Prop Thrust Calculation

Fred, where did you get that rule from? You've quoted it before but never given a source. And can you explain under what circumstances it might be valid?

 

where did you get that rule from? You've quoted it before but never given a source. And can you explain under what circumstances it might be

After 5 decades of boating its hard to quote a particular source that it came from.

It gives a rough idea of what an engine is producing at cruise with a rational engine and prop.

And can usually be used with other rules of thumb , gas engines make 10 hp/gal, diesels about 16 in operation (tho the commercials get over 20).

Useful sometimes ,



FF

 

That's an interesting rule of thumb. After a lot of playing about (thanks to invaluable advice from Rick Willoughby on the "pedal boat" thread), I have a prop and motor combination that delivers around 50lbsf per hp at cruise speed on my hull, a fair bit over the rule of thumb figure.

During all my testing and evaluation I found there wasn't a clear and straightforward relationship between thrust and power that could be used easily for prop/engine selection, as the dominant factors were the interactions between the hull resistance vs speed curve, the prop efficiency vs speed curve and the prop efficiency vs rpm curve. The whole process of getting a good match was iterative, with small changes in prop diameter, pitch or aspect ratio having disproportionately large effects on efficiency and therefore thrust per unit power at any given boat speed.

 

I have a prop and motor combination that delivers around 50lbsf per hp at cruise speed on my hull, a fair bit over the rule of thumb figure.

The rule is for conventional power boats, boats with huge prop diameter compared to the HP, slow RPM and usually more efficient 2 blades is way different from a 10-40 ton cruiser.

FF

 

Even more interesting, as my prop is a two blade, 13" diameter, running at around 700rpm.

I'm sure these "rules of thumb" have successfully been used for years to give a starting point for prop and engine selection, and give an installation that works (my guess is that this is how they came about in the first place, from trial and error over many years) but my own experience suggests that time spent in carefully optimising the engine and prop to the hull resistance curve reaps very big dividends in terms of thrust per unit power, and hence overall efficiency.

For some folk efficiency may not be that much of an issue, as long as the boat works, so maybe my findings are a just a function of my (some may say a bit obsessive, perhaps) striving to get rid of every last bit of wasted power in the whole propulsion system. Still going the same speed and only using 40% of the power (and hence energy) that the "20lbsf thrust per hp" rule of thumb suggests seems to make all this optimisation work worthwhile, at least for me.

Maybe it's time to revisit some of these old rules of thumb and see if they still stack up in today's more energy concious environment.

 

"Maybe it's time to revisit some of these old rules of thumb and see if they still stack up in today's more energy concious environment."


Most do change over time a gas engine used to be 10hp per gallon is now 12 with a brand new engine 14 without ethanol diluted fuel...

Diesel was 16 hp/gal but today with huge bucks and complexity 22 can be had .

Weather the loss in reliability of electronic high pressure fuel management is worth the risk is a personal choice.

For a yacht / cruiser , probably not worth the initial cost ,
for a ferry or tug or the ESSO Maru, fuel burn is tons per day , you bet.

FF

 

Some petrol engines are doing better than that figure now, by a fair margin, which illustrates how things have changed in recent years. The engine in my car, for example, has a BSFC of around 225 g/kWh, which equates to a bit over 16hp per US gallon, burning ordinary (non-ethanol) unleaded.

I think my main concern over the "20lbsf thrust per hp" rule of thumb is that is extremely hull shape and propeller selection dependent - it may well work OK for a high D/L ratio hull fitted with an off-the-shelf prop that's a bit on the small side, turning a bit on the fast side, but will be a long way out for a low D/L ratio hull fitted with an optimised large diameter, low rpm prop. The example of my own boat quoted earlier illustrates this, as her D/L ratio is pretty low at around 50, and this, combined with her low design cruise speed, large diameter propeller and low propeller rpm results in the big variation from the rule-of-thumb prediction.

 

I've looked at published "performance" data from a number of manufactuers of fuel consumption of 4-stroke outboards, and was surprised how close most are to 0.1 gal (US) per hour per advertised hp at wide open throttle. The best numbers I saw were 0.88 gal/hr per hp. I looked at engines from 2 HP to 250 HP, and used the wide open throttle fuel consumption numbers from boat/engine tests on the manufactuers web sites. There is variation for a particular engine model when tested on different hulls. One potential reason for the variation is the props on some boats may not be full loading the engine at WOT and thus the power may be less than the maximum for the engine tested.

The BSFC figure for your car engine is presumably at the best operating point, which would be at significantly less than maximum power and engine speed. Also, depending on which test proceedure was used the conditions may have some differences from "as installed" in a car.

My understanding is outboard power numbers are supposed to be the power at the propeller, while for a car engine it is the power at the flywheel/flex plate.

 

The example I quoted was extreme, the Toyota Prius Atkinson cycle engine, so not directly comparable to an outboard. As you say, that was a flywheel power figure, too, so again doesn't factor in transmission losses, which could be around 10%.

Having said that, an Atkinson cycle engine does have some attractive features for a marine application. It is around 35 to 37% efficient (in terms of converting the potential energy in fuel into useful work), but only over a limited operating rpm range. Its major drawback is that it is inflexible in terms of torque delivery, it delivers very little torque at low rpm which makes it unsuited to a conventional car drive system. However, because a prop presents a cube law relationship of power to rpm, it may well be that an Atkinson cycle engine might match a prop load pretty well. The low torque at low rpm wouldn't be a problem when driving a prop so, if well-matched (in terms of matching peak engine efficiency rpm to boat cruise speed), then it may well be practical to get BSFC figures of around 14hp per US gallon, even allowing for transmission losses.

My best guess is that there isn't such a powerful motivation for making outboards super-efficient as there is for cars, as the majority of outboards are probably used for leisure purposes (and possibly fairly infrequently at that) so the cost of fuel isn't a very significant factor when choosing a motor.

 

"My best guess is that there isn't such a powerful motivation for making outboards super-efficient as there is for cars, as the majority of outboards are probably used for leisure purposes (and possibly fairly infrequently at that) so the cost of fuel isn't a very significant factor when choosing a motor."

Perhaps on tiny motors , but most big power 4 stroke producers (100hp+)ARE auto engines that are re engineered to stand on end.

That these units can operate for hours at 4000-5000+ rpm is a testimony for todays auto engineering.

Fuel burn is important to the auto/marine guys , but perhaps not to the 4HP dink motor producers.

 

Rough calc for your review

Hi
Just read this thread to compare to my calc of thrust read for a cat.
From this site...http://www.ehow.com/how_7649204_convert-horsepower-thrust.html... I calculated the following...

Hull speed 7knts
Thats 708.88 feet per minute

Recommended motors are 50 hp per side.
50hp x 33000 = 1650000 lb.ft/min

1650000 / 708.88 = 2327.61 lbs per side.

Say 5000 lbs total.

Sound sensible for a 12.5 tonne cat with good lines??

Mick