longjohnsilver
Well-Known Member
That's exactly right Mike, all down to hull speed. Yes we do have to plan for tides, nothing new in that, raggies have been doing it for years ;-)
That's exactly right Mike, all down to hull speed. Yes we do have to plan for tides, nothing new in that, raggies have been doing it for years ;-)
Nautical
Well-Known Member
I have to be careful considering my association here but nevertheless, for ocean passages I would have to say a heavy displacement boat could be the right tool, great piece of kit and a battleship of a boat the Nordhavn in particular. Having said that there are (as with any boat) some downsides, one being round bilge D unless you have zero speed stabilisers (expensive) or deploy floppers every time you anchor they (as any round bilge boat) roll quite a bit, very sea sicky up top so high off the water but you do have that go anywhere in any weather feel to the vessel.
SD has its place in long distance cruising, for day long passages even in rough weather with the ability to get a move on if one is late for a bridge lift or the tide etc it is a real boon to just push the handles down and get there. 8 Knots against a 2 or 3 knot tide is painfully slow, 20 miles is 3 1/2 hours as opposed to 1 hour or a 100 miles in D mode takes 16 hours or 5hrs in SD mode. As one poster said after several days of it, it does become quite boring and tedious, the romanticism of it all soon wears off unless you have really considered the huge amount of time needed to get anywhere and how you will fill all that time several days at sea instead of half a day or worse a day. Again, nevertheless if you expect to encounter mountainous seas and extreme weather the heavy dispalcement route is possibily the way to go.
Having said all of the above the days when heavy displacement boats were considered the only option for extreme weather have gone, these days the RNLI to the US Coastguard all use hard chine semi D and even well founded and designed modified planing hulls can handle the worst mother Nature can throw at them, it might not be comfortable but certainly quite capable.
Aside from my assco, on a personal level I nearly completed on a Fleming when deciding on our Scandinavian adventure having looked at everything else D and SD, it is one beautifully built boat and exceptionally capable for what we had planed, the D route having done a few trips in one just brought the reality of the time factor into focus, a 1000 miles was 5 1/2 days at sea non stop. When we looked at where we would be going we were actually doing legs of no more than 300 miles in one go and never more than 100 miles from a safe haven so even if it looked like getting really nasty we could make port in a few hours if needed.
As far as handling/berthing is concerned it will never be as easy as with a twin engines SD but with practice and a bit of pre planing can be perfectly executed especially with good hydraulic thrusters and certainly someone with experience could get the swing of it quite quickly.
All IMHO of course, in the end it is what each is comfortable with and happy to have, having something that is brilliant for 10% of your cruising time is out of sync and similarly having something that doesn't just quite cut it 90% of the time mid Atlantic is also the wrong direction. They both have their pros and cons and I guess if one was ever going to buy one the trick to go for the one that has the lesser of the downsides but evaluating that could be a drawn out task but half the fun of deciding on a boat is planing and getting down to the nitty gritty of what you will end up doing, it might even surprise you that a planing hull boat might actually be the right tool if your distances are no more than a few hundred miles apart, I know several on here often do longer passages in planing boats and sometimes in not the best of weather, but at D speed have very favourable fuel burn as the engines are running at not more than 20% load, then if weather changes or you want to get a move on, zip along and be there in time for dinner
SD has its place in long distance cruising, for day long passages even in rough weather with the ability to get a move on if one is late for a bridge lift or the tide etc it is a real boon to just push the handles down and get there. 8 Knots against a 2 or 3 knot tide is painfully slow, 20 miles is 3 1/2 hours as opposed to 1 hour or a 100 miles in D mode takes 16 hours or 5hrs in SD mode. As one poster said after several days of it, it does become quite boring and tedious, the romanticism of it all soon wears off unless you have really considered the huge amount of time needed to get anywhere and how you will fill all that time several days at sea instead of half a day or worse a day. Again, nevertheless if you expect to encounter mountainous seas and extreme weather the heavy dispalcement route is possibily the way to go.
