Overpowered? mini cat plans

[home made tunnel]

MY FIRST POST ! yay!

So cliff notes, I've been designing my dream boat for the past few months, need opinions if i'm overpowering...


mini cat / tunnel hull type
15'hull (bow to transom) 18" setback outboard on back. (extended transom pod)
7' 6" beam.

so boat will be about 16.5' overall length.

My plan was to run a 150, then i thought about it more and more and decided on a 200!!!!

i see 17' bass boats with 300hp engines all the time, so i should be good? :o

I also see little 13' mini cats with only 70hp... which makes me think mine will be overpowered. :mad:


GOAL is to do 80-90mph, and be able to handle mild chop. I'm building it like a brick shlThouse to handle some rough stuff, and a 200 horse on the back, but wondering if it'll be too much?

I can scan and upload my plans later this evening. Work is very strick with internet.

[home made tunnel]

Just an idea, my boat looks very similar to that member's on here who built his own tunnel/cat. His was black with 017 on the side in white letters? I believe he was from overseas, but regardless, just for a mental image, his boat is similar to mine, but mine's on a slightly smaller scale.

[Iowafshr]

Sounds cool. Wish i had something more to add, but I'm looking forward to seeing pictures.

[baja200merk]

Sounds like way too much setback for the size I would think the less setback you can get the better. I know mrdemenor on here runs in the low 80s in a 13ft beil cat with a stock 130hp yamaha and hes got 2in setback.

[MercNuts]

At the risk of raining on your parade I think you are giving yourself 2 possible scenarios. In one case you may create something that will not perform as you expect and the other, you may very well build a deathtrap. Tunnel boat design has been an evolving science over 40 years in the making. In the early days of 125 horsepower and 70 MPH speeds there were still blowovers. While it is now possible to run over 160 in a small outboard tunnel and stay rightside up the evolution did not take place overnight and not without a lot of pain and failure in the process. The dynamics of a tunnel boat have all kinds of important factors. They are very weight sensitive and also one of the most critical details is the combinations of tunnel depth, width and difference in height of tunnel from front to rear which directly influences air entrapment and likelyhood of blowover. The center of gravity is also important. A member here Jimboat is his screen name, has extensive experience and information on tunnel design, but you may not find it cheap enough to purchase for a one shot home project. GlenL, the boat plans seller also has a tunnel in their list of plans. It is completely obsolete by today's standards, both in terms of styling and function but you may find it a worthwhile starting point that could be modified to your preferences.

A blowover in a tunnel with speeds that a 200 hp outboard can produce can be a fatal mistake. There are also other things to consider such as gearcase design, prop type and so on. If I was interested in playing with a tunnel I would try and find an older professionally made race boat and start from there with rigging and setup. You can probably find an old one that is completely obsolete by todays race standards that would be relatively safe with the horsepower range you are contemplating and would save you a lot of time and money.

If you do not have any tunnel design experience I find it extremely unlikely that you will have a successful first attempt.

[Action Dave]

That's a whole lotta setback and horsepower for a home-brewed tunnel boat of that size. I would start with an engine much smaller than a 200 and play with that untill you get the kinks straightened out. I see one of two scenarios happening if you rig it up with a 200.

1: The boat will sink. With all that weight in the rear plus all the setback, it's gonna be hard to float it.

2: The boat will float but handle terribly and possibly be dangerous.

[home made tunnel]

MercNuts (http://www.screamandfly.com/member.php?2695-MercNuts): Excellent points. All of which are true, and i agree 100%.

I've got the latest edition of "Secrets of Tunnel Boat Design" that i bought from aeromarineresearch.com, and i've read it cover to cover a dozen times.

After reading that book, my mini cat plans have changed at least a hundred times as far as dimensions, chine angles, deadrise, planing pads, and overall dimensions.
If it weren't for the $400 pricetag, i'd of bought the design program for it too.

Just an FYI, my boat design is 90% cat, 10% tunnel. You'll see when i upload some pictures this evening.
I cant emphasize enough how much research i've done, so i'll understand what you're talking about with any advice you all give. So have fun with all the terminology, i'm all ears!


I have lots of boating experience, but this is my first educated guess at designing a boat from the ground up.

The most high performance boat i've had thus far was a 150hp merc on my 16' checkmate diplomat with a 6" wide running pad. best of about 58 gps in Narraganset Bay.


