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Outboard Engine Modifications

Posted: Thu May 24, 2012 12:28 pm
by mmorasci
I hope this is the right forum for this topic (it seems to fit the best).

I am investigating modifying an outboard motor and before I spend too much time/money doing so wish to get some advise from the experts and devise a plan that has a better than good chance of success. My "doner" parts are from mid late 50's Johnson / Eveinrude motors.

Ultimately I am trying to achieve a tower between the power head and lower end of about 6" in total height. The hull of my boat is 3/16 steel plate (see my post under Ultra Pierre). My plan is to take the bottom 2.5" off of the tower section of a 35HP engine and bolt it to the lower end with the hull snadwhiched in between. This provides the basis for my boats' propulsion system. The lower end is fitted just behind the skeg and forward of the rudder and will not rotate as it is fixed to the hull. The power I wish to use is from an 18HP engine. I have designed the necessary components/housings to "marry" the powerhead to the tower section, actuate the shifter (F-N-R is maintained), and couple to the power head with the needed cooling water ports and exhaust gas passages, and can modify the drive shaft for power transmission. The "machanicals" all look good (on the drawing board, of course).

Where my concern lies is in the reduction of volume of the tower and in lowering the engine, do I:
1) Run a risk of wave pressure back flowing water through the cavity and into the combustion chamber?
2) By eliminating the port on the rear of the original tower where cooling water exits, do I cause excessive back pressure affecting perfromance?
3) With the vast reduction in tower volume in general, is engine perfromance comprimised?
4) Would a "backflow preventer" (springloaded or cable pull) be suggested for when the engine is not being run?
5) By eliminating the original tower and the subsequently the twist throttle, any ideas on rotating the timing plate for throttle control?
6) What else am I not thinking of that needs to be adressed?

There are many reasons I wish to do this modification and I have the mechanical design skills and access to the equipment to pull it off. While I have used these old outboards all my life and am fairly familiar with their overall use, I am by no means an expert, nor have I ever attempted this type of modification before.

I wish to throw this approach out there and get opinions and advise from the experts and so adress any/all issues/concerns that come up.

Thank you all for your consideration. ALL comments/suggestions are very welcome.


Re: Outboard Engine Modifications

Posted: Thu May 24, 2012 7:47 pm
by pamelalynne1
You will have several problems if I understand your proposal. The prop will by set too deep under the keel with a result in loss of efficiancy though drag of the lower unit and back pressure of the prop wash above the prop shaft. Read articles on surface piercing drives regarding loss of efficiency. You will have a mismatch between prop 35HP and power head 18 HP and will not be able to achieve maximum RPM levels causing further reduction in efficiency. Unless you have an alternate exit for the water relief you could have a problem with water backing up into the exhaust ports.
I have not seen your design; but I would think that a small diesel engine with a straight shaft or VEE drive and prop, or connected to a stern drive would be a much better option. You could also add a motor bracket to the transom and simply mount the 35 HP outboard.
In reality, you can make anything work to some degree; but some things are not worth the time, trouble or expense.

Re: Outboard Engine Modifications

Posted: Fri May 25, 2012 6:27 am
by mmorasci

Thanks for the feedback. I had not heard of surface piercing drives so I'll take a look this weekend. The max RPM due to mismatch of components doesn't have me too concerned (maybe because I don't know enough to be troubled by it yet). My thinking is that Hull speed is about 6 knots and I was looking to either find the best fit for a prop (I have several props for the 35 lower end in different pitches to choose from so I thought I'd start there or if needed, I can turn down the prop shaft to accepet one of the 18HP prop's I have). The water backing up is of great concern and I as yet do not have a remedy for this. The Boat is Ken Hankinson's Ultra Pierre Sail Version and he suggests 10HP is all that's needed to get the boat to hull speed. I was thinking an 18HP power head for a little added power if needed, they're fairly light (when stripped down), and I happen to have several lying around so they fit the budget nicely.

Part of my enjoyment in life is designing/building and in the beginning of this project (4 years or so ago), after much thought to a drive system, I penciled this idea in and then proceeded to complete the hull / deck / tank etc modifications around the concepet. I'm affraid now i may have "painted myself into a corner". Trditionally a pull up shaft would have been employed but with the cabin and centerboard trunk this is not really an option. An outboard in a well, takes up a lot of valueable remaining deck space and plugging the well and stowing the outboard while under sail don't seem appealing. I have thought about hanging a motor off the transom but can't seem to get the details right. Dory's have such raked and narrow transoms and with that big rudder hanging there I'm struggleing with bracket design and then how to subsequently mount/route the controls.

Now the real crux of the problem. The Admiral (and chief accountant) says the budget is about exahsted and it's time to finish this project and move on. While I have time to devote, some raw materials and the equipment, the cash outlay for an engine and outdrive system is going to be a tough sell at home. May have to cut a couple of ports in the combing, add a seat to the centerboard trunk and make up a set of oars for when the wind isn't blowing. I am trying to not have to undo much o fthe work that has been done so far, but there is a lot here to think about.

Thank you so much for the input, I think I have a lot to rethink and likely some parts to find.

As I continue to chug through this design any and all comments are very much appreciated.



Re: Outboard Engine Modifications

Posted: Fri May 25, 2012 8:33 am
by upspirate
This concept has been done some back in the 1950's/60's....I get some time this w/e I'll see what I can find

There also was a commercially produced boat produced with the same concept using OMC products(powerhead,lower unit mounted to the hull)

So your concept is doable.

The ignition needs to advance with the throttle

Re: Outboard Engine Modifications

Posted: Fri May 25, 2012 7:26 pm
by upspirate
Here is a small boat that they modified similar:

OMC also made a saildrive....similar concept.maybe you can see some of the hook ups here: ... 20Unit.pdf

This is a pic of the commercial boat they made with an OMC powerhead/lower was mounted forward of the driver.....can't find more on it right now
small inboard 2.jpg
small inboard 1.jpg
It can be to figure out how!!!

Re: Outboard Engine Modifications

Posted: Sun May 27, 2012 11:13 am
by mmorasci

WOW, Thanks. There is a tremendous wealth of info here in the links you posted. The adapter in the link looks very similar in concept to the adapter I've designed. Perhaps I was over thinking the exhaust chamber design a bit too much and it's affect on performance. My motorcycle background suggested that expansion chamber design had a lot to do with overall performance, but these approaches don't seem to have cared much.

Yesterday had me under/in the boat looking at a slightly different approach which may turn out to be easier. I got out the grinder, and by modifying the hull slightly, I can go back to the 18HP lower end, if I can "shrink" the tower to about 7" total height by cutting of the ends (4" from the top and 3" from the bottom) and rejoining them with a couple adapter plates, I'll be able to fit the assembly in the defined space.

The trick now is in re-packaging the mechanicals (I.e. driveshaft, coolant line and shift mechanism). Another week or so on the design board and maybe I'll see some light at the end of this particular tunnel. I will still have to find a clever way to rotate the ignition plate for throttle control as I have removed all attachments for the original throttle mechanism.

You have truly given me some hope and a lot of good information to carefully study.

Thanks again,