prop size?

[jsh]

Hello:

I am asking for advice on propeller size for a 22' deep V fully loaded weight should be 4800lbs +/-

Lwl:18'
draft 1.5'
bwl:7.5'

Power is a 7.4l crusader ch350 (through velvet drive 72c 1:1) develops 300 hp @ 4500
I would like to run the motor at the 3500rpm range developing 200hp+/- with lots of torque.

Shaft angle is 15 degrees and I have sufficient clearance for 18" diameter wheel

My confusion arises from reading about real world performance of inboards (Mako/Seacraft) who almost always complain about being under propped and from dealing with the people at Michigan Wheel who spec for my boat a 14x12 heavy cup.

I would like boat speeds in the 25knot range.

Any input is appreciated.

thanks

john

[daiquiri]

You can use a nice freeware prop calculator for Excel, by Surfbaud:
http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls
Go through various tabs and fill in the numbers in green cells. Since the weight is fixed, you will have to play with the gear ratio (i.e. commercialy available gear ratios) and/or boat speed until you get the maximum prop diameter you can install.

[Çemberci]

Your propeller size is Ok hull speed is higher than 33 -35 knot
Engine performance data
*********************************************
;* NavCad engine file HydroComp, Inc. *
;*********************************************

[NavCad]
Version=3.00
Release=PreRelease
Date=08/11/1994
Time=12:02:56.80
Precision=6
Description=CRUSADER mdl 350 316.5 bhp 300 shp 4400 rpm V-8 Cylinders

[EngUnits]
Power=Horsepower
FuelRate=GallonPerH

[EngineLib]
RatingPwr=316.5
RatingRPM=4400
GearEff=0.9479
PerfPwr=0,316.5,312.9,308.75,301.5,291.24,275,250.8,199.6,154.2
PerfRPM=4600,4400,4200,4000,3750,3500,3250,3000,2500,2000
PerfFuel=0,28.06,27.84,27.48,26.95,26.297,25,23.36,20,16.13

oktay çemberci

[jsh]

thanks for the response guy's.
I recognize the Numbers that come out of this propcalcs spreadsheet as they seem to be based on George Crouch's methods.

I questions these values because of common set ups often complain of poor efficiency (refer to the previous post 1980 23.6 Mako Prop size info)

I have a 14x12 prop to put on this boat but I don't think it the last one I will put her.

I don't have room for a gear reduction. The only way I can gain efficiency is with larger diameter/pitch

john

[Rick Willoughby]

Hello:

I am asking for advice on propeller size for a 22' deep V fully loaded weight should be 4800lbs +/-

Lwl:18'
draft 1.5'
bwl:7.5'

Power is a 7.4l crusader ch350 (through velvet drive 72c 1:1) develops 300 hp @ 4500
I would like to run the motor at the 3500rpm range developing 200hp+/- with lots of torque.

Shaft angle is 15 degrees and I have sufficient clearance for 18" diameter wheel

My confusion arises from reading about real world performance of inboards (Mako/Seacraft) who almost always complain about being under propped and from dealing with the people at Michigan Wheel who spec for my boat a 14x12 heavy cup.

I would like boat speeds in the 25knot range.

Any input is appreciated.

thanks

john

The attached shows what could be expected under ideal conditions. This analysis makes no allowance for the less than perfect weather, hull windage, appendage drag or additional weight. All these factors will reduce performance.

Basically it indicates that 25 knots should be achieved at 3500rpm with less than 60kW on the shaft. Top speed will be around 40 knots with the motor hitting the rev limit but no where near the power limit. So there is plenty of power to cater for the other loss factors.

The calculations are not too bad based on my personal experience with a 2.2 tonne boat getting 40mph with a single 125HP outboard. The hull was quite low with a well raked windscreen and there was a triangular planing surface.

The 14X12 prop is not a bad choice. A 2-bladed prop would be OK.

If the boat does not get somewhere near these results then you should look for reasons why. Most common causes are grossly under estimated weight and lots of draggy stuff under the water.

Rick W

[jsh]

thanks rick:

can we get better than 20% slip at 3000 rpm with a bigger diameter ?

john

[Rick Willoughby]

thanks rick:

can we get better than 20% slip at 3000 rpm with a bigger diameter ?

john

Not sure what you mean!

The 14" prop only has 11% slip. This is quite low for planing craft of moderate weight. Increasing the diameter without altering gearing makes for deteriorating performance for a practical prop because the blade area required to take the load results in a lot of unnecessary drag. The optimum 18" 2-bladed prop would have and EAR of only 8%. Such blades would simply not handle the forces. By comparison the optimum 14" 2-bladed prop has an EAR of 20%. Your existing prop is likely to have significantly higher EAR so it will be suboptimal for easy running conditions. This provides margin for head winds and rough seas.

