Propellor sizing confusion

[Trident]

So motoring my new to me catamaran up the solent she was impressively slow. She has two Yanmar 3GM30 engines and at 2500 rpm on both would only make 5 knots (and down to 4 against the solent currents). She has SD20 sail drives and currently 16/12 three bladed props which is what Yanmar recommend. She is 50 foot and only around 9 tonnes and the manufacturers blurb suggests 10 knots under engine, though that was with a shaft drive not sail drive. (Bottom was relatively clean, engines working ok etc) - so I was looking at the props - my last boat was also a cat and had a single sillette drive but the same prop and same gear ratio . She was also slow until I changed (against the advice of Sillette ) to a 16/14 and then she would push in to wind and seas like never before and make 2 knots more in flat calm.

Is there likely to be any issue with doing the same with the Yanmar sail drives? Its very firm about not going bigger (and I could fit a 17 inch but I guess they say no more than 16 for a good reason?) and strongly recommends the pitch I have now but could I really harm it if I change.

Sadly none of the prop calc software is designed for cats so getting a definitive answer on size and pitch is difficult - hence hoping for some more expert opinions on here

 

[CCJ]

what RPM are you getting with full throttle

 

[Trident]

It should be around 3400 but I never take it above 2500 rpm as the full throttle revs is only for short bursts. I appreciate its a useful figure for doing the prop calcs but I don't know and now she's out of the water for a refit so I can't test it

 

[Tranona]

It should be around 3400 but I never take it above 2500 rpm as the full throttle revs is only for short bursts. I appreciate its a useful figure for doing the prop calcs but I don't know and now she's out of the water for a refit so I can't test it

It is all guesswork until you know exactly what the current performance is. The engine is rated to 3600 rpm and I would suggest you need all of that. At 2500 you are only drawing 16 or 17hp out of each engine and trying to move 9 tons and probably a lot of windage (you don't say what boat it is) and a cruising speed of 5 knots is not far out. If you want to go faster or maintain speed against a chop then you need to apply more power.

Before making any changes suggest you do some runs in flat water at 200 rpm intervals from 2000 up and plot speed. Would expect a maximum of +/- 8 knots at minimum of 3500 rpm.

 

[RichardS]

So motoring my new to me catamaran up the solent she was impressively slow. She has two Yanmar 3GM30 engines and at 2500 rpm on both would only make 5 knots (and down to 4 against the solent currents). She has SD20 sail drives and currently 16/12 three bladed props which is what Yanmar recommend. She is 50 foot and only around 9 tonnes and the manufacturers blurb suggests 10 knots under engine, though that was with a shaft drive not sail drive. (Bottom was relatively clean, engines working ok etc) - so I was looking at the props - my last boat was also a cat and had a single sillette drive but the same prop and same gear ratio . She was also slow until I changed (against the advice of Sillette ) to a 16/14 and then she would push in to wind and seas like never before and make 2 knots more in flat calm.

Is there likely to be any issue with doing the same with the Yanmar sail drives? Its very firm about not going bigger (and I could fit a 17 inch but I guess they say no more than 16 for a good reason?) and strongly recommends the pitch I have now but could I really harm it if I change.

Sadly none of the prop calc software is designed for cats so getting a definitive answer on size and pitch is difficult - hence hoping for some more expert opinions on here

Judging from my experience, I think you are a bit on the slow side.

Our cat is 40 feet and 7 tonnes with two 3YM30 - 29HP engines and SD20 sail drives. She came with 16 x 13 fixed twin blade props and with 2500 on both engines would do between 6 and 6.5 knots in flattish conditions.

I've now replaced the fixed props with 16 x 13 Flex-O-Folds and I thought of going bigger but Tranona advised against it and suggested that I would get better performance out of the FoF's than the Radices ..... and he was right. At 2500 I'm now getting 7 - 7.5 knots.

I would have thought that with the correct 3-bladers and a much longer cat you should also be in the 6 - 7 category at 2500 but I'm no expert, as you can tell.

Richard

 

[geem]

So motoring my new to me catamaran up the solent she was impressively slow. She has two Yanmar 3GM30 engines and at 2500 rpm on both would only make 5 knots (and down to 4 against the solent currents). She has SD20 sail drives and currently 16/12 three bladed props which is what Yanmar recommend. She is 50 foot and only around 9 tonnes and the manufacturers blurb suggests 10 knots under engine, though that was with a shaft drive not sail drive. (Bottom was relatively clean, engines working ok etc) - so I was looking at the props - my last boat was also a cat and had a single sillette drive but the same prop and same gear ratio . She was also slow until I changed (against the advice of Sillette ) to a 16/14 and then she would push in to wind and seas like never before and make 2 knots more in flat calm.

Is there likely to be any issue with doing the same with the Yanmar sail drives? Its very firm about not going bigger (and I could fit a 17 inch but I guess they say no more than 16 for a good reason?) and strongly recommends the pitch I have now but could I really harm it if I change.

Sadly none of the prop calc software is designed for cats so getting a definitive answer on size and pitch is difficult - hence hoping for some more expert opinions on here

Hi Trident. A couple pf observations. Having been at a boatyard when a cat identical to yours was lifted, with cruising gear onboard it weighed 14t. 9t sounds very optimistic in my opinion. Those Yanmars are tiny for your boat compared to what is routinely fitted to modern cats of the same length. Twin 75 hp is not uncommon. Bare in mind a Lagoon 52 weighs 26t. Also witnessed at lift out. We used to routinly cruise our 30 hp engine in a Snowgoose at 2800-3000 rpm to do about 6 kts. She was 5.3t in normal cruising trim. All the best.

 

[tim]

I have a Solaris Sunrise 36 with twin VP 2003s. Like RichardS I've fitted FlexoFold 16x13s and achieve the same performance as him, although I typically run at 2000 revs and get 6 kts (lighter boat?).

 

[cueball]

Hi Trident,

What was the wind and wave conditions?

 

[Trident]

Perhaps I do need to use more revs but the diesel usage goes through the roof over 2400 rpm

Cueball - wind was quite light and no waves at all

Geem - you may be right - I have so far stripped out 3 skips worth of crap from the boat and she was carrying around 1000L of water - perhaps the will do better after my refit - a lot of weight is being shed...

 

[Tranona]

Perhaps I do need to use more revs but the diesel usage goes through the roof over 2400 rpm

The amount of fuel you use is directly related to the power you are drawing from the engines. You need a lot of hp to shift 9+ tons. Your engines are on the small side, but even if you have the larger 40hp engines you will consume the same amount of fuel for a given speed - just that the revs will be lower and the noise less. A monohull of that displacement would typically have 65-75 hp engine to get hull speed.

 

[QBhoy]

Hi there.
Being a pretty big geek in all things propeller related, can I strongly recommend you try and push your engines to the max recommend rpm ? A few essentials are required when selecting propellers accurately.
Current prop size
Gear ratio
Current rpm achieved
And max speed achieved gps.

Using these details you can usually pretty accurately size a prop up.

Can I also suggest that if your engine is rated to 3600 rpm, you should have no concerns taking to that rpm on occasion. In fact it’s quite good for a Diesel engine or engines to be worked hard from time to time. All diesels like to be worked hard every now and then, otherwise they carbon up and glaze the cylinders. No matter how harsh it might sound on the engine, it should do no harm to blow off the cob webs every now and then. You’ll find they burn more efficiently and keep in better health overall.