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-   -   length vs beam and trailer able (https://www.trawlerforum.com/forums/s37/length-vs-beam-trailer-able-7966.html)

h2orescuemedic 11-30-2012 08:45 PM

length vs beam and trailer able
 
How come all the trailer able boats that have a 9 foot to 10 beam are under 28 foot. Why don't they make a 32 to 36 foot long boat that is only 9 ft wide? I would think you could add a few feet to the length and have a very cool trailer able boat.

bfloyd4445 11-30-2012 10:02 PM

Quote:

Originally Posted by h2orescuemedic (Post 116401)
How come all the trailer able boats that have a 9 foot to 10 beam are under 28 foot. Why don't they make a 32 to 36 foot long boat that is only 9 ft wide? I would think you could add a few feet to the length and have a very cool trailer able boat.

I think you need a wider beam on the longer lenghts for stability. To narrow and it can be treacherous in rough water

GFC 11-30-2012 11:28 PM

Rescuemedic, first if all, thank you for keeping boaters safe. You're a hero.

I'm not sure why you want a 32' boat with a 9' beam. On most highways once your load is over 102" wide you need to have trip permits, flags, oversize load banners, etc. So if eliminating the requirement for all of those things is your goal, and you want to be legal, you'd best stay at 8'6" or less in beam.

I had a 330 Sundancer with a 11'5" beam and towed it all over the place. It's not much of a hassle getting the permits (many states sell them online) and then you don't have to worry about towing as long as you stick with freeways. If you get off the freeways onto 2 lane roads you're supposed to have pilot cars, but I've towed on 2-lane roads without them and have never been stopped..

Nomad Willy 12-01-2012 12:03 AM

Most people like wide boats w LOTS OF STABILITY. They think a narrow boat will tip over. Just look at the typical dinghy.

The narrow boat takes much less power and is FAR more pitch stable than a wide boat. Wide boats have 2 speeds .... fast and really slow. They wallow their bows high in the air requiring lots of power at 7 to 17 knots. Nice speeds to be going especially when it a bit rough. And when it is a wide boat will pound worse and porpoise much more.

medic,
Do it. Cut a boat in half at the best spot and add more boat. Make it as long as you want. I think it will be better in all respects however on the road you may drag the stern on driveways and such. Quite a bit of work though but one can find these boats really cheap. And then you can put the engine where it belongs ... on the transom. With the outboard outboard engine mounts the engine is too far aft. They are just a quick and dirty modification.

markpierce 12-01-2012 12:26 AM

It's not unknown for ships to splice an additional section to the hull, lengthening the ship but maintaining the same beam, and offering more accommodation. Some of the ships I've sailed on (as a passenger) had this "operation."

sunchaser 12-01-2012 10:45 AM

bfloyd says:
"I think you need a wider beam on the longer lenghts for stability. To narrow and it can be treacherous in rough water"

Really, now that you are a designer, what treacherous narrow boats are you referring to? For starters, there are some are some lobster boat guys that would beg to differ. Sailors too.

RT Firefly 12-01-2012 11:03 AM

Greetings,
Mr. sunchaser.... +1

bfloyd4445 12-01-2012 12:31 PM

Quote:

Originally Posted by GFC (Post 116457)
Rescuemedic, first if all, thank you for keeping boaters safe. You're a hero.

I'm not sure why you want a 32' boat with a 9' beam. On most highways once your load is over 102" wide you need to have trip permits, flags, oversize load banners, etc. So if eliminating the requirement for all of those things is your goal, and you want to be legal, you'd best stay at 8'6" or less in beam.

I had a 330 Sundancer with a 11'5" beam and towed it all over the place. It's not much of a hassle getting the permits (many states sell them online) and then you don't have to worry about towing as long as you stick with freeways. If you get off the freeways onto 2 lane roads you're supposed to have pilot cars, but I've towed on 2-lane roads without them and have never been stopped..

sundancers are great boats, i even took my 270 1989 da to lake tahoe twice.

Nomad Willy 12-01-2012 12:48 PM

Consider;
If you add a section of hull to a boat that section adds stability. Quite a bit. So when you lengthen a boat the beam remains the same and the length to beam to length ratio is higher so most would consider it a narrower boat yet it has increased stability. You could consider the added section to be water wings.

