CS 25

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CMG
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CS 25

Post by CMG »

Hello all,

I have a few questions regarding the CS 25 design. I think I am finally ready to devote the time and effort required towards building a larger vessel, and it appears that this design would work almost perfectly. I've read the many-many-many posts on lengthening this design and the how to PDF. I'm thinking a LOA of about 24 ft is what I am after, which I'm estimating is within the stated 10% tolerance. Is this accurate?

I'm a bit old school I guess and for a vessel of this type I really want dual power mounted on a bracket with the enclosed foredeck variation. I was thinking twin 4 stroke digitally controlled 200 HP motors, so about 1000ish pounds weight rigged and installed. I feel like the design is for motor(s) mounted on the transom and I am afraid adding this weight-especially with the stretching-might adversely affect the location of the LCG . I am aware of the scantling issues, but how about adverse trim issues and stability. With the increase in length, and thus displacement, I assume I would have some ability to shift internal components or maybe even ballast forward to compensate the extra weight hanging off the bracket. Thoughts? The design as is looks impressive, and both Peter's and Walker Run's boats look like they are perfectly balanced in the many posted videos. I wouldn't want to mess that up just for an extra few feet and/or twin power.

While searching through the last decade and a half of posts on this design, I noticed that Mr. Mertens mentioned something about a 35 foot Great Loop Boat design around 2019 or so if I remember correctly. Is this still in the works, and are there any details to be shared? That would possibly be the only design I would prefer more than the CS 25.

Thanks in advance!

fallguy1000
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Re: CS 25

Post by fallguy1000 »

It is a planing hull.

When you lengthen the hull with a bracket, as long as the bracket and engines are set at the right height, the hull should perform similarly. In the simplest context, adding weight to the stern will result in a boat that sits in more of a hole at rest, and so static trim might be affected.

You say stretch, but also say cs25 to 24' which is shrink, so a bit confusing.

Let's make some assumptions.

Assume your bracket is 2' and assume the center of gravity is x feet from the stern and based on a 600 pound engine.

If you add a bracket of 150 pounds at 1'+X and engines of 400# at 2'+X, you have altered the static trim by that much. And when the boat is underway and planing, none of that matters much, but sleeping you would have people forward. It is nearly impossible to make engine heavy boats perfectly level by moving fuel, water and batteries because of the massive weights aft. Some boat guys end up lead shotting forward. So, for example, to offset the weights aft, you would put the anchor locker forward and perhaps run a gang plank forward to move the anchor a foot further forward and pick up say a 50 foot pound moment. Then you can also put say 100 pounds of lead forward. But important to keep in mind the boat has only so much available displacement. Once you start adding, you must subtract elsewhere.

A way to do so could be building the non-structural aft sections in foam.

My boat has engines 100 pounds heavier on 6" setback 22# brackets. So, I am 244# heavy aft. And I am doing things to get weights forward as much as possible. But I may end up needing some lead shot in epoxy forward. We'll see. The Skoota is a prototype.
My boat build is here -------->

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CMG
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Re: CS 25

Post by CMG »

Thanks fallguy, but I really mean LWL. Thats my mistake. Sorry for the confusion. I estimate this design has a LWL not including the bracket of 22-23 feet. I'd like it to be at least 24 feet. When I look at other designs and production boats its not until that size that I seem to get the usable room I want from within the boat.

jacquesmm
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Re: CS 25

Post by jacquesmm »

You can scale the CS25 by a small amount. max. 10%, no problem there.
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TomW1
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Re: CS 25

Post by TomW1 »

An Armstrong Bracket only provides flotation at rest on plane it is above the water line and it does not provide any lift. So only the hull is providing the lift. Adding 10% to the CS25 will give you a 27.5 long hull. You will need to get the plans to see how it affects the LWL. You will need to make the calculations for moving weight forward to compensate for the extra weight of the two motors and to moving them back the two feet on the bracket.

So we are looking at 987lbs from the LCG vs. the 500 or so that was the design weight for the motors. So leaving the console where it is will make up part of this. Now we need to make up some more of that 497lbs by adding more lbs. in front of the console. And since it is so far back we need to add it as far forward as possible. It is a calculation 489 for the second motor from the LCG aft must be offset forward. You will need to determine the new LGC as you have stretched your CG25. Leaving the console in place will do a lot of good, but only part of your needs.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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