Thrust differences between petrol outboard and inboard diesel of same horsepower

[TANSL]

Of course, but it will depend on the exposure time. I don't think a rubber bellows, properly stored, will last as long as another, identical one exposed to the elements. And, honestly, I can't believe that its wear doesn't depend on use. But I'm probably misunderstanding you. For my part, I think this exchange of opinions, well backed up by facts, is not worth continuing.
Cheers.

 

[comfisherman]

Have a few anecdotes from over the years.

Few years back my family ended up with 2x boats with the same engines at near the same hours of operation. One had a 4.5:1 gear and a 50 inch prop the other a 1.78:1 gear swinging a 28 inch wheel. The former was a deep displacement hull that probably ran around light a bit over 110k pounds and a waterline length of about 48 feet. The other a shallow tunnel hull witb a waterline around 42 feet that weighted light in the upper 30k range.

The deep boat cruised almost 2 knots faster at the same rpm, remember at one point we wrapped a bunch of line up and towed the smaller boat in with the bigger boat to beach it and clear the wheel. Crew gave me grief as the deeper ratio engine moved the combo about as fast as the cruising speed of the little boat. It was always a bit surprising how much more oomph a big slow wheel could produce.

Our boats often chase shallow, it always blows me away to see speed and performance figures for the lobster boats back east. Have a friend in Maine with a boat longer and bigger than mine but with a 3:1 gear and a 36 inch wheel and the exact same main as me. His top end is a couple knots faster than mine, but im rocking a 1.6:1 and a deep tunnel with a 28 inch wheel. Wouldn't want to fish his boat here, but man Im always jealous of his ability for speed.

 

[RSD]

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.

OK that is good to know - having to pull the boat out every 200 hours was going to be a right pain in the butt - as that would mean the boat had to be pulled every month. Is there anything that needs to be replaced at intervals of less than 2 years when used for 200 hours per month? (things that would require the boat to be pulled out of the water).

How well does the new Volvo Penta Active Corrosion Protection system work? And does the electric current it uses cause any problems for aluminium boats?

 

[RSD]

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

Coming from a country (Australia) where UV degrades most things I can well understand that.

 

[RSD]

Have a few anecdotes from over the years.

Few years back my family ended up with 2x boats with the same engines at near the same hours of operation. One had a 4.5:1 gear and a 50 inch prop the other a 1.78:1 gear swinging a 28 inch wheel. The former was a deep displacement hull that probably ran around light a bit over 110k pounds and a waterline length of about 48 feet. The other a shallow tunnel hull witb a waterline around 42 feet that weighted light in the upper 30k range.

The deep boat cruised almost 2 knots faster at the same rpm, remember at one point we wrapped a bunch of line up and towed the smaller boat in with the bigger boat to beach it and clear the wheel. Crew gave me grief as the deeper ratio engine moved the combo about as fast as the cruising speed of the little boat. It was always a bit surprising how much more oomph a big slow wheel could produce.

Our boats often chase shallow, it always blows me away to see speed and performance figures for the lobster boats back east. Have a friend in Maine with a boat longer and bigger than mine but with a 3:1 gear and a 36 inch wheel and the exact same main as me. His top end is a couple knots faster than mine, but im rocking a 1.6:1 and a deep tunnel with a 28 inch wheel. Wouldn't want to fish his boat here, but man Im always jealous of his ability for speed.

Many thanks. I've seen some weird things with the performance of trawlers here too when they are all heading in to avoid coming bad weather - unfortunately I don't get to learn about what props etc they are using.

 

[RSD]

(Maybe) a better way of asking what I originally asked -

I read somewhere that water jets are the most efficient of all "reasonably common" propulsion types (so excluding things like surface drives etc).

