What is going on???

[cmngtit2277]

I just bought a 1987 Imperial 20ft. cuddy with a 260hp v8 chevy engine. I could not get the boat to go over 30 mph on a GPS so I took it into the marina.

Alright I just got the boat back from the marina and I am not happy. Work done...plugs, rotor, cap, carb rebuild, timing checked, and new prop. He changed it from a 21 to a 19. I certainly do have a better hole shot but my top end hasn't changed. Currently the boat is traveling at 4800 rpms WOT and again am using a 19 pitch. The boat is just barely breaking 30 mph. For a V8 whether its a 1987 or not seems slow. Please help.

 

[freddyray21]

Re: What is going on???

I would think water logged foam possibly adding a lot of weight. My 18ft Searay with a 120 hp will do a bit better than 30.

 

[cmngtit2277]

Re: What is going on???

Would it slow it down that much??

 

[erlindbl]

Re: What is going on???

Something definately does not seem right. I had a 1984 18ft with a 230 hp engine OMC outdrive and a 19p prop. At 4800 RPMs I was doing 49mph. If you find your gear ratio, you can go to one of the prop calculators and determine your slip. It must be very high to be doing 4800 rpms and only getting 30 mph. Since the GPS is probably not wrong, I would check the tach to be sure it is correct. You may not be getting the rpms you think you are. In that case I would check the compression on all your cylinders since you already have done the tune-up.

If your tach is correct, and the engine checks clear, that boat is pretty darn heavy.

 

[Dhadley]

Re: What is going on???

Generally speaking you are not going to go faster by dropping pitch. Obviously they dropped pitch to get you into the top of the rpm range. If you put your old 21 back on what speed & rpm do you get?

 

[MikDee]

Re: What is going on???

First I would suspect the tach being wrong, or maybe the prop slipping, and finally the engine coupler slipping, you now have a few things to check out. Your boat should do between 45-55, depending on condition of the engine, hull bottom, and if it's waterlogged or not. I had a 20' SeaRay bowider with a 305/228hp that would do 50, and a 24' SeaRay Cuddy with a 350/260hp that would do 40.

 

[cmngtit2277]

Re: What is going on???

I will put the 21 back on and see what happens. Let's say theoretically the boat is waterlogged wouldn't I still see more top end then what I am or is that realistic for the condition. Something is not making sense.

 

[Draven1772]

Re: What is going on???

I've had this happen to me before. Give it all the gas in the world and not go anywhere! For me, the problem was my prop spun off my hub. The hub was broke, I replace it (along with the prop) and now it works fine. It's an easy fix and easy to check.

 

[MikDee]

Re: What is going on???

I will put the 21 back on and see what happens. Let's say theoretically the boat is waterlogged wouldn't I still see more top end then what I am or is that realistic for the condition. Something is not making sense.

I guess you mean unrealistic? Yes, it would be,,, waterlogged means holding water (extra weight) in the built in foam flotation, or in the wood stringers. Remember, if your tach is right? Then you should be doing about 50mph with a 19" pitch prop, according to my formula: 4800rpm divided by 1.5 gear ratio= 3200rpm X 18 pitch (19 minus 1) = 57.6 minus 15% slip = 50.mph! I've been using this simple formula for years, it's pretty Dang accurate! somethings slipping somewhere? if not the prop, the coupler.