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Old 28-10-2014, 08:09   #1
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Speed question

Hello all
Ive a question regarding hull speed, if a longer waterline is faster than a shorter would it make sense to keep a boat laden heavily to submerge more of the hull, thereby increasing length in the water?
P
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Old 28-10-2014, 08:11   #2
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Re: Speed question

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Hello all
Ive a question regarding hull speed, if a longer waterline is faster than a shorter would it make sense to keep a boat laden heavily to submerge more of the hull, thereby increasing length in the water?
P
Then you're also increasing wetted surface. The amount you are going down is far more then what you are gaining length wise. It's not nearly as simple as you're making it out to be
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Old 28-10-2014, 08:40   #3
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Re: Speed question

A displacement hull is constrained in speed by its bow wave. Going faster will cause it to sink into its own trough. Weighting it down will not help this situation.
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Old 28-10-2014, 08:43   #4
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Re: Speed question

So is there an optimal load for a given hull design?
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Old 28-10-2014, 08:48   #5
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Re: Speed question

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So is there an optimal load for a given hull design?
Within reasonable limits, light makes right. especially in acceleration.
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Old 28-10-2014, 08:50   #6
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Re: Speed question

I just notice you have the same boat as me. Good luck speeding it up. They're wonderful boats, they sail well, they are comfortable, and fun but definitely not known for their blinding speed. I did have mine momentarily up to 10kts on a wing on wing run in high winds. She started shaking like crazy and the whole boat sank so low the freeboard at the quartes was only about a foot or two. Don't think I'd want to try it again. There's also a reason that for one of the most common boats out there you never really see them racing.
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Old 28-10-2014, 08:55   #7
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Re: Speed question

Lowering the vessel would increase the waterline and therefore increase the hull speed only very slightly. Using the formula of 1.4 times the square root of the length for a few inches of difference would not be a noticeable.

The problem is that by increasing the displacement that you are also increasing the wetted surface area (frictional drag). You would also be increasing the drag from turbulence (more loss of the laminar flow effect at higher speeds) and from wave making resistance, which is also a drag on the hull.

The net effect is that you would increase drag and therefore increase the amount of force it takes to move the boat through the water at a given speed.
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Old 28-10-2014, 09:09   #8
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Re: Speed question

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So is there an optimal load for a given hull design?
Yes, as light as possible.

Also with increased weight you decrease the freeboard making for a less seaworthy vessel.

With more weight you also increase the vessels moment of inertia, especially with pitching, which means increasing the tendency for the bow and the stern to dive deeper into the water. This is especially true in chop where the wave length is close to that of your vessels length.

Overall, more weight is a bad thing for boats. If one wants a boat which has a slower roll, pitch and yaw, the best solution is to get boat that is larger in both weight and physical dimensions and not to increase the weight of the boat that one already has.
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Old 28-10-2014, 09:55   #9
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Re: Speed question

Wouldn't trim also come into this?losing 1" at the bow may give you 2' at the stern if you have the older style boat


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Old 28-10-2014, 10:11   #10
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Re: Speed question

People who race cruising boats frequently empty the water tanks before a race.
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Old 28-10-2014, 10:36   #11
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Re: Speed question

Thanks for the reply's, I am curious because Ive been cleaning out my boat that still had loads of junk laying about from the previous owner and it got me wondering about weight and speed. I dont like a cluttered boat and if good housekeeping gives you better speed and stability that's another plus.
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Old 28-10-2014, 10:55   #12
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Re: Speed question

More weight = slower acceleration... so it takes more wind (or sail!) to get the boat to hull speed quickly. Some boats are designed to have a longer waterline when heeled...
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Old 28-10-2014, 12:08   #13
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Re: Speed question

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I just notice you have the same boat as me. Good luck speeding it up. They're wonderful boats, they sail well, they are comfortable, and fun but definitely not known for their blinding speed. I did have mine momentarily up to 10kts on a wing on wing run in high winds. She started shaking like crazy and the whole boat sank so low the freeboard at the quartes was only about a foot or two. Don't think I'd want to try it again. There's also a reason that for one of the most common boats out there you never really see them racing.
Tonight I'm going out on the last Tuesday night Cattle Drive of the season, where we race Catalinas from 27 to 36 feet. The Drive has steadily increased in popularity over the last two season, and now we see 12-15 boats out. There are no ratings or finish times, but we let the 27's start 3 minutes ahead.

There are several Catalina 30's racing, and they range from a stripped out boat with a feathering prop and good sails to floating condo's with tired rags. The Drive organizer has the 'racing' 30 and usually beats the bigger boats by a wide margin.

The fleet has measurably improved their boat handling and knowledge of the racing rules, and everyone realizes you don't need the latest superboat to have fun racing.
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Old 28-10-2014, 12:12   #14
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Re: Speed question

It might help to think of hull speed in a different way. If you took the sails from a 26ft boat and put them on a 30ft boat, assuming the two boats have the same basic hull design, the 26ft boat would be faster. It is a relative term. It still requires more force to propel a larger boat (or a boat with more wetted surface), it's just the larger boat can carry more sail.
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Old 28-10-2014, 12:25   #15
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Re: Speed question

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Tonight I'm going out on the last Tuesday night Cattle Drive of the season, where we race Catalinas from 27 to 36 feet. The Drive has steadily increased in popularity over the last two season, and now we see 12-15 boats out. There are no ratings or finish times, but we let the 27's start 3 minutes ahead.

There are several Catalina 30's racing, and they range from a stripped out boat with a feathering prop and good sails to floating condo's with tired rags. The Drive organizer has the 'racing' 30 and usually beats the bigger boats by a wide margin.

The fleet has measurably improved their boat handling and knowledge of the racing rules, and everyone realizes you don't need the latest superboat to have fun racing.
Good to know. I have only seen Cat 30's race a few times and compared to others they didn't fare well. I guess we fall in the latter condo tired sails category so I may be a little jaded. Don't get me wrong though I LOVE my boat. It sails well and is very forgiving. I Just have always associated her more as a tub then as a race car.
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