What hull will be most efficient between 8-12 kts?

[fallguy]

I could not afford to transport a catamaran from Minneapolis to San Diego. I had a bid of 55k. He said he was going to tip the boat, so not sure there is an issue with the cost for him.

If you do build this boat, please sea trial it well before setting out. Include a few runs out of surf.

I do think some sage advice has come from Mr E on engines. A couple of 9.9s is probably really best. Electrics get you a mile or 3 for a lot of battery weight.

Also, be careful around the rotational issues that present in cats. The beam sockets cannot be made from 2# pour foam; for one, but you seem to have some understanding here. I am nervous my beams should have been set in upside down for same reasons.

 

[dustman]

True, but not many survived with the length-displacement ratio you propose. Traditional vessels are what worked in their particular situation...not what didn't. As I have said before, there is a reason the Polynesians didn't discover the "Stormy". Tread carefully here.

True, but I'm not crossing oceans. People have crossed the Atlantic on what amount to beach cats that were smaller and less stable than what I'm proposing. The sea states are tame in the bahamas almost all the time. There are hundreds if not thousands of much less seaworthy boats buzzing around there every day.

 

[dustman]

Its a long way from Arizona to Florida, you really would spend a lot getting an over-size boat transported, I think you need to scratch that from the list, considering the budgetary constraints. Some kind of modular construction might allow a boat that is a good beam on the water, and really you do need beam with a slowish cat, if keeping it easily driven is paramount. Bonus a more stable boat.

I would be going from here to Corpus Christi, TX. A 24' x 12' cat can be mounted sideways on a car hauler with 18" deck and meet the regulations.

 

[dustman]

I could not afford to transport a catamaran from Minneapolis to San Diego. I had a bid of 55k. He said he was going to tip the boat, so not sure there is an issue with the cost for him.

If you do build this boat, please sea trial it well before setting out. Include a few runs out of surf.

I do think some sage advice has come from Mr E on engines. A couple of 9.9s is probably really best. Electrics get you a mile or 3 for a lot of battery weight.

Also, be careful around the rotational issues that present in cats. The beam sockets cannot be made from 2# pour foam; for one, but you seem to have some understanding here. I am nervous my beams should have been set in upside down for same reasons.

At 12' beam I can transport on its side on a car hauler without permit. I have access to a vehicle and trailer for this.

I'll be putting in at Corpus Christy, TX. Will be cruising around the bay first and poking out into the gulf to see how it behaves in the waves. Going as far east as possible using the intracoastal waterways, and otherwise will be sticking close to shore and ducking in when I need to.

I'll have 5kw of battery and 1kw of solar which should produce 4-5kw daily average. 9kw gives me around 60 miles of range at 3-4mph. Plus I will have a small sail. A 5kw tesla module weighs 55lbs.

The loads will be transferred to the foam by the aluminum spine within which is attached to the crossmembers, there will be no point loading on the foam or fiberglass skin. More worried about beam on wave impacts breaking off the front tip of the hull because the spine can't extend past a certain point where the hull is too narrow, but will manage that by intentional application of fiberglass.

 

[dustman]

If you have actually followed my threads on this, you will see that the structure weight for a cat does not follow the scale factor cubed, but the scale factor to the fifth. Even something as simple as a tramp Hobie gains weight to the fifth on length. Subtracting out the 200-300 lbs for engine and fuel does not leave anything for habitability. You need to do a detailed weight study.

You can be rest assured that I'll be doing a detailed weight study, down to the last fork.

 

[Mr Efficiency]

Your construction details seem eerily similar to one that was talked about on a thread, where a fella made a metal mesh ferrule, sprayed the foam on to it, then sanded it to the shape he wanted (sounds easy, but would be very difficult to get a fair result ) and finally glassed over that. Unorthodox, and really not sound, structurally, lightweight PU foam cannot be considered a structural element, but you are making it one.

 

[dustman]

Your construction details seem eerily similar to one that was talked about on a thread, where a fella made a metal mesh ferrule, sprayed the foam on to it, then sanded it to the shape he wanted (sounds easy, but would be very difficult to get a fair result ) and finally glassed over that. Unorthodox, and really not sound, structurally, lightweight PU foam cannot be considered a structural element, but you are making it one.

