question on placing reverse chines

[northerncat]

im looking at building a 6 metre assymetrical hull catamaran, i have a question is there any advantage to placing reverse chines( i think you guys call them steps) on the inside of the vertical tunnel wall?
sean

[Eric Sponberg]

Northerncat,

Steps run transversely across the bottom of the hull. Chines and lifting strakes run longitudinally along the hull. What you are referring to is a lifting strake. Yes, they work. Where do you put them--hard to say as no one has ever published any guideline on the shape and positioning of strakes. There is a lot of "voodoo" design out in the market, and people who are successful with the placement of strakes typically keep their design guidelines to themselves.

Typically, what we do know, however, is that on a powerboat (or a fast sailboat) the chine (either a simple hard corner or with a flat horizontal surface like a lifting strake) in the aft end of the hull should be slightly below the waterline at rest, and going forward it should rise gently towards the bow. How high the chine rises at its front end at the stem is open to guesswork. I have seen then stay absolutely parallel to the waterline all the way forward, and I have seen them rise all the way to the deck. In most cases, they end at the stem about 1/3rd to 1/2 the way to the deck. In cross section, the bottom surface should be horizontal (if the chine is shaped as a strake), but as the chine moves forward the horizontal surface should start to angle down slightly, never going more than 15 deg. below horizontal at the bow. 10 deg. is probably more typical. Intermediate lifting strakes (between the hull centerline and the chine) can follow similar guidelines, and from the middle of the boat going aft, they should follow the buttock lines. Going forward they may follow the buttocks or curve inward slightly towards the stem. Lifting strakes should not be placed directly in front of propellers--they should be either one side or the other from the shaft centerline, and they should end some distance forward of where the shaft exist the hull.

The interior strake that you propose would be like the exterior strake at the chine--submerged at the aft end at the still waterline, and rising slightly going forward. They are common on catamaran racing powerboats and hydroplanes.

Eric

[Eric Sponberg]

One more thing--the reasons for adding the strakes is that they help lift the boat out of the water at speed. This tricks the water into thinking that the boat is actually lighter than it really is--that is, there is less wetted surface and so there is less friction drag. Less drag = more speed.

Eric

[northerncat]

if my tunnel hull is vertical do they do anything? or would i need to put a slight outwards anglesay5 dgrees to encourage the water up to the strake
sean

[Eric Sponberg]

Vertical walls are OK--the water is going to ride up the hull no matter what. In molded hulls (hulls out of a mold) there is always a slight cant of about 2 deg in the inboard hull side to get the hull out of the mold easily and so it won't lock in. In a one-off, this does not matter. So if in your prototype you are thinking of eventually making molds for the hull, then you should add in the slight cant.

Eric

[northerncat]

thanks but im only after the one that i can build for myself to go fishing with
6m powercat

i will use strakes however to give me more lift now as i want to make my hull and boat as efficient as possible due to only wanting to use a 60hp motor, would a strake that is 25 mm be enough?
sean

[FAST FRED]

Strakes lift the boat , but require power to do so.

No free lunch in boats.

FF

[northerncat]

so then they dont use the power already being put into puching the boat through the water( i like a free lunch)
sean

[Eric Sponberg]

A 25mm strake seems a little narrow. My gut would tell me that 40mm would be better, and 50mm probably the max. You need to present enough of a "wall" for the water to turn against so that it changes direction as it flows up the hull and hits the strake. It is the changing of water flow direction that gives the boat the extra lift.

60 HP is not much. On my Cherubini Classic 20 monohull design, the builder installs a minimum of 225 HP in an I/O drive which gives it a speed of about 50 mph for about 2700 lbs of boat weight. I don't know what your boat weighs, that will largely determine what your max. speed will be with only 60 HP.

Eric

[northerncat]

with the 60 hp big foots i know in the torres straight they use these to push a 425kg 24 degree deep v fibreglass dinghy as well as 2 adult crew and 2 dugongs that can weigh up to 750kg and still get on the plane, lightly loaded they get 35knots out of the motor and this drops back to around 20 with a good load on
im hoping i can keep the weight of my cat down to arounf 3-400 kilos and would probably never have more than 5 people max on it, will it do it i think from a thrust point of view it will just depend on how easy it is to flip the boat up on to the plane i guess
as i said earlier im doing a lot of guessing which is why im asking questions on the forum
sean

[northerncat]

from the research ive conducted most normal 60 hp outboards with normal gearboxes will push about 800kg to about 30knots so im hoping mine becasue of the bigger gearbox and lrage block should be able to push around a bit more weight
sean

[rkpshenoy]

This is for all those who have prior knowledge of designing a passenger cat.
I have already done preliminary design of my catamaran design the way conventional ships
are designed by following similar steps but for sticking to the required L/B , S/L ratios for catamarans.
Having done that i saw that the lines plan i chose is inapplicable to my design.
The block coefficient of the hard chine hull form is far below the required value for the displacement calculated.
Later i learnt from some other guy in discussion forum that my approach in fixing the dimensions itself is wrong.
If so plz tell me how to fix the main dimensions like planing length..., trim, lift required etc
for catamarans.Also if you can suggest me any catamaran or a planing hull specific book it would be gr8.
Id let you know about the details of my project after anybody has replied.
Suggestions of all kind are invited.

[rkpshenoy]

Can the chine be totally immersed in water throughout the LWL of the demihull.
If so what will be the resulting planing characteristics.