I got a question about lengh of the boat

[fireant]

When I look at boats for sale, I notice many of them add 2-3ft to the sugar scope to add more speed.
My question how much of a difference in performance does it really make?
wont it take away some maneuvering in tight places with extra length behind the Rudders?

[catsketcher]

Sugar scoops are there because most of the time designers, builders and owners are too optimistic.
The designer thinks that he can design the boat and keep the weight down - so he designs a boat with a certain volume, thinking that this will be enough to float all the necessary stuff. He tells himself lies that the boat will be able to carry all the necessary weight with a certain volume.
The builder then builds the boat - adding more interior furntiture and lockers and extra stuff - thinking it won't add too much weight. He lies to himself that the boat is staying light.
The owner then adds solar arches to the stern, along with a heavy dinghy and extra fuel tanks and more - STUFF. He believed the fibs of the first two guys.
But the laws of Physics don't care for optimism, or blind stupidity. If you add weight to the boat, the hulls have to sink to float it. So if the designer didn't design in enough volume way back in the design process to float all your stuff, the stern sinks below the waterline.
But this looks bad - and is not fast in light winds. So owners get shipyards to add on an extra metre - extending the stern so now it looks clear of the water. So now it looks as though the boat was designed for all the stuff it has on it. BUt look carefully at the bridgedeck. On some models it slopes down aft at an angle that was supposed to be (when designed) totally level. That is because most sugar scoops extend the keel line - in reality slowing the boat at extra speed because of greater rocker and not really adding any worthwhile volume at all - it is usually for looks and low speed flow. The immersed volume of your typical scoop is a couple of hundred litres at most - not enough to make a significant effect on a boat overweight by a tonne or so. And certainly not enough to get it back floating level (if the solar arch, cockpit cover and dinghy have weighed her down too much by the bum).

I did all this on my little folding cat - I designed it, built it and then pretended that it didn't need an extension. But it did - I stuffed up. But to do a proper job I flipped the hulls over and then ground off the paint and added foam from the stern to up the about 40% of the way forward of the stern. Then glassed it and faired it up. Now it really has enough volume for its heavier than anticipated weight.

Don't worry about steering - scoops don't affect that at all.

When a boat is launched you DO NOT want it sitting on its lines. You want it sitting way above its marks - hopefully a little bum up. So when you start adding stuff it sinks little by little and then when you add all the extras (and extras tend to accumulate at the rear of the boat) the boat sits level. My 38 footer was about 22 cm high when she was launched empty (really empty). Her sisterships with the nice panelled lining and super sumptiuos interiors float much lower when cruising. They have more stuff and float too low - we have less stuff and float just right. And every year or two I clear out everything and only bring back the stuff we need. We go faster, we're cheaper and I can lend to boat to the kids - gotta love having less stuff.

[thinwater]

Don't forget ease of boarding. I think that is the real reason on many designs, if it is combined with a swim platform. Both my wife and parents had mobility problems, and adding sugar scoops made it not just easier but possible to get in an out of the tender.


Actually steers better in reverse now. If it cleans up the wake, it can add more speed than the change in length suggests. Just a pinch.



The reason an mileage vary with the boat design.



Sail Delmarva: Extended Transoms: The Process

[smj]

Having owned a Seawind 1000 both before and after a 4’ stern extension was added, I can honestly say the performance was noticeably better after. A cleaner exit, more waterline length and most importantly a much improved motion with hobby horsing and pitching negated.

[Thomas1985]

The biggest selling point of sugar scoops to me was swimming, dinghy, and rear dock access.

I would assume it’s going to minimally affect top speed, but I honestly don’t know…

[Bowdrie]

Originally Posted by catsketcher

When a boat is launched you DO NOT want it sitting on its lines. You want it sitting way above its marks - hopefully a little bum up. So when you start adding stuff it sinks little by little and then when you add all the extras (and extras tend to accumulate at the rear of the boat) the boat sits level

True enough, and not a new concept.
In the '30s William Atkin wrote about designing his boats to sit level with 2>4 people in the cockpit, and the boats back then had nothing in/on them like we have today.
The designers weight/balance study for my boat was for it to sit level and above its marks with 2 people in cockpit, all tanks 1/2 full, and a ~500 pounds of "miscellaneous stored/distributed gear".
Outside of a Table of Offsets, a comprehensive weight/balance study is perhaps the most demanding of a design effort.

[catsketcher]

Great comments from previous posters. An increase in length helps with pitch damping and also modern scoops are great for getting on board with all your stuff.

My mum had a Crowther 10 built in 1984. Lock never designed it for twin diesels and the builder for this one off was used to speed boats so didn't know light. So she sat bum down, way down. When she had metre long extensions added and folding props put on she was a totally different boat. Actually sailed nicely in lighter winds and looked quite nice. But she was way deep and the extra flotation did almost nothing for her stern down trim.

The ultimate for a designer/builder/owner is to get the transoms almost kissing when loaded for cruising, or even out of the water by a few centimetres. Then when going to windward the leeward hull still has a nice exit. There is another train of thought that immersing transoms a little is good too. This flattens out the rocker profile and gives a better top speed. This is probably better for racing orientated cats.

One thing to probably try and avoid is a flat section just out of the water. I talked to an owner with a very flat section transom about 7cm out of the water. He had added little pods to his underwater section. When I asked him about it he said it was because the flat sections banged in a chop and it drove him crazy. So keep the sections well rounded and try and get them close to the waterline.

