250HP outboard Speed prediction for heavy boat

[nacaman]

I have been looking at a proposed project for a 14m x 4m landing craft. Although inboards would have advantages, client wants outboards. Loaded boat is around 14 tonnes displacement and they want to go 20 knots. One unit proposed produces 2 x 250 hp outboards at 5500 rpm thru a 2:1 g/box ratio (ie 2750 rpm at the propeller). A 15" diametre propeller has been proposed with anywhere from 11" to 16" pitch (depending on who is asked)

One area I have struggled with is trying to find actual design data from outboard suppliers on what thrust the outboard prop actually produces at different speed or what the prop efficiency might be. It appears the solution proposed is to turn up with a bag of propellers when the boat is in the water and try to get the rpm right, so that the maximum outboard power can be achieved (this might be OK at the other end of the build, but doesnt help much for current issue )

Other than checking there is enough power, my concern is the propeller may be too small for the job and excessively cavitate. However Im not sure where I might look to find actual thrust or other prop design data on the outboards or the outboard props.
Anyone got any suggestions or suggest where to look ?

[Fanie]

Outboards have better power to weight ratio's than inboards.

What does your landing craft looks like ? 14 tonnes is a lot of weight for 20knots.

[Manie B]

You seriously need a NA

14 tonnes displacement and they want to go 20 knots

2 x 250 hp outboards

it just aint gonna happen

somebody is smoking thier socks

[rasorinc]

Look over Aluminum Boat Plans by Specmar, Inc. they show lots of landing craft of all sizes and weights--all out of aluminum. They have some unique craft. Just some info for you. Stan P.S>. Yamaha offers HP high power high
torque models--their 10 HP will outpull a 20-25 HP model of other manufacturers.

[terhohalme]

PropCalc (google Castle Marine) tells 16 1/2" x 11 1/4".

The boat is just in the limit of planing, Froude number 1.0. Use deadrise under 15 degrees and straight lines. Could be possible.

[Fanie]

Women weigh themselves without their clothes on so they weigh less (true) . Is the landing craft painted :D

[Rick Willoughby]

To do a more accurate design you need to provide some detail on the hull shape. As a first approximation the weight to drag on a low speed planning hull is around 8. So at 14t the drag will be a bit under 2t. Allowing for some fouling and protrusions lets say 1t (10kN) per engine. A flat bottom will plane more readily for example.

The attached shows what you can expect from a 15" prop. It will need to be pitched 15 to 16". I would go 15" in the first instance. The efficiency will be 68% and you will require 150kW or 200HP. So the 250s are OK for power.

The concern I would have for this application is the thrust capacity of the outboard leg. Outboard motors of this size are not intended to go this slow near full power so the thrust loading is likely to be well above design. Think about that tiny prop being constantly loaded at 1t. I have heard of heavy duty commercial legs in the past - maybe you need to investigate this.

Rick W

[Rick Willoughby]

I should add that going up in diameter improves efficiency. A 16" prop gets close to 70% and pitch drops to 14".

If you could swing 18" then efficiency 71% and pitch just over 12".

Ultimately it gets down to trial and error to get the best prop for the application because there are many factors involved. As an example you may not be able to trim the boat to design for the given load conditions. Another is that the surface finish may not be fair or is not always clean.

What I have provided should be a good starting point but you really do need to talk to the outboard makers regarding the application. It is certainly heavy duty. Each motor with a 15X15 prop will be capable of producing a peak thrust of 20kN at 13kts providing the leg bearings can transmit the force.

Rick W

[Rick Willoughby]

Here is the Savitsky prediction for the boat in flat bottom version. It is very sensitive to weight distribution according to Savitsky. If the weight is forward it will tend to dig a big trench and will take huge power to get over the hump.

The numbers for the conditions shown are close to my first approximation for drag being weight/8.

Rick W

[nacaman]

Thanks for all the feedback. The hull is intended to have a shallow vee around 7 degrees, with a small flat chine outboard. The Vee will remain fairly shallow up to near the bow, so that the ramp can be fitted.

[Rick Willoughby]

Savitsky indicates a slight 'V' reduces drag a little. Surprised me actually.

Rick W

[Fanie]

Flatter displacement.

If you want the boat to be able to do 20kn then in my opinion it must be able to do so reliably. If you have to do it downwind all the time you may not do it too often.

Also, if you have the motor work flat out all the time it is going to affect the life of the motor for sure. If you overpower a bit and you can do the same on 70% power you will save on fuel too and the motor should last twice as long.

[joakimhansson]

http://www.billmunsonboats.com/

[Joakim]

Here is the Savitsky prediction for the boat in flat bottom version.

The numbers for the conditions shown are close to my first approximation for drag being weight/8.


Using that Savitsky calculator needs some caution. It seems to calculate Lk and D clearly wrong. Lk is important, since this vessel might easily have insufficient LWL for keeping the bow section out of water. Savitsky method is not valid then and the calculator gives no warning.

However, the thrust seems to be OK and it can be as low as 16 kN with 7 degree V at 20 kn. That means only 160 kW of propulsion power. With propeller efficiency of 70% 2*160 hp should be enough to maintain that speed.

Joakim

[joakimhansson]

Thanks, I can´t find the savitsky calculator to play with....

[Joakim]

This is the one used here:

http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html

Are there better ones in the web? I have my own, which includes from Savitsky 2006 paper aero and whisker drag, both of which are not important in this case.