Having said all of the above the days when heavy displacement boats were considered the only option for extreme weather have gone, these days the RNLI to the US Coastguard all use hard chine semi D and even well founded and designed modified planing hulls can handle the worst mother Nature can throw at them, it might not be comfortable but certainly quite capable.
Aside from my assco, on a personal level I nearly completed on a Fleming when deciding on our Scandinavian adventure having looked at everything else D and SD, it is one beautifully built boat and exceptionally capable for what we had planed, the D route having done a few trips in one just brought the reality of the time factor into focus, a 1000 miles was 5 1/2 days at sea non stop. When we looked at where we would be going we were actually doing legs of no more than 300 miles in one go and never more than 100 miles from a safe haven so even if it looked like getting really nasty we could make port in a few hours if needed.
As far as handling/berthing is concerned it will never be as easy as with a twin engines SD but with practice and a bit of pre planing can be perfectly executed especially with good hydraulic thrusters and certainly someone with experience could get the swing of it quite quickly.
All IMHO of course, in the end it is what each is comfortable with and happy to have, having something that is brilliant for 10% of your cruising time is out of sync and similarly having something that doesn't just quite cut it 90% of the time mid Atlantic is also the wrong direction. They both have their pros and cons and I guess if one was ever going to buy one the trick to go for the one that has the lesser of the downsides but evaluating that could be a drawn out task but half the fun of deciding on a boat is planing and getting down to the nitty gritty of what you will end up doing, it might even surprise you that a planing hull boat might actually be the right tool if your distances are no more than a few hundred miles apart, I know several on here often do longer passages in planing boats and sometimes in not the best of weather, but at D speed have very favourable fuel burn as the engines are running at not more than 20% load, then if weather changes or you want to get a move on, zip along and be there in time for dinner
Last edited:
Neil_Y
Well-Known Member
No It's the William McCann, but now owned by the excelsior trust I believe.
piratos
Well-Known Member
one or two engines
Replies have come to the disussion full planning/Semidisplacement or fullDisplacements which is also interesting. All have Pros and Contras, but as to the question whether you could live with one engine if you are used to two is probably: Yes if you have to !
If you can live with the full displacementboats like Selene/Nordhavn twin engines is still on the option list, though it might cost you some of the fuel capacity. Probably the 2000nm range will be enough anyway.
Replies have come to the disussion full planning/Semidisplacement or fullDisplacements which is also interesting. All have Pros and Contras, but as to the question whether you could live with one engine if you are used to two is probably: Yes if you have to !
If you can live with the full displacementboats like Selene/Nordhavn twin engines is still on the option list, though it might cost you some of the fuel capacity. Probably the 2000nm range will be enough anyway.
Robin
Well-Known Member
As raggies with boat sold and intending moving to the USA to live aboard a trawler yacht we have been giving this a lot of thought. If we had the money a Nordhavn 47 would be very high on the list! The one we looked on at SIBS had a main engine on the centre line plus a spare 'wing' engine but I don't know if that could aid handling with both running or make it harder.
We have decided on twin engines and displacement speeds. Semi-displacement seemed like a good idea until we looked at the sums for the boats we liked, because fuel burn at 8kts was OK but at 14kts was horrendous and for little real speed gain. We are of course used to displacement speeds as raggies and to planning tides to best advantage. If we went the semi displacement route on the boats we are considering (very much like Longjohnsilvers) the engine size goes up from 2 x 135hp to 2 x 375hp. The bigger engines are turbo aftercooled and so servicing and repair cost are higher and life possibly shorter. Running such engines 90% of the time at displacement speeds would not be good for them either, just to have a bit more available for just 10% of the time.
Rolling has been mentioned and certainly Nordhavns do that without stabilisers or paravanes (they have a video of a trans Atlantic cruise in one where the stabilisers packed up). However there are hard chine trawlers about that are displacement ones, OK so the hull is really SD but there is no big engine option so that is not relevant.
