, we sail a somewhat pimped 31.5', ketch rigged motorsailer. This boat displaces 8.5 t in full fettle. She sports 770 sqft of sail, giving us a SA/D of about 18 on a 28.25' waterline. D/L is a hefty 360. We swing a three bladed, 18" fixed prop behind a streamlined deadwood and we let it spin freely. Rotation begins between 1.5 kn and 2 kn (come to think of it, I should be able to calculate actual prop resistance with this)

Surprisingly or shockingly, rather, we outsailed a Sun Odyssey 349 over 40 sm between Isle Brehat and Bas Sablons, St. Malo.

The data for the SO 349 are:
Length 34', LWL 31', D 5.35 t, SA 595 sqft, SA/D 18.44, D/L is 175. This boat had a shaft driven, folding/feathering prop. I know this, because I spoke to the owner in Bas Sablons.

Both boats left Lezardrieux at about the same time. We overtook the SO on the river under power and raised our sails as soon as we reached the Eastern channel under Brehat. By the time we cleared the last cardinal markers and were clear of the channel, the SO, about half a mile behind us, had also set it's sails and was beginning to, very slowly, catch up.
AWS was about 13kn, gusting 15, occasionally. Our boat speed was between 6kn and 6.5 kn on an initially close, later beam reach. The sea was pretty lumpy, due to a wind over tide situation; our tub is not overly fond of that kind of action.
Within the hour the OS had overhauled us to leward, as was to be expected, and very gradually pulled ahead.

Over the course of the morning the wind abated to between 8 kn and 10 kn AWS and the sea had settled. Our boat speed was down to around 5 kn and we were starting to catch up with the SO ahead. By the time we were off Cap Frehel, we were abreast of the SO and when we reached the cardinal off the Bas Sablon ferry terminal, they were some 2 miles behind.

The owner of the SO spoke to me when we met at the harbour master's office and wanted to know if we had been running the engine when the wind started to let up. We hadn't. I'm not sure he believed me.


My analysis of the situation is this: Both boats have a similar SA/D, even though the SO has a slight edge. She also has a marginally longer DWL, a feathering/folding prop and, arguably less wetted area.

Theoretically, two boats with a similar SA/D should reach their theoretical hull speed at the same time. At lower speeds, however, when resistance values for same length boats are quite similar, regardless of displacement, the heavier boat has an over abundance of relative sail area to overcome the same/similar resistance.

In the beginning, the SO was able to overtake us, because at 6 kn to 6.5 kn and with the choppy sea conditions our form resistance was already on the up, in as much as we were already at 90% of nominal hull speed and the SO clearly had the edge.

As the sea state abated and the wind died down, we were now in the sweet spot with a relative speed of 0.9 (5kn). Now we had comparatively more power to overcome the higher resistance of our prop and greater wetted area of the long keel, etc.
Ironically, this is also the precise range where a feathering/folding prop should provide the best return on investment.

The SO has a D/L of 175 and should easily outpace us in higher winds, but at lower wind speeds, we clearly had the edge, spinning, fixed prop, long keel and all. On a purely empirical level, my calculation that the speed deficit from our non-folding and spinning prop could be in the 0.06 kn to 0.2 kn range and compared to a feathering, may not be all that far off.
 
The rest.

On a purely mathematical level: the greater the displacement (or form resistance, as it were), the higher the relative speed and given a free spinning prop, the less effective/necessary a folder becomes.

Without wanting to incur any andersonesque connotations, this was not the first time we have experienced this outcome. There have been others, including a Pogo 30 (definitively with a folder), a Bav 32 cruiser, a Hanse 32 (now available on film) and some others I could not readily identify. All in light conditions, when, if one were to give credit to the general, empirical perception, our non-folder should have completely crippled us.

If we are slower, it is not at all noticeable, empirically or otherwise, least not in light conditions or in the context of other displacement types.
 
Can I add to this fascinating discussion that several other researchers have looked at this argument. The original one that I am aware of was a PhD thesis at Edinburgh(?) university. I have found this using Google but not for some years.

Some years after the YM tests a US man known as Maine Sail carried out his own version, using a catamaran raft towed behind another vessel, measuring the drag in a neat way with a setup of lever arms. This may also be found using Google, although he may now go under a different name.
 
You must log in or register to reply here.

Share this page

Top