Boat speed theory, in the wake of a major rebuild.

[Wotname]

Crikey Matt,

Don't worry about the LWL, haven't you realised the added height above sea level has made the extra weight aloft (with the stay extension thingy) that much higher and your ballast correspondingly closer to the surface. Your Swannie is in danger of becoming too tender and perhaps dangerously unstable. This is what should be keeping you awake at night...

Of course you can replace the removed weight by the addition of beer casks and when these are empty, you won't be so concerned.

Ya gotta keep things in perspective ()

[GILow]

Originally Posted by Wotname

Of course you can replace the removed weight by the addition of beer casks and when these are empty, you won't be so concerned.

...

Well, I HAD planned to load the keel with a few cases of The Hedonist, though I couldn’t be sure how long that extra ballast would last.

[Jim Cate]

Originally Posted by GILow

Well, I HAD planned to load the keel with a few cases of The Hedonist, though I couldn’t be sure how long that extra ballast would last.

Well Matt, if your holding tank is in the bilge area then it's a zero sum thingy... the beer only is above it's deep storage position for a short while, then returns to a stable height.

Jim

[chrisr]

Originally Posted by Jim Cate

Well Matt, if your holding tank is in the bilge area then it's a zero sum thingy... the beer only is above it's deep storage position for a short while, then returns to a stable height.

Jim

need to take into account the free surface effect when beer moved from canned storage to bulk liquid...

cheers,

[RaymondR]

Originally Posted by chasmains

I’d love to find a book that explains the theory in this thread. Any recommendations? Thanks.

Don't know about a book but there was a BBC radio series called "The Goon Show" which might help.

[GILow]

Originally Posted by RaymondR

Don't know about a book but there was a BBC radio series called "The Goon Show" which might help.

Particularly the bit “he’s fallen in the water. “

Great interview of Eric Idle on ABC Radio National Conversations recently. A slight mention of the Goons was made.

[GILow]

Originally Posted by Jim Cate

Well Matt, if your holding tank is in the bilge area then it's a zero sum thingy... the beer only is above it's deep storage position for a short while, then returns to a stable height.



Jim

Eeergh! Too much information.

[GILow]

Originally Posted by chasmains

I’d love to find a book that explains the theory in this thread. Any recommendations? Thanks.

Well, I’d start by asking some of the useful CF members. None of those left on this thread.

[El Pinguino]

Originally Posted by GILow

......

But then I got worried. I realise that my waterline is now a whole foot shorter than it used to be. .......

At first (speed ) reading I took that to mean the draft had decreased by 12 inches and I thought...

golly!

[GILow]

Originally Posted by El Pinguino

At first (speed ) reading I took that to mean the draft had decreased by 12 inches and I thought...



golly!

Nope, only 8.66 inches... so far.

But I’ve nearly got the Nissan out and that’s got to be worth an inch at least.

[Harry2k]

And you will also pick up waterline length when heeled further mitigating the already slight hit to your theoretic hull speed.

[Cadence]

Originally Posted by Harry2k

And you will also pick up waterline length when heeled further mitigating the already slight hit to your theoretic hull speed.

I'm not sure that applies to the theoretical formula? Just more wetted surface. which he would have anyway.
I think he is over thinking it.

[Adelie]

Originally Posted by Pizzazz

Does this make sense?
Hunter 31
SA/D = 16
LWL = 26

Formula gives 2.6+1.1+1.3 = 5.0 knots. I can definitely move faster offshore.

That’s the long term average, unless you start you motor anytime boat speed drops below 5.0kt.

There are plenty of things you can do to improve that. Drifter .... on a bow sprit, CodeZero, clean bottom weekly, pare down extra weight on the boat, ....

[Kolchac]

Originally Posted by chrisr

there might be a slight theoretical reduction in hull speed due to shorter waterline, however how often do you actually do hull speed ? i'm guessing with the boat in mind (swanson 42 ?) that 5-6 knots is more often the speed (?)
cheers,

Just about the "max" hull speed. It is theoretical. It means when a boat goes, it creates waves along the hull. If speed lower than "max", the boat is on several waves, if speed is close to "max" - the boat on 2 waves - bow and stern waves. If the boat goes faster than "max", she is on just one bow wave, and she has to "climb up" on it.
I have a very light 36' cruiser with a sail/hull ratio racer (so "max" is around 6+ knots). In the less than 2' sea and 20 kn wind she makes up to 8 kn close-hauled. With ginaker - 10 kn.

[rslifkin]

Originally Posted by Kolchac

Just about the "max" hull speed. It is theoretical. It means when a boat goes, it creates waves along the hull. If speed lower than "max", the boat is on several waves, if speed is close to "max" - the boat on 2 waves - bow and stern waves. If the boat goes faster than "max", she is on just one bow wave, and she has to "climb up" on it.
I have a very light 36' cruiser with a sail/hull ratio racer (so "max" is around 6+ knots). In the less than 2' sea and 20 kn wind she makes up to 8 kn close-hauled. With ginaker - 10 kn.

Hull design is a significant factor. Some designs will climb their bow wave more easily than others (think of a planing hull powerboat). So it's not a one size fits all rule of how hard it is to drive a hull beyond its "hull speed". For some hulls the power vs speed curve doesn't steepen all that dramatically, while for others you can quadruple the power input and you just plow more water and only gain 0.1 kts.