Super-Lightweight displacement boats Vs. old fashioned heavyweights.

rustybarge

rustybarge

Well-Known Member
Joined
9 Aug 2012
Messages
3,665
Visit site
The concept of a slim lightweight 'fast' displacement hull with small engines sounds attractive.
Too good to be true?

What do you think?

The Fleming 55'/ 30 tons / 16' beam/the gold standard passage makers, does 0.8mpg at 10kts with twin 500hp.....claimed max 20 kts.

View attachment 36083

The passagemaker-light 56'/ 23tons / 13'beam does 3mpg at 9.5kts cruise on twin 75hp diesels. 14kts max with twin 150 hp.......That's nearly 4 times better fuel economy!!:eek:

http://www.passagemakerlite.com/
http://www.tadroberts.ca/about/pdf/passagemaker-lite-pb-article.pdf

View attachment 36084View attachment 36085
View attachment 36086View attachment 36087
 
Last edited:
14kt max on a 23 ton boat with 150hp?

My 32' Scand running on one of it's 165hp engines would do about the same. I find that hard to believe TBH...
 
No Regrets said:
14kt max on a 23 ton boat with 150hp?

My 32' Scand running on one of it's 165hp engines would do about the same. I find that hard to believe TBH...
Click to expand...

Just a guess, a 150hp diesel will consume about 3.5gal/ hr at 2500 revs. Times 2 that makes 5gal/ hr for twins at say 10 kts= 2 mpg........:rolleyes:

Actually that might be a bit optimistic.
 
rustybarge said:
What do you think?
Click to expand...
TBH, "oxymoron" is the first concept that springs to mind when reading of "fast displacement hull", "swift trawler", etc.
Slow boating is NOT about saving fuel, is about a different way of cruising.
For those who just want to save fuel on a long passage, but can't live without the option of hammering the throttle for fast cruising, most planing boats are good enough, and there's no need to reinvent the wheel.
Anyhow, I have a gut feeling that neither 23 vs. 30 tons, nor the smaller engines can justify the fuel burn differences you mentioned.
Surely, the MUCH narrower hull plays a role - but that's bound to be reflected also in being one boat being actually bigger than the other, hence not directly comparable.
Besides, 10kts is surely above the ideal long distance cruising speed for the F55, while 9.5 might be just fine for the narrower hull.
When at the borderline of hull speed, every .1 kts matters, in terms of fuel burn.
Anyway, there's one thing I'm sure of: a builder claiming to build a "passagemaker" boat, and at the same time highlighting the advantages of a light construction, would never show anywhere near my personal top-10 list of passagemaker builders.
 
Last edited:
Anyway, there's one thing I'm sure of: a builder claiming to build a "passagemaker" boat, and at the same time highlighting the advantages of a light construction, would never show anywhere near my personal top-10 list of passagemaker builders.
Click to expand...

Ah yes, but this a fast displacement!
The advantage of old style displacement hulls was the weight, the more the better. Solid marble worktop surfaces, no problem sir. 150hp rib tender, great. 2000 gals fuel superb. Max hull speed is max hull speed, weight doesn't count.

Then there's the S/D boats, where weight doesn't matter at displ speed, but does at S/D speeds above hull speed.

And now: we have fast displ hulls, narrow and slim= low drag=small engines. Just like patrol boats.
It just might work.
 
vas said:
RB, care to explain what's fast displacement (other than a contradiction in terms?)
is it another way of calling the SD hulls or what?

cheers

V.
Click to expand...


Compare a destroyer to a container ship, both displacement.
Zap...zap...shrink....,,,,,''''........
Ah, we've got a fast displ. Boat........In theory!


Fast displ hull: View attachment 36091
 
Last edited:
thanks RB,

OK, went back to wikipedia and read the hull speed article in full (i.e. past the hull speed formula :p )
does make sense and I always thought this 1.34 times sqrt of waterline length as slightly odd and arbitrary...
Would be interesting to know if there's another formula that considers beam, weight and shape in general (last one is quite difficult.

cheers

V.
 
vas said:
thanks RB,

OK, went back to wikipedia and read the hull speed article in full (i.e. past the hull speed formula :p )
does make sense and I always thought this 1.34 times sqrt of waterline length as slightly odd and arbitrary...
Would be interesting to know if there's another formula that considers beam, weight and shape in general (last one is quite difficult.

cheers

V.
Click to expand...

I've just rechecked the quoted performance figures.

If these are true, that's seriously impressive, esp. At 9.5 kts.

3400 ltr/ 1700nm/ 10kts / 2100revs/ twin 75hp...... 2.25mpg fast cruise

3.23gal/hr at 9.5 kts/ 1900revs....2.9mpg economy cruise
Click to expand...
 
rustybarge said:
Ah yes, but this a fast displacement!
...
And now: we have fast displ hulls, narrow and slim= low drag=small engines. Just like patrol boats.
Click to expand...
Hang on a minute.
Don't be fooled by the fact that the "patrol boat" concept is getting somewhat popular also for pleasure boats.
Even the Dashews with their Wind Horse - an amazing boat in many respects - have some responsibilities, because they called her a "FPB", which she is not by any stretch of the imagination.
REAL modern fast patrol boats have neither displacemet hulls, nor small engines.
In fact, they are either propelled by huge diesels or gas turbines, if not both.
30+ kts cruising speed is pretty much the norm, with max well above 40.
And if there's anything those boats are NOT built for, that's fuel saving.

