Flats Skiff Catamaran Design Input wanted?

[Baywolf]

I'm a small builder, I like bay boats and skiffs, I found this little number in a pile of old papers and I think I'll finally get to refining it and building a plug. It is an extremely light, really simple effiecient design. It will likely measure 22' long, 8'6" beam, and use a 140-175 hp. The mid section on the tunnel are slight, about only a 1" height by whatever width at that point, and the gunwales aren't shown, but I wanted to see what reaction it would get and maybe some ideas, so the design doesn't look so 80's, lol....

My oldie is online at Baywolf Boats website.

[Baywolf]

:P Hers's another one, but I think it's asking too much, It might prove to be too far ahead for it's time...lol...

Actually this one is being molded already, the plug, ah, the plug, "the heart and soul of all your efforts, into something that you cannot fish off of", so sad, but necessary.

[Baywolf]

Here is one that toys with SS double quadrehedral, channels a high percentage of the water to the prop at low speeds to get a higher level of reponse in shallow water hole shot condition.

[xboats]

Stunning! Love the outer shape of your 3rd drawing (posted today 11:33), with your 2nd drawings tunnel -- least drag.

Question: Wouldn't widening the tunnel in the mid section, cause cavitation ?

[Baywolf]

Xboat, your right, but once youre on plane the hull rises on a set of pads and the tunnel becomes obsolete, the tunnell will only be needed during the first 10 meters or so of launching in a hole shot, supposed to be a shot pressurized water trying to exit at the prop in a hard sand bottom situation.

[Baywolf]

I was giving an old bay boat desing, and injecting a tiny bit of sea-doo to see what might happen.

[Village_Idiot]

10m sounds like a long time to get out of the hole. A lot of folks who run flats boats in TX talk about getting up on plane within a boat length.

You might check out the cat/tunnel design in the Flatscat - it is engineered to deliver water to the prop while the prop is running above the surface of the water. This is done with a reverse angle on the sides of the tunnel (quite the opposite of what you see on most factory pocket tunnels). www.flatscat.com

Also, to combat ventilation problems, stick with a prop that has a large blade area (swept blades) and a lot of cupping. Several prop builders along the TX/LA coast specialize in these types of props. Several I can think of are Louie Baumann www.baumannprops.com and Powertech www.ptprop.com, as well as www.propgods.com.

When it comes to standard factory props, four-blade props generally work better on tunnel hulls because they deal with the ventilation issues better. With that being said, I just replaced a 4-blade Trophy Plus prop on a tunnel hull with a Baumann 3-blade Blaster prop and it made a world of difference in holeshot and overall performance. The Blaster is a much better prop for that particular application. I think the main difference is in the cupping.

[Baywolf]

Village, your right, a hole shot in a boat length, I have a 21' with custom cavitation plate that, (when you hit the throttle) the transom lunges up out of the water alot like a hoovercraft might, which means it can lift itself out in less than the boat length, but I meant, 10m to when it is fully plained, and the tunnel becomes un-necessary, or mostly skimming the top at 35-40mph or so. I calculated the amount of water in that tunnel in about the boat length, and was trying to pressurize it, then removing the drag after it was on plain, which would take about 10 meters, but the hole should be seen alomost instantly.

[Baywolf]

Someone said something on a different forum about batmans boat so drew this thing, note the tubes for the torpedos, hydrofoils, tunnel for shallow draft, retractable suspension system, retractable keel. You just need to picture it all with carbon fiber and nano's.

[kengrome]

10m sounds like a long time to get out of the hole. A lot of folks who run flats boats in TX talk about getting up on plane within a boat length.Can I ask a stupid question? Why it is so important to get up on plane in such a short distance? Is it because you need to be on plane to get the propeller high enough to avoid damage when running fast in shallow water?

[Village_Idiot]

Sometimes you just have a small area of deeper water, say 18 inches, maybe a half-acre or less, that the boat will run in, and jump up on plane in. There may be hundreds of acres of shallow water around you, say 10-12 inches deep. Your boat will run in 10 inches of water but needs 16 inches to get up on plane.

Sounds like an iffy scenario, but I've been in it often enough to make a quick holeshot well worth it to me. Having a boat jump up 'out of the hole' in a 20-foot distance or less is good insurance when running coastal flats or shallow rivers, where water depths of less than a foot may be the norm rather than the exception.

[kengrome]

Thanks Village_Idiot, now I understand (I think) -- but let me ask this *theoretical* question just to be sure I got it right ...

If I design a boat that doesn't need any more depth to get onto plane than it already drafts when sitting at a dead stop, this would be a "good thing" in terms of moving around in such shallow waters, right?

In other words, if my design were to draft only 4-6 inches of water while the engine is off, and if I could simply start the engine and accelerate from a dead stop until the boat eventually gets onto plane -- all in the same 4-6 inches of water -- then I might have a boat the shallow water fishermen along the Gulf Coast will appreciate.