Joakim

[Rick Willoughby]

Using that Savitsky calculator needs some caution. It seems to calculate Lk and D clearly wrong. Lk is important, since this vessel might easily have insufficient LWL for keeping the bow section out of water. Savitsky method is not valid then and the calculator gives no warning.

However, the thrust seems to be OK and it can be as low as 16 kN with 7 degree V at 20 kn. That means only 160 kW of propulsion power. With propeller efficiency of 70% 2*160 hp should be enough to maintain that speed.

Joakim

Joakim
I do not like empirical models like the Savitsky predictor but I still point it out to people looking at planing hulls. It is about the best there is. I have only found one example out of about 20 or so where it was close to actual power. In practice it normally under predicts drag. The gap can be closed by considering windage and appendage drag. Some semi-planing hulls have very large keels.

I would always recommend having considerably more power than it predicts unless the design is meticulous in detail.

There is another predictor that uses analytical methods based on foil theory but it has not been widely used.

Michlet will also give good results for some hulls.

I suppose we both are suggesting that 2 X 250HP will be enough for 20kts.

Rick W

[Joakim]

In practice it normally under predicts drag. The gap can be closed by considering windage and appendage drag. Some semi-planing hulls have very large keels.

I would always recommend having considerably more power than it predicts unless the design is meticulous in detail.


My Savitsky program has appendage, windage and whisker drag models included, but most importantly it takes roughness into account. I think the Cf allowance suggested by Savitsky is not enough unless the boat is very big and slow. To reach it requires very smooth bottom in "normal" boats.

I'm not a NA and I have no own data to compare with, but I have calculated some existing boats with rather good results.

There are too many unknown factors like the real propeller efficiency, thus I agree that it is better to have have some extra power, but 2x250 should be easily enough, if the LCG is somewhere near 5 m.

How big errors do you usually see with Savitsky? Do you measure LCG and VCG? Could you give me an example I could try with my program?

Joakim

[Rick Willoughby]

My Savitsky program has appendage, windage and whisker drag models included, but most importantly it takes roughness into account. I think the Cf allowance suggested by Savitsky is not enough unless the boat is very big and slow. To reach it requires very smooth bottom in "normal" boats.

I'm not a NA and I have no own data to compare with, but I have calculated some existing boats with rather good results.

There are too many unknown factors like the real propeller efficiency, thus I agree that it is better to have have some extra power, but 2x250 should be easily enough, if the LCG is somewhere near 5 m.

How big errors do you usually see with Savitsky? Do you measure LCG and VCG? Could you give me an example I could try with my program?

Joakim

Jaokim
If you go back through this thread you will get enough data to compare with Savitsky.
http://www.boatdesign.net/forums/projects-proposals/hull-modifications-23205-4.html
Also this one:
http://www.boatdesign.net/forums/inboards/prop-size-again-24565.html

If you are interested in doing the full analysis then have a look at JavaProp. It takes a bit to learn it but it will give good results for the props.

Rick W

[Joakim]

Jaokim
If you go back through this thread you will get enough data to compare with Savitsky.
http://www.boatdesign.net/forums/projects-proposals/hull-modifications-23205-4.html
Also this one:
http://www.boatdesign.net/forums/inboards/prop-size-again-24565.html

If you are interested in doing the full analysis then have a look at JavaProp. It takes a bit to learn it but it will give good results for the props.


I didn't find the data needed for Savitsky method from those and at least the latter boat was quite far from being a prismatic hull. I was hoping to get some data that has all the ACCURATE parameters (beam, deadrise, displacement, LCG, VCG, propeller shaft angle and location, spray rail configuration, area of all underwater structures, windage area). If you also had the detailed data of propellers and engines, you could really compare.

If you don't even know the displacement and hull shape, it is obvious that any metdod will be very inaccurate. Assuming a prismatic hull with ideal LCG will obviously give much lower resistance than the reality for a hull that is not designed for planning speeds.

Javaprop is some kind of lifting line method? How do you know the geometry of the real propellers? What more do you get from it compared to calculators based on the Wageningen B-series propellers?

Joakim

[Rick Willoughby]

I didn't find the data needed for Savitsky method from those and at least the latter boat was quite far from being a prismatic hull. I was hoping to get some data that has all the ACCURATE parameters (beam, deadrise, displacement, LCG, VCG, propeller shaft angle and location, spray rail configuration, area of all underwater structures, windage area). If you also had the detailed data of propellers and engines, you could really compare.

If you don't even know the displacement and hull shape, it is obvious that any metdod will be very inaccurate. Assuming a prismatic hull with ideal LCG will obviously give much lower resistance than the reality for a hull that is not designed for planning speeds.

Javaprop is some kind of lifting line method? How do you know the geometry of the real propellers? What more do you get from it compared to calculators based on the Wageningen B-series propellers?

Joakim

It is not all on a plate. You have to go through the posts and fill in the gaps with what else you can find on the web. Make some engineering estimates.

JavaProp is an analytical method that is much more flexible than an empirical series. I started using it for a quick check on the small props I design and now find it easy to use. What it does not do is determine the impact of inclined shaft. You are also stuck with a limited range of foils and Re#. I normally work on an EAR of 50% unless I have actual data.

