Propeller tunnel: yes or no?

[marcochi76]

Could you please explain me the pro and cons of propeller tunnels?
I was looking at the Princess V52 (with shafts) and I noticed it has prop tunnels.
They claim better efficiency and reduced drag.

I see some boats use tunnel, other don"t.

Is this a good design?
Thanks

 

[tico]

My take on it - more than happy to be corrected...
Pro's - Lower shaft angle (more efficient) , less draft
Cons - Less buoyancy (Stern lift)

 

[PowerYachtBlog]

Negatives;
- higher low planning speeds, usually about 16/7 knots
- more susceptible to weight distribution, hence why a lot of boats who have them ride bow high

I think nowadays its near impossible to find a boat without them.
Although many Italian builders up till the noughties where not big fans of tunnels.
Even tough technically speaking they where invented by Sonny Levi.

 

[marcochi76]

PowerYachtBLog: and what are the real benefits?
thanks

 

[MapisM]

Even tough technically speaking they where invented by Sonny Levi.

I think you're mixing up hull tunnels with the LDUs (Levi Drive Unit), which were surface transmissions with semi-tunnels at the stern, above the propellers, working as rudders.
I can't recall any Levi boat with tunnels moulded into the hull - but I'm happy to stand corrected if you can name any.

 

[MapisM]

PowerYachtBLog: and what are the real benefits?
thanks

My understanding is that both pros and cons are marginal at best, with cons largely overweighing pros if the hull and its powerplant aren't carefully designed for tunnels from the ground up.
In fact, the main reason why as PYB said they are nowadays increasingly popular is that AOTBE they take a bit less space in the e/r.
And onboard space for anything else but the e/r is what sell boats...

PS: anyway, if the Princess V52 ticks your boxes, the fact that she has prop tunnels per se would be the last of my concerns.
And the same still stands if she would NOT have tunnels, mind!

 

[Portofino]

PowerYachtBLog: and what are the real benefits?
thanks

There are none ultimately.In terms of seakeeping and overall performance.40-60 ft range .

Starting from the gold std .This is it . You are looking @ a 23 degree dead-rise. Mid mounted engines in the centre first accommodation second .
Props hanging free in the water flow with very low shaft angles for greater efficiency fwd thrust , less lost thrust.



How ever these days the “ hotel “ side sells boats so todays buyers apart from a very few will not contemplate splitting up the accommodation .

In a compromising way tunnels allow the engines to be set further aft , as you move back the shaft angles without the tunnel will be excessively high so they simply sink the shaft + prop into the aft section’s hence the tunnel .Otherwise as you say the drag will be horrendous, so by tunnelling them they reduce it a bit .
Stern lift is lost , or turn around greater suction created , hence they need a higher planning speed and often ride bow high once up , like this .


You see dock walking various “add on s “what look like after thought bolt on ,none adjustable flaps to some boats.They add back the lost lift .Or these days permanent Humphrey style interceptors to again add lift .
Arguably more drag like lowering your movable flaps add s drag when they are down . But the net effect is a lower bow , more level running attitude but a slower boat for its given Hp .

You are also ( by shifting weight back ) messing with the CoG and its relation to the CoL ( centre of lift ) .
Builders can 1/2 offset this by placing tanks under fwd beds, or other weighty stuff Fwds .Depends on the size of the boat how much latitude this has .

Not forgetting extra noise as the water implodes on the tunnel sides , so in a 40-60 ft more vibrations , whether its noticed is another matter .

A perceived benefit is obviously better hotel accommodation.But it comes at a drop in performance.More NHV as said .
How much of a drop off of performance who cares , depends ?

From previous posts I note you are looking to improve on the sea keeping of your princess outdrive boat .

A lot of weight in the middle as close to the CoL and lower COG ( hardly any superstructure ie a sports cruiser ) makes for a wave crushing ride less pitching about .But this is just one segment of the orange in the overall package .

You need the rest of the orange segments to feel the real benefits of mid engined .True deep V being anther .

