Displacement speed Qs

[vas]

just did my calculations following last weeks 3day excursion to a couple of nearby islands.
that is 75nm
all done between 7 and 8knots
Boat has a planning hull and waterline length was accurately measured to 10.60m at waterline (13m overall)
D speed according to the 1.34Xsqrt(WL length in feet) gives 7.9knots
Finally remember we have no tides down here so speed is speed (bar going with or against the wind/waves et al)
Consumption was approx 3lt / nm for 12K kilos
Is this about right?

Now, I've read the theory with the bow wave and all that, but call me thick, I cannot (being on the boat) figure out if I'm at the right speed or a bit above it.
I'm asking as i'd expect a bit lower consumption and I'd also expect a much smoother exit of the water on the stern which looks like some very messy water popping about and reaching regularly the swim platform a good 25=30cm above the water (and 1.2m behind the stern).
I pic of the water at the stern follows, got a few video clips from mid boat on either side but they are too big (and useless imho) to bother and upload them, will probably try again from the dingy next time.

cheers

V.

 

[BartW]

Vas, there is calculated displacement speed,
but there is also the shape of the hull,
your boat (like mine) has a planing hull, and therefor is less optmised for displacement speed than a real D boat
the blunt stern fe,

but I look at it this way, the less wash / wave movement you generate, the closer you are at "your" displacement speed,
if you really want the lowest fuel consumption, move the boat with as little wash as possible.
every movement of water / waves consumes energy / fuel,

a few years back, on my way from SOF to Rome I have been testing during hours at one speed, 8, 9 , 10, 11 and 12 kn
and try to establish fuel consumption.
The answer is very simple, fuel consumption goes up gradually with speed,
there is not something as a optimum speed, choose your own optimum.

I like 10kn for my boat, that gives foam behind the boat just like on your pic,
fuel cons about 7..7,5l /nm
and is faster than many real D and sailing boats ;-)

we are in Venice right now, trip of 400nm, mostly at 10kn, 43hrs, 7l/nm average

 

[Hardmy]

I also cruise a lot at around 8 kts, this gives me 4.2l/NM. Boat is 48' SD hull, 25t with 2x3208TA CATs. A friend with the same hull (he has a Trader, mine is a Kha Shing) and CAT 3126 engines achieves 2.75l/NM. Such a big difference, only because of the engines...

 

[BruceK]

Vas what would your engines consume per hour at idle and neutral. No load? I ask because I seem to recall reading some where my KAD42s would consume 6 l per hour. i.e. The difference may then be smaller than you think looking at total consumption if that makes sence

 

[vas]

Vas, there is calculated displacement speed,
but there is also the shape of the hull,
your boat (like mine) has a planing hull, and therefor is less optmised for displacement speed than a real D boat
the blunt stern fe,

but I look at it this way, the less wash / wave movement you generate, the closer you are at "your" displacement speed,
if you really want the lowest fuel consumption, move the boat with as little wash as possible.
every movement of water / waves consumes energy / fuel,

a few years back, on my way from SOF to Rome I have been testing during hours at one speed, 8, 9 , 10, 11 and 12 kn
and try to establish fuel consumption.
The answer is very simple, fuel consumption goes up gradually with speed,
there is not something as a optimum speed, choose your own optimum.

I like 10kn for my boat, that gives foam behind the boat just like on your pic,
fuel cons about 7..7,5l /nm
and is faster than many real D and sailing boats ;-)

we are in Venice right now, trip of 400nm, mostly at 10kn, 43hrs, 7l/nm average

Thanks Bart, makes sense, will try again but atm I have my son and his gang as autopilot and I really need to finish the garmin a/p installation soon (or the hard top else I'm going to cook these guys and gals...)

I also cruise a lot at around 8 kts, this gives me 4.2l/NM. Boat is 48' SD hull, 25t with 2x3208TA CATs. A friend with the same hull (he has a Trader, mine is a Kha Shing) and CAT 3126 engines achieves 2.75l/NM. Such a big difference, only because of the engines...