Rick W

[Joakim]

I
JavaProp is an analytical method that is much more flexible than an empirical series. I started using it for a quick check on the small props I design and now find it easy to use. What it does not do is determine the impact of inclined shaft. You are also stuck with a limited range of foils and Re#. I normally work on an EAR of 50% unless I have actual data.


I have been trying JavaProp for a while and have some questions

1. I would like to compare it to a Wageningen B-series propeller calculator I have used a lot. How should I set JavaProp for this? Of course air->water, but what about profiles and angle of attack? At first look JavaProp seems to give clearly better efficiencies. Are they really achievable?

2. Quite often cavitation is the limiting factor in propeller selection/design. E.g. smaller EAR would give better efficiency, but is prone to cavitation. How to take that into account with JavaProp? It seems to design propellers with small chord at maximum R. This is not what most marine propellers are like. I would think the difference comes from cavitation and/or limited diameter.

Joakim

[Rick Willoughby]

Joakim
I am designing props for low power applications and use my own program for this as I can design for any foil section I choose to use. I use JavaProp for quick results as it is has nice interface and is reliable enough. It also allows for velocity ratio which my model does not. I work at such low loadings that I do not need to bother with this. In these circumstances pressure differences are very low.

JavaProp is certainly accurate for the range of foils it has when lightly loaded. For the large powered boats the only section that comes close is the MH116 at Re# 500,000. So I use this. For the given conditions I adjust the AoA until I get an EAR around 50%. Usually this is 0 or 1 degree on the MH116 foil.

I did compare JavaProp once with Wageningen B-series data and got reasonable comparison. Irrespective there are a number of people using JavaProp and achieving very good results. I have measured power data within 1% that shows I get prop efficiencies up to 86% and this is what I predict with analysis almost same as JavaProp.

In real life there are a whole lot of factors affecting props that I aim to avoid. My props essentially operate in open water. I always run the shaft horizontal. I have no strut or use a very tiny strut. I use the optimum diameter consistent with blade stress.

For my props the constraint on chord is blade bending strength not pressure differential.

I am aware that JavaProp usually indicates higher efficiency for most power boat applications but in the past efficiency was not a high priority and you see a whole raft of terrible power sapping layouts. How do you cater for a shaft angle of 15 degrees when you apply the Wageningen B-series. I hate the thought of running props on inclined shafts after what I have done to shafts at very low power level.

So JavaProp is very good if you are seeking efficiency. Just be prepared for a shape that is not like a Wageningen B-series prop. I can usually get 1 to 2% better than JavaProp by optimising the foil to the application.

Attached link shows a prop milled to my design.
http://www.boatdesign.net/gallery/showphoto.php/photo/11202/ppuser/18624
It achieves 86% efficiency at design condition.

Rick W

[Joakim]

My interest is not so much in designing propeller shapes, but analysing existing propellers and choosing a better one (just a hobby!).

I guess your designs are quite far from "normal" marine propeller due to very low drag and thus power?

How do you know EAR from JavaProp? Do you visually approximate it or calculate from the shape given?

I tried to compare the propeller of my sailing boat (actually it's a 2-bladed folding propeller which probably makes it far from optimal). It's a 14x8 and running at 1344 rpm 6.4 kn BSP, maybe 6 kn with wake factor. It's a strut drive thas no angle.

Using Wageningen B-series 50% EAR 3-bladed I get 800 N thrust and 54.5% efficiency. 800 N is very close to what my sailboat VPP-program predicts at that speed. The engine is 1GM10, so the real efficiency is much worse or there is something wrong with the engine, but that's another issue.

Using MH116 at 0.5 degrees and spinner dia of 0.112 m I get very similar results (about the same pitch using 1344 rpm and 800 N) with JavaProp, except that the efficiency is 61.2%.

So what is the explanation for this huge difference in efficiency? Which one would be more accurate for a normal good quality marine propeller I could buy? Are the Wageningen B-series really that far from optimum 14x8?

Joakim

[Rick Willoughby]

Joakim
If the prop is of uncertain shape you can actually load the blade chord and pitch angle at various radii into JavaProp and do analysis from there. I produce numbers as a text file and import from there. If it is poorly made it could, at worst, have the performance of a flat plate. So you can use this to check the worst efficiency for the particular pitch. This is not quite a flat plate as it has a rounded nose and pointed tail. This should be as bad as it can be unless the pitch is extremely poor.

Your motor is rated at 4.8kW so the predicted power of 3.6kW is not a huge difference - what is the gearbox efficiency, seal drag and auxiliary load. I would have a lot of faith in the JavaProp result and then go about looking for reasons why there is a difference. Check the motor torque curve to see that it is not being torque limited by the prop. It could be overpitched. I would measure the prop and load the actual chord and pitch angle data. I normally load 20 points but I would only check the pitch angle at three points to confirm it is correct and then calculate the points in between. I also measure the blade chord at a few points and then extrapolate.

The more detail you do like this the more you will close the gap. If you have a gap it will not be with JavaProp. It will be your inability to define sufficient detail. Once you have got the performance of the existing prop nailed then you can use JavaProp to produce a better design. This is why I like analytical methods because you know they are correct providing the input variables are accurate.

The Re# of 500,000 is somewhat low but the foil performance does not alter a lot between this and the actual you have. This is one area that my model handles better than JavaProp but it is not a huge difference - typically less than 2% in efficiency prediction.

