Omc 25h.p.

[JohnsonM50]

I do have a prop question before I go, I had to trim a prop to fit front to back. I trimmed away at the lower unit 1st as much as I could, then minimal y from the prop. As I mentioned it slips, you can feel it grab and release, it takes awhile to start, It does go but not with full solid traction, I tried lowering and tucking and got a small improvement. By eye it looks like its spilling off the sides too much rather than directing water to the trailing edge so I put more side cup on it and slightly curved the last 1/8 in of the trailing edge. Sometimes things are opposite to what seems right so am I on the right track is the question?

[Skoontz]

It's very difficult to fully grasp what is going on witho9ut seeing/driving/hearing the problem first hand, but to me, what you are describing sounds like blowout, which is what we called a problem the pre
'77 OMC thru hub 3 and 4 cylinder gearcases had. The design was such that at a certain speed, usually over 60, the water flatened out in front and caused an air pocket along the gear case. In '77-8 they changed housings, and their case looked more like a Merc than a Merc gearcase itself which solved that problem.

You are running a screw together housing but you epoxied all the holes and coned it, without seeing it in a test situation with colored water blown past the sides all you can do is play trial and error.

I can't imagine why a prop would slip from the hub by filing metal off a gear case. I'm not a prop guy but I';ll say that every time we ever ran any sort of side cup on a pre '77 gear case, it helped the immediate problem but did not solve what really needed to be done. The side cup as I understand has speed limitations in that it slows the rotation of the prop, thus takes valuable power. I had a hairbrained scheme for your rig that would be worth trying.

How about finding a C-D series hot dog gear case. So you don't want to drop the coin on it, thats fine. Photograph it and then shape the outside of your case with epoxy to look like the C-D housing. Then paint it and run it to see if there is a gain. You might have to cut off the skeg and heliarc a thinner one from a hunk of aludimon because you won't be able to reshape it's thickness and make it more effective.

Just a thought in the never ending quest for cheap speed. Trade dollars for labor and see what it gets you.

[JohnsonM50]

Thanks, I did copy-cat a D prop Ive got that I know works so maybe?? Labor for dollars, trial & error low cost speed.. yup

[RichardKCMo]

I hope this thread keeps going , very int. reading.
Richard

[JohnsonM50]

Me too, Ill run that hydro soon then soon after Ill run my runnabout so Ill be sure to have Q s maybe some A s. My runnabouts special to me, I built it, ran many motors on it inc. the 1 on the hydro. As Skoontz says the best $ is spent below the anticav. plate, hes right. I put a race lower on it awhile back. I just installed a balanced & mildly worked 35 OMC on it, graphite coated the bottom aft, put more roundover in the chine [used to be square for the 1st 1/4 in deep] and moved the fin back 5in [It spun once]. I doubt Ill get much speed from the speed coat but I hope it decreases the porpoise effect some and slides a little better in turns [rather than grab & hop]. My C hydro is definitly a faster hull Id bet this runnabout motor would make it fly! I need to get a flat bolt on mount, no clamp hooks cause Id have to cut the transome deep.

[Skoontz]

I had a brain fart for consideration on your 25.

Instead of epoxy in the screw holes and a cone, how about getting a hunk of aluminum tubing and welding silde brackets in the stock housing to slide it over, then attach the nose cone to the tube. Cut the top of the exhaust housing over the 5/8" bolt and add a hunk of 1 1/2 x 3" rectangular tubing just over the cavitation plate for an above water exhaust. If you need more propeller diamter you could add a inch and a half to the metal between the bullet and the bottom of the cavitation plate, then lengthen the driveshaft. Another trick would be to move the water pickup via aluminum tubing to the front bottom of the nose cone, then remove the water pump. You are going to force feed the water by using speed. Add a temp guage to see if you are heating up and if not, the removal of the pump would gain you 3-7 more usable HP (prop shaft HP).

[JohnsonM50]

Thats actually a cool set of ideas, I just got the hydro put away from a ride and it worked pretty well, GPS-ed 52.3 but not with the prop I messed with, it improved but still isnt quite there.
Now my airbornes ready so its goin on the trailer and Im going to pass this lower on loan to a pal who s got a wetback hydro and an omc 35. why would I let it go you ask... cause now I got a foot to make a real rat - heh heh.
I cant wait to go in the airborn, it can blow across the rough water of summer traffic without worrying about stuffing.

[Skoontz]

Now yer cooking. What brand lower unit and what gearing? You should still play with an open exhaust, and I think the rectangular tubing over the cav plate is the best route. If noise is an issue, find the water discharge and dump it into that opening.

[JohnsonM50]

I bought a used Y 102 hatchet 14 - 15 that I wanted for a spare and am going to see if it can be adapted to a shortened 25hp tower and a lower cut at the cav plate that will have a similar exhaust & water pickup as my 102 except this will have an omc pump. I have to ride some slow zones to play so definitly want a pump. I have a 1in relief hole in the tower maybe 1 more.
I test ran my airborne [runnabout] today man was it rough from boat wakes, from every direction - a freakin washin machine! The boat ran good, 2 hrs of break- in with the prop set as low as it can go and the timing set slightly retarded. Now I can set it up to go and try on a weekday eve when it might be calmer. After sayin I didnt have to worry about stuffin I took a splash over the bow and got hosed good. ha ha.

[Skoontz]

Heres a thought.....If you remove the water pump, through the slow zones what about using a hand held battery operated pump in the water pick up line? When done, put a clip somewhere in the boat to lock it down.