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Calcutta 360/390 Cat Conversion from Twin Diesel to Quad Outboards

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Calcutta 360/390 Cat Conversion from Twin Diesel to Quad Outboards

Old 02-13-2021, 10:44 AM
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Default Calcutta 360/390 Cat Conversion from Twin Diesel to Quad Outboards

We are converting from twin 315 Yanmar with Merc stern drives to quad 300 Suzukis on a custom bracket. I'm starting this thread to help me document my process, share my experience, thank those who are helping/advising me and hopefully receive some constructive input that might save me time, money and aggravation. This is by far not the first diesel to gas conversion. There have been many. But there are fewer planing cats converted. Even fewer (if any) planing cats converted to quads. It certainly will be the first Calcutta with quads. So although not a new concept, this application comes with few to no examples to guide me. The uncertainty is, hopefully, mitigated with research, careful considerations and advice.

The Who: Me, my family, friends and advisors. 100% recreational usage. From family floating 5 minutes from the house to cruise trips to hardcore offshore fishing and long range trips.

The What: Calcutta's Hull #1 (2007), 39' planing hull catamaran with a 14'-2" beam (yes, 14'). The original Diva. Truly. A hull decades ahead of its time with modern cat lines, curves, performance and space. I may have forgotten to mention but it has a 14' beam. It leans into turns. It tracks straight as an arrow. Following sea performance? Unbelievably good. It never dips the bow. Authority in heavy seas (fat bottom girl). Still unduplicated to this day considering the whole package. From a forward berth that can easily sleep 4 adults in AC/Heat, a casting platform above that big enough to fish over 10 people on the bow, a standup galley, a standup head, massive hard top, enclosed console area with very comfortably seating for 3 adults and room for 2 more standing during those July squalls in the gulf, hot water, 100gal/45gal wells, 1500qt plus cooler, generator, microwave, unbelievable below deck storage and more.... With minor configuration the boat could sleep nine (9) 6' tall straight-legged adults on cushions protected from the elements and in AC. I plan to do some of that in the near future.

The Where: Home port in Apollo Beach, FL (Tampa Bay Area) with a travel footprint from over 100 miles offshore to the oil rigs off Louisiana to the Dry Tortugas to the Abacos. Longer trips being planned....

The When: March 1st is go time.... This project is a long time coming. Years actually. Hundreds of hours of sketches, research, discussions, meetings, set backs, come backs, design problems followed by solutions. The goal (because you have to have one) is to be done, tested and tweaked by mid May of this year. Doable but very aggressive. In reality, I'd be happy with June or so. A schedule beyond that will be frustration fuel that will grow with each passing day. Come hell or high water, the project will get done and it will get done right. I have glass work to do, engines to order, an aluminum bracket (like none other) to have fabricated and all followed by install/rigging/testing/adjustment.

The Why: Ask me 4 years ago about quad outboards.... My response would be "sandbar motors. Impractical. Flashy. Underutilized. Unjustifiable cost. Honestly, stupid." Ask me now..... I'd say "Do you know what it feels like to be 200 NM from land (no exaggeration) 1:00am in morning staring at the temp gage on one of your two engines that is ticking hotter with every mile with 6 men onboard that have invested nearly 10k nonrefundable expense dollars for the trip? Or to be in the Abacos hosting a 2 week vacation with family and friends, about 500 miles from home and having a new, unwelcomed noise coming from a lower unit? " Well.... I know exactly what that feels like. Its a shit feeling that makes you seriously consider knitting or checkers. We made it in both cases. Ignorance is bliss among the crew but the captain is not afforded such luxury and puts a deal-breaking damper on his experience.... A handful of trips were spent coming home at 8knts. Not fun for me or the crew at that point. Quads aren't for everyone. Gas outboards aren't for every boat. They are for me. I don't care about top end. Well the inner 18 year old in me will giggling with a new top end but that is not the purpose. I need redundancy, power for heavy loads and reliability. Honestly, the current power and range is plenty for an average user. Even the average offshore fisherman and Bahamas cruiser. But some of my trips don't fit that mold, and I want to do more of those trips.... If your considering a diesel Calcutta, don't let this crazy project persuade you that it wouldn't be a fantastic boat that suits your needs and dreams.

