I hope to explain some of the issues being expressed about props. Mechanically HP is defined as Hp=torque x rpm the gear ratio converts the engine Hp (torque at rpm) to prop shaft torque and rpm. E.g. a 2:1 ratio the prop will have .5 of the engine rpm and 2 twice the torque. The prop shaft torque is used with the hull data (displacement +load) to size prop blade area. Once the best blade area for a boat and motor is determined the area is divided by two for a 2 blade prop or 3 for a 3 blade prop. A efficiency factor for the number of blades is applied to the areas, With the lower number of blades having the higher efficiency. (It is said that the most efficient prop is a one blade prop but there is a balance problem.)This usually means that a two blade propeller is about 1 inch larger in diameter than a 3 blade propeller.
The prop shaft rpm is used with other hull data (waterline length to find max boat speed) to determine the pitch. Since water can be pushed around and slips around the propeller a large adjustment is used for pitch calculations.
I ran 2 prop calculations for the same boat and motor so that only the difference between a 3 and 2 blade prop is shown:
Hull Speed: 6.7 kts. Power Required: 17.9 shp.
Available Power: 18.4 SAE hp ( 13.7 kW) @ 3600 RPM
Power Used: 18 SAE hp ( 13.3 kW) @ 3,564 RPM
Max Speed: 6.7 kts
RECOMMENDED 3 blade PROPELLER Diameter: 12.6 in. Pitch: 7.2 in.
RECOMMENDED 2 blade PROPELLER Diameter: 13.2 in. Pitch: 7.3 in.
Lets adjust diameter and see what happens
PROPELLER BASED ON INPUT DIAMETER
Diameter: 13.0 Pitch: 6.7 Blades: 3
Diameter: 13.0 Pitch: 7.5 Blades: 2
Since pitch only has 1 inch increments you could get a recommendation for the same diameter and pitch 13x7 depending on how the prop guy rounds the pitch number and what he has in stock.
This makes a person wonder about the present three blade prop fashion. There are good reasons for choosing a 3 blade prop but increasing power should not be one of them unless there is an extenuating circumstances as insufficient clearance around the propeller.
There are endless nuances around the art of prop selection and it is not an exact science.
Just a word about the electric powered boat. Hp mechanically is torque at rpm and electrically it is volts x amp. And therein is the electric motor prop selection problem. The prop does not see volts or amps but torque and rpm. The volts or amps needs to convert to torque and rpm. Since volts and amps are easy to measure the tendency is to use them when the prop only understands torque and rpm. If you look at a torque rpm curves the electric starts with high torque and goes low with rpm while the mechanical starts with low torque and goes high with rpm.
larry
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> I love it.so maybe my 6HP (horsepower) electric motor setup might be said to
> be 6000hp (hamster power)?
>
>
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of mkriley48
> Sent: Saturday, December 01, 2012 6:35 AM
> To: electricboats@yahoogroups.com
> Subject: [Electric Boats] Re: Question about a prop for a Columbia 9.6m
>
>
>
>
>
> I agree!! There seems to be a dearth of logic. also boat shape, hull speed,
> and whether the prop is out in the open or tucked behind a deadwood affects
> prop selection. A faq or wiki would be helpful.
> A hp at the prop at the same rpm is the same whether it is; gas diesel or
> hamsters.
> mike
>
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