Planing vs SD: Analysis, with pictures

jfm

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The SD vs planing hull debate is covered in other threads, so dont wish to repeat that. Gludy kindly sent me those pix of a Trader 575 and I've put them below, with some similar shots of Sq58 - same length, same hp, same weight if you carry same fuel /water. In this thread I'm asking the question: if you think a SD hull is more seaworthy than planing, then exactly what is it about the SD hull that makes it more seaworthy? Not picking any fight here, just curious.

FWIW my view is that there is virtually no difference in hulls or in seaworthiness (apart from the stabilisers). Look a the pix. Both have hard chines, deepish V at the bow, flatter aft, and a hard "chine" where the hull bottom meets the vertical transom. The 575 has a very flared bow and looks like it has more weight in the bow, which might make it lift less and crash through a head sea a bit more.

The 575 has a keel (looks like only one - not sure how you dry out. Legs?) and longer shafts and I suspect bigger rudders. And engines further forward. Plus stabilisers. So it has more nose weight and loads more appendage drag, which explains why the Sq58 does 33 kts compared with the Trader's 25kts, for the same bhp and same hull length and weight. So my theory is that there isn't much difference in seakeeping, and a SD is just a planing hull that's going a bit slower. Though the further forward engines might make a bit of difference. But in general, at 22 knots in each boat you are doing pretty much the same thing, whether that's planing or semiplaning.

I therefore think the thing that gives you the better ride in the 575 in bad weather is the stabilisers, not the so-called SD hull. That set up may well be worth having if your boating area demands it (as Gludy's does). But I think there is a big fuel cost to lugging all those appendages around, so with the same engines and the same passage times the Sq 58 is going to give better mpg over the whole season than the 575, isn't it? If not, why not?

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Gludy

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Re: Planing vs SD: Analysis, with pictures

JFM
At long last totally logical questions are ebing asked based on understanable views ... we may all get somewhere.

As an aside, the Trader dies out on legs - the keel is much deeper than the props so it sits on the keel and two legs.

The Trader is rated a class A boat for seaworthiness and the planing boat in the photo is probably no more than a class B so there are differences.

The speed you get with or without stablisers is not much different, they do not have a major influence overall.

What srikes me most about the two hulls is that the planing boat is very flat bottomed whereas the trader has maintianed the V shape throughout.

Even at 25 knots in the Trader the lift is hardly noticeable.

The rudders and keel is very much larger to stay dug into the water.

So the hull is differnt enough to give very different performances. If the trader was a planing hull I would get about 33 knots out of her, not 25 knots.

This is an intersting discussion and I look forward to what others have to say about the photos.
 

Wiggo

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Re: Planing vs SD: Analysis, with pictures

If what you say is true, then it's a deep V hull not SD. If you want to know about Deep V hulls in heavy weather, ask Don Shead (or Sam Snead, perhaps). Excellent rough weather performance, little lift compared to shallower planing hulls, very inefficient and thirsty at speed (is this sounding familiar at all?)

Bear in mind that the theoretically most efficient planing hull form is flat bottomed, as that will generate most lift, but it is obviously as uncomfortable as hell. The deeper the V, the more it carves through the waves (at the expense of throwing sheets of water everywhere) rather than riding up over them and slamming.

A true SD hull would, as has been said elsewhere (probably by you), tend to be round bottomed at teh bow and midships, tending to flatten at the stern to provide some hydrodynamic lift.
 

Alistairr

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Re: Planing vs SD: Analysis, with pictures

Tried to stay well out of this subject cause i don't know enough about it.
But, [ QUOTE ]
What srikes me most about the two hulls is that the planing boat is very flat bottomed whereas the trader has maintianed the V shape throughout

[/ QUOTE ]
I don't where you see that, but i think you've got it the wrong way round!!!
In this picture, and the one after it in jfm's post, you can see clearly a flat section at the rear of the hull,
DSCF0392.sized.jpg

Look closely and you can see 2 prop shafts, so that definately says its flattened off.
The Sqaddie can be seen clearly to have a 'V' all the way along, with possibly a very small flat bit at the stern.

As for you camparing Rudders, WHY??? When you can't even see the rudders on the Trader!!!!

