Need Better Acceleration

[davemoore]

Hello - newbie here... boat details:

1983 Marlin Mercury 18'
Mercruiser 470 3.7L Sterndrive
Family of 4 (two parents, 7 and 10 yr. olds)
Occasional Dog (72 lbs.)
Gear

Been out a few times, WOT around 3600-3800.

Tuned up motor today. Discovered prop is in really bad shape...

10 year old wants to learn to ski, but as of now, there is no way this boat is going to pull up a skier.

Reading about props... looks slightly hopeful. Checked prop. No markings. Measured it, about 14" diameter. Unknown pitch. Can't find the factory spec either.

If I pull it and find the markings, how much pitch change do I want? For example, if it's a 19, do I want to go to a 15?

Also, should I keep the same diameter?

Thanks for the help!

Dave

 

[hwsiii]

Re: Need Better Acceleration

Dave, 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, and most people don?t know the make and model of the prop, makes it much harder to propose a solution to the correct prop that you need. 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. 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. ALL 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 prop, how much gas and how many people in boat for test data
14. 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

 

[countvlad]

Re: Need Better Acceleration

Hello - newbie here... boat details:

1983 Marlin Mercury 18'
Mercruiser 470 3.7L Sterndrive
Family of 4 (two parents, 7 and 10 yr. olds)
Occasional Dog (72 lbs.)
Gear

Been out a few times, WOT around 3600-3800.

Tuned up motor today. Discovered prop is in really bad shape...

10 year old wants to learn to ski, but as of now, there is no way this boat is going to pull up a skier.

Reading about props... looks slightly hopeful. Checked prop. No markings. Measured it, about 14" diameter. Unknown pitch. Can't find the factory spec either.

If I pull it and find the markings, how much pitch change do I want? For example, if it's a 19, do I want to go to a 15?

Also, should I keep the same diameter?

Thanks for the help!

Dave

what is the recomended WOT range for the motor? diameter doest really mean much its all about pitch... prop beign in a "bad shape" definetly robs you or rpms.....

 

[anon1]

Re: Need Better Acceleration

...
8. Manufacturer?s recommended WOT range
...
H

Hello,

I'm going to fill in all of the info for my boat (in another thread), but #8 is not clear. Are you looking for RPMs?

8. Maunufacturer's recommended RPM range at WOT?

Thanks.

 

[countvlad]

Re: Need Better Acceleration

Hello,

I'm going to fill in all of the info for my boat (in another thread), but #8 is not clear. Are you looking for RPMs?

8. Maunufacturer's recommended RPM range at WOT?

Thanks.

call the dealer, get a owners manual... "To ensure safe operation and efficient performance, it is critical that your engine operates within the RPM range recommended by the manufacturer. Matching the right prop for the load is the most significant factor of RPM adjustment."

read here http://www.propline.com/Propeller-General-Information/Determining_RPM.htm

 

[davemoore]

Re: Need Better Acceleration

hwsiii:

Here's what I have...

1. Year, make and model of boat
1983 Marlin Runabout Bowrider

2. Length, width and base weight of boat, look for boat decal on back of boat
The boat is listed as 18ft. and measures 18'-8" overall. Width is 7'-0". Base weight is 1,250 lbs.

3. Number of people and gallons of gas normally on boat
2 adults, 2 kids (600 lbs. total) and 18 gallons of gas.

4. What do you use the boat for
Fishing, Tubing, want to pull kids on skis eventually

5. Is it a Deep Vee and if so how many degrees of deadrise
Yes, I think about 40 degrees. Not sure.

6. Year, make and model of motor
1983 Mercruiser 470

7. HP and gear ratio of motor IMPORTANT
170 HP, 1.84:1

8. Manufacturer?s recommended WOT range
4200-4600

9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
N/A

10. Is it a bass boat or does it have a pad bottom
No

11. Does it have a hydrafoil or trim tabs
No

12. Make, model, diameter, pitch and whether SS or aluminum prop
Unknown make/model, 14" dia., unknown pitch, aluminum

13. WOT RPM and speed from prop, how much gas and how many people in boat for test data
3500, same as #3

14. 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
Currently, there is no way I could pull up a skier as it accellerates too slowly, and I read that lowering the pitch would help with this.

And I've attached a photo (I hope!)

Thanks for any help,

Dave

 

[hwsiii]

Re: Need Better Acceleration

Dave, the numbers don't work right when I run them, something doesn't add up. I have a post on here that shows you how to measure the pitch on a prop, it is located here, if you can't find any numbers on the prop.

http://forums.iboats.com/showthread.php?t=332091

Also what was your past speed and RPM with the prop you have now.


H

 

[davemoore]

Re: Need Better Acceleration

H:

I have no idea what the speed was... my speedo sender is broken... the RPM was around 3500 ... maybe 3700 tops...

But, I just did a tune up and haven't been back out yet... I will find out Friday... hopefully it's improved.

I think if I pull the prop there may be some ID on the inside... ??

This will be my first time removing a prop, but I have the service manual so I should be ok...

 

[billy4hp]

Re: Need Better Acceleration

H:

I have no idea what the speed was... my speedo sender is broken... the RPM was around 3500 ... maybe 3700 tops...

But, I just did a tune up and haven't been back out yet... I will find out Friday... hopefully it's improved.

I think if I pull the prop there may be some ID on the inside... ??

This will be my first time removing a prop, but I have the service manual so I should be ok...