[thinwater]

Originally Posted by catsketcher

... There is another train of thought that immersing transoms a little is good too. This flattens out the rocker profile and gives a better top speed. This is probably better for racing orientated cats.

Once you exceed about 10 knots, the water exits cleanly off the back and the extra length is little benefit. What you do NOT want to do is curve the extensions upwards to get them above the waterline. That just sucks speed. You want the keel line to have almost no curve at all. Basically, the last 10 feet of fast cats have very little compound curve. They don't plane, but they can semi-plane over 10 knots. That's not the correct term, I'm sure, but you can feel the speed jump as they pull out of the hole at about that speed.

Thus, some multis don't benefit from extensions. This is boat-specific. But do you spend much time over 10 knots? Not unless you keep the boat light and you really like to push it!

But the sugar scoop boarding area is always nice.

[Fore and Aft]

Fireant we extended our Roberts 25 by one metre to make it easier to get on and off the floating caravan that she was. Immediately once back in the water at the same RPM on the engine our speed increased by 3/4 of a knot. Plus the wake was cleaner either sailing or motoring.
I have extended a fair few boats and the only failure was a big houseboat that we put new bows and sterns on. The failure was more due to the owner installing a new heavy interior that really destroyed the boats potential.
Cheers

[fireant]

Thanks that explain it to me. I was just wondering how much of performance was to gain.
I understand the part of more load carrying out back.

Thanks again for explaining. .

[CaptainRivet]

It was very common mod on seawind 1000 and 1100 as well as PDQ32 and FP32 Maledive to extend the sugar scoops by 3-5 feet to get less hobby horsing, raise payload by approx 300kg and get up to 0.5kn more speed. In this size it’s big difference if you have 32 or 37ft waterlength regarding behavior especially in ocean waves.

[catsketcher]

Originally Posted by CaptainRivet

It was very common mod on seawind 1000 and 1100 as well as PDQ32 and FP32 Maledive to extend the sugar scoops by 3-5 feet to get less hobby horsing, raise payload by approx 300kg and get up to 0.5kn more speed. In this size it’s big difference if you have 32 or 37ft waterlength regarding behavior especially in ocean waves.

Gday Rivet

I don't know of any Seawind 1100 and the I have never seen an 1160 with an extension.

Installing most sugar scoops doesn't really increase volume much at all so it doesn;t increase payload - it just raises the transom so it looks as though you increased payload - the boat is still sitting pretty much whereit was. My mate who makes Seawind 1000 sugar scoops for others shopwed me the construction of his. It would have about 40 litres of immersed volume in it. (Have a look at Seawind extensions - you can see that the old transoms line would still be under water and is not lifted appreciably) What is does do is clean up the water flow at low speeds and make it look as though the boat was designed for the load and helps with pitching. The only way to really get much extra volume is to push the extra volume forward of the old transom and this is a lot of extra work as so is very rarely done.

[thinwater]

Originally Posted by catsketcher

Gday Rivet

I don't know of any Seawind 1100 and the I have never seen an 1160 with an extension.

Installing most sugar scoops doesn't really increase volume much at all so it doesn;t increase payload - it just raises the transom so it looks as though you increased payload - the boat is still sitting pretty much whereit was. My mate who makes Seawind 1000 sugar scoops for others shopwed me the construction of his. It would have about 40 litres of immersed volume in it....
.

You slightly misunderstand the function. So did your mate. No, they do not lift the boat at the dock. In fact, my extensions were almost entirely above the water at rest.

Yes, the boat sits lower in the water, but with the extra length the water still exits well and it rides better.


More importantly, when driving hard to weather, the boat can sit almost entirely on one hull (my boat was turboed), which presses that hull deeper in the water (double the load plus wind down force from the heeled mast and twisted main). Then, they do add load carrying capacity. You need to measure the immersed volume when the boat is on one hull and sunk into its displacement wave--you will get a very different result.

Going to weather is the most important mode and thus the one you need to focus on. Monos lean, so the heeled shape is the important one. Multihulls load up on one hull, so you need to think just short of hull flying.

If this were not true, they logically transoms would be designed to be deep in the water at rest. They aren't.


---


My avitar is not my boat. Mine didn't do a rooster tail. Smoother and faster, with a 120% genoa, not a blade jib.

[Fore and Aft]

Catsketcher Seawind turned the 1000 into the 10850 with an 850mm extension but renamed it the 1000XL. She's a miles better boat than the 1000, an extra knot of speed was gained.
My best transom extensions were done by starting roughly under the bridge deck on cats and fairing back from there. I was always about making it look original and not just tacked on later as an afterthought.
Cheers

[smj]

Originally Posted by thinwater

You slightly misunderstand the function. So did your mate. No, they do not lift the boat at the dock. In fact, my extensions were almost entirely above the water at rest.

Yes, the boat sits lower in the water, but with the extra length the water still exits well and it rides better.


More importantly, when driving hard to weather, the boat can sit almost entirely on one hull (my boat was turboed), which presses that hull deeper in the water (double the load plus wind down force from the heeled mast and twisted main). Then, they do add load carrying capacity. You need to measure the immersed volume when the boat is on one hull and sunk into its displacement wave--you will get a very different result.

Going to weather is the most important mode and thus the one you need to focus on. Monos lean, so the heeled shape is the important one. Multihulls load up on one hull, so you need to think just short of hull flying.

If this were not true, they logically transoms would be designed to be deep in the water at rest. They aren't.


---


My avitar is not my boat. Mine didn't do a rooster tail. Smoother and faster, with a 120% genoa, not a blade jib.

A turboed PDQ32?