More food for thought.... The concept of this boat is based off a 5'x5' BOX! (the cabin space) to accomadate 3 adults sitting side by side. the rest of the boat is built around this.
Maybe i'll name her BOX-CAT.... or more entertaining... CAT BOX!!! hahaha


MercNuts and baja200merk, I look forward to hearing what you guys have to say when i upload the pics this evening. I really wasn't expecting any replies this fast!


I have seen the Glen-L designs, and oringinally was going to build a crackerbox, but it wouldn't be able to give me the ride and versatility of a boat i wanted with a big fat smallblock in the middle of the boat. I favor outboards, and cats/tunnels, so you could say the crackerbox was my inspiration to design my CAT BOX (litter box) haha.

[2.5-21]

Lets see some pictures! what are you using for build material?

[home made tunnel]

Lets see some pictures! what are you using for build material?

before dinnertime i'll have pics up haha.

and ply over frame is the build plan. Glassing all wetted and splash surfaces. sealed internals will just get some resin coatings.

From the 1st build, if all goes well, i'll be able to make some stencils for a stitch and glue method to build a 2nd hull.



If it performs like crap, i'll prob just use it to occupy my father's mooring in the harbour as a winter-stick haha.

[Coryc]

This sounds like a cool project. In regards to the 17' bass boats with 300's; They are v-hulls and are a lot heavier. They have a different center of gravity and a different ride. In my opinion 18" setback is a lot. especially considering the engine you were thinking of putting on it. If your design is somewhat similar to any other small cats then I would look at the setback and hp that they are using and start there. It's much better to put a 70hp engine on and see how it acts then put a 200 on and risk everything. Would love to see some pics!! Hope this helps.

[johnboy 88 vegas]

How exactly did you figure 18" of setbcack for a 400lb motor for this length boat? Like the others have said with a 70 or under I might could see it but a 200 is a blowover waitin to happen on a tunnel this length. Either I'd build the boat about 5' longer or remove about 14" of setback on this rig if your gonna run a 15" motor and bolt it to the transom. Just remember to compensate for setback if your gonna run an adjustable jackplate.

[Mr. Demeanor]

WAY too much setback.
Crazy crazy idea overpowering a little tunnel hull. :D

Buy this, hang a 200 and be done. Proven 95mph.
http://www.screamandfly.com/showthread.php?245215-Beil-Catamaran&highlight=Beil

[Mr. Demeanor]

Another problem with all that setback on a little cat. With a surfacing gearcase your going to get a lot of stern lift from the gearcase. As the bow begins to fly, the gearcase will be forced down. The further back it is from the transom, the further it moves downward with upward movement of the bow (think teeter totter).
When the gearcase goes under you lose your stern lift and the boat blows over without ever catching any wind.
I have experienced the mechanics of this with only two inches of setback and nearly went over.

[home made tunnel]

First, i'll show some pictures of my FIRST design, before i made a bunch of changes in dimensions.
It was 14', and 22* deadrise.


http://i1050.photobucket.com/albums/s405/turbobandit/Cat%20Box%20boat%20plans/originalBADplans.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat%20Box%20boat%20plans/originalBADframes.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat%20Box%20boat%20plans/model1frontisometric.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat%20Box%20boat%20plans/model1front.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat%20Box%20boat%20plans/model1rearisometric.jpg

[home made tunnel]

Second design... Widened the boat by 6", cabin included, deadrise to 16*, kept the tunnel height constant from Frame 3 to the rear, as opposed to decreasing, also added a foot in length behind the seat, and got some better measurements for the bow/deck curve.

Been thinking of putting the fuel tank up front, done know weather to put it there, or in back behind the seat.


ALSO.... AS FAR AS SETBACK.............. since this is the biggest concern... my original plan was because i did not want to have a splash well. i'm thinkin now that i'm going to have to regardless. :mad:
What else can you guys tell me about pro's / con's of setback?????
As to about how i came up with 18".... i came up with it as a ballpark number because i figured that's as much room as i would need without making a splash well ahead of the transom. I guess i did a bad doodle? heh heh.
Maybe a better solution would be to extend the back of the boat sides to equal the position/distances i have the motor mount area in relation to the seat? Meaning seat position and motor position stay, but extend the back of the boat to be equal to engine position... making the boat 16.5' total length?

