Semi displacement / planing, fuel prices

[MapisM]

Most mobo folk seem to have a rather irrational prejudice against power cats

Have you ever tried a cat in some significant beam sea?
It's a bit counterintuitive, but cats tend to roll much more, and much less predictably, than a stabilised monohull.
Otoh, it's true that they're inherently more efficient, but the trade off depends on the type of usage you have in mind.
I don't think that the efficiency alone will ever be enough to make them more interesting than monohulls, for most boaters.

 

[Deleted User YDKXO]

I don't think that the efficiency alone will ever be enough to make them more interesting than monohulls, for most boaters.

Agreed. The major problem is the difficulty in finding a berth for cats in popular areas like the Med, either a permanent berth or a visitor berth when cruising, and the cost of the berth when you do find one. This one point is enough to put me off them. The other big issue for me is that on most of them, the sleeping cabins are all squeezed into the hulls so they're all v narrow

 

[vas]

The other big issue for me is that on most of them, the sleeping cabins are all squeezed into the hulls so they're all v narrow

+1

most layouts I've seen (admittedly only in drawings, but it's enough for me at least to figure out what's happening in there!) are really limited if not godawful! Yes the hull size is good for running with less drag, but comeon there's no interesting spaces in them, resemble yachts a lot in principles.

If I'd go for a cat, it would be a sailing one and in need of a grounds up renovation so that I can get the layout I want out of it (or I'll start playing lotto to get me a 56MS )

 

[henryf]

The powercat 15 looks lovely inside and seems to have at least 1 bedroom of a decent size.

No layout plan though and of course no idea of prices so one can only assume you need to be sitting down when the gentleman hands over the velum envelope.

As has been said already berthing fees will probably be 150% the cost of a mono hull and I imagine a few places will simply turn you away on the basis you look too big. There is obviously some merit in twin hulls given the number of commercial fishing and diving Cats I see on my travels.

Henry

 

[gjgm]

Have you ever tried a cat in some significant beam sea?
It's a bit counterintuitive, but cats tend to roll much more, and much less predictably, than a stabilised monohull.

At SBS last Autumn, I had a look at a few. Southampton Water isnt going to get at all rough, but there was a wind blowing. I had to get off this pontoonmoored cat before I threw up. I assume you get used to the motion

 

[kcrane]

As has been said already berthing fees will probably be 150% the cost of a mono hull and I imagine a few places will simply turn you away on the basis you look too big. There is obviously some merit in twin hulls given the number of commercial fishing and diving Cats I see on my travels.

Henry

Having had a 38ft sailing cat in the Solent for two seasons we found there was no problem getting in to places, if anything marinas and harbours were welcoming, but yes, berthing was 150%. They tend to be 50% as wide as they are long, so ours was 19ft beam. That's a problem stern to in the Med, but not an issue where they moor off (Caribbean for example).

We also chartered a Lagoon 42 powercat in Florida which we liked for the space, the owners cabin is full beam at the stern, so huge. Flat out was about 15knts I think, but it rode well and we didn't have problems with beam seas.

Didn't buy a powercat tho' as we wanted more speed.

Separate point: does anyone in a planing boat really think about 'outrunning the weather'? The longest I am underway is 4-5 hrs.

Separate point: I've had the V48 out in some interesting weather, I don't think I'd be in a hurry to be out in worse conditions in any 50ft'ish boat, D or SD, I can't imagine it is an order of magnitude more comfortable. I'm saying I don't hanker after a slower boat so that I could go out in worse conditions than I can already go out in the V48. If I want to go slowly in the Princess I can (eg last Friday in the Solent, thick fog). Would a 50ft Linsen be burning significantly less fuel than me if we are both at 7knts? Enough less to make a difference to my overall running costs?

I can go out now in as bad weather as I'm interested in facing and I can go slow and get 3 or 4 mpg. The D upside is probably the range, if you have the time to use it.

Having said that, and showing how much emotion is involved, I always look twice at a Fleming!

 

[henryf]

In terms of outrunning the weather there are 2 occasions I can remember specifically. The most significant of which was a run back from St Peter port Guernsey to Portsmouth. As we left the fuel berth Jersey radio kept talking about a storm coming up from the south. We were able to stay ahead of it at 24 knots and had a good crossing. Had we been coming back at 8 knots I suspect we would have been in the thick of it mid crossing.

It's also the security of knowing if you set off the weather and sea conditions aren't going to change significantly in the next 2-3 hours. They could well do in the next 6-9 hours though.

Henry

 

[oceanfroggie]

We can cruise at 15-20kt in our SD boat but if the weather is nice we regularly cruise at 7-8kt in displacement mode. Quite like long slow lazy cruises and happy to start a long passage early in the morning. In poor weather into a strong head sea most will have to slow to displacement mode anyway. D cruising in nice weather can be so much more relaxing, boat quite, gentle movement, stick on auto helm and walk about the boat chilling out. It's nice to have the option to push up to 15kt for economic planing if we have some sort of a time window. Being semi-retired though I am lucky to be able to have some time flexibility.