He did it the hard way. You just aren't envisioning it. And the foam is not really a structural element, the 4" square aluminum tube inside it is the structural element. The load on any particular part of the foam will be very small. The fair result will be easy with the tool and jig I will be making, and that kind of foam is easy to sand. The shaping in one of the dimensions will already be done by the form. Artists use it to make sculptures, granted they use the 6lb so they can get fine detail. It's nothing like the eps foam you buy in home depot. Regardless, there will be plenty of experimentation prior to building.

I'd be interested to see that guys project if you can point me to it. Did he have problems with it? The problem with that approach is that you are then relying on the fiberglass to spread the load over the foam, and I imagine making the attachment to the mounting points was an issue.

Just so y'all know, I have been building and fixing things for a living for the past 20 years, so not totally talking out of my ***.

 

[fallguy]

@dustman

Have you considered a trimaran? The amas could be made really cheaply.

At 12' beam I can transport on its side on a car hauler without permit. I have access to a vehicle and trailer for this.

I'll be putting in at Corpus Christy, TX. Will be cruising around the bay first and poking out into the gulf to see how it behaves in the waves. Going as far east as possible using the intracoastal waterways, and otherwise will be sticking close to shore and ducking in when I need to.

I'll have 5kw of battery and 1kw of solar which should produce 4-5kw daily average. 9kw gives me around 60 miles of range at 3-4mph. Plus I will have a small sail. A 5kw tesla module weighs 55lbs.

The loads will be transferred to the foam by the aluminum spine within which is attached to the crossmembers, there will be no point loading on the foam or fiberglass skin. More worried about beam on wave impacts breaking off the front tip of the hull because the spine can't extend past a certain point where the hull is too narrow, but will manage that by intentional application of fiberglass.

the foam can't take the loads unless it has enough strength...most of the time higher loads require heavier foams...

I may be living in Corpus Christi by the time you launch.

 

[Mr Efficiency]

The way I understood it, the frame is not going to be attached to the GRP skin, there will be only PU foam in between ?

 

[Barry]

As I mentioned, and assuming your are going to have twin engines, outboards, the ability to run on one engine will be a significant range extender, at the cost of a little speed. What size engines do you have in mind ? You really need high-thrust engines for this speed range, and there does seem to be a bit of a gap between 9.9 and 40 hp with their availability, and 40 hp is way too much here

Yamaha has a High Thrust 25hp that fills some of the gap between 9.9 and 40hp

 

[Mr Efficiency]

Unless I am reading the specs wrongly, the 25 high thrust has a 2.08:1 gear ratio, which doesn't sound ideal

 

[Barry]

Unless I am reading the specs wrongly, the 25 high thrust has a 2.08:1 gear ratio, which doesn't sound ideal

My understanding is that the ratio is numerically higher and the prop diameter is larger as well. Yamaha was pushing their new 25, regular thrust outboard as lightest in its class, different ratio and standard diameter prop

 

[Mr Efficiency]

I have seen it stated as 2.08 in several places, in the high thrust, which does come in 25" leg too, they may be running more blades and less pitch and the reverse exhaust deflector, if it was a bigger reduction it certainly would be interesting. The only way that would work is if they were using the gearbox and props off say a 40hp, but with the bigger reduction, I am quite happy to be pointed to a source that shows it isn't 2.08

 

[Barry]

I have seen it stated as 2.08 in several places, in the high thrust, which does come in 25" leg too, they may be running more blades and less pitch and the reverse exhaust deflector, if it was a bigger reduction it certainly would be interesting. The only way that would work is if they were using the gearbox and props off say a 40hp, but with the bigger reduction, I am quite happy to be pointed to a source that shows it isn't 2.08

T25 High Thrust - Yamaha Motor Canada https://www.yamaha-motor.ca/en/water/outboard-motor/high-thrust/t25-high-thrust/T25-High-Thrust
Above is a link to a Yamaha manufactures page that shows the ratio at 2.42. There are other sites that show the 2.08. Certainly some confusion

 

[Mr Efficiency]

Thanks Barry, 2.42 sounds much better, but these Yamaha products websites are a shocker, I can't get much out of them.

Edit: I chanced of the relevant detail on the clunky website
29/12 (2.42:1)
Not a fuel injected engine, some other brands in that power range are these days I think