Rick W

[Rick Willoughby]

So what is the explanation for this huge difference in efficiency? Which one would be more accurate for a normal good quality marine propeller I could buy? Are the Wageningen B-series really that far from optimum 14x8?

Joakim

I would not consider it a huge difference but the answer to your last question is YES. The Wageningen propellers are made for a typical range of applications where there are draft constraints and significant thrust for the size. They are nuts for a yacht application that does not have a draft constraint and are lightly loaded.

I expect you could design a prop for your boat that would achieve an efficiency of 80% but it would need different gearing. Something like a 24X16 2-bladed with very low EAR would be practical but it needs to spin at half current speed. You should be able to get 75% with the current gearing. The higher the blade aspect the more efficient.

Rick W

[Joakim]

Yanmar 1GM10 is actually rated about 6.5 kW at propshaft: http://www.yanmarmarine.com/uploads/products/pdf/GM_YM/1GM10_TechData.pdf
http://www.yanmarmarine.com/uploads/products/pdf/Saildrive/Saildrive_20_TechData.pdf

The earlier version 1GM was probably that 4.8 kW. I have SD20, thus 1:2.64. The alternator is 35 A and I guess all else should be included in the rated power.

I know having much bigger diameter would give much better efficiency especially with lower shaft rpm, but it's a sailboat and primary interest is low drag while sailing. So I'm stuck to small size folding (or feathering) propellers, which obviously have high loading (close to cavitation limits) and I would assume Wageningen B-series would be rather good for that.

Also folding propellers needs heavy (very thick) blades in order to stay open while going backwards, thus the profile is far from optimal. I would guess flat plate is better than a very thick profile?

JavaProp doesn't "see" nor warn about cavitation. I guess that could cause significant errors, but in this test case it shouldn't be a problem.

Joakim

[Joakim]

Would it really be possible to get 75% with current gearing? With JavaProp I can easily get that with 0.6 m diameter and 4 deg AoA. But the Cl would be close to one and tip speed over 40 m/s, which would mean that Cp (pressure coefficient, not power!) of less than -0.12 would lead to cavitation. With AoA of 4 Cp is bound to be far less than -1.

Even with 14" propeller and tip speed of ~24 m/s Cp is limited to -0.35 and thus Cl to very low unefficient values.

With very low AoA (Cl) the efficients with JavaProp drop dramatically (to more realistic values?).

Or have I understood something wrong? What kind of tip speeds have you used with your efficient propeller designs?

Joakim

[Rick Willoughby]

Joakim
There are a lot of things to comment on.

The alternator would only draw significant power when it is charging battery. Typically just after a start so I would not get too concerned with this.

The drive leg could be sapping quite a lot of power. If the boat is taken out of the water you can check the seal drag. See what torque it takes to turn the prop when in neutral. You should be able to get around 98% on each set of gears. So you can do numbers on the drive leg.

We are making the assumption that the rpm is accurate and the motor is developing full power. These really need to be cross checked. Your fuel consumption can help with determining what power is being applied.

I would also do a drag analysis on the hull to see if the drag indicated by the prop loading is in the ball park. THis helps cross check rpm data and pitch data.

As far as the prop design goes if you get it more efficient then you can drop the engine rpm. Check what you can achieve with a prop doing 1000rpm at 3.3m/s having a diameter of 450mm. If you fiddle around with AoA at each of the 4 points available you should get very close to 75%. Now this is not the best foil available. It may not be a practical prop either because the blades will not be strong enough. I try to get down to 8% foils specifically designed for the required lift coefficient. I also optimise the chord and AoA at more than 4 points and include Re#. However my optimiser does not cater for velocity ratio. I cannot get a better shape for the MH foil then JavaProp is providing. However I have attached a text file that you can import to JavaProp geometry and try it. It will not be as efficient as JavaProp is getting but this is a shape that would have adequate strength for an 8% foil in your application. To use the file you set up all the prop parameters but do not do the design just load the file. You then use the analysis windows to get the data.

You need to end up with a propeller that is strong enough and it usually means a thicker section in the root.

Even your application has much higher velocity ratios than I normally operate at. Typically I am under 105%. I have not checked the minimum pressure in your application with the 450mm prop but the Cp is very low and dropping rpm reduces the loading.

I hope this gives you an idea of the options available. Working with JavaProp will give you greater insight then using series data.

Rick W

[Joakim]

Dropping to 1000 rpm would make the motor run at 2640 rpm with almost WOT. This is probably not a good idea for a motor designed to work at 3400-3600 rpm WOT.

I have a very comprehensive VPP program, which calculates the drag at any speed up to Fn=0.6 for my sailboat, for which I know very accurately all the parameters needed (displacement, Cp, LCB, LCF etc.). The hull is very smooth, since it's used for racing. At 6.4 kn the drag is about 750 N.

The propeller turns easily while on neutral. The rpm has been measured from engine sound using FFT, which is a very accurate method. The speed has been measured with GPS and an accurately calibrated paddle wheel log and there are no currents around here (well up to 0.5 kn can sometimes happen).

I have doubts about the power output of the motor, which is one of the reasons I'm interested in this.

If I measure my propeller and import r, c and alfa to JavaProp, what would that actually help? I don't know the profile and it is certainly not any of the ones available . There is a huge difference in thrust and power at same rpm, if I change the profile. Of course I could start from the "known" thrust, but it would be nicer to use the pitch. How is the pitch defined in commercial propellers? Is it a geometrical value or is it derived from zero thrust when V equals the theoretical speed from pitch.