Keep in mind, i want this to be more CAT than Tunnel. Even though it only has a 10" tunnel height


Anyway, here's my drawings / doodles:
My brain goes a million miles an hour, and i overlook things quite often.
Tell me what you think!!!!!!!!!

http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/profileFULL.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/TopViewFULL.png
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/frame1transom.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/frame2and3.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/Frame4.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/transom2.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/transom1.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/doodles.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/Cat Box boat plans/doodles2.jpg

[home made tunnel]

also, just to share, this was my old 16' checkmate w/ a 150 merc towed by my s10 with a corvette engine swap, and a big ol t70 turbocharger.
http://a2.sphotos.ak.fbcdn.net/hphotos-ak-snc3/13636_514053691946_182500493_30506793_1374840_n.jpg

[home made tunnel]

i'm drawing up some more plans of the stern, with less setback for comparison. Keep in mind, i don't plan on having this hull 'fly' like a traditional tunnel. I want it to be able to handle some rounger chop in a bay, like a jet ski can, and the offshore power cats do. But yes i know, it's only 15', so maybe the setback will give me some more stability, and limit the porpoising possibilities. It also severley aids in strengthening the boat by eliminating a splash well, and keeping the transom and back of the boat all one piece.

But this is what gave me the original idea for my 18" of setback:
http://continuouswave.com/whaler/reference/images/armstrongBracket390x298.jpeg

and some interesting reading on it: http://continuouswave.com/whaler/reference/engineBrackets.html
also: http://continuouswave.com/whaler/reference/standardTransomBracket.html

And in the thread of "little boats", there was a picture of a 17' mini cat (similar to my design) that had a 300hp on the back! WOO WOO! Awesome!

[Sheer Insanity]

Build it for twin 75's or 90's. Will be alot more stable and manoeuvrable. Plus would be the first mini with Twin's..

[home made tunnel]

Build it for twin 75's or 90's. Will be alot more stable and
manoeuvrable. Plus would be the first mini with Twin's..

That would be awesome, but that's x2 jack plates, x2 props, x2 rigging setups, and buying two outboards is 3 times the cost of just 1 200. Funny how two 90hp outboards are 3x the cost of a single 200hp

[cptunnel]

dude u have my full support no matter what! thats awesome

[home made tunnel]

You guys got me really thinking about my setback....
What research tells me is that 1" setback per 1' of boat length is the accepted "norm".
Also, this much setback DOESN'T work for tunnels because they're so light, and the moment leverage on the static stance floating at rest, and at speed is just too much. (500lb engine on 700lb boat etc). i'm expecting for my boat to be over 1000 lbs, and built TOUGH.


Is there anything else i'm really missing about the setback? I have 18" extension, but engine mount will only be 15" back, perfect for a 15' boat :)


Here's another picture of an outboard bracket with alot of setback on a small boat:
http://www.ifish.net/gallery/data/617/DSC01025.JPG

[FrenchPhil]

Don't worry about setback, you can always deal with that later if necessary with a jackplate & spacers. I'd make sure the stern is well reinforced, splashwell or not, cause on a full width cat keeping the transom and back of the boat all one piece is out of the question without proper framing.
What's your building material ?

[Mr. Demeanor]

Inch of setback to foot of length does not apply to cats that use air to lift the bow. Even big offshore cats use very little setback.

[home made tunnel]

Inch of setback to foot of length does not apply to cats that use air to lift the bow. Even big offshore cats use very little setback.
i dont plan on my boat having much aerodynamic lift at all as compared to hydrodynamic, my cat design is really for more stability than a v hull, and sexy looks too

[Sheer Insanity]

Every inch of set back will change the center weight of the boat. A 13' cat will way as much as a 200. coming off plane could get the motor wet and dunk the transom? :confused:

[Mr. Demeanor]

I have to be careful coming off plane with a sub 350 pound motor and 2" of setback.

If you think you wont have any aero lift from that thing....you are crazy.

[300x Stoker]

HMT in not going to find a better source for getting his questions answered than S&F. Base your design off the Beil and go from there. You can do it.

[home made tunnel]

So, less setback, more thinking about how to get that done, and maybe a gunwall in back?

I've seen tunnels on this site where the transom is actually underwater at rest!!


HMT in not going to find a better source for getting his questions answered than S&F. Base your design off the Beil and go from there. You can do it.

what's the Beil?

[Mr. Demeanor]

The one I posted a link to above in the classifieds and the one I own. Thats two of the only 5 ever built. For what he is asking, you will spend a lot more and may or may not have a great boat. His has custom paint and a nice trailer.

