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planing vs semi displacment


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I would like to find out the opinions of people as to weather they think a planing hull or a Semi-Displacment hull is best for the sort of cruising these kind of boats should do (a bit of river work and the rest a sea). I am talking about sea going cruisers from about 27 to 45ft.

From what i have read (not sure how true) a planing hull should be a bit quicker if all is equal on power weight size etc and that on the same bases should use less fuel but on the down side will be more prone to slamming in a head sea and rooling more than a semi disp at displacment speeds also a semi disp will be less prone to wind in the river at low speeds.

I have a turbo 36 and from what i have read these are a good sea boat and i would like to think so as well but would i have been better off with a semi displacment boat that is what i dont know.

cheers Barry

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Semi D for me all the way, a planning hull is very slow, very fast or running at speeds it was never designed to do which makes it uneconomical. A semi D can maintain 12 – 15 kts economically in reasonable comfort in a sea when a lightweight planning hull would be either shaking your fillings out and itself to bits by going too fast, taking too long to get anywhere by going slow, or using too much fuel and not handling well by maintaining a mid speed. Most 25 – 32ft sports cruisers are also on sterndrives which are not nearly as planted in any kind of a sea as the shafts normally found on semi Ds. You can find a Semi D with a protected prop in a bit of a keel which makes things much nicer on the river too. To be anything like comfortable in a sea a planning hull needs to be a deep V which brings its own problems with trim tabs constantly needing adjustment etc. Having had some of both I will never be drawn down the sports cruiser road again, unless I move to the Med that is.

There’s nothing like as much fun as blatting along at 30+kts on a calm sea and arriving at your destination before everybody else I agree, problem is we all boat on the East coast of the North Sea and those conditions are rare, certainly for outward and return trips.

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I have to say that to me an ideal cruising speed is around 20 knots, It means you can go just about anywhere you want on a single tide and makes passages a sensible length. :Stinky

If you are not bothered about speed buy a sailing yacht and get there for nothing, at least there is some advantage to that. :naughty:

Reality, unfortunately, is that if you split your speeds between the good and bad seas you probably average no more than the semi D but you stayed out there less time when it was nice, warm, sunny and flat and were out there a lot longer when it was cold, wet, windy and rough. which would you rather be out in?

surely in this day and age somebody can come up with a planing hull that doesn't use a fortune in fuel while in transformation mode, or a semi d that can get properly up and skimming to give some serious speed without the need for 1000hp engines.

It does seem that motor boat design stands still while all around moves on, lets face it would you spend 5k a new car without a screen demister or auto park wipers? so why 500k on a boat without them?

One thing I am fairly certain of though is I wouldn't go out there without a spare engine. cheersbar

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HI Antares

I can understand what you say about stern drives and light weight planing hulls like maxums bayliner etc. But my boat for exhample is around 40ft runs on shafts weighs about 10.5 tonnes with my gear on and 8.5 when it was new with no fuel, water or any equipment. now my boat will cruise at a low speeds filly on the plane at speeds as low as 14 knots and on to about 23 knots maybe 25 down hill wind behind ;);) .

When i was last out which was in a 4to5 with 20 to25 knot winds on a beam or following sea we cruised at 19 knots with ease although on the rear quater it did catch the rear a bit which was good fun. However on the run from gt yarmouth to lowestoft with the wind against tide on the inside channel i had to slow down to 8 knots as the holes were to big and the boat kept getting green water wash over the deck and in the prosses soak us completley. But i had an rya instructor with me on this occasion and asked him if i had been in a semi d would i have been dryer and his reply was no we would still be wet and we would still have fallen in the holes but the landings would have been a bit softer. cheersbar

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A 40 footer is a horse of an entirely different colour Baz, with a greater wetted area and LWL they will as you say plane at lower speeds and keep going, they are also bloody heavy and mostly fitted with shafts, surprisingly Semi Ds are often not dry boats at all and ours makes a lot of mess, one of the most capable boats in the world (Dale Nelson) is as wet as hell but as you have a propper wheelhouse instead of a tent it really is of no matter at all, taking greenies over the front is par for the course. Also there seems to be a bit of a misnomer about Semi D speeds, ours for instance will cruise at 20kts with a top speed of close to 25, it's just that I choose not to.

