Prop - Engine @ 3900 RPM @ WOT

[air0msw@yahoo.com]

I have a 01 Glastron GS249 with a 5.7 Volvo and it has a 15X17 pitch aluminum prop on it. Boat weighs about 4400 LBS. It will only go up to about 3900 RPM at WOT. I want to go with a S/S prop. What pitch should go with? Was thinking about a 17 pitch but didn't know if that would let me get to where the RPMs should be at WOT.

 

[Silvertip]

Re: Prop - Engine @ 3900 RPM @ WOT

Without a speed measurement we can't be of much help. Are you sure the engine is tuned up and running properly? Are you sure you are not lugging around several hundred pounds of water soaked foam? Is the hull clean? I suspect something other than the prop as you would need to drop 4-inches of pitch to raise engine rpm to the 4800 it should be at wide open throttle. Just going from 17P aluminum to 17P Stainless will not change that much. I really think you have performance issues other than the prop.

 

[air0msw@yahoo.com]

Re: Prop - Engine @ 3900 RPM @ WOT

It is running about 36 MPH. It does have bottom paint on it. It runs great, very smooth. Sounds and feels good at all speeds. Boat has been covered or in storage. Doesn't take on any water. What other performance issues can it have to cause this? The boat is a cruiser with the stand up cuddy and aft cabin. Any help is greatly appreciated.

 

[Silvertip]

Re: Prop - Engine @ 3900 RPM @ WOT

When was the last tune-up done? Many folks comment that their engine runs great but they can't figure out why it only revs to a little over half of what it should. The fact is they have an engine that is not firing on one or more cylinders and have no idea what "normal" is. I'm only asking the questions -- not accusing you of not knowing what an engine should sound like. An engine that seems to run good may have its timing retarded enough that it can't develop full power yet it would sound normal. It may be the engine is "tired" so a compression check might be in order. I ran your numbers through a prop calculator and something is wrong. Either your tach is not right or the speed is wrong. I used a 1.64 gear ratio and with a 17P prop at 3900 and 36 MPH you have about 6% slip which ain't going to happen.
At 4600 rpm however, slip goes to 20% so I suspect your tach is not correct and the engine is running properly.

 

[air0msw@yahoo.com]

Re: Prop - Engine @ 3900 RPM @ WOT

I am pretty confident the engine runs as advertised. So with a 17 pitch prop at 3900 RPM, what speed do you calculate it should be running? I guess hooking a tach directly to the engine would be the first item to check. After that, you have any other suggestions? Appreciate any advice.

 

[TahoeBlue]

Re: Prop - Engine @ 3900 RPM @ WOT

Do a compression test on it and check the plugs. How many hours are on it? You might want to have a shop check it out also. If its the stock prop then there is definately something wrong with your engine because the factory wouldn't over prop it that bad.

 

[Silvertip]

Re: Prop - Engine @ 3900 RPM @ WOT

If the tach checks out correct, something is wrong with the speed reading. If you are checking that with GPS then that leaves engine. In my view a 17P would seem to be at least in the ball park but not off by 900 rpm. Verify the tach and if checks within 100 RPM of the test tach, then do a compression check. If that checks ok, then check ignition timing. If this is a four barrel carb, verify that the throttle is opening fully and the primary throttle plate is standing perfectly vertical in the carb bore. The secondary is an air valve design so you can't make it open with the engine off. Air pressure above it makes it open.

 

[air0msw@yahoo.com]

Re: Prop - Engine @ 3900 RPM @ WOT

I will check the tach and compression first. This has a new engine with about 50 hours on it due to not winterizing it two winters ago. Will also replace the plugs and check carb. The prop is not in bad shape but I have a repaired spare that I will put on to check it with to see if there is any difference. The speed was checked by GPS. Sounds like this boat should run in the mid 40's from what I gather. I will post later what I find. Thanks