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Slow boat.


usaski1
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Thought I'd pick the brains of some BOS'ers and see if any ideas come about: I have a boat that is slow. Tops out at 38.8 or so mph on gps. Turning 5020 rpm, on a 13x13 3 blade OJ. No bad vibrations. Changed spark plugs, (and gaped them) today, no improvement. Feels kinda weak out of the hole. Indmar 325 EFI motor. 86 hours on the motor. Also seems like I'm getting poor fuel consumption. 30mph is 3700 on the tach. Thinking it might not be the motor if im getting 5k out of it. tranny is 1:1. Died 3x in the last 2 motor hours at idle. Ideling 650. (normal) Any leads?

 

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I'm sure @jody seal knows way more than I do but a few things come to mind. It might help to know the motor manufacture,boat manufacture,year and model. 86 hours so its fairly new? or just not used? Has it been sitting a while? Check the distributor and cables? Clogged fuel line?
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Bad prop? Try a different prop before you tear into the engine. I had a horrible prop - smooth but no power. A different prop cured the problem.

I also had an engine get really tired (2000+ hours always in the water) and become too weak to pull a good 36. A lower pitch prop (an altitude prop) fixed the speed issues and got me a few hundred more hours from that engine.

 

Eric

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Yeah, Started thinking about the trans last night. Need to check its fluid level. Could be the prop, don't reacall hitting anything. looks fine. Wish I had a spare to throw on and test. Dont know what happened to my signature, but boat is an 08 Moomba Outback. Boat didnt get used last summer due to TX drought. Could anything else be slipping?
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Lots of potential issues. The engine could have some problems related to fuel, ignition, and compression with sitting so long. Most obvious is the prop. An Acme 3-blade would make a world of difference on your boat. Shaft size? Typically a 13X12.5 .80 cup works well on that boat.
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Don't think so @Colo_skier, but has been this way for months. I can clean the hull, it is dirty. Im thinking Im 6mph off. I am getting the rpms... If it was a bad prop I wouldn't be getting the rpm's, right? Thanks for your tips guys... Anything else I can check? How can I tell if my trans is slipping?
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There are some key clues as to where to look: 1:1 trans implies that the boat should be running about 1 mph for every 100 rpm, so at 36 mph you should be around 3600+/- 300 rpm. WOT at 5000+ rpm indicates the engine has some grunt, but if only going 39 mph there is an issue as it should be mid to high 40's.

 

With that I would check the trans fluid and change if needed as it sounds like it might be slipping. Have you felt the trans to see if it is warmer than normal after running?

 

I would also check prop condition and condition of the shaft bushings (does the shaft bind up, needs to be wet to check if it has the black rubber bushings). Bad fuel economy and dying at idle (if in gear) indicate some drag on the shaft or a damaged prop (although I would expect vibration from a badly damaged prop). You could also check that for temperature, should be cool to the touch. Good luck.

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Eh, never on the flame arrestor... I'll have to check that. And Im going to check my tranny fluid tonight. I hope its low, cuz I want a fix, but I hope its not low, cuz I dont want a damaged tranny. I need to pull the prop off of it and have it checked at a shop. Dont recall hitting anything with it. Thanks guys, keep them tip coming.
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@usaski1, remember that the best way to solve this kind of problem is the process of elimination. Clean your hull first, check it off, test, do the next. Kind of like adjusting fins, one movement at a time if you want to know why. (although regular maintenance items as cleaning the flame arrestor should be done anyway). It would be easier to make suggestions once told us what you eliminated.
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Is This a DBW boat or mechanical? Does the throttle go to WOT?and linkage correct? Is the fuel pressure in speck? have you run a diagnostic scan? Does it have the correct Propeller on the boat? clean bottom? is the trailer still attached? :) . did you do a compression check? Correct spark plugs,cap and rotor? any exhaust blockage?

 

 

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DBW boat, throttle goes to wot as far as I can tell. Will hit rev limiter in neutral. (not sure of that number) Linkage? no linkage, DBW bot. Correct prop, yup. Clean bottom, will clean tonight. Don't know how to check fuel pressure. Compression check, no.. but this motor again would prob not be running 5k on the tach with bad compression.
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@usaski1, what kind of flame arrestor is it? Metal or paper? If metal, remove and spray from the inside-out with carburetor cleaner. Do this away from valuables and let it dry thoroughly before reinstalling. I know a former MC tech Indmar certified and that would be the first thing he would tell you to do if you are complaining about top end.
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Metal arrestor. Im gonna get some carb cleaner and clean it. I pulled it off tonight, didnt look too bad. but wasent perfect either. Thought about running it without the arrestor just to test real quick... then I came to my senses and decided maybe that's not a good idea! I also checked the tranny fluid tonight, and no issues there, unfortunitly. Getting mad. running out of things I can check. Thinking of pulling prop and taking it to a shop. Dont know what else to do.
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You are pulling 5020 rpm in the water and the boat is going 39mph with a 1:1 trans and a 13x13 prop. Given that data, the boat should be travelling roughly 1 mph for every 100 rpm regardless of how well the engine is running not .77 mph per 100 rpm so something between the flywheel and the prop is slipping. The two likely culprits are transmission and prop.

 

Transmission slip can be caused by lack of fluid, wrong fluid or high temperatures, so since the fluid level is good is the fluid correct (sounds like everything was okay with this fluid so that may not be the cause) or is the trans cooler plugged, kinked cooler hose or damaged or leaking? Smell the trans fluid to see if it smells burnt, check color (red or tan) and make sure it does not look like chocolate milk (water mixing with oil). If you still have trouble you could put a timing light on the coupler with a mark to see if it is spinning the same speed as the engine.

 

Look over the prop, make sure it is secure and the key is good so it can't slip. Also, another potential is cavitation so check anything in front of the prop that could be badly disturbing the water flow over the prop (bent tracking fin as an exteme example), you might also check to see that the prop shaft is straight and not bent.

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Going to check prop tonight. also going to clean flame arrestor. However, of my FA was the issue, I wouldnt be getting 5k on the tach. Im thinking now its mechanical with the prop, prop slip, or tranny slip. Im hoping to find out something good. Wish I had a new prop to test it with, so I could rule that out.
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Cleaned the FA... notably blackend in areas, an notably cleaner now.. but... no improvement. Still feels weak out of the hole. put a mark on the prop and shaft. took it for a spin, pulled 3 sets. Not for certain on it, I will have to repeat the test.. Might show some slippage. turned the prop by hand with boat in neutral. felt fine. prop and shaft seem to rotate as one unit. Checked for shaft binding against hull. No.
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Do they put zebra mussels on the SN200?

 

My hull gets incredibly overgrown. The boat has a reasonable hole shot but will not make enough RPMs to get to speed. Doesn't sound like Mark's problem.

 

Switch props.

 

Eric

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You indicated "might show some slippage", what evidence leads you conclude that? Is your prop keyed or splines? If keyed and there is some slippage maybe the key is cracked in two allowing the prop to spin but catch "near" where the broken portions of the keys intersect. Do you know what type of damper plate they use, some are kind of like a clutch plate and the springs do break, doubtful with only 86 hours, all the engine components should be in decent shape if the boat was maintained. Good luck.
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Solved!!!!!!!!!!! I put a new ACME 13x12 on it today... runs 46mph checked by GPS, and way better holeshot. IM A A HAPPY MAN! thanks for all your help.. and hey, I half tuned the motor up a bit too in my quest to solve this!
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