Thrust differences between petrol outboard and inboard diesel of same horsepower

[RSD]

What are people's thoughts on what the difference in thrust might be between a Suzuki 350hp outboard with their Duo Prop contra-rotating propeller system and a 350hp diesel inboard running a single prop? Would there be a big difference in thrust between them?

Application will be a 12 metre/39 foot aluminium catamaran fitted with twin of whatever option we end up going with.

 

[gonzo]

It depends on several factors. For example, are you calculating bollard pull, like a tug at zero speed? Do you need high acceleration like a ski boat?

 

[RSD]

It depends on several factors. For example, are you calculating bollard pull, like a tug at zero speed? Do you need high acceleration like a ski boat?

Neither of those ends of the spectrum - more in the middle - think 12 metre aluminium catamaran work boat needing to carry a 3000kg load on deck.

 

[TANSL]

I'd always believed that the thrust of a propulsion system was independent of its intended purpose. It seems I'm wrong.

 

[RSD]

I'd always believed that the thrust of a propulsion system was independent of its intended purpose. It seems I'm wrong.

Not wrong - just not understanding the question.

 

[TANSL]

Not wrong - just not understanding the question.

I think I understand the question perfectly. What I don't understand is the answer.

 

[powerabout]

The bit everyone gets wrong trying to compare an outboard and a diesel (unless BP as Gonzo mentions), is the reality when the boat is moving the diesel goes from idle to max rpm say 3000 and the outboard goes idle to 6000
Now put the gear ratio and prop diameter in there, miles apart.
Outboard has increasing torque curve to high revs, following hull resistance increase and diesel peaks very low in its rpm range ( why tugboats use diesels turbos can lift peak torque curve)

 

[RSD]

The bit everyone gets wrong trying to compare an outboard and a diesel (unless BP as Gonzo mentions), is the reality when the boat is moving the diesel goes from idle to max rpm say 3000 and the outboard goes idle to 6000
Now put the gear ratio and prop diameter in there, miles apart.
Outboard has increasing torque curve to high revs, following hull resistance increase and diesel peaks very low in its rpm range ( why tugboats use diesels turbos can lift peak torque curve)

Many thanks @powerabout - I guess in a way it is like comparing apples to oranges - they are both fruit, both will feed and sustain you for a short while etc.

I've been doing some more reading/research, and I'm now leaning towards something in between what I originally asked about - I've got more research to do but Volvo Penta's D6 DPI engines with stern drives offer the efficiency / convenience / less fire risk of diesel with contrarotating props - and they can be paired with Penta's joystick system which allows dynamic positioning. I don't know much about Volvo Penta, I know that stern drives don't have the best reputation and that you have to pull them out of the water every 200 hours or so to replace bellow seals etc - so not as convenient as outboards but swings and roundabouts I guess. The "ideal" option would be diesel engines with jet drives and dynamic positioning - but jet drives seem to be far far more expensive than other propulsion options unfortunately.

 

[powerabout]

Many thanks @powerabout - I guess in a way it is like comparing apples to oranges - they are both fruit, both will feed and sustain you for a short while etc.

I've been doing some more reading/research, and I'm now leaning towards something in between what I originally asked about - I've got more research to do but Volvo Penta's D6 DPI engines with stern drives offer the efficiency / convenience / less fire risk of diesel with contrarotating props - and they can be paired with Penta's joystick system which allows dynamic positioning. I don't know much about Volvo Penta, I know that stern drives don't have the best reputation and that you have to pull them out of the water every 200 hours or so to replace bellow seals etc - so not as convenient as outboards but swings and roundabouts I guess. The "ideal" option would be diesel engines with jet drives and dynamic positioning - but jet drives seem to be far far more expensive than other propulsion options unfortunately.

Long term the only thing to own is shaft drive diesel
Perhaps a charter boat can afford to replace the outboards at an interval less than the diesel, you can get joystick for them too.

 

[gonzo]

The bit everyone gets wrong trying to compare an outboard and a diesel (unless BP as Gonzo mentions), is the reality when the boat is moving the diesel goes from idle to max rpm say 3000 and the outboard goes idle to 6000

Correct gearing can adjust that. For example, if the propeller has to turn at 3000 RPM, a diesel would have a direct drive and an gas a 2:1 gear ratio. The propeller turns at the same speed with both setups. The torque curve is different though.

 

[gonzo]

I don't know much about Volvo Penta, I know that stern drives don't have the best reputation and that you have to pull them out of the water every 200 hours or so to replace bellow seals etc

They don't need bellows replaced every 200 hours, but maybe every 5 years. Impellers last a lot longer than 200 hours too. They usually get replaced every two or three years. I am a factory trained Volvo Penta mechanic.

 

[TANSL]

I am not lucky enough to be a "factory trained Volvo Penta mechanic" but I can say that no manufacturer talks about repairs or spare parts every xx years but rather every xx hours of normal operation.

 

[gonzo]

Bellows don't wear depending on hours of operation, but age regardless of use.

 

[TANSL]

Frankly, I couldn't have imagined it. I guess that's what they call "planned obsolescence."

 

[gonzo]

Rubber degrades with oxidation and UV damage. It is not planned.