Semi Displacement - best hull shape fuel efficiency

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gavinharris

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Hi. I prefer the fuel efficiency benefits of a semi-displacement hull. Problem is there are numerous hull configurations. Can anyone confirm/advise the best combination to achieve fuel efficiency? I have been advised that a Vee shaped forward and round further aft is more efficient than a Vee shaped forward and flat further aft. Cheers.
 
Welcome to the forum.
a search on this subject will give you a list of threads with many viewpoints.
But if fuel efficiency is your main target, SD hulls are possibly the worst solution, being full displacement the best, followed by planning hulls...
 
And Ak. Theres no answer so simple as pure efficiency. The Oxford/ Cambridge boats are probably very efficient. Though they seem to sink with regular occurence, even though just out on a river. I would think, that a plaining boat, with very small engine, would be most efficient, But you would not be able to steer it. So add a big keel and maybe a bigger engine.

The question is far to big and needs tying down to spacifics and usage.
 
As I see it semi-displacement hulls are only efficient at displacement speeds and it take a lot of power to get them over the hump, so they become inefficient. If you want to be economic you have to get a displacement boat. I have gone from semi-displacement to displacement and wish I had done it years ago.
 
Just adding a very simple point: for all types, whether semi-displacement, or whatever, lighter boat needs less fuel. But shoppers rarely look only for the lightest boat they can find.

Kelly
 
It depends on what you want the boat to do.

If you want to travel fast, say 25 knots, and be as efficient as possible then a planing hull is most efficient.

If you are happy to travel slow - say 8 knots at displacment speed Then a displacment hull is more efficient.

If you want to travel slow buty have the ability to travel fast when needed then the Sd hull is most effcient.

I had a planing boat that did about 0.55 mpg at 25 knots.
The same length SD boat that actually much more spacey and 25% heavier would do 0.38 mpg at 25 knots but would equal the planing boat's consumption at 18 knots. At 8.5 knots or so it would give over 2 mpg. Slow ttravel like that except in calm conditions is not really pracitical in the planing hull and the displacment speed of the planing hull is lower because its water line length is lower.

So first ask yourself what you wish the boat to do and then your question can be answered in more detail.

The range of SD hulls is large - some are alsmost displacment others are almost planing. I have chosen an SD hull because of their ability to provide a smooother ride, more space, more comfort and more slow speed economy than a planing hull. I can also go fast when I want to and use the same fuel consumption as the planing boat but have to accept that i travel at about 75% of the speed for that same mpg.
 
I can also go fast when I want to and use the same fuel consumption as the planing boat but have to accept that i travel at about 75% of the speed for that same mpg.


???/?????? /forums/images/graemlins/laugh.gif Is this crap. Or rubibsh. /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
Re: Semi Displacement - best hull shape fuel efficiency - re-sale

Re. semi-displacement hulls.
Would a SD hull at its true displacement speed be as fuel efficient as the same spec displacement hull?
What type of SD hull is most similar to a displacement hull?
I am planning on purchasing a launch in Southern Turkey then selling in UK.
For re-sale - what are more popular in the UK - SD or true displacements?
You guys are good.
 
Welcome AK.
Your last, sorry Your 2nd post is more relevant than Your 1st. /forums/images/graemlins/grin.gif
The amount of fuel consumed or the Transport cost may not be relevant to the Final Outcome.
Sneaky Antipidean curved ball as usual, trying to catch us Brits out eh! /forums/images/graemlins/tongue.gif
The vague Market Research question just asked is more to the point.
The correct purchase for Resale in the UK is more to the point.
So---" What can I buy in Turkey and sell in the UK and make a few Bucks and av a larf at the same time?"
That,s the adjusted Title of Your Post I reckon.
And the answer is---- I Dunno!
We can all give You clues as to what We like and what sells and doesn,t.
There are some Motor Boats made in Turkey which are very nice and would suit the UK market for some peeps usage,
Viki have been on sale in the UK for a few years now, seen one or two about.
Interesting Sketch.
I follow the thread with interest.
If You do end up in the UK.
Don,t bring any bleaters.
There,s enough ere already where I live.
NO MORE SNEAKY TACKLES NOW! /forums/images/graemlins/smirk.gif
G Day AK.
 
HLB
Nope its not crap at all just takes some thinking about. Its your jump in with both feet reaction that is crap. /forums/images/graemlins/smile.gif

I got the same fuel consumption in mpg at 18 knots in an SD boat than I got at 25knots ion a plaining boat - thats a simple fact.

I will spell it out to make sure you understand that you did not read the statement properly - in say a 100 mile trip both boats would use the same amount of fuel (that is have the same mpg) but the pl;aning boat wouild get there in about 4 hours whereas the SD hull would take a bit over 5 hours.

Therefore as stated the planing hull at speed is more efficient but the real life choice is that with an SD hull you can get there with a nicer ride but take longer whilst still using the same amount of fuel. or choose to go slowly at say 9 knots and take overe rwice as long but use one quarter of the fuel.

I was talking mpg - it simply that you did not understand the unit? /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif
 
Re: Semi Displacement - best hull shape fuel efficiency - re-sale

No, a well designed full displacement hull will probably be more efficient at displacement speed than any other hull type because it's only design requirement is to travel at that speed. A boat like a Nordhavn has a very specific hull shape which has been designed thru many years of experience to maximise fuel efficiency at displacement speeds. A SD boat will have to be designed for higher speeds and will incorporate hard chines, squared off transom and larger props well clear of the hull, all of which will cause drag at displacement speeds and reduce efficiency
 
Re: Semi Displacement - best hull shape fuel efficiency - re-sale

Mike - what you say is IMHO factually correct but you can only measure efficiency against a set of criteria.

