Prop Sizing - Who Should I Believe ?

[rotorman]

Hi all,
I am replacing my engine and my propeller. Initially I thought to keep the propeller, but than I accepted that it is easier to match the propeller to the engine than to try to buy the engine - transmission combination to match the propeller.
But now that I made my mind to buy a new prop I have conflicting recommendations from different suppliers.
These are my parameters:
-Boat is a 41 feet ketch.
-Engine HP: 55 at 3000 rpm
-Gearbox ratio: 2.61
-Displacement: not sure but between 10-13T
-Waterline length: 33 feet
-Max cruising speed according to manufactorer: 8.3
-3 Fixed bladed prop, right rotating
The Yanmar dealer's supplier recommended to use 19x13
Another supplier suggested to use 19x16
(Another recommended to use 19x13 for a gear ration of 2.1!)
I tried the propcal calculator. It recommends 20.56x16.29
I tried propcalc.xls. It recommends 20x14 or 19x16.

Whom should I beleive???

[SkiprJohn]

If it were me I'd believe the 19x16 and if your engine will not reach 2800 rpm in forward on your sea trials then have it repitched to 19x14. You are more likely to cruise at rpms from 1800 to 2200 rpm so when on sea trials see what your speed is at those rpms.
There are so many variables it really is a hard one to calculate.
kind regards,

[Ziggy]

First, make sure they all use the same input data, including the displacement and gear ratio.

Second, it's probably safest to go with the prop recommended by the engine dealer, that way you reduce the possibility of finger pointing if the engine does not perform as expected.

[rotorman]

Originally Posted by Ziggy

Second, it's probably safest to go with the prop recommended by the engine dealer, that way you reduce the possibility of finger pointing if the engine does not perform as expected.

My concern is that they try to be too much on the safe side (to make sure I can reach max rpm) and because of this I will run the engine on a higher rpm. Which results higher noise etc.

[Ziggy]

Originally Posted by rotorman

My concern is that they try to be too much on the safe side (to make sure I can reach max rpm) and because of this I will run the engine on a higher rpm. Which results higher noise etc.

Then I agree with SkiprJohn--you can go with the higher pitch and if necessary repitch the prop later. You should be able to easily change the pitch by 2".

[C4 King Cobra]

My set up is similar to yours except weight. Eighteen Tons, 41 ft. I have a 19 x 13 and find no problems thru rmp range.

[rotorman]

Originally Posted by C4 King Cobra

My set up is similar to yours except weight. Eighteen Tons, 41 ft. I have a 19 x 13 and find no problems thru rmp range.

Thanks for the info.
I run a few tests with propcalc.exe and seems like the weight does not have much affect on the pitch.
How can you tell that you are not underpropped, though?

[djmarchand]

Rotorman:

I ran your numbers through the prop calculator on boatdiesel. Sign up for $25 and you can do it yourself. But in any case browse the boatdiesel forum for free and you will get more than $25 worth of marine diesel knowledge.

First a couple of questions. What is the maximum diameter that will work with your boat? Remember Dave Gerr's book. It says that you should be 15% of the prop diameter away from the hull. Also can you increase the transmission ratio? That will help.

But plugging in your numbers and a 19", 3 blade prop gives a pitch of 14", right in the middle of what you are getting. The calculator says that the prop shaft rpm will be high and increasing the transmission ratio will help with prop efficiency. And you will have to increase the pitch as well- props with diameter equal pitch (square) are usually the most efficient.

How do you know if you are underpropped (or overpropped)? If underpropped you will be able to rev beyond 3,000 rpm in gear. If overpropped you will rev to less. Since your boat is heavy for that engine, I would prop right on the money or a little underpropped.

Although FWIW I have never seen a 33' lwl boat that weighs 13 tonnes. Ten is even high. And it does make a difference in pitch. I think you have more difference in the real weight of your boat than anything else discussed.

But you can repitch up or down if necessary. But why not install a MaxProp and stop worrying about pitch. Some MaxProp models can even be adjusted in the water. A haulout and repitching job will just about pay for the difference in a MaxProp vs a fixed pitch prop. And you get the advantages of more thrust in reverse.

David

[Bash]

I would want to know exactly what model engine you have before recommending a prop.

Some engines want to be run at higher/lower rpms than others. This applies even within a given brand of engines.

[rotorman]

Thanks all for the help.

Unfortunately I am not sure about the weight. According to a very old leaflet my boat is 7T nett. (BTW the boat is Amel Euros 41). On another document it is said that gross weight is 13T.
When I left from Croatia with full crusing gear (including exta fuel, dinghies, food supply for 1 year etc) the guy on the travellift measured me 15T.
The prop is enclosed.
The original prop was a 19x13 prop with a Volvo Penta MD21A with a gearbox ratio of 1:2.91.
And this is where its gets tricky. The MD21A has a max rpm of 4500. However for sailboats it was recommended to limit the rpm to 3000 by the prop. I never tried what rpm can I reach with the old engine but I have a memory running it at 3200.
I am replacing the current engine with a Yanmar 4JH5E. The highest gear ratio is 1:2.61 and that is what I ordered. I have to replace the prop because the old prop was LH.
I would prefer to be "proper" propped instead of being underpropped.
With the new gearbox my shaft RPM will be around 10% higher than it was before so if I have the same prop I already in a better position. However if I could have a more "square" prop than I would prefer to have it.

I hope this makes sense.

[rotorman]

Originally Posted by djmarchand

Rotorman:

Some MaxProp models can even be adjusted in the water. A haulout and repitching job will just about pay for the difference in a MaxProp vs a fixed pitch prop. And you get the advantages of more thrust in reverse.

David

Have been thinking about this but I don't like the extra complexity and maintance (and the price) of a Maxprop.

[rotorman]

Would like to add that normally I was running my engine at 2000-2200 and the only time I run it at higher rpm (at 3200) was when fighting a very strong current in the entrace of an atoll. With 2000 rpm I was doing 5.5

[Rakuflames]

Originally Posted by rotorman

Hi all,
I am replacing my engine and my propeller. Initially I thought to keep the propeller, but than I accepted that it is easier to match the propeller to the engine than to try to buy the engine - transmission combination to match the propeller.
But now that I made my mind to buy a new prop I have conflicting recommendations from different suppliers.
These are my parameters:
-Boat is a 41 feet ketch.
-Engine HP: 55 at 3000 rpm
-Gearbox ratio: 2.61
-Displacement: not sure but between 10-13T
-Waterline length: 33 feet
-Max cruising speed according to manufactorer: 8.3
-3 Fixed bladed prop, right rotating
The Yanmar dealer's supplier recommended to use 19x13
Another supplier suggested to use 19x16
(Another recommended to use 19x13 for a gear ration of 2.1!)
I tried the propcal calculator. It recommends 20.56x16.29
I tried propcalc.xls. It recommends 20x14 or 19x16.

Whom should I beleive???

Have you contacted prop manufacturers? I just got a new prop from CDI and they had a chart that told them what prop for my boat with my new engine.

[rotorman]

Originally Posted by Rakuflames

Have you contacted prop manufacturers? I just got a new prop from CDI and they had a chart that told them what prop for my boat with my new engine.

Yes. And the manufacturer recommended by the dealer suggested the smaller prop and another independent suggest the bigger prop.

[daddle]

I recall (???) that under-propping is detected by applying full power when stationary and noting that the RPM is slightly lower than that RPM which the manufacturer specifies must be attained. Then, applying full power while underway at cruising speed and noting that the specified RPM is attained. The idea is that a stationary vessel somewhat overloads the prop while a vessel at speed does not.