If you went up to an 18" prop then it would need to be spun at 2000rpm at 25 knots for optimum efficiency of a little over 84%. Slip with this prop would be 7%.

There is an optimum slip for a prop. Making the blades larger to reduce slip increases the viscous drag on the blades. So you gain on one factor to the detriment of the other.

Rick W

[lazeyjack]

as always good stuff Uncle Rick
However 300shp, I doubt v much if you stuck that engine on a dyno it wouldmake 2o0, that is the nature of ohv, petrol engines of USA manufacture, I had the dubious pleasure(actually the thing was a dog) of driving the Chev with the 8l plus engine, that was the latest suberban thing , with inj, or whatever they call the model of ute
and I can tell you it felt abt 50% of the hp of a 4l mercedes, so , there you a re

[daiquiri]

I have my own xls spreadsheet for the calculation of Wageningen B-series props, based not on Crouch's method but on tests. It works in combination with the engine&gear data, so I could make a simulation of your boat's performance if you give me 3 or 4 RPM-BHP points for your engine.
I've tried to find some data about Crusader CH350 in Internet, and have found nothing.

[Rick Willoughby]

as always good stuff Uncle Rick
However 300shp, I doubt v much if you stuck that engine on a dyno it wouldmake 2o0, that is the nature of ohv, petrol engines of USA manufacture, I had the dubious pleasure(actually the thing was a dog) of driving the Chev with the 8l plus engine, that was the latest suberban thing , with inj, or whatever they call the model of ute
and I can tell you it felt abt 50% of the hp of a 4l mercedes, so , there you a re

Stu
I did qualify what I stated above with power at the prop shaft and power figures are in kW not HP.

I have the impression that the specs for some of these engines provide rated power at the top of the piston. All the losses from that point to what eventually gets taken off at the end of the shaft has to be accounted for by the user.

This is one of the other factors that needs to be accounted for of course when trying to reconcile actual with predicted. I agree with you that it is not trivial. Could easily be more than 10% of rated power going into other things just to make the engine work.

Even if it can put 200HP into the prop the boat should get close to 40 knots barring flybridge and underwater protrusions that sap power. (Have no idea of this detail)

Have you got an example of powering a 2 tonne deep 'V' planing hull? The prop seems small but without gearing it is spinning awfully fast.

Rick W

[jsh]

hi rick:
I read off of your attachment efficiency at 3000 rpm is 80% I figured the remaining 20% was slip. no? ok 11% ok good result.
I get it diminishing returns slip vs drag.

If you went up to an 18" prop then it would need to be spun at 2000rpm at 25 knots for optimum efficiency of a little over 84%. Slip with this prop would be 7%

This is what I want best speed attainable at lowest possible rpm with highest efficiency possible
do you think it is possible to achieve 20-25 knots with an 18" prop with under 3500rpms?

More information on the boat, requirements and constraints:
22' deep V splashed/modified off Bertram molds in the 70's
final stage of total rebuild /refit
never been in the water so I have no performance baseline
Intended usage is long range summer coastal commuter vessel 200nm round trip with at least half the trip expected to be rough going
To this end I chose a heavy deep v and a big motor
appendage drag would be limited to strut/shaft/raw water intake/rudder/tabs
windage would come from the center console/t-top


hi daiquiri:
I will post power,torque,fuel curves tomorrow

[Rick Willoughby]

I have looked at an 18" prop of practical proportions. It is 2-bladed and probably the lowest EAR you can get hold of. EAR for the data shown is around 20%.

This does not achieve 84% because to achieve that you would have narrow blades unable to handle the loads.

A pitch of 16" gets the motor to 2200rpm and provide more power in reserve. An 18" pitch might bog the motor. You would need to see what power is possible at 2200rpm to check this. IC motors typically get peak efficiency at 75% torque and 75% rated rpm so roughly half full power. Your rpm will be down on this but torque should be about right.

I have also attached the power demand curve for the prop at 2200rpm and varying boat speeds. This allows you to assess if the motor will bog down under adverse conditions. Even with 16" the motor will not be able to rev out so it is likely top speed will be down on what you would get with the 12" pitch.

A couple of other things:
1. The low EAR prop has higher efficiency but you have to select a prop that can handle the force.

2. The prop analysis is based on a prop aligned with flow not angled at 15 degrees. I do not like steeply inclined shafts because it introduces nasty out of balance forces. If you are stuck with this angle for practical reasons then a 3-bladed prop will have less vibration than a 2-bladed prop.

