Displacement Hull Power Catermaran

Not at the weights you're talking.
You seem to be concerned with fuel consumption, go with 9.9's or even 8's.
Or go with longer hulls to avoid planing and enjoy higher displacement "hull speeds".

BajanS once again gives good advice.

 

Yes, the bridgedeck connecting the two hulls will be 6" thick. I will weld 6" tall .125 aluminum flat stock at 45 deg angles to the bulk heads that are perpendicular to the length of the boat. Once the bottom and top aluminum sheet is attached it should provide enough torsional rigidity. A small aluminum cabin and helm will be on top of that. The span between the hulls will be only 4 ft 6" max. I suppose the cabin could be designed to be a structural support, but I didn't think it would be necessary.

 

Really? You are relying on a 6" deep aluminium sandwich to hold the hulls together AND provide enough torsional rigidity?

I think the only way this would work would be if the cabin is designed to be an integral part of the structure connecting the hulls together.

 

https://www.pinterest.com.au/pin/364017582352552785/
The catamarans beams in the link look to be some where between 5 to 8 inches in an oval shape. My bridgedeck will span 18 ft of the length of the boat essentially creating a structural 18ft x 8.5ft x .5ft beam. Plus my catamaran design is narrow. My aluminum sandwich of a beam will have a honeycomb effect that gives it strengh both in torsion and normal bending. The aluminum stringers in my sandwich beam will not be as abundant as plastics that use the honeycomb effect to achieve rigidity, but it will serve the same purpose. Pontoon boats connect there 2 hulls with 2.5" tall beams at 24" off center and then screw plywood on top to create a shear panel. They seem to be doing fine over the years. While I want to be safe I also do not want to over build due to weight.

 

Willy, for comparison, try drawing a midship section to scale of a typical pontoon boat (18" or 24" diameter tubes?), and compare it with a midship section sketch of your boat (with 4' deep hulls) at the same scale.
What do you notice?
Think about the possible bending moment at the joint of the hull and bridge deck - it will invariably be much greater on your 4' deep hulls than on a much smaller circular pontoon.
A good rule of thumb to use is 'if it does not look right, then it probably isn't right'.
Re-draw your midship section so that it 'looks right' visually (ie it does not look like it will break in half easily), and you are getting there.
Post it on here, and ask for comments.
Or, (better still) buy an existing boat, or a proven set of plans, or ask folk on here to collaborate with you on the design of a boat that would be fit for purpose for what you want to do.

 

My pontoon boat I built for water sports on a local lake has 25" diameter tubes and a 3" tall deck. This is pretty much the norm for the industry, I have no idea what safety factors were built into the design, I just copied it when I constructed it. The increased bending moment from the taller hull on my Florida boat will be almost twice as much, but I can easily more than double the Area Moment of Inertia of the deck beam to compensate for this added bending moment with out making the deck more than 6 inches thick.

That being said, looking at the beam dimensions of the Seawind 24 is a better option in my mind. The 3 beams look to be oval of a size around 5x7 inches. Further research shows that this is a proven design and has handled the pounding of crossing oceans.

 

Fantastic news!

Shouldn't that read Florida / Bahamas?
I see a bit of potential misdirection going on...
You are designing it for crossings, right?

Speaking of design, I think it time.
May we see the plan?
Would you like collaboration or are we done here?
That is, your question has been answered and you don't like the answer.

 

I am sensing attitude, did I do something wrong? Am I not allowed to disagree with the assessment that a 6" thick bridge deck is impossible? Misdirection?

As far as the plans, I am being told that my plans won't work, so I've been trying to figure out what I am missing regarding bridge deck strength. I need to keep the CofG as low as possible for an 8.5 ft beam and enough distance from the water to the deck that I don't get tunnel slap. So things are up in the air, lol... Also I am not looking for a "collaboration", I am just looking for answers that I can't find through research on the internet. So I am learning on my own and will ask again when I need help. In this case I wasn't sure if the hull shape I proposed would plane and become a rough ride if I pushed the speed to 10 mph, 3 mph past the Froude max hull speed of 7 mph for a 23 ft boat. I think the answer was, no, it would not plane and we would still have a nice ride.

On a side note, I have been looking for used boats for sale, damn prices are high. I even started looking for a used Seawind 24, lol. But I generally like projects so who knows....

 

Willy, we have been trying to politely offer suggestions re your design, but you do not appear to be taking these suggestions on board.

Have you come across the C-Dory Tom Cat?
https://www.c-dory.com/series/catamarans/
He is 25' x 8'6" - note that his hull form is very different to yours. There is a reason for this.
You would do well to take note of the design features of this vessel, and incorporate them into your boat.

Here is one for sale on Yachtworld
2022 C-Dory Tomcat 255 Power Catamaran for sale - YachtWorld https://www.yachtworld.com/yacht/2022-c--dory-tomcat-255-8156777/

Please be aware that even these damn high prices are most probably less than the cost of building a boat new, even if you do not factor in the cost of your own labour.

Here is a 23' Glacier Bay for sale in Florida for US$35,000 - you could always put smaller engines on something like this if desired, although you might be pleasantly surprised how fuel efficient these cats can be at higher speeds. Note that these Glacier Bay are displacement cats, not planing.
2001 Glacier Bay 2270 Isle Runner Power Catamaran for sale - YachtWorld https://www.yachtworld.com/yacht/2001-glacier-bay-2270-isle-runner-8245111/

Or a 26' Grady White Tiger Cat that is still 8'6" wide, with an asking price of US$23,000, including two elderly 2 stroke Johnson O/B motors.
1998 Grady-White F-26 Tigercat Power Catamaran for sale - YachtWorld https://www.yachtworld.com/yacht/1998-grady--white-f--26-tigercat-8174882/

Edit - this cat probably comes pretty close to what you have in mind - except that she is 3'6" too wide for trailering.
2011 Catamaran Power Catamaran Power Catamaran for sale - YachtWorld https://www.yachtworld.com/yacht/2011-catamaran-power-catamaran-7559756/

 

All the power to you willy13.
Build it.
Best of luck with your endeavour.