Not sure how I would feel about displacement speeds after a long time used to planing speeds however. For us and what we plan to do, displacement is a no brainer.
We have decided on twin engines and displacement speeds. Semi-displacement seemed like a good idea until we looked at the sums for the boats we liked, because fuel burn at 8kts was OK but at 14kts was horrendous and for little real speed gain. We are of course used to displacement speeds as raggies and to planning tides to best advantage. If we went the semi displacement route on the boats we are considering (very much like Longjohnsilvers) the engine size goes up from 2 x 135hp to 2 x 375hp. The bigger engines are turbo aftercooled and so servicing and repair cost are higher and life possibly shorter. Running such engines 90% of the time at displacement speeds would not be good for them either, just to have a bit more available for just 10% of the time.
Rolling has been mentioned and certainly Nordhavns do that without stabilisers or paravanes (they have a video of a trans Atlantic cruise in one where the stabilisers packed up). However there are hard chine trawlers about that are displacement ones, OK so the hull is really SD but there is no big engine option so that is not relevant.
Not sure how I would feel about displacement speeds after a long time used to planing speeds however. For us and what we plan to do, displacement is a no brainer.
D
Deleted User YDKXO
Guest
Trev, fair point about the rolling tendency of round bilge d hulls at anchor but won't the heavier weight and deeper draft of a typical round bilge d hull boat like a Nordhavn mitigate this tendency? Also what I'm trying to understand is whether there is any difference between the d performance of something like a Nordhavn and a hard chine semi d hull like a Fleming or Outer Reef. Which is more fuel efficient? I'm guessing that might actually be the hard chine semi d hull because that type of boat is generally lighter and the hull displaces less water. Also is there any difference in the motion in a sea way?
Robin makes a good point about the engines. If you run a semi d boat at 7-8kts for 90% of it's operating hours, what effect does that have on the engines?
Robin makes a good point about the engines. If you run a semi d boat at 7-8kts for 90% of it's operating hours, what effect does that have on the engines?
D
Deleted User YDKXO
Guest
V good point about the engines, Robin. What boat have you decided to go for, if you don't mind me asking?
I was wondering myself about whether you could use the wing engine to aid manouvering but I don't think the relatively small prop is going have a big effect
I was wondering myself about whether you could use the wing engine to aid manouvering but I don't think the relatively small prop is going have a big effect
Robin
Well-Known Member
Top of our wish list are Defever 49 Raised Pilot House, Defever 48 (same as Longjohnsilvers of this parish) and the Defever 44. These are all from the late 1980s although they are still made in similar format. The 49 is available as a round bilge or hard chine (there were two different builders) and the 48 is actually the same hull as the 49 hard chine. The 44 is a round bilged hull. All of these (I'm advised by those in the know in the USA) are best with stabilisers and about 50% had them from new. Most of these have 2 x 135hp Ford Lehman natural aspirated 6 cylinder diesels, a few newer ones have Perkins similar sized. We looked at this 1988 DF49 last Easter which is now sold.
We also looked at Grand Banks 46 and 49s, both of which can have small or big engines, about 30% in the USA with Ford Lehman 135hp and 70% with Cat 3208TA 375hp. The speed difference with the bigger engines is not much, 14kts say versus 9kts, for a huge difference in fuel burn.
The Nordhavn people at SIBS said the wing engine with a saildrive folding prop still gave 5kts or so?
Bajansailor
Well-Known Member
I think that for similar sized vessels, a displacement hull at displacement speed will usually be more efficient than a hard chine semi-displacement hull.
This is because at these speeds the prismatic coefficient has a lot of say in the resistance of the vessel - most of this is form resistance, with virtually no wave making resistance.
A planing boat has quite a lot of form resistance at displacement speeds - just look at all the turbulence behind the transom at these speeds, compared to the much cleaner wake of a more displacement craft.
