prop size?

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

 

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.

 

prop size

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

 

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

 

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

 

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

 

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

 

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.

 

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

 

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

 

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

 

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.

 

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.