Hull Waterline Length to Beam

I read that the beam should be 4:1 waterline length. I know that it is possible to increase that, but what are the effects of increasing the beam substantially? thanks. Say 48' waterline length, and 18' waterline beam.

 

What type of boat?. What speed to length ratio? What is the mission? There are no absolutes in boat design. Everything is a compormise.

 

I'm designing a sportfishing yacht that is currently 48ft at the wl, and 18ft beam, and currently beginning a new file with 16ft beam. I want to cruise easily at 35 knots. When I started I didn't mean to end up with an 18ft beam, but once I got there, that made me curious about effects on performance. I am a novice designer, but a descent captain, and know what I want. As far as the calcs go, I'm lost. Speed to Length ratio: Can I design towards a specific S:L by horsepower, or is that a constant?

 

No problem , just be sure to spec a pair of 2000+HP MTU.

Plaining boats work on vastly different principals than displacement boats that push thru the water, so closer to a box is OK .

FAST FRED

 

Great thanks. Currently I have C18's in my drawings, may not be enough hp for the design.

 

noway

If u use such a small L/B ratio it will be catastrophic for the performance of the hull. Resistance due to waves may reach 12 times !!!!! resistance due to friction. U can see this in Bailey's study about small vessels hydrodynamics. In order to reach a speed of 15 knots u will have to power the hull with an engine of at least 600 HP nominal power.
Think about it again !!

 

thanks for the input! It's very helpful!

 

plain_sailing shows it in it's worst light. What happens to a planing hull (using Series 62 and 65 data) of low L/B is that as L/B decreases below about 4, there is an increasingly large resistance per pound of displacement (R/W) hump you need to get over at a Displacement Froude Number ( V/(g*volume of hull ^1/3)) of about 1.4. As you increase speed to a Displacement Froude Number of ~ 3 this hump goes away and all L/B values tend to converge to a R/W of 0.12.

So for your boat with a displacement of ~ 45,000 lbs and 35 knots the Displacement Froude Number is about 3.5, R/W is ~0.13 which is 5850 lbs drag which is an ehp of 638 for the hydro alone...say 150% for airdrag and roughness and 50% efficency gives a minimum installed hp of about 1914 shp continious.

But R/W for a L/B of 2.66 at a Displacement Froude Number 1.4 (about 14 knots) will be about 0.14 which is a higher resistance per pound (6300 lbs resistance), but a lower ehp because of the slower speed. I.E. an ehp of about 270 or a required minimum shp of about 810 to get her over the hump.

Now this is for a fixed CG location relative to the planing area centriod and does not address the effect of changing the CG location on lift and resistance which can really change the results. If you really want to get into this, try to get a copy of the Series 62 and Series 65 planing hull model tests which took L/B ratios from 2.0-7.0. As an aside, this data show that a L/B of about 4.0 is a pretty good compromise for Displacement Froude Numbers from about 1 to 5; which I think brings this discussion back around full circle.

 

Great information! I've began to slim her down a little, to reduce the resistance. I guess it's all a compromise. I want a fairly wide beam and a pretty fast boat, so somewhere between 3-4 L/B sounds like there is a beam that will work. thanks for the input.

 

Remember, slimming the boat down itself does nothing for the cruise speed you are considering. What slimming does is only lower the "hump" resistance, but if your cruise shp is greater than the hump shp then narrowing the hull gets you nothing except better economy at off speed. In order to decrease the total resistance at cruise speed, you need to make the boat lighter, not necessarily narrower.

Enclosed are two figures from PNA showing the Series 62 shape and data.

 

L/b Ratio

I think that there is more to consider than resistance when determining the L/B ratio. Obviously by reducing it the workable deck area will be somewhat increased, however by increasing the breadth the transverse stability of the vessel is improved.

This on first though would be a good thing to improve the stability, but may infact cause very high accelarations which is not a good thing for a fishing boat where the crew are working on deck.

 

Yes, I really am interested in a more stable hull in big choppy seas, at 4-6 knots, but also having a higher cruise speed (35+ @ 2100-2300rpm or most efficient for the engines), then most 45-50 sportfishing boats on the market. I guess I'm toying with the design, and the numbers to see if there is a solution that would fit my needs. Currently I have in my drawings twin Cat C-18s, which produce right at 1000 hp each, and I've been playing with the C-30's that will produce 1550 hp each. The interior will be geared for fishing and not so much luxury like most major manufactured yachts, so hopefully with some smart design, I can build beautifully, clean, AND light. Just from experience, the more stuff that's inside the cabin in 4-6ft seas, the more broken stuff you have to fix and clean up when you get home. So, anyway, I just don't really know what's possible, except that what rules of thumb I read about seem to be broken, when I inspect actually sportfishing boats on the water, and that's where my question came up.

 

Just paperwork is probably not going to give the best boat. When you look at working offshore fishermen you are seeing a boat that was evolved to meet both the ocean and the market at the time it was built. The Carolina boats have to leave the dock in the morning, get out to the gulf steam pretty fast without making the customers sick or injuring them, have slow speed motion/handling in the waves compatible with trolling and fighting fish plus not making the customers sick and then get back to the dock in daylight plus, maybe, outrunning a storm. There is a lot in there other than boat efficiency.

The waterplane aspect ratio is a better way to look at the hullform than beam/length since it treats the actual shape more accurately. Most of these boats will have a ratio near the range of 3.5. For many boats, this will be similar to a B/L of about 4:1.