blocking off the top of water pickups. rule of thumb?

[conig]

i have read and heard of people blocking off the top 2-3 holes of the factory water intakes on lower units. Is there a rule of thumb for how much one can block off. example 1/3 or 1/2?

its a full manual tiller on a cmc with only a tachometer.

[rock]

Before you do anything I might suggest you try a set of scoops. They are small plates that cover the holes but let enough water in to pressurize the area allowing you to run higher on the transom. I have had good luck with them.

Rock

[conig]

i like the idea. where can I find a set?

[JUPITER PULSARE]

i like the idea. where can I find a set?

They were made my Mercury but are no longer manufactured as far as I know.

[conig]

They were made my Mercury but are no longer manufactured as far as I know.

the motor is a 2005 omc 40 2 stroke. the last one before the etec. i googled abd couldnt find a pic.

[home made tunnel]

There's a thread on this site somewhere that tells about the thread size, drill size, tap size, and stainless NPT allen-head plug size to do it nice and proper. I did it on my lower for the top 3 holes, and never had a problem.
Do some digging - it's a good write-up

[quicklt1]

I blocked top 4 of the 8 on my 200 works great, find thread...

[DanUmbarger]

I couldn't find anything here for the OMC's but did find the one where they used the allen plugs for the Merc's. The Merc holes are smaller ( and more of them ) than the OMC's so on the OMC's I think you'd have to epoxy the top couple of holes...can't imagine finding plugs big enough without cutting off volume area in the inner channel area.

[delawarerick]

I have a v4 with 4 holes I block the top and raise engine till I loose pressure that down 1/2 inch. I also widen the slots.

[sharpeye Mike]

Not a big deal. It's only to minimize the amount of air the pump sucks in. Just rough up the paint and fill the holes and grooves with either silicone or epoxy. they both work good.

[FMP]

What's the best way to attach some scoops? My intakes are low on line with propshaft but was thinking about helping them grab. I've seen the ProMarine Merc scoop and making some doesn't seem a problem , drill and tap I guess is about the most secure way to keep them put but drilling this area or weld a small scoop then shape? Maybe shape and get more inlet area as mentioned above to keep clean water to the prop?
Not meaning to jack the thread just putting it out as an option to your plugs.

[conig]

on this motor it has plastic grates. they cover only 2 holes at the top which are roughly 3/8 inch holes running vertically up the lower unit.

I removed the grates and wrapped the top of them in one strip of electrical tape and then screwed the grates back down. the tape is tightly supported at all corners. I was able to secure trim the prop till it blew out without the stream getting weak or stopping. it does take alittle longer to start flowing when first started.

now I need to replace this rapture prop with something more efficient. its a fine fishing prop but i know something faster exists. i am running 40mph 5500 rpm 2.32 gear case 19pitch rapture 14ft flat bottom 40hp.

[Slimm]

On my old engine I drilled all eight on each side larger by a couple sizes and then tapped the top 4 and installed some allen set screws with red loctite to make sure they didn't come out. I did got the idea from a post on here but can't find it.

* I just remembered, before I did all that I used some black duct tape that my brother uses in his heating/AC job to block off the top holes just to make sure it did what I wanted. It worked and to honest I left it on there for almost a whole spring, summer and fall. I was having too much fun fishing to be bothered with doing it the right way and I finally felt bad enough to take the tape off and do it properly...it just happen to coincide with cold weather and heavy rain. :D I was rewarded by a couple more weeks of good fishing after that so I stand by my decisions!

And for what it's worth I almost didn't get that damn tape off! Whatever that black commercial duct tape is made of is some tough stuff!


Slimm

[nitro_rat]

http://www.screamandfly.com/archive/index.php/t-165557.html

[phillnjack]

just takes off your grates and on the inside of them fill with epoxy resin. quick dry 5 minutes stuff will do then go try it out,
if it dont work then simply take the resin off. if it works then leave it alone, or get a piece of plastic and make your own grates.
if you use something like hdpe you can bend it with a hot air gun to whatever you want...

[home made tunnel]

http://www.screamandfly.com/archive/index.php/t-165557.html

That's the one.
5/16 SS set screws and a tap set to tap threads in the top holes. Make sure you get the NPT threaded set screws (pipe thread, they're tapered and will tighten).

[home made tunnel]

MCMASTER.COM has a million - good prices too.

[71V153]

Before ya do all that crap. Install a water pressure gauge, and test your hi-temp alarm(s).

That'll tell ya how high you can go.

[loop]

On my old engine I drilled all eight on each side larger by a couple sizes and then tapped the top 4 and installed some allen set screws with red loctite to make sure they didn't come out. I did got the idea from a post on here but can't find it.

* I just remembered, before I did all that I used some black duct tape that my brother uses in his heating/AC job to block off the top holes just to make sure it did what I wanted. It worked and to honest I left it on there for almost a whole spring, summer and fall. I was having too much fun fishing to be bothered with doing it the right way and I finally felt bad enough to take the tape off and do it properly...it just happen to coincide with cold weather and heavy rain. :D I was rewarded by a couple more weeks of good fishing after that so I stand by my decisions!

And for what it's worth I almost didn't get that damn tape off! Whatever that black commercial duct tape is made of is some tough stuff!


Slimm
Did the same thing with plain ol duct silver tape once. This was on an 80+ mph boat. Was surprised how long it held up and it gave me a good idea of what i needed to block off. Carefully applied Gorilla tape would probably hold up much better.