what position?

hi i've got a 70's i think 55 hp chrysler, it's on an ancient 16 foot duracraft runabout. how would i or do i determine the proper height of the motor on the transom? there were several holes bored in it already, i had/have trouble getting it to plane it's hard on gas but i've noticed that after installing a hydrofoil it planes a bit better however i have it trimmed out on the last possible hole (manual) and to me it seems that it sits a bit low still. i guess my question is how far below the hull is the anti cavitation plate?? ( the thing the hydrofoil bolts onto ) supposed to be? thanks alot eh!!

Re: what position?

That thing you bolt the the doelfin on should be level with the bottom.
The motor sounds like it has the wrong prop.Was it used on a pontoon or a lighter boat?What rpm can you achieve and what pitch is the prop? J

Re: what position?

the motor came off of a 14 foot aluminum i believe. my boat is light enough for me to position it on the trailer by lifting it with my arms so i don't think it's a weight issue. the only numbers i could find on the prop are 0 3/8by 12 1/2. will an inch below bottom with the fin make a lot of difference? also it seems to be quite hard on fuel i am wondering if the fuel/air mixture (rich/lean) screw might be too far to the rich side? i would like to get a newer motor but the whole boat isn't worth it and would like to tune this one a bit. thanks for the reply's i appreciate every bit of help i can get.

Re: what position?

anti cavitation plate AND hydrofoil must be out of the water when running at full throttle; so the they must be positioned 1 or 1.5 inches above the bottom of the hull.

Re: what position?

thank you

Re: what position?

The one is missing! the prop is a 10 3/8 diameter by 12 1/2 pitch. That is a little much for a 55 on a 16 foot boat. If you can find one, a 10 3/8 X 11 1/2 might give a bit better performance. Check on ebay or The Chrysler Crew. I don't believe iboats carries those props. HOWEVER: If they do carry props for early Force 50 HP engines, they are the same prop with splined shaft and thrust pin drive. Again, spending 80-100 bucks for a new prop is not really worth experimenting unless you intend to keep the boat. Unless you can get a really good sale, better to try the auctions.

Be careful with the idle needle. Yeah! Leaning it will give a little better economy and a better idle. BUT the top cylinder normally runs a little leaner than the bottom. Go too lean on the low speed needle and the money you save in gas will be spent on a new piston. I strongly recommend that unless you have a specific problem, you leave the low speed needle alone.

Outboards normally are fuel hogs. If you can't afford the gas, then you can't afford the boat.

As far as the hydro-foil: If it improves performance, then go with it. However, with it setting below the surface of the water at planing speeds, it will increase drag and lower top speed. That engine is not so heavy or powerful that you can not test run it without the bottom two bolts. SO: For test purposes only. I have done it a number of times. A safety chain would be a good idea though. Raise the engine on the transom in increments by putting a spacer under the clamps and tightening the clamping screws. At each increment, vary the tilt setting to see what happens. When you find the point where the engine starts to cavitate while on plane and in a straight line, lower it about 1/4 inch. This will most likely give the best all around performance. Once you find this sweet spot, install the bottom two bolts.

Re: what position?

Outboards normally are fuel hogs. If you can't afford the gas, then you can't afford the boat...... by hard on fuel i mean like almost 5 gallons for a 6to 10 mile run i know outboards are hard on fuel but that seems a bit much. so i shold start with the foil level with the bottom of the boat and eperimen from there huh? i'm assuming that if i getthe motor set to right height there will be less drag therefore more plane and top end? boy it wouls be saweeet if i could get this thing to work right, i've had it for 6or 7 years and have had grief from the start, just too poor to buy new boat and sort of just settled on poor performance till i found this site! thanks frank!

Re: what position?

Yeah! That is a little much for fuel consumption. My older (1967 ) 55 on a 15 foot deep vee runabout used to cruise for roughly two hours at about 3/4 throttle and about 20 miles per hour on a six gallon tank. That works out to about 6 1/2 MPG. Never checked it at full throttle, but I would not be surprised if fuel consumption doubled or more.

So: if you are having drag problems and the boat is plowing through the water, It is conceivable that you would be using a lot of gas to go a short distance.

I would start with the cavitation plate and hydro foil even with the bottom of the hull and go up from in steps from there. First step would normally be one inch. If you encounter no problems, then raise up 1/4 inch at a time.