Boat is slow

[cmngtit2277]

I just recently bought a 1987 20' Imperial cuddy with a V8 260 hp engine. I would expect this boat to travel atleast 40 mph if not closer to 50 with the size of the engine. However I can only get 30mph on a GPS. Is there any one thing that could slow this boat down that much or would this be a combination of several issues? The boat is currently in the marina getting a tune up and carb rebuild. The weight of the boat is 2415 lbs. Any ideas would be greatly appreciated.

 

[Firestar]

Re: Boat is slow

Size prop? Engine RPM's at WOT?

 

[cmngtit2277]

Re: Boat is slow

Sorry about that, I forgot to mention that the tach is getting fixed as well while in the shop. I will provide that data as soon as I can.

 

[Windykid]

Re: Boat is slow

The weight seems low for dry weight. Have you weighed the boat?

Water in the bilge?

Tune up?

You will have to post the WOT RPM.

Prop size and check it for slip.

 

[cmngtit2277]

Re: Boat is slow

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.

 

[traveling man]

Re: Boat is slow

i would get the boat weighed.

 

[cmngtit2277]

Re: Boat is slow

I will weigh it...if I come in higher is there a solution to waterlogged foam?

 

[cmngtit2277]

Re: Boat is slow

Bottom of boat is clean and while running I do have the bow up. When I bring it down I lose about 2 mph. If the boat is waterlogged is it worth doing all the work to repair a 1987 or just use it for a couple years as is and then maybe trade it in??

 

[Don S]

Re: Boat is slow

You may just have the wrong gear ratio in your outdrive. Amazing how many DIYers stick a drive on a boat because it looks like the same thing as the old one. Never realizing there are different ratios.
You may get the proper WOT rpms, but you always loose something with the wrong ratio.

 

[cmngtit2277]

Re: Boat is slow

How do I go about checking that and what am I looking for?

 

[Don S]

Re: Boat is slow

Best thing to do is pull the spark plugs, put the shifter in forward, turn the engine by had about a full turn or so to get all the slack out of the system. Mark where the prop is, and where the crank shaft is. Then turn the crankshaft (Normal direction of engine rotation) and count exactly how many full and partial turns it takes to get the prop to turn exactly 1 full turn.
What it's supposed to be depends on which kind of outdrive you have.
You never mentioned if it's a Merc, OMC, or Volvo or model.

 

[Maclin]

Re: Boat is slow

Using a simple prop calculator to come up with the "theoretical" drive ratio I plugged in in 4800 rpm, 30mph, 10% slip and 19 pitch it shows a drive ratio of 2.5......not sure if that is even offered in any configuration.

Seems something is out of kilter with your numbers, maybe the tach is not showing the correct rpm.

In the same vein, how was it determined that a 19 pitch would be better than a 21 pitch without some good numbers for WOT rpm's and speed?

 

[cmngtit2277]

Re: Boat is slow

My tach is still not working however the tech at the marina put his portable on temporarily to check it. He gave me every reason under the sun as to why it would not give me a higher top end...waterlogged, the fact that the previous owner layed the new floor over the old floor and it's an 0lder v8 and thy are only rated for 220 hp at the prop (260 at the engine). By the way it's all Mercruiser, sorry for not mentioning that before.