Displacement Hull Power Catermaran

Discussion in 'Boat Design' started by willy13, Mar 16, 2022.

  1. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    I am planning a build of an aluminum, trailerable, displacement hull power catamaran. Attached is a drawing of the hull shape. Before I go further with the design I wanted to ask a few questions. My wife and I decided that slow and steady is better for our purpose of exploring the Florida keys and eventually exploring the Bahamas. We plan to trailer the boat from NY to Florida and live on the boat for a week to a month during the winter. The current boat length is 23 ft which limits our efficient speed to 7 mph according to Mr. Froude. We chose a flat bottom to reduce draft. First question, am I correct that since it's a displacement hull and the flat bottom will be under water that the ride will not be harsh like a jon boat? I chose a wave piercing bow and a hull that is parallel to the water to prevent planning if the 15hp x 2 outboards can push the speed significantly past 7 mph. Is this correct, or will the hull possibly raise above the water soon after the 7 mph Froude hull speed number? From my experience with displacement pontoon boats I think this boat might reach 14 mph with 15hp x 2. It's hard to predict but fully loaded for live aboard we should be at 5000 lbs to 5500 lbs displacement. I'd like to have the option to cruise slightly above the Froude hull speed, 10 mph. The other reason I chose a flat bottom is it's easy to fabricate from aluminum. If ride quality is an issue I can change to a round bottom. Thoughts? IMG_20220316_091311054.jpg
     
  2. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    1st: I am no professional so gut feel comments not to be taken as truth but purely as comments to advance discussion.

    -I don't think you will complicate the design much by sloping the front up. Either way I doubt it will slam horribly as the bow is pretty narrow.
    -With no rocker and deep straight hulls the steering will not be the easiest.
    -30hp will probably beat the 7mph easily witch such narrow hulls - even at 23ft on flat water.
    -square cross section is for sure the easiest to make but flaring sides out (wider at the top) would give you much more interior space without compromising waterline beam AND give progressive buoyancy. Also arguably it would be stronger by nature.
    -keeping the sides parallel gives advantages in modularity of the build etc. but I think you would get better performance with curved sections
    -Extra chine might reduce wetted surface enough to be worth while. If still parallel sides the extra work should be relatively little.
    -the rear "rocker" is pretty significant and as the hull keeps the width to the rear that is bound to suck the rear down quite a bit. Keeping the aft wide is ok on an efficient hull but you need to slope up gently (far straighter section). I know the LDL hull by Irens below is quite different in every aspect but still using it as an example. If you want to keep the horizontal straight flat mid section then canoe stern might be a better option.

    8ft is pretty narrow imo. Where does the "living" happen? In a superstructure above or in the 4ft tall hulls? (I assume superstructure).
    Did you calculate the displacement on your drawing?


    [​IMG]
     
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  3. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    Thank you for your thoughts. 8ft wide is certainly a compromise, 8 ft 6" is max for NY towing without special signage and what not. The living space will be in the superstructure. This is a vacation live aboard so except at night we plan on enjoying the nice weather.

    I sloped the rear up so I can reduce running draft when we get in shallows around islands. I assume the prop needs clean water. The outboard height will be adjustable for when in deeper wavy waters. I certainly can make a more gradual slope if I needed. Or canoe the stern if that will prevent the stern from sucking down. That is something I was not aware of so thanks for bringing it up.
     
  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Just doing some very rough 'back of a fag packet' calculations gives me an approx displacement of 4,300 lbs at a draft of 1.5' (ie wth the transom just immersed, according to your sketch).
    The hull depth seems to be about 4'?
    The immersion might be about 400 lbs / inch (for both hulls), so your load draft might be a tad under 2' - 50% of the hull depth.
    Even if you can build a bridgedeck connecting the hulls that is only 12" deep, then you only have 12" of bridgedeck clearance above the waterline - not a lot when you are bashing to windward.
    You might well find that the underside of the bridgedeck slamming is more of an issue than the hulls slamming.

    If your main constraints are approx 24' long and 8' beam, then I think you would be better off with a monohull - that will have an L/B ratio of 3, which is fairly average for a monohull motorboat.
    And you will then be able to more easily get the necessary volume of displacement for your loaded condition, with a more seaworthy hull shape to boot.