Joakim

[Joakim]

Here is a rough measurement of my propeller. The last column is the profile thickness %. The pressure side is completely flat and I measured alfa along it. Both the leading and the trailing edge are equally blunt (about 6 mm thick 8 mm from the edge at r=0.1 m) and the profile looks symmetric.

I tried using MH 126 at two first section and the MH 114 at two outer ones. This reflects the thickness rather well. At 6.5 kW I get ~1300 rpm, ~1100 N and 52% at 6 kn. All else is fine, but thrust is way off compared to VPP. The sections have a bit concave pressure side and clearly asymmetric profile, thus not what my propeller is.

Then I tried flat plate. Now 6.5 kW results to over 1700 rpm, almost 1000 N and 46% at 6 kn. At 1344 rpm the figures would be 2.3 kW, 400 N and 52%.

Then I tried Clark Y, which is flat on the pressure side and got at 6.5 kW a bit over 1500 rpm, over 1000 N and 52%. Using 1344 rpm I got almost 4 kW and a bit over 700 N and 56%.

Thus now I got 6 different values to choose from. Flat plate is obviously wrong. I have very hard to believe in drag of 1100N, thus maybe the Clark Y option is the closest one. Would the symmetric profile cause the power need to jump from 4 to 6.5 kW? Or is the motor outputting only 4 kW. Or a bit of both? Maybe I could play around with xfloil to find the difference.

And that one is also very close to what I already had from Wageningen B-series.

Joakim

[Joakim]

I did some tests with xfoil. I started from naca 4312, which looks rather similar to Clark Y. Then I made it almost symmetrical (trailing and leading edges identical). Xfoil doesn't survive from totally blunt trailing edges, thus only almost.

At zero AoA naca 4312 had Cl = 0.43, Cd=0.0077 and the symmetrical 0.63 and 0.011. The symmetrical one had much flatter pressure distribution and despite higher Cl Cp minimum was at 0.7 compared to 0.9 of naca 4312.

According to JavaProp Cl decreases from 0.5 to 0.15 at the tip and Cd decreases from 0.019 to 0.014. Is there induced drag included or why are the Cd's so high in JavaProp?

Setting Cl to 0.2 puts Cd of naca 4312 to 0.0094 and the symmetrical one to 0.012. Again the symmetrical one has clearly "better" Cp with a maximum of ~0.5 compared to ~0.7 on suction side. On very tip of the pressure side the Cp's are 0.8 for the symmetrical and 1.4 for naca 4312.

Thus according to xfoil the symmetrical profile would have about 30% more drag at the same lift. How would that transfer to propeller efficiency?

But it seems the symmetrical profile would be clearly less prone to cavitation.

Joakim

[Rick Willoughby]

Joakim
You have provided a lot to comment on.

The one comment I will make now is that you are assuming the power demand for the prop is the rated power. This is unlikely to be the case. I expect it will be designed to have reserve power to cope with high wind. You may have experience that supports or counters this.

It is not unusual to overprop a diesel to get its cruising operation at lower revs so you get maximum efficiency. I expect the motor will go close to achieving rated power less the gear train losses. It should easily achieve your stated speed if the drag is correct.

I will look at the blade information you have provided.

The intention with providing the text file is so you are aware of how to load your own data. The other thing that many people do not realise is that the pitch angle does not need to be perfect. In fact the optimum will have different AoA at different radial points so the best blade will not have perfect pitch angle. So where do you actually measure the pitch? I determine it at the 75% radial position for the given foil usually in its standard configuration. Obviously a cambered foil will still provide thrust under these circumstances. Most of my blades work with slightly negative AoA when optimised.

Generally I find the standard foil sections available within JavaProp are thicker than what ends up optimum. That is why I made my own program so I can choose any foil section. I have never bothered to see if JavaProp can accept other foils. I know there is a version that has more sections than others. Also the software has changed a little since I have been using it. However it is worth pointing out that for the low angles of attack typically seen on water props the foil shape does not make a huge difference to efficiency. A flat plate will be significantly lower like 10% or so but most foils will be within 5% of each other.

JavaProp certainly accounts for induced drag.

If you are also looking at different foils you should try JavaFoil. I found the interface much easier to work with than Xfoil. There is a lot of similarity with JavaProp. I have compared JavaFoil on a number of foils with test data from Selig and it aligns well. Both these programs can be downloaded and run directly from your computer without internet connected.

I commend you with the precision taken with measurements. With a little more perseverance you should be able to close the gap between what JavaProp is producing and what you are getting with the boat.

Rick W.

[12553Jack]

Hi First post this, it may or may not be useful. We fit fishing Cats with Honda Outboards largest so far 2 x 150hp on 10 mtr x 4 mts cica 6 ton displacment, for performancre I have used the Castle Marine prop calc program ( free from their website ) for the prop size I use Honda technical to get somewhere near and then fine tune with another prop if necessary. Some customers require absolute top speed others best fuel economy for a given speed,because of all the variables trial and error is the only way I have to go on. I think manufactures are very reluctant to make staements on performance just in case it does not perform as predicted.We have not conducted sea trials yet on the 10 mtr mentioned but if anyone is interested I will post the Data.