The Alaska Marine Highway lengthened two ferries about 50' (I believe). I would love to know but don't know how that changed performance. I think they kept the same engines.

bfloyd4445 12-01-2012 01:12 PM

Quote:

Originally Posted by manyboats (Post 116579)
Consider;
If you add a section of hull to a boat that section adds stability. Quite a bit. So when you lengthen a boat the beam remains the same and the length to beam to length ratio is higher so most would consider it a narrower boat yet it has increased stability. You could consider the added section to be water wings.

The Alaska Marine Highway lengthened two ferries about 50' (I believe). I would love to know but don't know how that changed performance. I think they kept the same engines.

The width of beam is increased as well as the length for proper stability. If you lengthen a boat and keep the beam the same it would only improve stability if the beam were already much wider than it should be for the current length. I've had experience with a long narrow boat and that thing was dangerous in a beam sea. it was very, what's the correct term, roily. I think the Alaskan ferries in your example was already of a very wide beam to facilitate loading and likely too square so the additions of 50 feet to the hull would likely bring the beam to length ratio back into the realm of correct proportions for stability. Can you imagine trying to handle a boat configured like a pudgy rectangle?

no, i cannot believe that making a skinny boat longer will improve its stability

theran5317 12-01-2012 01:30 PM

I'm thinking there is a lot more to this question than just the beam.

All the mass calculations related to Center of Gravity vs Center of Buoyancy that are affected by the weight distribution in the boat.

bfloyd4445 12-01-2012 03:24 PM

Quote:

Originally Posted by theran5317 (Post 116592)
I'm thinking there is a lot more to this question than just the beam.

All the mass calculations related to Center of Gravity vs Center of Buoyancy that are affected by the weight distribution in the boat.

absolutely! Height, width, length ratio of beam width change in relation to length overall, waterline length, tons displacement, ....geezz...could go on and one. Ok out there, what's the answer u engineer types? I guess many of the variables could be dropped out of the equation as they have little effect for standard trawler configuration.
Ever heard the term, she handles like a pig? Heard this often used to discribe an under powered wallowing vessel. Are trawlers pigs?

sunchaser 12-01-2012 03:52 PM

bfloyd says :
"absolutely! Height, width, length ratio of beam width change in relation to length overall, waterline length, tons displacement, ....geezz...could go on and one."

You havn't come up with the important ones yet, keep guessing.

Tad Roberts 12-01-2012 04:06 PM

It gets complex.....remember we are talking about a boat shaped object. Most above are talking about transverse stability, but remember longitudinal stability is also an issue.

Transverse stability is dependent on a number of factors. The displacement(V), the center of buoyancy (all underwater volume)(B), the vertical center of gravity (the vessel, her crew, fuel, water, etc)(G), and the waterplane area and distribution (Iw). There's another point involved called the metacenter (M), it's mythical (an invention) and it's the point around which the boat is said to rotate when heeling.

The distance between G and M is called GM, and it's length (in feet or metres) is another basic measure of stability.

When a hull is floating level and upright the center of buoyancy (B) is on the centerline somewhere below the waterline. Not far below if the hull is a shallow barge, a long way below if its a deep and narrow type. G is (hopefully) also on centerline somewhere above B. In modern powerboats it's well above the waterline, in ballasted sailboats it's close to or slightly below the waterline. M must be above G for the hull to be positively stable.

As the hull heels B moves to the low side and the area of the waterplane changes shape (depending on hull form). In a flat shallow barge B moves quickly while in a narrow and deep hull B moves slowly. If you visualize B as an upward force and G as the downward force you see that as long as G stays on the centerline and B moves to one side there is a righting force created between the two.....Forcing them back into line(upright).

The distance (in feet or metres) between those opposing forces is called the righting arm (RA or GZ). The RA multiplied by V (displacement) is called the RM, righting moment. Most stability curves are of RA values.

You can increase stability by lowering G or raising M. Lower G by adding ballast on the keel or removing weights up high (dinghy on the roof, etc). Raise M by changing the shape of the waterplane or the displacement.

