Beam to length ratio?

[Danielsan]

I think I will go and rescale the hull to 7.50 x 2.60, that cuts of 25 cm on each side, that won't be that big of a deal? I only would have to redesign interior parts, maybe it is just enough to move them 25cm inwards?

When done this I will put some pics of it.

Greetz,

some "research" on the net

The decisive gain from reducing the wetted beam
Residual resistance (Rr) varies as the square of the Beam (B)and the first power of Length (L): Rr = B^2L.
Consider the following: a molecule of water pushed by the bow will follow the path of least resistance until it is out of the hull's way. In this course it will push other molecules that have been pushed aside before, and those molecules will push others that were pushed before etc.
This is why Rr varies as the square of the Beam (B^2) but only at the first power of Length, and small changes in the beam result in big changes in Residual resistance.


BTW any one knowes how to determine C as a constant, and not by doing the reverse calc from the crouch formula?

[FAST FRED]

IN the US boat expenses are figured on LOA. So a wide box of a boat 3 stories tall is prefered to somthing usefull.

Pleasure boats seldom operate 100 hours a year ,
so the value of a BIG waterfront cottage is prefered to a usefull all weather vessel.

FAST FRED

[yipster]

BTW any one knowes how to determine C as a constant, and not by doing the reverse calc from the crouch formula?

i'm still only a boat science amateur, only from experience i mentiont the 260 HP to be on the low side.
from what i understand in Crouch constant is the C is taken from similar boats and is not a real constant?
(hope not have have made type errors)

C=Crouch constant
V=Speed in Mph
W=Weight of vessel in pounds
P=Shaft horsepower
V=C
W / P

HP=W / {V/C}2
C=V / (P/W}.5

to get planing speed (around 17-20 knots) you require 150 hp per ton ot thereabouts.

seems a lot faster to calculate right?

[cyclops]

Right on, Fred. Our yachts all look like white plastic shoeboxes with a point at one end. And they have that lovely cheap mass produced feeling for a non-necessity item. I actually find all natural aluminum more attractive because it just looks different.

[Danielsan]

Hi Ypster,

I don't now but I had calculated this at home I used 140 for C as this was lower than the number used for a specific type of power boat(150)

I put this in v=C/((displacement/HP)^0.5)

140/((4800lbs/260hp)^0.5)->32 knots? Or am I wrong

This is for not knowing C exactly, Isn't there a way to determine C more precisely?

Greetz,

[yipster]

Danielsan,

looking again at your drawing i asume your planning the maximum size in stich and glue on a basket mould? dont think the registration taxes are for selfbuild boats not intended for sale within 6 years or something. did you check max beam, trailor weight vs legal tow vehicle? fresh up my memory than!

didnt check your sum but looked at some C values. you used C 140 but i read here thats for smal speedboats. i'm only guessing yours is more like C 180??? didnt calculate that one eighter but did see your also low on the easy "thereabouts" formule. maybe its enough hp, i dont know. deadrise is important too: a flat bottom gives most lift and is faster but lower as 17 degrees and you'll be bumping over the waves with poor control, defenitly use lifting strakes.

must say i find this interesting stuff but dont have the Grouch constant spreadsheet but recall many questions asked about this subject in the past, maybe a good search on the forum (under option one?) brings it up? perhaps some pro's here want to share these numbers, can refer to a book, add info?

edit: that request still stands but reading instead of talking i found http://boatdesign.net/forums/showthr...=&threadid=615

150 - average runabouts, cruisers, passenger vessels
180 - 185 Small runabouts
190 - high speed runabouts, very light high-speed cruisers
190 - 205 Multi-step Hydros
205 - Seagoing Monohulls to around 40'
210 - race boat types
210 - Single-step Hydros
220 - three-point hydroplanes, stepped hydroplanes
220 - to 270 Sea Sleds
230 - Seagoing Monohulls , 70' to 80' increasing to
230 - racing power catamarans and sea sleds

so your right in your calculation Danielsan but you use lightweight and fully loaded may add an extra ton.
i asume youve read allready http://www.hamjet.co.nz/files/Jet%20Torque%2012.pdf

[Danielsan]

I will have to rescale the hull and interior as for towing the thing it can not be larger than 2.55 meters!!! otherwise it will be " UITZONDERLIJK VERVOER "

So this overrules everything. Neverless the HP stuff has to be verified

Greetz,

Yipster, are you going to the HISWA? some members here are going, I don't know when I will go but I am going.

