weight vs rpm

[sabastianunf]

is there any rule of thumb as to increasing weight to decrease rpm. I have a 19 foot cuddy cabin running a 120 Hp merc sterndrive. Any experience or input is greatly appriciated.

 

[dreamchaser]

Re: weight vs rpm

huh? why not try a more pitched prop ? that will decrease ur rpm

 

[sabastianunf]

Re: weight vs rpm

Thinking about adding fat sacks - water filled bladder to increase size of wake for wakeboarding. Not sure how much water to add and what pitched prop to use with added weight. I know adding weight/load will eventually cause rpm's to drop - just wondering how much weight will cause rpms to drop and how quick.

 

[Nova II 260]

Re: weight vs rpm

I can fix you up with my some of my wife's girlfriends. They love the water and bring plenty of weight.

 

[Silvertip]

Re: weight vs rpm

You are asking two questions. First -- does adding weight decrease RPM? Of course it does. Every ounce of weight you add will decrease the engine rpm (but you may not see it or feel the difference). For your second question regarding increasing the wake: Decreasing RPM does increase the wake. A change of boat attitude, such as moving everyone to the stern and trimming the motor up to create a severe bow high attitude will increase wake. If that doesn't work, take Nova up on his offer of his wifes girl friends. d

 

[sabastianunf]

Re: weight vs rpm

I know that adding weight decreases rpm but at what rate? If nothing else - help me out nova and throw a couple of those fat honeys on the scale then put them in the boat and tell me how many rpm's you loose. the plan is to put the sacks or fat chicks in the back to raise the bow generating more wake. This will cause rpm's to drop so I need a new prop - 4 blade to improve hole shot and lower pitch to compensate for rpm drop. Lots of factors and vaiable but just trying to find a good starting point. I am thinking of going from a 17 pitch -3 blade to a 15 pitch 4 blade but am concerned that if I do not add enough weight that I may end up over reving the engine and spending more $ to get the prop repitched.

 

[Nova II 260]

Re: weight vs rpm

Sabastian, I found this.
Boat Weight/Speed Calculator and
Prop pitch calculator

Hope this may help ya and let me now where to send the fat chicks, we have way too many around here.

 

[Nova II 260]

Re: weight vs rpm

I can send you a truck load in bikini's right away, Let me know if you need more weight.