Design thoughts?

[Stumble]

If anyone remembers I used to post here a lot before life got in the way. Since then I have stoppeed practicing law and am back in school getting a tertiary degree in Naval Architecture and Marine Engineering. As part of my Senior Design class we have to generate a vessel for a 'clint' who wants a shallow water boat as a photography platform in the marshes and near coastal waters of Louisiana. I am posing my design here to see if anyone has any thoughts or criticisms. Feel free to be brutal, I certainly don't mind well intentioned criticism.

If anyone is curious the parent craft is Kurt Hughes 21' Power Cat.

 

[HJS]

Length, width, speed, total weight including payload, maximum, minimum?
JS

 

[Stumble]

Woops...
Length 21'
Width 8'6"
Draft ~6"
Speed TBD, 6kn minimum
Weight: ~600lbs lightship, plus 800lbs crew/supplies

 

[fallguy]

Draft will be everything.

Noone can comment on anything without some data. Length, displacement, draft, loaded weight, dry weight...

My gut hunch is draft is too deep. Gut hunch is too heavy..

 

[Stumble]

Draft will be everything.

Noone can comment on anything without some data. Length, displacement, draft, loaded weight, dry weight...

My gut hunch is draft is too deep. Gut hunch is too heavy..

I am a little concerned about the weight as well. When I do the volume analysis I won't be surprised if I need a little more volume for the displacement. The plan is to stretch it to ~23' so the hulls can still be made from 3 sheets of ply. But the nominal weight isn't far off from the parent craft, if distributed a little different.

 

[bajansailor]

@Cajunpockettunnel built a flats boat recently, and documented the build on here in this thread -
Pocket tunnel bateau https://www.boatdesign.net/threads/pocket-tunnel-bateau.64358/

It is very shallow draft, with the propeller in a tunnel - a vessel like this might be suitable for what Stumble's SOR says?
And being a monohull rather than a cat, then you would have no worries really about the volume of displacement available for the weight.

 

[fallguy]

In general, catamaran forms struggle as flats boats. The tunnels excel. This must not be overlooked. I am planning a move to Texas and would have loved to build a Woods Skoota18, but the draft is simply too deep versus monohull..

 

[Stumble]

In general, catamaran forms struggle as flats boats. The tunnels excel. This must not be overlooked. I am planning a move to Texas and would have loved to build a Woods Skoota18, but the draft is simply too deep versus monohull..

It would have been an interesting option to be sure. But I will bet a dollar my boat ends up being shallow draft. He is planning on using a 70hp outboard, so the tunnel is just allowing clean water flow. To fully immerse the intakes the draft of the skeg is 14" under the waterline while with a 9.9hp the skeg is only 11.5 inches. To go any shallower than this would require a jet or an inboard in the tunnel. Even if I have to both extend the hulls and accept a deeper draft I don't think I will get to 14"

In my case though I don't have near the budget to buy the 70hp motor he is planning on going with. My all up budget is $20,000 with the assumption I will be paying $100/hr for construction, so a $10,000 motor just won't work. Right now the total material list is around $10,000 with the rest reserved for labor.

 

[rberrey]

I would draw in a trolling motor for the area the boat will be used . K-Design has 2 power cat designs that might work if the cabins are knocked off , I think the 18' is around 8" or 9" , low hp , not sure what the draft on the Skoota,s are , but no cabin should = less draft .

 

[Stumble]

I would draw in a trolling motor for the area the boat will be used . K-Design has 2 power cat designs that might work if the cabins are knocked off , I think the 18' is around 8" or 9" , low hp , not sure what the draft on the Skoota,s are , but no cabin should = less draft .

Ya, I keep trying to figure out where to put a trolling motor. The current plan is to have it on the port bow, but to get one that can self deploy and remote steering starts to impact the budget. The next option is mounting it close to the doors, which would allow simpler operation and a cheaper drive. But is less functional. So at some point it becomes a budget issue not a design one.

I am a big fan of the ones that can hold position, but they would require being mounted on the center of the forward beam which has other issues.

 

[bajansailor]

To fully immerse the intakes the draft of the skeg is 14" under the waterline while with a 9.9hp the skeg is only 11.5 inches. To go any shallower than this would require a jet or an inboard in the tunnel.

But this is still relatively deep draft (I did say relatively) - when compared to a flats boat like Cajun's.
Have a look at his thread - I can't remember what his draft is but I think he can operate in very skinny water - like 6" - and not have to worry about the outboard engine leg hitting the bottom.
I am very much in favour of cats for most purposes, and I do like the look of your cat - but I also think that for what you have in mind a typical flats boat would do the job more effectively than your cat.

 

[rberrey]

You don't have many goodies to cut to save on budget . Anchor winch could go , hand line on that size boat , half top instead of full hard top , half glass with no hinged door , that would be only a couple 100 bucks . Any real savings will be on the motor , labor , materials , so that will be in the sourcing and not so much in your design . I was working up the Mobile River a couple years back and got checked by Coast Guard in one of our work boats , a horn would be nice to have on a boat .

 

[fallguy]

Autopilot can hold position. The electric trolling motor just adds weight. You'd be better off going with two smaller engines versus 70 and electric. The battery weights are non-negotiable.

The desired draft for flats boats is all one foot or less to skeg bottom. The Skoota draft of the hull alone is 14". The problem is the hulls have rocker and while this makes for some great bay boat and offshore business; flat or skinny water ain't havin it. The hulls performance would be another matter; if the bows lift, the engine lowers, etc.

Anyhow, a catamaran hull is not ideal for skinny water.

 

[fallguy]

Here is a 2" hull draft design.

https://www.boatbuildercentral.com/StudyPlans/XF20_STUDY.pdf

 

[Stumble]

But this is still relatively deep draft (I did say relatively) - when compared to a flats boat like Cajun's.
Have a look at his thread - I can't remember what his draft is but I think he can operate in very skinny water - like 6" - and not have to worry about the outboard engine leg hitting the bottom.
I am very much in favour of cats for most purposes, and I do like the look of your cat - but I also think that for what you have in mind a typical flats boat would do the job more effectively than your cat.

A current 70hp Yamaha has a distance from the water intakes to the bottom of the skeg of 14.5 inches. His may be a little different given its age, but its going to be somewhere around there. At some point the prop size starts to dominate and you have to have enough draft to immerse the props no matter how shallow the hull is. I run very shallow water in Louisiana too, and at least here you can tolerate the skeg hitting the bottom somewhat since its mostly mud, but anywhere with rocks and its a different story. I suspect his hull form isn't actually any shallower than mine, he mentions its about 6" and mine is in that same range, 5.75" with 1 person, but the smaller motor means I won't be hitting the ground as soon.