[home made tunnel]

The one I posted a link to above in the classifieds and the one I own. Thats two of the only 5 ever built. For what he is asking, you will spend a lot more and may or may not have a great boat. His has custom paint and a nice trailer.

Oh, there's a link? I can't see links or pictures at work :mad:

[Mr. Demeanor]

Oh, there's a link? I can't see links or pictures at work :mad:

Go to classifieds less than 20' and look for Biel.

[Mr. Demeanor]

Oh yeah...



AND GET BACK TO WORK !!! :D

[home made tunnel]

Oh yeah...



AND GET BACK TO WORK !!! :D

haha too late! i'm out of work at 3:30!!!!

[home made tunnel]

SO rather than just drawing up some different transom plans for offset / etc etc, i just built another small model... just the framework though. I came up with this idea::: Just by extending the waterline under the transom extention i have, i created a swim platform, and reduced the offset! Nice, isn't it?

Now my questions are this... to put the gas tank up front, or in the back? i want something around a 23 gallon. I will be making it myself. I did welding for a living on nuclear submarines for years.


Here, you can see the framework, and the swimplatform / hull extention so that i don't have 15" of setback anymore. Engine will be mounting right at the back of the boat's waterline now.

http://i1050.photobucket.com/albums/s405/turbobandit/P1090148.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/P1090149.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/P1090150.jpg


Here's some fun with a sharpie, i plan on having a door to access the front compartment here. And building a side pod for the steering wheel/controls / gauges. WIth the cabin being 5' wide and 5' long, there's plenty of room for 3 full sized adults to side side by side with elbow room. The boat now measures in at 16'.

http://i1050.photobucket.com/albums/s405/turbobandit/P1090152.jpg
http://i1050.photobucket.com/albums/s405/turbobandit/P1090151.jpg

[FrenchPhil]

251300


I like the swim platform idea. If your building in ply, I'd readjust the cockpit width so your longitudinals run as far back as possible.

[home made tunnel]

251300


I like the swim platform idea. If your building in ply, I'd readjust the cockpit width so your longitudinals run as far back as possible.

Hmm, i think i'm confused about what you mean... The longitudinals of the cockpit, meaning the left and right sides?? Or you say the width? are those 2 different dimensions you're talking about?
I'm using the tunnel sides as a vertical strongback for the shape of the hull, as well as the sides of the hull itsself in Ply stitch and glue construction with full epoxied fillet joints.
Also, the cockpit sides, and the 2 pieces that run longitudinally from the engine mounting bracket, to the seat back will be vertical strongbacks too. So that's 8 vertical strengtherners in the back of the boat that i'm thinking of...
1) cockpit sides
2) tunnel sides
3) Hull sides
4) transom gusset pieces

I didnt extend the cockpit sides all the way to the rear of the boat, because i wanted a low point for the bilge pump, and to keep it all open in the back. I was planning on putting the bilge pumps in the back left and right corners.


What exactly were you suggesting about width, and extending the longitudinals??

Thanks for your input!!!

[FrenchPhil]

The pic didn't upload...

[home made tunnel]

you suggest making the cockpit skinnier? I like my 5' wide though lol. Then i couldn't call it the Cat Box, it would have to be the Cat Rectangle! haha.

[Mr. Demeanor]

Everyone wants a cat to look like a cat with a pickle fork style bow. With these small cats though, you need as much lift as possible which means the longest, widest tunnel you incorporate. You might want to make your tunnel longer toward the bow. Your sponsons will not "stick out" as far but you will get more lift and a drier ride when the tunnel "sneezes" in rougher water. I wish my sponsons stuck out in front just a little to give it more of a cat look.

[FrenchPhil]

Structurally, on a ply cat it's easier if the cockpit is no wider than the tunnel. On a 16' foot cat
your looking at 6-7 foot beam so a 4' wide cockpit is already pretty large for 2 people.

[home made tunnel]

Structurally, on a ply cat it's easier if the cockpit is no wider than the tunnel. On a 16' foot cat
your looking at 6-7 foot beam so a 4' wide cockpit is already pretty large for 2 people.