 

[IDAMAY]

Planing vs SD

To return in some degree to the OP. We changed two years ago from a 47ft planing boat to a 45ft SD one. We cruise fairly extensively and usually do 1000NM to 1500NM per annum. Overall our average speed in the planing boat was 10kts (from which you will infer that we spent quite some time at displacement speeds). In the SD it has been 6.5 kts (from which you will infer that we don't get over the hump very often).

Over the life of the planing boat (six years) we averaged 0.8NM per gallon and in the SD (two years so far) 1.4 mpg. Both these consumption figures include considerable generator use which I can't factor out but which will be pretty much equal. So, the SD boat gives 50% improvement in consumption - worth having but not a "killer" factor.

Our perception of the two boats is that the planing hull was more fun and got us places quicker when we wanted to. The SD gives us more confidence and is more comfortable in head (particularly) and OK in stern seas but not when the sea is anywhere on the beam. We are not stabilised so always increase speed in a beam sea in order to give some additional dynamic stability. This doesn't do anything for fuel consumption.

We don't aim to be out in bad conditions and, being retired, we don't have to. On the odd occasions when we have been out in poor conditions, the SD hull and heavier construction gives us real confidence in its abilities and I don't think we will be going back any time soon.

When I was working, though, and cash reasonably rich/time poor I certainly preferred the get me there quick ability of the planing form.

Richard.

 

[Nick_H]

So, the SD boat gives 50% improvement in consumption - worth having but not a "killer" factor.

That's not really correct though. Going slower (average 6.5 kts instead of average 10 kts) gives you the 50% improvement in comsumption, not the change to a SD boat.

 

[IDAMAY]

Planing vs SD

Point taken, Nick but I don't think I would have been comfortable at average speeds of 6.5kts in the planing boat unless I had spent all my time on lakes or canals!

 

[henryf]

I think JFM made a valid point when he said he pottered around for some of the time and would see if that increased to the point where slowing down permanently ever made sense.

Maybe we'll do the same. See how things go on nice calm days.

Like most things in life we will know as and when we are ready.

Henry

 

[jfm]

he said he pottered around for some of the time and would see if that increased to the point where slowing down permanently ever made sense.

Thing is, I can't see it ever being permanent. I think it will be no more than say 50% of the time, max. Problem with a D is that you are committed to slow running: you have no choice. Hurricane is the same - he appreciates the benefits of slow running and pootling, but still only say 1/3 of the time

I think permanent compulsor slow running would cut down my mileage enormously, as in "I can't be bothered to go to Corsica becuase it's 12 boring hours, again...")

Hence I prefer to have the slightly suboptimal D running of a P hull, compared with a pure D boat, as a quid pro quo for being able to do a relaxed 23kts when I want to. And at rest, which is most of the time and when you have the most fun on a boat, I can't see any particular advantage of D over P. Therefore I still think, long termishly, a stabbed P boat is the best overall answer in the 15-24m segment

 

[kcrane]

We can cruise at 15-20kt in our SD boat but if the weather is nice we regularly cruise at 7-8kt in displacement mode. Quite like long slow lazy cruises and happy to start a long passage early in the morning. In poor weather into a strong head sea most will have to slow to displacement mode anyway. D cruising in nice weather can be so much more relaxing, boat quite, gentle movement, stick on auto helm and walk about the boat chilling out. It's nice to have the option to push up to 15kt for economic planing if we have some sort of a time window. Being semi-retired though I am lucky to be able to have some time flexibility.

Modest edits to your post to reflect a P boat, point being I changed very little and what I changed improved the picture

We can cruise at 24-30kt in our P boat but if the weather is nice we can cruise at 7-8kt in displacement mode. Quite like long slow lazy cruises and happy to start a long passage early in the morning. In poor weather into a strong head sea we have to slow to displacement mode. Slow cruising in nice weather can be relaxing, boat quiet, gentle movement, stick on auto helm and walk about the boat chilling out. It's nice to have the option to push up to 26kt for economic planing if we have some sort of a time window.

 

[stillwaters]

Absolute best bet has to be a 10 year old well kept F55 or its ilk, cheap motoring, and the upside , limited numbers and a steady demand from true blue cruisers which doesn't seem to deminish even when the market is on its bum !

Didn't I read somewhere (not certain but,it may have been one of Piers' items about his F55,Play d'eau) that the stabilisers on the F55 aren't particularly effective at displacement speeds and,if so,do you know if this is typical of SD hulls which don't have the benefit of the newer technology stabilisers?
The point of the question is that,having experience of displacement,semi-displacement & planing boats,SD hulls appear to be the least economical being especially thirsty at planing speeds - so,if the 5-10 year old stabilisers only work at planing speeds,maybe the SD becomes the worst of the compromises rather than the best.
My current boat is a 43ft SD,great in almost all conditions but would certainly benefit from stabilisers to keep the crew happy. The real downside of it,though,is that it is cursed with fuel-flow meters (standard with Cummins) and,having spent about 700 hours watching them laugh at me every time my right hand slips forward (3mpg@7kn/2mpg@14kn/0.5mpg@23kn max) they don't half encourage slower progress. Also,as I'm now thinking about sizing-up for longer and more comfortable forays,I've just been considering a 70ft SD - it appeared to be a good idea until I heard it's consumption was only 0.2mpg @ mid-teens cruising speed.That's about £30/mile at current prices!!!
Pretty certain that an equivalent planing boat could better that by quite some margin?