The displacement hull will probably have a prismatic coefficient of around 0.60 - 0.65, whereas a semi-displacement hull will have a higher coefficient, perhaps 0.65 - 0.70, and a pure planing hull would tend to be higher still.
The prismatic coefficient is the ratio of the immersed volume to the area of the midship section multiplied by the waterline length, and is a measure of the 'fullness' of the hull.
Changing tack, re handling a big single engine boat, my only experience of this was running one of these Damen Pushy Cat tugs for a while -
http://www.damen.nl/PRODUCTS/DAMEN_PUSHY_CATS.aspx?mId=8652
Although she was 40' and perhaps 18 tonnes, she could easily be handled and manoeuvered singlehanded. She had a huge propeller, with lots of prop walk which would kick the stern in nicely. We would get a spring line on with the midships bollard and just hold her alongside on that.
Last edited:
AndieMac
Well-Known Member
I recently heard fuel consumption figures of 1 litre per N Mile for the round bilged smaller Nords, and the feeling is certainly round is more efficient than hard chine.
Last edited:
Nautical
Well-Known Member
Hi Mike, interesting question, I am no hydro dynamics expert so can only relay personal experience over 20 odd years 10 of which were with SAR so had the opportunity to skipper many round bilge full D and hard chine SD in everything from flat clam to some serious up and down stuff.
In my experience, full D round bilge hulls tend to roll quite a bit, I think partly down to many deep keeled D hulls have a lot of ballast in the lower keel which tends to set up a pendulum affect especially when beam on. I owned a round bilged ex Solent class displacement lifeboat for several years which had a hull of monumental proportions and while she was a bulldozer of a boat she did roll like a pig beam on, at times quite hairy , dipping her gunwales every swing. Probably a little unfair as Nordhavn's and Selene's have a much higher free board but many similarities in their design. Of course my displacement boat did not have stabilizers which if I were having one now would be top of the options list. I have also had the privilege of helming Trents, Severn's and Arun's all Hard Chine SD and they all without doubt rolled less beam on and at displacement speed there wasn't much to choose between either D or SD. What I can not say is whether the roll on a round bilge displacement hull would be much the same as SD when fitted with stabilizers, I would assume they would significantly reduce the roll on D hulls .
Again I would not knock D hulls for their intended purpose and to many they are the ultimate for long distance heavy weather cruising, you just have to re think your cruising habits and accept that everything takes a lot longer.
As far as turning and assistance in turning with the wing engine engaged I think that might hinder rather than help, the grip from that little prop is going to be no where near the main prop so I would guess you would have to be giving it a lot more throttle to have any affect against the main engine and whether the prop wash would disturb the main prop again would need to be considered. I would think a bit of practice with just the main engine and that big rudder would be the preferred mode.
Hereunder some fuel burn figures for a recently delivered Outer Reef 63. All figures were taken from live data from on board meters and CAT's engine management. Average speed from two way run.
2 X CAT C9's 503 BHP D rated
RPM....SPEED...USG port....USG stb....ENGINE LOAD port....ENGINE LOAD stb
1100....7.6.........1.9............2.1..............11%.......................13%
1200....8.1.........2.2............2.4..............12%.......................14%
1300....8.8.........2.5............3.0..............13%.......................17%
1400....9.3.........3.0............3.7..............14%.......................17%
1500....10..........4.0............4.7..............15%.......................18%
Bear in mind these numbers are USG PH so you need to multiply by 3.785 to get LPH
I hope that helps, I am sure some one will have numbers for full displacement fuel burn.