Just an FYI boat speedometers are notoriously inaccurate, a GPS (either hand held or permanent mount) is far more accurate...

Good luck...

 

[hwsiii]

Re: Need Better Acceleration

Dave, Billy is right speedometers are notoriously inaccurate, even a handheld GPS or some phones even have one. The prop ought to have markings on the side or after you pull it look on the inside of the prop for the numbers. Also having the new tuneup you did will help us get it right as well, good job. Just get the numbers for WOT RPM and speed with the pith of the prop and I will run the numbers.


H

 

[davemoore]

Re: Need Better Acceleration

Ok, I've got good news and bad news...

First of all, I want to say THANKS to Bubba1235 who pushed me to do a FULL tune up... I did, and the performance today was just amazing!

As it turns out, I don't think I will need to change the prop pitch after all! It was just the poor performance of the motor. I can't believe the remarkable difference between last time out and today's acceleration. We are going to pick up a wakeboard this weekend for the kids.

We took the GPS with us and clocked the speed at WOT and the RPMs. 38 MPH and 4100 with our full load (family and gear). That's pretty close to the manufacturer's specs (4200-4600).

And you're right - the speedometer on the boat isn't worth a darn...

The one thing that started happening though today that didn't happen before was one time when I shut her off there was some run-on (or dieseling or whatever it's called). And it was really hard to start sometimes... but the temperature never goes over 160-165, so that's good.

NOW - for the BAD NEWS... somehow - don't know how this happened - the left Hydrofoil where it bolted to the lower unit got broken off (along with that fin on the lower unit). I have no idea how this happened, but I do have one idea. I may have raised the outdrive with it turned so the hydrofoil hit the little step platform bar when it came up, and snapped it off... this made the boat almost uncontrollable though... tomorrow I'm going to take off the right hydrofoil and see how it handles, and start looking for a new lower unit. :-(

I'm very encouraged though about the performance of the motor. I will probably just replace the prop with a new one of the same pitch, once I figure out what it is... what do you think H?


Dave

 

[hwsiii]

Re: Need Better Acceleration

Dave, I think that is just about right without everybody in the boat you would probably turn about 4,300 to 4,400 RPM and that is fine. I am glad it is running great, and I am sorry to hear about the anti vent plate.


H

 

[hwsiii]

Re: Need Better Acceleration

Dave, after rereading your post I decided that something went wrong with the boat when you did the full tuneup. I can't say what it was, but it certainly shouldn't be dieseling and hard to start after a full tuneup. Recheck the points, timing and advance, plug wires on right plugs and run some seafoam through the engine to clean any excess carbon out of the cylinders. This could cause major damage to the engine and I would hate to see that.


H

 

[davemoore]

Re: Need Better Acceleration

Yeah, good point H... Honestly, I am not SURE I have the correct plugs in there...

Dave

 

[hwsiii]

Re: Need Better Acceleration

The plugs that i found said BPR6FS, NGK.


H

 

[davemoore]

Re: Need Better Acceleration

Ok, I got the new plugs in - Chamion RV9YC (they didn't have the NGK's in stock) - which are acceptable according to the service manual. Checked (and reset) the timing again too. Running very good. I also put the SeaFoam in the fuel tank. Am taking her out again tomorrow morning and will let you know how she performs.

I discovered today when I was working on removing the right "fin" of the whale tale that the part that broke off had been broken off and repaired before... It may have very well been weak and just the force of the prop and water could have snapped it off... don't know... at any rate, if I can get on plane without the hydrofoil (whale tale), I'm not going to worry too much about it.

But the prop for sure has to be replaced. It's really pretty bad.

Dave

 

[hwsiii]

Re: Need Better Acceleration

I agree with you Dave.


H

 

[steelespike]

Re: Need Better Acceleration

Don't go back to the foil.If you need help planing get trim tabs.Something like smart tabs are reasonable,easy to install ,work well and have excellent factory support.

 

[davemoore]

Re: Need Better Acceleration

Yeah Steelespike, that's what I'm discovering in everything I read and watch (YouTube).

Thanks...

 

[davemoore]

Re: Need Better Acceleration

Ok. Once again, I have good news and bad news! Sheeze - I'm tired of saying that already! Haha...

So, without the whale tale, I had NO problem getting on plane - even with an extra person (200 lbs) and a dog (75 lbs) in the boat... we were almost at capacity (according to the nameplate). This motor has a lot of torque and planes out very quickly!

Now the bad news - lost the impeller today. Got it out though and will replace it this week.

Of course, I pulled the prop to pull the lower unit, and found some weird stuff...

First of all, the prop that was on there is a 14.5 X 19. However, if you look at the photos below, something tells me it doesn't go with this boat. Also, what is all that white junk? I couldn't figure out what it was - very strange.

I will DEFINITELY be ordering a new prop this week, but I want to know for sure what I should put on there.

What if I went to a 14 X 17? That would get me more power to pull up skiers, right? Or should I stick with the 19 pitch?

Also, it looks like someone filed the little fin (the mfr. calls it a trim tab in the service manual) to allow clearance for the prop. Is 14.5 a standard diameter? I think this boat probably came with a 14...

The other thing that perplexes me is that even after replacing the plugs with the correct ones and setting the timing and point gap again, I had a lot of "run on" and hard starting. Not too sure what to think... Could it be the SeaFoam? I read that other people complained about run-on with SeaFoam treatment... any thoughts?



Thanks Again,

Dave