Inboard Diesel Prop Choices

[Snore]

Dug

Before hauling do some research on what prop the manufacturer recommended when they designed the boat. It is possible that the PO put a prop he could find, versus the right prop on the boat.

That is what happened to me, found it out during the survey and fixed it when I had the bottom painted. Big difference when you have the right prop!

Bill

[Dug]

Originally Posted by Snore

Dug

Before hauling do some research on what prop the manufacturer recommended when they designed the boat. It is possible that the PO put a prop he could find, versus the right prop on the boat.

That is what happened to me, found it out during the survey and fixed it when I had the bottom painted. Big difference when you have the right prop!

Bill

This is something I am hoping is a problem.
Bad because I have to replace it-
Good because it is the problem and I can replace it-

Good point...thanks!

[Dug]

Originally Posted by MarkSF

The 180 degree turn is for use with the GPS, not the knotmeter, the principle being that if there is current the effect will be averaged out.

For example :

Current 2 knots. Boat speed through the water 5 knots. In one direction the reading on the GPs will be 5+2 = 7 knots, in the other direction it will be 5-2 = 3 knots.

Average speed = (3+7)/2 = 5 knots.

Also using this method the tidal current can be precisely determined. It's half the difference between your two speed measurements.

The knotmeter is very unlikely to be accurate anyway, at least until you calibrate it. How do you calibrate it? Using the GPS and the above measurement technique.

Without calibration the average knotmeter could be 20% out.

Excellent idea!
Hadn't considered the knotmeter calibration...

[River Cruiser]

The choice of prop pitch - diameter is determined by the wot (wide open throttle) rpm spec of the manufacturer. If the spec is 3000 rpm you should reach that + 3 to 5% on a boat with a reasonably clean bottom & running gear with a properly tuned engine. Each 1" of pitch will change wot rpm about 100 rpm, more pitch = less rpm & less pitch means higher rpm. If you are over pitched and can't reach the proper rpm it leads to a shorter life for the engine, it is like a car going up hill in to high a gear.

[Dug]

Finally hauled out of the water and found a two bladed prop. Wouldn't a three bladed prop be more efficient?...with the correct pitch of course....

[jeremiason]

You will find that you will loose power with a three blade prop, due to the limited horse power of your engine....

Find out what your engine/transmission recommended prop pitch is and see if it is correct... My guess is that it is to much prop pitch...

If it is wrong, or you can't determine what it is... Take it to the local propshop and they can re-pitch the prop. It will cost much less than a new prop.

[roverhi]

Two blade props are nearly as efficient as a 3 blade. What you gain in performance with a three blade will be eaten by loss of speed under sail in light to moderate conditions. The three blade props are smoother.

A properly pitched prop, either two or three blade won't make a big difference in available power.

Prop pitch and diameter are determined by available space for the prop, transmission reduction gear, and horsepower/torque curve of the engine. Diameter is limited by space and how fast the prop is spun. A large diameter prop cannot be spun as fast as a smaller prop because the water passing the tips starts to cavitate above a certain rotational speed. Smaller diameter props tip speed is lower for the same rpm so can be spun faster with a higher pitch to make up for smaller diameter. Most diesels have a fairly hefty reduction gear so they can swing a larger diameter prop because they develop more torque at lower rpm than gasoline engines. The old Atomic 4 had less torque, direct drive or low ratio gear reduction and made max hp at a higher rpm so tended to have smaller diameter props. Assume that's why the PO went with a lower ratio transmission (less reduction/higher shaft speed) and stayed with a fairly small diameter propellor when they converted to diesel.

[Dug]

Originally Posted by roverhi

Assume that's why the PO went with a lower ratio transmission (less reduction/higher shaft speed) and stayed with a fairly small diameter propellor when they converted to diesel.

The original paperwork and maintanence reciepts points to an 8 hp 1cyl diesel. Was an Atomic 4 gas inboard an option on the 1984 Cal 24's?

[jeremiason]

Originally Posted by roverhi

Two blade props are nearly as efficient as a 3 blade. What you gain in performance with a three blade will be eaten by loss of speed under sail in light to moderate conditions. The three blade props are smoother.

Peter

The orginal post said he had a 8 hp deisel engine...

What you are saying is that an 8 hp deisel will power a three blade fixed prop with the same efficiency as a two bladed prop provided they are properly pitched?

If that is true, I stand corrected...

[jeremiason]

Dug

Here is a link with the 1984 CAL 24 specs.... It looks like it came with a 7.5 HP Yanmar as the in-board option.

http://pages.sssnet.com/go2erie/cal24.htm

[Dug]

I believe one of the reciepts for the diesel calls it an 8hp yanmar. Would there be a nameplate on the engine itself to verify this?
I'll take some pics next time I go down to the boat and maybe someone on here may have a eureka moment when they see it-

[roverhi]

Have only seen pictures but there was a single cylinder 8 hp Yanmar that looked to be mounted on it's side. Believe it was YSB-8 but quite probably could be wrong on that. Don't know if it's parts availability or low horsepower but have seen a number of them advertised real cheap, even free. No idea of those engine's condition, however. Knowing Yanmar, the 8 might not be the horsepower.