SD Hulls and Fuel Consumption

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Heavy weather does cost fuel - I think it can add 20% or so to the fuel bill.
at 8.5 knots can drop to 1.5 mpg.

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Still good for a boat of that size and comfort /forums/images/graemlins/smile.gif

It gets even more complicated as you need to juggle time savings avoiding wind against tide by an increase in speed and loss of fuel economy to save dropping to 1.5mpg as the seas mount.

First 50 nm @ 19 knts to make the headland before the tide changes.
drop to 8.5 for the next 3 hours and run with the tide
But the wind is due to veer in two hours so you best push it up to 19 knots for 30 mins to save dropping to 1.5 mpg.

one day delay puts the trip back to Monday so need to check when the wind is going to swing back and ooo /forums/images/graemlins/ooo.gif should really try to get back to the office by 1400 hours where you can earn more than the fuel wasted /forums/images/graemlins/wink.gif
 
Mike
I agree that there are a whole range of SD hulls and hence sea characteristics.

I think you alos highlight a good point - slow speed with SD hulls can lead to rolling etc but that is very much the case with a slow speed planing hull as well..... even usually worse I really think the stabiliser has changed all that. It allows better handling in following seas, prevent rolling at high and low speeds etc.

I could never comfortably helm Drumbeat from the lower helm in any sort of decent sea - now i can go a whole day in the dry and comfort in rough conditions at the lower helm.

I think that probably the SD hull is less efificent like for like at all speeds - but the planing hull relies on its speed for stability.

The SD Hull (with stablisers) disadvantges:-
Lower top speed.
About 25% Less fuel efficient at higher cruising speeds

Advantages
Much more comfortable ride at all speeds
Better sea keeping generally and in particular into a head sea.
Allows long range slow speed crusing at very good mpg figures
Greater range at low speed 0discounting that as an option on a planing boat)

That is how i see it.
 
DAKA
You are spot on with all that.
That is just how it is.
Recently with Padstow closed I wanted to do the 165 miles to Falmouth in one leg.

At low speeds this was about 17 hours - through the night and so needed crew to take watches etc - hence better to go at 19 knots and do it in the day with no extra crew. After that for the rest of the trip we juggled things exactly as you say. Its one of the reasons I have now discounted the Nordhavn option - I need the flexiblity.
 
Agree with that, gludy. One thing that separates a SD boat and a planing one is that SD boats are generally built heavier with less attention to weight saving because they are generally not designed to go at fast planing speeds (even though they can if you throw enough horsepower at them). That extra weight means that SD boats are probably going to be more comfy in head sea conditions irrespective of any differences in hull design and, of course, a heavier boat is likely have larger fuel tanks which is a big plus for cruising
IMHO for weekend coastal cruising and the occasional longer trip, a planing hull is better suited simply because most peeps have limited boating time and need to get to their destinations in a short time. But for extended cruising, I agree with your thought processes. A heavily built SD hull with good range, stabilized for displacement running and capable of higher speeds when necessary is the way to go
 
Mike
You also mentioned another very valid point - time.
The fact is that I hope to retire this year and my boat plans are aimed at having a lot of time.
When you have to do your boating at the weekend only you have to get where you are going fast and so the planing hull comes more into the picture.
I bet that the average age of SD hull owners is higher than that of planing hull owners.
 
Last season, we did quite a lot of slow (displacement) cruising - mainly to ensure that we arrived at the right place at the right time - Guernsey to meet the tide for refueling etc.

Also, Paul, I'm already retired so can often afford the time to go slower.

By keeping the speed to about 8 knots, our 51/52 foot boat seems to durn very little fuel - I guess a full range of about 600/700 miles.

What would be really nice would be to shut an engine down completely thus saving the "running" overhead for that engine. I've tried it before but when I had a look in the engine room the shut down engine is (obviously) running all its gear at the same speed as the powered engine - this must be very inefecient. It would be really good to find an efecient way of running on a single engine - I would then expect to get a massive range 700 or 800 maybe even 900 miles.

I can hear people saying that what we actually need is a displacement boat - but I have to say that most of our cruising (cross channel etc) is done at 23/24 knots so we are very happy with a planing hull.
 
My biggest saving is running on one engine. Displacement speed of 7.5/8 knots requires 2 engines using 1.5 g/h each whilst one engine only requires 400 more revs and uses 2 g/h.
Fortunatly I have a big keel that makes single engine running easy. The noise levels and vibration are also much less.
When running inland at river speed on one engine I get around 7 mpg
 
I am intrigued by the assertations above that some planing boats are more efficient at displacement speeds that semi-displacement or displacement boats.

A simple and very basic way of assessing efficiency is to just look at your wake. All else being equal, then the more wake you produce, the more $$'s are spent to acheive a given speed.
In my limited experience, displacement boats generally seem to have less wake (at less than hull speed) than planing boats of the same length. Of course once you start approaching displacement hull speed, then the planing boat is going to start becoming more efficient.

Changing tack (as I am a raggie at heart), nobody has mentioned displacement and planing powercats yet in this thread - I think that power cats have a lot to offer in every aspect.
They are very spacious, roll much less than monohulls, invariably have excellent seakeeping and usually seem to be more fuel efficient (than monohulls), as two relatively narrow hulls (L/B ratio of 8-10+) are often easier to push through the water (or up on to the plane) than a conventional relatively tubby monohull (which typically might have a L/B ratio of about 3).

