What is "Hull Speed" Anyway

[Cadence]

Originally Posted by malbert73

I mostly agree with this as a summary but it overly generalizes which I think this thread demonstrates. There is probably no one size formula which determines every sailboat’s “maximal displacement speed”

The formula quoted numerous times above proves repeatedly unreliable for boats with narrow beams, or light displacement, or full aft sections, or powerful sailplans, or some combination of above.

But for non-planing sailing, the statement that I and others have said- which is that we regularly exceed hull speed- really just means our hull speed doesn’t calculate well by the 1.34*sq root LWL. That formula is too simplistic for more modern designs, or older ones with overhangs that immerse, or any other number of variables. Our “hull speed” is some different calculus.

And regardless, all hull speeds can be exceeded in the right conditions.

It's just a hypothetical mean no. (1.34} that seems to have worked for years on displacement hulls as a guess. So it seems you nailed it. I'm surprised no one has argued fresh or salt water.

[rgleason]

Since bow bulbs help at certain speeds (not at slow speeds) I wonder if we'll ever see inflatable bow bulbs of some sort on sailboats?
--Probably not because they'll be planning (modern design) or hydrofoiling.

[Cadence]

Originally Posted by rgleason

Since bow bulbs help at certain speeds (not at slow speeds) I wonder if we'll ever see inflatable bow bulbs of some sort on sailboats?
--Probably not because they'll be planning (modern design) or hydrofoiling.

Probably only depends on your age. I'll bet cruising sailors will be around for awhile.

[Polux]

Originally Posted by Joli

At the end of the day it's mostly about waterline length although it's possible to press a boat and sail above theoretical hull speed. Most cruisers seem to be happy to sail to 70% of theoretical hull speed and very very few are willing to press the boat for additional miles even if it's a planing hull (which makes a crappy cruising boat for all sorts of reasons).

It is disencouraging to post on forums!

When it has clearly established that there was not not a single hull speed for the same LWL, but a fixed one for boats with a D/L of 240 and over and another one, a variable one, for boats with a D/L lesser than 240, one that for the same LWL is increasing with the lightness of the sailboat, your post implies that nothing of this is a sound reality.

What you say is only true for heavy boats where the hull speed depends only on the LWL. For lighter boats the hull speed for the same LWL increases as decreases D/L. It is not only dependent on the LWL but on LWL and D/L.

So, it is not mostly about LWL but mostly about hull speed that varies for the same LWL with the D/L for most modern boats.

Regarding most cruisers to be happy sailing at about 70% of the hull speed, that is not true. That can only be true for the owner of an heavy sailboat.

Again it has to do with the easiness a boat reach hull speed and that was to do with boat sail power (stiffness) and with D/L. A heavy boat needs much more wind to reach hull speed than a light boat.

On a 40ft boat the traditional hull speed is about 8k, 70% of it is 5.6k. If upwind a performance cruiser can do that with about 6.5/7 kt winds and even if one can sail with that weak wind, or even less, certainly the owner of that boat would not be "happy" with that speed and would sail normally way faster than that.

[Pete7]

Originally Posted by Steve1944

Hull speed is the calculation of maximum speed of the hull of a boat through water. A mariner can improve or have that speed diminished by effects like tide and winds. It’s often confused with the maximum speed of a boat. It’s a professional term.

I think you need to add the words "for displacement craft".

Pete

[TJ D]

Originally Posted by Polux

It is disencouraging to post on forums!

When it has clearly established that there was not not a single hull speed for the same LWL, but a fixed one for boats with a D/L of 240 and over and another one, a variable one, for boats with a D/L lesser than 240, one that for the same LWL is increasing with the lightness of the sailboat, your post implies that nothing of this is a sound reality.

What you say is only true for heavy boats where the hull speed depends only on the LWL. For lighter boats the hull speed for the same LWL increases as decreases D/L. It is not only dependent on the LWL but on LWL and D/L.

So, it is not mostly about LWL but mostly about hull speed that varies for the same LWL with the D/L for most modern boats.

Regarding most cruisers to be happy sailing at about 70% of the hull speed, that is not true. That can only be true for the owner of an heavy sailboat.

Again it has to do with the easiness a boat reach hull speed and that was to do with boat sail power (stiffness) and with D/L. A heavy boat needs much more wind to reach hull speed than a light boat.

On a 40ft boat the traditional hull speed is about 8k, 70% of it is 5.6k. If upwind a performance cruiser can do that with about 6.5/7 kt winds and even if one can sail with that weak wind, or even less, certainly the owner of that boat would not be "happy" with that speed and would sail normally way faster than that.

