need new 4 blade prop. not sure

[kartman]

so, i took my boat out today after i put my gauges in. it runs 36mph at 5600rpm. its a 80hp merc on a 16' v-hull. with a 15 pitch 3 blade. it rockets out of the hole, but wont hold speed for nothing, especially when im pulling someone. i was thinking about a 17 pitch 4 blade and venting it. or is this to extreme?? im looking for a prop that will jump on plane quick, but also give me a better top speed, and speed holding is a must. not sure what to do, as this is my first boat.

 

[hwsiii]

Re: need new 4 blade prop. not sure

Kartman, 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
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

 

[kartman]

Re: need new 4 blade prop. not sure

1. 1976 glastron v-162 futura
2. 15'6". 72" wide. not sure on the weight. its really light
3. usually 3-4 adults. and 20 gals of gas
4. its a standard v hull
5. 1979 mercury 800
6. 2.3 gear ratio 80 hp
8. 5000-5500rpm
9.
10. its not a bass boat, but im not sure
11. no hydrofoil or trim tabs, although im thinking of putting a hydrofoil on
12. 13 1/4 dia 15pitch 3 blade aluminum. not sure on the brand. it doesnt say
13. 5700 rpm 33mph 3 adults
14. about 2500 feet
15. looking to lower top rpm, and not loose any out of the hole performance. plane at a lower speed than 17mph,and would like to raise mph also. i was thinking of putting a vented 17pitch 12.5"dia 4blade alum prop on.

 

[junior1113]

Re: need new 4 blade prop. not sure

17x4 would drop rpm by 450 you dont need that. i would work on setup. lifting motor. where is cavitation plate in relation to bottom of boat? can you post a pic? i would say a 15"pitch 4 blade if it were mine. but let h do a prop calc to confirm that. i would keep rpms 5300-5600 with todays gas, to keep hole shots good. 4 blades hold the water a lot better. thats why i run them.

 

[hwsiii]

Re: need new 4 blade prop. not sure

Kart, I see the numbers are a little different this time, you gave me some real numbers instead of me guessing wrong ones last time. I show you ought to be able to turn an 18" pitch prop, but when I run the numbers for what you gave me I show you can only use a 17" prop. With the RPM numbers you give me with 3 people in the boat my best guess is you are able to turn about 6,000 RPM with only 1 person in the boat. Using that as the basis by changing to a 4 blade in a 16" pitch you ought to be able to turn that prop at about 5,500 RPM with one person.

I am showing you only have a prop slip of 6% right now and that is very good but your RPM are too high. You can try a 17" 4 blade but it will put you at about 5,100 to 5,200 RPM with one person in the boat.
You shouldn't lose any hole shot because you will have 4 blades instead of 3 and you might gain a little speed, but not much.

H

 

[kartman]

Re: need new 4 blade prop. not sure

any idea on who makes a 16 pitch 4 blade aluminum prop?? the only 16 pitch ones that i can find here are ss. im going to try to pick up a 17pitch 3 blade used prop locally, but what i think would work the best is a 16 pitch 4 blade after talking to local prop shops. i just dont think that a ss prop is worth the investment. thats a quarter the cost of my boat!!!

 

[hwsiii]

Re: need new 4 blade prop. not sure

You don't need anything but aluminum. I will see what I can find for you.


H