As an example, the aluminium powercat in my avatar photo is 49' long, 16' wide, with 2 x 115 hp O/B motors. She has high deadrise (25 degrees at the transom) assymetric hulls and can do approx 22 knots flat out when light and about 13 knots when loaded with 40 passengers.
Her hulls are very slender, with a L/B ratio of 15 at the waterline, and she produces minimal wake as a result, even at maximum speed.

BTW, re running on one engine - this ally power cat tracks beautifully on one engine with no helm needed, and no tendency to wander - one characteristic of assymetric (mirror image) hulls.
The downside though of this excellent directional stability is that if she is surfing down a swell in the open ocean, and the stern gets pushed round, she does not respond immediately (time delay of perhaps a few seconds) when the helm is applied to bring her back on course again.
 
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another very valid point - time.


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Time = engine hours 3 or 4 times more at slower speeds

If I put 100 hours a year too many on my boat I will soon put off some buyers and quickly loose 10% boat value, how would you factor into running costs ?
 
Sone good popints made in that lot.

I have no doubt that a D power cat is more efficient in fuel than D,SD or planing - I started the thread to just give my results for planing and SD.

Single engine running means you have to lock the free prop and that is a complicated opertation - if you do not it will cause a lot of extra drag. Best solution is a third small engine and that I am looking at for the Marlow.

Slow ttravel does add hours to the boat - I allowed for the cost of maintaiance but not the cost of extra deprecation. However with fuel at say just £3 a gallonh, the differential in fuel costs in my example are in the order of £4.50 per mile or say at 9 knots approx £40 an hour so the saving in fuekl dwarfs any extra depreciation costs so I do not think that such extra costs are significant.

For my boat I need a boat that can say get across the channel at 18 knots and go a lonmg way, over 1000 miles at low speeds - all in comfort. Rgar is why i have chosen the SD hull and the figures worked out about as planned. The low speed option may well be the only future option given fuel prices and that option is open to me as a viable alternative.

I can really see the case for many choosing a planing hull and I am not knocking that in any way. I think if i was to be based in obe place with weekends and short breaks then so far the planing hull has made sense.
 
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Slow ttravel does add hours to the boat -with fuel at say just £3 a gallonh the saving in fuekl dwarfs any extra depreciation costs so I do not think that such extra costs are significant.

.

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You are most likely right, I havent given a great deal of thought to it but if you put 1000 hrs on a boat in a season as opposed to 250 hrs and loose 10% value the lost value could be £100 k in 750 hrs @ 8knts = 6000 nm which is £17 hour, a bodged calculation but enough for some real thought as the gap closes.

I hope I am wrong and sincerely wish you a happy retirement /forums/images/graemlins/smile.gif

Pete
 
A related question or two perhaps.

In a vehicle we measure miles travelled and base service intervals on that, but in a boat we use engine hours run. Obviously 100hrs at 8 knots is only 800 miles but at 25kts is 2,500 miles, hugely different.

So, taking two similar boats but one with smaller engines for displacement use and the other with larger engines for SD use, over the same distance travelled say over 10 seasons, is the smaller engine one with many more hours (but same miles) more or less knackered than it's bigger engine counterpart?

I ask because I have a possible interest in buying a trawler style MY in the USA to live on for a year or two after retiring, maybe doing the Intracoastal circuit 'great circle' up the east coast and back down through the Great Lakes and rivers to the Gulf of Mexico, through/round Florida and back to the start, all in about 5000/6,000mls I believe. Looking at potential boats from say 43ft up to 50ft, many come with 2 x 120hp engines (max say 10kts) yet some (same hull) have 2 x 300/375hp engines with max speed say 15/16kts. Bearing in mind speed limitations anyway in many places, not to mention fuel consumption, range and costs, I wonder which is the better choice, big or small? Would a big engine one at low speeds be more or less fuel costly?

PS some of the ones I looked at on the net are 'Marine Traders' dare I ask if these are related to yours?
 
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Single engine running means you have to lock the free prop and that is a complicated opertation - if you do not it will cause a lot of extra drag.

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But surely a locked prop/shaft will also cause underwater turbulance/friction thus providing a similar inefeciency. I'd be interested to hear if just locking the shaft works.
 
Thanks, Bajansailor, for the interesting input from a boat designer - and the actual experience of a power cat at sea.

And as to locking shafts - or not - of 'free' engines, we'll see what people say, as this question often comes up on sailing boats...!

And Robin asks interesting questions as to whether a big engine at slow speed is best, or what... We'll see if anybody has any ideas!
Best wishes
 
Agree entirely and I wish I could afford a suitably designed motor cat.
A cat of about 40 x 14 (to fit a standard berth) with two smallish inboard diesels would be nice.
 
Ribin
I thik it is difficult to generalise on engine sizes - it all must depend on the characteristics of each individual engine.

I think yiou can say that a larger engib=ne turning slower will probably use more fule than the smaller enhine turning faster providing that the smaller engine is well within it comfortable crusing revs.
 
DAKA
I do not mthinl that thethose extra deprewction figures are in the right ball park.

Two boats both well serviced one with 250 hours one with 1000 hours would not be £100k different in price.

Mind you I would still guess that half of my cruising will be done at high speeds.
 
There is a comprehensive list of power catamaran builders and designers at http://www.powermultihulls.com

Go to 'Boats' when you arrive at their home page.

The Highland 35 from Fountaine Pajot is a rather fine looking power cat - and with two relatively small inboard diesels I am sure she must have excellent fuel economy - have a look at

http://www.fountaine-pajot.com/rubrique174-en.html

However I think they were only introduced a few years ago, hence second-hand prices are still going to be rather high in comparison with older monohulls of equivalent space down below (I think this is a better yardstick rather than just using length).
Her bigger sisters all look rather nice as well.
 
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