What should my top speed be?

[Blue]

I finally got the new boat in the water. Sold the 19' aquasport and now I have a 1973 20' CC with a 1993 Johnson 150. The difference is incredible! Like night and day!

The last boat had a slightly older 150 but flew.

This seacraft has a 14.5 x 19 stainless prop. On the GPS I am getting 32-33 with a 1/4 full tank and one person. My tach has issues and as far as I can tell can't be trusted. It shows idol at 1500 and WOT off the gauge, like 7000 +.

This seems slow to me. Who out there has a similar set up or who can give me insights on where I should be.

Thoughts on what to change?

I looked through past threads and am finding that prop as a good choice so I turn it over to those who know the boat.

Thanks for the help

[lost2a6]

Blue, You have a few issues going on here. Your tack is defiantly off. It sounds like it is on the wrong setting. If you look on the backside of the tack, you'll find a little dial. It should be on the highest setting, which is #6 I think. They have these different settings for different size engines. Once you get your tack right I think that you'll find that you're not turning enough RPM's as a 19 pitch prop on a 20ft SC with a 150 is usually too much, you'll most likely use a 17 pitch. I would say that your top speed should be in the upper 30's to 40MPH, of course with the engine trimmed up just before cavitation starts. [img]/images/graemlins/wink.gif[/img]

[Blue]

Alright I borrowed a 15 x 17 last night and went for a rip. I got 37mph WOT. it felt much better out of the hole and sounded happier at top speed.

Do I stop here or should I find a 15in to try? Is there any reason that would be better?


I tuned the back of the Tach but it is not an OMC so it its numbers stopped at 4. On One it seems as close as it will get. Moving at idol the tach now reads 700 not 1700 and wide open it reads 4400. Ideal for that engine is 4500 - 5500 acording to the book. Does this mean I should size down one more time or assume with 37mph that the tach is faulty and stick with a 17?

Thanks for the help.

[RS]

Hi Blue,
I think you should try a 15". My 20 with a 130 does 40+ with a 13.25x15" @ 5700 RPM. And that's with the motor mounted too low.

I'm surprised the 150 isn't spinning your prop faster. Is the gas old?

[Abe's Rocket]

Hi Blue,
I think you should try a 15". My 20 with a 130 does 40+ with a 13.25x15" @ 5700 RPM. And that's with the motor mounted too low.

I'm surprised the 150 isn't spinning your prop faster. Is the gas old?

i'll second all the above...i have an old 115 on my 20 and i can touch 31mph

[Blue]

Alright here's the latest. I took the boat out with a 15x15 and was surprised to see I was just making 31mph. Came out of the hole quick but I guess the 17 is the best option. I still find it surprising that it isnt faster. The gas is a few months old. I didn't add more in an effort to get a feel for the acuracy of the fuel gauge. I will be filling it up today for the weekend trip out to the vinyard so we'll soon see what effect it has.

Thanks for the help.