Working Out Putt Putt Displacement/Weight

Yobarnacle:
At estimated 1500 lbs the calculator gives me a 14.7" x 13.8", in real life a 15" x 14".
For 2000lbs it gives 14.7" x 12.8", real size 15" x 13".
For 2500 lbs it gives 14.7" x 12", that is 15" x 12".

So, yes, by a maximum load rule-of-thumb I could run a 15x12, but ...... I don't want the motor revving with too small a pitch in its high torque range for no good reason (a) if I'm wrong about the weight or (b) when I'm going a moderate speed with only myself on board. I have a 10hp motor which really only needs 5 hp to push this little boat, so I'm trying to select a prop pitch which will keep revs as low as possible rather than just thumping away making noise and fuel consumption.

And, I already have a 15 x 13 prop from the old 4hp motor. But I am sure it is too "flat" for this 10hp motor. I am guessing I now need something around a 15x16 or a 16x15. To re-pitch the existing prop costs about the same as another second hand prop. Hence I need to accurately know weight of the boat to "break the rules"! And I don't particularly want to pay for the wrong sized prop based on sloppy guestimates.

 

To abcdefg:
Good idea. What is the method for weighing something you can only get under one end of at a time? Is it done by weighing each end and simply summing them up?

 

As long as the object is supported at the same points you are measuring the loads at that would work. ie boat supported only at bow and stern, with load cell at bow and stern.

 

Why pay? You have the middle of the road estimate prop, 15x13. Try it out.
The prop is matched to the boat. Pushes the boat. The engine just spins the prop.
No surprise the same prop you already have, is the recommended one.
Meanwhile, keep your eyes open for a sweetheart deal on a slightly larger or more aggressive prop.
People get lucky when they are LOOKING to get lucky.
Later when you luck into a stronger prop, try it out.
Changing a prop underwater isn't for fun, but not impossible either, only needs fore thought and planning ahead of time.
Keep good records of fuel economy and speeds for later comparison.
There is a lot of science and engineering in boat design. In final analysis?
It's still more art than science. Consider definition of artisan. Art is learned by doing. Good luck and fortune smile upon you.

Why did you switch 4 hp diesel for 10 hp petrol?

 

I switched motors for many reasons:

The old motor was partially seized; very corroded on the exterior; a 1964 Yanmar NTS85 is now too hard for parts in Australia; it was a ***** of a crank handle to start.
I know far more about petrol engines than diesels. Old diesels are bloody smelly and rattly. I plan to put a half cabin on the open boat, sleep in it, and don't want the stench of diesel around. The diesel had to run at 1700 rpm for maximum hull speed. I wanted more power than I needed, rather than too little. The 10hp revs max at 1200 but I can run it around 500 for hull speed (... that is with the right prop of course). I got a great deal on the 10hp Simplex ... they are considered the Rolls Royce of Putt Putt motors. And I ended up getting a good sell price for the Yanmar from a collector of antiques!

Aahmm, nuff reasons?

See link for video of my new motor. The big green one not the little red one.
http://www.youtube.com/watch?feature=player_detailpage&v=qLYCdt3fxjk

 

If you had said "Cause wanted to." would have been sufficient reason.

Pretty, pretty engine.

I like my 40 year old Albin diesels. Parts still available.

Good luck.
There is a possibility for more speed out of the same prop.
A CVT scooter transmission.
Set it up so at top end, where the 10 HP is, you are in overdrive.
1200 rpm engine and maybe 1600 rpm on prop.
OR
You might luck into a variable pitch prop and control box tranny from an Albin Vega.
OEM was 10 HP Albin diesel.
Or 10 HP Volvo (Albin diesel with Volvo label, made by Albin for Volvo)

 

Just put the old one back on. It is not too flat (low pitch) since the new motor revs higher, but I wouldn't want anything less than 13" pitch. My first thought when you told us the power and rpms of the old and new was that the old prop would be adequate if it had enough area, and it does. By choice, I'd probably go with a 13 or 14" dia prop because most of your cruising will not be at 70% power, but rather 50% or less. I push a 38', 17000# sailboat around with a 14" two-bladed prop with .4 area ratio using a 27 hp diesel. Contrary to what most people think, this is just about optimal because I rarely use more that 5 hp.

Keep in mind that a gasoline motor doesn't have a governor. It will not want to hold constant rpms. For trouble-free cruising, you want a fairly high absolute slip rate on your prop for good handling and a lesser torque feedback to the motor. Your fuel consumption will double vs the old diesel, but big deal - It will take from sunrise to sunset to burn $10 worth of fuel.

I calculate a 14 ft/sec slip at speed and full power. That's fine. An outboard would be running a 9" prop or so at 2500 prop rpm and more that 20 ft per second slip. I wouldn't go any less than 13 pitch at 1200 rpm. If the motor won't red-line, just use a belt sander (with the prop free-spinning, or being driven slowly) to remove diameter 1/8" at a time until it will.

 

Thanks Phil for your input.

Just to clarify: the old engine ran at 1500rpm through a 2:1 gearbox to the prop -- therefore at 750. The new motor runs at 1200 rpm max but at 1:1 to the prop.

So, I do now intend to just put the old prop back on first, to see what I get with it. I can fit up to 16" under the transom and was always under the impression that a larger prop diameter with lesser pitch was better than visa versa. One prop shop here advised me I would need an 18 x 20, or a 15 x 23 for a 10hp!!! (I've subsequently figured they are way wrong).

So, as some have indicated, my problem is presently academic ... until I test the new motor, with the old prop in a boat of unknown weight in the reality of the water! And I guess calculations for prop slip are also irrelevant now until I test her out.

Thanks everyone for your input. Problem solved (for now).

Why would a boat engine need a governor? For thick patches of water, maybe But, the flywheel of these monsters is about 30 kgs (see video above), so there ain't much power fluctuation.

And I laughed heartily at your methodology of belt sanding a prop down by increments! I was actually thinking of doing that if needed, but would never have told anyone, lest they accuse me of butchery and unbalancing a prop! The problem will be to find an underwater belt sander, a very long power cord, and to swim behind fast enough to grind it down at the same time.

 

Underwater sanders are far to common in Florida. Electric, diesel, and gas powered. We call em sandbars. Will sand your prop paper thin, and reduce diameter at same time.

 

Glad to see that we are enjoying a good sense of humor in this thread.

Yobarnacle, I have sanded a few props here in Florida myself, opened a few oysters with the prop too.