What is a realistic top speed for this setup?

[Josh in Pa]

I have a 21' bowrider that weighs about 2900 empty. It is more like an 18' boat with a swim platform on it. It has a 350 magnum that is freshly rebuilt with high compression (10:1) pistons. It runs excellent. I think the gears are 1.50:1 and it has a 21" prop on it. The other night I had it wide open, and it could only do about 45 (gps). With the throttle wide open, it would only rev to about 4400 rpm. I was thinking it would go at least 10 mph more, and also thought that a 21" prop might be underpropped. I am thinking that perhaps my secondaries (4 barrel) are not opening? I am not real familiar with boats, but I cannot feel or hear them kick in. All electronics are new, and the engine starts and runs perfect. Are my expectations of 55+ unrealistic, or is something not quite right? Any advice is appreciated. Thanks,
Josh

 

[Limited-Time]

Re: What is a realistic top speed for this setup?

I've got a 19' bowrider 5.7 LX (4 BBL) running a "hurt" 19 pitch 3 blade Alum prop.........Sat I clocked 54.7 MPH (gps) @ 4800 RPM with just me and and about 5 gal of gas on board.

 

[LubeDude]

Re: What is a realistic top speed for this setup?

Are you familiar with how to use the trim on the outdrive? Once you get her on plane with the outdrive down all the way, are you trimming the outdrive out some? This will lift the bow out of the water some and should gain you some needed RPM and speed!

 

[SuperNova]

Re: What is a realistic top speed for this setup?

I agree with lubedude; you probably need to work on getting the trim right and get the boat out of the water a little more. I would say that reasonable expectations would be somewhere between 50 and 55. My old bowrider would do about what you are saying until you started trimming it, then it would all of a sudden pick up another 10 mph and about 600 rpm.