aquasport 246cc diesel tunnel drive

[gran398]

lets see.... 2550 boat, 1300 motor , ruder , shaft etc... 500 fuel, 500 passengers, thats 4850.
the shamrock 246 with gasoline weighs in at 4300 dry with gas engine, throw in the optional yanmar 6lia  and its well over my figure...  im not looking for a speed demon, just a good solid and sea worthy hull, hopefully i'll get a chance to take her out before heading down to baja in late september, but i have no problems taking her and doing sea trials down there....

That is exactly what I was figuring. Right at 4900#.

Main thing....fuel placement. I called Albemarle today on your behalf.... they're here in NC.  On their inboard-powered 24 jackshaft  (original Albemarle 24) they place twin fuel tanks outboard of the engine for a reason. The tank forward idea on your hull....I'm thinking she's gonna dig. And dig bad in a following sea. And not like you can trim her up. Tabs are no help....tabs offer only downward trim on this application.

If she'll never go in the ocean...a lake boat....or if your ocean there is like a lake, not so much of a problem. Weight placement/running attitude/safety is less of an issue.

But...if a bow heavy boat in the ocean digs hard going downhill  in a big following sea...and the boat is thrown sideways...big problem.

 Over here, we may get two slick days out of thirty.

You asked for opinions...hope this gives additional consideration.

[saltfly]

I agree with Scott. Rough seas are a norm here as well. I like the saddle tanks. Outboard of the engine. You would be able to fine tune the boats balance point by moving the tanks a little forward or aft as needed. A digging bow would be a problem for me up here. I normally power up a wave and back off the throttle  going down a wave to lift the bow , so I’m ready for the next wave. If fuel were to roll forward, starting down the wave it would  push the bow down and backing off the throttle wouldn’t help me. The boat would settle to much keeping the bow down. It wouldn’t be pretty. But which ever way you go, this will be one of the more interesting builds. I’m looking forward to it. I’m also looking forward to seeing how she sit and handles once finished.

[GoneFission]

The 222 was offered as early as 1975 with a gas or diesel inboard with a Paragon or Velvet Drive transmission, so a 6BT Cummins on a 246 should work.     The 6BT is about 38" long and that should fit under the console.  Saddle tanks amidships or toward the rear would make sense to me and would keep it from being nose-heavy.  

Is the gearing 1:1?  You really don't need/want a gear reduction for most diesels.  If the gear is 1:1, it seems like a 17-19 pitch prop would be about right for a 2500 RPM cruise.  

Are you going to use raw water cooling, or install a heat exchanger and go with fresh water cooling - some diesels don't do well with raw water cooling...     Water to air intercooler or air to air?  Air flow over the intercooler could be an issue with air to air, but space could start to be a problem with water to air...    

Looks like a cool project!    Keep posting pics!

[dburr]

A relatively simple exercise to see what is going to happen is to set up a spreadsheet and look at some numbers..

The hull in its present state has a known weight.  If you do a quick search of the threads here you will find more than one where the topic of weighing a boat comes up (not on a weigh station scale, axle weight won’t help).  In short, the hull must be level and supported by at least three points.  The locations of the scales must be known as accurately as possible for a datum point.  Standard is a 0 reference at the forward perpendicular (furthest forward point of the bow).  

So for the sake of discussion, you level up the boat and have figured out how to set up a bathroom scale so you don’t blow the springs out of it like a bad Bugs Bunny gag .  At the forward jack point which is located a measured 4.5 feet aft of the FP you see 1875 lbs.  The aft two jack points are right under the chine at the outside edge of the transom at 24.5 feet.  On the left you see 980 and on the right you see 1000.

The math..

Arm x weight = moment.  Weights are summed (Wt) and moments are summed (Mt) then to find the calculated CG (arm) you divide Mt by Wt and that gives CG..  
In this example 1875*4.5=8438 then 980*24.5=24010 and 1000*24.5=24500.. Finish up with the sums we have an empty hull that sits an empty weight of 3855 with an arm of 14.772 (moment is 56948).

So now what..   ?
If you set up a spread sheet that has a reasonable number of stations and start putting weights in places it is really interesting to watch what happens to the arm, the one I did starts at 0 and goes to 24.5 in 1.5 increments..
Put a 700 pound engine :thumbleft: at 13.5 and the arm moves forward to 14.57 feet.  Put the fuel, say 75 gallons forward of that at 10.5 ft and now the CG is at 14.14 ft moving forward and as Scotty said she will be a mother with a following sea..

