Not sure where to post this but have a problem with my boat. Motor is a 31hp Vanguard/Mudbuddy on a 16ft alumacraft. Out of the water the motor runs great revs up in neutral and drive. In the water it wont go above 2800-3000rpm in drive. Any idea as to what would cause this?

I have a 31 hp Vanguard on a GT. Is it stock or have you done any mods to it? Have you been able to solve the problem or is it still bogging down? Is this something that just started or is it a new condition? Have you made any recent changes...prop, fuel tank/lines, filter, etc?

It kind of sounds like you may have too much prop. What are you running?

If it just started happening and there were no other changes- ensure you are firing on both cylinders. You can pull the cowling, unplug the ground wires to the coils and run it and see if the problem goes away- this will isolate the coils from the kill switch, oil pressure switch, key switch, wire harness issues. With the wires pulled you won't be able to turn it off by the key- wear gloves and pull the plug wires.

If its been getting worse over time- have a compression and leak down test done on it.

Since its a 31, it has the old stock style coils and the diodes on wiring harness for the coil ground wires. When the diodes go out, they can cause the coil to either stop firing or fire intermittently both of which rob power- the diode going bad is a common problem over time. New front wire harness is less than $30 and you can just cut the end with the diode off the new harness and splice it into your existing one if you want to do it quickly.

Valves leaking on the heads is also a common problem- thus the compression and leakdown test.