If the criteria is to cruise at displacement speed then a displacement boat must be the best and most efficient choice.

If high speeds then the planing boat is more efficient.

If you want displacment with the option for occasional high speed then the SD hull is most efficient. This is why SD hulls sell so well. It is what pilot boats are SD hulls as are lifeboats etc.
 
Lots of answers and all quite plausible.
I think the best advice is to decide if you wish to go fast or slow (max' 8 knots) and buy either planing or full displacement accordingly.
Many of us end up with a compromise because we don't always wish to cruise slowly, viz Gludy's comments. The SD is a good compromise but with Gludy's boat the downside is that his 60 ft (??) Trader needs over 1000 hp to make it go quickly which is never going to be efficient at displacement speeds where 200 hp would probably be all that was required by a displacement boat.
My next boat will be full displacement but I shan't buy it until close to retirement when time will not be an issue.
 
Re: Semi Displacement - best hull shape fuel efficiency - re-sale

What size boat are you thinking about, depending on the boat length I would make different suggestions. Also what kind of accomodation do you require, these factors may force you in one direction more than another.
 
Mike
I think you are missing my point - probably because i did not explain it well enough.

I think that to polarise the issue into choosing displacement or planing is not right.

Having experienced an SD hull I would never choose any other.

I would prefer to travel at 18 knots in comfort using the same fuel as the planing boat at 25 knots in discomfort. No other hull offers that choice.

When I choose to travel at displacement speeds get incredible fuel consumption - over 2 mpg for a 60 foot boat!!

So yes - the SD hull is a compromise but it is the only hull that offers the choice that reflects my real world boating. If I need to hurry to avoid weather or get to a lock in time - I can. Yet I have a boat that can potter along economically at 9 knots when I want to.

The Nordhavn does not have great fuel consumption and I am beginning to think that it may be possible that an SD hull with stabilisers may offer better fuel consumption at low speeds than the Nordhavn but it cannot offer the same sea keeping as the Nordhavn.

I actually think a planing boat may offer even better fuel consumption at low speeds albeit the speed would be lower because iof its reduced WL length. The drag in the water would be very low but the sea keeping would be very poor and so would the rolling.

The SD boat I owned was almost a planing hull with a keel and stabilisers - without those stabilisers it could really roll.

So having though about it more - getting 2.4 mph from the twin 700hp cats at 8.5 knots is not bad at all and i think displacement boats may struggle to achieve that ...... I am now going against my own earlier points in this very thread. /forums/images/graemlins/smile.gif
 
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I would prefer to travel at 18 knots in comfort using the same fuel as the planing boat at 25 knots in discomfort. No other hull offers that choice.


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I dont agree with this. The displacement catamaran is in a different class when it comes to comfort and efficiency at these sort of speeds
 
Well I agree with you - the displacement cat beats them all for efficiency - it in a class of its own and can trevel both fast and economic.

I was just comparing one monohull with another and during the process realising that displacment boats are not that fuel efficient at low speed - other boats beat them for efficiency at displacment speed!!1 That is a surprise to me.
 
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I was just comparing one monohull with another and during the process realising that displacment boats are not that fuel efficient at low speed - other boats beat them for efficiency at displacment speed!!1 That is a surprise to me.

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I can't go along with that statement at all. I have a 35ft, 18 ton boat, and at 6 knots that's a whole 900 rpm, it would take me nearly 4 hours to use a gallon. At 8 knots, 1800 rpm I use ¾ of a gallon an hour. Flat out at 2200 rpm, 9 knots and trying to dig a hole in the sea, I burn nearly 1½ gallons an hour.

At slow speed my boat is extremely efficient. I challenge you to find any SD or planing hull of a similar size and weight that can better that low speed efficiency.
 
I can rrally understand why you say that - i thought that and said so at the beginning of this thread.

To do any comparison you have to compare like to like.

You claim:-
at 6 knots 24 mpg.
8 knots 10.6 mpg
9 knots 3 mpg.
What is the boat - what beam?

Taking the current MBY mag review, this month, with the Nordhavn 43 footer:-

6.9 knots 3.59 mpg
8 knost 2.17 mpg
8.7 knots 1.67 mpg

I hope those figures are UK gallons but if not then the actual consumption of that boat is:-

6.9 knots 2.88 mpg
8 knost 1.736 mpg
8.7 knots 1.36 mpg


I can tell you that the fuel consumption on my last 60 foot SD boat beats those figures by a long way.

Taking the boat owned by my friend - a 42 SD boat he gets over 4 mpg - much better than the Nordhavn D hull.

Those who own a planing boat at these low speeds would probably do even better because their is less drag - no deep hull, no keel to drag through the water.

OK they cannot use their boats at such a low speed in any sort of sea as the boat depends on speed for its stability but the SD hull with stabilisers is stable, not as good as the D hull but OK.

I am as surprised as you may be by these figures - all I am doing is presenting the facts that are there in review after review for anyone to see.
 
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What is the boat - what beam?

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Tech Details.

Cygnus Hull 35ft Long Keel 12'6" beam.

Lister HRW6M 6.25 Ltrs 88hp with 75Kg flywheel, 3-1 reduction box to a heavy 30" prop.

Trawlers with this hull can go to sea for 3 or 4 weeks with under 300 gallons.

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I can tell you that the fuel consumption on my last 60 foot SD boat beats those figures by a long way.

[/ QUOTE ]

You have to compare it to another 60'er, to get a more like for like comparison.
 
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