How fast before SD hull P's?

[Its_Only_Money]

How fast before SD hull P\'s?

Out of interest at what speed would an SD hull be said to be fully planing?

Does it depend on engine power (ie with enough power ANYTHING will plane) and SD hulls never have enough installed power to fully plane by design, or is being SD an inescapable function of the hull form (bit like the displacement speed rule)?????

 

[Gludy]

Re: How fast before SD hull P\'s?

The sd hull cannot really plane. Past a certain point you are just chucking fuel at it to make it go no faster. It digs itself into the water and with the keel etc simply cannot lift itself out.

 

[omega2]

Re: How fast before SD hull P\'s?

our Aquabell 33, weighs about 6 tons, we have 2 180's and when we open up from standstill we leave a great big hole behind us then we build a hump and then at about 12 knots we leave it behind us, at 15 knots we have a rooster tail, and at 20 knots we have a long flat wake, but we are not truly planing as in a true defininition of the word. What we do have however is a none slamming ride and a lovely platform to fish from even in heavy weather, which we avoid if possible, but if caught out we feel comfortable.

 

[Gludy]

Re: How fast before SD hull P\'s?

I was once told by a boat designer that few, if any boats actually do plane. It would take a speed of well over 40 knots for a 40 footer to be considered truly planing

Anyway - does your boat have a reasonable sized keel?

 

[jimg]

Re: How fast before SD hull P\'s?

[ QUOTE ]
The sd hull cannot really plane. Past a certain point you are just chucking fuel at it to make it go no faster. It digs itself into the water and with the keel etc simply cannot lift itself out.

[/ QUOTE ]
Not sure that is really true, I take you back to the Seaward 49. If you look at the fuel consumption figures they reach a point where the mpg is constant regardless of speed and she will reach 27 knots. I am sure it would go faster if you put even bigger engines in.

 

[Gludy]

Re: How fast before SD hull P\'s?

I am only guessing myself but with e deep keel I really think planing is impossible.

Of course there are all forms of SD hulls, some nearer to planing, others nearer to displacment so maybe some, without much of a keel might be bale to plane.

 

[omega2]

Re: How fast before SD hull P\'s?

Now I have got some sort of response, how about this, I know of another Aquabell 33 same spec and weight but she has 2 275's and that one will exceed 30 knots pulling the same 2450 revs with of course bigger props, so it would appear that power = speed in a SD just as in a planing hull.

 

[Gludy]

Re: How fast before SD hull P\'s?

Yes power does equal speed up to a certain point but that must depend on the hull shape. SD cover s a very wide range.

In a planing boat, you actually use more fuel just before the hump as when over the hump and the water drag suddenly decreases when over the hump and you begin to plane. Increase power generally results in increased speed and this hull form is most efficient at the higher speeds.

This is not the case with an SD hull - if an SD hull ploanes, it is not an SD hull but a planing hull by definition.

Look at the jeel in the photod of the Trader - there is no way that is ever going to plane but it can do 25 knots and it does that mainly in the water, not on it. The penalty for that is it uses much more fuel that the equivalent planing boat at 25knots. However because it can do it in the water, it must require more energy in a wave to overturn it than the planing hull on top of the water and without anything in the water.

 

[Bandit]

Re: How fast before SD hull P\'s?

i had a Halmatic 34 o With a keel originaly with two x 180 Mercruiser diesel which soon made good deadmen for a swinging mooring , probably the best use for them. With these the boat dug a big hole and did 15knots.

I repowered with a pair of 265 Sabres and the boat would pull itself out of the water and be clear untill only the back half of the boat was in the water . ok but well out at 22knots with a good rooster tale, but at 25 knots it pulled even further out of the water and used to lean out on turns very alarmingly.

I understand that you can define the hull speed by a length beam and speed correlaton to well exceed this the boat will plane and planing is when the boat lifts partly clear of the water and the pushing aside of that water gives the lift leaving far less volume of water displaced boats hull than the weight of the boat .

Anything within reason will plane if it does not dig itself in and you give it enough power.

 

[Gludy]

Re: How fast before SD hull P\'s?

I think that is right but to plane you cannot have much of a keel and to plane you really need a boat that is designed to do otherwise , as you say, you can have a hairy ride and some probelms in bends etc.

 

[mjf]

Re: How fast before SD hull P\'s?

What about hydrofoils then????????????

 

[Gludy]

Re: How fast before SD hull P\'s?

hydrofoils are basically planing hull forms on the end of fins .... They are limited in what seas they can plane in.

There is no way around the simplicity of the general point that I am making

 

[Its_Only_Money]

Re: How fast before SD hull P\'s?

[ QUOTE ]
There is no way around the simplicity of the general point that I am making

[/ QUOTE ]

Do you have to make the same point on every thread though? Anyone doing that on "your" threads gets lynched!!!

 

[Its_Only_Money]

Re: How fast before SD hull P\'s?

So if we either:

a) added a keel to a planing boat or

b) removed the keel from a SD

would it change either to the other type (assuming power levels adjusted to suit the new top speed of each)

Anyone know of a single hull made in both modes, I seem to remember there may have been "V" and "Y" versions of certain Princess (maybe not??) boats in years gone by, were any of these close enough for direct comparison or were the different hulls more different than that?????

 

[Gludy]

Re: How fast before SD hull P\'s?

I simply answered a question that was to do with the subject of when boats plane.

 

[Gludy]

Re: How fast before SD hull P\'s?

I think there is more to it than that - I would myself be interested in any boats you can find that offered the option.... just to see what was claimed for them.

If planing boats were as stable as displacment boats at displacment speeds then I can see no role for displacement boats. One boat performs better in its mode and the other in its mode.

The problem is there are a whole range of say SD hulls, some of which I think you could almost do as you say and put a keel on, otherwise they may even get to planing - others of which are nearer to displacement hulls and could never plane.

I am not claiming to know the answers here, I am trying to find out the answers.

 

[BarryH]

Re: How fast before SD hull P\'s?

What about the Corvette. 30'ish foot and it palns, but sure it has a keel. I'd go for a corvette over a trader or Grand Bankes if I was in that market.....which I'm not

 

[Gludy]

Re: How fast before SD hull P\'s?

Is the keel a small one at the back?
I had a Princess 385 with a small keel section at the back and it planed.

 

[Its_Only_Money]

Re: How fast before SD hull P\'s?

No - the "general point you are making" was about SD vs P seaworthiness, nothing to do with speed transitions and everything to do with hydrofoils being "limited in the seas they can plane in"

 

[Gludy]

Re: How fast before SD hull P\'s?

OK - slap my wrists ... I was still just answering a question but you are right ... caught out fair and square!