Planing boat at D speeds

[oldgit]

. Would any Semi D boat in that position be a lot better? Dunno.

Going from a displacement hull to a planing hull was a convincing refutation of the myth about displacement hulls being sea kindly at low speeds.
My Princess 35 was far more stable in any sort of wash and would settle down far more quickly than my larger heavier Broom which would still be rolling like pig long after the wash had vanished over the horizon.. .

 

[Sticky Fingers]

Going from a displacement hull to a planing hull was a convincing refutation of the myth about displacement hulls being sea kindly at low speeds.
My Princess 35 was far more stable in any sort of wash and would settle down far more quickly than my larger heavier Broom which would still be rolling like pig long after the wash had vanished over the horizon.. .

That's interesting. Must be due to the wider flatter hull shape of teh planing boat compared to a D hull.

 

[oldgit]

Possibly.
The final arbiter of this matter is the wife who much prefers the action of the Princess in anything over F1 weather conditions.

 

[Sticky Fingers]

... The final arbiter of this matter is the wife who....

That is of course the deciding factor in so many things...!

 

[Portofino]

Possibly.
The final arbiter of this matter is the wife who much prefers the action of the Princess in anything over F1 weather conditions.

No it’s due to the hard chines and other horizontal sections like wide lifting strips further aft .
A semi D has not got as pronounced features as a good P hull .
Theses flatter surfaces resist roll when a P boats cruising at D .

Once the P boats start to gain speed those harder chines and lifting strips start adding in dynamic stability a greater rate over a semi D s softer rounder chines and the P boats firm up .
So at the difference from 6 knots to 10 knots OGs Prinny s chines and lifting strips are disproportionately adding stability compared to say a Hardy ( semi D ) of similar length.

 

[Sticky Fingers]

No it’s due to the hard chines and other horizontal sections like wide lifting strips further aft .
A semi D has not got as pronounced features as a good P hull .
Theses flatter surfaces resist roll when a P boats cruising at D .

Once the P boats start to gain speed those harder chines and lifting strips start adding in dynamic stability a greater rate over a semi D s softer rounder chines and the P boats firm up .
So at the difference from 6 knots to 10 knots OGs Prinny s chines and lifting strips are disproportionately adding stability compared to say a Hardy ( semi D ) of similar length.

Right thanks. Probably need to get out in a Semi D one day to experience the difference. TBH I've no idea what I'm doing having come to mobos rather late in life after 5 decades sailing.... but that's not going to come as a surprise.

 

[piratos]

No it’s due to the hard chines and other horizontal sections like wide lifting strips further aft .
A semi D has not got as pronounced features as a good P hull .
Theses flatter surfaces resist roll when a P boats cruising at D .

Once the P boats start to gain speed those harder chines and lifting strips start adding in dynamic stability a greater rate over a semi D s softer rounder chines and the P boats firm up .
So at the difference from 6 knots to 10 knots OGs Prinny s chines and lifting strips are disproportionately adding stability compared to say a Hardy ( semi D ) of similar length.

I have personalyl no experience with Hardy's but of course this Hardy has Spray Strips, and not lifting strips - and she can not be a SD, because you say Semi D has "rounder chines" - other features like Center kiel or Bilge kiel doesnt count. ...

To me a boat is either in displacement mode or she is not. and if the boat is exceeding her "hull speed" she is planing. You call it Semi Displacement, or as others call it Half Planing. For me you may even call it heavy planning or poor planning. Fact is that a "SD" is designed to perform best at speeds exceeding the hullspeed - factor 2,5 to 3 where as the planning boat should reach 5 (Sq LWL x 5)

Piers Flemming is a SD as well, and has no round chines ! Difference to "full planning" is that SD will usually have some kind of a Kiel, plus bigger rudder enabling the boat to steer well even in full Displacement mode.
Engines are usually sized to fit this speed mode, and yes - plenty SD boats have ben built with much bigger engines than required for a SD. (an example was the Hardy 62 with twin 1200 hp)
In the seventies (yes long time ago) the norwegians built the typical norwegian Snekke - Full Displacement and speeds of 6-7 knots. A simpe plate over the props made the boats what they called "Half Planing" and with same engines speed doubled!

I know your world is high speed, but 50 knots is not a lot of fun in a f50 or 60 ft sportboat .- compared to 15 in a 3m Rib

Have a nice day. I am preparing my holiday trip

 

[rafiki_]

No it’s due to the hard chines and other horizontal sections like wide lifting strips further aft .
A semi D has not got as pronounced features as a good P hull .
Theses flatter surfaces resist roll when a P boats cruising at D .

Once the P boats start to gain speed those harder chines and lifting strips start adding in dynamic stability a greater rate over a semi D s softer rounder chines and the P boats firm up .
So at the difference from 6 knots to 10 knots OGs Prinny s chines and lifting strips are disproportionately adding stability compared to say a Hardy ( semi D ) of similar length.

I’m no hydro dynamicist, but at D speeds, any of these features will have little impact. The flow is just too slow.

