Raw water intake size Thru hull

Cheers guy's, The boat will only be on the water for one or two days at a time so I'm not worried too much about growth it will however be able to travel at 50-60 MPH would that be an ok pressure to put on the engine, reading the link that Stumble posted and I take it this is for yachts so what would it be like for a powerboat I guess with two 4" exhaust pipes it may not be such a big problem.

This is from the link.

Beyond these photos many boat yards, some builders, and DIY's install the scoops style strainers BACKWARDS on sailboats. Many a sailor have actually caused their engines to become hydrolocked and dealt with water in the cylinders because of the improper installation of a scoop style strainer. If a scoop strainer faces forward you risk physically scooping sea water up and over the siphon break then filling the exhaust and eventually the cylinders of your engine with sea water. This is not healthy for the engine or your own piece of mind.

This strainer, like many out there, is another potentially expensive mishap waiting to happen. All it takes is some rough weather and a good surf down a wave to fill your engine with salt water. Facing them backwards is better but can also add unnecessary vacuum on the raw water pump. Sailboats should ideally have round, non-directional strainers, not scoops, or my favorite, none at all.

 

You can use them' just ensure that the preassure leaving the water pump doesn't exceed around 15psi (not sure for your engine). If it does it can cause the pump to eat impellers, or hydrolocking. Some manufacturers are more tolerant of these scoops than others. Personally I don't see an advantage, unless the cooling system was designed for them which to my knowledge oly occurs on high end racing boats.

 

There is a small instruction paper that comes with this strainer. It suggest you install them backwards on slow speed boat and the reverse on high speed boat.

We have given up this thru hull strainer on high speed boats (32-36 knots). We install flush type mount thru hull. Really flush with a recess on the water side and extra reinforcement on the dry side.

The engine manual suggest a raw water strainer if the boat is to operated on sandy beaches or areas of high folliage. A talk with the engine reps says it is "reccommended but not supplied" and will void the warranty if the engine damage is caused by ingestion of foreign objects.

From experience, we had foulings of the raw water intake by jellyfish or seaweeds even in relatively "clean water".

It is best practice to use a separate raw water strainer that you can clean by shutting off the seacock. The better one is having a capped tube that rises above the waterline with a Y connector going to the raw water strainer/water pump. That way, in case of foul ups, you can clean the intake side without shutting off the seacock or diving underwater to clean the intake side.

 

Mine is still the best and dont have to have a hole in the hull !! hardly nothing goes inside and with a inside strainer its no problem !!. Remember bigger is better and the slower the water moves in the pipes the less chance of things getting stuck tight and having to dig them out!! so with a strainer before the other strainer and not much chance of anything getting in wow shes the coollest of them all !!.

 

If a scoop strainer faces forward you risk physically scooping sea water up and over the siphon break then filling the exhaust and eventually the cylinders of your engine with sea water. /QUOTE]
Man you must have some strange sort of plumbing for such a thing to happen !! if you looked closely at the scoop i posted a picture of, it would not creat enough pressure even if the boat was doing 100 mph down hill with a cyclone up its rear end !! scoop is almost debateable if you could call it that !!
In the case of water getting into the exhaust yes have delt with two such events like that both in Tahiti while i worked there . one was a water lock muffler (made and installed by the french navy ) mounted above the motor height and a baffel broke inside and alowed all the water to drain into the pipe and into the motor .
Another time a blade missing off a water pump impellor and alowed the water to flow straight through into the exhaust and back into the motor !! was a chance in a million it happened and stopped in the right place . But the canadian couple was it had happened before and no one was ever able to explane why .Yes it was a yacht and the motor was below the water level . they had left after a motor repair in Tahiti and i sad bye !! 3 days later they were back with a motor full of water . I said immediatly after they told me check the impellor in the pump and sure enough 1 blade missing !!!!

 

This was taken from the westerbeak installation manual.

Scoop-Type Intake
Where generators are installed in very high speed vessels (50-60 knots), it has been necessary to use a scoop-type raw water intake to insure an adequate supply of cooling water to the generator. Generally, this is not recommended, but if it needs to be done, the engine must be mounted high enough so that the exhaust piping has a continuous downward pitch from the muffler to the exhaust outlet in the hull. The pitch should be adequate so that the muffler will continuously drain and no water can back-up into the engine (see illustration).

Note that they specifically exclude hydrolocking from the warranty though. I have to admit i didn't think they were ever a reasonable option, but if the boat is traveling in this speed range there is a basis for scoops, so long as reasonable precautions are taken.

 

This is the engine if it helps any, and I hope it remains above water level as the risers are above the boat

 

Are we all on the same page? Are we talking of engine cooling via intercooler or in the case of fresh water boats, engine cooling is direct to the engine block via the thermostat/bypass valve,water pump, around the block and out? In which case engine above or below waterline would not matter/

Or are we talking of a wet exhaust system where the engine and its associated plumbing differs when it is above or below waterline? From the picture Sean has posted, it looks like it is a WET exhaust system sans the waterlock canister.

 

no problem with that install
It would normally have 2 small slot type pickups (I will try to find a photo) going to a y fitting and then single feed to the pump
PS Does that engine still have the EGR valve connected??

 

Yes the ERG is connected, I am new to this type of engine and only had automotive parts to get it running so the plan when I can afford it is to get a marine distributor, carb, and alternator "not cheep"

 

It is perhaps a good idea to remove the EGR. All engines perform better if they are not forced to breath their exhaust gases and for marine engines it is no requirement.

 

Sean,

I would highly recommend getting rid of those car parts before you crank up the engine for the first time! The reason marine parts are so expensive compared to car parts is that they don't spark, or have flame arresters. On a car isn't not much of a problem since gas vapors are heavier than air, and so fall out of the vehicle. On a boat they tend to explode since there is nowhere for them to go.

Most marine insurance for instance will not cover damage due to non-compliant parts if they cause the damage.