Jack

[Rick Willoughby]

Hi First post this, it may or may not be useful. We fit fishing Cats with Honda Outboards largest so far 2 x 150hp on 10 mtr x 4 mts cica 6 ton displacment, for performancre I have used the Castle Marine prop calc program ( free from their website ) for the prop size I use Honda technical to get somewhere near and then fine tune with another prop if necessary. Some customers require absolute top speed others best fuel economy for a given speed,because of all the variables trial and error is the only way I have to go on. I think manufactures are very reluctant to make staements on performance just in case it does not perform as predicted.We have not conducted sea trials yet on the 10 mtr mentioned but if anyone is interested I will post the Data.

Jack

Jack
I will certainly be interested in the data. Verification of weight at test and a bit more detail on hull shape would be of interest as well.

Also do you know if outboard manufacturers offer a heavy duty leg for these heavier loads? Is there anything special with the mounting and gearing?

Rick W

[Rick Willoughby]

Here is a rough measurement of my propeller. The last column is the profile thickness %. The pressure side is completely flat and I measured alfa along it. Both the leading and the trailing edge are equally blunt (about 6 mm thick 8 mm from the edge at r=0.1 m) and the profile looks symmetric.

I tried using MH 126 at two first section and the MH 114 at two outer ones. This reflects the thickness rather well. At 6.5 kW I get ~1300 rpm, ~1100 N and 52% at 6 kn. All else is fine, but thrust is way off compared to VPP. The sections have a bit concave pressure side and clearly asymmetric profile, thus not what my propeller is.

Then I tried flat plate. Now 6.5 kW results to over 1700 rpm, almost 1000 N and 46% at 6 kn. At 1344 rpm the figures would be 2.3 kW, 400 N and 52%.

Then I tried Clark Y, which is flat on the pressure side and got at 6.5 kW a bit over 1500 rpm, over 1000 N and 52%. Using 1344 rpm I got almost 4 kW and a bit over 700 N and 56%.

Thus now I got 6 different values to choose from. Flat plate is obviously wrong. I have very hard to believe in drag of 1100N, thus maybe the Clark Y option is the closest one. Would the symmetric profile cause the power need to jump from 4 to 6.5 kW? Or is the motor outputting only 4 kW. Or a bit of both? Maybe I could play around with xfloil to find the difference.

And that one is also very close to what I already had from Wageningen B-series.

Joakim

Joakim

I have now looked at your data. The attached spreadsheet gives your data in the form required to import into JavaProp. You copy the cells shown in yellow and paste into the import window available on the geometry page. JavaProp will extrapolate.

I have selected the Clark Y foil because it has a 12% section and likely to be the closest section available within JavaProp to your prop given it is around 12% thickness.

With this set up JavaProp gives me 3.19m/s, 4.75kW and 800N at 1344rpm. The efficiency is 58%. I believe you will find this will be quite accurate unless the blade is nothing like a foil. The Clark Y foil is flat on one side and curved on the lifting face.

I also tried a flat plate and modified it by increasing the angle by 3 degrees. The efficiency drops to 53% and power goes up to 5kW. I expect this would be the lowest possible efficiency.

You could spend more time to accurately measure the angle of the blades rather than assuming the pitch is accurately set. Then replace the beta I have calculated for import to JavaProp.

I expect the JavaProp data to be reliable. If you accept this then it would be possible to design a prop that will do better. I believe you could get close to 75% efficiency running at 900rpm on a prop that has sufficient strength. Strength at the root will be the limiting factor on efficiency. Power required will be under 3.5kW and engine torque 14Nm. This is still well below the available torque.

If you were inclined to do this then I expect you will improve fuel economy substantially but you are unlikely to get the engine to rev out. Top speed should be a little better than now.

The attached link shows low EAR 2-bladed props in the right size range:
http://www.australpropeller.com.au/fixedyacht.htm
I have never handled one of their props but the proportions are OK.

If you have access to fabrication facilities it is not too hard to make a prop. I can get small ones milled specifically but they are expensive and your loading is quite high so would need stress analysis. A fixed bladed prop will certainly impair sailing performance even if the blades are narrow.

Will be interested in anything you find that helps close the gap between your test data and the JavaProp derived data.

Rick W.

[Rick Willoughby]

Something else I should comment on.

Known foils have a defined shape and the pitch angle for a prop is given for the foil at zero angle of attack or the nominated offset. If the foil is cambered then it will have lift at zero AoA even if there is no offset.

Hence it is not unreasonable to adjust the blade angle until you get the required thrust at the given rpm. JavaProp will give you the power to achieve the thrust at the rpm.

I design my own blades and am not too worried about what I define as zero angle as long as the manufacturing matches my reference. If I was using a Clark Y I would use foil data with the back side at zero because the flat surface provided an easy reference. So all my data then relates to this.

If you get around to measuring the actual blade profile on the Yanmar then you can do lift curves for it with JavaFoil and find what matches it best.

Right now I am saying your prop efficiency will be between 53 and 58%. If you do more measurement you may be able to get a better answer.

Finally, symmetric foils do not make for efficient propellers. I know they are poor but I do not think they are as bad as a flat plate but please note I have not validated this. I have never bothered to create data for a symmetric foil to make the comparison.

Rick W.

