I got a question about lengh of the boat

[Apollo19.5]

Originally Posted by catsketcher

........

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.

Yep, agree. My transom is out of the water and has all the clean wake, light air performance, etc, but at anchor with the tide being stringer than a bit of chop from the opposite direction, does not make a pleasant situation with shutter from waves hitting under the stern. Not a problem when sailing or any other anchoring situation, just a PITA when it does happen. We anchor to avoid now or set a stern anchor.

[catsketcher]

Nice thread,

Thinwater - I remember talking to my designer - Robin Chamberlin - when I launched my boat. I told him I still had 4cm of immersion before my transoms touched the water. He told me to keep the boat light as the weighted hull when going to windward would immerse it anyway.

Then again, way back in the 80s. We all looked at the then new John West - designed for racing by Lock Crowther. We all wondered if Lock had stuffed up as the transoms were immersed about 8cm at rest. He told us were were thinking too slow. As a racing cat it was more important for the rear rocker (release angle) to be as flat as possible so that the cat could attain a better top speed. He did not design the boat to do anything under about 6 knots as it would do that in almost anything.

So for me - I reckon that a boat that reaches downwind a bit and wants to go fast will have transoms that are close to the water at rest. Downwind, the leeward hull is only lightly pressed and extra rocker will slow the boat down at higher speeds (but be beneficial at lower speeds). Going to windward in heavy going, I want good pitch resistance before less drag as there is masses of power anyway so I don't think that one needs to worry so much about having excess clearance for heavy weather. Most cruisers go downhill too so I would slant performance to that anyway if it made a difference.

Fore and aft - I reckon that is best practice - go forward of the transom and foam up to reduce release angle and increase immersed volume more. I know the XL is a nice boat and it would be good to see the diff between the stern sections on the scoops my mate does and the Seawind ones.

This is obviously a big thing for me - having spent a few months part time designing and building a new set of stern sections for my own little cat. But way way back - a new friend was putting a sugar scoop on a Twiggy tri. He knew Lock Crowther and asked him how to do it. Lock told him not to keep going with the keel line (because the transom was close to the water already) but actually reverse the upward sweep so that the boat didn't squat. So she had a kink in her keel line - and went very fast. Lock was a better designer than almost anyone.

On my little 7 metre folding cat, I was searching for volume. I had already put on a scoop and it didn't help flotation issues. So I fattened up the stern sections, deepened the aft keel line and actually put the new transom slightly in the water. The idea is to get the stern sections fuller quicker so she doesn't sit bum down. In the end, your boat should get the bum it needs - mine needs volume so she gets foamed up, widened, deepened and slightly immersed. But other boats need different approaches.

[mikereed100]

While scoops improve access I would caution against making them too low to the water. I designed my boat with the lowest step about 6" up from the waterline. Makes it really easy to get back on the boat after swimming but when anchored in a bump that lower step scoops up a ton of water that makes quite a racket as it flows off the back and wreaks havock with my wa if I am in the cockpit. If I were to do it again I would leave the lowest step at least a foot above the waterline to avoid this.

[Factor]

Originally Posted by catsketcher

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

There was never any seawind 1100. There is at least two 1160s with stern extensions that i know of in QLD. One an 1160 Deluxe and one a LITE, the deluxe also had reverse bow extensions. It was markedly quicker than ant other Deluxe I have sailed - mind you it had **** hot sails as well

[David Mathis]

Powerboats have the same problem. Several years ago when we had a 32 trawler. We were docked next to a 36 which was the same length give or take a couple inches. How did this happen? His swim platform and anchor pulpit were molded into the hull while mine were bolted on. His extensions counted as part of the hull length. The boats, minus the swim platforms and the anchor pulpit, were the same length and had the same interior space. He paid much more for his "36" than I did for my "32" even though the weights and space were the same or very close.
You may have noticed some of the high performance boats with a long needle bow. We have an 18' performance boat that is more like a 14' boat with a deck extension.
It's also notable that some boats are measured along the gunnel rather than measuring centerline length.

[singlespeed]

Besides the dampening effect for hobbyhorsing of poorly designed hulls and the ease and convenience of ingress and egress from dinghies, kayaks and paddle boards, the bottom steps are a great place to hold the various "tools" you need when cleaning the hull bottoms while at anchor. And no, I have never post a tool off my scoops. Not having to climb a ladder to retrieve that scraper you find you need, after you are already in the water is a plus. Added benefit...... Your significant other doesn't have to interpret exactly what tool you are requesting from where and not having puddles of sea water through out the cabin when you actually go and get it yourself.