I see what you're saying. Well, the beam at the back transom is 7' exactly. Beam midship by the frame at the front of the box is 7'6". I'm not concerned with making it easier/harder to build, i just want it comfortable for 3 people. Mainly so i can pull a skiier/wakeboard/tube, and have a 3rd person in the boat watching. Ya know, so it's legal and all. Oh, and it has to fit in my garage so i can build it. (i'm looking at the 12"x12" tiles on the floor here at work visualizing a 3.5' wide cockpit haha)

I like your suggestion, but i'm happy to stay with the 5'x5' cockpit box. It should be structurally plenty strong enough. <-- was that proper english? haha.

The tunnel is only 3 and a half feet wide, and that would make things really crowded inside if i made the cabin that skinny, and also make the boat look silly and unproportionate. (i think so at least).
If ya pay me, i'll build one identical to mine with a skinnier cabin for ya! :)
I agree with you, it would make it stronger doing it that way, but i'll work a little harder to get that extra cabin space.

[home made tunnel]

Everyone wants a cat to look like a cat with a pickle fork style bow. With these small cats though, you need as much lift as possible which means the longest, widest tunnel you incorporate. You might want to make your tunnel longer toward the bow. Your sponsons will not "stick out" as far but you will get more lift and a drier ride when the tunnel "sneezes" in rougher water. I wish my sponsons stuck out in front just a little to give it more of a cat look.

i know plenty of getting wet in the chop! haha. I've had a few flat bottom boats out on Narraganset bay, heading to Newport, and boy do you get wet! haha. I do plan on riding in the chop alot with this boat, and i have been contemplating extending the bow like you said. This is really why i beefed up the back end with gussets, more frames than needed for a ply on frame stitch&glue method, but like i said, i want it tough to take the chop.

MY SECOND dilema is this... if i make the bow longer, or extend the tunnel with shorter forks, then i risk more of a blowover. I designed the bottom chine with 4" wide pads. 8" total pad width with 1 each side (obviously, for a cat).
I chose to design a cat for a couple reasons... 1- stability, and 2- THEY LOOK AWESOME!!!!
You're definately right it would keep you dryer by extending it, and get more aerodynamic lift, but i'm mainly going for hydro-lift because i will be in choppy water quite often on the bay, and i imagine my bow will be lifting fairly often.

then again, it has to be short enough to build in my garage, and tough enough to handle some fun choppy stuff. AND IT HAS TO LOOK COOL!!!!!!! haha.
Dilemma dilemmas.

Anyone have a video of a tunnel / cat SNEEZING ? it's definately fun on a flat bottom, but i've never even been on a cat (except for the Martha's Vinyard high speed ferry!!!!)

[2.5_stoker]

[quote=mr. Demeanor if you think you wont have any aero lift from that thing....you are crazy.[/quote]

x2!!!

[home made tunnel]

FUN FACT TIME because i'm bored on lunch break, and have been doodling a bit.

Wanted to guesstimate weight, so the concensus online is that 1/2" plywood weighs about 1.56lbs per square foot.


these are just ballpark area dimensions:

84"x180" Top and Bottom x2 ..............105ft2...... 210ft2
96"x24" tunnel sides x2 .......................18ft2 ........36ft2
27"x180" boat sides (L,R) x2 ................33ft2 ........66ft2
27"x84" frames x3.5........................... 15ft2 ......52.5ft2 (frame 2 is only side pieces)
(18"x21")+(18"x180")Transom x5 ..........13ft2......... 65ft2 <--5 layers of 1/2" ply
36"x18" transom gussets x2 ..................4ft2 ..........8ft2
...................................................................437.5ft2 TOTAL
...................................................................437.5ft2 x 1.56lbs = 682.5 lbs Hull weight


16' Cat Box built like a brick shiz house hull BALLPARK weight to be 680lbs. Just an educated guess.
Yup, deff need a 200hp on her!!!!

680lb hull
+ fiberglass = overr 700lbs.

~700lbs
+ 500lbs engine
+160lbs fuel
+200lbs for jack plate / battery / wiring / stereo / steering / anchor / safety eq. / seats / etc.

Guessing i have to build a trailer to handle 1600lbs! Sweet.

[Capt.Insane-o]

http://img.photobucket.com/albums/v83/LynnMackjr/m_5a12d0f25ebc4d1dffd6e0abba12a308.jpg (http://smg.photobucket.com/albums/v83/LynnMackjr/?action=view&current=m_5a12d0f25ebc4d1dffd6e0abba12a308.jpg)

[Mr. Demeanor]

The only thing that is going to blow you over is the trim button.
You would be amazed how far I can trim up without it getting sketchy :)

70mph making a pass by the S&F gang on the beach. Rumor is some of them were running for cover. Hey, if the Action guys can do it, why not me...what could go wrong?