 

[James L]

View attachment 117272

I have personalyl no experience with Hardy's but of course this Hardy has Spray Strips, and not lifting strips - and she can not be a SD, because you say Semi D has "rounder chines" - other features like Center kiel or Bilge kiel doesnt count. ...

To me a boat is either in displacement mode or she is not. and if the boat is exceeding her "hull speed" she is planing. You call it Semi Displacement, or as others call it Half Planing. For me you may even call it heavy planning or poor planning. Fact is that a "SD" is designed to perform best at speeds exceeding the hullspeed - factor 2,5 to 3 where as the planning boat should reach 5 (Sq LWL x 5)

Piers Flemming is a SD as well, and has no round chines ! Difference to "full planning" is that SD will usually have some kind of a Kiel, plus bigger rudder enabling the boat to steer well even in full Displacement mode.
Engines are usually sized to fit this speed mode, and yes - plenty SD boats have ben built with much bigger engines than required for a SD. (an example was the Hardy 62 with twin 1200 hp)
In the seventies (yes long time ago) the norwegians built the typical norwegian Snekke - Full Displacement and speeds of 6-7 knots. A simpe plate over the props made the boats what they called "Half Planing" and with same engines speed doubled!

I know your world is high speed, but 50 knots is not a lot of fun in a f50 or 60 ft sportboat .- compared to 15 in a 3m Rib

Have a nice day. I am preparing my holiday trip

An SD hull is basically a planing hull that keeps more of the hull in the water.
As it doesn't produce as much lift at the same speed as a full planing hull, you end up using more fuel as more of the boat is in contact with the water, but you don't leave the water as easily as you would in a full planing hull so in rough weather you are going through the waves and throwing a lot of water about rather than over the tops of the waves so you have less vertical motion and less pounding as your hull never leaves the water so it never has to land.
The difference in hull shape comes because they can design the hull specifically for that speed range without worrying about stability at 30 or 40 knots or efficiency etc. So you end up with more volume aft to give a more level ride at those speeds where a planing hull would be trying to stick its nose in the air etc etc.

 

[rafiki_]

Scala, find a boat you (by this I mean The Admiral) really likes, and accept that every boat is a compromise. many of us on here have happily used a P boat at D speeds. You couldn’t say vice versa.

 

[Homer J]

Yachts have great big keels and huge flappy bits that stabalise them very well indeed but you know when that 50 foot Princess comes tootling along at 30 knts for a close pass, say half mile off, you still see the indolent and disgruntled peasants go into uproar and start throwing the teapot in their general direction. What kind of miracle are you hoping for? The walk on water kind or the Halleluja! I only lost half my gin kind?

I deleted my comments as I got the wrong end of the stick the first time.

 

[Sticky Fingers]

Scala, find a boat you (by this I mean The Admiral) really likes, and accept that every boat is a compromise. many of us on here have happily used a P boat at D speeds. You couldn’t say vice versa.

You’re right of course. Aside from the pitching and rolling the rest of the D speed trip was grand. And I really don’t want to be changing anything so soon.

 

[Portofino]

View attachment 117272

I have personalyl no experience with Hardy's but of course this Hardy has Spray Strips, and not lifting strips - and she can not be a SD, because you say Semi D has "rounder chines" - other features like Center kiel or Bilge kiel doesnt count. ...

To me a boat is either in displacement mode or she is not. and if the boat is exceeding her "hull speed" she is planing. You call it Semi Displacement, or as others call it Half Planing. For me you may even call it heavy planning or poor planning. Fact is that a "SD" is designed to perform best at speeds exceeding the hullspeed - factor 2,5 to 3 where as the planning boat should reach 5 (Sq LWL x 5)

Piers Flemming is a SD as well, and has no round chines ! Difference to "full planning" is that SD will usually have some kind of a Kiel, plus bigger rudder enabling the boat to steer well even in full Displacement mode.
Engines are usually sized to fit this speed mode, and yes - plenty SD boats have ben built with much bigger engines than required for a SD. (an example was the Hardy 62 with twin 1200 hp)
In the seventies (yes long time ago) the norwegians built the typical norwegian Snekke - Full Displacement and speeds of 6-7 knots. A simpe plate over the props made the boats what they called "Half Planing" and with same engines speed doubled!

I know your world is high speed, but 50 knots is not a lot of fun in a f50 or 60 ft sportboat .- compared to 15 in a 3m Rib

Have a nice day. I am preparing my holiday trip

Arh “ planning “ has been mentioned.

Short answer= It in a nutshell generally defined by the lowering of the angle of attack , ie trim angle starts dropping .

longer Ans =
Planing

A planing hull is simply one so shaped that a degree of dynamic lift is added to its natural buoyancy during the time when its speed of advance exceeds that rate at which solid water can close in abaft of it .


I think we all first need to agree that there is no one binary point of planing. It is not either on or off. It is a transitional regime, and therefore, any attempt to define a point of planing is somewhat nonsensical.