I mean imagine in a car forum if you said “ seen some cars with the engine seemingly in the middle …what’s the benefit of this mid engined malarkey …..?

It feels just like that when you helm one , you can immediately tell the difference from a rear weight bias set up boat at sea through waves / other folks wake .

 

[PowerYachtBlog]

I think you're mixing up hull tunnels with the LDUs (Levi Drive Unit), which were surface transmissions with semi-tunnels at the stern, above the propellers, working as rudders.
I can't recall any Levi boat with tunnels moulded into the hull - but I'm happy to stand corrected if you can name any.

Yes this is correct. But the Levi Drive Unit, as first fitted on the Italcraft Drago was the first use a tunnel above the propellers.
It is in theory and use a different concept to what is used today on the majority of boats, moulded propeller tunnels.

Even though Sonny Levi was consulted to fit tunnel drives on some of the more traditional Deep-Vee Italcraft C-series released in the mid eighties, like the C-58 and C45.

I think Hatteras where the first to fit a moulded tunnel propeller in the hull in the eighties.

 

[PowerYachtBlog]

PowerYachtBLog: and what are the real benefits?
thanks

Tico told you a couple.
Even though the five inches of draft difference (average for a fifty footer) really do not deserve a mention and will not give you (me) a sleepless night of happiness.

The real benefit would be cleaner water at above 22/24 knots, which is when I think you start feeling the advantage.
Then after that it would have to be a case to case report.
If I remember well the Princess V52.II has a nice bow angle,

Propeller Tunnels.
Do they improve seakeeping? No as you need faster speeds to lift plane.
Do they improve economy? Probably you get a .2 to .4 litres per nautical mile above 22/24 knots
What they improve? Probably better direction stability as the trust of the propellers has cleaner water.
Why are they used so much? Mystery of the third kind.

 

[Elessar]

There are none ultimately.In terms of seakeeping and overall performance.40-60 ft range .

Starting from the gold std .This is it . You are looking @ a 23 degree dead-rise. Mid mounted engines in the centre first accommodation second .
Props hanging free in the water flow with very low shaft angles for greater efficiency fwd thrust , less lost thrust.
View attachment 165318


How ever these days the “ hotel “ side sells boats so todays buyers apart from a very few will not contemplate splitting up the accommodation .

In a compromising way tunnels allow the engines to be set further aft , as you move back the shaft angles without the tunnel will be excessively high so they simply sink the shaft + prop into the aft section’s hence the tunnel .Otherwise as you say the drag will be horrendous, so by tunnelling them they reduce it a bit .
Stern lift is lost , or turn around greater suction created , hence they need a higher planning speed and often ride bow high once up , like this .
View attachment 165321

You see dock walking various “add on s “what look like after thought bolt on ,none adjustable flaps to some boats.They add back the lost lift .Or these days permanent Humphrey style interceptors to again add lift .
Arguably more drag like lowering your movable flaps add s drag when they are down . But the net effect is a lower bow , more level running attitude but a slower boat for its given Hp .

You are also ( by shifting weight back ) messing with the CoG and its relation to the CoL ( centre of lift ) .
Builders can 1/2 offset this by placing tanks under fwd beds, or other weighty stuff Fwds .Depends on the size of the boat how much latitude this has .

Not forgetting extra noise as the water implodes on the tunnel sides , so in a 40-60 ft more vibrations , whether its noticed is another matter .

A perceived benefit is obviously better hotel accommodation.But it comes at a drop in performance.More NHV as said .
How much of a drop off of performance who cares , depends ?

From previous posts I note you are looking to improve on the sea keeping of your princess outdrive boat .

A lot of weight in the middle as close to the CoL and lower COG ( hardly any superstructure ie a sports cruiser ) makes for a wave crushing ride less pitching about .But this is just one segment of the orange in the overall package .

You need the rest of the orange segments to feel the real benefits of mid engined .True deep V being anther .

I mean imagine in a car forum if you said “ seen some cars with the engine seemingly in the middle …what’s the benefit of this mid engined malarkey …..?