    Edit - I think you would be hard pressed to find a 'better' 24' power cat than a Woods Skoota 24 for the usage that you have in mind. Note that Richard says that she is 'trailerable' (she is 13' wide), so this must involve some dis-assembly. But Richard is very good at designing multi-hulls that dis-assemble easily.
    Sailing Catamarans - Skoota 24 trailable centre cockpit weekender https://www.sailingcatamarans.com/index.php/designs-2/6-powercats/263-skoota-24
     
    Last edited: Mar 16, 2022
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  5. ziper1221
    Joined: May 2018
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    Location: florida

    ziper1221 Junior Member

    How will this differ from a pontoon with custom superstructure, or from a small catamaran houseboat?
     
  6. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    IMG_20220316_204319564.jpg
    I canoed the back of the hull. It will be even easier to build.
    With 15" of draft, displacement is 5616 lbs. I think I did it correct, volume of hull in water (ft cubed) x weight of water (62.4 lbs/ ft cubed).
    The 2 hulls will share bulk heads. A bulk head at every 2 to 4 feet, can't decide. With 1" flat stock aluminum stringers. Plan is to use .125 aluminum, 5083 or 5086, for everything. The bottom of the cabin floor will be 27 inches above the water. Should that be high enough to reduce slapping in Gulf waters?
     
    Last edited: Mar 16, 2022
  7. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    Good question, do you think my hull shape will be a more efficient displacement hull shape than a round pontoon hull with a snubby front nose? My guess is yes, but I am not a Naval Architect. What I do know is that a problem with a pontoon hull shape is that once the water line is half way submerged, so 12.5" on a 25" diameter pontoon, you start loosing buoyancy per 1" of draft. Also I am trying to get the floor higher off the water than a pontoon boat. Luckily my wife and I are perfectly happy with a tent size cabin, so the structure above the hulls shouldn't make the CofG to high.
     
  8. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    So you want to cruise around the keys. Florida bay on the west side of the key chain is skinny water but has lots of interesting things to see and do. The east side is not so friendly in marginal or bad weather. That is the Atlantic side.

    The sketch you have done needs some help. As Bajansailor has politely implied, you have not built a safe enough reserve displacement into the boat. He has also given some very sound advice, which is, build a monohull rather than a multi. A mono will serve your purpose better in that size range.

    Your "wave piercing" bow design will give you a very wet topsides if you get into much more than calm water. Even dangerous in a following sea as you have drawn it. And as suggested by others, it will not steer well in any kind of sea state.

    There are some very experienced and highly competent members here Willy13. They are almost always honest and helpful. Please abandon the notion of designing you own boat. You may be an intelligent and educated person but your first attempt at boat design reveals that you are insufficiently expert in that kind of endeavor.

    You could actually go to the Bahamas in a 23 -24 foot boat. But it had damned well better be a stout well designed boat if you get into some weather. For example if a northeaster approaches while you are crossing the Gulf stream you are in for a day you will remember forever, if you are lucky enough and a skillful enough seaman to survive.

    With all that I do hope that you will build a boat and enjoy some wonderful times in the Florida Key areas. Just stay on the west side and find your way up he intercoastal waterways to Miami or around the west southern tip of the bay of Florida and up the coast to Fort Myers or other nice place to visit.. Not to suggest that there might not be some challenges in terms of weather in Florida Bay.

    Stay safe. Get the plans for a proven professional design and build it.

    Best of luck to you
     
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  9. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Willy13,

    I can see what you're trying to do here, I get it.
    I've experimented with similar ideas in disposable, rebuildable wood for racing hull sea trials.
    The difficulty in turning was ridiculous.
    The 15's would help, but hitting reverse on one, and full throttle on the other, at speed, is not pretty, or overly effective.

    All the other points raised are valid. Sorry

    It can't be worded delicately, Bajansailor came close, it's a terrible idea, given the circumstances.
    Go with the advice offered.

    I know it's counterintuitive given the Forum name and all.
    Go with a proven design, in aluminum, and enjoy your sea-time after that long trailer drive.
    Please.
     