So when you get to practical considerations of inclined shaft, 3-bladed prop, EAR of around 50% the efficiency will drop off for the 18" prop.

I would not bother with anything other than the prop you have until you have some performance data from it. This gives a more solid basis to work from.

Also if you get the chance to weigh the boat then do this as well. Make realistic allowances for crew, fuel and provisions and review the drag data.

Take what I have provided as what is possible. You have to temper it with what you can actually achieve working within the constraints you have.

Rick

[jsh]

Daiquiri:

here are the power,torque,and fuel curves.

john

[daiquiri]

Daiquiri:
here are the power,torque,and fuel curves.
john

Got it. I'm doing it in my spare time so I'll give you the results tomorrow, at about this time. It will be based on the data you have posted above.
I agree with Rick that you should evaluate your boat's weight as accurately as you can and make a test run with the actual prop you have.
Take a note on max speed with full throttle and engine RPM. You can measure the speed with GPS or you can perform the test on a measured mile.
Your engine's max RPM is 4600 (from the curves you have posted above). You shouldn't be able to exceed it on full throttle.

[daiquiri]

Hello:
I am asking for advice on propeller size for a 22' deep V fully loaded weight should be 4800lbs +/-
Lwl:18'
draft 1.5'
bwl:7.5'
Power is a 7.4l crusader ch350 (through velvet drive 72c 1:1) develops 300 hp @ 4500
I would like to run the motor at the 3500rpm range developing 200hp+/- with lots of torque.
Shaft angle is 15 degrees and I have sufficient clearance for 18" diameter wheel

My confusion arises from reading about real world performance of inboards (Mako/Seacraft) who almost always complain about being under propped and from dealing with the people at Michigan Wheel who spec for my boat a 14x12 heavy cup.

I would like boat speeds in the 25knot range.

....
....

This is what I want best speed attainable at lowest possible rpm with highest efficiency possible
do you think it is possible to achieve 20-25 knots with an 18" prop with under 3500rpms?


I have made an analyisis with a spreadsheet created by me, which uses the Wageningen B-Series props test data from the original report, Savitsky method for resistance, engine curve data corrected for various losses, wake factor and cavitation calculations from Dave Gerr's book.
I've used Michigan Wheel props, series DJX for 3 blades and DQX for 4 blades.
These are the results:
1) the prop that's been recommended to you (if it is, as I presume, a common 3-bladed 0.61 E.A.R. model) has been calculated for maximum speed at maximum engine RPM. Theoretically, it should give you 39 kts at 4500 RPM. In practice it will cavitate at full throttle and will probably not be able to give you that speed.
2) Your engine torque curve has a maximum at about 3500 RPM and then decreases until 4500 RPM. Having this in mind and assuming that most of the time you will navigate at cruising speed, If I were you I would use the following approach:
- calculate the prop such that at 75% throttle (cruise) it doesn't cavitate and the engine works somewhere between 3700 and 4000 RPM. In that region the engine revs are self-regulated, i.e. a slight increase in resistance (due to increase in waves encountering frequency or height, for example) will cause an RPM decrease, which will cause a torque increase. Hence, the engine automaticaly compensates for increased resistance.
- with that prop, check the maximum speed you can obtain (and the corresponding RPM), keeping an eye on cavitation.

The prop which best satisfies these conditions is a 4-bladed 14"x12" E.A.R.=0.735 (Michigan Wheel model DQX).
It works at 3730 RPM at cruise, has the efficiency of 0.65 and gives you about 2.01 NM per gallon. The Gerr method says it is not perfectly cavitation-free, but is very close to the limits of acceptancy. And since the method itself is certainly not perfectly accurate, it's ok. ;)
At maximum speed (39 kts) this prop will work at about 4400 RPM, thus close but slightly below to engine's maximum RPM, which is technicaly a desired condition.

I have also included few other models which come close to the desired condition.

The last model (3-bladed 18"x12" EAR=0.61) is the one which more closely satisfies your initial requests. It gives 25 kts cruise at about 2650 RPM, and 32 kts max. speed at 3300 RPM.
It is cavitation-free and gives the best NM-per-gallon ratio, but the engine is not self-regulated and you will not be able to use it at max power, which means that your boat will be overpowered for the way you intend to use it.