[12553Jack]

Hi Rick

I will post the data when we launch, as we will crane the boat in the crane will having a weighing device fitted. The boat is made by GRP fabrications of Ashington England they have a web site and you may be able to obtain data from them. However we have also fitted 2 x 130 Hondas to a 8 mtr Cheetah Cat this is a commercial fishing vessel out in all weather and worked HARD, performance about 28 knots fully loaded Disp circa 3.5 tons. We have lots of Honda 90s working commercially with 1 pair I am told having run in excess of 7000 hrs. I would suggest that the modern outboard engineering is robust enough to withstand very hard commercial operation.I have to say we are exclusively Honda and it is the outboard of choice for most commercial fishermen but that is not to say other types are not as good.I would also say I am not theoretical in my marine engineering I fit it if it works I record the data and pass it on more a mechanic than a scientist. On the mounting side they are usually mounted on pods bolted to the transom these have to be robust and the forces calculated for the vessel and power output. The gearing is determined by the factory with no options ( that I know of ).I did consider setting up a data base where users could input what they have and how it performed. Say 23 foot Boston Whaler fitted with 200 hp honda size of prop fitted top speed max revs and fuel consumption it would then allow anybody considering that combination to compare to say a guy who fitted a 150 to the same boat and perhaps anther with a 90. Out of interest we fitted the 200 to the whaler it goes rather quickly !!!

Jack

[Joakim]

Rick
I did already measure the propeller (see message #31). I measured the angles using a 25 cm protractor along the flat pressure side. I guess the accuracy would be about 0.5 degrees with that method.

However, it seems Clark Y does not have horizontal pressure side. It is at about 2 degree angle to the reference plane in these co-ordinates: http://www.ae.uiuc.edu/m-selig/ads/coord/clarkysm.dat

So maybe 2 degrees should be subtracted from the angles measured, but can I trust this is really the reference plane in JavaProp?

Any of this has rather little to do with efficiency, which should be with all different approaches a bit over 50%. But I'm still have not been able to clearly define the operating point of the propeller. Using JavaProp and starting from the measured propeller angles and blade widths I can get many different results regarding thrust and power at the same rpm.

Now the propulsion I should need is that 750 N at 6.4 kn which makes 2.5 kW and thus less than 5 kW at assumed efficiency.

I should have 6.5 kW at propeller shaft, thus things are not adding up. I would really like to know where is the error. I know rpm and speed are very accurate. Thrust required should be OK, but maybe 10% error is possible. So what is left are that the engine is not able to output that 6.5 kW or the propeller efficiency is only ~40%.

Joakim

[Rick Willoughby]

Joakim
As you have determined, if you are referencing the angle to the flat side then you need to adjust to get it to align with the standard foil section. The Clark Y data will indeed be out by 2 degrees when using the flat side as reference.

I did not realise your third column was degrees. I have now entered your data adjust for the 2 degrees in my spreadsheet. I have redone the JavaProp data and copied it into the spreadsheet to produce a thrust and power curve for the prop at various boat speed for 1344rpm. It indicates that the thrust is about 850N and power is 5kW. I expect you will eventually determine this to be very close.

The conclusion from this is that you are not absorbing 6.5kW at the prop.

You are not reaching full engine speed but this could be governor setting rather than torque limit. It could be that the governor setting is not very precise. A better indication of the power will be the fuel consumption. The data sheet indicates 2.3lph at full throttle. Have you measured fuel consumption for sustained periods at full throttle. If it is only producing 5kW then fuel consumption should be around 1.8lph with prop doing 1344rpm.

Rick W

[Rick Willoughby]

An indication that the motor is working hard is that you start to get poor burning and the exhaust will show some black. If you are not seeing this when running at full throttle it could be another indicator that the maximum power is not being achieved.

Also if the engine revs are not dropping in a strong breeze it also indicates that there is more torque available and the engine is being governed at 1344rpm at the prop.

It is a mechanical governor and I could imagine it being imprecise and may need regular reset. Someone more familiar with the Yanmars may be able to comment on this.

Rick W.

[Joakim]

Rick
The file I included earlier was a file to be directly imported to JavaProp. JavaProp ignores the last field, which was the profile thickness.

I had already planned to measure the fuel consumption, but haven't found spare time for that. Now the boat is up for the winter until May. I try to measure the fuel consumption and probably even do a towing test in order to verify the thrust needed next summer.

There are two adjusting screws in the governor. One for maximum rpm and one for maximum fuel output. Someone has probably tampered the governor settings, since the sealing wire connecting the screws is missing. The engine revs a bit over 3800 rpm on idle and it used to rev over 3700 rpm with the 14x6 propeller the boat came with, thus it shouldn't be hitting the rpm limit in governor. The 14x6 propeller was put by the earlier owner and the 14x8 is the original setting.

There is no black smoke. Should there really be black smoke, when the engine reaches the design rpm? I thought black smoke is related to overloading the engine on lower rpm, but I'm not a diesel expert.

Joakim

[Rick Willoughby]

Joakim
You will see in my spreadsheet I have added 2 degrees to the blade angle you provided in your little table. The curve I have provided should now be very close to what your prop is doing at 1344rpm.

Black smoke is not a good thing as it indicates poor fuel burning. However if the motor is working at maximum torque and not being run lean you should see the exhaust darken up at any point in the rev range. If the motor is easily able to get to full revs under light load then it is clearly not being governed back. It could be the maximum fuel setting is low if it has been fiddled with.

