help with io to outboard conversion, engine sizing for boat

new to boating.i have just been given a boat ,it at one time had an inboard but has long since been removed.a freind of mine has put an out board on it but im not sure if he has done it correctly (he is a bit of a bogde artist).it is 18 ft long fiberglass with a cabin it had a 1600 ford petrol engine in it. he has done something with the transom and bolted on a 40hp outboard which is now broken and been removed.i want to know what size engine would be suitable (fishing on the south coast of england southampton)and what would be needed to modify boat from inboard to out board if i was starting from scratch to do it properly thanks for any advise . money not really an issue as i got boat for free boat is in very good condition other than installing an engine.

 

need a bit more info to be able to help. Some photos of the boat would be good.
Items to consider are:
weight, moving from inboard to outboard is moving a lot of weight to the stern of the boat, can it accommodate this? What is the weight of the boat? Is it a lightweight hull or a heavy displacement type of hull? Was the original engine connected to a standard prop and shaft or was it a stern drive? How far forward was the original engine?
horsepower, what kind of hull is it, planing or displacement? A planing hull will need a lot more horsepower than a displacement hull. For a displacement hull eight horsepower would be fine, for planing 40 may be adequate but is probably on the low side for decent performance.
transom construction, is it strong enough to support the weight and thrust of an outboard motor?
With some answers to these questions then a more helpful reply can be formulated.

 

hi thankou for the responce ,sorry to be vague but this boat has come to me with no info but on further investigation i think it is a sunseeker corsare /poole power boat 18 the engine was right at the back behind the transom and a stern drive ,i hhave been told the hull is chine?? i have a freind who was a grp laminator at vospers who is going to rebuild the transom after the initial bodge the other person has made as this is going to be used for fishing its not going to be thrashed. i have seen a 75 horse power johnson old but in good working order i am thinking about buying. the concern i have now is that since the engine was an inboard and now going to be outboard will i have to put more weight in the back to compensate as i assume the inboard was a lot heavier .will it be a waste of time trying to get this boat back in the water???

 

Hi Tony,
the problem, if any is not the actual weight of the engine but its location. Even though the outboard may be lighter in weight, its location on the outside of the transom is furter back than the original engine by at least two and possibly three feet. The additional distance of the outboard from the center of buoyancy of the hull will sink the stern of the boat further than the original engine weight did. This can be compensated for by moving other weights forward, such as the batteries and the fuel tanks, so it is not a fatal flaw in your plans. Probaly the best way for you to proceed is to get the engine and mount it, and then sort out the balance issues by trial and error after the boat is launched.
One other thing to bear in mind is the transon angle. When designed for use with an outboard the transom of a boat is angled aft about 10 degrees from the vertical when compared to the plane of the waterline. Your boat was designed as an inboard/outdrive so some modification of the transom to get the correct angle may be required. Most outboards can be trimmed to accommodate more angle but not less.
I hope these comments are of some use.
All the best with the project.
David

 

90-115 hp is pretty much standard for this size.

A transom wedge under the mounting plate will get you more angle (14-15 degrees is standard) on the transom, if needed.

http://www.bobsmachine.com/

look on the index under Transom; Transom wedges
$30 a pair.