Hardmy, that's slightly confusing, your engines are around 400hp each and the others are smaller, similar or what?
TBH, I'd expect similar fuel consumption on a similar hull carrying similar load and same length especially at those speeds that AFAIK prop demands should be similar. Care to elaborate?

Vas what would your engines consume per hour at idle and neutral. No load? I ask because I seem to recall reading some where my KAD42s would consume 6 l per hour. i.e. The difference may then be smaller than you think looking at total consumption if that makes sence

Bruce, that's an interesting POV, IIRC MiToS does around 4kn with both engines engaged and idle (around 650rpm) The 8kn I'm travelling is around 1300rpm. I know things are not really linear but at 8kn means both engines drink 8X3lpkn = 24lt or 12lt each at 1300rpm which is not really much. Makes me feel slightly better, although I'll try 7kn on a night passage and see what I get.

cheers

V.

 

[MapisM]

I also cruise a lot at around 8 kts, this gives me 4.2l/NM. Boat is 48' SD hull, 25t with 2x3208TA CATs. A friend with the same hull (he has a Trader, mine is a Kha Shing) and CAT 3126 engines achieves 2.75l/NM. Such a big difference, only because of the engines...

I would agree with Vas, that sounds weird.
In principle, two similar hulls similarly loaded need a similar amount of power on the props, for any given speed.
Which type/size of engine is actually providing such amount of power to the props is not so relevant.
Ok, a modern electronically controlled engine could have a slightly better specific fuel burn (g/kW-hr), but the difference can't be anywhere near your numbers. Besides, while the 3208 is older than the 3126, also the latter still has mechanical governor, and I'd be surprised if the difference in specific fuel burn between the two would be meaningful.
Are you sure that your 3208 are tuned correctly, and that the boat is properly propped (on top of the usual stuff like load or dirty hull)?

 

[Hardmy]

I would agree with Vas, that sounds weird.
In principle, two similar hulls similarly loaded need a similar amount of power on the props, for any given speed.
Which type/size of engine is actually providing such amount of power to the props is not so relevant.
Ok, a modern electronically controlled engine could have a slightly better specific fuel burn (g/kW-hr), but the difference can't be anywhere near your numbers. Besides, while the 3208 is older than the 3126, also the latter still has mechanical governor, and I'd be surprised if the difference in specific fuel burn between the two would be meaningful.
Are you sure that your 3208 are tuned correctly, and that the boat is properly propped (on top of the usual stuff like load or dirty hull)?

What I can say, is that hull and props are always completely clean because as soon as the boat is unused for more than 2 months, I lift her out of the water. I also always get my 19 kts with taps wide open.

This important difference is indeed surprising and I cannot give you any answer about it. My engines run smoothly, they produce no soot or smoke (well the first minutes after a cold start only, but that clears up very quickly). My engines are the TA version with 475hp, he has the 250hp natural.

So you think I should investigate this?

 

[MapisM]

My engines are the TA version with 475hp, he has the 250hp natural.
So you think I should investigate this?

Wow, I wasn't even aware that the 3126 existed in NA version.
In a sense, that's even more surprising, because while NA engines usually have a reputation for durability and reliability, purely in terms of efficiency turboed engines should have an edge.
Don't just take my word for it, but in your boots I would explain the situation and the comparison numbers to a good Cat engine and get his opinion.
You might also pm LS1, just in case he didn't see this thread...

 

[Nick_H]

just did my calculations following last weeks 3day excursion to a couple of nearby islands.
that is 75nm
all done between 7 and 8knots
Boat has a planning hull and waterline length was accurately measured to 10.60m at waterline (13m overall)
D speed according to the 1.34Xsqrt(WL length in feet) gives 7.9knots
Finally remember we have no tides down here so speed is speed (bar going with or against the wind/waves et al)
Consumption was approx 3lt / nm for 12K kilos
Is this about right?

