Flats Skiff Catamaran Design Input wanted?

[kengrome]

What would keep the tunnel from feeding the prop air?

Ah, you want the details! There will be plastic pipes at the top of the tunnel directing much of the air-filled water out the other ends of these pipes. This won't extract all the air-filled water but it will help.

Yes, a test is in order.

[Village_Idiot]

If the performance is there, I'm sure it will find a market. By performance, I'm not talking speed and power, I'm talking cruising performance, accessibility (launchability) and fishability. Do you think you could cruise at 15-20mph with a 20-25hp engine? 20mph is fast enough for most people if the boat will get them where they want to go in a reasonable amount of time, especially if they can get over 5mpg while doing so. If it won't go over 10mph, I don't think many folks would bother with it. I'm sure many anglers would give their eye teeth for a small economical boat that would run shallow at a decent speed and that they could launch from almost anywhere. Witness the success of the Gheenoe despite its strange name, looks and (lack of) marketing.

I like the idea of the engine under the console. I saw many tender boats built like that down in Tahiti.

I think the biggest issue in your design is modulating the proper water/air mixture going to the prop, especially as the boat increases speed. A little too much air and you end up with too much ventilation and no thrust...

[kengrome]

I think the biggest issue in your design is modulating the proper water/air mixture going to the prop, especially as the boat increases speed. A little too much air and you end up with too much ventilation and no thrust...

If the rest of the concept is reasonable I can always re-design the forward / amidships section of the tunnel so it will force air out the sides of the hull instead of entrapping it and funneling it toward the propeller. This should eliminate the issues you and Fred have mentioned with aerated water. I would still use the surface drive prop of course, because that's a requirement of the design when half the prop is out of the water -- but at least the water getting to the lower half of the prop will no longer have air in it.

[kengrome]

Okay, I got rid of the funnel-shaped bottom on my other boat. This new version pushes aerated water away from the centerline instead of funneling it into the tunnel and propeller. This should eliminate the concern you guys expressed regarding aerated water in the tunnel and propeller.

This is a smaller version, only 12 feet long by 4 feet wide. I used these dimensions for two reasons, first because of fewer seams during construction so the boats should be cheaper to build, and second because this boat will be more efficient (or faster) with the same power since it is 25% narrower than the previous 5 foot wide version.

The drive system is exactly the same since that's what makes a boat like this unique. I'm guesing the boat will weigh 200-300 pounds including the engine. The waterlines shown are 4 inches at 485 pounds, and 6 inches at 850 pounds, so it's really a one-person boat that can haul two people if it has to, but it will be faster and more fun with only one aboard.

My concept is to put a fold-up seat in front of the console (or just a cooler) and a leaning post behind. With two people on board the captain will stand between the console and the leaning post, and the passenger will sit on the folding seat or cooler in front of the console.

Obviously it's not a rough water boat but it has an 18 inch bow height so it should do okay in a chop that's not too big. Figuring 50 pounds per HP this boat would plane with only 10 HP when lightly loaded, and there's a cheap 10 HP air-cooled China diesel I'd like to try on the boat just to see how it works. It may just have enough power to make the boat plane with only one person on board.

[Baywolf]

Here's a little rig, fixing to be know all around, as the Baywolf Boats SWA, a hard sand bottom will little more than an afterthought, each time time you "hit it", you'll fly, sand, mud, chop, doesn't matter, she's gone.

[rossiroller]

your first drawing is awesome and strangely reminiscent of the mach ?10? in the movie speed racer

[Ilan Voyager]

I'm open mouth...very clever solutions for a difficult problem. I didn't know anything about this kind of boats and I'm enjoying this thread. Thanks for the very interesting links. Gives me some ideas for beaching boats.

[srimes]

Hey Kengrome, what are you planning to use for reverse? I like the your idea for engine and surface drive, but don't know of cheap and availible transmissions.

[kengrome]

Hey Kengrome, what are you planning to use for reverse?

How about a paddle or some oars or a pole?



A transmission would add $1000 to the cost ... and for what? So the boat can cost $5000 instead of only $4000? Or maybe so the captain can back up without using his own muscles?

My goal was to design a very cheap, light boat -- one that can avoid unnecessary frills that production boats normally come with. Let's not forget the fact that this particular model is only 12' long and less than 300 pounds including the engine ... so it's not like it's going to be difficult to push it with a pole or oars in the few situations where 'going backwards' is desirable.

Sometimes I think of having reverse on a boat like having reverse on a motorcycle. It may be a convenience on the big ones, but it is certainly far from being a necessity.

[srimes]

Ok, that's what I was thinking too. Just wondering if you had a trick up your sleave for a larger boat.

Originally Posted by kengrome

How about a paddle or some oars or a pole?



A transmission would add $1000 to the cost ... and for what? So the boat can cost $5000 instead of only $4000? Or maybe so the captain can back up without using his own muscles?

My goal was to design a very cheap, light boat -- one that can avoid unnecessary frills that production boats normally come with. Let's not forget the fact that this particular model is only 12' long and less than 300 pounds including the engine ... so it's not like it's going to be difficult to push it with a pole or oars in the few situations where 'going backwards' is desirable.

Sometimes I think of having reverse on a boat like having reverse on a motorcycle. It may be a convenience on the big ones, but it is certainly far from being a necessity.

[kengrome]

Just wondering if you had a trick up your sleave for a larger boat.

Since people are willing to pay a lot more for larger boats, I would simply use a F-N-R marine transmission in a larger model. The one I had planned to use for the larger version of this boat is the ZF 6 M:





http://www.zf-marine.com/ZFR/Transmi...tion&GN=ZF_6_M

The last time I checked, they cost about $700 in this country, but I think they cost about $1200 in the USA.