As a matter of interest what speed did the boat do with the smaller pitched prop? You could do a check on operating point for this prop as well . This will help confirm the boat data.

Rick W.

[Joakim]

Rick
I only used the 6" prop for two weeks when I bought the boat (it was the lifted up for winter). I only changed the folding blades, which actually came with the boat. I have already sold the 6" blades, thus I can not measure them, but they looked identical except the pitch.

I didn't do very thorough measurements with 6", but it reached about the same boat speed as the current 8", clearly over 6 kn anyway. The rpm was well over 3700 rpm, but I can't remember the exact value. Calculating with these, the thrust by JavaProp would be only ~700 N, depending on the exact rpm and the way I reduce the pitch of the measured propeller.

These propellers are actually sold by Volvo-Penta and probably manufactured by Radice. It is now "the old 2-blade version" of Volvo-Penta, since there is a new one, which came 2008.

Joakim

[Rick Willoughby]

Joakim
I posted the attached image on another thread. It belatedly answers your question regarding which angle is used to define pitch. You can see there are three possible.

Rick W

[bydgroup]

Hi Nacaman!

We have currently in the design board a new motor yacht of 51 ft and a displacement around 12.5 tonnes and are thinking on using 2x300 Hp outboards, to achive a speed of 20-22 kts. All numbers seem to be right and pwer prediction as well, but never had the experience with a heavy boat with outboards.

As I've seen that you had the same question I would like to ask you a few things..

Did you had the chance to test your boat? Do you have some conclusions on the use of outboards for heavy boats? Did you found the appropiate propeller.

The engine manufacturers say that is not an usual thing so they dont want to confirm or deny the performance, and also I'm concerned on the durability/relayabililty of the engines!!

Thanks!

[nacaman]

Hi Bydgroup,
The boat is under production but not tested. This whole area of the outboards for heavy displacement boats is a minefield due to the lack of technical data on outboard prop performance. There is a far bit of uncharted water here. From a technical standpoint an inboard with prop installation would be a safer and more robust long lasting installation, albeit with higher initial costs. If the operating hours were higher, the diesel would in the longer term reduce this cost difference. In our case hours were lower and client did not want to pay the extra initial cost.
For the vessel we were originally intending to use 2x250 hp engines. After discussing this with outboard manufactures, we got various opinions and responses.
The complications we came across were

1) The thrust loading on the props is high and much higher than would acceptable for a commercial prop. Some cavitation is likely, which may affect prop life. I have read this should not cause thrust breakdown, but it is uncertain how much effect this will have on the thrust generated. The most suitable outboard appears to be the higher reduction ratios with the biggest props, however none we found were fully satisfactory for blade loading. Despite this the warm fuzzy feedback we received from outboard prop suppliers was that the speed was possible to achieve with these outboards
2) It appear the speed prediction for a heavy vessel based using outboards is best determined by assembling lots of old salts at the pub, plying them with drinks, and comparing against performance of other unrelated speed boats and averaging the answer.
3) We were unable to obtains data on the propeller efficiency for outboard at our proposed trial condition. Other guys I talked to were using ballpark QPC's of 0.4-0.5 for props. In reality this will vary with different boats and speed. Further the outboard manufactures we talked to, could not match a vessel resistance curve against the performance of their outboard to predict a vessel speed. Again comparison against there trials data was done, the data we saw did not include heavier planing boats.
The prop size we looked at marginally above Gerrs suggested limits for minimum prop diameter for maneuvering

Operationally
We have ended up increasing the power to 2 x 300 HP outboards. This was based on feedback from the outboard manufacturer that outboard g/box on the 250 HP was not robust enough to do the job and would prematurely fail. We also find out that as outboards are generally intended for pleasure boats that some manufacturers assumed only around 120 hours per year of operation. This may be fine for a pleasure boat, but could be a concern for more commercial operation.
The path is a bit risky and if you do go down this path, there are uncertainties regarding the final vessel performance. If you can get the outboard manufacture to agree to change the outboard to a larger outboard if it fails to perform, it would be a worthwhile assurance to have.

[bydgroup]

Thanks Nacaman for your comments!!!

I would love to hear your impressions once you test the boat with the outboards.

On the meantime we have not received from any manufacturer any thrust data, the only ones that answered us with some helpful comments is Mercury, that they believe that their mercury verado 300 will do the job

Those are some of their comments:

On a normal V-bottom (this boat appears to have a flatter bottom) planing hull at 13.5 tons with 600hp total inboard power, the speed calculates at 30 MPH. With Zeus/600 HP, the speed would be approx. 36-37 MPH.
With twin 300 Verados, the predicted speed would be about 27 MPH; however, it's important to note that you will need 12" pitch propellers. These props are only available in right-hand rotation, thus you would need both engines to be right-hand rotation.

The 300 Verados consume 28-30 Gallons/per hour each at WOT (wide open throttle). At cruise,(3000-3500 RPM) the consumption is 10-11 GPH.
The Verados will work well at 9 and 20 knots.


Regards

[powerabout]

Asking an outboard to do the job of an inboard diesel is asking for trouble.
Just look at the gear ratio and the prop sizes and you will learn that big outboards are not for pushing houseboats.
Your lefthand gearbox will not like all that load even if you can get a prop?
Prop slip will be huge.