Now, I've read the theory with the bow wave and all that, but call me thick, I cannot (being on the boat) figure out if I'm at the right speed or a bit above it.
I'm asking as i'd expect a bit lower consumption and I'd also expect a much smoother exit of the water on the stern which looks like some very messy water popping about and reaching regularly the swim platform a good 25=30cm above the water (and 1.2m behind the stern).
I pic of the water at the stern follows, got a few video clips from mid boat on either side but they are too big (and useless imho) to bother and upload them, will probably try again from the dingy next time.

cheers

V.

By way of comparison i'm currently on a friends sailing boat, 45' loa, 11 tonnes displacement, and under power he gets 8 kts from a 53 hp engine. He uses 8 litres an hour, equivalent to about 4-5mpg

It's really comfortable under power in a big beam sea as the heavy keel gives it a long roll period, and it's equally good into a head sea due to the fine entry of the bow. With such a small engine it still has as much space below as a 45' mobo, despite the narrow bow.

I'm starting to think the ideal mobo is a sailing boat without sails ?

 

[BruceK]

By way of comparison i'm currently on a friends sailing boat, 45' loa, 11 tonnes displacement, and under power he gets 8 kts from a 53 hp engine. He uses 8 litres an hour, equivalent to about 4-5mpg

It's really comfortable under power in a big beam sea as the heavy keel gives it a long roll period, and it's equally good into a head sea due to the fine entry of the bow. With such a small engine it still has as much space below as a 45' mobo, despite the narrow bow.

I'm starting to think the ideal mobo is a sailing boat without sails ��

et tu? Lately I have been cruising around at 7 knts and really enjoying it and have also sneakily looked at a few raggie affairs in the 40 foot class that can be sailed short handed. I imagine I should be publicly pilloried and flogged for my deviant transgression, but tempted still.

 

[Nick_H]

We already do lots of our cruising at 8 knots, and i was ready to be blown away by sailing, but actually we've decided it's the slow, quiet, comfortable cruising we like, and the addition of sails seems like a lot of faff! I'm clear that our next boat will be optimised for slow cruising though, but maybe with the ability to speed up occasionally.

 

[longjohnsilver]

just did my calculations following last weeks 3day excursion to a couple of nearby islands.
that is 75nm
all done between 7 and 8knots
Boat has a planning hull and waterline length was accurately measured to 10.60m at waterline (13m overall)
D speed according to the 1.34Xsqrt(WL length in feet) gives 7.9knots
Finally remember we have no tides down here so speed is speed (bar going with or against the wind/waves et al)
Consumption was approx 3lt / nm for 12K kilos
Is this about right?

Now, I've read the theory with the bow wave and all that, but call me thick, I cannot (being on the boat) figure out if I'm at the right speed or a bit above it.
I'm asking as i'd expect a bit lower consumption and I'd also expect a much smoother exit of the water on the stern which looks like some very messy water popping about and reaching regularly the swim platform a good 25=30cm above the water (and 1.2m behind the stern).
I pic of the water at the stern follows, got a few video clips from mid boat on either side but they are too big (and useless imho) to bother and upload them, will probably try again from the dingy next time.

cheers

V.

Hi Vas, have you tried slowing down just a tad more to say 6.5 - 7kts. If so I suspect your fuel consumption will drop significantly from that at just about maximum displacement speed where you will be pushing a significant bow wave.

Our 48' weighs in at around 25 tons and we generally cruise at about 7kts, and we then get about 2+ nm per 4.5 lts. I also reckon that we are then only using about 50hp from the 270hp available, 1600rpm being the most efficient running revs, max around 2300, although I've never pushed up to those revs in the 9 years we've owned Eos, seems pointless increasing fuel consumption by approx 2-300% to gain an extra knot or so.

 

[Trundlebug]

Our 33ft, 7 ton displacement cruiser uses between 3.5 L/h (@5kn) and about 7 litres / hr (@7.3kn) That's between 6 and 3.5mpg. Total, not each engine. Read it and weep!

