Altering a Planning Motorboatl?

[MapisM]

in your option would the installation of stablisers alone work sufficiently.

Absolutely, not a single doubt about that.
You can't really believe how effective fin stabilizers are unless you give them a try.

And btw, I fully agree with Piers suggestion.
Which incidentally would still stand ALSO with the smaller engines.
Just mentioning this because fin stabs are indeed an expensive bit of kit, probably a bit above the 40k you mentioned.
But smaller engines and stabs are not alternatives.
If you want to cruise at D speed (and with the smaller engines you would have no other choice), stabs are highly recommended regardless of what you've got in your engine room!

 

[MapisM]

Not really, as I said. You can not compare it with a small runabout where you have a hump speed and then you can reduce the throttle and still maintain a high speed on plane. The Canados and most boats of that kind are like plowing water and have not travelled over the hump so to speak, it is more like a semi displacement boat even in higher speeds.

Hang on, if that would be the criteria for separating P from SD hulls, you could as well say that there is no 20m+ P boat anywhere on this planet.
Of course the hull length affects the transition from D to P attitude (the longer the hull, the "softer" the transition), but so what?
Name me a 20m+ boat that jumps out of the water like a 25' sportboat, and plainly refuses to cruise at any speed between D and P, if you can.

 

[Designo]

Hang on, if that would be the criteria for separating P from SD hulls, you could as well say that there is no 20m+ P boat anywhere on this planet.
Of course the hull length affects the transition from D to P attitude (the longer the hull, the "softer" the transition), but so what?
Name me a 20m+ boat that jumps out of the water like a 25' sportboat, and plainly refuses to cruise at any speed between D and P, if you can.

So, whatever my answer, it will be wrong..

 

[henryf]

Someone will be long shortly with the calculation for hull speed but you won't be cruising at 15 knots. That will cost you a fortune in fuel. You're talking under 10 knots, I'll guess 8-9 knots plucked out of the sky. At this speed your range will be measured in 1,000 plus miles, I bet at 15 knots it would be 2-300 miles depending on the tank size(s).

You simply can't defy the laws of physics unless you throw a lot of energy at the problem.

Henry

 

[Mr Googler]

So, whatever my answer, it will be wrong..

 

[Portofino]

Someone will be long shortly with the calculation for hull speed ............

You simply can't defy the laws of physics unless you throw a lot of energy at the problem.

Henry

Square root of the waterline length Lets try that .?
So 20 M. 64 ftr -you looking @ 8 knots before fuel bill starts the rise

 

[omega2]

THERE ARE TWO LEVERS SOMEWHERE ON THE DASH BOARD, just back them off a bit, you will be astonished on the fuel burn rate.

 

[MapisM]

So, whatever my answer, it will be wrong..

LOL, if that's your feeling, who am I to argue?

Jokes aside, maybe we are just at cross purposes.
Your statement "most of the planning 20 m boats with MAN 1200:s are not really planning" seemed to imply (in my understanding, but maybe I was wrong?) that such power is not enough to make a 20m boat plane, hence my question in post #11.
And then you argued that 20m boats behave and plane differently from a runabout.
Which is something I fully agree with, but that is true regardless of power - which obviously affects the top speed, but not the transition behaviour of the hull.
So, I hope you'll forgive me if I'm still struggling to see what point you were trying to make with your first statement...

 

[Designo]

LOL, if that's your feeling, who am I to argue?

Jokes aside, maybe we are just at cross purposes.
Your statement "most of the planning 20 m boats with MAN 1200:s are not really planning" seemed to imply (in my understanding, but maybe I was wrong?) that such power is not enough to make a 20m boat plane, hence my question in post #11.
And then you argued that 20m boats behave and plane differently from a runabout.
Which is something I fully agree with, but that is true regardless of power - which obviously affects the top speed, but not the transition behaviour of the hull.
So, I hope you'll forgive me if I'm still struggling to see what point you were trying to make with your first statement...

My first statement was more of an understatement that most boats of that type have a hull and engine combination that makes them plow instead of plane, to put it simple. My latest design is a semi displacement doing 38 knots. Destriero made 70 knots without planing.

 

[Hurricane]

Most on this forum will know that we do just this.
We have a 20m planing boat with 1200 hp MTUs
A quick comment about changing for smaller engines - what would the effect be on the hull with a lighter engine room?

Anyway back to the point of using the existing engines
Lots of benefits here
Much cheaper - long term than changing the engines
Better resale value
With the bigger engines, you can "make a quicker run cover" if the weather turns nasty.
At displacement speed, the savings on smaller engines aren't that great.
Here's some figures
Our MTUs consume 175 litres per hour per engine at 25 knots
At 10 knots it is down to 22 litres per hour per engine
Ay 6 knots (tick over) they will be burning about 10 litres per hour.
We hold 4000 litres of fuel when full - so you can do the maths - 7000 litres would be plenty to do what you want
So, you can see from these figures that we can just about reach 300 miles at planing speed

So, my advice is go for the planing boat and "go with the flow"
We don't have stabilisers and IMO we don't need them
The money that you would spend on stabilisers or replacement engines would pay for a HUGE amount of fuel.

