Tunnel Questions

I think you're going to find yourself heavily overpropped, and your wish to swing a large diameter prop is driving your design criteria into some awkward contortions.
Most deep vee boats like yours are running props of more like 18" diameter.
You can overprop to some extent, provided you do not intend to do any travelling at high speed (not likely achievable anyway) , or high throttle settings. This is a popular concept on trawler type boats, and there is quite a bit of calculation involved to successfully pull it off.
You may also find your waterline at the stern significantly deeper as you add weight and subtract hull volume, ie; the last picture in your first post.
There are some darn good reasons why so many boats similar to yours are outboard powered.

 

Thank you. Your advice is worth gold to me. I was planning to be over propped slightly. I would much rather be overpropped and have the tunnel arc slightly too large than be underpropped and have the tunnel arc too small. I was thinking I might need to downsize to a 20 inch or so prop. I designed the tunnel to accommodate the MAX prop I think I could possibly need.

I once roughly calculated the volume of water (displacement) I am losing from the lost tunnel volume and calculated it to be no more than a couple hundred pounds of displacement I was losing.

The hull will be thicker and the boat will be a little heavier in the stern, but I am hoping not to add much more weight than what was in there before. The engine arrangement will actually be lighter than what it was setup for before. The boat was previously setup for twin Volvo penta stern drives (around 716lbs each... total 1432 lbs for twin stern drives). Moving to a single Cummins 6BT which is 210 HP and weighs 1120 lbs. I am shaving a few hundred pounds of weight I think, but it might equal out with the weight of the shaft, rudder port, log etc. I am really not 100% sure how it will play out weight-wise.

Perhaps I should do some float tests once I get the engine/grear in but before I finish building out the inside of the boat, just in case I need to move some weight around.

Thank you again for your input. You are on to something.

 

I forgot to add that the 1" thickness you are seeing is most likely due to the overlaps at the transom and at the keel, possibly also at the chine.
If the entire hull was 1" thick, it would be extremely heavy, I would guess at roughly half of that or less over most of the area.
The fiberglass pipe suggestion is very good, whether you use it as a mold or the actual tunnel. Again, 1" thick is way overkill.
I have used sections of PVC pipe as molds, just cut out the hole carefully, and insert the pipe tight under the hull, then glass over.
Don't stress over the radius at the tunnel edge, just build it up strong on the inside, then grind a nice radius at the outside edge. It isn't rocket science, and everything on a boat is a compromise anyway, especially when you factor in the actual offshore operating conditions in your area!

 

prop too big

hi, i have built 3 successful tunnel drives, a uniflite mega 28 with a cummins , a aquasport 246, with a cummins and a uniflite express 27 with a cummins, here is a link to some pics of the aquasport, if you jump around my pictures / folders you can see other pics of the other boats as well.
right now your prop is too big, you wont be able to pull that... with that tranny ratio and your weight you probably need a 18 or 20 pitch.
here is the link to my picasa album... dont listen you people who live in books, theory is great but unless you get your hands dirty you really dont have a feel of a boat in the real world...
my uniflite 27 does 20 knots at 2800 rpm from a marinized P7100 dodge cummins 6bt, my aquasport cruises at 24 at 2000 rpm and tops off at 35 on a marinized dodge cummins rotary pump.

https://picasaweb.google.com/104905260722286217761/Aquasport246Cc

 

gon2sea,

Please check out my boat build thread at http://www.bdoutdoors.com/forums/threads/28-foot-force-project.408292/page-12 Wondering any advice or pointers you can offer.

I tried to email you but it says you are not accepting emails. Thanks.

 

On aluminum boats I have left the tunnel wall/hull sharp and they work just fine.... well, at least to 30 knots.

 

Thanks. I was advised by a respected boat builder that has experience with tunnels to "sharpen the lines" so the water releases, just like you said. That is what I'm planning to do. Keep them sharp. Sound right?

Here are some recent update pics of the project:

http://fiberglassin.com/pics/2016-08-10/

http://fiberglassin.com/pics/



It currently has the following layers of fiberglass which were done in 3 sessions:

1. 1.5 oz mat - (38 x 74) full sheet over tunnel
2. 18 oz roving - (38 x 74) full sheet over tunnel
3. 1.5oz mat (22 x 33) - reinforcement sheet for rudder
4. 1.5oz mat - (32 x 33) reinforcement sheet for strut
5. 18oz roving (20 x 31) - reinforcement sheet for rudder
6. 18oz roving - (30 x 31) reinforcement sheet for strut
7. 1.5oz mat - (38 x 74) full sheet over tunnel

8. 1.5 oz mat - (38 x 74) full sheet over tunnel
9. 18 oz roving - (38 x 74) full sheet over tunnel
10. 1.5oz mat (22 x 33) - reinforcement sheet for rudder
11. 1.5oz mat - (32 x 33) reinforcement sheet for strut
12. 18oz roving (20 x 31) - reinforcement sheet for rudder
13. 18oz roving - (30 x 31) reinforcement sheet for strut
14. 1.5oz mat - (38 x 74) full sheet over tunnel

15. 1.5 oz mat - (38 x 74) full sheet over tunnel
16. 18 oz roving - (38 x 74) full sheet over tunnel
17. 1.5oz mat (22 x 33) - reinforcement sheet for rudder
18. 1.5oz mat - (32 x 33) reinforcement sheet for strut
19. 18oz roving (20 x 31) - reinforcement sheet for rudder
20. 18oz roving - (30 x 31) reinforcement sheet for strut
21. 1.5oz mat - (38 x 74) full sheet over tunnel

We now have 6 full sheets of 1.5oz mat, 3 full sheets of 18oz roving with many more (smaller) reinforcement layers over the rudder and strut areas. It took us 3 glassing sessions to get this far. I think we are about halfway to the thickness we need to be. Overall thickness is currently around 1/2" or so, but it's much thicker in the rudder/strut areas.

Full tunnel sheets run up the stringers 3-inches. Gonna go over the stringers with 4 layers mat, 2 layers 18 oz roving once the tunnel layers are done.

I filled the crack between the tunnel and hull with q-cell/cabosil thickened VE resin. I am going to start doing sheets up the inner transom wall now that the crack between the transom and tunnel are filled.



How thick should I actually make it? Does 1-inch thick sound good?

Any other advice?