It's been significantly thirstier since I had the props re-pitched to nearer the recommended setting, interestingly - the figures above are before re-pitching.
Used to be 12in pitch, I gave them all the specs of the boat and they recommended 10in pitch, but I plumped for 11in. Glad I did, it would have been way under propped at 10.

I'm planning to go back to 12in when the boat comes out this winter.
Although the max revs it will achieve with 12 are 2900rpm (rated at 3600) general day to day cruising is better with reduced consumption. In a displacment boat, who uses max revs anyway...

 

[Hardmy]

We already do lots of our cruising at 8 knots, and i was ready to be blown away by sailing, but actually we've decided it's the slow, quiet, comfortable cruising we like, and the addition of sails seems like a lot of faff! I'm clear that our next boat will be optimised for slow cruising though, but maybe with the ability to speed up occasionally.

Me too, what I miss the most from my sailing times, is the total quietness.

Here in the Ionian Sea, it feels from time to time like a lake with no wind and waves. I guess it wouldn't need much HPs to move our boat to 5-6 kts in such conditions.

Would it be realistic to mount e.g. 2 x 5KW electric motors via gear belts on my shafts and power them with my Onan 12.0 genset? Would be a dream, complete silence and drinks only 4.5 liters an hour.

Is it a crazy idea to retrofit this on an old boat?

 

[MapisM]

We already do lots of our cruising at 8 knots, and i was ready to be blown away by sailing, but actually we've decided it's the slow, quiet, comfortable cruising we like, and the addition of sails seems like a lot of faff! I'm clear that our next boat will be optimised for slow cruising though, but maybe with the ability to speed up occasionally.

LOL, as I've been saying for decades: it's not a matter of IF a boater will eventually appreciate slow cruising, just of WHEN. :encouragement:

 

[vas]

thanks for the comments gents,

Hi Vas, have you tried slowing down just a tad more to say 6.5 - 7kts. If so I suspect your fuel consumption will drop significantly from that at just about maximum displacement speed where you will be pushing a significant bow wave.

Our 48' weighs in at around 25 tons and we generally cruise at about 7kts, and we then get about 2+ nm per 4.5 lts. I also reckon that we are then only using about 50hp from the 270hp available, 1600rpm being the most efficient running revs, max around 2300, although I've never pushed up to those revs in the 9 years we've owned Eos, seems pointless increasing fuel consumption by approx 2-300% to gain an extra knot or so.

J. I'm still experimenting, next long trip (180nm or so) will be done at 7 or under 7kn and will do the calcs. Have to admit that at 8 it's noisy from the exhausts and not so appealing to sit on the aft deck and chat, I feel that 1 or more kn down the aft deck will be a much liveable space...

Me too, what I miss the most from my sailing times, is the total quietness.

Here in the Ionian Sea, it feels from time to time like a lake with no wind and waves. I guess it wouldn't need much HPs to move our boat to 5-6 kts in such conditions.

Would it be realistic to mount e.g. 2 x 5KW electric motors via gear belts on my shafts and power them with my Onan 12.0 genset? Would be a dream, complete silence and drinks only 4.5 liters an hour.

Is it a crazy idea to retrofit this on an old boat?

Slightly crazy, but why not think a bit about it it. First you need to do the maths and make sure that two 5KW motors will actually shift the boat to 5-6kn, then find proper clutches and things to disengage the g/boxes as you definitely don't want to turn them around as well and then we are talking.
Maybe some special coupler/decoupler matched with a pulley exists and can run the system, maybe even one motor in the middle running two pulleys one for each shaft (and swap around the rotation direction on the one :argh: )

would like to see what the experts have to say!

LOL, as I've been saying for decades: it's not a matter of IF a boater will eventually appreciate slow cruising, just of WHEN. :encouragement:

proportionally analogue to cost of diesel innit?
TBH cannot say I get any kicks from running at the massive speed of 20kn, I'm better off driving my fiat coupe if I want some speed.

cheers

V.