 

I have, that is why I have not posted any further plans of my build because I am trying to figure out why a Seawind 24 can have (3) 5" x 7" aluminum beams to support its 3.5 ft tall hulls at a 16 ft span. Yet you are telling me I need a 12" thick deck. My current plans require a 6" deck to get high enough off the water without causing CofG issues. I found the Seawind example after taking your suggestions to do more research. And I feel that this example is proof that I can make a 6" deck work. Yes, the Seawind 24 will probably be lighter, but its has twice the beam span.

THAT BEING SAID, lol, that Glacier Bay you posted is exactly what I have been looking for. So thank you for finding that. I will need to research this boat for sure. There are so few power catamarans with cabins. So thanks again!

 

Caution regarding storing gasoline below deck. We keep gas on our vessel for the motorcycle and chainsaws but it’s always stored above deck. A bit of fumes and the internal arc of a light switch can make your day more exciting than pleasant.

 

Willy13:

I did a little preliminary math.

Your hulls would have to draw maybe 18 inches of water to get approximately 5400 lbs of displacement. Remember. There are two of them.

I agree with most of the criticism of your hull design. The deep, flat bottom bow is likely to cause some chaotic turbulence, as some of the water that is supposed to follow the curve of the bow decides to go under it instead.

If it did this evenly, no problem. But that's not likely to happen. What is more likely to happen is that more will sneak under one side than the other. This will cause a lateral pressure difference which will cause the hull to turn.

Trying to correct for this will likely not work, as the pressure difference could quickly switch sides. And this situation would be worst in a following sea.

Far better to curve the bottom up at the bow to atleast the maximum load waterline. Even better to curve it up a few inches higher.

This curve should start at atleast 1/3 the hull length aft the stem.

The curve up to the transom should start atleast that far forward of the transom.

With just 30 hp total, there is no way you're going to get this beast to plane. So don't worry about that.

If you took the material, you cut off to get the upward curve, then welded it to the top, atleast at the bow, you would get a lot more local buoyancy there. And this would be a great help.

The last thing you want to happen is for one of the bows to dig into the back of a wave. With the greater local buoyancy there, this is far less likely to happen.

I don't think think you need all that much displacement. Since your boat is almost certain to never plane, it doesn't need the ultimate strength and rigidity of one that is.

I think you can whack off maybe 1,000 lbs, and have the hulls, deck, and cabin come in at say 2,500 to 3,000 lbs. The engines and fuel could add maybe another 600 lbs. This would allow you 40 to 50 gallons. This would give you atleast 112 miles of range with both engines running full throttle.

This would leave you with around 900 lbs for yourselves and your gear.

OK. That seems a bit skimpy, so maybe your total displacement goal should be closer to 5,000 lbs.

 

Thanks for the explanation sharpii2. I have moved away from the flat bottom and do appreciate the further expanation of possible down falls. Still not sold on needing a 12" thick deck.

I have started to look at the hull design of the Glacier Bay catamarans. They look to be true displacement hulls and this is confirmed through there own marketing and reviews of the boat, both proffesionally and owners. Even the people that did not like the feel of the boat said it was the softest ride, which cements my decision to stick with a displacement hull. The complaints seem to describe how a typical displacement pontoon boat handles that has enough power to push it past its theoretical efficient hull speed. I have owned such a boat and am fine with that. I couldnt find any fuel economy numbers on the 2270, but I found a performance bulletin on the bigger 2780 (27ft, 8800 lb, 300 hp total). It was getting 2.11 mpg at 17.7 mph at 3000 rpms. The only other data I could find was on displacement hull pontoon boats (no liftin strakes), they had smaller outboards, obviously singles, but were half the displacement (no gear, fuel,water). I could predict how much HP I would need for my heavier boat by finding the crouch constant for that style hull. But I wasn't sure how fuel economy would compare on a heavier boat. I guess it could be figured out once I determined the size of motors I would be using. Now that the hull shape has moved to a shape that has been proven at speeds above the froud maximum hull speed, I am looking at twin 50hp to get to a top speed in the the 15 to 18 mph range. I won't run that fast, my thinking is the outboards might be more effiecient at 3500 rpms all day rather than the upper range with twin 15 hp. My desired cruising speed (7 to 10 mph) has not changed.

The main thing currently keeping me from buying a Glacier Bay 2270 is the weight. It has a trailer weight of 6900 lbs. The hull alone weighs 3600 lbs and I figured an aluminum version would be 1000 lbs lighter. The other is wood rot. While I can certainly learn how to work with fiberglass, I already have metal working skills.

I am going to assume that a combination of the economic crash of 2008 and the obsession with fast cats killed the business, because they seem like nice well built boats?

 

Have you tried copying the basic Glacier Bay cat hull design, and adapting it for aluminium?
The shape is proven, and it is much better than what you have shown of your design so far.
Have you done a fairly detailed weight estimate for an aluminium version, re your assertion that it would be 1,000 lighter than a fibreglass Glacier Bay?

World Cat took over Glacier Bay in 2009, and they were producing some of the Glacier Bay cats for a while, but these seem to have been discontinued now.
Our History - World Cat https://worldcat.com/advantages/history/