So to counter this you put a 40 gallon live well at 23 ft and plan to run it full all the time (343#) you only shift aft to 14.70 ft.   Move the engine to 15 foot mark and use wing tanks on either side for a total of 75 gal and you are now at a 15.34 ft arm..

And an “oh by the way” the weight of the boat is now 5346# and there is not a screw or a new drop of resin hold any of this stuff in place.

The point of this is to take a careful look at where you put stuff, you’re gonna be fine with a little care..  Without knowing where the CG is of a good riding and floating boat is it is almost a pointless exercise.  If you could splash your hull to see how it floats it would be an immense help.. Take all the kids with you and have them move around until she floats level and put them back in the boat for the weighing with dire threats to hold still until you are done and then you are in business!:mrgreen:!

 What would be totally cool is if one of the Brethren that has a 246 that floated well HAS figured out where the CG is.  That way we would have a record of the arm that makes the hull sit pretty and ride well.  

Then one of us Wingnuts could put an 8V92 TA in a 246( just to watch her go of course) and come out ok where she sat.  Waterline might be a little deep but so what… 8)

This WILL be a cool project!!

Edit: My Apoligies! I just reread the thread and I would have used your weights if I had been paying attention!! !!

[dburr]

While we are spending your money for you check these guys out..

http://www.evolutionmarine.com/detail.html?id=3001&cart=137528800735814165

 I don't work for these guys nor am I beer drinking buddies with them (have met then and they are worth having a beer with though..).  I have seen the shaft systems and have worked on boats that have them. They cost more but are a really slick way to go if you want no dripping stuffing box, low maintenance, efficiency and comfort…     !!

[saltfly]

I don’t want to sound like I’m high jacking this thread, but some one did bring up a jet drive. Here’s a pic. Of a boat I’ve never seen in the marina I use. The lines of this boat are what drew me to her. Once I started to look her over I saw she had twin jet drives. I would love to talk to the owner to see how she does in the Atlantic. I know it would be a great boat on the Chesapeake. But back to my reason for posting it. You obviously have a lot of experience with tunnel hulls. You must also be going to use your 246 in shallow water, or why the tunnel. So I thought you may like seeing this boat. Like I said, since some one did bring up jets.

[dburr]

The 2550 number you have off the capacity tag I assume?

If so, it is supposed to be the difference between the maximum displacement (design number) and the boat weight. The boat weight is supposed to be (33 CFR 183.33):

The combined weight of the hull, deck and structure, permanently mounted stuff (“appurtenances” is the Fed word, when’s the last time you used THAT in a sentence?? ) and weight of “full permanent fuel tanks”..

So, when the boys do an overhaul and gut the boat they are in effect changing the design parameters of the hull.  Some cases reduce the boat weight by a few 100 pounds when using new lite cored materials and going with a smaller fuel tank.  A lighter boat weight means more machinery/cargo weight..
How much?  Again, without the design weight we can just guess.. I attempted a search but got skunked.  Going to look through Bob’s CD to see if it is there..

As far as HP ratings, that’s based on math involving the boat length and max beam towards the stern so that isn’t effected by the rebuilds..

You are however dead right about the more steel you put in the less beer you can carry and it all increases the power required to achieve spry performance..  The size of the wake could get big enough to surf on !

Rick if we (I) are/am flogging the mule you can delete this post .

[Capt. Bob]

I've read this thread and I've absorbed a lot about weight, performance and seaworthiness so to throw a curve (but not derail) let me ask this.

What's up with the "tunnel hull"?
Not being in the boat business for 30 or 40 years and never worked on any boats other than I've owned, my understanding of the creation of the tunnel is at best, weak.
Understanding the concept is easy enough but the steps needed to complete are lacking.

Is the original hull cut and re-glassed or is additonal glass added to form the tunnel?
If it's cut, how is the original design performance shape affected?

No one has hit on this aspect of the rebuild and I'd like to know. After all, I know that older CCPs and other model Aquas had the inboard option but from the photos I've viewed, none were tunneled. I'll do a little internet soul searching but I would like "gon2sea" to hit on this aspect of his project.

Thanks and good luck.  

