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Re: Replacing Inboard with Outboard
Our boat is also 27ft and displaces close to 8000lbs, she was original designed for an 8hp inboard. The inboard was a block of rust and I didn't want to pay close to 3x what we paid for the boat to put a new inboard in place, so we mounted an outboard off the transom.
We were told you needed 1 hp per ton of displacement, which would mean we needed at least 4 hp. Rather than "risk" it, we went with the Tohatsu 6 HP Sail Pro, which has an extra long (25in) shaft and a small alternator, we bought it new for roughly $1500. The motor performs wonderfully and pushes our boat at about 5.5-6 knots at 20% power, which is better than the original inboard would do. With current and wind, that obviously changes, but I've never seen it make a major impact. You do NOT need anything over 6 HP, I've seen 15hp outboards on the back of 23ft sailboats before and it is way overkill. With the outboard, you are looking to make and keep momentum, it isn't going to push the boat beyond hull speed.
The biggest drawback is the prop coming out of the water with any kind of wave action, resulting in a loss of thrust. It has not been an issue for us yet, but it can make getting in and out of places with a lot of big waves complicated. This is the biggest concern, but is manageable if you learn to compensate by sailing the boat. The 25in shaft definitely helps, but it's not perfect. We've pushed against 20 knot headwinds with small swells fine, but you aren't gonna have the same performance you will with an inboard and you can expect to have issues with this at some point.
You will want a good outboard mount with plenty of travel, don't cheap out here, we did and I regret it (we are replacing it next week). I looked around and at the recommendation of James Baldwin of AtomVoyages, we went with the Garhauer OB-125 mount ($250, roughly). The mount is spring loaded to help with weight and has close to 15in of travel. I should be able to comment on it's performance in a week or two, after I pull the old one, fill the holes, and put the new one on.
As for your negatives:
1. We didn't notice any significant balance changes. The motor weighs 75lbs, which should not have a huge impact, but it might. You can always add weight up front to compensate.
2. Ours is over 1/2'' thick and we reinforced it with a 1in backing plate to be safe. There will be a lot of pressure on the transom at this point, so reinforcing it as much as possible is definitely recommended.
3. We filled the stern tube with epoxy and glassed over it, if you really want to get fancy, you can build a wood filler for the rudder and keel that removes the empty space where the prop normally sits. We haven't done this yet, but James Baldwin has some info on his site about doing this. We also removed the thru hull and seacock for the motor.
As for using your existing fuel tank, I'm not sure I would do that. If you are going to pull the A4, you might as well remove the risks of having gas in the cabin all together and find a convenient way to mount the fuel tank outside. We removed everything and it opened up a lot of storage space, also removing odors associated with inboards completely.
Last edited by Shinook; 10-18-2013 at 06:56 PM.