understanding operating RPM questions

[ATF]

Hello All,<br />I've got a 2000-150hp optimax that I bought last year which I run on a 20ft deep V aluminum walleye boat. I'm trying to understand operating RPM so I can dial in a prop.<br />I thought I had it figured out last year when I tried a 19p tempest plus prop. I got 5400rpm at wot and about 47mph hoever when testing I didn't have the boat weighed down as much as normal.<br />Now I went out this year with 3 guys and a full load.... and can only get 43mph and 5100rpm at wot.<br />Should I be looking at trying another prop in the same pitch or go down another size? I don't really want to lose the top end speed.<br />Can going down a size let you bring up the rpm without losing top end?<br /><br />Thanks in advance.

 

[DJ]

Re: understanding operating RPM questions

ATF,<br /><br />Excellent question.<br /><br />Here is how I would explain it.<br /><br />You want your engine to be able to hit the upper end of the manufacturers recommended rpm range with a light to medium load in the boat.<br /><br />With that said, if your boat is always heavily loaded, you should take that into consideration to also hit that upper end of the rpm range.

 

[G DANE]

Re: understanding operating RPM questions

I see it this way:<br /><br />When you'r alone in an empty boat, it should be able to hit or slightly overrun the manufacturers maximum WOT RPM ( by 200-300 RPM ). You wont cruise past MAX RPM for excessive time. That way motor will still be in powerband with a normal load in boat, You will gain speed, gain hole shot, save fuel and avoid lugging your motor.

 

[walleyehed]

Re: understanding operating RPM questions

Look into raising your engine off the transom before purchasing more props...generally you will gain about 150 RPM per hole up.<br />On your set-up, the 3rd hole may be as high (if that) as you can go without adding some set-back.<br />The Tempest performs well at the surface, it's something you might want to consider...

 

[ATF27]

Re: understanding operating RPM questions

Thanks for the input guys. Its truly appreciated.<br />I put the boat in the water today and tried 2 more props. I still can't break 5500rpm's. They were all 19P merc. props.<br />I have 2 more to try but I'm starting to lose hope.<br />If I have to go to a 17P I'm going to be really disgusted with the top end speed I'm sure.<br />Walleyehed; Thanks for the tip about raising the motor. I'm on the second hole from the top but maybe raising it one more will make the big difference I'm looking for. Any advice about what I should be looking at in relation to the cavitation plate and the bottom of the boat?

 

[DJ]

Re: understanding operating RPM questions

The cav. plate should be even to an inch or two above.

 

[Spidybot]

Re: understanding operating RPM questions

Agreed. And don't take lower top end with a 17" for granted.<br /><br />All else equal it will perform the same at +400 rpm.<br /><br />As you are into testing, why not test one or more 17" props?

 

[Mercathode]

Re: understanding operating RPM questions

ATF <br />Often where and how your load is set will affect performance. If your passengers like to congregate to the front causing your bow not to lift can effect overall perform. Is the fuel tank towards the front of the boat etc. Keep in mind that Mercs like to spin I would go with a 17!

 

[ATF27]

Re: understanding operating RPM questions

Well I have nothing left to lose now. I think I've tried pretty well all the 19p props the dealer can think of. The 2 I tried yesterday had the same results as the rest, nothing to crack 5500rpm. I will try to get a 17p from him to see what will happen.<br />He also suggested it might be worth considering moving the engine up one hole.<br />As for weight distribution I tried that too. The gas tank is subfloor and towards the back of the boat. I tried relocating my passengers and gear to diminished performance.<br />Thanks again to all who've posted. Hopefully I'll get this properly setup before its time to winterize.

 

[walleyehed]

Re: understanding operating RPM questions

You may be surprised at what that one hole up in your case does..listen and watch for vetilation with trim changes, turns and the like.<br />I've been down this road, and a little time and patience (playing) will show improvements.<br />I started with a 115 spinning a 15 pitch, and with some time and a few tricks from a Gent. here on iboats (Dhadley), I went clear up to a worked 18" SS and ended up with 250 more RPM and a load more top-end. There are ways to maximize performance without maximizing cost.<br />G DANE has posted some good numbers for referance above, If you can get 5600 with a normal load, I feel that would be a good target figure