 

[Nick_H]

Me too, what I miss the most from my sailing times, is the total quietness.

Here in the Ionian Sea, it feels from time to time like a lake with no wind and waves. I guess it wouldn't need much HPs to move our boat to 5-6 kts in such conditions.

Would it be realistic to mount e.g. 2 x 5KW electric motors via gear belts on my shafts and power them with my Onan 12.0 genset? Would be a dream, complete silence and drinks only 4.5 liters an hour.

Is it a crazy idea to retrofit this on an old boat?

No idea on the technical question, but we're currently sailing ariund your home waters, corfu to cephalonia, and it's the most amazing cruising grounds we've been in. Why are there so few motor boats when the winds are often light and waters are calm?

 

[Hardmy]

Thread drift “ON”

Nick_H, me neither I don't understand why there are so few mobos in the Ionian. Some would say that there is a lack of infrastructure and “mauvaises langues" would add that there are no places for gin palaces to show off.

The only downside of the area is in my view is the suboptimal flight offer and, in the high season the fact that flotillas confiscate some town harbours for themselves and prevent other boats to berth. Which is of course not OK, since all town quays are free to everybody and they cannot be blocked nor reserved.

The best time for sure is May-June and September-October when temperatures are fine with few boats being around. Would you for example consider moving your boat in this area? Compared to SoF, it is quite cheap to keep a boat there. But it needs some time to find the good tradesmen for completing jobs on the boat.

I promised myself to post a photo reportage, but I must overcome my lazyness. So a few pics just to give some impressions:

Re weather. The conditions are in general benign. But from time to time, some unforecasted storms come out of from nowhere and you can see charter boats with destroyed sails. Also, typically during the night on anchor, a big but short blow in the complete opposite direction will test the resetting capabilities of your anchor. This was the very reason, why I changed my anchor in the middle of the season. I also witnessed several sunken boats in such events. On Ithaca, Corfu, Preveza all in September last year. Locals say that weather was more predictable in the past. I cannot judge, since we spent only 2 seasons here.

Sunseeker in Corfu (here on the dry in Preveza):

Princess on Ithaca:

SY in Preveza:

Meanwhile we have pretty much seen most of the Ionian, and our plan is to move on next year. e.g. either round the Peloponnese or cross the Corinth canal then head north towards the Chalkidiki area.

Thread drift “OFF”

 

[Hardmy]

Slightly crazy, but why not think a bit about it it. First you need to do the maths and make sure that two 5KW motors will actually shift the boat to 5-6kn, then find proper clutches and things to disengage the g/boxes as you definitely don't want to turn them around as well and then we are talking.
Maybe some special coupler/decoupler matched with a pulley exists and can run the system, maybe even one motor in the middle running two pulleys one for each shaft (and swap around the rotation direction on the one :argh: )

Not sure that I need to disengage the gearbox during electric operation. I guess, it can just be in neutral provided that a separate pump lubricates/cools it.
Seems that there are lots of choice in respect to electric motors driving shafts via a belt. No need to reinvent the wheel…

Anyway, I didn’t do any maths, since it is really not a priority at the moment. But ones always needs some projects / new ideas. It will be done at the latest when I will give in to my parents who want to relocate the boat on the Saimaa Lake (the homewaters of Scubaman) for one summer.

 

[vas]

Meanwhile we have pretty much seen most of the Ionian, and our plan is to move on next year. e.g. either round the Peloponnese or cross the Corinth canal then head north towards the Chalkidiki area.

If/when you do give me a shout as I think Pagasitikos bay and the n. sporades are definitely worth a visit, or even a stop over for a year. Not sure you'll have too many things to do in Chalkidiki, only visited from land, must be rather crowded.
Not sure though you can lift your stabbed boat on the trolleys we have in the local yards, but any patents or machinist work you want, you can definitely do it on the cheap down here.

cheers

V.