[Raggi_Thor]

Anyone heard of the "30-30" rule? I like simple rules for estimates:

1) 30lbs/hp -> 30 knots (plus/minus for shape, length etc, this is for a "planing hull")
2) Hp vary with the square of the speed, double the speed and you need 4 times more HP.

So if 18 knots is OK, you can push about 80lbs/HP.
More weight than this will be hard to get on a plane...

[yipster]

no, i did not heard of it, do like simple rule of thumbs thou
trying the calculators in the Crouch constant test showed to my surprise the same result as Willallison and you mention: it takes a lot more power to go gliding faster but what i now wonder about as flat and wide glides earlyer, is how to know a rough minimal planing speed for a certain boat?

[cyclops]

Again, There are the rules of type ( location of engine and transmission ) of hull. I just do not have the memory. Basic easy planning is to keep all ( most ) of the heavy mass in the center of balance or flotation. British boat designer Scott-Paine proved this perfectly, as his 70', 40 ton boats accelerated at a level attitude to planning speed. His boats were durable in heavy seas. By now all good designers know what hull shapes to use for 90 % of a customers happiness. They will never know how to distort it into a most profitable shape for the companies bottom line. It is like telling them, " put the fire out with gasoline ". It could be done. But at great risk to all.

[lobsterman]

if you would all like to see some vessels that take the length to beam ratio to some extremes without sacraficing stability, you should check out some of the newest boats being built for commercial fishermen in Nova Scotia and Newfoundland.
nowadays they are mostly in the 2 to 1 range ( i.e. 40' long by 20' wide ) and they are getting wider every year, they may lose a little on the speed capabilities, but they certainly do not lose any stability, as most of these vessels are built to withstand some of the worlds worst weather.
i would say that they are built like a battleship although battleships usually have a ratio of 4 to 1, check out... www.trinav.com , there are some quite impressive designs and pics on this site.

Pat.

[Eric Sponberg]

Actually, the reason that the Canadian fishing boats are being built so wide is because of a screwy tax law that limits vessels to 65' Loa. Beyond that length, taxes go way up, so the only way to increase catch capacity is to make the boats wider and deeper. This has been taken to the extreme, and, in fact, makes the boats unsafe. By being so wide, yes, the boats have a lot of initial stability, but that is not necessarily a good thing. It makes rolling motion particularly uncomfortable (very fast with high accelerations). Also, in waves, the boats tend to ride with the face of the wave as it inclines, rather than stay more upright as a much narrower boat would do. The propensity to capsize therefore, is much higher with a very wide boat. Also, once upside down, a very wide boat is much more stable than a very narrow boat.

To read more about this problem, both for commercial craft, but primarily for sailboats, read "Seaworthiness, the Forgotten Factor" by C.A. Marchaj (pronounced MAR-ki). It is an eye-opener.

Eric

[lobsterman]

i believe the 65' limit came about as a result of restrictions being imposed on the commercial fishermen in order to restrict their growth,though i think that the tax situation that you refer to does apply to recreational vessels, and you are right that some of the older vessel "conversions" ( sponsoning and lengthning) were downright unsafe do to the low freeboard and reserve bouyancy and even some of the newer designs still have to have anti roll tanks installed onboard, but i would rather spend day in and day out on a vessel that has a little bit of a roll to it, rather than the constant pounding of most light weight hard chined recreational vessel designs.
all boat designs are suceptable to some degree of yaw in a following sea regardless of the beam, thats were experienced boat handling comes into play as most commercial / work boats do not have enough reserve HP or speed to keep themselves at the optimal point of the waves, but in the same respect they do not have to worry as much about applying too much speed and thus risking pitch poleing. oh and by the way most of the 65' vessels have not exceded a ratio of 3 to 1, yet!!!!


pat.

[yipster]

browsing this whaler site i noticed a simple formula for Maximum Rated Horsepower using only beam and length.



(2 X L X W) -90 = rated horsepower

Where:

L=boat length
W=transom width; if the boat does not have a full transom,
the transom width is the broadest beam in the
aftermost quarter length of the boat.

The rated horsepower may be rounded up to the nearest "5".
Here is an example of how the rating formula works in practice, applied to an older Boston Whaler boat, the V-20 model:

Boston Whaler V-20
Length = 19' 10"
Width = 7' 5"

Thus:

(19.83 x 7.42 x 2) - 90 = 204.15
Rounding up = 205 maximum rated horsepower