That said, it's very well known that the hull speed depends also on the width, though to a lesser extent than LWL.
As a consequence, AOTBE a narrower boat has a higher max hull speed, which in turn means a lower power required at any speed below that, and eventually less fuel burn.
But a beamier boat has many other advantages, and in fact in the recent past the trend has been towards beamier, rather than narrower hulls.
Not to mention that the best way to exploit the higher efficiency of narrow hulls has been around for some time - it's called catamaran... :)
 
Last edited:
MapisM said:
Hang on a minute.
Don't be fooled by the fact that the "patrol boat" concept is getting somewhat popular also for pleasure boats.
Even the Dashews with their Wind Horse - an amazing boat in many respects - have some responsibilities, because they called her a "FPB", which she is not by any stretch of the imagination.
REAL modern fast patrol boats have neither displacemet hulls, nor small engines.
In fact, they are either propelled by huge diesels or gas turbines, if not both.
30+ kts cruising speed is pretty much the norm, with max well above 40.
And if there's anything those boats are NOT built for, that's fuel saving.

That said, it's very well known that the hull speed depends also on the width, though to a lesser extent than LWL.
As a consequence, AOTBE a narrower boat has a higher max hull speed, which in turn means a lower power required at any speed below that, and eventually less fuel burn.
But a beamier boat has many other advantages, and in fact in the recent past the trend has been towards beamier, rather than narrower hulls.
Not to mention that the best way to exploit the higher efficiency of narrow hulls has been around for some time - it's called catamaran... :)
Click to expand...

We had a new boat cruising the Shannon this year. It's a american patrol boat built in Germany in 1948. What's so interesting about this particular design is that it was built and designed a a lightweight fast displacement hull. It's 84' long by 12' beam, 3mm steel hull plating, twin 280hp 12 ltr deutz marine engines.

Colm the new owner collected the boat in Holland, and brought back to Ireland via the south coast. 900 revs/ 5 gal hr/10 kts...............nearly an exact match with the numbers for the lightweight fast d. Boats. As a matter of interest the whole center of the boat is taken up with the massive engine room, to keep the weight low in the hull, leaving a tiny amount of room forward for a saloon, and at the back for cabins.

I was only onboard when it was stationary, but i did notice it rolled very easily in quite small wakes from passing boats. But then it is a round bilge design.

That just highlights what you've said, a narrow hull rolls easier than a beamier one.
 
Several things to say here. First the Fleming is a semi displacement boat, not a displacement one, so its hull is designed to take the greater loads from higher speed cruising and may be heavier as a result. Second, as has already been pointed out, it's beamier so it's accommodation will be superior and, third of course, the Passagemaker Lite is just a CAD concept and the actual built weight is likely to be greater in reality
 
K
Deleted User said:
Several things to say here. First the Fleming is a semi displacement boat, not a displacement one, so its hull is designed to take the greater loads from higher speed cruising and may be heavier as a result. Second, as has already been pointed out, it's beamier so it's accommodation will be superior and, third of course, the Passagemaker Lite is just a CAD concept and the actual built weight is likely to be greater in reality
Click to expand...


I compared the lightweight displ. Boat concept to the Fleming because they've quoted 14 kts max with twin 150 hp. Now no displ. Boat of 56' that i am aware of can get into double figures. So it seems reasonable to compare with a Fleming because most owners seem to cruise around the 10-15 kts range even though it is a S/D boat capable of 20kts max.

Yes a Fleming is beamier, and for the same length needs 3 times the power and consumes more than 3 times the fuel at 10 kts. Although i've heard owners complain that the accommodation on a Fleming is very tight compared to the big displ. Cruisers like the Nordhaven.

The reason a slim/lightweight/low powered passage maker is relevant is for one reason only: fuel consumtion! Who can afford 1 mpg or less, more than €700 for every 100 miles. :eek:
 
I compared the lightweight displ. Boat concept to the Fleming because they've quoted 14 kts max with twin 150 hp. Now no displ. Boat of 56' that i am aware of can get into double figures. So it seems reasonable to compare with a Fleming because most owners seem to cruise around the 10-15 kts range even though it is a S/D boat capable of 20kts max.
Click to expand...
OK fair enough but 14kts with 300hp in a 56ft boat sounds a tad optimistic to say the least but I would keep an open mind on that

Yes a Fleming is beamier, and for the same length needs 3 times the power and consumes more than 3 times the fuel at 10 kts. Although i've heard owners complain that the accommodation on a Fleming is very tight compared to the big displ. Cruisers like the Nordhaven.
Click to expand...
Yes you're right, the accommodation in the Fleming 55 is tight, mainly in the saloon area which is small compared to similar length planing boats. However that is mainly due to the fact that the Fleming is designed as a proper passagemaker with wide sidedecks that you can walk down in complete safety. If the PassagemakerLite has similar sidedecks, its going to feel very pinched in the saloon indeed and you're going to need a very accommodating SWMBO to put up with that! Yes, if you compare, say, a Nordhavn 55, that has a slightly bigger saloon than the Fleming 55, although it still feels small, but Nordhavn achieve that with asymettric decks; in other words, there is no sidedeck on the port side

The reason a slim/lightweight/low powered passage maker is relevant is for one reason only: fuel consumtion! Who can afford 1 mpg or less, more than €700 for every 100 miles
Click to expand...
I tend to think that anyone who could afford to build a one off 56ft boat like this is not going to be over concerned by fuel consumption, especially when the trade off is potentially less accommodation and possibly more rolling in a beam sea. In any case, long range D boats are able to take advantage of cheaper fuel locations. For example, if you had a Nordhavn 55 in the Med, you'd fill up once a season in Gibraltar or Montenegro at low prices and not think again about refuelling until the end of the season.
Still the PassagemakerLite is an interesting concept for the future because one thing is for sure and that is that fuel prices are only going one way and thats upwards
 
You must log in or register to reply here.

Share this page

Top