[CaptainRivet]

I am not a naval expert but there seem to be several length of cats were a longer sugar scoops have lot of advantages and others were its really negligible.
And that sweet spot seem to be getting 32 till 36ft into 38 till 40 and 44 till 45 to 48ft length.
Even the manufacturer doing that eg Seawind made an 1000XL and outremer extended their 45 to 48.
On the other hand i am not aware of any 40ft got extended to 44ft at the sugar scoops...sure there is more then juat length.
i am by far not an expert. just my observation when looking to buy and keep an eye on the market for smaller size and performance oriented cats and what typical mods and enhancements were.

Really surprised thats not a common mod on a FP Mahe 35 to get it to 39-40ft. Was my favorite in the small cats and to improve hobby horsing it seem a good idea to add 4 or 5ft but its not commonly done but eg on PDQ32 and 36 its a common mod to do.

[Bill_Giles]

If your boat is 34ft and you add 2ft to its waterline length then you will have added 0.2kts to its theoretical top speed.
Speed in knots is the square root of the waterline length in feet x 1.34

[catsketcher]

Originally Posted by Bill_Giles

If your boat is 34ft and you add 2ft to its waterline length then you will have added 0.2kts to its theoretical top speed.
Speed in knots is the square root of the waterline length in feet x 1.34

Gday Bill. That is not really the case in a good cat. If you keep the hull fineness ratio to about 10-1 LWL to beam then hull speed is not a thing at all and it less of a thing for wider cats. My cruising 38 footer will happily sit at 10-12 knots on flat water on a reach in a nice moderate breeze and can do up to 14 if the wind gets up more -but she likes a swell to do more than 14s .

Cheers

Phil

[Icarus]

Originally Posted by 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?

Scoops are good as a swimming/diving platform...
That's about it.
No difference in performance, why would one expect performance improvement?

[Fore and Aft]

Icarus, I assume you are saying that "No difference in performance, why would one expect performance improvement?" in jest? Roughly speaking the longer, the waterline the faster the boat. Particularly in the old days with heavier hulls and older designs. Our own yacht gained 3/4 of a knot for a one-meter extension and I never heard any of my clients (except one) say they did not get a performance improvement on their transom extension.
David I never understand why some owners have a big duckboard (swim platform) built and not a hull extension. It's even more perplexing when you see big trim tabs fitted. Hard chine hulls with flat transoms are some of the easiest shapes to extend.
Cheers

[lifeofreilly57]

Originally Posted by 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

Ease of boarding depends on the situation.
We found that in many Caribbean anchorages if yoour anchored in a cut where there was a nice steady breeze to sty relatively cool had some chop, no problem for our boat but made boarding a boat with a sugar scoop rather clumsy and difficult. We have a side entry ladder which is much easier to use in any situation. Once while out to dinner a storm front came through, waves up to 3 feet in the anchorage, while others were having a devil of a time boarding there sugar scoop sterns we were able to easily board our side ladder, just wait for the wave to rise and step on the appropriate step.
If you have some mobility issues, then the sugar scoop makes sense, but make sure to find a totally protected anchorage (hot with no breeze).
If I end up with a boat with a sugar scoop I'll add a side boarding ladder to ease loading in a chop.

[smj]

Originally Posted by Icarus

Scoops are good as a swimming/diving platform...
That's about it.
No difference in performance, why would one expect performance improvement?

Been explained if you’d read the entire thread.

[JustMurph]

https://www.graingerdesigns.net/the-...r-performance/

This is a good read.

[rob denney]

That analysis is the accepted wisdom, but ignores some of the nuances. For instance, a flat (or near flat) bottomed hull on a cruiser does not require floorboards and can consequently be lower. The weight and windage (and hard work to build) saved usually outweighs the relatively small increase in wetted surface.

The wetted surface/speed argument re rocker holds true at rest, not so much when the boat is moving. The side view of a rockered hull is a mirror image of the top surface of a wing which provides lift in the direction of the curve. To see how it works, loosely hold a spoon by it's handle near running tap water. Hold the convex part of the spoon just into the flow and feel it get pulled further in. Same happens with rockered hulls when they start moving.

A wide flat transom on a rockered hull converts this 'lift ' in to downward and aft components. Whether the aft component is offset by the front section pulling forwards depends on the relative widths (wider will lift more) and rocker angle (steeper the angle the more forward/aft the pull).

Next hold the bottom end of the flat side of a knife in the flow. It will be pushed outwards. ie upwards if it is an unrockered bottom.

There are other considerations, but from a performance view, this may provide some food for thought/discussion.