speed question--lake water temp?

[cobra 3.0]

I've noticed my boat being slower too when the water was real cold. Others claim this to be false and that cold temps mean higher speeds.

 

[roscoe]

Re: speed question--lake water temp?

a 15* water temp change can't have that kind of impact. <br />Same for air temp. Density, humidity, and altitude may have a slight effect.<br />I've experienced a bigger change in temps just from going from harbor or enclosed bay, out into the open water. Or from a cold morning launch to a sunny afternoon. Never noticed any rpm change like that.<br /><br />I think the bigger factor here would be the design changes between the props, and the change from ss to aluminum.

 

[bayman]

Re: speed question--lake water temp?

I have seen that a cold engine doesn't perform like a warm one. Once warmed up though they perform on the same par. Could that be it?

 

[mattttt25]

Re: speed question--lake water temp?

i'd go with a 22" ss prop. my guess is that it will put you exactly where you want.<br /><br />edit- you didn't mention if you changed the diameter of the prop?

 

[dicktaber]

speed question--lake water temp?

Okay, here's my situation:<br /><br />Nitro 17', Mercury V-6 XR2 150HP, came with a 20" SS prop. (Been having issues with carbs/ignition so I haven't had it out as often as I'd like, but a recent move from IL to TX should extend the boating season some.)<br /><br />With the 20" prop it hits 6000 rpm at 50 mph, and I had to "watch it" not to exceed 6000. Some informal polling at the lake with other 17'ers with 150 HP showed that 25" was common. Found a 25" SS on eBay and bought it. Also found a 23" aluminum on eBay and bought it. Decided to start with the 23.<br /><br />Due to the other issues (see above) it's been a month or more between outings. Several weekends ago I got it out on the same lake as last time; differences were: 23 inch prop vice 20, and air temp about 10 degrees cooler, water temp at least 15-20 degrees cooler.<br /><br />Results--55 mph at 5200 rpm, WOT. I always read that 1" pitch equals 200 or slightly less rpm. My 3" change should have given me 5500-5600 (noting that I could exceed 6000 with the 20"er).<br /><br />Does the water temp (density?) affect the RPM appreciably? Should I keep the 23 until the water warms up, at which point I'll be perfect, or should I be looking for a 22 or 21? Got all winter, and eBay's always open <br /><br />Thanks again, Rich

 

[dicktaber]

Re: speed question--lake water temp?

Thank you for all the input.<br />--Engine temp is "normal", at least as normal as my temp guage allows. I see from 160-200 depending on speed, load, throttle position.<br />--Original 20" SS prop was 13 3/4, 23" is about 14 1/2.<br />I'm eyeballing a 22" SS right now...stop me before I buy again.

 

[KCLOST]

Re: speed question--lake water temp?

I'd go with a 23P Trophy... 13-3/4" with vent holes... <br /><br />BTW, are you trimming all the way out? That will gain you more rpms at WOT...

 

[dicktaber]

Re: speed question--lake water temp?

don't know about "all the way out" but I nudge it until I get max rpm without cavitating (which it seems to do more easily now--any connection?). It has a doel-fin device on it--I recall reading that removing it will increase top speed, but what effect will it have on trimming/cavitation? <br />Thanks again, Rich

 

[KCLOST]

Re: speed question--lake water temp?

The fin, if mounted properly really should only affect hole shot... At WOT and trimmed out some, the fin should be actually out of the water or right on top of it which won't affect drag to slow you down...Should have no affect on cavitation, that's just motor height/prop related.<br /><br />So the 5200 rpms was with the motor trimmed out?<br />A larger prop will tend to cavitate sooner that a smaller one (diameter)...<br /><br />That's why I recommed a Four blade trophy (more bite compared to a 3-blade)...You should be able to run a 23P with no problems on your rig... That 23P aluminum prop you have on there now is a completely different animal compared to a good SS...

 

[dicktaber]

Re: speed question--lake water temp?

...and, for the latest installment. Got out last Tuesday (the day after the 35 mph wind, and the day before it snowed!) by myself, I hit 60 mph at 6000 rpm. Perfect. And now I know what you mean by "trimmed out"--I was nudging the button and nothing was happening, so I guess that's all I get. I also recently bought a 4 blade 22P ComProp on eBay and am looking forward to trying that with the two of us in the boat (but first, by myself I may end up with a 23 SS, and I thank you all for your help. <br />PS--supposed to be 67-70 Friday so maybe I get to try it then--ever tell you how much I like Texas?