What actually defines planing using the most widely accepted definition is —-

The fact that if the speed increases the trim angle will decrease.

If you aren't planing, increasing speed in the sub planing regime results in an increase in trim angle, for a planing hull. Again, for a semi-planing hull that might not happen at all, but for a true planing hull, the speed where it attains planing status is the point where the trim angle decreases as the speed increases.

Look guys this is pretty simple.
Think about an airplane wing. To supply a given amount of lift at a given speed you need a specific angle of attack. If the speed increases you have to decrease the angle of attack or you will get an increase in lift. As you approach planing speed the trim angle doesn't decrease and indeed, the lift is increasing since you are supporting an ever increasing portion of the hull with hydrodynamic lift.
But once you are on the plane, even though the hull may be heavy and still have a significant portion of displacement lift, if &the trim angle drops as speed increases, it has attained planing status.
This is why using arbitrary measures like % of weight or amount of lift, are not appropriate, a heavy hull might have a different lifted height when compared to the same hull when lightly loaded. But in both cases, when the trim angle decreases, planing has been realized.

How much of the displacement volumn must be above the surface to be "on plane"? all of it, most of it, half of it, any of it? What % if any ?
Since planing begins to occur when hydraulic forces lift the hull, how much until it is actualy "planing"? Any lift replaces displacement forces with dynamic ones, so some say the begining of planing occures when there is any lift not associated with displacement.

Therefore to some they see no reason for any of the qualifiers at all. Many shapes can plane to one degree or another, no reason to put those conditions on it. I also do not see why 50 percent is the magic number either, if any portion of the weight is lifted out of the water by hydrodynamic lift you will reduce the drag and increse speed.

Some say that planing occures if any of the weight is suppored by the dynamic forces on the hull from the water. A little or a lot of the weight being supported is illrelevant, the fact that the hull is not fully in displacement mode means that lift forces on the hull are partially supporting the weight of the boat.

I think the real problem is the word "planing" itself is obsolete and based on archaic ideas about fluid mechanics. The origin of the word assumes it is even possible to be above or "on" the plane of the surface. We know a lot more about the process and forces involved but are stuck with these obsolete terms.


Getting back to the topic some suggest taking into account the “subjective feeling “ of the persons on board, I believe that anyone who has been on board a planing vessel can witness that it's movements became "stiffer" and more jerky when encountering waves at high speeds than it was during low-speed navigation. A mathematical explanation for this behavior is that the perturbation of hydrostatic lift component is in linear relationship to the vertical speed of the incoming wave disturbance, while the dynamic lift component increases as a square function of the vertical speed of the perturbation. Hence, the vertical accelerations become much more severe in high-speed regime.
It makes me think that perhaps it can lead us to a completely different criterion for the definition (by convention) of the planing, based on the vertical acceleration response of the vessel to incoming waves, or to a single standardized perturbation which could be reproduced in towing tanks. The planing regime would then be indicated during sea trials by the on-board accelerometers, rather than through the GPS readout .

Just some food for your creative minds.Risky on here I know


I think that at the end, it is a matter of semantics and not physics or engineering. It would be better to say, for example, "this vessel at a speed of 54 knots has 93% dynamic lift to displacement ratio", rather than argue whether it is planing or not

I think we can all agree that, when looking trim angle it becomes pretty clear where planing starts.

The more you think about it one soon realises this becomes different for every hull, and for every load condition, every thrust angle, and for every offset of the thrust vector from the line of the planing surface, as well as things like prop rake. It's possible to significantly change the onset of planing by modifying those variables.

When all things are considered, it is far more complex than a simple % of lift. For lightly loaded short, wide hulls, the change in trim angle happens much more quickly with increasing speed. For much larger hulls with heavier bottom loading, the curve will likely be more of a gentle hump, but the reality is, when you are past the peak of the trim angle curve the hull is now in the planing regime. Since there a any number of variables that can push the actual speed at which the trim angle starts to decrease - that's not a bad thing to use for a rule of thumb .

Maybe planing is like time: we all know what it is, until we think about it

 

[BruceK]

What?! You dont have a FB. Just give it some welly. If you cant see over the bow you're not planing yet. If you can see something in the water 100 yards in front, you're planing.

 

[Don107]

My boat is semi D and rolls like anything when hit by the wake of a larger boat. Especially when I’m at anchor and the ignorant to others big boy boaters come thrashing past at 20 knotts only 50 yards from us. Even if they are further away the rocking and rolling is pretty uncomfortable. To the point where we are forced to pull the anchor and stop fishing. TBH I don’t think any hull would be comfortable when subjected to that kind of selfish manoeuvres.

How would one of these go

Regards Don

 

[ST840]

How would one of these go

Regards Don

That’s twice in a week ive seen this video on here. Are you promoting them?
A very good system I’m sure, shame they look like a dogs dinner though.

 

[Pinnacle]

thank you ? pity I screwed up my timing and missed you lot yesterday!

You were missed! ?

 

[Sticky Fingers]

You were missed! ?

Awesome ? I think! One day.