Up front Q&A: (I have had many questions along the way. Maybe listing them below will save everyone time.)
1.) Why not go with bigger diesels?
  • Although this cat has gobs of room around the motors for maintenance compared to other cats and even other inboard monos, you do run out of room rather quickly. Weight is also a consideration.
  • This is a sterndrive setup. There are horsepower limits to sterndrives that don't meet my needs.
  • The HP limit on one motor that would work here would still be 400-500HP at best. Even carrying spare low pitch props, an engine failure would result in me coming home at 8knts. The second engine is destined for failure trying to come in from 100 miles offshore at any speed. 100 Miles at 8 knts. Do the math. Your trip is ruined.
  • Getting the power outboard, frees up ample space for additional fuel storage, which I need. When the project is done, Ill have 500-600 Gallons of fully plumbed below deck fuel storage. Giving me a range of 400-500 planing miles before needing bladders. Why would I need more? A trip to Mexico is being considered....
  • Replacement cost is half of a diesel. I plan to have this boat for a long time and power replacement will be part of my future.
  • Weight per HP is half of diesels.
  • Warranty on outboards is double that of diesels with no usage limits. At 400 hours of usage per year (probably more after this), this is big for me.
  • Mechanics for inboard diesels are in short supply and thinning. They just don't want to crawl around in a tight dirty bilge when they can work on an outboard.
  • Oddly, gas is lighter than diesel. Not a big deal for most boaters but when your talking about 600 gallons on board (with some math rounding) that's almost 600lbs of weight savings. Not dismissible.
  • Some cons to admit, gas is more expensive than diesel. It is more volatile (especially considering temporary fuel bladders). Economy is expected to be less. Partly because of higher cruise speeds but diesels burn less fuel. All these are conscious considerations and manageable.
2. Why quads?
  • As explained above, even with twin 450 outboards you can see it wouldn't be wise to push 17k-20klbs (boat and full load) any distance on one motor. Even with spare low pitch props. Twin Mercury 600's...? um...not on my budget....
  • Why not trips? Possible but this would require an engine centered on the tunnel. Doable, and its been done, but there are performance drawbacks. I'm already miles into uncharted territory and I'm not interested in going deeper.
  • Even with three motors, I've seen time and time again, people trying to come in hot on two and blow a lower unit, set an engine on fire, blow a power head. Granted, I don't think they changed props, but generally just too much of a load on the remaining working motors.
  • Redundancy. I will still carry spare props but I'm confident that if we lose one engine in Isla Mujeras, we aren't cancelling the rest of the trip or trying to get work done in another country. Roll on....
3. Why 300's?
  • I've done a lot of research on the various manufacturers, HP, etc... but mainly talking to people with experience of various motors. In short, the higher the HP of an outboard, the less reliable. 300HP seems to be the tried and true sweet spot for a naturally aspirated engine. Very highly regarded by all users I have talked to. Strong, affordable, reliable, repeat customers. Going above that and you are simply packing too much power into the system. All of the manufactures come with less reliability on the 350's+ than the 300's. Even if its all covered under warranty, I'm not interested in losing power offshore, haul-outs, time off for repairs, etc... Now start adding in superchargers.... well... I simply cant subscribe to a theory that I wont have some major headaches at the end of a 6 year warranty period with supercharged engines that might have 3k hours on them. I know repowering at a sweet spot in the hour meter could mitigate this risk, but Ill save that trial for another day.
4. Why Suzuki?
  • Why not? They seem to come with equal user testaments to their performance and reliability. I know of multiple boats very well running 300 zukes. They are lower costs than others and I have a connection that is financially assisting. No brainer for my particular conditions.
