351W Optimum Max RPM?

Man I love this place. A couple weeks ago we were talkin' to a guy in Isreal about his platform and now we got Brunei on the line. WOW. And I used to think it was cool to talk to BKH in CA....john


ps to BKH; it's still cool to talk to CA also.

Hey Guys, that DWSA joint looks pretty cool. I had no idea there were those kind of facilities in the middle east. Guess I have to get out of the midwest a little more often. FYI,By my calculations gas is $1.36 per gallon in Dubai.

Thanks for your replies everyone. That's why I ask before I commit. Correction for you:
Brunei is on the island of Borneo in south-east asia, not Dubai which is where I'm going as soon as I finish typing this! (don't feel bad - half my family still thinks I live in the Middle East.) Fuel happens to be a give away here too, as it's all just off the beach.

What rpm's should the Mercruiser pump turn then? And the boat is at my place!! The guy is an exceptional engineer, no question, but I guess this wasn't one of his finer ideas!
Cheers everyone.

Just to clarify 18C is 64.4F. 1 deg C is equal to 1.8 deg F. The conversion formula is: F=C*(9/5)+32.

The engine in my '79 is far from stock and I have never had a cooling problem with the stock raw water pump.

Additionally with the design of SN hulls you're not going to get really fast top speeds no matter how high your rpms or hp are. 50mph is about it. There is a lot of the hull in the water and drag increases with speed exponentially, along with the power needed to overcome that drag.

These guys are on the right track on all accounts. Even if the motor could turn 5500 occasionally without issues it is not going to be making its max HP at that point unless you change the cam, heads, ignition, etc. Therefore you would be cutting down your top speed, and decreasing your fuel economy (yeah I know the stuff comes right out of the ground over there but it still has some value). The only reason I would see for doing this with a stock engine ever was if you were pulling massive loads and wanted to develop your max power at a lower boat speed, but I seriously doubt your loads are that extreme and would justify the high end performance loss. If extreme towing loads were the issue you would not want to be cutting down your propeller as you would need all the face area you could get, the proper way would be to lower the pitch.
    If I was going to do that I would oversize the pump pulley to slow down the rpm in the pump, not speed it up as your "marine engineer" is suggesting. Speeding it up too fast will result in cavitation and decreased water flow (especially if you do not increase the size of the hoses and pickup), and more importantly decrease pressure at the circulation pump, meaning the cooling water will take more time to enter the system when the thermostat opens. This combined with the extra heat generated by turning 5500 rpm (engine and transmission heat both being dissipated by the same flow of water) is most definitely going to leave you looking for a new engine long before you should be.
     My main reason for kicking in my two cents to the above correct replies is that I have worked with Volvo Application Engineers designing while boats for a couple of manufacturers during my contract engineering stage, and not only are your source’s comments wrong but they aren't even logical. Volvo does not choose the outdrive/prop combination for a boat they just approve it. If a boat is over propped it is because the boat maker wanted to be able to publish better fuel economy or top speed numbers, and is willing to sacrifice low end power. Depending on the engine Volvo will typically approve a lightly loaded top speed RPM within a 1000 RPM range leaving the boat builder with a lot of leeway to pick how they want the boat to perform. If Volvo were trying to limit RPM to keep down warranty issues and limiting RPM causes inadequate cooling, then Volvo is not too bright. They aren't my favorite marine engine manufacturer but they haven't gotten their market share by accident.
      If you were really just worried about the extra heat and longevity, run one of the better full synthetics in the engine and the tranny. If it helped you sleep at night you could buy a higher rated pump and run it with a pulley that matches its highest rated flow at your top rpm, increase the line size, add a second pickup, and make sure you have a good high flow thermostat. Get your money back from that “marine engineer” and use it for gas, beer, and water toys.
-Joe.

79; As always, straight to the point. Can't believe that there is alot of replacement motors in Brunei. Where in H is that anyhow?...jbear

Speed or Ski? That's the question you have to ask yourself. Why does a ski boat need 550 rpm's?

I live in the South (similar or hotter than Brunei waters....surface temps of shallow ski lakes are easily greater than 18C) and we pull slalom at 3200 to 3600 rpm in Tournaments (an old 289 and a new 350) so the engine has a heavy load at very warm water temperatures and have never had a heating problem...although if you pay close attention to the thermometer you can see slight swings as the thermostat adjusts to hot surface water.

If you want to see your pump in action...hook it up to a garden hose and run the rpm's to about 3000....that pump will outrun supply and collapse the hose! Maybe the eccentric was worn out on the pump your mechanic has experience with???

Morto, what is your main objective here? Is it to pull more skiers at the same time and get out of the hole faster or just to watch the needle on your tach climb higher? You need to state this first! Yes the 351W will turn 5500 RPM safely, but how often are you going to be doing this. Because, you are going to reduce the longevity of this engine by twisting it that tight all the time. You have to remember that a marine engine operates under a constant load all of the time (unlike a car). It's like always climbing a steep hill and never reaching the top. For this reason, marine engines wear much faster than automotive engines to start with.    

Also there are other things to consider. For one - if your transmission output is 1:1 you are going to be turning the shaft at this same speed. The stuffing box and strut are not designed to handle much more than 4000 RPM shaft speed. Also, you're talking about turning a pump with a rubber impeller at 8000 RPM. That's not going to last long either.

Ask your engineer if he has made these mods before and if he can show you a working example that is still holding up.