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Barry, comparing yours to a semi D is probably not that much different in terms of handling, just the pure weight of Osprey will make her fairly planted on the rivers and at sea there is only a small window, between about 10 knots and 14 knots that you can't run at for ecconomy reasons so why worry?

IMHO the big differences are in the 30' boat class and then the two are a world apart. Comparing Clanny to Kingfisher is a bit like the classic tale of the hare and the tortoise, totally different animals. the only difference being instead of a sleep under the tree Clanny stops at Gillingham to refuel. :grin:

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Well I wouldn't exactly call 20kts cruise a tortoise Ian. :naughty:

It’s just that we are no longer able to do the 40 knots of our last boat or the 45 knots of our one before that – which is a shame on the rare occasions when you can have a blast about – like doing nearly 50 through the Thames barrier!!!!

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The Fairline Turbo is pretty well regarded as a decent sea boat Baz there is a fine line between what constitutes planing and what a semi D in many cases.

Basic definitions are:

Semi Displacement Hull - "A hull design featuring a vee shape forward and flatter bottom aft. This compromise design offers the vessel a somewhat more comfortable action at slow speeds, and allows planing at higher speeds".

Planing Hull - "A hull shape that allows a powerful fast boat to ride up on the after part of the hull, lifting the bow above the water and allowing the boat to move much faster".

I am a semi D man our ship is over 7 tonnes unladen and would do 18-20 knots from its single engine when new but this top end has come down with assorted junk accumulated. We cruise at circa 14 knots with reasonably fuel economy. Ours has a deep V front section and a dead rise of around 18 degrees allowing 'planing' at higher speeds. Mind you I did have to put my trim tabs down at 8 knots ish down when transiting the Oostende to Niewpoort canal to get the nose down :norty: We dis have a lock to catch :naughty:

The beauty of a Semi D is that you can keep making decent progress without slamming even when it cuts up a bit, oh and ours surfs quite well in a decent following sea amazing to see the log increase by 4 knots or so :grin:

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Thanks for that people :) .

Osprey is a planing hull thats for sure but it is nothing like what i have had before which are a few small ribs a shakespere 14 a shetland blackhawk with a 90 on the back(great fun by the way) then a fjord 24 weekender with two 170hp's on sterndrives (used more fuel than osprey and thats no lie) onto what we have now it feels planted quite slow to turn at speed but gets from a slandstill to on the plane in 7 secounds which i think is fantastic for the size of boat.

I am hoping to do some criusing with wayne and tracey when they get there new boat (eventually :grin::grin: not this weekend though :( ) which being a birchwood of the same size and speed i think we will be suited to cruise together and tracey is a good cook and magot makes loads of tea so even better :)

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With a top speed of 19kts Wayne's cruising speed will be 12/15kts so you made need to run a bit of tab Barry. This is an ideal speed for the TS and will maintain this speed in pretty much any sea.

Plaining or semi d. I have had two plaining hulls and also quite a few sports cars. I now have a TS37 with a semi d hull and a Grand Cherokee, at 41 years old I prefer comfort and practicality, though I do have the rib when I need a fix. :)

Jonathan

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Just look at the progressive buoyancy in the bow design on them, no wonder they seem to be landing quite softly, if you did that on a cruiser you would lose a lot of accommodation. On balance I’d trade that off just like I will with a flat foredeck and good side decks. But I wonder how many would go for the accommodation over sea keeping? I think that is answered just by looking at most cruiser designs at any boat show.

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Blimey!!

Theres a lot to be said for knowing what your boat can handle!!...........Mind you, I certainly wouldnt volunteer for testing............give me a flat calm and making our own waves thanks!!

........or rather more experience!!

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After all i have read on here my conlutions are.

I still strongly believe that my turbo 36 will be more economical at a fifteen knot cruising speed than the same sort of size and power semi disp as i am on top of the water and the semi is pushing it out the way with still quite a bit still in the water.

In the roughest weather the semi will have softer landings and maintain a higher speed for longer.

i have witnesed so i know that the wash off a semi plaing is far worse at any sort of speed.

the planing hull is a bit more fun.

And my last point is that princess,sealine,sunseeker,fairline,windy,etc all use mainly planing hulls wich are all young people's dream boats. Where as hardy(but i do like these) aqua star,broom,etc use mostly semi planing hulls. so on this basis i will get a semi planing in another 20 years maybe :lol::lol::lol::lol::lol::lol::lol: not realy maybe 10 ;)

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