Confusing data....Kennnnnnnnnnnny

[Texasmark]

So Friday I'm out in my boat and run her up. Water temp is now 80 degrees. Instead of the 52 I got in January, I'm getting 50. <br /><br />I understand the why on that, but what I don't understand is that the rpm's are the same at 5600. If hotter ambient temp reduced engine performance, why aren't the rpm's off proportionally to the 2 mph loss in speed? Says slip increased as a function of temp??????<br /><br />Thanks,<br /><br />Mark

 

[walleyehed]

Re: Confusing data....Kennnnnnnnnnnny

I don't have any good answer for that, Mark. As you say, it indicates more slip.<br />My RPM DOES drop with temp rise...I dropped from 6250 to 5900, 50 degs. to 94 degs.<br />Maybe Dhadley may have something to add but I can't answer that.......???????? sorry!!!

 

[93 TRACKER WITH A RUDE]

Re: Confusing data....Kennnnnnnnnnnny

Are you hauling more wait or equipment??

 

[Silvertip]

Re: Confusing data....Kennnnnnnnnnnny

Race cars run faster on cold days than hot days. Even though you may develop the same engine RPM, horsepower on a hot day comes down a bit and since the air is dense, the boat pushes harder. Just as track temperature affects racecars, water conditions can affect boat speed. Pushing harder is where the slip enters the equation. Does it equate to 2 MPH is anyones guess but definitely a factor. Figuring out the exact cause for a slight performance loss takes some fairly detailed testing. Different fuel, humidity, temperature (air and water), water conditions, slightly different trim, are all factors as is a little extra stuff on the bottom of the boat. My 2 cents worth.

 

[Boilermaker]

Re: Confusing data....Kennnnnnnnnnnny

Density decreases as temperatures increase, although I would find it hard to beleave a 2 MPH speed decrease/slip increase from water density change of 28*F.

 

[walleyehed]

Re: Confusing data....Kennnnnnnnnnnny

horsepower on a hot day comes down a bit and since the air is dense, the boat pushes harder.

I think you mean "less dense" at higher temps, that's why the power loss, and for the same reason, the boat has LESS air resistance in summer than it does in winter. Exact reason we fly at the highest available altitude in aircraft...less dense air means faster ground speeds.<br />I don't know what water density change would occure, or if it would have any noticable affect.<br />I'd still like to hear from Dhadley on this...he may shed some light.

 

[Ron G]

Re: Confusing data....Kennnnnnnnnnnny

i lost more than 2 mph last year.im interested also.

 

[Texasmark]

Re: Confusing data....Kennnnnnnnnnnny

Thanks for the inputs guys. The "racetrack" is the same. Same rig and loading, same 1000 acre lake; winds light and variable, same trim....max.<br /><br />This boat has about a 25 degree deadrise to the transom....unlike the one I read about the other day that had 0 (best I could tell). What's nice about it is when you dump the throttle, you don't get any following wake over the transom....she just pops right up.<br /><br />With the engine trimmed so that the plate is parallel to the hull, the front of the skeg is 13" out from the transom at the bottom; no jack plate or the like but Merc does have one whopping transom bracket on this sucker. Guess they needed room to tuck the trim unit inside it.<br /><br />So my "20 degrees of trim" Merc advertises, craps out when I'd like to have another 5 degrees or so. Would at least like to make the prop blow (loose speed) then tuck her back in till it stops....but I can't. But that's ok.<br /><br />Mark

 

[Silvertip]

Re: Confusing data....Kennnnnnnnnnnny

Yup -- I'll take 50 lashes with a wet noodle on the density issue. Something is different but 2 MPH can be just an anomaly.

 

[imported_bjs]

Re: Confusing data....Kennnnnnnnnnnny

humid air changes the effect of any engine me and my brother race cars to and its a fight to get that old small block chevy to run just right in that summer air.jet jet jet that carb.its very hard to get just right.