25 kts at 3500 RPM is, imho, not attainable under your specifications. Your boat requires about 80 HP to run at 25 kts. Vice versa, your engine gives 285 BHP @ 3500 RPM, which become about 255 after transmission losses are accounted for. It means that you're looking for a prop with an efficiency of 80/255=0.314 (31.4%). That's pretty low. It would mean stalling blades - the exact contrary of hydrodynamic efficiency. ;)

Few conclusive words. The prop choice is a matter of compromise. And is a kind of wandering in the dark when it comes to small boats, because very rarely is some test data available for a boat under consideration. So one has to use some generic empirical methods, the accuracy of which is proportional to the experience of the person using them. And even the experience is not a warranty against errors on this scale of boats.
Basicaly, if you ask 10 persons for advice, you will probably get 10 different answers. :D

[daiquiri]

If you went up to an 18" prop then it would need to be spun at 2000rpm at 25 knots for optimum efficiency of a little over 84%. Slip with this prop would be 7%.

Rick, please don't get angry on me for this comment, but I really doubt that a 18" prop will ever get to have 84% efficiency in the real world, under the hull.
84% is a big goal even for carefully optimized tankers or bulk-carriers, with their slow-reving engines.
In the Wageningen basin test report, there is not a single prop ever going over 80% in ideal, free-flow conditions.

[Rick Willoughby]

Rick, please don't get angry on me, but I really doubt that a 18" prop will ever get to have 84% efficiency in the real world, under the hull.
84% is an big goal even for carefully optimized tankers or bulk-carriers, with their slow-reving engines.
In the Wageningen basin test report, there is not a single prop ever going over 80% in ideal, free-flow conditions.

daiquiri
I acknowledge this by adding the qualifications for practical consideration like shaft angle and appendages noted in earlier post #12. Our conclusions are much the same by the way. On the other hand ships have a serious problem with flow characteristics off the hull that can be avoided on a small craft.

I am achieving prop efficiencies as high as 87% for low power applications but there is a lot of effort and careful design to get this result. This link shows what can be done with as little as 120W:
http://www.adventuresofgreg.com/HPB/HPBmain.html
I have attached a photo of the prop I designed for the boat. This particular one has an efficiency of 86% at design condition. I have my own software for optimum prop design but it is not a lot different to JavaProp, which I used for the above analysis.

My method and JavaProp have the advantage that they are based on analytical methods and do not depend on empirical data. It is much more flexible and shows what is possible. There are no inefficiencies already wound into them. Helps focus the mind on eliminating losses.

So efficiencies in the high eighties and maybe even low nineties is possible but you have to know what you are doing to achieve that result.

Rick W

[daiquiri]

Fantastic adventure and great result! I loved reading it. :)

And that prop is a true example of what can be achieved by avoiding the standard schemes.

What airfoil sections did you use for the prop, if it is not a secret? I'm an aerospace engineer, you know, so I'm really affascinated by these things... ;)

[Rick Willoughby]

Fantastic adventure and great reasult! I loved reading it. :)

And that prop is a true example of what can be achieved by avoiding the standard schemes.

What airfoil sections did you use for the prop, if it is not a secret? I'm an aerospace engineer, you know, so I'm really affascinated by these things... ;)

The sections close to the hub were a little bit thicker than the majority of the blades. They start with a 20% thickness right at the hub and quickly reduce to 8%. The section is a modified NACA 4 series of my design. They have similar performance to an MA409 section but simpler shape. This is a highly regarded low Re# foil.

At the time the prop was designed I was hand fabricating them from stainless flat bar and the design was suboptimal to simplify hand fabrication. Greg was able to find a friendly machine shop able to produce blades from my CAD file. I did a bit of rework on the design for CNC production but did not take it as far as I could have. With complete freedom on section shape I could get 87% with a CNC prop at these power levels and even better efficiency with slightly higher power on an easily driven hull.

My own hand fabricated prop was also tested on the boat and was between 1 to 2% less efficient than the CNC prop. I calculated it would be 1% worse. It is a matter of how much time you spend to grind the profile. I fabricate a complete prop in about 4 hours including cutting, welding and profiling.

My objective is for high efficiency at relatively low power for electric drive and solar/wind energy collection so I have done a lot of work aimed at reducing losses. Understanding the basic physics and using good analytical tools gives reliable results. Performance gaps can be eventually nailed down and reduced. On Greg's boat, the lowest loss component we considered was 0.23W lost in offsetting the rudder drag due to the offset thrust. So we looked at what would normally be regarded as minute components. The shaft is 1/4" spring steel unsupported at the outboard end. I am yet to find anyone who can appreciate this without seeing it work. But it means the 2 to 3W strut loss is completely eliminated.

Rick W

[jsh]

Daiquiri/Rick:
Thanks for the sound technical advice.
I work in shipbuilding and repair and would return the favour should the need arise.
I will repost real life performance figures when they are available.
john

[Rick Willoughby]

Daiquiri/Rick:
Thanks for the sound technical advice.
I work in shipbuilding and repair and would return the favour should the need arise.
I will repost real life performance figures when they are available.
john

John
I will look forward to some test results. Always good to get feedback and it will provide you with a good point of reference for next prop if you choose to go that way. Don't forget to weigh the boat if the opportunity presents.

