Top RPMS and speed?

[olmpilot]

Just looking for some ideas, since this is my first boat and first O/B. I have a 18' Seaswirl Cuddy, with a 120 HP Evinrude on the back. I took it out on the bay yesterday, and with only 2 adults, 1 child and minimal gear, I was only able to top out at about 28 MPH. I'm thinking I should get more power out of it than that. Also, the dealer told me to keep the RPM's around 4500, so I was running at about 4900 to get as close to 30 MPG as possible. I can put the throttle down more, but that kicks the RPM's up to about 6000, which I was scared to run it at. Is the 4500 too low for this motor/boat combination? It has a 14x7(?) prop on it right now. <br /><br />Thanks!

 

[LubeDude]

Re: Top RPMS and speed?

If the dealer told you to run it at that speed, RPM) and no more he is in the wrong business. If he on the otherhand was trying to give you the best fuel economy then he was about right. Sounds like you are a little under propped, you should be at about 5,500 at WOT. that would likly give you a little more top end.

 

[Jack Shellac]

Re: Top RPMS and speed?

28 MPH is not all that bad for a heavy boat with a 120 turning at 4900 RPM. It's important that your motor be able to turn near the maximum rated RPM to assure that it's propped right. Sounds like you could use some more pitch ,if it turns 6000 RPM, to get it down to 5500. This should also give you some more speed. The dealer is correct in that you usually get the best fuel milage at around 4500 RPM.

 

[olmpilot]

Re: Top RPMS and speed?

What kind of prop should I look into? There is a 2nd prop that came with the boat, but when we test drove it with the dealer, that was the prop that was on it and he said it was a ski prop and changed it for the 14x7 that was in the boat. There isn't a stamp on the 2nd prop that I could find. The boat was a consignment, so the dealer didn't know "alot" about it, but seemed to be trying to be helpful without putting much into the deal, since it wasn't his boat. There was also no manual, so I'm not sure what the "rated" RPM's are for the boat. Can I find a manual or order one somewhere?<br /><br />Being new, I don't want to end up blowing a very expensive motor my first season by over reving it.<br /><br />Thanks for the information!

 

[Dhadley]

Re: Top RPMS and speed?

Lets start by looking at your existing prop. I'd almost bet its a 17". What style/brand?<br /><br />6000 rpm for a V4 looper is no big deal. Just about right actually with an average load. The rev limiter is at either 6200 or 6700 depending on which power pack you have. What year is the motor?

 

[olmpilot]

Re: Top RPMS and speed?

I will need to look at the prop later this evening (the boat is stored at my girlfriends house). I was told that the motor is the original motor that came with the boat and is also a 1989 motor. The Model number is E120TLCEM.

 

[Dhadley]

Re: Top RPMS and speed?

Yep, thats an 89. Great motor. You'll love it.<br /><br />Next time out jot down the top speed (gps if possible) and rpms at top speed. We can do some quick calculations to see what kind of slip percentage (efficiency) we're getting.

 

[olmpilot]

Re: Top RPMS and speed?

Thanks, should I push the 4500 to see what I can get for top speed with higher RPMs? Like I said, it will push much farther than the 4500-4900 that I have been running it at. Also, I don't have GPS, but will have a depth finder installed this week so I can compare that to the spedo in the boat and see if they read the same.

 

[Dhadley]

Re: Top RPMS and speed?

We like to see the set up to allow that motor to reach 5800-6000 rpm with an average load. If your rig likes to cruise at 4500, thats great.

 

[olmpilot]

Re: Top RPMS and speed?

Dhadley, <br />I checked the props in the boat. The one currently installed simply has "13 1/4 x 17" stamped on it. The other "spare" doesn't have anything stamped on it that I can find. It does have an obscure "011002" stamped on the inner ring. Neither one have any identifying style or brand and being new, I don't know of any way to identify them. They are both black (assuming they are aluminum, not SS).<br /><br />The reason that I cruise at 4500 right now is that is what the dealer told me to do, and I don't have any paperwork that said it was safer to go higher than that. It will certainly do it, as I constantly had to throttle back to keep it in the 4500-4900 range.<br /><br />My "load" right now has consisted of 1 cooler, 2 adults, 1 kid and a crap trap and fishing pole. So, very little gear is on the boat right now.

 

[Dhadley]

Re: Top RPMS and speed?

You can run the motor at any rpm depending on what makes the boat handle comfortably. If you like the way the boat rides and handles at 5500 -- go for it. <br /><br />I usually cruise my Dusky at 3000. It just has a nice easy ride and the wind noise isnt too bad at that speed. If we're trying to outrun a storm, we can zip her on up to the 5800.

 

[olmpilot]

Re: Top RPMS and speed?

Great, thanks for the advice. I will do some experimenting next time I go out and see how she handles. I just didn't want to blow her out the first weekend on the water.