It feels just like that when you helm one , you can immediately tell the difference from a rear weight bias set up boat at sea through waves / other folks wake .

And you have to go through the wake because unless you slow down you can’t steer.

“Gold standard”

If you had a sailing boat it would be an Anderson 22.

 

[MapisM]

Props hanging free in the water flow

Aside from being disturbed (and rudders even more so) by an awful P-bracket installation, as your pic clearly points out.

 

[MapisM]

Sonny Levi was consulted to fit tunnel drives on some of the more traditional Deep-Vee Italcraft C-series released in the mid eighties, like the C-58 and C45.

No idea about the C45, but one of the many boats I considered during my last search was the C58, and I'm pretty sure that she has plain vanilla shafts with no tunnels.
Its only peculiarity, aside from the high deadrise, are the transom rudders - both arguably pointless on what was meant to be a sedate flybridge boat, but...
...pretty sure at Italcraft in those days they weren't shy to adopt pointless solutions!
like the Aermar with full flagged LDUs, which made the boat a maneuvering nightmare...

 

[Portofino]

Aside from being disturbed (and rudders even more so) by an awful P-bracket installation, as your pic clearly points out.

What’s your issue with the P bracket installation ?

 

[MapisM]

C'mon Porto, you know exactly what it is, 'cause we already discussed it in the past.
You may pretend it's not so relevant, and obviously I couldn't prove it is with any numbers.
But if the simple observation that all half decent boatbuilders always installed the brackets flush with the hull bottom means nothing to you, who am I to try to convince you that Amati also made schoolboy mistakes, sometimes?

 

[ChromeDome]

Just a bit of real life observation:

My 30' twin diesel planing hull has tunnels and engines and a 600-liter fuel tank placed almost central in the boat. 3.44 m beam, 7½ ton with full tanks and ready to go.

It is directionally stable at all speeds, even at a waterline speed of 7 knots. It does not have a distinct planing threshold and runs quite flat when at a stable plane. Top speed is 24 knots and it has trim tabs, but at this weight, the effect is limited.

The keel and bow design line helps with reasonable comfort in rough seas.

 

[Portofino]

C'mon Porto, you know exactly what it is, 'cause we already discussed it in the past.
You may pretend it's not so relevant, and obviously I couldn't prove it is with any numbers.
But if the simple observation that all half decent boatbuilders always installed the brackets flush with the hull bottom means nothing to you, who am I to try to convince you that Amati also made schoolboy mistakes, sometimes?

Others might be interested so I will attempt to enlighten them .You are most welcome to come along for the ride .Metaphorically speaking .

look at this vid ( apologies for the wind on phone mic early on btw )

The primary wake is caused by water being displaced laterally.Water flow with deep dead rises is hugely displaced sideways .More so then shallow dead rise even more so around and above 30 knot cruise the designed running speed of this boat .

It doesn’t run parallel to the hull like a flat bottom SUP or surf board .Theses have zero to little wake as you all know .

This huge lateral displacement as well as creating a huge wake creates a negative pressure at the stern between the P brackets .Other shallow dead rise say a 16 degree for example will suffer reduced lift , but at certain deadrises ( deep ) that reduced lift at certain speeds turns into negative pressure .

The props are as far back as they can sensibly be placed , this also reduced shaft angle .But in this delta we are interested in the neg pressure area where the P brackets sit ,which is tiny now.
In this area any real hydrodynamic interference, like the fwd edge of its P bracket not faired in is insignificant .
Indeed the interference might actually counter somewhat the neg pressure build up .
So it’s deliberate or it’s drag insignificant to even productive in terms neg pressure reduction .
Suggesting Amati made a “ school boy “ error illustrates lack of knowledge .

Re watch the vid with this in mind .

First the primary deflection or initial wake is caused by the violent lateral impact and displacement of water .

The mini rooster tail immediately after the transom is caused by the neg pressure release sucking up water .Counterintuitively it’s NOT caused by the props throwing water ….I know it appears so .