251332

[ridgerunner]

You got your glass figured at only 20 lbs?? Two gallons of epoxy and a small piece of what kind of glass??
Re Setback. On a 20' V-hull production boat I have a 30" setback bracket.
While it is true that the water is cleaner and the motor height can be raised for efficiency the lever action created by the motor hanging off the bracket causes the static trim to be outta whack. In other words it sits low at the stern at rest. The link you gave to Boston Whaler's article on brackets is one I had seen before. The one Whaler made for their boats "Whaler Drive" mimics simply bolting on another 2' of boat. Gil brackets are nice but offer no floatation. The write-up is solid info for normal V-hulls and Cathedral hulls but has no place on a mini-cat. My boat is no speed demon and has very little by way of aero lift. The additional leverage to raise the bow with trim was an accurate statement in the article, but you shouldn't need the additional leverage. Cats don't rely on trim to fly high they rely on aerodynamics and hydrodynamics. I saw you changed the plans to change the setback. You may want to just square it off because I believe you will need the additional displacement with a 200 and the passengers sitting at the stern. That fuel tank will need to go up front IMO. That is how I got the 30" bracket to work well on a 20' boat, shifted as much weight as possible fore. I attained near perfect balance with a lightweight 200. Heavier motors hurt the balance.

If you read the book on tunnels you understand how the roof of the tunnel and top deck form a wing and then they spell out how air rushing into the tunnel
gets compressed, adding further lift. How can you say your cat won't have aero lift?

I am intrigued by your design/willingness to move forward and wish you all the best. 251314

[home made tunnel]

The only thing that is going to blow you over is the trim button.
You would be amazed how far I can trim up without it getting sketchy :)

70mph making a pass by the S&F gang on the beach. Rumor is some of them were running for cover. Hey, if the Action guys can do it, why not me...what could go wrong?


From research, i've found that aerodynamic coefficients don't even come into play until over 50mph. The biggest factor is the angle of attack from trim, relation of the cavitation plate to bottom of hull angle... It creats ALOT of hydrodynamic drag, and hurts top speed alot, but helps it CHEAT in the way of getting more aerodynamic force. The book was stating how it's really a catch 22, and basically trimming up only defeats the porpoising effect of running nose down.

So yea, i'll most likely just trim it up with a 200 out back heh heh.

[home made tunnel]

You got your glass figured at only 20 lbs?? Two gallons of epoxy and a small piece of what kind of glass??
Re Setback. On a 20' V-hull production boat I have a 30" setback bracket.
While it is true that the water is cleaner and the motor height can be raised for efficiency the lever action created by the motor hanging off the bracket causes the static trim to be outta whack. In other words it sits low at the stern at rest. The link you gave to Boston Whaler's article on brackets is one I had seen before. The one Whaler made for their boats "Whaler Drive" mimics simply bolting on another 2' of boat. Gil brackets are nice but offer no floatation. The write-up is solid info for normal V-hulls and Cathedral hulls but has no place on a mini-cat. My boat is no speed demon and has very little by way of aero lift. The additional leverage to raise the bow with trim was an accurate statement in the article, but you shouldn't need the additional leverage. Cats don't rely on trim to fly high they rely on aerodynamics and hydrodynamics. I saw you changed the plans to change the setback. You may want to just square it off because I believe you will need the additional displacement with a 200 and the passengers sitting at the stern. That fuel tank will need to go up front IMO. That is how I got the 30" bracket to work well on a 20' boat, shifted as much weight as possible fore. I attained near perfect balance with a lightweight 200. Heavier motors hurt the balance.

If you read the book on tunnels you understand how the roof of the tunnel and top deck form a wing and then they spell out how air rushing into the tunnel
gets compressed, adding further lift. How can you say your cat won't have aero lift?

I am intrigued by your design/willingness to move forward and wish you all the best. 251314

As far as aero lift, i want the boat to run more flat, as opposed to having to trim the motor up to get speed. Like i mentioned in my last post, i read alot on angle of attack too with hydrodynamic drag, etc etc.
I did read up on the air going over the top of the boat/deck, but with my windscreen design, it really spoils the whole surface. I don't have a "wing" shape at all, and will basically be residing in ground effect at any higher speeds. The bottom of the tunnel under the "cabin box", and the top of the deck, is all flat/parallel to each other. The windshield and cabin could be like one of those giant wings found on ricer mobile chinese ching chong meow racing wings hanging off the back of an inner city 4 colored honda import haha.

