calculating prop. Need help.

A 12x8 may work, but isn't recommended. Optimum diameter is 11 inches. You may not have enough torque to swing a 12" prop. Top speed with an 8 pitch may be too low. A competent prop shop can reduce diameter and re-pitch a 12 inch prop to suit.

For example :

The prop on the left is our old OJ 12x16p LH 3 blade Nibral propeller. It's our best prop, but we
wanted to increase our cruise RPM by several hundred RPM, so we had a new OJ propeller
reworked and labbed to an 11x14p.
​

Refer to the Pitch Ratio Test line in the spreadsheet. Each gear/prop combination is tested to ensure that the shaft speed is acceptable and flagged as 'OK' or 'Fail'. Disregard any props flagged with 'Fail'. These props will work, but they are not a good match according to the Optimum Pitch Ratio Formula :

If you rerun the spreadsheet with decreasing and increasing values for Required Speed, you'll easily see when the 'OK' button lights up indicating a good match.

____________________
Paul Kane
Kane Custom Boats Ltd.
Chelsea, Quebec

Building the Glen-L Hot Rod : http://www.kanecustomboats.com

Glen-L Boat Video Directory : http://www.kanecustomboats.com/pages/video_directory.html

 

calculating prop.need help.

If you need to reduce a propeller diameter in half inch increments to get a good clearance I would take that choice only if a better solution was not available and the prop was very cheap.
Final tuning of a propeller by some adjustment however good the maths and theory is can improve performance.

If you take a close look you may easily be able to adjust the shaft angle to get better clearance by inserting a wedge under the shaft log which is a common practice.

I belonged to the local Traditional Small Craft Club and a fellow member had a beautifully constructed Hacker I uploaded a picture of it onto the Classic photo Gallery on boatdesign site.

 

Paul,
I've been playing with different values and i must admit it almost addictive. Like finding the best gearbox ratio's on race weekend
I was wondering, when the prop diameter is to big i might not have enough torque, but what are the possible problems when it gets to small? If i put 100hp @ 5500rpm in the calculator, i get a 9X10. Is there such a thing as a "too small" prop?

Tom,
can't find that picture, do you have a link?

Joris

 

Too small of a prop, is like having tires that are too narrow. It will spin in place and not be able to absorb all the power from the engine

 

Hi Joris
In the long run, it might be better to hire someone to do a proper calculation. More info on the hull form, engine, etc would be needed. As Gonzo noted, a too small prop (diameter) will be like putting golf cart tires on a truck- you could literally 'spin your wheels' due to lack of blade area.

 

JSL,
I had a feeling i wasn't going to find a decent solution without help. While searching for a company that can modify props i ran into a company that has been very helpful in helping me decide what would be the best solution and while it involves changing the transmission (as most of you here suggested) i'm sure i won't regrets it.
Tonight i will see how far i can go in modifying the shaft-angle as Tom suggested, hopefully that will allow me tof fit a prop larger than 13" so i can use a 2:0 transmission.
If anyone is after a Paragon P31L, let me know, i know someone who has a spare now
Joris

 

10 1/4" diameter.

Estimating speed is hard because knowledge of drag is necessary. If your boat planes with a lot of wet surface sitting relatively deep in the water (with buoyancy still carrying a good fraction of the load), which I guess is likely, a speed of 27 mph would be possible. Again, it depends on how much drag. I probably don't have a boat with comparable drag to scale your numbers properly. At 27 mph @ 4000 RPM the right pitch for a flat high pressure blade side is about 8-9". If you're running slower than 27 mph then you need proportionally less pitch. The calculation assumes that there is not a big fat keel or anything else in front of the prop to disturb the inflow.

The calculations are from correct hydrodynamic scaling, not from any tables or formulae you can find anywhere.

 

To absorb 85 shp at 4000 RPM with 2:1 you'll need 15.5" diameter. Impossible with your limitation.

 

Right ballpark.