 

[jfm]

appeared to be a good idea until I heard it's consumption was only 0.2mpg @ mid-teens cruising speed.That's about £30/mile at current prices!!!
Pretty certain that an equivalent planing boat could better that by quite some margin?

All fin stabs can work better in big seas if you go faster, because they work by water flow over the fins. The latest ones tend to have bigger fins and hydraulics, as is needed for zero speed work, so they generate bigger anti roll forces underway too. And moderm lectronics are better for sure

Ref your comment above on mpg, BartW reports 15litres per mile =.3mpg at 21/22knots planing in his 60-70t Canados, measured roughly by checking the tank sight gauges. I get 16-18 litres per mile = say 0.27mpg, measured by electronic read outs, in a 57-60t Sq78 with fin stabiliser drag, at 19-25knots. So, yes, that suggests the SD hull you were looking at might be the worst of all worlds, eek

 

[Deleted User YDKXO]

I've just been considering a 70ft SD - it appeared to be a good idea until I heard it's consumption was only 0.2mpg @ mid-teens cruising speed.That's about £30/mile at current prices!!!
Pretty certain that an equivalent planing boat could better that by quite some margin?

You'd probably be right. I'm guessing a 70ft planing boat would be doing about 0.3mpg at 20kts so about 50% better. It all hinges around whether that 70ft SD boat would be more comfortable at D speeds than the 70ft P boat and the jury seems to be out on that one although my personal opinion is that the additional weight and the design of most SD hulls (keel, sharper forefoot, more rounded bilges) do make SD boats better at D speeds. Then, as I said in one of my previous boats, IMHO most SD boats are better laid out and engineered for extended passagemaking and have greater fuel capacity.
In the end it all comes down to what kind of boating you do. If you do planing speeds 90% of the time and d speeds 10%, then you're better off with a planing boat. Vice versa and IMHO, a SD boat is better. If you do 50/50, don't ask me

 

[BartW]

I've just been considering a 70ft SD - it appeared to be a good idea until I heard it's consumption was only 0.2mpg @ mid-teens cruising speed.That's about £30/mile at current prices!!!
Pretty certain that an equivalent planing boat could better that by quite some margin?

My Canados 70 uses 15l/nm at planing speed (20kn), which is 0.3mpg
I have no figures at D speed

 

[BartW]

Ref your comment above on mpg, BartW reports 15litres per mile =.3mpg at 21/22knots planing in his 60-70t Canados, measured roughly by checking the tank sight gauges.

yes I'm very sure about the 15l/nm,
In Cassis the fuel pump is very slow, so I could fill her up very accurately, after each long weekend...
and also after the long leg to Rome,
every time, consumption exatly at, or just below 15l/nm

never 22kn though, (avoid going over 2000RPM)
most of the cruising was done at or around 20kn
long trip to Rome:
20.5kn fuel tanks full
21.5kn light weight
exactly 15l/nm

sorry for the pedantry

60T-70T yes I'm very curious to find out how much it is exactly,
hopefully soon at next lift out for antifouling and hi lo platform mounting job

 

[stillwaters]

All fin stabs can work better in big seas if you go faster, because they work by water flow over the fins. The latest ones tend to have bigger fins and hydraulics, as is needed for zero speed work, so they generate bigger anti roll forces underway too. And moderm lectronics are better for sure

Ref your comment above on mpg, BartW reports 15litres per mile =.3mpg at 21/22knots planing in his 60-70t Canados, measured roughly by checking the tank sight gauges. I get 16-18 litres per mile = say 0.27mpg, measured by electronic read outs, in a 57-60t Sq78 with fin stabiliser drag, at 19-25knots. So, yes, that suggests the SD hull you were looking at might be the worst of all worlds, eek

Surely though,if the zero speed stabs work well at zero speed,presumably they also work more effectively at slower speeds than standard stabs,therefore comfortable displacement speed on an SD becomes more of an option for more of the time,maybe?

Also,as your figures confirm,equivalent planing boats do use less fuel and additionally,because their top speeds are typically in the low 30s rather than low 20s for SD,cruising speeds are also significantly higher whilst returning better economy. I guess much of the decision making comes down to which compromise seems to make sense on the day.
However as generally speaking,SD boats are no thirstier than others at D speeds,if D speed for much of the time was acceptable coupled with the ability to go at say 2.5x the speed when necessary,then maybe this compromise could work for some?