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

[GILow]

So, I was walking back and forth today as I slowly remove my engine, and paying a lot of attention to the waterline of my boat.

It has become a bit of an obsession of mine lately, and although I know some of the weight I am removing will be replaced by the new engine, I’ll still end up nearly 200 kg lighter. This on top of the nearly four tons I’ve removed so far.

But then I got worried. I realise that my waterline is now a whole foot shorter than it used to be. I don’t know what it was originally, I guess around 36 feet for this canoe stern 42 footer, but I can clearly see the new waterline is a foot shorter than the old.

Does this mean I’ve just reduced my effective top speed?

Is it time to start loading up the boat with crap so that I can go faster?

[Pauls]

Your practical max speed, aka hull speed, for a displacement hull, is a function of waterline length, as you state. The square root of the waterline length is the function that drives hull speed for your particular hull. So, if you had a 36' waterline and you've reduced it to 35, the square root of 35 divided by the square root of 36 will be the ratio of how much your hull speed has been reduced. And that value, for the above numbers, is .986X.

If you can notice a .014X reduction in your speed then you are a very perceptive sailor. Or, in other words, it doesn't make any practical difference.

[coopec43]

Now I'm certainly no expert but if you put the two ton back as ballast then your yacht would be stiffer, your sails would develop more power and you would sail faster.

Now let's hear it from the experts.

[billknny]

It's more complicated than that.

The calculation is for comparing two baots of similar design, of different lengths. It is not precise, and is not a hard rule. As your boat picks up speed, you'll have a bow and a stern wave that will rise up, and make your effective waterline longer, regaining the waterline you "lost" when you put your boat on a diet.

Adding weight will NOT make you go faster. It will slow you down. Do not go down that rabbit hole!

But even that is a bit situational... When you are sailing downwind, more weight is always bad. If your weight is really low enough to improve the stability of the boat, it MIGHT help you go a bit faster close hauled... but just don't go there unless you can add it to the bottom of your keel!

[GILow]

Well, I was being a little tongue-in-cheek about adding weight, but I think we can see from the learned responses here, it is kinda ambiguous.

Pauls’ point about the trivial difference is, of course, the most material of all. No way will I spot that difference, but then, I come back to overall displacement and feel that should override the theoretical speed loss.

Anyway, I’ll resist the urge to add any more crap, and since this old girl has already had a ton or more of lead added to the keel I will also resist the urge to add any more weight there.

Just thought it was an interesting conundrum to get our brains going.

Meanwhile, it’s back to dissecting the old engine for me. Trying to get to the point where the heaviest single component is under 100 kg.

[RaymondR]

Immersion decreased so both form and skin drag have decreased so faster boat. Remember that the hull speed formula is where the resistance curves up and if your sails can't drive your boat to and above that speed it doesn't much matter what it is.

The centre of gravity tends to be low in ballasted boats and consequently much of what you removed may have been above the VCG so it may not have tenderized it much anyway.

By the time you load the hair drier, iron, shoes and extra water needed to service the average female the boat will be back down to it's original marks anyway.

[Jim Cate]

Matt, when pressed I suspect that your WL will be extended by heeling, possibly to much the same length as before. Even if not quite, the difference will be small as stated upthread.

Meanwhile, your performance in less than hull speed conditions will be enhanced quite significantly, especially in lighter winds. You're gonna love it, mate, but it will keep you from enjoying the new engine so often!

When we switched from our original Insatiable to I-2, we offloaded 17 years of accumulated packraterism. We gained about 6 inches of freeboard. We then sailed her from Yamba to Manly to put her on the market. On that trip north we were astonished at how much better she performed in ALL conditions. Poor dear had suffered the death of a thousand acquisitions! I blush to admit that I-2 has suffered some of the same affronts...

Jim

[slug]

Originally Posted by GILow

So, I was walking back and forth today as I slowly remove my engine, and paying a lot of attention to the waterline of my boat.

It has become a bit of an obsession of mine lately, and although I know some of the weight I am removing will be replaced by the new engine, I’ll still end up nearly 200 kg lighter. This on top of the nearly four tons I’ve removed so far.

But then I got worried. I realise that my waterline is now a whole foot shorter than it used to be. I don’t know what it was originally, I guess around 36 feet for this canoe stern 42 footer, but I can clearly see the new waterline is a foot shorter than the old.