But this guy says not .Theres amongst other things a whole chapter on propellor positions and wake effects .


The second V , v in the wake further back approx a boat L ( go look at the vid a third time ) is the added effect of the primary V rolling back meeting the initial rooster tail ( that’s caused by neg pressure ) and adding water , summing up it .creates a another mini wake , and enlarges the initial rooster tail .

So why don‘t they all do it ? Builders I mean .Deep V s , true deep V by accepted definition is over 20* dead rise create huge wake which induced drag and requires a lot of HP .HP is expensive and adds weight .

This little 13 M WL ( 14.8 LOA ) hull is powered by relatively huge by boat size engines .13 L inline 6 s 700 Hp .Compare those dims to Elasars boat .Furthermore the slightly by 1/2 M longer Itama 46 ( bit beamier ) by V8 15 L 800 Hp the same size in MapishM s but shorter and lighter .
You loose out on efficiency in terms of Hp needed and this impacts on fuel consumption the greater the dead rise you go .Engines are expensive.Not just the engine but everything to support ….gearbox , shafts , props , tanks eg I have 2000 L in three tanks etc etc .

As i said at lower deadrise fwd props slower speeds like MapishM boat or Elasars chugging round at low twenty knots , then the smooth water flow around the faired in P bracket might have benefits .
The lower dead rise , with conventional p bracket / shaft and prop hanging + stern flatness means a lower reduction of neg lift or more stern lift . The water flow will be more longitudinal less lateral so it’s important to fair in in .

Unless of course it’s tunnelled which is where we began



Probably not explain this well ( nothing new there ) .Sorry chaps .

Watch the vid .

 

[oldgit]

ACM 38 with tunnels. 2 x 240HP. 6500 Kilos Fuel 1000K
Princess 35 . No tunnels. 2 x 200HP 6300 Kilos Fuel. 600L
Observations for ordinary mortals with run of the mill boats.

The P35. Basically its open the throttles wide and its bow up rendering any forward vision impossible until it gets over the hump where apon it sort of flops down onto the plane.
You can then throttle back to your required cruise speed. Adjusting the trim tabs makes barely any difference speed wise but need to be fully on to achieve anything noticeable.

The ACM. You only need open the throttles enough to gently get onto the minimum planing speed without drama or fuss.
Speed can then be increased as required.
Trim tabs do make a difference to speed but not that much to trim angle.

Minimum planing speed lower on the ACM than the Princess by a couple of knots and a far more comfortable and quiet cruise, the P35 always felt to be working hard to keep up the momentum.

Going astern in hurry . The Princess did make a bit of a fuss, the ACM sounds and feels like the props are about to join you inside the boat !
The P35 responded more decisively when changing direction at speed but heels more ..
Ability of both boats to maintain a heading when crossing wash about the same.

Both boats at WOT not cruising speed.
ACM

P35

One of these boats is having trouble lifting its bum out of the water and its not the one with tunnels.

 

[Portofino]

Just a bit of real life observation:

My 30' twin diesel planing hull has tunnels and engines and a 600-liter fuel tank placed almost central in the boat. 3.44 m beam, 7½ ton with full tanks and ready to go.

It is directionally stable at all speeds, even at a waterline speed of 7 knots. It does not have a distinct planing threshold and runs quite flat when at a stable plane. Top speed is 24 knots and it has trim tabs, but at this weight, the effect is limited.

The keel and bow design line helps with reasonable comfort in rough seas.

Lovely boat but i see the stern sinking when running exactly illustrates what tunnels do .

Heres mine running @ 27 knots .( fouled late season ) Note the water level .

 

[Elessar]

Others might be interested so I will attempt to enlighten them .You are most welcome to come along for the ride .Metaphorically speaking .

look at this vid ( apologies for the wind on phone mic early on btw )

The primary wake is caused by water being displaced laterally.Water flow with deep dead rises is hugely displaced sideways .More so then shallow dead rise even more so around and above 30 knot cruise the designed running speed of this boat .