In my experience, full D round bilge hulls tend to roll quite a bit, I think partly down to many deep keeled D hulls have a lot of ballast in the lower keel which tends to set up a pendulum affect especially when beam on. I owned a round bilged ex Solent class displacement lifeboat for several years which had a hull of monumental proportions and while she was a bulldozer of a boat she did roll like a pig beam on, at times quite hairy , dipping her gunwales every swing. Probably a little unfair as Nordhavn's and Selene's have a much higher free board but many similarities in their design. Of course my displacement boat did not have stabilizers which if I were having one now would be top of the options list. I have also had the privilege of helming Trents, Severn's and Arun's all Hard Chine SD and they all without doubt rolled less beam on and at displacement speed there wasn't much to choose between either D or SD. What I can not say is whether the roll on a round bilge displacement hull would be much the same as SD when fitted with stabilizers, I would assume they would significantly reduce the roll on D hulls .
Again I would not knock D hulls for their intended purpose and to many they are the ultimate for long distance heavy weather cruising, you just have to re think your cruising habits and accept that everything takes a lot longer.
As far as turning and assistance in turning with the wing engine engaged I think that might hinder rather than help, the grip from that little prop is going to be no where near the main prop so I would guess you would have to be giving it a lot more throttle to have any affect against the main engine and whether the prop wash would disturb the main prop again would need to be considered. I would think a bit of practice with just the main engine and that big rudder would be the preferred mode.
Hereunder some fuel burn figures for a recently delivered Outer Reef 63. All figures were taken from live data from on board meters and CAT's engine management. Average speed from two way run.
2 X CAT C9's 503 BHP D rated
RPM....SPEED...USG port....USG stb....ENGINE LOAD port....ENGINE LOAD stb
1100....7.6.........1.9............2.1..............11%.......................13%
1200....8.1.........2.2............2.4..............12%.......................14%
1300....8.8.........2.5............3.0..............13%.......................17%
1400....9.3.........3.0............3.7..............14%.......................17%
1500....10..........4.0............4.7..............15%.......................18%
Bear in mind these numbers are USG PH so you need to multiply by 3.785 to get LPH
I hope that helps, I am sure some one will have numbers for full displacement fuel burn.
Bajansailor
Well-Known Member
Errrm, Trev, the Arun class lifeboats are not hard chine - they are round bilge semi-displacement (sorry, just being pedantic).
Your Outer Reef is getting 1.9 miles per gallon at 1,100 rpm, steadily decreasing to 1.15 mpg at 1,500 rpm.
What are the max rpm on the Cats on this Outer Reef? I am guessing that you could probably get an extra 0.5 knots perhaps, while drinking double the fuel consumption at 1,500 rpm?
One litre per mile (3.785 mpg) for the smaller Nordhavn's does sound pretty impressive! They do have fuel consumption figures on their website, and this figure is probably for one with a big slow Lugger - I noticed that the Nordhavns with the bigger faster engines did have much less mpgs.
Although still better than the 30' - 33' Grady White cruisers here - there are a few of them with twin Yamaha 250 hp outboards (one even has three on the back....), and they seem to average around 0.5 mpg at cruising speeds - ouch!
Your Outer Reef is getting 1.9 miles per gallon at 1,100 rpm, steadily decreasing to 1.15 mpg at 1,500 rpm.
What are the max rpm on the Cats on this Outer Reef? I am guessing that you could probably get an extra 0.5 knots perhaps, while drinking double the fuel consumption at 1,500 rpm?
One litre per mile (3.785 mpg) for the smaller Nordhavn's does sound pretty impressive! They do have fuel consumption figures on their website, and this figure is probably for one with a big slow Lugger - I noticed that the Nordhavns with the bigger faster engines did have much less mpgs.
Although still better than the 30' - 33' Grady White cruisers here - there are a few of them with twin Yamaha 250 hp outboards (one even has three on the back....), and they seem to average around 0.5 mpg at cruising speeds - ouch!
AndieMac
Well-Known Member
The Lugger was 105 h.p in a 46' at 6 knots.
I had a 34' hard chine with a 120 h.p. Ford Lehman, and she would average 7 litres and hour at 6 knots, so I guessed it was right.
