Prop Sizing - Who Should I Believe ?

[Maine Sail]

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???

Prop sizing is not what I consider to be a proven practice on sailboats and few ever get it spot on with the first try. Get a written guarantee from the prop maker that the prop will allow your engine to spin max rated RPM in flat current free water with a clean bottom and prop or what ever your engine maker wants to see. If not, they will re-pitch or fix for free.

This is how I buy props and not one shop has ever balked at it. Of course when they "F" up, which they do occasionally, I eat the cost of a short haul or diver to change it..

My last prop, for my own boat, was purchased from Norm at West By North the Campbell Sailor maker. He pitched it wrong and sent me a brand new prop ahead of time so I could change it in the water and then send the old one back! That is GOOD customer service, BUT, we had the "conversation" ahead of time..

Also had an issue with a Flex-O-Foild on a customers boat, that was over propped, and Flex-O-Fold made good but again I had the "conversation" up front with them. No problems..

From my perspective the bottom line is to try and find a shop willing to stand behind their calculations and re-pitch or fix the "issue" at no cost to you when their calculations are wrong.

[Rakuflames]

Originally Posted by rotorman

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

What's an "independent?"

[David M]

The engine MUST be able to reach maximum RPM regardless of which propeller you choose. It is a common fallacy that over-propping a boat saves fuel or is okay to do if you never plan on running the engine at full speed anyway.

Determining over or under propping is done with the boat making way with a clean bottom and a healthy engine...not at the dock. A stalled foil takes less energy to turn which could result in a false indication that the boat is not over-propped.

What over-propping does is cause premature wear and causes carbon to accumulate in the combustion chamber.

If anything it is better to be slightly under-propped than to be over-propped.

[djmarchand]

Rotorman:


Ok, you have an Amel 41. That helps. I found an elevation view of an Amel 41 listed for sale on Yachtworld and see the huge overhangs. That explains how you can get so much weight in 33' lwl. BTW that owner listed his weight at 18,000 lbs.


Travellift scales are notoriously inaccurate and 15 Tonnes (metric, right) equals 33,000 lbs. I can't believe an Amel 41 even loaded for bear weighs that much. My guess would be more like 25,000 lbs.

So if I plug that into boatdiesel's calculator, I get 19.0 x 14.3. I understand that the max transmission ratio for your engine is 2.6 and 19" is as big as you can go. If you really weigh 33,000 lbs then the pitch drops 1". As others have said you can usually repitch a prop up to 2" either way.

The advice about pre-negotiating a free repitch is good, but the haulout (or diver) cost is likely to be yours.

Think harder about the MaxProp. Yes you have to grease them at each haulout, but that is all the maintenance really needed. Maybe a rebuild after ten years- about US$1000. You will love the improved thrust in reverse. I could get off most soft groundings by reversing with a MaxProp whereas with a fixed blade prop I probably couldn't.

David

[rotorman]

Originally Posted by Rakuflames

What's an "independent?"

I meant "not the one recomended by the engine dealer"

[rotorman]

Originally Posted by djmarchand

Rotorman:


Ok, you have an Amel 41. That helps. I found an elevation view of an Amel 41 listed for sale on Yachtworld and see the huge overhangs. That explains how you can get so much weight in 33' lwl. BTW that owner listed his weight at 18,000 lbs.


Travellift scales are notoriously inaccurate and 15 Tonnes (metric, right) equals 33,000 lbs. I can't believe an Amel 41 even loaded for bear weighs that much. My guess would be more like 25,000 lbs.

So if I plug that into boatdiesel's calculator, I get 19.0 x 14.3. I understand that the max transmission ratio for your engine is 2.6 and 19" is as big as you can go. If you really weigh 33,000 lbs then the pitch drops 1". As others have said you can usually repitch a prop up to 2" either way.

The advice about pre-negotiating a free repitch is good, but the haulout (or diver) cost is likely to be yours.

Think harder about the MaxProp. Yes you have to grease them at each haulout, but that is all the maintenance really needed. Maybe a rebuild after ten years- about US$1000. You will love the improved thrust in reverse. I could get off most soft groundings by reversing with a MaxProp whereas with a fixed blade prop I probably couldn't.

David

Thanks David,
this is really usefull info.

[rotorman]

Update: I decided on a 19x15 low BAR (0.38) propeller. I am very excited. I hope the size will be correct. Probably will be back in the water in 3 weeks and will see.

[rotorman]

Update: I had the sea trials the day before yesterday and happy to report that everything worked out well.
The engine can reach 3050 @ WOT (and can reach 3100 in neutral), the boat can do 8+ knots.
I am really glad that I did not went with the recommendations of the supplier working with Yanmar. If I would have I would be seriously underpowered. They recommended 19x13 prop and the one I have now is 19x15. The new prop is spot on. The techincal inspector from Yanmar was pleased as well.
I think the supplier working with Yanmar really wanted to err on the safe side to make sure that I can reach max rpm.
Thank you all who responded to this thread. It was really helpfull. Fair winds, Rotor is Out.