No doubt, it will have 800% (not literally) more lift than a V hull because of the tunnel, but it wont be an F1 formula tunnel racing at 150+mph.

I really like the static look of the back end squatted in the water with a big engine on the back. So that's not a real concern for me, but i'm still doing alot of reading about offset.

Yea, i know i'll be using more than 20lbs of fiberglass on the whole boat... i just liked the nice even "700" for the hull. That is just a HUGE GUESS on weight. but little bit educated guess.

And i agree with the fuel tank up front. It will free up alot of space in the back, and keep things nice and tidy, and more balanced.

Thanks again guys for all the help! This website has exceeded my expectations as far as comments/input from you all.

[FrenchPhil]

I thought you wanted to go fast with a minimal load. If you want to pull a skier & have 3 people onboard it's
going to be tough considering small full tunnels don't carry loads very well. You estimate 700lb but I
think it's going to be lighter than that - around 500lb. 1/2" ply is fine for framing but not for planking
where 1/4" is generally used. Add a 500lb motor plus gas & getting on plane will to be difficult.
A Modv type hull would carry loads better but they are more complex to design/build.
Nevertheless, good luck your project.

[home made tunnel]

I thought you wanted to go fast with a minimal load. If you want to pull a skier & have 3 people onboard it's
going to be tough considering small full tunnels don't carry loads very well. You estimate 700lb but I
think it's going to be lighter than that - around 500lb. 1/2" ply is fine for framing but not for planking
where 1/4" is generally used. Add a 500lb motor plus gas & getting on plane will to be difficult.
A Modv type hull would carry loads better but they are more complex to design/build.
Nevertheless, good luck your project.

I didnt think 3 people was alot of people in a 16' boat... that is a nominal load, no? Even with a fuel tank mounted forward? Especially with a 7' beam?

I'm building it for speed/durability. To be able to handle a chop. (as realistically practical for a 16' boat). I'm still young / dumb/ fullacum, so bouncing around is still fun for me. I'm not building/designing it for skiing/wakeboarding, but i'm definately going to try it when its done!
Hey, Jetskis weren't designed for towable watersports either, yet they still do it!


About the Ply thickness.... I was planning on doing it all out of 1/2" ply.... You don't think 1/4" would be too thin? even at 80mph in a chop with some pounding???
My 700lbs was a very rough guess, i hope it will be a little lighter, but we'll only truly know when i get all my material etc etc.
Maybe i should use 3/8"? From what i read, 1/2" ply is actually slightly smaller than true 1/2". And 1/4" is also just shy of 1/4" thickness... That worries me a little bit for structural integrity for the conditions i plan on running in (Narraganset bay / newport). My hull is built with minimal frames, and i was incorporating the cover panels to add structural integrity. (This is a half and half method of ply on frame vs stitch and glue).
Your opinions are DEFINITELY taken into consideration, and thaks for your input!!

By the way, what is a ModV hull?

[2.5_stoker]

three people in a 16 foot Vhull wouldnt be a bid deal with the right power but tunnels lack the hydrodynamic atributes ie the wedge shape to force them out of the water...a modv tunel has a center sponson to help cary the weight but still traps air like a tunnel...heres a pic of my modv

http://i43.tinypic.com/w99shx.jpg

[home made tunnel]

three people in a 16 foot Vhull wouldnt be a bid deal with the right power but tunnels lack the hydrodynamic atributes ie the wedge shape to force them out of the water...a modv tunel has a center sponson to help cary the weight but still traps air like a tunnel...heres a pic of my modv

http://i43.tinypic.com/w99shx.jpg

Ooooooo, i understand about Modv now...

Well, in the plan, i designed it with a flat 4" wide running pad at the bottom of each sponson at the back 4'. That should help a little with slower speeds, and all around i assume... Perhaps if i increased it to 6" on each sponson? that's a total of a foot wide running pad~!!!!
i excpect it should be able to plane at around 20mph?

What's the slowest planing speeds for most small cats / tunnels? (with/without planing pads)

ps- a 200 hp out back would likely help the slow speeds with some extra OOMF i imagine.