As others have said, displacement speeds aren't for everybody, nor is crossing oceans.
Nautical
Well-Known Member
Your absolutely right, it was 15 years ago, the old grey matter is not what it was, still a fine boat and I would actually say a better boat than the Trent which to me felt a little flighty at top speed.
1500 rpm and burn rises to 8.7 USG PH for 10 knots which in imperial is 1.39 MPG
Her best speed for fuel burn is 1100 rpm which gives 2.289 MPG and 7.6 knots.
Maximum RPM is 2500 rpm and 15.5 knots but fuel burn is high of course, if you want to do that speed all the time you are better off with the C12's which are much more efficient at higher speeds as the engine load is 70% as opposed to the C9's which are at 98% load. at 15.5 Knots on C12's she is about as efficient as your GW with twin 250 hp outboards which when you think about it is pretty impressive for a boat hugely bigger in size and 10 times heavier.
1500 rpm and burn rises to 8.7 USG PH for 10 knots which in imperial is 1.39 MPG
Her best speed for fuel burn is 1100 rpm which gives 2.289 MPG and 7.6 knots.
Maximum RPM is 2500 rpm and 15.5 knots but fuel burn is high of course, if you want to do that speed all the time you are better off with the C12's which are much more efficient at higher speeds as the engine load is 70% as opposed to the C9's which are at 98% load. at 15.5 Knots on C12's she is about as efficient as your GW with twin 250 hp outboards which when you think about it is pretty impressive for a boat hugely bigger in size and 10 times heavier.
grumpy_o_g
Well-Known Member
I'm always intrigued by the difference in power between an auxiliary on a sailboat and the equivalent on a displacement powerboat. I'm guessing that 70' foot would give the same level of accommodation no problem and I would have thought 150 HP was more than adequate even these days.
A quick look at some ads though shows that they have 200 HP at least on the likes of the Turkish commercial gullets and even Oysters, Swans, etc. seem to get that too so maybe it's not that far off.
The one thing that strikes me on both hard chine and round bilge is the submerged square transom. I'd have thought beam, displacement, waterline length and the entry and exit were all more critical than the shape of the cross-section for efficiency.
Oddly enough my very limited experience has sometimes been the opposite of Nautical's and other peoples. I sailed fairly regularly on a Waarship some years ago - okay a different animal completely but I remember vividly that it was an absolute pig in a certain size swell. It would follow the water it seemed - as the swell got under the chine it would lift it very quickly, pause at an angle and then drop it the other way before repeating the process on the next swell. Even things like Squibs and Hunters would be comparatively steady and the Stellas and the like just bobbed straight up and down. I guess the keel and sails makes a huge difference but that was one situation where hard chine and beam seemed to work against us.
A quick look at some ads though shows that they have 200 HP at least on the likes of the Turkish commercial gullets and even Oysters, Swans, etc. seem to get that too so maybe it's not that far off.
The one thing that strikes me on both hard chine and round bilge is the submerged square transom. I'd have thought beam, displacement, waterline length and the entry and exit were all more critical than the shape of the cross-section for efficiency.
Oddly enough my very limited experience has sometimes been the opposite of Nautical's and other peoples. I sailed fairly regularly on a Waarship some years ago - okay a different animal completely but I remember vividly that it was an absolute pig in a certain size swell. It would follow the water it seemed - as the swell got under the chine it would lift it very quickly, pause at an angle and then drop it the other way before repeating the process on the next swell. Even things like Squibs and Hunters would be comparatively steady and the Stellas and the like just bobbed straight up and down. I guess the keel and sails makes a huge difference but that was one situation where hard chine and beam seemed to work against us.
D
Deleted User YDKXO
Guest
Thanks, Trev, I recognise those figures from the OR website! I don't know how that compares to a round bilge boat like the Nordhavn but 2.2 nm per imperial gal @ 8.1kts for a 63ft boat is impressive. How do the Cat C9's like being run at 1200rpm for most of their life?
