new boat possible problem

usspforman · Jun 24, 2012

Hey guys new to the forum. I have a few questions. My dad and I bought a new 24 foot south bay last year with a 115 merc. We took it out a few times and it would do 21 MPH at wot. It would also cavitate ( I think ) in turns at higher speeds and cause the motor to overheat and shut itself off if you didn't straighten out in time. Even with the motor all the way down it would do this. Our wot rpm is around 6000 if I remember correctly which according to mercuries website is about right but the 24 ft Bentley that they tested got around 30mph. We though all this was because of the stock aluminum prop that came with the motor so we got a ss prop but it didnt change a thing. I cant remember the specs of the prop will have to check another time. So I'm curious what we could do to fix the speed and possible cavitation problem. Or what's a good prop for this boat and engine setup.

Thanks in advance.

WaterDR · Jun 25, 2012

Re: new boat possible problem

Your speed is WAY too low! That is a red flag. Sounds like the wrong prop for sure. I have a 25 footer, a 115 Yam and make 30.

Let us know the specs of the prop. Is it a four blade or three blade?

lncoop · Jun 25, 2012

Re: new boat possible problem

Yeah, thirty sounds perfectly reasonable for that rig, especially brand new. Max RPMs aren't excessive (I don't think), so I don't know that a different prop would accomplish anything. If you're ventilating in turns and losing cooling I wonder if the motor was installed too high. What does the dealer say?

crb478 · Jun 25, 2012

Re: new boat possible problem

First can your motor be lowered? If it can I would try to do that first. What kind of prop do you have. I personally like four bladed props on pontoons especially if they cavitate in turns. I have an 1998 crestliner 24 foot pontoon with an 1986 yamaha 115 and I have hit 31 mph on gps with clean toons in smooth water. you should really be going faster.

usspforman · Jun 25, 2012

Re: new boat possible problem

The Dealer has caused us nothing but problems so I prefer not talking to them bout it, I dont think they really care after they make the sale. The prop thats currently on it is a stainless steel 13.4x14 silverado 3 blade, what the dealer recommended

. The motor is mounted as far down as it can be, the brackets resting on top of the transom. I've heard that cupped props help for cavitation/ventilation problems, would that be worth exploiting? I measured from the top of the bracket to the ventilation plate to check the shaft length ( I think thats how its done ) and its 25 inches give or take. Let me know what you guys think.

usspforman · Jun 25, 2012

Re: new boat possible problem

Also full throttle doesnt really seem like full throttle, by that I mean when the motors at full power the throttle can move another 2 or 3 inches forward till its all the down. Could the dealer have messed up the throttle linkage making it so i cant get full throttle. I would think the motor should be at full power with the throttle all the down in the forward position. Not a few inches from all the way down in the forward position.

WaterDR · Jun 25, 2012

Re: new boat possible problem

Intersting.....something not adding up. Your prop is in the right ballpark. Almost the same size as mine (13.5 x 15). Your rpms are correct....your speed is just too low. BTW, I can trim my motor up or down all I want and my speed does not change that much....sure, it gets better or worse, but just by 2 to 4 mph. The fact that you are hitting 6,000 rpms would seem to suggest you are at correct WOT.

Could you have water in your logs? I know that is not likely, but just trying to help sort this out.

usspforman · Jun 25, 2012

Re: new boat possible problem

I'm not quite sure the 6000 is correct I just remember it being some where round there. I'm probly gunna take her out tomorrow just to figure out exactly what the rpms are.

Any insight on the post about the throttle. Two posts up.

How do you check for water in the logs?

Water logged · Jun 25, 2012

Re: new boat possible problem

My 08 Merc 115 hp 4 stroke is rated at 6400 wot. On my 22 Party barge which is actually 24' I turn 6400 at about 31 mph gps. If I'm not careful it will wind to 6600 with a 13.75 by 15 prop. You should be getting more rpm and speed.

Glenn

usspforman · Jun 25, 2012

Re: new boat possible problem

Hmmm I'll take her out tomorrow morning and get back to you guys with exact numbers. Your all really helping me out, so thank you. What kind of prop are you using waterlogged?