BTW
We call this technique "POOTLING"
Here's a youtube video clip of ours whilst "POOTLING"



Good luck
Mike

 

[rustybarge]

Our MTUs consume 175 litres per hour per engine at 25 knots

....either your injectors need servicing, or your fuel consumption is tax deductible!!!

 

[MapisM]

My first statement was more of an understatement that most boats of that type have a hull and engine combination that makes them plow instead of plane, to put it simple. My latest design is a semi displacement doing 38 knots. Destriero made 70 knots without planing.

I see what you mean, and I don't disagree that ANY 20m monohull plows through the water to some extent, surely much more than a runabout.
Which still plows much more than a turbine powered air entrapment catamaran, for that matter - so, where should we draw the line?
And that's even more true for a 60m LWL vessel more akin to a military ship, rather than a pleasure boat, of course.
Btw, where did you hear that her max speed was 70 kts? Afaik, the only "official" numbers were "40+ heavy, 65+ light".
There are folks at Fincantieri who pretend that she actually topped even more than 70, but I never saw it confirmed by any reliable source.
Not that it matters one iota in this context, but if you would tell me that you were onboard during her seatrials, well, that would indeed make me a tad envious!

Fact is, at the end of day it's mostly a matter of semantics.
I don't think anyone around here of those who own 20m+ vessels with 1000+ hp engines would call them SD boats.
Otoh, Piers said that his Fleming has a SD hull, and pretty sure most folks (if not all) would agree.
And if you're referring to the D80 as your latest design SD boat, while as I said I can see what you mean, I'd be curious to hear how many boaters who saw her in the videos which were posted would call her a SD vessel...?

 

[Designo]

I see what you mean, and I don't disagree that ANY 20m monohull plows through the water to some extent, surely much more than a runabout.
Which still plows much more than a turbine powered air entrapment catamaran, for that matter - so, where should we draw the line?
And that's even more true for a 60m LWL vessel more akin to a military ship, rather than a pleasure boat, of course.
Btw, where did you hear that her max speed was 70 kts? Afaik, the only "official" numbers were "40+ heavy, 65+ light".
There are folks at Fincantieri who pretend that she actually topped even more than 70, but I never saw it confirmed by any reliable source.
Not that it matters one iota in this context, but if you would tell me that you were onboard during her seatrials, well, that would indeed make me a tad envious!

Fact is, at the end of day it's mostly a matter of semantics.
I don't think anyone around here of those who own 20m+ vessels with 1000+ hp engines would call them SD boats.
Otoh, Piers said that his Fleming has a SD hull, and pretty sure most folks (if not all) would agree.
And if you're referring to the D80 as your latest design SD boat, while as I said I can see what you mean, I'd be curious to hear how many boaters who saw her in the videos which were posted would call her a SD vessel...?

My point if you still have not understood what I wrote, is that since the kind of boat in question does not have a hump-speed as a smaller boat, it works well to keep the original engines and just reduce the speed to save fuel...

About Destriero, I have been onboard and I have met the designers and builders and owners, but I have not been on any sea trial. The 70 knots was not the point either, but the fact she isn't planing at WOT. And yes, Fincantieri is saying 70 knots; https://www.fincantieri.it/cms/data/browse/news/000458.aspx

The D80 is not fully planing either, but of course you can find boats that size that does. I can see some pretty big and fast boats outside my windows quite often; http://en.wikipedia.org/wiki/CB90-class_fast_assault_craft

 

[Hurricane]

....either your injectors need servicing, or your fuel consumption is tax deductible!!!

Neither, I'm afraid !!!

 

[rustybarge]

Neither, I'm afraid !!!

78 gals at 25kts =0.32 mpg, or more than £2000 per hundred miles.[Days outing:couple hours up the coast, and return]

I don't often read YBM magazine as it deals with the big stuff, but I reckon their fuel estimates are way too conservative.
The same happened at MBM, where they underestimated the mpg by 30% or more.... They then did a big comparison test a couple of years ago between the main mid power contenders in identical boats and got quite a shock.

I now understand why twenty year old Super yachts are so cheap!

 

[MapisM]

if you still have not understood what I wrote

Yeah, yeah. And beyond a shadow of a doubt, you perfectly understood what I wrote, right?
No worries, let's put this to bed, life's too short.

 

[MapisM]

I now understand why twenty year old Super yachts are so cheap!

LOL, and you don't want to know how much oil goes in those blocks at each change!

 

[Hurricane]

LOL, and you don't want to know how much oil goes in those blocks at each change!

I buy 200 litres for an oil change

 

[MapisM]

LOL, Q.E.D.!

 

[rustybarge]

I buy 200 litres for an oil change

Can you buy a 40 gal barrel of lubricating oil, and what sort of price is it /ltr?
I have got a feeling that the OP is going to get another shock!