One thing I have learnt is that the Selene and Nordhavn hulls are slightly different. The Nordhavn seems to carry it's round bilge shape all the way aft whereas most Selenes, the current ones anyway, are round bilge forward but morph into a hard chine form aft. Selene say this is to offer better resistance to rolling (and give a slightly higher top speed) which tends to bear out your assertion that round bilge hulls roll more than hard chine ones.
I agree with you about the wing engine. I think it would be useless for assisting slow speed manouvering
One thing I have learnt is that the Selene and Nordhavn hulls are slightly different. The Nordhavn seems to carry it's round bilge shape all the way aft whereas most Selenes, the current ones anyway, are round bilge forward but morph into a hard chine form aft. Selene say this is to offer better resistance to rolling (and give a slightly higher top speed) which tends to bear out your assertion that round bilge hulls roll more than hard chine ones.
I agree with you about the wing engine. I think it would be useless for assisting slow speed manouvering
Bajansailor
Well-Known Member
I would agree that round bilge hulls generally tend to roll more than similar hard chine hull forms - this happens with sailing yachts as well.
For instance our 35' (sailing) boat has a wineglass shape midship section and she is effectively double-ended at the stern in way of the waterline (in order to achieve a 'good' prismatic coefficient for 'slow' speeds).
Compare this with a more modern Ben, Jen or Bav sailing boat, with 'U' shaped midship sections that flatten out and stay beamy going aft, all the way to the transom.
In a slight swell our boat at anchor will be rolling furiously, while the BenJenBavs nearby are hardly moving in comparison.
The BenJenBavs have more form resistance at slow displacement speeds, as they are 'dragging' a broad stern with them, hence will need more power.
However they would tend to have a larger engine (than ours) and a more powerful sailing rig (than a hull form like ours) to allow for this.
And they also then have the ability to surf a wave going downwind, and will generally have better helm control down wind than the IOR influenced pinched stern sailing boats of the 70's, which can apparently get a bit wild when sailing down wind hard.
Going back to motor boats, on a planing boat with constant deadrise between midships and the transom, the hull form might almost be parallel mid body the whole way aft, hence it will have a relatively high prismatic.
(If you chopped the hull in half amidships, the prismatic coefficient of the aft section would probably be greater than 0.9 - for comparison, the prismatic of a rectangular block of wood is 1.0).
If you have variable deadrise going aft from amidships (eg such that there is zero at the transom, ie the hull is flat), then the prismatic coefficient will be less than for constant deadrise aft.
Which is good from a resistance point of view - but not so good if you want to have high dead rise the whole way back for good sea keeping.
And if you reduce the waterline beam while going aft from amidships with this decreasing deadrise then the prismatic will be reduced some more.
But then the effective planing surface will also be reduced......
So as usual, everything is a compromise in boat design!
For instance our 35' (sailing) boat has a wineglass shape midship section and she is effectively double-ended at the stern in way of the waterline (in order to achieve a 'good' prismatic coefficient for 'slow' speeds).
Compare this with a more modern Ben, Jen or Bav sailing boat, with 'U' shaped midship sections that flatten out and stay beamy going aft, all the way to the transom.
In a slight swell our boat at anchor will be rolling furiously, while the BenJenBavs nearby are hardly moving in comparison.
The BenJenBavs have more form resistance at slow displacement speeds, as they are 'dragging' a broad stern with them, hence will need more power.
However they would tend to have a larger engine (than ours) and a more powerful sailing rig (than a hull form like ours) to allow for this.
And they also then have the ability to surf a wave going downwind, and will generally have better helm control down wind than the IOR influenced pinched stern sailing boats of the 70's, which can apparently get a bit wild when sailing down wind hard.
Going back to motor boats, on a planing boat with constant deadrise between midships and the transom, the hull form might almost be parallel mid body the whole way aft, hence it will have a relatively high prismatic.