Assuming that to be true - and using for this example a 350hp motor (petrol or diesel as appropriate) that efficiency is defined as "thrust gained vs fuel used when the correct ratio and size prop/impeller/propeller/whatever similar is used on a given hull powered by the same HP engine", lets say that the efficiency scale we are using is from 0 to 100 - and that 0 means zero, and 100 is the efficiency of a water jet - given this what would the efficiency of

• a 350hp Suzuki outboard running Suzuki's Duo-Prop counter rotating props
• a 350hp inboard diesel engine mated to Volvo Penta's current stern drive that runs counter-rotating props
• a 350hp inboard diesel engine mated to a correctly geared and propped shaft drive
• there are probably some others that I have forgotten

I know that the above is somewhat generic for want of a better term and that it will probably confuse some but hopefully it will make sense to others.

 

[DogCavalry]

Efficiency defined like this, where work done is divided by energy(fuel) input, give water jets extremely poor efficiency. They aren't used for mileage, but are often the best solution for a specific engineering challenge.

Thrust is force applied in the direction you want it applied. In a water environment, you get that force by momentum transfer. Accelerating water in the other direction, changing its momentum gives you a reaction force, like Newton's 3rd law :mass times velocity change. "mv"
Unfortunately that gives the water kinetic energy "½mv²" A merciless relationship. Accelerating half as much water twice as fast gives the same thrust but takes twice as much energy. Water jets accelerate small amounts of water to high speed. For "efficiency" in the thermodynamic sense, you want to maximise mass flow. Biggest prop, turning slowest. Unfortunately, that generally doesn't meet the SOR.

 

[RSD]

Water jets would be my preferred option for this but had been ruled out as the boat needed dynamic positioning and the water jet systems that had dynamic positioning controls were the high end ones that were simply too expensive. However... by pure chance a couple of days ago I came across an aftermarket system that will work with almost any water jet system - this system is from a company called Glendinning - Marine Waterjet Propulsion Controls - Glendinning Products LLC https://www.glendinningprods.com/products/complete-controls/waterjet-controls/ - potentially this now opens up the possibility for water jets to be an option once again as it means that we could fit water jets that aren't crazily expensive.

 

[BlueBell]

...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?

Duo Prop hands down for trection.

EDIT: Just realized the comparison was inboard to outboard.
That changes everything.
It depends how big a prop you can fit on the inboard.
It also depends on a bunch of other variables not elaborated on.

So, not enough info given.

 

[DogCavalry]

Yes indeed. Extensive testing by NACA showed that duoprops gave an average of an extra 11% thrust with the same power and diameter. They worked by recovering the rotational energy lost after the first prop. I often see them poorly implemented even in 2025. The second prop must have a different number of blades, and have ~10% smaller diameter.

 

[gonzo]

OK that is good to know - having to pull the boat out every 200 hours was going to be a right pain in the butt - as that would mean the boat had to be pulled every month. Is there anything that needs to be replaced at intervals of less than 2 years when used for 200 hours per month? (things that would require the boat to be pulled out of the water).

How well does the new Volvo Penta Active Corrosion Protection system work? And does the electric current it uses cause any problems for aluminium boats?

The active corrosion protection is an applied voltage. It protects all parts of the boat that are connected to it.
I would suggest changing the impeller yearly. It is also an opportunity to grease the input shaft, u-joints and gimbal bearing.

 

[baeckmo]

.........I would suggest changing the impeller yearly. It is also an opportunity to grease the input shaft, u-joints and gimbal bearing.

Fair enough about impeller changing period, but as far as I know, there is NO VOLVO I/O DRIVE WITH COOLING WATER IMPELLER; is always located to front of engine!
The only I/O drive in production today with impeller is the Mercruiser Alpha and possibly some clone thereof. These are not suitable for the application discussed here.

 

[gonzo]

I know the location of the impeller. It is quite rude to scream at me (all caps). However, taking the boat to the shop is a good opportunity for the yearly maintenance.

 

[TANSL]

Sure, but that's another matter.

 

[baeckmo]

I know the location of the impeller. It is quite rude to scream at me (all caps). However, taking the boat to the shop is a good opportunity for the yearly maintenance.

In both your posts no 11 and 26 you mention the impeller in the context about I/O drive removal and service. For the uninformed reader I think that might (wrongly) be interpreted as if there were cooling water impellers somehow integrated into the Volvo drive legs. I find my use of caps to notify the actual fact was relevant, since a misunderstanding might lead to incorrect selection of components.