How much power

Kerosene. I've dived in Belize and Mexico. I know the boats. I've made the same kind of boats in Playa del Carmen for dive shops. The Belizean shops know their job, and start from a polyester mexican panga, a very forgiving design in matter of center of gravity, they do not design the waterlines nor the hulls. They do only the diving amenities on bare hulls bought to IMEMSA. One exception was the late Carlos Cruz from Sarteneja, able to build a whole boat, a sailing traditional fishing boats, classical runaboats and more modern. He used plans given to him by a an american guy and also copied several boats. I'm afraid he was the last survivor of a gone era.

 

The comparison is flawed.
They were not NA with diploma and certificates, but they were NA with an enormous corpus of tradition, heritage and lots and lots of experience of sailing and building. They learnt in the hard way, if the boat was really bad everybody was drowned. The Australian NA was an idiot, you do not enter in such a race of bancas without a deep experience of these boats, and a big bunch of races for learning. The guys know their boats and how to drive it....
That's the difference with most of the rookies in this forum; experience and tradition in a kind of boats.
I have worked with several builders of wooden fishing boats. No formal education, just primary school. All had the following common traits. All had good experience of navigation and fishing. Some were fans of regatta on sail boats, Tertu was a formidable tactician. They made their apprenticeship in excellent traditional shipyards and learnt deeply their trade, a result of hundreds of years of tradition, empiricism and observation. All were extremely intelligent and able to learn. A guy was considered as able naval carpenter after 4 years of apprenticeship, and after 4 to 6 years in a shipyard, taking more and more responsibilities. That means 8 to 10 years of formation. Plus a good eye and capability of integration of several factors in their kind of boat, knowing perfectly the effect of a minimal change in the dimensions. They had built dozens of similar boats. All knew very well the limits of their domain of expertise, and did not played outside. Alexandre Tertu was able to design and build a 39 meters 128 feet 700 metric tons fishing boat in wood, but he always refused to build a sailboat for a customer without the plans of a NA specialist in yachts in classic wood...And he did not hesitate to hire a NA or a NE for specific problems.
None played with an innovative motor trimaran, or motor multis...
I have no doubt you'll be able to design a motor trimaran after some experience on multis sail and motor, and tons of theoretical learning. Because for such a boat you'll need to make a lot of calculations, as there is not a long tradition and empiric formula. These little beasts are delicate and sensitive. That does not need high mathematics, just a good secondary level in calculus. You'll laugh of your first drawings. And you'll do not need to go in a forum and ask "How much power"...
Meantime if you want to go fast and be sure of the results as you are risking your hard earned money, better to go for an experienced NA in the kind of boats you want.

 

Ilan Voyager said:

"I saw in Russia, in 1991, a small mono on foils, 40 feet 4 tons 240 HP Volvo 38 knots top speed, 32 knots cruising. Able to cruise at 28-30 knots in 3 feet waves. That was a proto nicely built in alu, foils in titanium. A true flying carpet, very smooth."

Hello Ilan,

I have ridden in one of these boats here in Canada!
Exactly as you describe.

Two of them were here for the summer of 1998 trying to get into whale watching.
A 3 foot wave was all they could handle with 7 people on board (10 - 12 seats total).
More than 3' and the wave would hit the hull dropping it fast, inducing huge drag.
Then, a 4 foot wave came through my front right window, shattering it!
20 gallons on board. We headed to the marina immediately.
I never road in one again.
The next summer they were gone after not doing any business.

They were dark green and only had forward foils, the V-stern stayed in the water
with the Volvo-Penta, inboard/outboard leg.

Crazy fast but not suitable around here with all the logs in the water, BANG...
Blub - blub - blub...

On calm waters they would have been great, but the day I went out, it was too rough.
Another reason why they couldn't do any business here, we can get weather here,
even in the summer.

Ahhhh, thanks for the trip down Memory Lane.
Sorry for the thread-drift.

 

That is incredibly light for a 46' x 16' power boat ! I cannot imagine the motion of that boat, at that weight, at the quoted speed, in any sort of sea. The word unnerving comes to mind.

