(semi)Displacement hull vs planing hull at displacement speeds??

[wipe_out]

Looking at boats for fuel efficient and longer distance cruising.. Obviously this lends itself to displacement or semi-displacement speeds.. In the lengths I would be looking at (10-12m) there are not that many options for SD or D hulled boats but lots of options for planning hulls..

So in the real world what is the difference between running a planning boat and semi/displacement boat of the same length at the same D or SD speed in terms of fuel consumption and ride?

 

[Medskipper]

Looking at boats for fuel efficient and longer distance cruising.. Obviously this lends itself to displacement or semi-displacement speeds.. In the lengths I would be looking at (10-12m) there are not that many options for SD or D hulled boats but lots of options for planning hulls..

So in the real world what is the difference between running a planning boat and semi/displacement boat of the same length at the same D or SD speed in terms of fuel consumption and ride?

Ive always felt that any boat cruising at under ten knots is a more comfortable ride with displacement or semi displacement. Planing boats can be quite uncomfortable at displacement speeds, usually because the sea state is bad and you are forced to come off the plane.

 

[Bouba]

I have an 11m sd hull, to me it feels good in a lumpy sea but I have no comparison. The other thread on the same subject is about boats upto 80ft, and the conclusion is that a p or sd is about the same. They are probably right, at that size and weight and stabilized to boot, I would think in the 30ft range a p hull would be more uncomfortable than a sd but I'm just guessing.

 

[petem]

Be aware of what the engine is doing at 9kts or so. For some boats it means a noisy supercharger may be running.

 

[rafiki_]

My 12m planing hull is quite happy at D speeds. Also the option of popping onto the plane when needed. Can't do that with a D hull.

 

[Bouba]

My 12m planing hull is quite happy at D speeds. Also the option of popping onto the plane when needed. Can't do that with a D hull.

Can with an SD

 

[Greg2]

Don't think it makes much odds in the 10-12m range. We had a semi-displacement two boats ago and current boat is planing with a small skeg keel - both Brooms and both good sea boats. We run the current boat at displacement speeds when it suits and I would be hard pushed to differentiate between the two.....except that the semi-displacement rolled a lot more, particularly in a quartering sea but she was round bilged.

May well depend on the hull shape because 'planing' actually covers quite a range so perhaps some are better than others?

 

[stillwaters]

There are actually a wide variety of hull shapes in each of the three so-called types, and sometimes the differences can be quite slight. However, in simple terms, the sharper bow entry with rounder mid sections of D & SD hulls generally give a softer ride than P boats in rougher conditions. Reason being is that whereas P boats are designed to more easily lift and travel over the surface of the water, D boats cut through the water. SD boats similarly cut through the water at D speeds, but unlike D boats they have flatter sections aft and can therefore also plane given enough power. The downsides to SD boats though, tend to be the heavier fuel usage compared to P boats, which can even be true at D speeds but especially at P speeds, as well as their tendency to roll more in quartering seas if not stabilized, plus they tend to ship more water over them. The advantages though are that whilst all but the largest P hulls find it hard to resist bobbing over even quite small waves or other boats' wakes, SD & D hulls tend to cut through them and therefore don't pitch quite so much. And in rougher conditions when the flatter sections of the P hulls start to slam into the waves, the SD hulls have softer entries which usually don't slam - but they can keep your windscreen wipers busy.
Boats are all about which compromise suits you best at the time.

 

[Bouba]

There are actually a wide variety of hull shapes in each of the three so-called types, and sometimes the differences can be quite slight. However, in simple terms, the sharper bow entry with rounder mid sections of D & SD hulls generally give a softer ride than P boats in rougher conditions. Reason being is that whereas P boats are designed to more easily lift and travel over the surface of the water, D boats cut through the water. SD boats similarly cut through the water at D speeds, but unlike D boats they have flatter sections aft and can therefore also plane given enough power. The downsides to SD boats though, tend to be the heavier fuel usage compared to P boats, which can even be true at D speeds but especially at P speeds, as well as their tendency to roll more in quartering seas if not stabilized, plus they tend to ship more water over them. The advantages though are that whilst all but the largest P hulls find it hard to resist bobbing over even quite small waves or other boats' wakes, SD & D hulls tend to cut through them and therefore don't pitch quite so much. And in rougher conditions when the flatter sections of the P hulls start to slam into the waves, the SD hulls have softer entries which usually don't slam - but they can keep your windscreen wipers busy.
Boats are all about which compromise suits you best at the time.