5. Nobody is senseless enough to ask this but me.... Why not a hull extension and an integrated euro-bracket? Have a 40+ modern cat with quads and gorgeous lines?
  • Very good question sir! I wanted to... I literally wanted to extend the boat a few feet. Glass in an integrated bracket, relay in new stringers across the joint and tie-it all together as one solid tank of a boat. I wanted to. Bad. After over a year of contacting about 15+ local builders and custom glass shops, nobody wanted to do it. Some have inhaled too many years of resin while others were economically oriented and up front. Of course Calcutta was my first call but they have their plate (and space) full with some awesome projects that aren't ending any time soon. At the end of the day, everyone is busy with projects that don't take up as much space in their yard as Mad Natave would. Not cost effective for them. To make it cost effective for them, is not cost effective for me and outside my budget.
  • I did talk to some CNC builders but for a one off project, it just wasn't economically sensible. If I had more owners in with me, it would be a great idea. All other owners appear to power that suits their needs well.
  • Structurally changing a boat (extending it) is frowned upon among banks and insurance carriers. Adding a bracket is too but palatable. With a heavy insurance budget....
  • Going with a bracket, it is entirely feasible to convert back to diesel if me or a future owner ever wants to. Who knows.... fuel might be $6 a gallon soon. If so, 1.5 mpg vs 0.9mpg might hold more weight than it does now.
6. What is the performance expectation?
  • My uneducated guess is a planing speed between 20-25mph, cruise around 35-45mph(depending) and a top end between 55-60mph. Probably closer to 55 but hoping I can pack the tunnel with some pressure and reduce drag and displacement.
  • At cruise, my guess on fuel burn will be about 0.8 to 0.9 mpg. I hope I'm being conservative.
  • Aside from ample power to run heavy loads long distances, a top performance goal of mine is a low and comfortable planing speed. The boat plans off nicely at around 17knts. Even with heavy loads. When mother nature says, slow down, you slow down. I don't care how big and bad you are. Waves turn into ramps and walls at 30mph+. Many boats in this class do not like to be between trolling speed and 30mph. I don't want to be in that category. Many times I was glad to be running a very comfortable 17-22knts straight into a 4-6' head sea chop with autopilot on and feet propped up on the dash. Quads on cats don't leave much room for trim tabs. However, I have a few creative solutions on trim that I'll save for a later date. They could be failures but could turn out to be very, very cool and innovative....
7.) Why not buy another boat or have one built?
  • I am very fortunate to be where I am in life but I don't have the budget to have someone build me a new boat with similar specs.
  • Cost effective and used 40' modern cats with quads are about as common as unicorns.
  • Also, aside from a new model from Calcutta, there is no equivalent on the water. There simply is not another boat in its class that suits our needs and style. Some people are cool with a million dollar rig that comes with a port-a-potti but the family spends a lot of time on the boat and they need (Full disclosure.... I do too) a little but more creature comforts without sacrificing fishability.
8.) Have you talked to Calcutta, engineers, architects, experts, Dads...?
  • Yes, yes, yes, yes, yes.... All have been very gracious with their time and have given project-saving advice. Discussions involved hull design vs speed, engine placement, weight rebalancing, structural reinforcement, practicality, etc.... Ad nauseum...
  • At the end of the day though, all are supportive of the project but notably cautious since this has never been done on a Calcutta. The asymmetrical hulls and very aggressive deadrise on the outboard side leave a little guess work to engine placement but I think we have a good plan moving forward with engine adjustment contingency.
With luck, this is a step towards that elusive perfect boat. Everyone's perfect boat is different. This will be perfect for us.

