What is "Hull Speed" Anyway

[Dockhead]

Originally Posted by estarzinger

This. Even smallish waves both impart significant energy and change the dynamics. It is probably an explanation for much of the 'disagreement'.

There are some decent calculations in the drogue literature about the energy imparted from waves, and at surprisingly small waves it will surpass the sail energy. Basically you have the boat's mass sliding/falling down an incline, plus any crest jet effect.

. ..

Sounds like you understand something about how surfing works. Care to enlighten the rest of us?

[Don C L]

Originally Posted by rgleason

I think we all like to go fast, whatever that is, but your big boats are acting like a laser, 420, Viper 640, Vx One or Evo, whatever..., maybe not a sailboard or kiteboard, but it must be a thrill in a big boat like that. Do they get skittish and sensitive on the helm at that speed?

I crewed on a Frers 65 many moons ago. The boat didn't get skittish at the helm. We did!

BTW I remember from my memory banks somewhere reading that some theorized that square riggers lost at sea may have exceeded their hull speed to the point that the ensuing water pressure generated around the hull simply pulled it under. Anyone heard of that? Know a reference for it?

[Cadence]

Originally Posted by Don C L

I crewed on a Frers 65 many moons ago. The boat didn't get skittish at the helm. We did!

BTW I remember from my memory banks somewhere reading that some theorized that square riggers lost at sea may have exceeded their hull speed to the point that the ensuing water pressure generated around the hull simply pulled it under. Anyone heard of that? Know a reference for it?

It was the sea serpents. Probably as likely.

[Panope]

Not sure if it was mentioned up thread, but I believe that prismatic coefficient has a significant effect on hull speed. Comparing two boats with the same LWL, beam, and displacement but differing PC, the higher PC boat (fuller ends) should have a higher hull speed.

Also, a boat with large "overhangs" should be able to exceed hull speed to a greater degree than a boat with IDENTICAL underwater shape AND displacement but without overhang. The boat with overhang has the benefit of having the WL "grow" as the boat develops bow/stern waves and/or heels. Of course, the overhang boat would be even faster if it instead used it's longer overall length to put more of it "in the water" as modern boats do.

Steve

[EveningTide]

I have experienced some interesting examples of hull speed. My 21.2 foot waterline length gives 6.20 knots using the factor of 1.34. Since my boat is heavy with long keel this factor may be correct. Example #1 was sailing alongside a Cal 25 while beam-reaching in 15 knots of wind. We were maxed out and neither boat could gain as we sailed together for four miles. The Cal 25 has more sail area to displacement, about half the displacement but slightly less waterline length.

In example #2 the other boat was a Laguna 26 but the wind was lighter. Again we sailed for several miles and neither boat could gain. They were much lighter displacement but slightly shorter waterline length. So in both cases we seemed to be locked together by our similar hull speeds.

I consider that if I am making 85% of hull speed (5.3 knots) I am doing very well. This can be exceeded when going downwind. Once we averaged 6.2 knots for 20 miles in wind gusting to 32 knots with lots of surfing. The max GPS reading was 9.7 knots (for an instant only). Another time we averaged 6.8 knots for 45 minutes in 18 knots of wind (no surfing). Since I don’t have a speed-thru-the-water indicator I assume some current was helping us a lot in both these cases.

[B00B00]

Originally Posted by Panope

Not sure if it was mentioned up thread, but I believe that prismatic coefficient has a significant effect on hull speed. Comparing two boats with the same LWL, beam, and displacement but differing PC, the higher PC boat (fuller ends) should have a higher hull speed.

Also, a boat with large "overhangs" should be able to exceed hull speed to a greater degree than a boat with IDENTICAL underwater shape AND displacement but without overhang. The boat with overhang has the benefit of having the WL "grow" as the boat develops bow/stern waves and/or heels. Of course, the overhang boat would be even faster if it instead used it's longer overall length to put more of it "in the water" as modern boats do.