Is this correct?

[Village_Idiot]

Yes, that is correct.

Many of those TX flats anglers are used to drifting in less than a foot of water and running in 8 inches or less, so there you have a target to shoot for.

FWIW, I run a 25-foot flat bottom tunnel hull with a 60-inch bottom width. It drafts 8 inches at rest, plus about 6 inches of prop and another 3-4 inches of skeg. When on plane in deep water, it probably drafts around 16 inches. However, when running up on a flat or submerged sandbar as speed, the compressed water raises the boat so that the bottom of the boat is almost level with the ambient surface of the water, leaving me with a "dynamic draft" of 8-10 inches of prop/skeg. If you've done much running between deep and shallow waters with a planing hull, you'll know what I'm talking about. Airboat pilots have much experience with this phenomenon - and it is why their boats perform much better in a few inches of water as opposed to deep water.

Good luck designing a planing hull that drifts as shallow as it runs. ;)

[kengrome]

Many of those TX flats anglers are used to drifting in less than a foot of water and running in 8 inches or less, so there you have a target to shoot for. Good luck designing a planing hull that drifts as shallow as it runs.

Actually I already designed it months ago, I was just hoping I had designed something that's actually useful and possibly also desirable ... :)

http://www.bagacayboatworks.com/linkfiles/flats15-67.jpg

This is a small, very light (probably 350 pounds with engine) 15' by 5' flats boat designed to be powered by an inboard engine installed beneath or in front of the center console (not shown yet). 10 HP should make it plane with a light load and 20 HP should make it plane fast(er).

The waterline shown in these drawings is 4 inches draft at 750 pounds displacement so there's 300-350 pounds of payload capacity in this boat before it starts drafting more than 4 inches -- and at 6 inches its displacement is 1250 pounds, so there's still enough capacity for several people plus their gear even at 6 inches draft.

The trick to this boat is that the prop is located completely above the bottom of the hull, so it doesn't draft any more whether the engine is running or not. It uses a surface drive propeller to take advantage of the fact that it will be running in aerated water sometimes too.

By keeping the prop above the bottom of the hull at all times it will never get damaged, nor will you ever have to worry about adjusting the prop or engine for different water depths. And since you can run the boat at full power in such shallow water with no risk of prop damage, there's no need for a hole shot any more. If you're drifting in 6 inches of water and you want to go somewhere else, all you do is start the engine and go!

:)

With no need for a hole shot there's no need for any excess "hole shot horsepower" either. This means the boat can use a MUCH smaller engine and still go everywhere any other flats boat can go ... but the guys who own these will save on fuel.

Here's a picture of how I superimposed a ZF Surface drive mechanism onto my design. I had actually created the design before I did this superimposition, so it was a pleasant surprise to learn that what I had designed so closely matches the ZF's dimensions:

http://www.bagacayboatworks.com/linkfiles/zf-flats.jpg

I'm not going to use a ZF drive of course, they cost too much and I can accomplish the same thing with a much lower cost traditional prop shaft in a tube.

The whole idea of this boat is economy -- not only during construction but while the boat is in use. That's why I spent a lot of time considering everything about this boat that I could think of. Things like avoiding the need for propeller adjustment by designing the hull to regulate the depth of the water hitting the prop means I can avoid the cost of an adjustable propulsion system. This saves money and keeps both production and retail prices low.

Inboard engines can be had for 1/4 the cost of outboards. They may not make the nice sound of a 4-stroke outboard but with a custom muffler they are no noisier ... and they can be fixed cheaply any lawnmower shop.

There is one really nice feature about this boat that does not exist on outboards, too. With the engine installed in the middle of the boat, the entire transom area is completely free from obstruction. This means you can walk all the way around the boat when fighting a fish and nothing will get in your way ... :)

Granted at only 15' by 5' this is not a very big boat, but I think it may be big enough for three people, and I'm sure it is big enough for two. I also designed a smaller 12' by 4' version although this may be pushing it in terms of what shallow water fishermen will tolerate as far as small size is concerned. Then again, a 12x4 version might be ideal for those who almost always go out on their own without a companion.

I lived in Texas before, and I know that some of the 'good ole boys' there are into power and speed ... so I'm not sure a 15x5 flats scooter with such a small engine would attract them even if it performs well. What do you think?

[FAST FRED]

What would keep the tunnel from feeding the prop air?

With the scoop setup you propose Air might be entrained at a very low speed and propulsion would cease when the prop is unable to get even one blade wet.

Sounds like time for another model test.

FF

[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. :D

[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.

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://zf-marine.com/Images/Gears/ZF10M.gif


http://www.zf-marine.com/ZFR/Transmissions/index.cfm?DS=Description&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.