Engine / Speed / Revs / Fuel Burn

[alenka]

I have always felt that our 43DS is a little under prop'ed.  It takes around 2,700 rpm to achieve 7kts using a flexifold feathering prop.

I was even once told by another owner that he was getting 8kts from his smaller 54hp engine at just 1,800 rpm (fixed prop). But when he finished the sentence with  'the boat goes very nicely once it is on the plane'  I tended to question some of the wisdom of his comments.

However, having recently found the owners manual for the flexifold prop and become aware that the pitch in both forward and reverse can be adjusted I am now really starting to believe its current settings are not at the optimum. Before I start tweaking it would be nice to know what others are achieving.... Don't try and fix something that isn't broken is very much in my mind.

Anyway...

43 DS (standard keel) Yanmar 75hp Turbo engine and flexifold prop. Cruising at different speeds and using the engine hours recorded on the taco always work out at around the same fuel burn of 2,7 lts/h.

At the moment I work to the following. Speeds are GPS in calm conditions.

2,000 rpm  - 5kts
2,300 rpm  - 6kts
2,700 rpm  - 7kts


Accepting that 8.2kts is the theoretical hull speed for the boat and that the engine needs to be set above 2,500 rpm for the turbo to properly kick in and prevent sooting an engineer friend seems to think these figures are good.  But as the engine can be taken up to a very uncomfortable 3,500 plus revs I am not so sure.

Am I expecting too much to hit 8kts whilst keeping revs around 2,700?

[rene460]

Hi Alenka,

The engine is selected to provide enough power to drive the boat at a reasonable speed, usually limited by wave making to something near that "theoretical" hull speed. The engine provides its maximum power at full throttle and at a rated rotational speed that you can find in the engine specifications.

At reduced throttle opening, the engine produces less power, and if it could drive the hull at hull speed at lower rpm, the manufacturer would have selected a smaller engine if one is available.

It sounds like your engine has a rated speed of 3600 rpm, and if at wide open throttle, you get about 3700 rpm, then your propellor is about right. Then, at 2700 rpm a typical engine will produce considerably less power, the propellor will require less torque, and provide less thrust. You may be able to find it in the engine book, particularly if there is a performance curve provided somewhere. If your engine easily reaches that 3700 rpm, it may indicate that a bit more pitch on the propellor would be useful. (I would not push it to see the maximum engine rpm you can get at wide open throttle, just enough to get 5 to 10% above the rated maximum speed.)

So I would suggest that expecting to get hull speed at 2700 rpm is quite optimistic.

The speeds and rpm you have recorded are very useful however. If you find you can't get that 3700 rpm then most likely the propellor is fouled, increasing the load on the engine, or possibly the engine needs attention.

If you find the engine revs needed to reach those speeds, or the hull is slower at those rpm figures, then the hull is fouled. This will affect your sailing speed in any given conditions. You have to decide how much speed reduction you will tolerate before cleaning the hull.

Rene460

[alenka]

Hi Rene,

Thanks for your comments.

Yes I can certainly get well over 3,500 rpm but it is not comfortable at such high revs and I would certainly not run it for any duration at this setting just to get 8kts.

The standard engine fit for the 43DS was just 54hp and the 75hp version was an upgrade, presumably to give an extra reserve of power to drive through bigger seas if needed.

My thoughts are if 54hp 'can' get the boat to 8kts, then running the 75hp engine at lower revs should produce a similar power output to the smaller engine - Hence less revs required by the 75.

Both the hull and prop are kept clean and the engine has had expert attention all of its life.

I suppose the big question is... Does the 54hp engine really get the boat to 8kts and at the low revs claimed by some.

Would anyone out there with the 54hp configuration like to confirm their experiences please?

