My Small Bluewater Trimaran

I don't think you're right about the "very high waterplane area", it is ridiculously deep draft for a 1200 lbs boat, I think he quoted 3.5 feet. The waterplane area would in fact be less than a typical boat of that displacement. All of which is academic anyway, the practical difficulty of moving it around with a tolerable degree of efficiency seems insurmountable.

 

Bizarre!

I think any messages after post 17 are pointless

Richard Woods

 

The surprising thing is the OP proceeded to the stage of building something radical, that he clearly has not properly evaluated as a scale model, which would have been a cheap, fast way of establishing whether it had "legs", which is does literally, but not figuratively !

 

We haven't heard from the OP in a while. I hope all the negative replies haven't discouraged him from posting as it looks like he has already put quite a bit of time and money in his project and I for one am looking forward to seeing what will actually happen when his boat is put in the water. I'm not sure how much accurate data you could get from a scale model without some real sophisticated equipment. If you built a 1/6 scale model it would be 24" on each side of the deck triangle and the sides of the pyramid triangles would be 12". 1/6 of the proposed 1200lb displacement would be 200lbs which I'm pretty sure would sink the model so would just have to weight it to sink it about 7". Since one of the main concerns is that the boat will have considerable drag in the water and speed will be really limited so lets say you were going to use a 60 horse motor on the boat again you would have a real problem scaling this, you obviously couldn't use a 10 horse motor it would be way to heavy and too much power for the model. My opinion would be that unless you were a naval engineer with a whole lot of sophisticated equipment the model wouldn't really show you much and the OP says he has tested a model as much as possible. Prismatic, how is the project going and do you have an idea when your boat will make her maiden voyage. Good luck!

Dave T

 

He says he scale-modelled it and it survived a "bath-tub tsunami", which might have told something about its stability, but nothing about the drag involved in propelling it.

 

Dave T; The weight conversion for a model is scale factor cubed. In this case, one sixth scale. The cube of six is 216. Divide 1200 pounds by 216 to get 5.555 pounds which will be the scale weight of the model.

 

If the OP knew enough about mechanical similitude to properly scale the bathtub toy, he'd have likely also consider other, plainly obvious things in the development of this craft (loosely held term). He'll be very lucky to make 5 MPH with this ill conceived contrivance.

 

From tthe 'for sale' ad

"professionally designed and built by Aluminium Marine Brisbane. "

Oooh - I wonder if they know their name is now mud !!!

 

Not many 30 foot motorsailers about with twin 90hp outboards !

 

or add-on pontoons to make sure it doesnt sink.

 

Read this: www.cruisersforum.com/forums/f48/beaus-boat-9093.html
Those "pontoons" were the design brief from the start - a trimaran with small amas. They demount and the weird shape is to fit the whole craft into a shipping container.

 

In the owner's words..............

"I ended up buying two Honda 30 Hp outboard which will give me a 10 knot cruise speed with one motor and 20 knots with both motors."

It seems the correct response to that should have been, "dream on", the claimed speed is now 15 knots with twin 90's, which is a major revision !

 

Ah, the mystery is explained

Also, 'professionally designed' seems like a bit of a sales pitch in the sale add.

To quote the owner "I had a lot of difficulty convincing the structural engineer and the boat builder that i had to have FLAT surfaces. ."
That and I bet a lot more arguments'

I wonder which 'professional designer' will put up his hand up for this one.

 

This is a troll king thread me thinks.... the OP is actually welding up a buoy.... we've all been tricked... 10ft and blue water are mutually exclusive...

 

Well, it could always be used as a drogue, wouldn't matter where you attached the line, the drag would be tremendous !