Any advice is welcome. If I choose a different route than your advice, it will still be considered, appreciated and respected.

More to follow....

Thanks,
Nick
Mad Natave





Old 02-13-2021, 10:51 AM
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In....seems like a solid plan for good reasons. Best of luck.
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Old 02-13-2021, 11:05 AM
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Looks like a fun project. I’d be looking at a set of the Mercury 300 V8s with CMS midsection. They’re just over 100 pounds lighter each than the Suzuki 300 with more torque that you’d want in the event you’re down to 3.

Last edited by davidwademarine; 02-13-2021 at 02:05 PM. Reason: Typo. I know the difference between your and you’re
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Old 02-13-2021, 12:52 PM
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Yes, I've been impressed with the weight and power of Mercs 300 V8. I'm engaged to Suzuki and don't see why they won't meet my expectations but the bracket geometry is being designed to accommodate all manufacturers and up to 400hp. In case I want to switch gears down the road.

Last edited by Mad Natave; 02-13-2021 at 12:57 PM.
Old 02-13-2021, 01:13 PM
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sounds like a cool project
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Old 02-13-2021, 01:27 PM
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Cool. Good luck
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Old 02-13-2021, 01:38 PM
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in
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Old 02-13-2021, 01:51 PM
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I read the why. Question. Multiplying the motors can possibly create more headaches during those long trips, no? Either way, looks to be a cool project. Best of luck
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Old 02-13-2021, 03:50 PM
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Sounds like a fun project.
I didn't see anywhere that you listed a generator, but if you're planning to run the AC/Heat off inverters, the Mercury 300s have much higher capacity alternators than Suzuki 300s. With 4 running you could run 2 AC units and never tap into the house bank. Suzukis wouldn't do that.
Old 02-13-2021, 04:05 PM
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Cool project, would love to see some pics of the cabin. What kind of performance did you get with the Yanmars?
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Old 02-13-2021, 04:07 PM
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Looks like an awesome project, following this completion
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Old 02-13-2021, 04:36 PM
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Aggressive project for sure, and it will be very interesting to follow.
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Old 02-13-2021, 04:46 PM
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Quad 300v8 would be my pick too. Torque will help if your down a motor. They are sweet motors. Looking forward to seeing this come to light. Hard to re-dress such a legend of a Calcutta but I can totally see why
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Old 02-13-2021, 05:41 PM
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Yes, we have a panda 4000 Generator. Small footprint. Very nice to have. Runs anything we want and without it, we wouldn't shut engines off on offshore trips. This allows to keep engine hours down by not running them all night.
Old 02-13-2021, 05:43 PM
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Cool project your going to miss that open transom tho
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Old 02-13-2021, 05:43 PM
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Originally Posted by KongerUp View Post
I read the why. Question. Multiplying the motors can possibly create more headaches during those long trips, no? Either way, looks to be a cool project. Best of luck
Kind of a double edge sword. More motors, more headaches. But we are leaning towards the more motors, more redundancy side of it.
Old 02-13-2021, 05:49 PM
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Originally Posted by BST24351 View Post
Cool project your going to miss that open transom tho
Yes we will. The dive platform on calm days was a popular place to bottom fish. There will be a nice open section in the middle for one person with a nice diver ladder. The tuna door will still open and I think we can slide pelagics in on the Starboard side (if needed). Also, the boat fishes well all the way around so there is ample space for any bottom fishing. With the bow rail, the bow is also a popular spot for people to post up. Especially jigging or popping.
Old 02-13-2021, 05:57 PM
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Originally Posted by BayHouse View Post
Cool project, would love to see some pics of the cabin. What kind of performance did you get with the Yanmars?
Some interior pics attached. There should be more pics on Mad Natave facebook page. Yanmars planed the boat at 17 knts, crusied at 22-24 knts (1.3nmpg) and top end around 32 knts. Mind you, I carry tons of gear, tools, spare parts and spare fluids. So tripping the boat down to an average load would of course get a little better. I couldn't ask for better fuel burn on a rig that big but it is time to move on.





Old 02-13-2021, 06:19 PM
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Cool boat. Project should be interesting. I think you’ll get an honest .8 mpg. I wouldn’t put much credence in the screen grabs people post of quad boats getting 1+, it’s tough to get 1 offshore with a load. I’ve had plenty of Suzuki’s and they’ve been reliable. I think you’ll be happy with them. Maybe not as happy as with the mercs though 😜
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Old 02-13-2021, 08:34 PM
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Can’t argue with a man that’s done his homework. Congrats on this project, looking forward to seeing more pics.
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