[vasko]

There is s very easy way to undestand if a boat over or under or exact proped :

If you get in dead calm water at full throttle and clean hull a bit less then the max revs stated by the engine manufacturer then you are OK if you can get max revs or over you are underpropped, if you cannot get close to max revs you are overpropped

Persinally I prefere overpropped - currently I go to 7.5kt+ at full throttle and cruising at 6 with 2200 - my stated max revs are 3100 but I can get max almost to 3000. - my boat is 34ft with 2000h on the clock - engine is 1990 - volvo 2003 - 28HP the perfect prop is 16x12 - I overpropped to 17x13

[hoppy]

FWIW, Jessabbe with the non turboed version of that engine would do

RPM Speed (kn) Consumption (l/h)
1800 5.1 1.7
1900 5.7 2.0
2000 6.0 2.3
2100 6.4 2.6
2200 6.5 3.0
2300 6.5 3.4
2400 6.8 3.6
2500 7.1 3.9
2700 7.4 4.6
3000 7.9 6.0


Speed based on calm test and consumption from the manual

I would imagine that you should be able to get similar figures with the turbo compensating for the extra size to push around.

Do you know the specs of your prop? Could be interesting to hear what they would recommend for your engine/gearbox combo now.

[alenka]

Will dig deeper into the manuals to see what the max revs are stated for the engine.  Prop details will have to wait until next year when I am back on board.

Thanks

[puravida35]

Nov 12, 2018 17:03:20 GMT alenka said:

However, having recently found the owners manual for the flexifold prop and become aware that the pitch in both forward and reverse can be adjusted...

I didn’t think Flexifold props had adjustable pitch. I thought you had to buy/install different blades to get different pitch with Flexifold?

[rene460]

Hi Alenka,

I suspect that when two engine options are offered, it means the predicted power requirement lies somewhere between the available sizes. The owner is then able to choose the lighter one if the main interest is racing, or the more powerful one for the cruiser, who may want to make best speed for a long distance (though at the cost of fuel consumption) or more likely needs maximum power to pull off a Lee shore against wind and high seas. In this case the larger engine would not normally need to run at maximum speed for cruising.

I am interested in why you feel the engine is not comfortable at high rpm. Diesels generally are made to run continuously at high load, and there are normally more problems caused by running at low load too much of the time.

When you look out the specs, you will find a confusing array of figures. Look for a rated speed, and also a separate slightly lower maximum continuous speed. The manual for my engine even includes different figures for the gearbox.  Obviously the lower one must govern.

Running at maximum rpm will not be good for fuel consumption, and will probably feel uncomfortably noisy, but there should not be any excessive vibration. It should run happily at maximum continuous speed all day without issue. On the other hand, too slow causes several problems with a diesel, and should be balanced by regular periods of full throttle operation.

That said, most of us prefer to back off from the maximum continuous speed for reasons of fuel consumption, noise and a (usually mistaken) idea that it is "easier" on the engine, almost certainly from our understanding of an automobile engine.

Rene460

[alenka]

Rene460,

You summed it up pretty well.  Anything above 2,500 RPM is tiresome after a while.  On really long legs it would be nice to manage 7+ kts without being mentally worn out by noise.

I guess what prompted me to create this thread are comments from those claiming 8kts is achievable very *easily; And in some cases at 1800 RPM. From comments clearly not all engines are designed to run at the same max RPM - 1800 RPM could be close to max throttle on different engines.  I will start digging into the specs. 

Puravida35,

I didn't realise this myself until I spotted it in the manual that there is pitch adjustment stop for both forward and reverse.


* One of my crew has pointed out to me that there is sometimes an element of wishful thinking when it comes to claimed speeds. This summer we ourselves had a guest who took great delight is reading out the paddle wheel speed even though he was constantly reminded that it was in error by 1.5kts above GPS.

[hoppy]

Alenka,

Having stripped a flexofold 3-blade down and reinstalled it, I can tell you that there is no way of adjusting the pitch other than by replacing the blades.

Perhaps you have some old outdated model or you are misunderstanding the manual? The 3 blade model Jessabbe has can't be adjusted.

[alenka]

opps... School boy error.

It's a Variprop!!

[hoppy]

Nov 14, 2018 15:24:55 GMT alenka said:

opps... School boy error.

It's a Variprop!!

Makes more sense...

Increase the prop's pitch.

