Advice Needed: Cruiser Yacht with Outboards

[Jim Cate]

Bish, in case you didn't know, Boat Poker (who has been giving some advice about both the generic construction of cored hulls and transoms and your specific marque of boat) is a professional surveyor. I'd pay very close attention to his warnings, and not just because they expand upon mine way upthread, but because the results of the sort of damage he mentions could literally rip the transom out of your boat without warning, with obvious results.

It's easy to say you will never stress the boat or have it out in rough conditions, but things do happen that you don't anticipate. A for instance: you mention that if a storm was to build unexpectedly you could be home in around a half hour. Often in such situations folks will floor the throttle to hurry things up, putting max stress on the transom and other parts of the hull. So, speeding along, you hit one of the building waves a bit off course, fly off the top and crash into the trough... adding a lot of inertial load to the thrust loads already there. That's when the weakened or missing bonds between skins and core will fail, and you really don't want to be there just then!

Please don't take this as an attack, but a warning of severe consequences presented by a very experienced surveyor and a sorta experienced yottie.

Jim

[transmitterdan]

Listen to Boat Poker.

Thrust from the original engines was transferred from the transmission and engine to the boat via the stringers attached to the hull down low in the bilge. The transom was never intended to accept the forward or reverse thrust of propulsion. It’s very difficult to imagine that the cost of rebuilding the transom to transfer thrust to the hull would be economical on a 30 foot Cruisers Yachts boat.

A thorough sounding and moisture survey of the hull above and below the waterline would be mandatory as the boat hangs in the slings. It would be a huge surprise if the hull does not have water inside the laminate somewhere.

[dasreboot]

looks like it is an I/O boat, so a outboard retrofit is probably doable. A better forum to ask this question is probably "the hull truth " forums. someone there has probably done it.

[Saranda]

Hello my friend,
I have just log into my first form.
I have being boating for some time and have had several boats. Some people are lucky to have been brought up in the boating scene, but I wasn't and the consequence was and still is, a learning curve.
I had a 6 meter boat that was a inboard with a 165 hp Holden v6 engine coupled with a sterndrive. I changed the boat to a outboard with a boat builder modifying the stern. (transom). I thought that the inboard was under powered , so I stuck a 225 Yamaha outboard on. The consequence was, two much power, a transom that had to be strengthened, the center of gravity was higher resulting in it rolling more side to side. I used the engine cavity as storage more fuel tanks and dive bottles. Also to equalize the extra weight on the back, I moved the fuel tank forward. The main reason for going to outboard was , the sterndrive was to much maintenance and would never have another.
Outcome from this was great, Boat is far more responsive, easy to maintain and I don't regret it.
However, what the others have been saying about - Strength of transom, stability are correct.
One thing I did was join the coastguard as a volunteer. They put me through lots of course and practical training. Over 13 years have past, with several boats later and am now a skipper for the coastguard teaching others.
Hope this helps.

[OleBird]

Bish,


I'm in the middle of just that conversion - IO to OB - two Merc 165 hp 4 cyl, to two 175 hp Suzuki's. I ran the I/O's for 10 years with no problems, but age caught up with them, along with noise & vibrations. I have consulted with a number of others who have made the switch - most are very happy with the results. My OB's are mounted on a custom designed aluminum bracket bolted thru the transom with high strength bolts. We put a lot of engineering into the set up taking into account changes to weight distribution as well as buoyancy. Total empty weight is down by nearly 300 lbs - but the CG has moved to the stern by 20" +/-. My hull is not cored, but solid fiberglass. A cored hull could represent a real weak spot. I would research that very carefully. If your target is a balsa core design - check it for moisture very carefully. Any sign of moisture would be a show stopper for me. I've had a few issues of wet cores in decking. That can be fixed, but a wet cored hull (converted or not) is just not acceptable to me.



It's true the transom was not likely designed to handle the engine (thrust & torques) and should be reinforced with structure to transfer those forces to the original engine mounts and stringers. Should be pretty obvious looking in the old engine room. Mine consist of 4 vertical - 2' wide epoxy/coosa board cored gussets between the transom and hull and horizontal "U channel", gunnel to gunnel, from the same material - which also serves as a battery shelf. Because of CG issues, I had to move starting batteries to the transom. Also added two 3/8 Aluminum gussets that are bolted to the bracket (thru the transom) and secured to old engine mounts. I've added a couple of strain gages on these gussets to monitor for any movement.


I'm not back in the water yet, but two others that I know of are and both report good handling results, no unexpected problems. One had to change props for proper engine loading, but otherwise it worked pretty well out of the box.


Soooo, it's not unheard of, done more often than generally noted. It can be done with good results, but does require some thought and engineering. There are some potential serious problems. I would have a recognized surveyor look it over carefully.


Old sage, 'Never fall in love with something that can't hug you back - or you wouldn't drill a hole in - especially a boat!!"


Good Luck