86 Cobalt prop question UPDATED

[94transam25th]

Ok so i am new to owning a boat, my dad has had boats all my life but i just bought my first boat last week and had it out today and doesnt seem right i think its a prop problem

here it goes.

on the boat when i got it it had a 19pitch prop and those little dolphins on the outdrive... it takes quite awhile to plane and seems like it will run way too high of rpm. if i let it go wide open throttle it would go all the way to 5500 or maybe even above just didnt want to try and only doing 32 mph gps... also i pretty much had to keep the trim all the way down otherwise the rpm would get too high

my dad gave me a 23p mirage stainless steel prop do you think i will even be able to plane with this since i had trouble planing a 19p?

the boat is an 86 cobalt with 5.7v8 260 and alpha one seems to me it should do faster than 32

any suggestions?

 

[hwsiii]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Transam, we will try to help you as much as we can, but there is certain information that is needed in order to help you. 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 availableas 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
8. Manufacturer?s recommended WOT range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Does it have a hydrafoil or trim tabs
11. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from prop, how much gas and how many people in boat for test data
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

 

[steelespike]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Welcome. Something is up.Slow hole shot and wild rpm don't go together.
Need more detail on the boat With the thousands of manufacturers over the years and their 100s of models its pretty much impossible to know each.
A description of the boat will help evaluate your problem.If you read the stickies at the top of the page and the helps at the iboats prop store page
it will help you understand prop selection.We're looking forward to helping you.

 

[ramster]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Unless it's a really big heavy boat, a 5.7 should easily turn that 23" mirage since you already have it give it a try.

 

[94transam25th]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Unless it's a really big heavy boat, a 5.7 should easily turn that 23" mirage since you already have it give it a try.

the boat is a 1986 cobalt 21BR
it has the 5.7v8 not sure who makes it but i believe it to be 260 hp
yesterday had 7 people and took it just to go run over to some islands to hangout

i also have a little scag that is right where the arrow is pointing that is stopping me from being able to put the mirage prop on... is it possible to take it off? most boats i see just have a little plate where that is

 

[94transam25th]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

i also found out the boat weighs 3600 pounds dry no gear or people

 

[hwsiii]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Transam, there is a reason we ask for the information we do. I need to know the gear ratio.

H

 

[steelespike]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Transam, there is a reason we ask for the information we do. I need to know the gear ratio.

H

That tab is to aid steering and it is an anode to combat corrosion.
you can change it to a plate or block.
While you did give us the weight A "21 BR "doesn't mean much.
is it Bow Rider?, Is it deep V modifiedV or??.
Please sit down and list us the details;so its not spread over 1/2 a dozen posts.Was slow hole shot with 7?
Did you try to balance the passenger load?Do you trim in for hole shot?
Just to remind slow hole shot and wild rpm don't go together.
If you hole shot was slow with a 19 it will likely be very slow with a 23.
With the info we ask for we can determine if you have a prop problem,boat preoblem,motor problem or tach problem.

 

[94transam25th]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

That tab is to aid steering and it is an anode to combat corrosion.
you can change it to a plate or block.
While you did give us the weight A "21 BR "doesn't mean much.
is it Bow Rider?, Is it deep V modifiedV or??.
Please sit down and list us the details;so its not spread over 1/2 a dozen posts.Was slow hole shot with 7?
Did you try to balance the passenger load?Do you trim in for hole shot?
Just to remind slow hole shot and wild rpm don't go together.
If you hole shot was slow with a 19 it will likely be very slow with a 23.
With the info we ask for we can determine if you have a prop problem,boat preoblem,motor problem or tach problem.

i put the 23p prop on today but i will need to go look at the boat tommorow to find the rest of the details you ask for

i just got the boat and only know it is a 86 cobalt and 21BR is actually the model of the boat. it is a 21ft bowrider (pretty creative name eh lol?) but i dont know how to find what gears are in the outdrive

 

[94transam25th]

Re: 86 Cobalt prop question

Re: 86 Cobalt prop question

Transam, we will try to help you as much as we can, but there is certain information that is needed in order to help you. 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 availableas 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
8. Manufacturer?s recommended WOT range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Does it have a hydrafoil or trim tabs
11. Make, model, diameter, pitch and whether SS or aluminum prop
11. WOT RPM and speed from prop, how much gas and how many people in boat for test data
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