My understanding is that the Pitch number represents the number of inches the prop moves forward in one full rotation. So you can in theory calculate the pitch adjustment you need to make to get your desired speed/rpm

[NZL50505]

Don't know if it helps but my 42DS (not 43) with normally aspirated 54hp engine can reach 8.3 kts in flat water at WOT (which is rated as 3,000 but I can get 3,200). If I pull back to 2,400-2,600 then I am in the mid-high 7s depending on conditions. Then around 2,000 I'm in the 6s.

Volvo 3B folding prop (which by the way is nuclear in forward but total rubbish in reverse).

[NZL50505]

Just noticed this earlier post - which is a good reflection of my post just a few moments ago i.e. confirmation but repetition!

Nov 13, 2018 8:49:05 GMT hoppy said:

FWIW, Jessabbe with the non turboed version of that engine would do

RPM Speed (kn) Consumption (l/h)
1800 5.1 1.7
1900 5.7 2.0
2000 6.0 2.3
2100 6.4 2.6
2200 6.5 3.0
2300 6.5 3.4
2400 6.8 3.6
2500 7.1 3.9
2700 7.4 4.6
3000 7.9 6.0


Speed based on calm test and consumption from the manual

I would imagine that you should be able to get similar figures with the turbo compensating for the extra size to push around.

Do you know the specs of your prop? Could be interesting to hear what they would recommend for your engine/gearbox combo now.

[rene460]

Hi Hoppy, for a different point of view on how a propellor works, personally I don’t find it very useful to think of the pitch as the distance the propellor moves foreword in one revolution. The propellor is fixed relative to the hull by the thrust bearing, so that would imply the distance the hull moves foreword for each revolution of the propellor. Mind you, it is probably the most common written explanation, included in almost every book or article on the topic. It is often quoted in conjunction with a “slip factor”, a fudge factor which “corrects” for the difference between the calculation and observation. We all grew up on this explanation. But it is very easily demonstrated as not helpful.

If you observe the boat, and hence propellor motion, in the pen when you run the engine in gear with the mooring ropes still attached, so zero hull velocity, and the boat motion when motor sailing on a good broad reach at the same engine rpm and then at normal speed in calm water at the same rpm, you can see the slip factor can have any value you care to give it, so does not give much in the way of useful information.

A more useful concept, is to describe it as the distance the water is moved back with each revolution of the propellor. The difference is that the water can and does move back relative to the propellor, regardless of whether the hull can move. We have all observed this. And the mass of water moved times the velocity change of the water gives change of momentum of the water, which can be used to calculate the force on the hull (through the thrust bearing). The hull then accelerates through the water in response to the thrust until the drag equals the thrust, after which it continues at uniform velocity while the engine runs at the same speed. (With the mooring ropes on, the calculation gives the tension in the mooring ropes.) This explanation is a direct consequence of Newton’s laws of motion, and the conservation laws of physics.

Of course, a fudge factor, or efficiency, is still required to account for the difference between the mass of water actually accelerated by the propellor compared with the amount calculated using pitch, diameter and rpm. The biggest factor determining this efficiency is possibly the blade area compared with the area of a circle the same diameter as the propellor, though the blade shape, thickness profile and surface roughness also contribute to some degree. As in all energy transfer, friction and other losses due to turbulence and the viscosity of water means we never get 100% efficiency.

But if we think of a propellor as a thrust producing device, and the hull moving in response to that thrust, we are much closer to an explanation that follows the laws of physics. I find it a better explanation for propellor action.

From this understanding, we can say that increasing the pitch will provide more thrust at any given rpm, which will make the hull go faster in line with its drag-speed curve. It will also load up the engine which is not a bad thing if it can run above its rated maximum speed at the current setting. If the engine is really a little bigger than required for the hull, this increased pitch will help load up the engine more at lower rpm, when a propellor set for maximum rpm generally does not match the engine so well.

Rene460

[hoppy]

Nov 14, 2018 22:04:03 GMT hoppy said:

So you can in theory calculate

Like I said, in theory you can calculate it. Obviously it's a very complicated calculation process. BTW you forgot the affect of temperature on water density.