But hey what do i know!!
 

jfm

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Re: Planing vs SD: Analysis, with pictures

Gludy: I'm gonna take a leaf out of your book and reply para by para:

"The Trader is rated a class A boat for seaworthiness and the planing boat in the photo is probably no more than a class B so there are differences." Agreed (I think we established in the other thread that the Sq58 is a B). . Not disputing that, just trying to determine why. Incidentally, at the boundary the diff between a Class A and a Class B is a Rizla thickness. We dont know if these boats are top/bottom/middle of their resp categories. So let's not carry on this discussion on the assumption there is a big seakeep difference between the 2 boats, as the class A/B coding doesn't prove that

"The speed you get with or without stablisers is not much different, they do not have a major influence overall." As a guess, I would have thort the drag on those babies at 30knots is a lot, like 2knots worth of speed.

"What srikes me most about the two hulls is that the planing boat is very flat bottomed whereas the trader has maintianed the V shape throughout." Yep, agreed, esp at the transom. This is surely the thing that makes the 575 a semiD, I mean it doesn't lift as high ontot he plane. But I doubt the hull V affects the if both boats are in heavy weather doing say 11 kts?

"Even at 25 knots in the Trader the lift is hardly noticeable." Yep - a function of the less flat hull, as above. Probably improves steadyness at SD speed say 18kts - goes through waves not over them. Big fuel penalty at 18kts though.

"The rudders and keel is very much larger to stay dug into the water." Can see these will improve steadiness in rough weather, but big fuel penalty. The Sq58 rudders are tiny.

"So the hull is differnt enough to give very different performances". Is it? I would say that we have still only identified somewhat minor differences so far as seakeeping is concerned.

"If the trader was a planing hull I would get about 33 knots out of her, not 25 knots." Agreed, if you flatten the aft hull surfaces and took off the appendages you would get say 33kts. That's quite a big difference in fuel and speed (in terms of outrunning the weahter). But I'm sceptical that the appendages and the aft V hull make for better seakeeping, at slow speed like 11kts in heavy weather. So I still think the big factor is the stabs, not the hull shape
 

Gludy

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Re: Planing vs SD: Analysis, with pictures

I think you are right ..... its a fairly complex issue.

I am doing some reading on it so i can get it all sorted out in my mind.

What I do know is that going to sea in the Trader was totally different with or without stablisers on. I do not think that the hull shape can ever get it to plane ... in fact i know that. As I increased speed in the Trader from 5 knots to 25 knots there was no hump, I could set the speed at anything and it would sit there happily. It was a much more comforatble ride that opened up the possibility of everyone passing time the way they wanted to when underway.

I look at it this way .... using the same fule i will cruise at 18 knots in the Trader, a drop in my normal cruising pseed of 25 knots in calm weather but in rogher weather I really could still maiantain the 18 knots whereas I have to drop well down in the squaddie - so sometimes its faster and sometimes it slower. However its always more comforatbel for everyone.
 

jfm

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Re: Planing vs SD: Analysis, with pictures

Hmm you are dead right alistair, the 575 is flat hulled, aft of the keel. Strange. The Sq58 apears to have flat bits, it's mostly a result of the prop tunnels - the flatty bits are the tops of the tunnels
 

jimg

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Re: Planing vs SD: Analysis, with pictures

There are quite a few variations on the SD hull shape and the Nelson hull is closer to what I beleive is the "best" variant. Over the past 25+ years I have owned various boats including one of the first Flemings in the UK back in 1991. The Fleming hull is similar to a Trader or Grand Banks but none are a match for a Nelson. I have been out in the Seward 49 many times wirh the owner and we have been in really rough water and we just kept going at 25kts. All that happened was we threw up even more spray that usual! The Fleming, Traders are just not in the same league I am afraid to say as far as sea-keeping goes. You can see the hull shape on the Seward web site:
http://www.seaward-m.com/49photos.html
 

Chris_d

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Re: Planing vs SD: Analysis, with pictures

Gludy, the Trader is a planing hull, it wouldn't achieve 25knts unless planing, the problem is it won't go any faster because it can't generate enough lift and the keel, large rudder etc.. produce too much drag. The shape however does allow it to operate at speeds which are awkward for a planing boat, say 10-15knts and these speeds are sometimes the most comfortable speed in certain conditions.
 

