How to calculate hp on an outboard?

[MercFan]

Hi guys!<br /><br />Had the boat back on the dam Saturday. Seems like all is finally well!!! Thanx to all the inputs from you guys. It's a bit hard to start and takes a good 10 secondes of cranking before it fires up, but all is well from then on.<br /><br />I did lift the motor 1in last week and it looks good. The anti cav plate runs just ontop of the water at plane. Hole shot with the vent holes is fantastic and gets up to WOT rpm a lot faster.<br /><br />I also did a theoretical speed calc and it's very close to the onboard speedo. 60km/hr on the onboard speedo and calc speed is 63km/hr 17in pitch at 4800 rpm.<br /><br />Is there any way that I can calculate how much horsepower this is using? I can have the boat weighed if I need to.<br /><br />Any1 got any ideas on how to do this? Or can refer me to a website?

 

[Silvertip]

Re: How to calculate hp on an outboard?

If the engine is turning only 4800 RPM at WOT you need to get that raised up a bit. Since we/I have no idea what motor you are talking about, we have no idea what to recommend. Fact is you are lugging the engine.

 

[Texasmark]

Re: How to calculate hp on an outboard?

Once you "unlug" your engine (reduced pitch prop to bring your rpm's up to the top of the suggested WOT operating range), your hole shot should be fantastic. Like 4800 to 5250 or 5500 (where a lot of engines top out), at 200 rpm gain per inch of pitch decrease calculates out to be about a 3 inch drop. You'll scream out of the hole with that if you have a good shot now. <br /><br />Additionally, per Kenny's prop testing, lugging a 2 cycle is extremely harmful to them and bad on you wallet (high fuel consumption).<br /><br />Mark

 

[Texasmark]

Re: How to calculate hp on an outboard?

Forgot the main topic.<br /><br />To calculate hp you need torque and rpm's. Most engine mfgr's don't publish torque so you might as well forget it. Fix the prop.<br /><br />Mark

 

[MercFan]

Re: How to calculate hp on an outboard?

Thanx guys

 

[MercFan]

Re: How to calculate hp on an outboard?

I was hoping that there's a way to ensure that I am indeed getting all the ponies from my engine, and that I'm not missing 15 or so of them.<br /><br />All I have at the moment is the fact that I know the compression is over 130psi when hot on all cylinders. I have rebuilt all the carbs. I have rebuilt the fuel pump including non-returns. The engine's electrics were redone. And full Link and Sync as per Clams' post. But there's no way that I can verify that this is the best the engine will do, or if I missed something. Reed valves maybe? Any way that I can test them in the engine? Will they have an influence on performance?<br /><br />It's a 1970 1150 running on a 18ft Bowrider. Deep V hull, approx 2200-2300 pounds with engine and fuel.

 

[croSSed]

Re: How to calculate hp on an outboard?

There is no way I can think of to accurately calculate outboard h.p. on any boat. There's too many variables with the hull of the boat. The best thing to do if you want to know how many ponies that beast is REALLY making (and this is going to cost some money) is to find a boat repair place that has a prop-shaft dyno, and have them run and tune it on that. <br /><br />As far as after-market reeds are concerned, there is very little (if anything) you can do to RELIABLY up the hp on those old IL6's. I don't think there are any aftermarket reeds available for these engines.<br /><br />BTW, if your motor is running right, it should make around 100 propshaft hp.<br /><br />TG