However, with the known pitch, engine rpm at recorded speeds, you can come up with a fudge factor that covers all water issues and even the gearbox ratio and then apply the fudge factor to come up with a new pitch to try. Probably no a good idea to make a pitch adjustment that will give 8knts at 2000 rpm

[windspeak]

Anecdotaly I can not resits I fitted a four blade Kiwi prop on my SO DS 43 with my 75 Yanma turbo and obtained 12 knots in moderately flat water
Mind you I was going round Portland at the time seasonal greetings Jollybob

[zaphod]

I would suggest you contact the prop manufacturer and tell them what engine and transmission you have. They should then be able to tell you exactly what pitch to set the prop to. That would be better than the trial and error method.

[alenka]

Okay folks. Thanks. Much good info here.

Clearly, the 54hp engine is capable of getting the 43DS to its hull speed of 8 kts, so the 20 extra hp of the turbo should (in my mind) be able to get the boat to 8 its with rpm in reserve.

Over the next few days I will check rpm/power output of the 75 and try and work out at what rpm equates to 54hp.

Zaphod,

Good call. I think it wise to talk to Variprop and see what they recommend as a starting point.  I do wonder if the prop was taken out of the box and just fitted at the factory default setting.  Far too often I have seen, so called, professionals just go for the easy option.

Windspeak,

I have heard good and bad things about the Kiwiprop. The four blade option, capable of handling 75hp, was not available when my boat was as re-propped.

However, 12kts? Really?? Even a 60 footer would not have a THS close to this.  To get a boat 50% above its hull speed, as this would be for the 43DS, it would take an enormous amount of power. Drag is a square root law and there comes a point where no matter how much HP you apply the increase in speed is minimal. 

[windspeak]

Try Portland race Jollybob

[rene460]

Even without the local knowledge, the mention of a location had to have special significance with that speed.  Presumably a current of five knots or more.

Skin drag varies roughly as speed squared over a limited range, and close enough for small differences in speed.  At very low speed it is nearer linear.  But at higher speed wave making becomes more significant.  The hull speed is approximately where the “hull catches up” with its own bow wave, and has to climb onto it to go faster.  This is the reason for the big increase in power required to get the boat up on the plane.  The speed at which this occurs is not only dependent on length of the hull but also on the shape, so it’s not a very precise number.  It comes from Froudes law and is useful enough to help us understand what is going on and to scale model behaviour up to full size.

OK, OK, we don’t all need that much detail.  I just thought I would throw it in.  Motion of a boat over the water is complex, but interesting.

rene460

[sailbleu]

Nov 13, 2018 21:04:04 GMT rene460 said:

Hi Alenka,

I suspect that when two engine options are offered, it means the predicted power requirement lies somewhere between the available sizes. The owner is then able to choose the lighter one if the main interest is racing, or the more powerful one for the cruiser, who may want to make best speed for a long distance (though at the cost of fuel consumption) or more likely needs maximum power to pull off a Lee shore against wind and high seas. In this case the larger engine would not normally need to run at maximum speed for cruising.

I am interested in why you feel the engine is not comfortable at high rpm. Diesels generally are made to run continuously at high load, and there are normally more problems caused by running at low load too much of the time.

When you look out the specs, you will find a confusing array of figures. Look for a rated speed, and also a separate slightly lower maximum continuous speed. The manual for my engine even includes different figures for the gearbox.  Obviously the lower one must govern.

Running at maximum rpm will not be good for fuel consumption, and will probably feel uncomfortably noisy, but there should not be any excessive vibration. It should run happily at maximum continuous speed all day without issue. On the other hand, too slow causes several problems with a diesel, and should be balanced by regular periods of full throttle operation.

That said, most of us prefer to back off from the maximum continuous speed for reasons of fuel consumption, noise and a (usually mistaken) idea that it is "easier" on the engine, almost certainly from our understanding of an automobile engine.

