Beam to length ratio?

[Danielsan]

Hi there,
Just wondering, as I was looking at some specs mostly dimensions of comparable boats. I foud out that most boats with a 7m50 LOA have a beam of 2.10 to 2.50m. Now my question, I have designed a 7.40m LOA with a beam of 3.1m!!! How will this affect the behaviour of the boat?
Any comments?

Greetings,

[Eric Sponberg]

Daniel,

In general, for a given length and depth (freeboard plus draft), beamier boats will be slower (more wave drag), will have more weather helm when heeled, and will be harder to steer. They will have more initial stability, but will have lower range of ultimate stability. You have to satisfy yourself that whatever you are putting inside and on the boat is worth the compromise in performance.

The extremes of beam can be seen in the open class 50s and 60s where in many designs the hulls are very shallow but very wide. They have very good initial stability, but generally poor ultimate stability. They sail to weather very poorly and are difficult to control. Narrower open class boats are much easier to sail and so are easier on the crew and autopilot. They point to weather much better. The wider boats are faster downwind, but the narrow boats are faster upwind. And the changes in the rules to prove self-righting ability (with the crew member inside the boat changing the location of the ballast to effect self-righting) are a direct result of the poor stability of the boats in their earlier incarnations in the 80s and 90s.

In your boat, will the performance be bad? I don't know, it might be just fine, all things considered. But the features above are the basics.

Eric

[cyclops]

Very well put, Eric. Except for the few people who order a boat that can do everything perfectly and are willing to pay the price for it.

[yipster]

Just wondering, as I was looking at some specs mostly dimensions of comparable boats. I foud out that most boats with a 7m50 LOA have a beam of 2.10 to 2.50m. Now my question, I have designed a 7.40m LOA with a beam of 3.1m!!! How will this affect the behaviour of the boat?

what comparable boats and what behavior? no deadrise mentioned, from these dimensions my guess is it must be a planing boat?
sail or motor? a picture can tell a thousend words...

[Danielsan]

Thx Eric,

I forgot to give some more details about the boat. Is a small daycruser à la bayliner. should go with jet drive and a Steyer 256HP engine. I asume indeed that the wider beam will need more power than one with a smaller beam.
Are there some recomendations about the L/B ratio? what's good or not?

greetings,

LOA 7.50 LWL 6.50
Beam 3.10 BWL 2.90
Displacement 2.500
speed 30-35max

[Grant Nelson]

Sailboat or Moterboat? Hard bilge or soft (curved)? What is the displacement (weight)? Fast or slow boat? Some of the key issues might be wind resistance, a lot of water resistance due to the large wetted surface, and 'comfort' (steming from the 'stiff' stability) as the boat will travel more on the waves, than through them. In anycase, your dimensions are not impossible, look at some Catboat designs, which have similar dimensions.

[CT 249]

Daniel, I've always wondered why modern powerboats are so beamy (apart from the issue of the A/R of the planing surface), so can you tell me why you went for a short and beamy boat of 2,500 kg, rather than stretching it to a long but slender boat of similar internal volume and cost?

I'm just curious; is it marina costs or what that drives these decisions? Does a 7.4m x 3.1m boat allow you a better interior layout than (say) a 10m by 2.3m boat (similar volume and weight therefore AFAIK similar or lower cost)?????? I don't normally get to ask designers about these issues.

Thanks

[Danielsan]

There is one major reason for the 7.50m As this is a personal project and a one off, when going over the 7.50 i have to pay 2.500EUR taxes only to get the papers to put it on the water. Let say taxes if I go beneath it doesn't cost a cent.

The reason for it being 3.10 is by mistake. Somewhere I must have forgot to change some dimensions!!!!

If the 3.1 would not be a problem, I might leave this as it is. It could give transportation problems? Ypster you could now something about that? We are neighbours?

CT249" I am flattered you call me a designer, but I am not, or let's say amateur "designer"

Greetz,

[Eric Sponberg]

Sorry for my misunderstanding, thinking your boat was sail instead of power. Similar things apply however, regarding drag and stability. The wider power boat will have more drag and more initial stability, but the ultimate stability will be less. As for speed, the wider boat will require more power to get up on plane.

In general, narrower is more fuel efficient than wider. You can go narrower in the quest for powering efficiency to the point of making the boat impractical to live on or reducing stability to the point of ridiculousness. So practicalities to make the boat useable force the compromise.

To answer CT's question, boats are paid for primarily by length--purchase price, taxes, insurance, marina fees, etc. Therefore, if you can pack more accommodation into the boat for a given length (at otherwise constant ownership cost), the depth and beam have to be larger. The point is to have the installed power and performance at reasonable values for the compromises that are made. If the entire package makes a useable boat, then the design is satisfied.

Eric

[Danielsan]

The only problem is that all the interior stuff is designed around this or better said inside this dimensions.

I could easely scale the 3.10 to 2.50, but then I will have to change all interior parts.

[yipster]

3.10 by 9 (hull) is our 27 sea ray and i like that old boat and size better than the new 2.60 beam witch is legal for trailering. thats why many boats are 2.60. trailering heavyer boats require special tow vehicles like a hummer, all in law somewhere but try a ramp with a bigger boat and you (may) be cured. bigger size boats are by special transport flat trucks. for your design 256 hp may be a tad on the short side for planing when fully loaded, did you check crouch formule for power?

the drawing is fine but why not look for a bayliner? (get one from 2000 up) building one yourself is gonna cost you really a multifold and holds many risks and you wont beat bayliner. having a "boat" still means: "break out another thousend" for so many things. after i get our SB drive fixed you are invited this summer to camp free a few days on our boat and get even more adicted but planing around for fun easely cost half a grand a day just for fuel here, can you hear me crying also?

[cyclops]

In the USA, boat length equals more money for a boat. More width does not. A wider boat can hold more people and passes safety requirements easier and more cheaply. We have the widest bowriders in the world. Our boats look like the Ace of Spades symbol. They ride like blocks of wood in any chop. Most Americans figure a bigger boat in any shape, is better than a narrow boat.

[D'ARTOIS]

Joop, nothing can beat the thrill of creating something like a boat, lot's of emotions involved, look at the number of threads and posts here; what CT 249 says cuts wood nicely. Beam on the Wl costs power and speed, to get planing speed (around 17-20 knots) you require 150 hp per ton ot thereabouts.
We have an Admiral's Barkas as a workboat that goes about 14 knots with a 150 3D6 engine and that is only 2.80 mtr on the Wl (on 13.50 mtr Length)
see photo

[yipster]

D'ARTOIS, your more than right, sorry Danielsan, only wanted to help you. got inside drawings to show?
i got carried away seeing the drawing had very much in common with a bayliner.

We have an Admiral's Barkas as a workboat that goes about 14 knots with a 150 3D6 engine and that is only 2.80 mtr on the Wl (on 13.50 mtr Length)

i envy your specs. to me nowaday's such a boat makes more sence.

[cyclops]

A generalized word about WIDE boats of W to L ratios of 1 to 2.5 or less. They have fantastic slow speed stability. They can flip the boat very easily in choppy windy conditions because the new drivers feel and remember the low speed safe handling. Driver alone in bad conditions has to be taught the boat is really a roof shingle at +50 mph. You are either lucky or not. Once she starts to go up up, there is no time or maneuver to stop her. " Blowovers " can occur at any time in wide fast ( water and wind speed combined ) boats.---------------O. K. I know that first hand. Sterndrives ARE very bad in this respect.