Does weight affect WOT rpm

[madolive3]

If I have three or four hundred extra pounds in my boat wide open throttle rpm stays constant that doesn't change with the weight, is that correct

 

[SkiDad]

if you are propped right it won't change a ton - my boat only drops 100 rpm with a full boat. I think it depends on the boat. I have heard of some that drop quite a bit.

 

[gm280]

Of course the RPMs change with the load. Otherwise you could prop for no load and get the same performance when fully loaded. And I can tell you that isn't going to happen. It is the exact same with vehicles. You get better gas mileage when you have no passengers, but that drops with the more folks you hull around. And boats are the same... And that is because the engine has to push more weight through the water and that takes more force to accomplish the same speed. And the fact that a heavier boat sits deeper in the water per the displacement. So YES the engine RPMs will drop as weight is added...

 

[Sea Rider]

Weight does affect wot rpm, less weight + rpm, more weight - rpm with a given prop. You should be propped to achieve at least middle to max wot rpm, that's boating inside safet wot rpm range for your engine. Is using a digital tach, there's always a slight +,- 50 rpm variation compared to a needle tach which looks nearly steady.

Happy Boating

 

[madolive3]

Ok thanks. So when I choose a prop, I should be at the upper end my rpm range so when the boat is loaded I don't drop below min. Specs.

 

[Sea Rider]

In your particular case go for a wot spin on flat calm waters with the amount of boaters that will be riding that boat, check achieved wot rpm with a tach, compare numbers to your engine safe min-max wot range, can go from current in use prop maximizing pitch to suit your boating needs. This is not exact rocket science, must be dialed under trial and error.

Are you just asking or want to maximize a prop for your combo in which case more information is needed regarding boat type engine HP, prop size & pitch, number of passengers.

As an example while being at wot : If you select a prop that gives full wot rpm for that engine while being highly loaded, engine rpm will stay same, OTOH if you add more weight rpm will drop accordingly, if you reduce weight, rpm will raise accordingly. Once best pitch is dialed for that number of passengers you can run engine throughout the entire throttle range without worrying.

Happy Boating

 

[madolive3]

I just want a clear understanding before I buy a prop, I replaced the current Chewed up 15x17 with a used prop of the same size and pitch, only in good shape. This will give me my baseline. After this weekend I should have enough info from my boat and from this forum to decide how I want to proceed.
Yes, the concept is pretty simple i'll admit, but I'm relatively new to boating.

 

[Texasmark]

The name of the game is hull to water friction. Like Ski Dad said, depends on the hull. Pad hulls or stepped hulls that have a small wetted area with a lot of HP can maintain the boat on that pad with big changes in load whereas a conventional semi v with a mediocre engine gets the hull wetter which produces more friction and speed and rpm loss as long as prop slip remain constant. If the slip increases due to turbulence then the rpms can come up with a loss in speed.

As Ski Rider was saying, you have to decide where you want to prop and adjust accordingly when things change. If most of your running is alone, go for the higher pitched prop and expect sluggishness when you load up with folks. If loading up with folks and water toys is you game, prop for that, I like the upper end of the recommended rpm range, and when alone just manage the throttle to hole the rpms in check.

 

[Tail_Gunner]

Of course it does here play with this a bit you will find it quite accurate.. http://www.go-fast.com/boat_speed_predictions.htm

 

[Texasmark]

Of course it does here play with this a bit you will find it quite accurate.. http://www.go-fast.com/boat_speed_predictions.htm

While the viewer is on the site, pick up their prop slip calculator. Not only gives you slip but you can play around with the numbers and get an idea as to what does what to what. Been using it as my standard for many years. Yes it's accurate....as the data you feed into it.

The Fast V bottom entry sounds like a bass boat with pad, jack plate, set backed hull and all that which, as I said above, gives you a little latitude on adding some weight before you have to pay a premium in mph.

 

[madolive3]

Thanks guys. It'll be awhile before I can post any results. I broke my arm in 4 places at work the other day.
I like to bring my threads to a conclusion, has to wait a couple of weeks.

 

[Texasmark]

Thanks guys. It'll be awhile before I can post any results. I broke my arm in 4 places at work the other day.
I like to bring my threads to a conclusion, has to wait a couple of weeks.

I bet your boss was thrilled about that. I know where I worked, accidents were a really big deal. Hope it clears up ok for you.