Prop pitch problem or other

"Hello everyone,

Have a qu

"Hello everyone,

Have a question for you as I am puzzled and getting different answers.

I have a 17' Sylvan Bowrider with a 120 HP Mercruiser I/O. Right now it is running a prop with 20 inch pitch. My RPM's at full throttle are running about 3500 and only getting 25 MPH. I've been told that I should be getting at least 4000RPM and in the high 30 MPH or maybe even 40 MPH. I just had the boat in for repairs, and the place that did the repairs says that he believes the RPM problem is because of the prop pitch. He feels I should be running around a 17 pitch prop. I called another Marine repair place in my area, they said they think the 20 inch prop pitch is about right and it sounds like something else is wrong.
Was hoping maybe some of the great minds in here could shed some light on this and tell me what YOU think is right and what is wrong. Is it a Prop problem or something else? Sure don't wanna spend that kind of money on a prop only to find out that wasn't the problem. Especially since I have 3 props sitting here that are 20 pitch.

I'd really appreciate it if anyone could share their thoughts and expertise about this with me.."

"I am by no means an expert, b

"I am by no means an expert, but I believe that is WAY too much prop for that boat. I used to run a 17 pitch on my old 17' Glastron ( same engine and drive setup as you describe ) and it performed nicely. Good hole shot and ideal RPMs on top end. To give you an idea, I only run a 21 pitch on my 25' 454 MAG Cheetah."

"It may very well be too much

"It may very well be too much prop but we only have half the info. 2 items to consider are pitch and prop diameter. A 19p 13-3/4" = 17p x 14" More pitch or diameter will NOT give you more speed. In order to make speed you must turn the engine, and output shaft at the proper rpm.

You lose 250 to 300 rpm per pitch and 1/2 inch. 3500 is not far off. Try a 19p."

"I concur with Dailed In, you

"I concur with Dailed In, you have way too much prop. I'd start with a 14x17 and tweek from there depending on what kind of performance you're looking for:
- a 14x15 for better holes shots / pulling skiers out of the water (not much top end speed and not so good fuel economy cruising).
- a 14x18 for higher top end speed and/or lower fuel consumption while cruising (longer to get on plane with slower acceleration).
Also:
- trim tabs will help get the boat on plane faster and will be more efficient(effectively makes the boat longer with no added weight, displaces the same amount of water weight over a larger area while moving)
- a hydrofoil will help make the prop more efficeint... to a point (while moving, it increases the pressure at the prop resulting in less cavitation."

"Okay, thanks. To start, the b

"Okay, thanks. To start, the boat already has trim tabs on it. I just bought this boat 2 weeks ago. I will look in a little while and get the numbers off the prop. I just can't afford to go out and buy a prop, then find out I need a different one, OR, the prop wasn't even the problem. But, I've heard a few times now that it sounds like too much prop.

One other thing, a boat this size, what would be the norm for the size of gas tank in it? I just had to get it looked at and this guy said there was water in the gas. I had to dispose of the gas and he gave me about 16 US Gallons of gas, which means about 56 liters of gas or more. Sounds like a pretty huge gas tank.... ??? Especially since there was only about 3/4 a tank of gas left."

"Lee
First, If you have a goo

"Lee
First, If you have a good dealer or prop shop near by most of the time they well lend you some props to try so you can find the one that works the best for you're boat. Of course they expect you buy one at some point. Second, a 20 gallon tank is not much in a I/O boat.Third, Beware of the old we found water in you're gas and had to drain it rip off. Even more so these days. If there was that much water,best find out where it came from."

"I've been reading a lot o

"I've been reading a lot of the threads in here today and I saw one that talked about bad gas. (water in gas) In that thread, it talked about the motor running funny if there was water in the gas. Would the motor run funny, or stop running completely? I'm asking because the reason I took my boat to get looked at is, I was cruising along at about 8MPH (speed limit in river) when all of a sudden, the boat just shut down. Wouldn't start again after. Could this have been due to water in the gas like this guy says I had?? Hate to think I just paid $600 for a water in the gas problem and it wasn't the problem at all..

Just to update on my prop thing, the numbers on the prop I have now are 13.75 x 21P. So, a 19 pitch prop should be better I assume but what about the 13.75 part? Should that stay the same??"

"Well that depends.
How long

"Well that depends.
How long had the boat been running? More than five minutes I doubt it was water related since water is heavier than gas and is in the bottom of the tank with the fuel pick up. So the water would have been pumped up pretty fast if it had enough to stop the engine.
Sometimes there is just enough water that it stays below the pick up then it is sucked up when the boat moves around. This most times will cause the engine to run rough,spit,cough.
At 8mph I think the boat must have been pretty smooth unless there was rough water. Even then the chance of getting enough water to stop the engine without it running rough before it stopped would be rare.
The water should have been sucked up at start up if it had that much water in the tank.

Without being there and seeing what happened it is really hard to know. This is all based on past experience.

Just beware is all Im saying.