Gludy

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Re: Planing vs SD: Analysis, with pictures

I think you are probably right in what you say there.
My hunch would be that the Nelson would beat the Trader for sea-worthiness.
However that is only oine factor and I accept a loss in one area for a gain in another area.

"I have been out in the Seward 49 many times wirh the owner and we have been in really rough water and we just kept going at 25kts."

Could you please tell us more about what sea conditions when you were able to maiantain the 25 knots .. against tide... what?. What engines are in her?
 

jimg

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Re: Planing vs SD: Analysis, with pictures

It is the boat on the web site ( the only one built so far ) and has the fairly new CAT 710 hp engines. We went thru the Portland race just so we could see how she behaved, and she was brilliant! I been on a Trader and my old Fleming in similar seas and we had to slow to displacement speed to avoid falling off the large waves. The down side is relatively limited internal accommodation for a 50 foot boat.
I was there when the MBM boys did their boat test and they were pretty impressed, and so much so that they made it their SD boat of the year.
 

tcm

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Re: free board is the key imho

i think it's the freeboard that makes it "more seaworthy", big wave up the bum of the sq wd sploosh over the swim platform so needs decent/better drainage in time for the next one, whereas the trader deck is up a load of stairs and cd take bigger waves before deck sploosh and swamp. I mean, in the limit, it would be impossible to plane over really rough sea as the waves lurch up and stop the planing effect - so all wd be trundling along at disp speeds imho.

As an aside, i don't much like these deck hatches at the stern of the trader - seem a smashable if F8+ really is the order of the day?
 

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Now, I am no expert on one hull v t'other. However a semi displacement hull is also a semi planing hull, or as stated in the report I read about an American owned trawler type yacht, at speeds around 18knots and above the vessel was clearly seen to be planing. At low speeds she wallowed and rolled in moderate conditions until stabilisers were deployed etc etc.
 

whisper

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Re: Planing vs SD: Analysis, with pictures

Re Seaworthiness - classes are I believe as follows :-
A)Ocean - designed for extended voyages offshore......may exceed force 8.....wave hts 4m and above...... vessel largely self sufficient.
B)Offshore - offshore voyages .....up to and including force 8....wave hts up to and incl. 4m
Are we saying that the Squaddie 58 is class B or am I misunderstanding what seaworthiness classes you are talking about ? If I am correct then I'm amazed as our humble tub is rated B
 

duncan

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Re: Planing vs SD: Analysis, with pictures

So's mine.......... /forums/images/graemlins/crazy.gif

Seriously thought displacement might be a big differentiator but it appear the Trader is only about 10% more than the Squaddie.

To me the big big difference in the hulls is the keel which, as hull speed increases, will stiffen up the lateral motion on the Trader significantly - and of course even more so with those stabalisers.
 

rickp

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Re: Planing vs SD: Analysis, with pictures

whisper has quoted whats in the actual RCD document (Annex I)

Category A is wind in excess of F8, and significant wave height exceeding 4m. Of course, it says nothign about the ride or comfort at that.

Rick
 

rickp

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Re: Planing vs SD: Analysis, with pictures

Yeah, realised and corrected then saw you post. Still, I'm not convinced that anything the RCD means it would be safe of prudent to be out in a F10 just because you have a 'A' boat.

Rick
 

Gludy

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Re: Planing vs SD: Analysis, with pictures

There is no way I would be out in an f10 in any boat Class A or not.

Going back to roots what I am claiming is this:-

For a given length of boat a well designed Class A DS boat will be more seaworthy than a well designed planing boat.

However, all this is to a large extend besides the point because I would never intentionally head out into evan a f8.

What really matters to me is the difference in ride and comfort and that I have experienced and to some extent knpw about.

I frankly do not know much about hull shapes etc, I have a lot to learn and I have now set myself the task of learning.

These threads have been a good laugh and some good banter .... a few days ago I looked at the forum and decided that it needed to be livened up ..... I think that has been achieved..... my fingers are beginning to need a rest as when these things start I very rarely get much work done.

If any of you are heading Swansea way ,look us up and I will make you a class A cuppa tea.
 
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