Ok here it goes
1. 1986 Cobalt 21BR
2. Deep Vee
21'1" Centerline
96" Beam
17" draft
3600 pound average weight
60 gallon capacity
3. Usually start out with full tank of gas and about 5 people
4. Use for cruising to islands and out to dinner/ beaches
5. Deep vee 20 degree deadrise
6. The motor is an 86 350 v8 with 260hp
7. dont know the gear ratio of motor
8. Pretty sure from what i can find 4600 is recommended for WOT
9. Inboard
10. Has a hydrofoil on now
11. Right now has a 23 pitch Mirage Stainless Steel pretty sure 14.5" dia
12. With 3 people on the boat and 3/4 tank of gas i can hit 40 mph (GPS) in kinda rough water but can only get to about 4200 RPM tops and takes a bit to get on plane

looking forward to the advice,

thanks,
Brandon

 

[hwsiii]

Re: 86 Cobalt prop question UPDATED

Trans, I cant find your gear ratio so the best I can do is show you how many RPM you will gain by changing to a 21" pitch prop with the 3 people and 3/4 tank of gas you will be turning about 4,600 RPM and with only 1 person you may overrev the engine RPM a little, but if you buy a 4 blade prop the RPM will go down to about 4,450 and you will have a lot more pulling power and acceleration available at all RPMS.


H

 

[94transam25th]

Re: 86 Cobalt prop question UPDATED

Trans, I cant find your gear ratio so the best I can do is show you how many RPM you will gain by changing to a 21" pitch prop with the 3 people and 3/4 tank of gas you will be turning about 4,600 RPM and with only 1 person you may overrev the engine RPM a little, but if you buy a 4 blade prop the RPM will go down to about 4,450 and you will have a lot more pulling power and acceleration available at all RPMS.


H

how would i be able to figure out the gear ratio?

 

[94transam25th]

Re: 86 Cobalt prop question UPDATED

how would i be able to figure out the gear ratio?

ok i just figured it out i have 1.50 Ratio

 

[94transam25th]

Re: 86 Cobalt prop question UPDATED

Trans, I cant find your gear ratio so the best I can do is show you how many RPM you will gain by changing to a 21" pitch prop with the 3 people and 3/4 tank of gas you will be turning about 4,600 RPM and with only 1 person you may overrev the engine RPM a little, but if you buy a 4 blade prop the RPM will go down to about 4,450 and you will have a lot more pulling power and acceleration available at all RPMS.


H

Ok here it goes
1. 1986 Cobalt 21BR
2. Deep Vee
21'1" Centerline
96" Beam
17" draft
3600 pound average weight
60 gallon capacity
3. Usually start out with full tank of gas and about 5 people
4. Use for cruising to islands and out to dinner/ beaches
5. Deep vee 20 degree deadrise
6. The motor is an 86 350 v8 with 260hp
7. 1.50 gear ratio alpha one
8. Pretty sure from what i can find 4600 is recommended for WOT
9. Inboard
10. Has a hydrofoil on now
11. Right now has a 23 pitch Mirage Stainless Steel pretty sure 14.5" dia
12. With 3 people on the boat and 3/4 tank of gas i can hit 40 mph (GPS) in kinda rough water but can only get to about 4200 RPM tops and takes a bit to get on plane


i have the manual that came with the outdrive and it reccommends a 19 pitch and says it should hit 42-50... my experience with a 19 pitch is that it over revs as i trim it out and then all of a sudden just jumps really high once i hit 1/4 of the way up from full down trim

 

[hwsiii]

Re: 86 Cobalt prop question UPDATED

Brandon, after reading and rereading the posts my opinion is you have a bad tachometer. Like Steele says a very slow hole shot yet overreving the engine does not go together. IF your gear ratio is 1.5:1 then you were showing a prop slip of 51% on the 19" pitch prop and the 23" is showing a prop slip of 34%, and I don't believe either one of them.
GET RID of the hydrafoil, if you want something to raise the stern get some Nauticus Smart Tabs SX, they work right.
Get a shop tach or another tach and a GPS and go out by yourself and get me a WOT RPM and speed. Go behind the tach and turn it back and forth a few times as they get corroded and the electrical signals get messed up and they give you wrong readings, just make sure you set it in the right mode when you are finished.
I believe you probably should be using a 21" pitch prop but I am not positive because the data I have so far is definitely not correct and I cannot make a correct analysis with bad data. I think you will have to agree you do not have a 51% prop slip with a 19" pitch prop turning 5,500 RPM and a BAD holeshot. But get rid of the dolefin before you run these tests.

Prop Slip

H