There are some real bad dudes in the marine business. They pray for people that know little or nothing about boats.
How hard is it to say I had to drain the tank? Could have then maybe not.
Easy money for a guy not on the up & up."

"Before running at the 8MPH sp

"Before running at the 8MPH speed, we had just finished about a 5 to 10 minute trip across the lake, (ruff water) which brought us to the mouth of the river (calm water) where the speed must be reduced to 8MPH. We had been running at 8MPH for about 2 minutes, then suddenly, she died. This guy said that it "looked" like water in the gas, but didn't "test" like water ??? Even said that it could be a result of running HIGH OCTANE UNLEADED Fuel through it. (I had been told that the high octane would be better for the boat than regular unleaded - is this true?)

He gave me all the gas that he pumped out of the boat to dispose of. But, after talking to someone in the know, I placed all the gas in white, 5 gallon pails. By placing the gas into the pails, the water, if any, should seperate and go to the bottom of the pails, allowing me to see that there was in fact water in it. But, they've been sitting since yesterday and so far, no seperation can be seen. If there was THAT much water in the gas, I wouldn't think he was able to get it all out and some should still appear in the gas that I have."

"Sounds like there was no wate

"Sounds like there was no water in the gas.
First - don't go back to that guy, he's priming you for bigger bucks down the line... don't be his next cash cow.
Second - buy the seloc manual advertised on this site (or equivelant) and start reading. Lots of good info in the first few chapters on how things work and how to keep them that way.
I'm not trying to cop out of helping you out, but if you're going to be doing some work yourself, you need the info... and if you're not going to do the work yourself, you need the info.
Regarding the prop question, 13-3/4" vs. 14", generally, I'd go with the size prop that fits the drive without hitting anything, then adjust the pitch accordingly to achieve the performance you're looking for."

"Oh, by the way, it terms of f

"Oh, by the way, it terms of fuel consumptions in marine engines... 16 gallons ain't much. I would think you would have a 30 or 35 gallon tank. 20 seems small, but I'm used to feeding a 5.7L and my 50 gallon tank is very limiting."

"Dry gas that tank. Every fuel

"Dry gas that tank. Every fuel tank makes water every day. Condensation period. The dry gas will surround the water blob in the tank and break it up into small pieces the system can digest instead of a wad of water ending up in the carb and stalling you out. One bottle for each 15 gals. Treat the next 3 to 4 tanks. You will see it run better with each tank full.

That prop you have sounds like an outboard OD and pitch. I agree with others here, 1" of pitch is equal to 200 rpm. I would go a little bigger in OD say 14 or 14.5 and the 19P"

"First, I would like to [b]THA

"First, I would like to THANK EVERYONE here for all your help, feedback and opinions. You have no idea how much it is appreciated and how much it has helped.

Troy- I did as you suggested and just ordered that manual. Thanks for the tip.

Harves Marine- this "dry gas" you're talking about. Is that the actual name of the stuff? Just trying to figure out where I would get it. If you know of any stores that might have this stuff, let me know the name, and hopefully they have that store in my area. I found a guy that had a used 17 pitch prop but there isn't anyone in my area that allows you to try props. Guess they all just want to make their bucks and that's it.

Anyway, once again, a HUGE thanks to everyone here!!!!!!"

"Lee.
Dry gas as it is called

"Lee.
Dry gas as it is called can be found at any 7 eleven walmart target on & on. One name brand well known is Heet. It's about a dollar a bottle.
As for the water in the buckets,if it was there it would have shown up right a way. I have to wonder just how this person was able ot remove all the water from the tank since it sounds as if it is built into the bottom of the boat. This is not easy because there is no access unless one was built into the floor over the sending unit for the gauge. As the others have said I think it is a good idea to stay clear of this shop. On the open water running,again I would think the boat would have been missing or show some kind of sign you had water in the fuel if there was that much in the tank. On the octane,this depends on the year and model engine. Some do better with higher octane and others you are wasting you're money. Just like cars. Best of luck to you"

"Just curious, would this &#34

"Just curious, would this "dry gas" be the same as a gas line anti-freeze?? I went and checked out WalMart in my area and they've never heard of dry gas. (not surprising with the folks around here...lol)

That's why I was asking about the gas line anti-freeze."

"Try any good auto parts store

"Try any good auto parts store. Have you considered having your prop repitched at a prop shop? Much cheaper than a new one. If it's aluminum, they can add/ subtract about 2 inches of pitch.


Jeff"

"Well, just to update. Fastjef

"Well, just to update. Fastjeff, I found a place that would repitch one of the props I have. He'll take it down to a 19 pitch. I aslo had good luck, in the sense that, the one prop I had, was brand new. The guy I bought the boat from (2 weeks ago), had just put this prop on the boat when I bought it. The place he bought the prop from, was willing to exchange it for another one, since I hadn't used it yet. So, they exchanged it for a 14.5 x 17 pitch. Now I figure I should see a good improvement."

Dry gas is fuel antifreeze.

Dry gas is fuel antifreeze.