(If you chopped the hull in half amidships, the prismatic coefficient of the aft section would probably be greater than 0.9 - for comparison, the prismatic of a rectangular block of wood is 1.0).
If you have variable deadrise going aft from amidships (eg such that there is zero at the transom, ie the hull is flat), then the prismatic coefficient will be less than for constant deadrise aft.
Which is good from a resistance point of view - but not so good if you want to have high dead rise the whole way back for good sea keeping.
And if you reduce the waterline beam while going aft from amidships with this decreasing deadrise then the prismatic will be reduced some more.
But then the effective planing surface will also be reduced......
So as usual, everything is a compromise in boat design!
AndieMac
Well-Known Member
Good report Martin......You notice the change from 70's and 80's planing hulls, with the beam tapering aft, to the volume conscious contempory designs.
Nautical
Well-Known Member
Hi Mike,
The C9's in the spec we order them are pretty happy tipping along at 1200rpm no problem, there are owners out there that have racked up 1000's of hours with no issues. On a personal preference I would probably run the engines at 70% for an hour every so often but they are quite happy to do hour after hour at 1200.
Fuel burn SD V's D, well I guess that is all subjective based on how slow you are happy to cruise at but the below nicked from MBY :-0
Full displacement 55' long distance trawler with 1 X John Deere 330 BHP
RPM............Speed...............MPG
1200............6.9..................2.44
1300............7.4..................2.20
1400............7.7..................1.85
1500............8.3..................1.61
1600............8.7..................1.34
1700............9.2..................1.22
1800............9.6..................1.05
2200............10.8.................0.77
You can see as long as you keep below 7.5 knots the fuel burn is comparable to a twin engined SD, if you push D more than an extra knot or so the fuel burn increases dramatically and if you push near to maximum hull speed for D you are burning alot more than SD at the same speed.
The C9's in the spec we order them are pretty happy tipping along at 1200rpm no problem, there are owners out there that have racked up 1000's of hours with no issues. On a personal preference I would probably run the engines at 70% for an hour every so often but they are quite happy to do hour after hour at 1200.
Fuel burn SD V's D, well I guess that is all subjective based on how slow you are happy to cruise at but the below nicked from MBY :-0
Full displacement 55' long distance trawler with 1 X John Deere 330 BHP
RPM............Speed...............MPG
1200............6.9..................2.44
1300............7.4..................2.20
1400............7.7..................1.85
1500............8.3..................1.61
1600............8.7..................1.34
1700............9.2..................1.22
1800............9.6..................1.05
2200............10.8.................0.77
You can see as long as you keep below 7.5 knots the fuel burn is comparable to a twin engined SD, if you push D more than an extra knot or so the fuel burn increases dramatically and if you push near to maximum hull speed for D you are burning alot more than SD at the same speed.
mjf
Well-Known Member
Here are the Bandido 75' numbers
Bandido 75' Speed Engine Consumption
kn RPM l/h ****
Port / at anchor *
Port / at anchor **
Minimal speed 1 engine *** 3.2 600 6
Minimal speed 2 engines *** 4.2 600 12
Cruising speed 1 8.0 1.200 48
Cruising speed 2 10.1 1.600 98
Cruising speed 3 11.5 2.000 193
Maximum 12.0 2.100 226
Sorry the tabulation is skew-whiff but you can see 600rpm is 6 ltrs/hr 3.2kn
Bandido 75' Speed Engine Consumption
kn RPM l/h ****
Port / at anchor *
Port / at anchor **
Minimal speed 1 engine *** 3.2 600 6
Minimal speed 2 engines *** 4.2 600 12
Cruising speed 1 8.0 1.200 48
Cruising speed 2 10.1 1.600 98
Cruising speed 3 11.5 2.000 193
Maximum 12.0 2.100 226
Sorry the tabulation is skew-whiff but you can see 600rpm is 6 ltrs/hr 3.2kn