I wish I could have summed it up as eloquently as stillwaters has:encouragement:

 

[rafiki_]

Can with an SD

I have no experience with SD, so just giving my personal view. ��

 

[paul salliss]

Personally I would not buy a D boat. They are designed to cover huge ground economically and unless you are retired and adventurous I am not sure how that works. Better to have a faster boat where you can choose. Just my opinion

 

[Cathy*]

Our first boat was an 8m SD. We used to sit happily at anchor for a cup of tea and, although we didn't travel far from home, she felt stable in the water. After a year we wanted another 4 ft and another engine so we could go further and stay onboard longer so moved up to a 10m twin engine planing hull. She was not particularly comfortable in anything more than a smooth/slight sea at less than planing speed. Consequently there were times when the sea was bigger and we had to slow down and I felt like I was on a combination of rollercoaster and waltzer. At anchor it felt like we were a cork on the surface at the mercy of every little swell.

We have just moved up again to 11.5 metres SD. I'm hoping that this will be the last move, if she doesn't suit I'm going to take up golf. We achieved 19 kn on the sea trial and I'm sure at that speed would guzzle fuel but we don't intend to go that fast. We've only done 2 very short runs out into Plymouth Sound last week while we get used to the boat. There was about a metre swell and plenty of traffic making waves which we hardly noticed so so far so good

 

[curiouskb]

Our first boat was an 8m SD. We used to sit happily at anchor for a cup of tea and, although we didn't travel far from home, she felt stable in the water. After a year we wanted another 4 ft and another engine so we could go further and stay onboard longer so moved up to a 10m twin engine planing hull. She was not particularly comfortable in anything more than a smooth/slight sea at less than planing speed. Consequently there were times when the sea was bigger and we had to slow down and I felt like I was on a combination of rollercoaster and waltzer. At anchor it felt like we were a cork on the surface at the mercy of every little swell.

We have just moved up again to 11.5 metres SD. I'm hoping that this will be the last move, if she doesn't suit I'm going to take up golf. We achieved 19 kn on the sea trial and I'm sure at that speed would guzzle fuel but we don't intend to go that fast. We've only done 2 very short runs out into Plymouth Sound last week while we get used to the boat. There was about a metre swell and plenty of traffic making waves which we hardly noticed so so far so good

From what I read before. Generally planning hulls tend to roll a lot more in beam seas when going at displacement speed, compared to D hull shapes. However when up on plane the planing hulls gain lateral stability and thus they are more comfortable with beam and following seas.

 

[paul salliss]

There are actually a wide variety of hull shapes in each of the three so-called types, and sometimes the differences can be quite slight. However, in simple terms, the sharper bow entry with rounder mid sections of D & SD hulls generally give a softer ride than P boats in rougher conditions. Reason being is that whereas P boats are designed to more easily lift and travel over the surface of the water, D boats cut through the water. SD boats similarly cut through the water at D speeds, but unlike D boats they have flatter sections aft and can therefore also plane given enough power. The downsides to SD boats though, tend to be the heavier fuel usage compared to P boats, which can even be true at D speeds but especially at P speeds, as well as their tendency to roll more in quartering seas if not stabilized, plus they tend to ship more water over them. The advantages though are that whilst all but the largest P hulls find it hard to resist bobbing over even quite small waves or other boats' wakes, SD & D hulls tend to cut through them and therefore don't pitch quite so much. And in rougher conditions when the flatter sections of the P hulls start to slam into the waves, the SD hulls have softer entries which usually don't slam - but they can keep your windscreen wipers busy.
Boats are all about which compromise suits you best at the time.

Your last line is perfect. We can debate many of these topics to
kingdom come,
but it is all a giant compromise as to what suits you and your family at a particular stage in life.