So adding length might change the waterplane if new volume is greater than the structural weight added (boat floats higher). The added displacement will increase RM, righting moment, because that's our RA multiplied by displacement. But unless we make the boat wider RA will not change. So it will take more force to heel the boat, but she will capsize at the same heel angle as the shorter boat........

bfloyd4445 12-01-2012 04:32 PM

Quote:

Originally Posted by Tad Roberts (Post 116621)
It gets complex.....remember we are talking about a boat shaped object. Most above are talking about transverse stability, but remember longitudinal stability is also an issue.

Transverse stability is dependent on a number of factors. The displacement(V), the center of buoyancy (all underwater volume)(B), the vertical center of gravity (the vessel, her crew, fuel, water, etc)(G), and the waterplane area and distribution (Iw). There's another point involved called the metacenter (M), it's mythical (an invention) and it's the point around which the boat is said to rotate when heeling.

The distance between G and M is called GM, and it's length (in feet or metres) is another basic measure of stability.

When a hull is floating level and upright the center of buoyancy (B) is on the centerline somewhere below the waterline. Not far below if the hull is a shallow barge, a long way below if its a deep and narrow type. G is (hopefully) also on centerline somewhere above B. In modern powerboats it's well above the waterline, in ballasted sailboats it's close to or slightly below the waterline. M must be above G for the hull to be positively stable.

As the hull heels B moves to the low side and the area of the waterplane changes shape (depending on hull form). In a flat shallow barge B moves quickly while in a narrow and deep hull B moves slowly. If you visualize B as an upward force and G as the downward force you see that as long as G stays on the centerline and B moves to one side there is a righting force created between the two.....Forcing them back into line(upright).

The distance (in feet or metres) between those opposing forces is called the righting arm (RA or GZ). The RA multiplied by V (displacement) is called the RM, righting moment. Most stability curves are of RA values.

You can increase stability by lowering G or raising M. Lower G by adding ballast on the keel or removing weights up high (dinghy on the roof, etc). Raise M by changing the shape of the waterplane or the displacement.

So adding length might change the waterplane if new volume is greater than the structural weight added (boat floats higher). The added displacement will increase RM, righting moment, because that's our RA multiplied by displacement. But unless we make the boat wider RA will not change. So it will take more force to heel the boat, but she will capsize at the same heel angle as the shorter boat........


whew. but thats almost exactly what i said the diference being u know what i am talking about and i only know from my experiances at the helm. I would love to see the math. Thank you very much for the explanation. I gotta think about ewhat u posted...distance between G and M????

Tad Roberts 12-01-2012 06:08 PM

1 Attachment(s)
This might help.......

Attachment 14389

bfloyd4445 12-01-2012 06:10 PM

Quote:

Originally Posted by Tad Roberts (Post 116657)
This might help.......

Attachment 14389

Thanks. I get the picture.
from your picture it is obvious a narrow beam in relation to length is gonna cause stability problems

RT Firefly 12-01-2012 06:36 PM

Greetings,
Slightly tardy welcome aboard Mr. Roberts. Um, your vessel wouldn't happen to be named "Reluctant" would it?

theran5317 12-02-2012 12:09 AM

Quote:

Originally Posted by bfloyd4445 (Post 116658)
Thanks. I get the picture.
from your picture it is obvious a narrow beam in relation to length is gonna cause stability problems


I'm thinking that a well balasted keel, would make for quite a long righting arm, and make the boat more stable.

https://www.businessupnorth.com/images/virginia2.jpg

Tad Roberts 12-02-2012 04:12 PM

Quote:

Originally Posted by bfloyd4445 (Post 116658)
from your picture it is obvious a narrow beam in relation to length is gonna cause stability problems

Well.....perhaps.....But we recently did a stability study over on the Woodenboat forum of a Bolger Windemere design. This is a flat-bottomed plywood box about 30' by 8', draft is a bit more than a foot. Her stability was surprisingly good, more than expected for a sheltered water cruiser. Part of the reason for that is her relitivly high sides, and also no double-decking or raised cabin, the house is set down in the hull. Stability is as much dependent on height vs beam as anything....Low is good, high (tall or double-decked) is less good.....


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