Steve

My thoughts would be that the boat with the longer overhang at the stern would also be harder to then get planning as it would suck down rather than breaking free?

[roland stockham]

Originally Posted by Panope

Not sure if it was mentioned up thread, but I believe that prismatic coefficient has a significant effect on hull speed. Comparing two boats with the same LWL, beam, and displacement but differing PC, the higher PC boat (fuller ends) should have a higher hull speed.

Also, a boat with large "overhangs" should be able to exceed hull speed to a greater degree than a boat with IDENTICAL underwater shape AND displacement but without overhang. The boat with overhang has the benefit of having the WL "grow" as the boat develops bow/stern waves and/or heels. Of course, the overhang boat would be even faster if it instead used it's longer overall length to put more of it "in the water" as modern boats do.

Steve

Absolutely but I would add a couple of points. Theoretical top speed is based on what the boat could do given enough power and without considering either sea state or heal. The idea of overhangs (ignoring when it has been used to 'cheat' rating rules) is that when upright at lower speed whetted surface is less so the boat can be quicker in light airs. when the wind picks up , so sails develop more power, the water line extends increasing top speed. This works well on traditional heavy displacement boats but is limited to purely displacement hulls with no planing ability.
Block (or prismatic) coefficient is complex, I don't think that high BC necessarily equals higher hull speed. If that where the case the fastest hull shape would be a lighter!!! At the stern it can create lift and allow a hull to plane but at the stem it creates high wave resistance requiring more power for the same speed in any sea. At 1/3 distance from the bow a high PC makes hulls prone to slamming which, apart from being uncomfortable, also slows the boat down. So it is more a case of the PC must 'fit' the overall design.

[Pete7]

Originally Posted by B00B00

My thoughts would be that the boat with the longer overhang at the stern would also be harder to then get planning as it would suck down rather than breaking free?

My thoughts too. If you want to plane then you need flat aft sections to provide lift to get over the hump and a sharp edge to the transom so the water peels cleanly away from the hull reducing drag. The long overhanging counter stern will encourage water to follow the hull shape, the bow to rise and the stern to sink.

Perhaps we are seeing the design of yachts change from rounded canoe shapes with fins to ones with flat panels and hard chines as the demand for ever greater speeds becomes mainstream. Will these fast yachts kill off existing canoe hull designs just as the fin keel made full keeled yachts extinct? In the future will mono hulled cruising yachts then pay as much attention to unnecessary additional weight as the cat owners try to do?

Pete

[Polux]

Originally Posted by Pete7

.....

Perhaps we are seeing the design of yachts change from rounded canoe shapes with fins to ones with flat panels and hard chines as the demand for ever greater speeds becomes mainstream. Will these fast yachts kill off existing canoe hull designs just as the fin keel made full keeled yachts extinct? In the future will mono hulled cruising yachts then pay as much attention to unnecessary additional weight as the cat owners try to do?

Pete

On my blog I have several articles about the advantages and disadvantages of different types of modern hulls.

Many assume because a boat has all or almost all the beam pulled back they have similar hulls. That is completely wrong. Basically in what regards pure performance you have two types of transoms, the ones for solo racing and the ones for crewed racing....and all the others in between.

The ones designed for crewed racing offer a slightly best performance at the cost of a more difficult control downwind. The ones designed for solo racing have a slightly worse performance and offer easier control that translates in speed when sailing solo because you need a crew to take the extra advantage that the other hull type can offer (and an experienced one).

The type of hulls that has a bigger influence on cruising hulls are the ones that are used on solo racing because cruising boats are not normally sailed by an experienced crew and easy speed, easy control and sailing with little heel is the goal in what regards designing cruising boats, that and a big interior space that happily is a consequence of this same type of hulls.

[Kenomac]

Polux,

Can you please send me the link to your personal blog.