 

[Deleted User YDKXO]

From what I read before. Generally planning hulls tend to roll a lot more in beam seas when going at displacement speed, compared to D hull shapes. However when up on plane the planing hulls gain lateral stability and thus they are more comfortable with beam and following seas.

Not sure thats true. A narrow beamed round bilged D hull is going to roll like a pig in any kind of beam sea whereas a wider hard chined planing boat is going to resist rolling better. I hesitate to use the S word but stabilisers are probably essential if you are going to use any type of hull, D/SD/P at D speed in a beam sea for extended cruising

 

[Robin]

Not sure thats true. A narrow beamed round bilged D hull is going to roll like a pig in any kind of beam sea whereas a wider hard chined planing boat is going to resist rolling better. I hesitate to use the S word but stabilisers are probably essential if you are going to use any type of hull, D/SD/P at D speed in a beam sea for extended cruising

Here in Doodleland the doodlers are paranoid about a ripple and stabilisers are common, even in the ICW. We did not have them in our SD high sided, top heavy 'fast trawler' and never felt the need but then again never tested that in offshore beam seas, round here a typical Channel crossing would probably have them filming 'Perfect Storm 2' They get paranoid about crossing the Gulf Stream if the wind opposes the flow and the guides issue dire warnings of it.

 

[Deleted User YDKXO]

Here in Doodleland the doodlers are paranoid about a ripple and stabilisers are common, even in the ICW. We did not have them in our SD high sided, top heavy 'fast trawler' and never felt the need but then again never tested that in offshore beam seas, round here a typical Channel crossing would probably have them filming 'Perfect Storm 2' They get paranoid about crossing the Gulf Stream if the wind opposes the flow and the guides issue dire warnings of it.

Yeah I've noticed that. On the BoatsTEST.com website they often point out they're testing the boats in 2-3ft swells as if its half a gale and I'm often wondering what they'd think of a wind against tide sea state in a F6 in the E Channel

 

[Robin]

Yeah I've noticed that. On the BoatsTEST.com website they often point out they're testing the boats in 2-3ft swells as if its half a gale and I'm often wondering what they'd think of a wind against tide sea state in a F6 in the E Channel

Their forecasts broadcast swell as 'wave heights' so 20ft swells, common enough in western approaches offshore sound like a truly veritable wall of water to overcome. I'm not sure I will dare to return to real boating again!

 

[MapisM]

From what I read before. Generally planning hulls tend to roll a lot more in beam seas when going at displacement speed, compared to D hull shapes.

Not sure thats true.

I'm sure it's not, if by "roll more" we just consider the rolling angle.

But the rolling period is as relevant for confort as the angle, if not more.
That's where D hulls, even if not stabilized, are "better" than P hulls, which are flattish, hard chined, lighter, and have no keel - compared to D hulls which are the opposite in all these respects. With the partial exception of hard chines, which are indeed used in some D vessels, but mostly commercial ones, and more for higher internal storage space than any other reasons.
This leads to a more jerky and uncomfortable rolling movement, with the hull that tries to "follow" the surface motion, rather than cut through it - even if the roll angle can be, and quite often is, lower.

That said, we are talking of differences relevant for those who measure passage times in days, rather than hours...

 

[Bouba]

I'm sure it's not, if by "roll more" we just consider the rolling angle.

But the rolling period is as relevant for confort as the angle, if not more.
That's where D hulls, even if not stabilized, are "better" than P hulls, which are flattish, hard chined, lighter, and have no keel - compared to D hulls which are the opposite in all these respects. With the partial exception of hard chines, which are indeed used in some D vessels, but mostly commercial ones, and more for higher internal storage space than any other reasons.
This leads to a more jerky and uncomfortable rolling movement, with the hull that tries to "follow" the surface motion, rather than cut through it - even if the roll angle can be, and quite often is, lower.

That said, we are talking of differences relevant for those who measure passage times in days, rather than hours...

What they talk about on the trawler forum is the snap back after the roll. There the argument is between D and SD nobody has a P! While they admit a D rolls a lot more than a SD they say (the D people) that the roll is pleasant and easy to live with while the snap back of a SD is uncomfortable. The snap back being of course the righting