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Old 17-03-2011, 14:23   #1
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semi-displacement vs displacement

can some one explain in detail the difference and similaritys of these two? in a power boat or sail how does it affect the boat? handling GPH not numbers but in general.

i did the search and have not found a thread about them.

thanks guys and gals

John
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Old 17-03-2011, 15:12   #2
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Re: semi-displacement vs displacement

There is not a hard definition but in general a semi-displacement vessel still has a hard chine for most of the length of the hull, a deep V entry with a good amount of deadrise, possibly 10 degrees or more at the transom. They might also have full length or partial length keels to reduce yawing. Semi-displacement vessels are a compromise between the planing efficiency of a flat bottom hull and a comfortable ride.

For sailboats it is much the same with a hard turn at the transom and less deadrise than a full displacement hull. Semi-displacement sailboats may or may not have a hard chine at the turn of the bilge.
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Old 17-03-2011, 16:52   #3
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Samson,

As far as motorboats go, I would suggest that the key difference between 'displacement' and 'semi-displacement' is the treatment of the aft underwater lines. A true displacement hull will have a smooth run aft rising towards the waterline, encouraging water to flow around and past the hull with the minimum of disturbance - in that respect rather like most cruising sailboats.

A semi-displacement hull will invariably have much flatter aft sections that do not rise progressively towards the waterline, but continue almost horizontally until truncated by a fully immersed transom. The effect is twofold: the flat lines aft mitigate against squatting as speed increases and the sudden termination of those lines at the immersed transom encourages the water to break away from the hull cleanly leaving the characteristic clean wake of a planing hull.

As others have remarked, such designs usually have a fine entry and may or may not have a hard chine. They are usually sea-kindly, not particularly fuel-efficient, and invariably wet in any sort of sea! However, the fact that nearly every modern commercial Pilot launch has this type of hull speaks volumes for the seaworthiness of this type of hull.
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Old 17-03-2011, 17:01   #4
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Re: semi-displacement vs displacement

Semi displacement has a much finer bow entry to cut through the waves and a planeing type stern. They are usually quite narrow and roll at anchor. They are wet, underway.
They use less power up to about 15 knots for a 36-40ft length.
They are prone to broach with following seas because the bow is in the water and the stern twists to following seas.
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Old 17-03-2011, 19:43   #5
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Re: semi-displacement vs displacement

so is it not even any good? i see some pocket cruisers on yachtworld with semi displacement hulls. is the way to go with any cruiser be it power or sail. does semi displacement get worse GPH then a full displacement hull lets say same size boat same weight just one is semi and one is full.

thanks john
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Old 17-03-2011, 19:54   #6
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Re: semi-displacement vs displacement

They are just different.
I had one years ago, a 36 ft masters fly bridge cruiser.(7 ton)
Only 10 ft wide(narrow) but with one 280hp perkins it would do 18 knots on a good day and cruise at 15 knots.
A similiar planeing hull would need 400 hp minimum just to get on the plane but would cruise at 20 knots.
The semi disp technology came before planeing boats (1948), but with cheap fuel in the 60's planning power cruisers took over.

Today the newest technology is to use narrow Catamaran/trimaran hulls (1-16 w/l) which are considered fast displacement.
They don't plane but can do 30 knots plus with low power.

Speed cost more fuel.
A displacement boat at hull speed only uses 5 hp/ton.
A planeing boat need a min of 50 hp/ton just to plane (go faster)
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Old 17-03-2011, 20:34   #7
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Re: semi-displacement vs displacement

50hp/ ton isn't necessary correct...

A 1987 Chris Craft Constellation 501 that measures 50ft has 600 total hp (2xV6.92TA) and planes at 14mph.... it weighs 24.5 ton so according to your figures...

24.5T x 50hp =1225 hp.... when in truth the 24.5 ton vessel only has 600 total hp.. which in fact is 24.5T / 600hp = 24.5hp/ ton

The deadrise on the stern is 4...
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Old 17-03-2011, 20:43   #8
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Re: semi-displacement vs displacement

Quote:
The deadrise on the stern is 4...
Nobody makes boats like that anymore.
Most, if not all planeing boats now have a deep v hull. which uses more power.
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Old 17-03-2011, 20:58   #9
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Re: semi-displacement vs displacement

Functionally, you can expect a semi-displacement hull to deliver greater speed from a given waterline length, as they can break through the 1.34 * root LWL maximum speed ratio (feet and knots) of full displacement hulls.

How much faster they will go faster depends on power, weight, design, heel, shape and sea conditions. But the Ross 930 (a lightweight) will do about 1.55 times heeled (8.5 kts), increasing to around 3.0 times (15+ kts) surfing under sail. This is on a LWL of ~27-30 feet.

You can pay a big price for this as sea conditions deteriorate in directional stability in following seas, motion (particularly as waves come under the stern), and rudder lift / stall when heeled (single rudder versions). But in better conditions you are sailing at good speed when others are motoring ...
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Old 17-03-2011, 21:08   #10
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Re: semi-displacement vs displacement

Although manufacturers don't produce boats like that anymore doesn't mean those boats are out there and for sale.
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Old 18-03-2011, 01:45   #11
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Smile Re: semi-displacement vs displacement

Quote:
Originally Posted by beau View Post
Nobody makes boats like that anymore.
Most, if not all planeing boats now have a deep v hull. which uses more power.
No so. Two examples that were originally both based here in Guernsey are Seaward Marine Seaward Marine Limited, seaward motor cruiser and Aquastar Motor Yachts Aquastar Motor Yachts Both companies are have grown significantly, based on selling semi-displacement motor boats. I'm sure there are plenty of other examples.
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Old 20-03-2011, 14:05   #12
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Re: semi-displacement vs displacement

The OP focusses on both sail and motor vessels. So I just note that hull design trends, and how they perform in practical terms, are quite distinct.
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