length vs beam and trailer able

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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.
 
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
 
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..
 
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.
 
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."
 
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.
 
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.
 
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.
 
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
 
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.
 
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?
 
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.
 
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........
 
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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????
 
This might help.......

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

virginia2.jpg
 
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.....
 
I'm thinking that a well balasted keel, would make for quite a long righting arm, and make the boat more stable.


Be careful with that idea.....it might be good up to a point. If you add ballast to a boat not intended to have it, she will float deeper in the water. This means the rail goes under water sooner as she heels, which could cause early flooding at a lower heel angle. Usually peak stability is just as the rail (deck edge) goes under water.

Yes, adding ballast to lower G is increasing stability, but whether the lower capsize angle is worth it is a question.....
 
Most designs that are narrow it seems are priority wise leaning toward more efficiency and weight even for a full disp hull is of great importance. The most common rule of thumb for determining the amount of power required for a disp hull is weight/power. More or less directly proportional. So we see most narrow boats built low w little top hamper.

So ballast has it's price.
 
I'm thinking that a well balasted keel, would make for quite a long righting arm, and make the boat more stable.

virginia2.jpg

Yes but thats another part of the equation that was taken into consideration when the center of gravity was calculated right?

oh, and veyr nice boat
 
First thanks Tad, That was very help full. I did a little more looking but you got me in the right spot to look. And the right frame of mind too. For some reason when i made this post i was looking at the boats for sale. When I found the off shore "race boats" the 100mph or +, 2 and 3 engine boats and some of them are very long even 38 to 40 ft but only 11 feet wide. ....... I know just because it fits on a trailer and its a boat. its not the same lol I do understand but just got the wheels turning upstairs a bit.

Until i retire the only way i can justify a trawler or any other boat is I have to be able to put it on a trailer I can pull with a 1 ton truck. One of the down sides to living in the middle of the USA in Missouri.

Thanks again guys.
 
First thanks Tad, That was very help full. I did a little more looking but you got me in the right spot to look. And the right frame of mind too. For some reason when i made this post i was looking at the boats for sale. When I found the off shore "race boats" the 100mph or +, 2 and 3 engine boats and some of them are very long even 38 to 40 ft but only 11 feet wide. ....... I know just because it fits on a trailer and its a boat. its not the same lol I do understand but just got the wheels turning upstairs a bit.

Until i retire the only way i can justify a trawler or any other boat is I have to be able to put it on a trailer I can pull with a 1 ton truck. One of the down sides to living in the middle of the USA in Missouri.

Thanks again guys.

I don't think it takes much to get a permit to go over nine feet wide. I know in California it dosent
 
Eric
As to the Alaska Ferries being extended and gaining speed. Yes, As I recall, the extension increased speed by almost a full knot at original setting.
In conversation with a now decested old tug boat master who knew engineers on the ferries, the increase in hull speed allowed the ferries to reduce RPM allowing maintaing the schedule speed (Fexablility when required, like making up for lost time during incidents) This was during the 70's fuel crunch when saving fuel was a factor. The Malaspina on the Bellingham run saved tons of fuel on each voyage. I believe that system was then running PS300 Lt Fuel (actually heavy fuel, but did not require quite the heat to make it flow from the tank to engine) Currently the system is running pure diesel.

Tad- On a earlier thread, I had opened a discussion on ballast as it related to vessel rolling. You echoed knowledgeable posters with your diagram and explanation. The conclusion of that discussion has me convinced to add ballast. The situation is locating ballast at a palatable cost. I have discarded the thought of using 60# bags of cement/sand mix (contained within plastic garbage bags till atmospheric moisture set them hard) and now am budgeting for 60# commercial lead cannon ball trolling weights. (almost $2.00 per #) How many? Darn'ed if I know, just keep adding till results are gained!
Thanks for the refresher.

A.M.(Al) Johnson-Ketchikan
27' Marben
 
For most trailering folks staying under 8ft 6 ,no permits, is the easy solution.

An allowed 40 ft trailer length will allow a pretty large boat , but many states limit the overall length of the combination to 65ft. A few are at 75 , a few in the NE are 55.

YES ,commercial trucks go way longer , but non commercial IS limited.

For our use we built up a 35 ft bus conversion (reinforced frame , 400hp engine) and the trailer LOA limit caused us to select a 23 ft boat , remember the fuzz will be tape measuring to the end of the tilted up out-drive.

There is also some space required at the trailer ball end to be able to go round corners.

As it is impossible to back a trailer that can not be seen, the simple solution is a front mounted ball hitch , that makes launch recovery a snap.

Works on cars and trucks too!
 
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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.
Why don't 'they' make long narrow trailerable boats?
It's not a question of stability or seakeeping or performance, it's because nobody would buy them. Side by side not one American in a million would pick the skinny boat, especially if his wife and kids had anything to say about it.

Things are different elsewhere.
Narrow boats for sale, Canal boats for sale, narrowboats
 
Jeff,
Boats were narrow in the past mostly because they were easily driven when narrow and high power engines were not available. It's hard for me to believe most all boats are still wide (including my own) the way people complain about fuel consumption. And people don't like "tippy" boats ... even yachtsmen. Don't see any narrow inflatables. Actually I think there is one that is fairly narrow but I've never seen one.

But if your'e happy at 6 knots wide can be driven quite easily too ... almost as easily as narrow. And if my own Willy were widened to 12' beam my present engine would not need replacing and for all practical purposes Willy's not overpowered.

But if you get even close to hull speed narrow = easily driven.
 
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Tad- On a earlier thread, I had opened a discussion on ballast as it related to vessel rolling. You echoed knowledgeable posters with your diagram and explanation. The conclusion of that discussion has me convinced to add ballast. The situation is locating ballast at a palatable cost. I have discarded the thought of using 60# bags of cement/sand mix (contained within plastic garbage bags till atmospheric moisture set them hard) and now am budgeting for 60# commercial lead cannon ball trolling weights. (almost $2.00 per #) How many? Darn'ed if I know, just keep adding till results are gained!
Thanks for the refresher.

A.M.(Al) Johnson-Ketchikan
27' Marben

I don't suppose you have a foundry up there? Pig iron is about the cheapest/densest thing currently (bought at retail prices) available. Common cement bricks from the building supply (not hollow ones) will work if you have space. Steel flat bar cut into sections and painted with Primacon will stow nicely. The advantage with using steel is that it's easy to drill and bolt down. Here we have a recycler that will sell scrap lead (not perfectly clean) for about a dollar a pound.....

Whatever you use make sure it absolutely cannot move even if the boat is upside down.....
 

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