1996 force 75hp prop selection

OK, have a 1997 starcraft superfisherman. Aluminum boat, weighs about 1200 pounds dry. Running a 1996 force 75hp with 15 pitch prop.
The boat does about 30mph. and rpms, i think, are too hight, running almost 6000 of open it right up.
Should I go too a higher pitch prop. Seems like it winds up too easily, I thoaught the boat would go faster than that.
If I tow my kids on a tube. It full blast too get 24mph.
Thanks for any help.
mike

Re: 1996 force 75hp prop selection

Mike, 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. Working over the internet and not being able to see the boat or prop the question is about makes it much harder to propose a solution to the correct prop that you need, and most people don?t know the make and model of the prop . When we get information in bits and pieces we continually have 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. 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 best prop we can 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. Most of my calculations are done assuming an aluminum 3 bladed prop will be used, when changing to a SS prop or changing within SS props we need to discuss what you are trying to accomplish.

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
7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
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 your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS
14. Are you at sea level, or a much higher elevation
15. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

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

Re: 1996 force 75hp prop selection

Wow, thats a lot of info you want.
Sorry here is all I know
1997 starcraft superfisherman 170sc
17 foot
weight, 1200 pounds dry weight.
holds 15 gals of gas.
Has 1996 force 75 hp outboard motor, with dolphines.
Running 15 pitch mercury prop now.
wot right now is 5800 at 30 mph (gps)
Motor should be more like 5400 rpms at wot. I dont run it wot because of the rpms.
I would like more performance from the motor, what about a stainless prop and i was thinking 17 pitch would be better for my rpms.

Re: 1996 force 75hp prop selection

actually, just checked its a 14 pitch prop....

Re: 1996 force 75hp prop selection

Mike, if you change to a 15" pitch your RPM will be right at 5,400 RPM and you will lose very little hole shot and maybe gain a mile or two an hour. Here are the numbers.

Prop Change




H

Re: 1996 force 75hp prop selection

Mike, after going back over your information, I find that you ought to be going faster than you are unless you have a very tired motor or it needs a tuneup. Without the gear ratio I can't be sure but I just thought I would let you know.


H