Rene460

Maybe slightly OT , yet important to all of us , please trow in your insights on the red statements Rene .
It's just that , the way I see it, a slow rev engine will go on and on and on .It has always been the wisdom that slow rev engines last longer .
I just can get rid of the thought that a high rev wears down a lot faster than slow rev.
I never accede 2000 revs , try to keep cruising at 1500 -1600 ( preferably somewhat lower) and that will get my 50HP & 3 blade prop to do 5kts , calm sea no head-wind or current.However , the last few years I detected some change and loss in that rev-speed balance .
I'm aware of the issue of glazing cylinders at low revs and rubbish piling up in the exhaust but would very much like to know more about the accuracy of the overall proclaimed advise to make the engine ' sweat ' now and then .
I take my exhaust elbow off with intervals ,clear the raw water holes inside , and free the engine's exit of a solidified pile of soot that in my view can be caused by fuel injectors that have lost quite some functionality .
Before leaving the boat in September I took out the injectors and overhauled them in my workshop
After pressure testing them first I found out some were leaking and several nozzle-holes of all 4 of them were clogged and covered with crystallized oil or diesel ( took everything apart and ultrasone-cleaned them ) I even needed to change the shim's inside due to bad spray-patterns so they could get back to the prescribed 225 bar .They are now bagged and tagged and ready to accompany us to the boat at the end of the month .
Having seen the state they were in I'm sure it will have an effect on the engine's performance , and most likely also the rev-speed ratio .

Regards

[rene460]

Hi Sailbleu, I am glad that you are looking in, I always admire your practical approach to these matters. I for one don’t feel qualified to service my own Diesel engine and admire you for the way you just get on with it.

My career experience is in the heavy industrial end of the engine range. I was involved in specifying, selecting buying and commissioning them for very specific purposes, and had many opportunities to talk with the manufacturers in putting together commissioning procedures and using the relevant parts of the manuals to write the plant operating instructions. Always brand new. So not directly associated with the small marine diesels in our boats. However, I put this background along side the manual from Yanmar for the engine in my boat and try to draw some useful conclusions.  So yes, could be over the top.

Yanmar is very clear on the need to run the engine hard enough to get it properly hot, to prevent carbon build up. If you have a turbocharged engine it has additional requirements. Certainly cylinder glazing is often quoted, but the carbon build up you describe is quite consistent with reasoning suggested in the Yanmar recommendations. And I suggest it is also consistent with the reduced engine power that your performance monitoring suggests. Unfortunately I do not have the engine mechanics experience of seeing the state of many engines to say definitely. It just appears to me to be a consistent story.

Diesel engines for marine and generator service are designed to run fully loaded for long periods. The bearings etc are properly designed for the continuous rated power and speed. Industrial engine are usually recommended to be run at least 80% load, at least 80% of the time. Most of us do not feel able to do the advanced servicing that you do, and I don’t really know if you save much wear and tear on an engine designed to run hard.

Most of us started with cars that used to need gentle running in when bought new. Their ratings for continuous running are well below the maximums the salesmen like to sprout. And we are normally served well by treating them gently, even though they no longer need that running in. Besides the normal car running cycle involves widely varying speed, and the odd occasions we actually push them hard, are usually followed by a slower period which allows any excess heat to dissipate. Only on the race track are they really pushed to the limit, then they receive much more servicing than most of us want to pay for. Looking after cars by not pushing them too hard generally pays off, though short trips where the engine never gets hot, the stereotype short shopping trips and church on Sundays is well known as a problem there as well. So a very different running cycle from a marine engine which may be expected to push the hull at reasonable speed for long periods without slowing, so has to be properly rated for this. Similarly a generator drive, where the electrical frequency is determined by the engine speed within very close limits and load can be quite steady.

Then there are the race sailors and a lot of cruisers, speed restrictions in channels to get in and out of the marina, then up with the sails and shut down the engine. No chance for the engine to ever warm up.

So it’s a complex subject. Trying to put together sensible running strategies for very different operating cycles. And understanding how the engine are rated and intended to be run is difficult. The more we discuss it and all contribute our experience, the more chance we have of getting to understand how to operate our engines.

Yanmar say if you can’t run fully loaded for a short period every hour, to race the engine in neutral a few times to warm it up before shutting down.

