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  #91  
Old 01-21-2017, 01:48 AM
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waikikin waikikin is offline
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Originally Posted by DennisRB View Post
This is the charter cat. The sport boat style wind and sun on your face helms don't suit the SOR of a non performance charter boat which is driven on autopilot 99% of the time, and has divers and swimmers walking past where those helms are at anchor. That helm position is more suited to the fast cruiser you drew. This may be better with a single offset helm for cost and simplicity, but that seems hard with your FP style sloping roof. Not sure if a pop top would work either though, as the rear viability would be bad with the slope of the cabintop.

Also can you explain why the slope of the cabin top seems so aggressive? Is the cockpit raised compared to the salon? What as the thinking behind this?
The helm/s could just be on the cabin bulkhead with a cutaway or hatch/cover for when not in use, The area outboard of the cockpit looks good for kicking back/daybed but the hatch might interfere- maybe a helm bench over could let the area still be used? I'd tend to just have engine controls to stb only, then there's positioning of headsail sheets/winches unless that's been covered already.
Jeff.
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  #92  
Old 01-21-2017, 02:32 AM
bscatam bscatam is offline
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Higtdrawings

Explaining design futures.
1. Aft helms. General idea is for family cruising cat design with possibility of 2 up to 4 guests on board. So I make deck and cabin space integrated to accommodate 8-10 person. In case of main bulkhead helm it is not possible.
2. Cabin roof slope. Attaching drawing. The aft part of B deck is doubled and raised with 125 mm for cables to achieve wire link between starboard and port steering wheels. I will publish my solution soon.
3. This is a cruising cat so 90% of time is on autopilot.
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40 ft cruising cat design. First steps.-hight.jpg  
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  #93  
Old 01-21-2017, 09:30 AM
valery gaulin valery gaulin is offline
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Really nice Design. Love the look of your catamaran. Dont have much experience with boat design but aftrt reading all the comments there is some valid point and some other not! Make sure you dont listen to every comments before some contradict others. Any boat is all about compromise. C'ant have a crujsing - racing catamaran that will do both well!!!! All about compromise. But keep going, learn and try if you can get to study other design to see their flaws and strong attribute to learn from other people mistake. Of curse the best way is to learn from our own mistake but when it is about building our own boat we dont really have many tries. Few person will built more than 1 boat, that being said few will even get to finish that one boat they are building.
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  #94  
Old 01-21-2017, 01:24 PM
bscatam bscatam is offline
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Some additional layouts
Attached Files
File Type: pdf pdf.pdf (1.04 MB, 43 views)
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  #95  
Old 01-21-2017, 02:20 PM
valery gaulin valery gaulin is offline
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Dont know much but are you surr your hull has enought volume for the strucure you are putting on this Catamaran?
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  #96  
Old 01-21-2017, 02:40 PM
bscatam bscatam is offline
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Parametric study.
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40 ft cruising cat design. First steps.-par.jpg  
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  #97  
Old 01-21-2017, 04:06 PM
valery gaulin valery gaulin is offline
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I am not a designer but it look like your prismatic coefficient is on the low side. Check with a designer! From what i understand a low prismatic coefficient is never a good thing to undershoot. Always better to overshoot especially for catamaran because they are very weight sensitive. From my understanding a to low of a prismatic coefficient can introduce hobby horsing problems and nose burrying.

Your bruce number seams to demonstrate a descent performance for a crusing catamaran. The only other thing is I think that the displacement is on the low side for this size of a catamaran with this structure and all the cruising gear. But again check with a designer, It would probably be very worth it to pay a couple hours of consultation with an experience designer to fine tune your design.

Other wise it is a great looking catamaran, exactly the way I like them. You are on the right path. Just some little compromise here and there to refine your design and it is there.

Good job!!
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  #98  
Old 01-21-2017, 04:20 PM
TANSL TANSL is offline
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Originally Posted by valery gaulin View Post
From my understanding a to low of a prismatic coefficient can introduce hobby horsing problems and nose burrying.
In another thread you have expressed a similar opinion but I am not able to understand why this behavior. Could you please, or someone knowledgeable on the subject, explain that relationship?. Thanks.
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  #99  
Old 01-21-2017, 05:14 PM
valery gaulin valery gaulin is offline
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Part of the answer is here.
http://www.catamaransite.com/catamar...ensioning.html

Primatic tells you the fulness of the stern and bow compare to the midsection.
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  #100  
Old 01-21-2017, 05:16 PM
valery gaulin valery gaulin is offline
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A barge would have a primatic coefficient of 1.0.
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  #101  
Old 01-21-2017, 05:42 PM
TANSL TANSL is offline
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I'm sorry but I think it has to do with the volume distribution of the submerged hull, and the position of the center of gravity, rather than with the prismatic coefficient.
A barge can have a prismatic coefficient equal to 1.0 , we agree, but that does not explain anything.
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  #102  
Old 01-21-2017, 06:05 PM
Emerson White Emerson White is offline
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Originally Posted by TANSL View Post
I'm sorry but I think it has to do with the volume distribution of the submerged hull, and the position of the center of gravity, rather than with the prismatic coefficient.
A barge can have a prismatic coefficient equal to 1.0 , we agree, but that does not explain anything.
The prismatic coefficient is a measure of the distribution of the volume of the submerged hull.
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  #103  
Old 01-21-2017, 06:35 PM
UpOnStands UpOnStands is offline
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Quote:
Originally Posted by TANSL View Post
I'm sorry but I think it has to do with the volume distribution of the submerged hull, and the position of the center of gravity, rather than with the prismatic coefficient.
A barge can have a prismatic coefficient equal to 1.0 , we agree, but that does not explain anything.
The importance of the prismatic coefficient depends on the assumption that the hull shape immediately above the water line is not significantly different to the hull shape immediately below the water line. A very fine bow is likely to have a low rate of increase in displacement with immersion above the design (static) water line. This is expected to yield significant bow bury into waves; the bow does does not rise quickly enough but sticks in and the boat slows. If the hull has significant rocker at both ends(also yields low prismatic coefficient) it has little damping and will tend to pitch fore and aft too freely.
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  #104  
Old 01-21-2017, 06:48 PM
TANSL TANSL is offline
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Originally Posted by Emerson White View Post
The prismatic coefficient is a measure of the distribution of the volume of the submerged hull.
No, with the same block coefficient you can have multiple positions of the center of buoyancy.
I think the same reasoning serves to respond to UpOnStands. But I'm afraid this is not the right thread for this topic.
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  #105  
Old 01-21-2017, 06:54 PM
UpOnStands UpOnStands is offline
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true, the prismatic coefficient is a very "rough" measure of volume distribution. The maximum immersion cross-sectional area can be placed at the designers pleasure. The general assumption is that its placed close to the center of hull length - but got to give the designers something to play around with
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