    Last edited: Mar 17, 2022
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  10. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    I now understand the issue with the wave piercing bow, and honestly thought it may be an issue, so the bridgedeck structure and cabin is set back 5 ft from the bow. I thought that there would be enough buoyancy to pop the bow back up before the water slapped the bridgedeck structure with it set back and at only 7 mph. The reason I tried to make this design work is because I thought it would be more efficient than a planning catamaran hull. There are plenty of old aprox. 8ft wide, 23ft long, planning catamaran hulls available with bad motors, that I could throw 2 small outboards on and slowly explore the keys. And maybe thats what I will do, but I am worried about fuel economy with such a setup. I do not understand the recommendation to go with a monohull of the same width when I thought a catamaran of the same width is more stable in the water due to having twin hulls and thus a center of buoyancy for each hull? I also don't understand the reserve buoyancy comment. I doubled checked my numbers and the most draft would be 15" with 27" between the water and bottom of the bridgedeck structure. Certainly at some point the distance between the center of cravity and center of buoyancy may become a stability issue, not sure when that is, but we do not plan on having a tall cabin, and will be storing a lot of water and gasoline as low as possible in the twin hulls. So if we changed the bow to a more traditional planning catamaran bow, is there still no reason to go forward with this design?
     
  11. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Let's see this through with an open mind.

    Please show, if even simply a sketch, your lay out, preferably to-scale, three-view (front, side, top).
    Your tank arrangements and all loading (ex. batteries, engines, oil, provisions, safety gear, fenders, crew, guests and their gear, fuel, water, blackwater, hulls, cleats, docklines, etc).
    Looking for total, worst-case scenario here: wiring, lights, fire extinguishers, dogs, cats, everything. ( flooding? )

    You'll want to round, or somehow crown your hull tops for easier re-floating and water shedding (added buoyancy) once submerged.

    And let's clear up this rookie mistake right away: planing vs planning.
     
    Last edited: Mar 17, 2022
  12. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    https://www.boatdesign.net/gallery/a0-sport-lounge-boat.7613/full?d=1493444241
    This is weird, I log back on and this picture is on the front page. It looks similar to what I want to do but not fast, slow and steady. Did this boat ever get built or did it remain a concept only?

    BlueBell, I can get more sketches if there is still hope, lol, I only included pictures that related to the specific question I had.
     
  13. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    I will entertain your curiosity.
    It's your call.
    What was the question?
     
  14. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    I wanted to know if my design would start planing off if I had enough HP to push the speed past the Fraud Hull Speed equation of 7mph. To keep a pleasant ride I was hoping I could hit 10mph without planing. I thought that a wave piercing hull of that shape might allow the hull to remain displacement longer.
     

  15. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    You have answered yourself very well here. If you are just pottering about at 6 knots or so, one of these old GRP cat hulls will work fine for what you want to do.
    Ok, it might have a bit more resistance than a well designed displacement hull (like the Skoota) but it probably would not have more resistance that the hulls you have drawn.
    As you say, buy one of these (hopefully for a song), throw on a couple of 20 hp O/B motors and you are away, for a fraction of the cost of buying all the materials and welding consumables to build your own boat.
    And it will be a lot more seaworthy than your design as well.

    All else being equal, yes, a 25' x 8' cat should be more stable than a 25' x 8' monohull - but this comes at the price of having less cargo carrying capacity. And your average 25' x 8' monohull will be fairly stable anyway. I think your weight estimates are probably very optimistic - it is very easy to add weight to a boat, and very difficult to get it off again (same as with many people :) ) And even for simple cruising, you will be amazed at how your weight adds up.

    OK - 15" + 27" = 42"
    The overall depth of the hull is 48".
    This implies that your bridgedeck depth is 6" - or am I missing something here?

    The version with the canoe stern is not going to plane.
    The version with the broad stern will plane, but you will be losing your fillings and teeth when you start going over tiny waves at speed (ok, this is a bit of an exaggeration, but any rectangular box will pound when planing).
    Ergo, you need a hull form that is best suited for the conditions that you are likely to experience.
    Ask yourself why you have not seen any hulls like your design proposal around - if it was a good idea, somebody else would have built it by now.
    How about starting with something like a Skoota, and adapting it for aluminium construction, if you are adamant that this is the material of choice for construction?
    Or, how about seeing if folk on here can join you in a collaborative design exercise to design this boat (but be aware of too many cooks can wreak havoc to what is cooking). Anybody interested? :)
     
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