 

BlueBell. Thanks for your post. That was long time ago. The boats you're talking were fluvial boats for the Volga River like the big ones designed in the 60ties. I'm amazed that some guy took this model to Canada as these boats are absolutely not made for sea and less for whale watching.
The foiler I evoked had foils front and aft and maybe was a development. It had a Volvo but with a rather complex custom transmission on the aft foil. It was designed for rivers, lakes and an eventual escape on the Black Sea. The limit of 38 knots comes from the foils profiles which begin to cavitate after 40-42 knots. But on the Volga River that was a astonishing flying carpet.

 

Hi Kerosene,
people are too cynical nowadays. Defeats the purpose of having a forum. I could slap a 200HP motor for testing as I have a spare one and see( but was expecting responses from "more knowledged" individuals). Personally, I wouldn't want a world where everything is bought from a catalog. I encourage my kids to think outside the box and be creative. Push the envelope but learn from experience. Consider carefully advise from wisdom derived from experience.
But nonetheless thank all for their input.

 

In any event, a super skinny boat is not the last word in seaworthiness, why not build a cat and get something that has the best basic form for high stability and seakindliness , in a lightweight boat, the snag being the need for twin engines.

 

Wanna build efficienct ( Single engine) boat. Trimaran fits the bill

 

You know, Standman, if you go box hull (square cross-section versus round) you can approach 20:1 vs 16:1 for the same displacement.
Either way, I'll bet as little as 100HP inboard diesel would give you 20 knots.

You could simulate 80% diesel engine weight and sea-trial varying HP outboards.
Or, just go with your 150HP diesel and see what happens, I don't think you'd be disappointed.
Stabilized monohulls are good at almost any speed, there is no "hump" to speak of...

Why a diesel?

 

Hi IV...how ya doing?

Fully concur with your analysis, and as always those with a "dream/vision" have that but little else, as they are guided by emotions, not facts.
There are no emotions in engineering!

Just a quick look through this thread, there are major glaring omissions. Such as 1) no SOR and 2) What he's drawn is a Trimaran, which is poles apart from a stabilised monohull.
He's just playing with shapes, which anyone with a pencil and paper can do, but what it represents and what it is "designed" to achieve (the SOR) is light years away from his mind set. Thus never the twain shall meet. Just my dog is bigger than your dog pointless argument - hardly an engineering debate!

Had so many of these type of threads on this forum it becomes tiresome. Why seek advice (on a project that is lacking data/information on so many levels) if you're unwilling to listen except to only those that blow smoke up your backside!
Same old MO...

 

I don't think so. He says 5000 kg, Do you think a 10,000 kg cat of the same length would get 20 knots with 2 x 100 hp ? Not even close, as a cruise speed. More like 2 x 200, I would say.

 

What about... frustration?

 

Hi BB,
The box hull would make the build much, much easier. I chose a round or some dead rise for added structural strength but since you brought that up, ease of build might trump it and just add sort of a kiln. So thinking about your box hull, I had a small motor ( Honda) and a jet pump that came out of a 700cc jetski. Laminated some Biax and made a long skinny hull. I knew the motor was underpowered for the jet pump but, why not give it a try. I took a small PVC pipe ( The kind for house plumbing) roughed up the inside with sand paper, put WR800 and csm 300, wet it and inserted it in a larger diameter pvc pipe for use as AMA arm. It was surprisingly strong that we could lift entire boat with it. I thought it would be a fun boat for my 12 yr old son. So I took it out for a sea trial and it went much faster than i thought. i didnt have my son drive it yet as it go far sooo quick that i would have a hard time getting to him if he got into any trouble didn't have time to lower my boat as it was getting dark. Estimate it exceeds 15 knots. Will take more data soon .
diesel because I have an existing engine and diesel fuel is costs about 20% less than gasoline here

 

Hi, I managed to build her. However, I can only manage 10+ knots on a 160HP Diesel. I do notice quite a bit of cavitation. I've attached a pic of the shape where the prop is and prop is position below the bottom most part of hull angled at 8 degrees and 70% of it (24" dia) is below the hull. Hoping to find out if hull shape or the prop design is causing the cavitation.

 

It is a little hard to tell from the pics, but the taper of that under body aft, does seem abrupt, unless I am looking at it the wrong way. That would create a lot of resistance, do you have a drawing of that area, or more pictures ?