Thanks

[IdoraKeeper]

Originally Posted by Panope

Not sure if it was mentioned up thread, but I believe that prismatic coefficient has a significant effect on hull speed. Comparing two boats with the same LWL, beam, and displacement but differing PC, the higher PC boat (fuller ends) should have a higher hull speed.

Also, a boat with large "overhangs" should be able to exceed hull speed to a greater degree than a boat with IDENTICAL underwater shape AND displacement but without overhang. The boat with overhang has the benefit of having the WL "grow" as the boat develops bow/stern waves and/or heels. Of course, the overhang boat would be even faster if it instead used it's longer overall length to put more of it "in the water" as modern boats do.

Steve

Nice thought..I always thought overhangs were about dry decks and appearances. Once again if it looks good, it's likely to be good

[El Pinguino]

Originally Posted by Dockhead

Sounds like you understand something about how surfing works. Care to enlighten the rest of us?

I'll try and keep it simple.....

Its not about 'surfing' as such..... although my wee boat once did 17 knots while surfing sideways...

Have a look at page 8 http://www.mar.ist.utl.pt/mventura/P...w%20Design.pdf

Now.... consider the wave overtaking your yacht.... maybe its only half a meter high or less...

No... your yacht isn't 'surfing' but as this wave passes through it modifies your bow wave in the same manner that the wave generated by a bulbous bow would modify it.

Resistance decreases as the crest of the wave goes past... boat goes faster..


If you want to really make your brain hurt search for 'bulbous bow', they are still working on designing the 'perfect bulb' .... https://www.marineinsight.com/naval-...g-bulbous-bow/

To see the effect of bulbs compare the bow wave of a (bulbless) tug doing 8 knots with the wave generated by a large containership doing the same speed ( 8 knots is 'dead slow ahead' on many large ships.....)

[Cadence]

Originally Posted by El Pinguino

I'll try and keep it simple.....

Its not about 'surfing' as such..... although my wee boat once did 17 knots while surfing sideways...

Have a look at page 8 http://www.mar.ist.utl.pt/mventura/P...w%20Design.pdf

Now.... consider the wave overtaking your yacht.... maybe its only half a meter high or less...

No... your yacht isn't 'surfing' but as this wave passes through it modifies your bow wave in the same manner that the wave generated by a bulbous bow would modify it.

Resistance decreases as the crest of the wave goes past... boat goes faster..


If you want to really make your brain hurt search for 'bulbous bow', they are still working on designing the 'perfect bulb' .... https://www.marineinsight.com/naval-...g-bulbous-bow/

To see the effect of bulbs compare the bow wave of a (bulbless) tug doing 8 knots with the wave generated by a large containership doing the same speed ( 8 knots is 'dead slow ahead' on many large ships.....)

It is my understanding the bulbous bows disturbs the surface tension thus the amount of bow wave? Someone with an education in fluid dynamics may agree or did agree?

[El Pinguino]

Originally Posted by Cadence

It is my understanding the bulbous bows disturbs the surface tension thus the amount of bow wave? Someone with an education in fluid dynamics may agree or did agree?

In simple terms it creates a secondary wave which cancels out much of the bow wave as can be seen here. https://www.marineinsight.com/naval-...e-bulbous-bow/

Interesting thing in one link that I posted earlier.... big fast ships that are now 'slow steaming' due to increased capacity/reduced demand are having their original bulbs replaced with ones designed for optimum effectiveness at the slower speeds and they are saying that they are paying for themselves within 12 months... so all bulbs are not created equal.

[NevisDog]

Originally Posted by El Pinguino

In simple terms it creates a secondary wave which cancels out much of the bow wave as can be seen here... .... big fast ships that are now 'slow steaming' due to increased capacity/reduced demand are having their original bulbs replaced with ones designed for optimum effectiveness at the slower speeds and they are saying that they are paying for themselves within 12 months... so all bulbs are not created equal.

In order for the bulb to cancel out that bow wave, it must create an equal and opposite wave to match (as you have said), which only works at one specific speed. So, change the 'cruising' speed and you must also change the bulb.