It doesn’t run parallel to the hull like a flat bottom SUP or surf board .Theses have zero to little wake as you all know .

This huge lateral displacement as well as creating a huge wake creates a negative pressure at the stern between the P brackets .Other shallow dead rise say a 16 degree for example will suffer reduced lift , but at certain deadrises ( deep ) that reduced lift at certain speeds turns into negative pressure .

The props are as far back as they can sensibly be placed , this also reduced shaft angle .But in this delta we are interested in the neg pressure area where the P brackets sit ,which is tiny now.
In this area any real hydrodynamic interference, like the fwd edge of its P bracket not faired in is insignificant .
Indeed the interference might actually counter somewhat the neg pressure build up .
So it’s deliberate or it’s drag insignificant to even productive in terms neg pressure reduction .
Suggesting Amati made a “ school boy “ error illustrates lack of knowledge .

Re watch the vid with this in mind .

First the primary deflection or initial wake is caused by the violent lateral impact and displacement of water .

The mini rooster tail immediately after the transom is caused by the neg pressure release sucking up water .Counterintuitively it’s NOT caused by the props throwing water ….I know it appears so .

But this guy says not .Theres amongst other things a whole chapter on propellor positions and wake effects .
View attachment 165402

The second V , v in the wake further back approx a boat L ( go look at the vid a third time ) is the added effect of the primary V rolling back meeting the initial rooster tail ( that’s caused by neg pressure ) and adding water , summing up it .creates a another mini wake , and enlarges the initial rooster tail .

So why don‘t they all do it ? Builders I mean .Deep V s , true deep V by accepted definition is over 20* dead rise create huge wake which induced drag and requires a lot of HP .HP is expensive and adds weight .

This little 13 M WL ( 14.8 LOA ) hull is powered by relatively huge by boat size engines .13 L inline 6 s 700 Hp .Compare those dims to Elasars boat .Furthermore the slightly by 1/2 M longer Itama 46 ( bit beamier ) by V8 15 L 800 Hp the same size in MapishM s but shorter and lighter .
You loose out on efficiency in terms of Hp needed and this impacts on fuel consumption the greater the dead rise you go .Engines are expensive.Not just the engine but everything to support ….gearbox , shafts , props , tanks eg I have 2000 L in three tanks etc etc .

As i said at lower deadrise fwd props slower speeds like MapishM boat or Elasars chugging round at low twenty knots , then the smooth water flow around the faired in P bracket might have benefits .
The lower dead rise , with conventional p bracket / shaft and prop hanging + stern flatness means a lower reduction of neg lift or more stern lift . The water flow will be more longitudinal less lateral so it’s important to fair in in .

Unless of course it’s tunnelled which is where we began

View attachment 165404

Probably not explain this well ( nothing new there ) .Sorry chaps .

Watch the vid .

I have to say that’s a good explanation.

It explains why I would never have a deep V boat as they are inefficient and for me offer zero benefit. My boat is more capable than I am and that’s good enough.

I know you love yours though so I’m glad it exists.

The explanation (assuming it’s correct) mitigates the impact of not fairing the P brackets but it doesn’t excuse it. It looks terrible! It unfathomable why they didn’t even if it makes little or no performance impact. You can’t see them when you are enjoying the boat though so it’s fine.

 

[benjenbav]

I think I just watched a video clip of a dog’s water bowl?

Tunnels allow engines to be placed further aft in the hull. That creates more usable real estate forward of the engines. Likewise v drives for that matter.

Given that - in comparison to, say, automobile design - naval architecture is a mix of back-of-a-fagpacket inspiration and post-hoc rationalisation the consequential effects on running angle, comfort and handling then get addressed by lumping weight forward, adding interference etc.

Hull shape is a whole other kettle of fish. Cigarettes might be fast but it’s tough to sell them to folk who want to drive their family and friends to a quiet anchorage to spend the day playing in the water and enjoying fine food and wine.