Edit: Something like this?
http://www.youtube.com/watch?v=TYnIolOAJwo

[saltfly]

Capt bob not trying to step on any toes here, but from my experience . They where built to facilitate getting into shallow water as well as to make trailering easier. Pen Yan built a lot of them. I still see a lot of those boat around, don’t know if they are in buss. Any more though. As for building one, you have to cut out the old hull and form your own tunnel to fit. We built one on an old boat for a duck hunter friend once. We built the tunnel, then put it up to the hull and traced it on to the hull and then cut. Then fit and glassed it in. That worked for us. I know their are a lot of people that like inboards and want to get as shallow as they can. So the tunnel lets them do that.

[Capt. Bob]

Thanks salt.
I edited my post and added a You Tube vid of a guy doing one on another boat. That's just what he did.
The tunnel concept of reducing draft makes great sense and is probably what "go" is planning.

However this works out, it's an interesting rebuild to say the least.

For an old doper like me, this is great stuff. We have a good deal of "out of the box" rebuilds going on.
Superior information and input for all Forum members. Thanks to all participating

[dburr]

What's up with the "tunnel hull"?
Not being in the boat business for 30 or 40 years and never worked on any boats other than I've owned, my understanding of the creation of the tunnel is at best, weak.
Understanding the concept is easy enough but the steps needed to complete are lacking.

Is the original hull cut and re-glassed or is additional glass added to form the tunnel?
It it's cut, how is the original design performance shape affected?

Edit: Something like this?
http://www.youtube.com/watch?v=TYnIolOAJwo

Short on time so short answer.. For a little history take a look here at William Atkins plans:
http://atkinboatplans.com/Utilities/Everhope.html.

If memory serves, it all began with the Seabright Skiff.  They were used in the surf and beach net fisheries of the Jersey shore and were prime for the transition from oars to power in the first days of marine power.  Easy to drive, very efficient hulls that did well with low power and could hit the beach flat with no damage to the running gear.  Advent of formable plastics morphed to the idea of a tunnel vice the built down section of the skiff on production built boats, as Walt said Penn Yan built a pile of them.

For modifications to existing hull shapes, tunnel types range from the shape of a big pipe (to form a mold) pushed up into the hull on an angle to give you room to swing a wheel size that matches the power you have, (guy at Whaler Central did that to a banna hull 22 Outrage to run skinny with an outboard) to the shape that the gent on the link used to allow for a smooth transition for the shaft.  Ideally the shaft angle is set up to be as close to 0 as possible for maximum efficiency.  You do have a slight loss of buoyancy due to the decrease in displacement volume of the tunnel and there is a slight increase in drag because of the increase area of the wetted surface.  The drag we would not notice at the speeds we run.

[seabob4]

Tunnel hull and prop pockets are the same...allow for  a much lower placement of the motor in the hull, using, say  a 6 or 8 degree down angle transmission coupling, allowing the prop the clearance it needs, giving a more direct "straight line" of propulsion as opposed to traditional I/B installs (usually under the helm with long shafts to allow the least downward deflection).  The only true drawback to tunnel drives, or prop pockets, is the loss of stern lift due to the reduced running sections of the hull aft.  This can be compensated for by tabs, especially out of the hole.

V-drives are certainly viable in this setup, especially if they are on hand, and either cheap or free.  The down angle of the output flange will dictate the size and location of the pocket.  But V-drive installs are meant to locate the engine aft, as their concept is rooted in creating room that would otherwise be used up for machinery.  So a console or leaning post locale for the motor would dictate, just by common sense, a straight shot I/B.

I do agree with Scott in regards to the Albemarle jackshaft setup that they have been using for  years.  That's what I would do...but it seems the Cummins is a bit averse to the more commonly available outdrives out there.  Oh, they're out there, but cost $$, and it seems that the OP has a lot of hardware on hand that he can put to use, rather than drop another $7500 on a drive that will make the Cummins happy.

So I say throw the Cummins in there, get the tranny angles right, create the pocket, see what she does.  Why not?

[Capt. Bob]

Sink or run like a scalded dog remains to be seen.In between is fine.
Keep it coming go2sea.You have started a very technical rebuild and for that we thank you.
Now this thread is really taking off.
Thanks to everyone on the tunnel primer. Just what I needed.

[seabob4]

IIRC, back in the early 90s Sea Ray was dumping their exhaust into the tunnels...anybody remember that, like on the Sun Dancers?

[icemanbryan]

I think this is an awesome thread and there is a lot of really good points here.
Keep it coming, I look forward to the final result, I think it will work out fine.