Mi duckdown · Jun 25, 2012

Re: new boat possible problem

handheld GPS for true speed.

usspforman · Jun 25, 2012

Re: new boat possible problem

There's a Garmin GPS built into the dash that you can see the speed on. Its also what sends the speed information to the regular speedometer on the dash because if the GPS isn't on the speedometer doesn't work just keeps going back and forth from peg to peg till you realise your an idiot for not turning on the GPS lol

WaterDR · Jun 26, 2012

Re: new boat possible problem

Just to be sure, i would compare it to another GPS.

usspforman · Jun 26, 2012

Re: new boat possible problem

Okay guys I took her out today and got a bunch more information to help us figure this out. Unfortunately I don't have a handheld gps to verify it with and we were the only ones on the lake today otherwise I would've asked another boat to drive along side of me. Anyway heres the info i gathered.

Passengers and boating gear- Around 375 pounds
Approximately 10 gallons of fuel

Speed at
3000 rpm- 8 mph
4000 rpm- 15 mph
5000 rpm- 18 mph
WOT 6400 rpm- 21 mph/22mph against wind/with wind.
All with motor all the way down.

Engine temp stable at 144 degrees at WOT.

I can turn the wheel 2.5 turns to the right putting the boat in a very tight turn at WOT. This drops the RPM to 5900 at 15 MPH. Engine Temp remains at 144 degrees.

I turn the wheel 1.25 turns to the left at WOT RPM 6300 speed 20 MPH Engine Temp begins climbing. If I turn any further left Temp rises faster and engine sound changes. Eventually I have to straighten out or the motor shuts itself off and alarms go off. When I straighten out and slow to 5-10 MPH engine cools in a matter of seconds.

Here our some pics, not the best but what I can do.
The boat 2011 922CR

The motor Mercury 115 efi

Motor at idle in water

usspforman · Jun 26, 2012

Re: new boat possible problem

Motor at WOT all the way down

Sorry I forgot to take pics while turning. Hope all this information helps.

rickdb1boat · Jun 26, 2012

Re: new boat possible problem

Boat and motor look WAY deep in the water..

rickdb1boat · Jun 26, 2012

Re: new boat possible problem

This is how mine looks. How does your's compare?

usspforman · Jun 26, 2012

Re: new boat possible problem

Our motor does look a bit lower than yours. Mind you I was standing on the back deck at 160 pounds and my buddy that's probly 200 was sitting almost in the back of the boat. I wouldn't think that would be enough to make a difference though.

Is there a measurement of some kind that I can do relative to the surface to see if its sitting deep enough? And I still don't understand why it overheats when i turn left but not right.

rickdb1boat · Jun 26, 2012

Re: new boat possible problem

I really don't know why it is overheating when you turn to the left. Might be a water pickup problem on the lower unit that should be looked at by a dealer or any number of other things. And indeed 360 lbs at the back of the boat can make a difference in how it sits. So that may explain why it looked a bit low. Something still seems amiss though. The numbers you posted just don't add up. I would guess you still have a prop issue. I use a 13 3/4 X 15 Black Max Aluminum Mercury prop on my 20'er with a 90 4 stroke (essentially the same motor you have) and I get 29 MPH at 5900 RPM's with a light load. I can turn either direction as hard as I want and the prop does not ventilate, nor does the motor change temps at all. At 6400 RPM's, you should see at least 25 MPH, and I say 25 because your toon is heavier than mine. And with the motor being as low as yours is, there should be no ventilation unless the water flow to the prop is disturbed more than usual. Hopefully others can offer up some other suggestions..

usspforman · Jun 26, 2012

Re: new boat possible problem

I'll take her out again tomorrow before work. Mainly to get a picture of how it sits from the dock with no weight other than fuel on the back of it. Maybe it is sitting to low. The research I've done on the internet hasn't given me a clear answer as to where the anti ventilation/cavitation plate is supposed to be in relation to the water line at WOT or in relation to the toons. I don't really wanna buy another prop that may or may not work, so I'm kinda at a loss at this point.

new boat possible problem

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