Does this mean I’ve just reduced my effective top speed?

Is it time to start loading up the boat with crap so that I can go faster?

Yes

Shorter waterline lower hull speed

The calc is a little more complex in that a factor is wetted surface

If the bow went up and the stern sank you now have more wetted surface You need more energy to move

Stern up , bow down is less wetted surface

Less energy needed

[RaymondR]

Originally Posted by Jim Cate

Matt, when pressed I suspect that your WL will be extended by heeling, possibly to much the same length as before. Even if not quite, the difference will be small as stated upthread.

Meanwhile, your performance in less than hull speed conditions will be enhanced quite significantly, especially in lighter winds. You're gonna love it, mate, but it will keep you from enjoying the new engine so often!

When we switched from our original Insatiable to I-2, we offloaded 17 years of accumulated packraterism. We gained about 6 inches of freeboard. We then sailed her from Yamba to Manly to put her on the market. On that trip north we were astonished at how much better she performed in ALL conditions. Poor dear had suffered the death of a thousand acquisitions! I blush to admit that I-2 has suffered some of the same affronts...

Jim

Hi Jim,

Yep, sneaks on only one handful at a time but continuously over many moons.

You've bought the new boat, offloaded the old one and then comes the hard part - what do you want to keep.

When I changed boats a few years ago it was obvious that nothing had ever been thrown away. On the bottom was stuff from when it had been launched in Florida, on top of that stuff with instructions in French and Spanish and on top of that another layer with instructions in English and Hebrew. It was like an archaeological dig.

[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 (?)

in such case i'd suspect that being 2mt lighter you will be faster nearly all the time, plus reach hull speed sooner and more often (planning down those waves !)

cheers,

[GILow]

Originally Posted by Jim Cate

...


Meanwhile, your performance in less than hull speed conditions will be enhanced quite significantly, especially in lighter winds. You're gonna love it, mate, but it will keep you from enjoying the new engine so often!

When we switched from our original Insatiable to I-2, we offloaded 17 years of accumulated packraterism. We gained about 6 inches of freeboard. We then sailed her from Yamba to Manly to put her on the market. On that trip north we were astonished at how much better she performed in ALL conditions. Poor dear had suffered the death of a thousand acquisitions! I blush to admit that I-2 has suffered some of the same affronts...

Jim

Yes, I can say performance is remarkably different. I’ve actually had to relearn how to sail the boat. Steering is particularly different, more responsive, but also requires a bit more attention. Light wind performance is way better, but I don’t know how much of that is thanks to the Kiwi Prop.

Your six inch freeboard gain is very relatable, I’m currently at 240 mm higher (a little under 9 inches), and that’s before I do the engine swap.

I’ve lost track of where I got rid of some of the weight, but certainly removing all the unwanted “stuff” that was on board helped. I think there were at least ten pairs of thongs (flip flops for those non-Aussies who just freaked out).

[GILow]

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 (?)

in such case i'd suspect that being 2mt lighter you will be faster nearly all the time, plus reach hull speed sooner and more often (planning down those waves !)

cheers,

Yep, 5 - 6 knots was the norm. Recent tests suggest 6 - 7 may be on the cards though.

Not sure planing is gonna happen tho...

[Jim Cate]

Originally Posted by GILow

Yep, 5 - 6 knots was the norm. Recent tests suggest 6 - 7 may be on the cards though.

Not sure planing is gonna happen tho...

Yep, you will need to jetison a few more pairs of thongs to get Manera planing in a regular basis... but planing is highly over rated, at least amongst the Swanson crowd.

JIn

[Fore and Aft]

GiLow you could take a power plane to the hull and reduce the hull thickness by half an inch at least. Those Swanson’s are built like tanks. With a lighter hull and putting a vertical bow to increase the waterline you might pick up an extra knot.
Cheers

[GILow]

Originally Posted by Fore and Aft

GiLow you could take a power plane to the hull and reduce the hull thickness by half an inch at least. Those Swanson’s are built like tanks. With a lighter hull and putting a vertical bow to increase the waterline you might pick up an extra knot.

Cheers

Mine is worse than most. The hull was commissioned by a steel boat owner who didn’t trust this new-fangled fibreglass stuff and had them toss in a few more barrels of resin. Total dead weight.

But I don’t lose a lot of sleep over osmosis.

Anyway, I may not get there fast, but I sure get there in comfort.