rpm, wot, and top speed

[7-up]

I have a 1985 Johnson V6 175 hp. Right now I am turning 5000 rpm at wot and getting about 46 mph with a full tank to half tank of gas and just myself on board. <br /><br />The boat is an 1990 18' Cobia bowrider, shallow V. Weight is about 1300 pounds not including gas. Have a 35 gal. gas tank, not sure how much the gas weighs. I have a 15x21 prop.<br /><br />Is this as fast as I should get or should I get more speed? With my old 1990 100hp Merc I got about 38 mph at5500 rpm under the same conditions.<br /><br />Also should I be getting higher than 5000 rpm at wot?<br /><br />Any advice is greatly appreciated<br />Thanks,<br />Barry

 

[Hooty]

Re: rpm, wot, and top speed

That engine should turn up 5800rpm or so. Where is the cavitation plate in relation to the bottom of the boat? <br /><br />c/6<br /><br />Hooty

 

[7-up]

Re: rpm, wot, and top speed

Hooty:<br /><br />Thanks for the reply. The cavitation plate is about online with the bottom of the boat. I can play with the trim and it doesent make a difference with the rpm. Also does it seem like I should be getting more top speed?<br /><br />Thanks,<br />Barry

 

[DJ]

Re: rpm, wot, and top speed

Barry,<br /><br />I would try lifting the engine one notch on the transom.

 

[7-up]

Re: rpm, wot, and top speed

Thanks djohns19, I'll bring it back to the guy who installed it and ask him to do that.<br /><br />Barry

 

[DJ]

Re: rpm, wot, and top speed

Barry,<br /><br />Be careful when you test it out. If you have a water pressure guage, WATCH IT very closely.<br /><br />If not, make sure your hot horn is working and be listening for it.

 

[7-up]

Re: rpm, wot, and top speed

Thanks for the advise. Is that in case the water intakes are too high and not taking in enough water?<br /><br />Thanks for all your help. This site and you guys rule!<br /><br />Barry

 

[chilifriedpork]

Re: rpm, wot, and top speed

Try re-proping. Possibly a propellor with a smaller pitch will give you a li'll more rpm's, and possibly top end speed.<br /><br />Good Luck

 

[rabidfish]

Re: rpm, wot, and top speed

Installing a lower pitch prop will certainly give you more RPMs, but you might actually lose some speed. The prop that gives you approx 5500 RPMs at WOT is the proper one for your rig, no matter what its pitch. The trick is to find the prop that gives the best performance while maintaining that RPM.

 

[7-up]

Re: rpm, wot, and top speed

Thanks for the replies, I am probably gonna try a smaller pitched prop to get those rpm's up.<br /><br />Barry