Spot on, Paulo

[Polux]

Originally Posted by rgleason

Since bow bulbs help at certain speeds (not at slow speeds) I wonder if we'll ever see inflatable bow bulbs of some sort on sailboats?
--Probably not because they'll be planning (modern design) or hydrofoiling.

As usual with most things on cruising sailboats, the shape of bows comes from the development of racing boats at high level. On the mini class the winners have a completely rounded bow:



That was nothing to do with hydrofoiling but with increasing hull stability and bow buoyancy. However that gives a worse and most of all (in what regards cruising) a very uncomfortable motion upwind with waves.

The minis are used mostly on downwind races, even if that bow proved also to be competitive on traditional offshore races.

On other racers the bow has become rounded but not so round.

We cannot take as measure (regarding efficiency) the bows on the 40 class racers and IMOCA because the rules do not allow bows similar to the ones on the mini but on IRC racers, where the problem can be a worse handicap, they don't use them at least till the moment.

I don't know of IRC racers with the kind of mini racer bows (even if I had seen designs and proposals). A good example of a contemporary bow is the one of the Rambler 88,one of the fastest monohulls for its length :





Most new designed cruisers have already a slightly rounded bow, some more than others, like it is the case with the Pogo 36 (below). I bet the tendency will be to follow racing boat's bow shape, not the ones that are used mostly on downwind races but general purpose ones.

[roland stockham]

While this is a fun discussion in the real world of cruising people carry heavy ground tackle, a month or more of stores and everything from a sewing machine to maybe scuba tanks. In the racing world they cut their toothbrushes in half to save weight. Cruising boat are not lightweights once they set off. On traditional hull this makes negligible difference. The more you move toward lightweight race bread hull-forms that rely on planing for speed the more of an issue it becomes.

[Don C L]

They also rely on moveable ballast... the human kind

[TJ D]

Originally Posted by roland stockham

While this is a fun discussion in the real world of cruising people carry heavy ground tackle, a month or more of stores and everything from a sewing machine to maybe scuba tanks. In the racing world they cut their toothbrushes in half to save weight. Cruising boat are not lightweights once they set off. On traditional hull this makes negligible difference. The more you move toward lightweight race bread hull-forms that rely on planing for speed the more of an issue it becomes.

Well, I think that this is true of smaller boats, but on bigger ones, the weight matters less. We live on our 55, carry 2 anchors/rodes, have plenty of spares and tools, and actually carry absolutely everything we own on board. While we're obviously not achieving the speeds that the boat achieves dead empty, she still sails very well. The 'required' gear inventory for cruising is sort of the same regardless of size, so the impact is felt much less on the bigger boats.

We do take a different approach to how much crap we feel like we need to carry than we did on our 'traditional' boats- We are a lot more weight conscious, but we're not living like monks, either. We still see speeds well above 'hull speed' on a regular basis, and not just surfing.

So, size does matter a lot here. It's hard to make a general statement about performance.

[TJ D]

Originally Posted by Don C L

They also rely on moveable ballast... the human kind

Water ballast is the ticket! It's far preferable to having a bunch of folks sitting on the rail. Also, we don't have to feed them or listen. When we don't want 'guys' on the rail, they are returned to the sea...

[30yearslater]

I'm sure glad my 1976 Oday 27 can't read. Her 'hull speed' is 6.4 knots. She couldn't do this.

[Steve1944]

Hull speed is like horsepower. Make of it as is your sophistication of it.

[Cadence]

Originally Posted by 30yearslater

I'm sure glad my 1976 Oday 27 can't read. Her 'hull speed' is 6.4 knots. She couldn't do this.

? I had one. A nice little boat 6.4 might even be stretching it?

[Kenomac]

There seem to be many owners of displacement boats who don’t understand that the ocean and seas have currents which add and subtract boat speed. We travel through these currents and eddies all the time, and when I see the speedo go up over hull speed.... I know how to interpret the data. It doesn’t mean I have a boat which can magically go 2-4 knots above hull speed... Oh boy, I now have a racing boat!

The ocean water is in constant motion with currents, it’s not just sitting there stationary as your displacement boat travels through it. It’s hard for me to believe the boaters on this forum are having difficulty understanding this concept. Your boat travels against the current... it goes slower than you’d expect; your boat travels with the current... it goes faster than it would otherwise. This isn’t rocket science...

Planing hulls, hydrofoils and unusually narrow hulls is another game.... they can go faster.... but not displacement hulls which need to move the H2O out of their own way.