A Little Background On The Boat
In March of 2012, I purchased a boat which capsized off of Kauai and subsequently washed up on shore. The boat had a lot of damage but also had potential. The cab was destroyed, but could be replaced. The transom ripped out when the twin Volvo Penta outdrives fell out, but a new transom is certainly do-able. Once the boat flipped over, I guess the engine mounts weren’t designed for the boat to beat upside down on the rocks, which it did for a few good hours before ending up on the beach. When the engines fell out, their outdrives ripped most of the transom off. Because of the angle of the hull, most of the damage was confined to the last few feet of the boat (specifically, the transom and the most rear part of the gunnel). The hull was basically undamaged except for a few teeth marks from a front-end loader used to get it back on the trailer.
Many of the boat pieces like hatch lids, cushions, chairs, etc., washed up on shore and were recovered. Things that that can be reused will definitely be reused. There is a good amount of rod holders, throttle controls, hatch lids, fuel tanks, trailer, hardware/brackets that can be reused which is going to save a lot of money.
To put new Volvo Penta engines in, I’d be looking at $52,000K just for the engines. Used would probably be half of that, but still out of my league, especially for a damaged boat like this. Once the boat is fixed, it should be cherry. I am trying not to sacrifice quality on anything, but I am definitely doing it on a budget because it’s too easy to throw good money away on boats.
Single or Twin Engines?
Here we are on December 8, 2015 and I am nearing 4 years of owning the 28 foot Force. After a lot of research, I have learned a lot about boats. I’ve learned that the vast majority of ocean-going vessels are single engine diesels. Diesels are best, especially for a commercial fishing boat like this. Single diesels are more efficient than twins. If you know how to anchor correctly, you should be able to get away with a single engine boat, in my opinion. I think I will be just fine with a single. I will just be careful anywhere there is an onshore wind and I plan to always have multiple anchor systems with extra heavy chains if I ever do get into these situations (which I will avoid, especially with a strong wind). I also plan to constantly have multiple EPIRBs mounted directly to the boat that are tested. So, given these two things, why not go with a single diesel instead of twins? It will be half the upfront cost and almost twice as efficient when it comes to fuel consumption and maintenance.
The Cummins 6BT
After a lot of research on Boatdiesel.com, the Cummins 6BT was chosen because they can be found for about half the cost of a Volvo Penta engine. After a year or so of searching, I ended up purchasing an early 90’s Cummins 6BT 210HP with unknown hours from a guy on Oahu. It included some new filters, a serviced heat exchanger, a new raw water pump and paint job. It starts up just fine and seems to run good. The fuel pump leaks, but that because it’s missing a fuel pump bracket. Even if the engine does have to be rebuilt down the road, that is OK with me. I got it for a fair price and it is a good project engine even if it does not hold up. I am HOPING it runs as good as it sounds bolted to the pallet. At half the cost of an “unauthorized” Cummins reman engine, I should do OK either way. But, it sure does seem to run good the few times I have ran it.
The Cummins 6BT is turbo charged, but it is not super charged or after cooled. If you add an after cooler and bigger pistons, this block will produce in the 250-330HP range. With a super charger and after cooler, the engine will produce 370HP. If you Frankenstein the block, you can get 1000HP out of it. So, when you run this school bus engine at 210HP, the reliability is great. They are efficient and very dependable to say the least, but still having a lot of power and torque. These engines produce too much torque for a Bravo or Volvo outdrive. For an outdrive, you’d have to get a military-grade Konrad outdrive, but the Konrads are 1:1, so you still need a transmission. That is why I decided to go v-drive instead, but sometimes I wish I would have looked at this option a little more.
Because I have the 210 HP version, I am not getting the faster RPM’s that I would with a supercharged engine. Because of this, I am going with a larger (22×20 3B) prop than I could with a supercharged 370HP version. Having a large 22-inch prop with 3-inches of clearance under lowest part of the hull gives means I have at least 25 inches of “junk” hanging off the very lowest part of the hull, which sucks big time for trailering your boat around Hawaii. Having the boat 35+ inches high on the trailer is no good for many reasons. So, in order to save 7 inches, we decided to go with an 8-inch tunnel cut into the bottom of the hull.
The tunnel will decrease the overall draft and decrease the shaft angle, both of which can increase the efficiency of the boat overall. The problem with a tunnel is you lose displacement. I did some calculations a while back and estimated a tunnel 5 feet long and 8 inches deep would be about 150lbs of water weight I am losing. Because we are going from Volvo Penta’s to a single Cummins, we are actually shaving a few hundred pounds of weight in the rear, so losing some displacement from the tunnel could be beneficial as far as that standpoint.
I did a lot of online research on tunnels over the last couple years using Boatdiesel.com, Boatdesign.net and Donald Blount’s Tunnel paper. One of the most important aspects I have surmised is that you don’t want the entrance of the tunnel to steep because the water entering the front of the propeller should not be disturbed for best efficiency. In fact, less than a 15 degree change in slope is recommended for the tunnel entrance and less is even better for efficiency.
Since I plan to run a 22-inch prop with a 3-inch clearance, I am going to cut a bunch of semi circles with that diameter that fit on this table I made:
I will cut a bunch of these semi-circle shapes and taper them smaller and smaller at one end, making sure the tunnel entrance does not exceed 15 degrees. Once they are tapered, I will use a nail gun to nail thin slats longways across the semi-circles, basically making a mold for the tunnel. I cut circular hole on the transom with a 1-inch larger diameter to accommodate for the thickness of the tunnel plug itself. The plan is to make the plug about an inch thick so it is ridiculously strong, but also “locks” underneath the transom and rests on top of the hull (on the inside). This arrangement of the tunnel inside the hull will naturally lock it in place.
I’ll make a vinyl-ester “peanut butter” putty to fill in any gaps and then start layering copious amounts of fiberglass on the inside to build it up over another inch or so, tabbing it all together with the width of the hull. My plan is to basically line the whole inside of the boat with roving/mat/roving/mat/etc/etc/etc so that it is ridiculously thick around the are of the tunnel. Every area will be tabbed properly onto the adjacent areas. I am using vinyl esther for most of the job, but I might switch back to polyester once it is time to put in the new bulkheads. I do have to keep the stringers as symmetrical as possible as I keep laying glass, so I’ll just have to keep that in mind as I keep laying fiberglass down.