Outboard conversion on displacement hull

[Allan]

I'm looking for some good information on converting an older displacement hull (Davidson Chugger) that had a 1 cylinder 8.5 hp diesel (beyond repair) to small 4 cycle outboard. The boat is 18' long with a 6.5' beam and a full keel. Has a flat transom, made of fiberglass and a small cabin. Replaceing the diesel, plus fuel tank, prop and shaft and various other contol parts (which are all trashed) appears to be a very expensive option. So I was thinking of using a 9 to 10 hp four stoke outboard as a possible alternative. Would a transom bracket be a good way of mounting the outboard or should I think of making a proper motor well into the existing transom. Any thoughts or possible direction to some information on this topic would be appreciated. Most threads I've read seem to be dealing with outborad conversion on semi-displacement or planing hulls.

Thanks

[tom28571]

Here is a Chugger for sale that has a 9.9 high thrust outboard on it.

http://www.buysell.com/root/0/287365...ultdetail.aspx

[Willallison]

For starters, I'd go for a production type outboard bracket - one that will allow you to lift the engine clear of the water.... it makes beaching and trailering safer, and if you have to leave the boat in the water it eliminates any fouling / corrosion problems.
Given the size (and weight) of the engine you're replacing, and also given that this is a displacement boat, I doubt whether the resulting change in the boats centre of gravity will have much effect - you'd get far greater movements as a result of movements by the crew.
As Tom eluded to - I'd definitely go for a high-thrust outboard. This doesn't necessarily mean that you have to have a mototr with a different lower unit. We have a high-thrust prop that we occaisionally fit to our 15hp tender outboard. It allows us to quite easily tow our 25 ton cruiser around! Quite clever really.... being a merc, the exhaust exits via the prop hub. To ensure that the exhaust doesn't diminish the props efficiency in reverse, the prop sits on the spline with a special nut at the rear. When you go into reverse, the prop slides aft along the shaft until it hits the nut, sealing the normal exhaust outlet and forcing it out the front of the prop (which is now the rear as you're in reverse...! ).

[Tim B]

very,very clever idea, Will. I have to wonder it it's worth it, though.

On the origenal topic, if you're replacing an inboard with an outboard, the CG position will change. you may need to play about a bit to get the trim correct. Since it's a small weight, this should be quite easy. There's various methods, but for the first attempts a bag of gravel weighs about the right amount. It'll have to be somewhere up-front, but where is hard to say.

Tim B.

[Allan]

Thanks for the replies. I was thinking of using a Honda 9.9 long shaft, high thrust unit, with remote control. But with most of the outboard brackets I've seen so far I was wondering how to set up the steering and remote controls on these units. That made me think that maybe cutting into the existing transom and building a proper motor well might be the best option. But I haven't yet found any really good information on transom/motor well construction. I've basically stripped out the interior of the existing boat so modifying the weight distribution shouldn't be a problem.

[Willallison]

Regardless of how you do it - either as a built in outboard well, or a bolt on bracket - running the controls should be a relatively simple matter: you just have to ensure that there are no sharp bends in the cable. What kind of steering are you contemplating?

Tim: re the prop... it is most definitely worth it - the high thrust prop turns our normally sporty inflatable into a tug boat! Changing the prop takes no more than 5 minutes, and whilst we don't exactly need to do it every day, it has come in handy more than once. (btw - this is an off-the-shelf quicksilver product, not a diy prop....)

[Tim B]

fair enough, if it works. I just don't like added complication below the waterline, that's all.

Tim B.

[Allan]

I found some of the information I was looking for in this book "Fiberglass Boats" by Hugo Du Plessis if anyone is interested.