More top end!!!!

[stcroixjoe]

Hey guys I have a 21' Sylvan center console (alum) with a 1989 force 150 hp hanging on the back I can only get abot 35mph @WOT @4800RPM's the pitch of the prop is 13x19 MWC alum. I though the operating rpm range was 5000-5500 rpm's now which prop can give me more top end speed it had a ton of tourqe and take off .

 

[hwsiii]

Re: More top end!!!!

Joe, welcome to Iboats prop forum and we will try to help you as much as we can, but there is certain information that is needed in order to help you. When we get information in bits and pieces we are continually having to look back and forth to try and piece the information together. The following form has all of the information that is needed to help you with your problems in finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planing hull boat that needs this information, and most other people who are trying to help you will need the same information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the exact prop you need for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed and RPM.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
8. Manufacturer?s recommended WOT range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from prop, how much gas and how many people in boat for test data

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

H

 

[countvlad]

Re: More top end!!!!

Hey guys I have a 21' Sylvan center console (alum) with a 1989 force 150 hp hanging on the back I can only get abot 35mph @WOT @4800RPM's the pitch of the prop is 13x19 MWC alum. I though the operating rpm range was 5000-5500 rpm's now which prop can give me more top end speed it had a ton of tourqe and take off .

have you ever had the boat running past 4800???? are you sure your tach is working correctly??? when was lasrt time that the boat had a decarb?

if everything works fine then you are overproped.... i would definetly switch to either a 16p or 17p.... you will probably get a few mph and your motor would be in the recomended range....

get a manual find out for sure whats the recomended range.... if max is 5500 try to get as close as you can to 5500, maybe 5300-5400... if the range is higher, maybe from 5000-6000 you wanna get to 5800WOT...

 

[hwsiii]

Re: More top end!!!!

Joe, I show your motor to have a 1.93:1 gear ratio with a WOT RPM of between 5,000 and 5,500, please verify this, if this is true then here are some numbers for you. You are showing a prop slip of 22% and if you change from the 19" pitch prop you are running now to a 17" in the same make and model of prop you will gain right at 565 RPM to 5,365 RPM.


Prop Change

Prop Slip

Photobucket is down so I will post the backup later.


H

 

[stcroixjoe]

Re: More top end!!!!

Thanks for the replies, but i'm really not understanding everthing correctly, you say my prop is slipping 22%,now if I put on a 17p will I gain 22% more power or will that put me in the proper WOT RPM range ?? Excuse me if I sound stupid but I just really don't understand props and pitchs that good ,but if you want to talk about walleye fishing ...... Thats a different story lol

 

[hwsiii]

Re: More top end!!!!

Joe, we all have knowledge in different areas, this area is just not one that you know well. By changing to a 17" pitch prop it will raise your RPM closer to the recommended factory specs, which will help prolong your engine life and help give you maximum HP and torque. This is totally separate from prop slip.It is doubtful that you will gain or lose any speed but you might lose a mile or two and hour, but your holeshot and acceleration to cruising speed should be much quicker. If you have any other issues with the way the boat handles or not planing at lower speeds we need to know that too.

Prop slip is the percentage of speed that is lost because a prop is turning in water versus a solid. It just means that if a prop has a pitch of 19" and a prop slip of 22% that for every revolution it turns in the water instead of moving forward in the water 19" it only moves forward about 15", and the difference between the 19' and the 15" is 4" and is expressed as a percentage of forward movement, which in this particular case is 22%. Normally by lowering pitch in a particular make and model of prop you will see a decrease in prop slip in any particular boat and motor combination if it is not attaining full RPM. I hope that made it easier to understand for you.


You need to run the boat and give us a Wide Open Throttle RPM and speed number from a GPS, so we have some real numbers to verify what pitch you need. After you get the right pitch prop then we can work on raising your motor to get maximum performance out of your boat and motor. In most cases we would work on getting your motor setup right first, but when you are 700 RPM below where you should be we know we have to change pitch tpo get you close enough to maximum RPM. Also if the prop has been dinged up much or is bent we need to know that as well. Please verify your gear ratio.

H