Personally, when I get out of the channel from my marina, I generally run it up to 3600 rpm for a few minutes, the back down to about 2800 if I am motoring any distance. On our lakes that means from 1 to 4 hours. When the wind allows I cut back to just steering ability while I raise sail then shut the engine down. So far the mechanic is not finding anything untoward. I have no experience in looking at engines that have followed this procedure, and comparing with engines that have not. It’s where we have to all share our experience. Please see this as an attempt at discussion, not a lecture. I hope it clarifies where I am coming from.

Rene460

[sailbleu]

Rene ,

my apologies for the misunderstanding in my first sentence with the " OT " , I meant my Off Topic , not your ' Over The Top ' by that .
Having cleared that , I can tell you that my fetish for saving/sparing the engine at all cost goes as far that I even try to anchor straight out of a sail and also leave the anchorage on sail. But I'm not the only one doing that .
All that is done in conjunction with safety first towards other boats in the bay of course .
It's been said that a cold start equals the wear and tear of 10 warm running hours .
Maybe not that much considering the lifespan of our diesels , but how many times do we approach or leave an anchorage ?!
Imagine a small outboard for just these maneuvers (wind allowing) , docking/undocking included. Have already done some tests with petrol and electric outboard you know . Was thinking of a stern-mount for that , would be a nice project . Silly , I know .

I never heat up my boiler water running the engine on extended anchorage stays .
I use my Eberspacher ( Webasto) in combo with a heat exchanger for that .
Solars are my batteries best friends and in case of emergency I run the genset .
So maybe my fetish is more of an obsession , we all have our own cross to bear .

Thanks again for the detailed explanation .

Regards

[rene460]

Hi Sailbleu, no apologies needed. Now that I think about it a bit more, I think people use OTT (with two T’s) for over the top, so I certainly contributed by misreading it. Never mind, I don’t think of it as very far off topic either, as Alenka’s original question was about an appropriate speed for the engine to be running, though he was concerned about prop pitch settings and reasonable hull speed expectation. The question does seem aimed at optimising speed for quite low rpm,

So, taking my usual approach, born of a career which always required an in depth grasp of equipment that had never run before, I tried to summarise some of the important factors in determining reasonable rpm. Then perhaps we can see a reasonable basis for our conclusions.

I should have mentioned that my little 3YM20 has a rated maximum speed of 3600 rpm and a maximum continuous speed a bit lower. (I am still unclear about which one I should aim for 80% of, but 2900 rpm moves us along quite nicely, and is in the right range. Some of the larger engines seem to be rated for around 3000 rpm, so 80% of that.

The other aspect of the question is how different engine rpm affects the load the propellor puts on the engine. This also introduces the topic of what the engine throttle lever actually does. What happens to the load on the engine as we reduce engine speed? That is also important to selecting reasonable running speed.

As for saving on cold starts, again, I am in total agreement. Your practice develops incredible skill. I need a bit more courage to try it, and practice, out in open water where there is nothing to hit if I misjudge it. And nothing I have ever seen recommends running the engine at low load to charge batteries or heat water. It works if needs must, but I also feel it should be avoided where possible. I am with you 100% on the little separate water heater, and solar, and if really necessary a little portable generator for when the sun just will not cooperate.

Also as one who has sailed trailer sailors for over thirty years, I don’t think having a little outboard on the back is silly at all. The brackets are readily available, so it is mostly about devising a suitable strong mounting on the transom. Room for lots of ingenuity there. You might construct a track or a linkage to lift it up to the rail when not being used, and preferably it could fill a dual purpose as the dingy engine, so not even be extra weight to carry. I often wonder if one of those little electric thrusters, like Vasko uses on his dingy, would be enough. If all else fails, the old fishermen used to move quite heavy boats quite nicely with one oar over the back.

And in any case, having sails as reliable alternative power is ok in the text books, but is of limited use in confined situations, a little outboard, say 9 or 15 hp, would be very useful in most circumstances. But that is getting off topic.

Perhaps we had better let Alenka tell us if we are getting close to the information he needs. We can always start a new thread on other interesting topics.

Rene460