Rick W

[RangaTang@sea]

it might not apply but the bigger the prop the slower it needs to spin compaired to a smaller prop to provide the same amount of thrust. there is a very big difference and the bigger prop the more efficent

[daiquiri]

Daiquiri/Rick:
Thanks for the sound technical advice. I work in shipbuilding and repair and would return the favour should the need arise. I will repost real life performance figures when they are available.
john

You're welcome. If you have any other need/question ecc. you know where to find us. ;) :D

[No Joe]

You can use a nice freeware prop calculator for Excel, by Surfbaud:
http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls
Go through various tabs and fill in the numbers in green cells. Since the weight is fixed, you will have to play with the gear ratio (i.e. commercialy available gear ratios) and/or boat speed until you get the maximum prop diameter you can install.
Ok , soooo,,,, where is the option for single, twins or trips?
Do we use total HP for the 2 engines, & divide the prop pitch in half ???not :confused:

[tranmkp]

Albin 27 - Westerbeke 70hp - 3800 rpm max power

original owner prop - 18x15 - only 3000 rpm acheived mello cruise @1800 for hull speed - 7knts - begin to overheat

re-pitch prop 18x10 - spools to 3600 maybe 3800 but scary - about 13knts - I think it will spin more - might be a bit under prop. Cruise now at 2600. Not as mellow.

see chart for power curve - is 2600 where I want to be? Im not trying to get the best fuel burn - Id rather get have what the engines really like to spin.

http://www.artshot.net/w70.gif

http://www.artshot.net/w70a.gif

[daiquiri]

Ok , soooo,,,, where is the option for single, twins or trips?
Do we use total HP for the 2 engines, & divide the prop pitch in half ???not :confused:

:D :D :D
You're right, it has none of these options. It's a very simple propeller calculator and performs a preliminary calculation for a single prop-engine combination. It doesn't take into account the increase in effective power for multi-engine boats, and I suspect it doesn't take into account the wake factor either. Yet it appeared to be suficiently usable for what the initial request was.

[Jango]

According to Propcalc, the correct prop for your application, "Planing Heavy Cruiser" w. 300HP @4500 rpm and 4800lbs is a 13 x 10, 55% D.A.R min. - providing max of 32.5 Knots w. 12% slip.
With 200 HP @ 3500 rpm, correct prop would be a 14 x 11 providing 26.5 Knots w. 14% slip. You will note with higher shaft speeds, i.e. 1:1 ratio, recommended Prop diameters are smaller.
I would suggest a 14 x 10 since 3500 rpm efficiency is preferred.

[jsh]

thanks jango:

I purchased a used 14x12 for cheap and will be launching in Feb some time.
I will post some performance figures when they are available
john

[bit]

Rick W

hello, and thank you Rick
I: density = 1000; kinematic visco = 0.0000013, and the result is the attached image.
What's wrong? :confused:

[Rick Willoughby]

hello, and thank you Rick
I: density = 1000; kinematic visco = 0.0000013, and the result is the attached image.
What's wrong? :confused:

I expect that your result is based on the default foil section, which is a flat plate. This would be the most conservative.

If you want something more realistic then you should select an appropriate foil. JavaProp has a limited range included. For this sort of application I usually select the MH 9.8% at Re#500,000. You need to then adjust the angle of attack until you get the blade area near something that matches the EAR you are interested in. This could mean the AofA is negative because the cambered foil will still provide lift down to about half the camber.

With any propeller you have to be mindful of the resulting blade shape. It has to have enough metal to take the imposed loads.

One minor aspect is the density. If is for salt water then use 1025. The viscosity is temperature dependent as well but it will not make much difference in the normal range.

Rick W

[bit]

Thanks Rick
Excuse if advantage of your patience.
If you look at the attached file and give me some other advice ...
Thank you

[Rick Willoughby]

Thanks Rick
Excuse if advantage of your patience.
If you look at the attached file and give me some other advice ...
Thank you

You need to tell me what advice you want. If you want to check the data you could compare with Wageningen series data.

The prop is very large and slow revving for the power. Also for a practical prop you would need much larger cord and heavier section at the hub. Efficiency is low for such a large diameter. It will have quite high EAR. Blade aspect is about 2. You could possibly reduce the EAR by operating at higher angle of attack and still have blades that can be made strong enough.

The hub for a prop of this size will be much bigger than you have. At least 0.2m and maybe even 0.3m.

Rick W