Bristol 32 Electric Conversion Project

Bow heavy with engine removed

Since I've been sailing Intrepid and I started this blog, the biggest thing that has happened has been the project to convert the boat to run with an electric motor. This was a pretty big move that took some time to see through. When I acquired the boat, Intrepid came with a well functioning Yanmar 3GM30 diesel engine. The engine served us well for awhile but then one day there was an oil leak and the engine ran with low oil pressure long enough to cause some damage. The engine was taken out of the boat and after a long and inconvenient downtime period and at great expense, it was rebuilt and reinstalled in the boat. It was while the engine was out of the boat, that I first began to consider the alternatives and question the sense of having a diesel engine in a sailboat. When the newly rebuilt engine still gave me trouble, it was then that I made the commitment to make the switch. So one day I just hired a crane, pulled the engine out and went to work... Once the project was completed, enjoying the results and then hearing of world events such as the Gulf of Mexico oil spill, fully strengthened my resolve that converting to electric was absolutely the right move.





Before: Yanmar 3GM30 diesel engine


Converting s/v Intrepid into an electric boat, I have to say honestly, was a real pain in the #$^&. But in hindsight it was definitely worth it and it can be (and was) done by an inexperienced amateur. Working part time by myself, and making lots of mistakes, the project took me approximately 9 months to complete and cost roughly the resale value of a Yanmar 3GM30 diesel engine. Since the work was all consuming, I often forgot to take pictures and didn't take very good notes. What little I present here, is not meant to be instructive, but really just of interest especially to those who have sailed on Intrepid since and want to gain an appreciation for what all went into this green sailing wonder.

Since the marina charges a lot to be hauled out, I decided to do the entire project in the water at the normal slip. The engine was lifted out of the boat at the seawall and then we had to sail back into the slip. The winds were just right and we found that sailing around the dock is really no problem at all. What was interesting though was seeing the boat in the water with about 400 pounds taken out of the stern. S/V Intrepid would float front heavy, with her butt in the air, for the next several months.

Engine mess

One of the first things that had to be done was remove and de-tox the boat of all the diesel engine components and grime. All of the fuel, exhaust and cooling system components had to be removed of course. Removing the 25 gallon fuel tank opened up a large area under the cockpit. Also, the area where the conventional house and starter batteries used to be had to be removed. This opened up storage space in the cockpit lockers. Once all this area was cleaned and painted, a new 25 gallon water tank was able to fit in the same space where the fuel tank once sat. The area where the batteries used to sit on a shelf was cleaned and made into a dedicated dive gear storage space.

Removing and replacing the previous 12 volt DC system (batteries, cables, charger, monitor, etc.) was a big project. Of course re-doing the electrical system is really what this project was all about. As tedious as it was, it was actually very interesting at the same time to see how everything used to work (or really didn't!) and come up with a plan for how it really should work. I should've known my old system better, but the new system is so much simpler, actually. While re-doing the electrical system I was able to re-run the wiring the way it should be, neatly and was able to trace and inspect every wire. All of the supporting components, such as the new 48 volt battery charger, DC-DC converter, and eventually a 48 volt inverter and regulators for solar panels and wind generator are now all located in one convenient and protected area that is close to the motor controller and 12 volt distribution panel. All the electrical system components are now protected and easily accessible.

Motor and battery mount under construction

Another big project- actually the biggest and hardest, was the placement of the batteries. Ideally, you want the heavy batteries to be low and center in the boat and away from the motor, especially if they are wet cell, like on Intrepid since they can give off explosive hydrogen gas. Of course on Intrepid, there is absolutely no other place practical to put the batteries, except on the old engine mount- right next to the motor! The best place would be above the keel, but Intrepid has a very shallow bilge and this would  require cutting up the cabin sole and taking up already cramped living space. Another option would be under the settees, but again, the space is cramped and the hull is thinner there and I don't think I'd want to add that much weight to the side of the hull like that. The area where the engine sat is the most reinforced area of the hull and is the only place I really felt comfortable placing 400 pounds of weight. The idea then was to build a box for the batteries that would adequately separate the batteries from the motor and prevent gasses from being ignited by sparks.

Motor and battery mount complete

Once the engine mounts were laid bare, they were cut to shape and support an FRP reinforced heavy duty plywood box that was made to fit the four EB-160 batteries that were to be used with the system. Transverse supports were glassed into the engine mounts to make a (tic-tac-toe shaped) matrix to support the weight over the mounts. They were very carefully measured so the weight would be evenly distributed over that area. My biggest fear was hauling out one day and seeing a huge crack in the hull from the weight being improperly supported. I think it's overkill now though, as I laid several more layers of glass, and in some places epoxy layup (epoxy over FRP, not the other way around, as I learned- thanks Don Casey, my best friend through this whole process!) to support the box in a sturdy matrix over the engine mounts. The box was then bolted to the matrix with stainless steel brackets, in case I ever want to remove it, I can.

Asmo Marine Thoosa 9000 installed

The motor by the way, the rails that come with the Asmo Marine Thoosa 9000 system were too long to fit on the old engine mounts the way I have the battery box mounted, so I had to have custom made motor rails for my system. Luckily there is a very good machine shop in Okinawa and they know me very well over there now. The custom made, shorter rails are actually thicker and probably stronger. The motor, with the custom rails fits just right behind the battery box.

Battery box

Finishing the battery box, first a cover had to be made that would protect the batteries, contain/ properly ventilate explosive gasses and also look nice. So I chose to make a teak cover that fit over the top half of the box and supports a storage box that acts as a step for the companionway. This also required that a ladder be made. In my humble opinion it looks alot better and is more functional than before. As for ventilation... this is a tough subject. There is a removable wall that completes the separation of the motor from the batteries (and cabin). Really, the batteries are part of the cabin and any gasses would ventilate mostly into the cabin. Originally, I had cut a big hole in the back of the top of the battery box to lead a duct hose, connected to a blower fan to ventilate out to the cockpit. But I haven't gotten around to finishing the blower fan and so far, in 2 years of operating the boat, it hasn't blown up yet. I really think that the natural ventilation through the top of the battery box is probably good enough, but it is still something that I'm cautiously watching.

Completed system

One last design issue, regarding the battery box placement was what it did to the surrounding cabin space. Originally there was a large icebox next to the diesel engine that had alot of thick insulation that was old and probably didn't do much. The usable space in the icebox was actually small so it was really a waste of space. I always thought it would be nice to have a proper sit down navigation desk so... in order to fit the battery box under the companionway, I had to knock out the old icebox, which very conveniently allowed for the placement of a sit down nav desk. It is very small and cramped, but good enough and the desk is big enough to navigate with and I made it open up to fit my laptop computer. Everything I ever wanted!

There's a lot more, I'm sure to say about the conversion and the design considerations. This is really just a summary of 9 months of work. In a future post I'd like to share some things about the design considerations for the propeller and shaft.  Of course more posts later about performance and... even better- all the fun we're having aboard s/v Intrepid now!