Propeller question from a rookie boater!

[lucky7]

Just bought a 1971 Sportcraft 25' cuddy cruiser. Has 305 Chevy inboard/outboard. The problem is when I go full throttle full trim down, I only get about 2500 rpm. It hits 4000 rpm at half throttle out of the water. Someone suggested I change props. Any and all advice please!!

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lucky7

[JohnMickish]

Try trimming the motor up and watch the mph vs. the rpm. To get max speed you might have to trim it up alot more than you think. Keep bumping it up until it starts to porpose (bounce) then trim it down a click or two. Ala, max speed on smooth water.

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It still beats workin'

[Surface Tension]

We need to know more.
Will the boat get on plain?
Is the bow plowing water?
Do you have trim tabs?
Weight distribution?

[lucky7]

Surface:

The boat won't plain, the weight distribution was something we messed with, and I do not know what a trim tab is? Help!

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lucky7

[Surface Tension]

What diameter and pitch prop do you have?
I noticed your from Colorado. Whats your elevation and is the boat you purchased from that same area?

The trim tabs will be mounted along the bottom of the transom on both sides of the lower unit. They could be spring operated with no adjustments or have a hydraulic ram. These will aid the boat to get on plain. Once on plain you would raise them up to lift the bow. You can also use them independently while running to take out any list.

[lucky7]

Surface:

Thanks! I will try your advice and let you know how it turned out! Again, thanks!

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lucky7

[Surface Tension]

lucky7, Your welcome but I'm not done giving you advice yet.
Do you have trim tabs?
They should be lowered, once on plain you'd throttle back to a good cruising RPMs. About now the bow should be plowing water, raise the tabs to get the bow up. You'll be fine tuning throttle, trim tabs and the outdrives trim for optimal performance.

If your taking that boat from low elevations to your new home in high elevations you'll need to get the carb re-jetted. After testing you may find you need a new prop as well.

[Mueske]

This is just an example of what must be done when bringing a motor to high elevations (and why I must beg or rent rides when I return to near sea level). Mine is an Evinrude 140 h.p. outboard but the principles remain the same. Four-stroke engines are probably more forgiving but for best performance should be adjusted.

I had smaller jets installed to go along with the smaller amount of oxygen at the elevations I go to. I can't go to Lakes Mohave or Havasu (max. 450-650 feet above sea level) but can hit everything above that. Nearby lakes vary from 8,000-4,500' in elevation and easily in the range of my jetting. I'd hole the pistons if I went to a lake lower than 1,000' as a friend with an identical setup did a couple of years back. The jetting and changing prop pitch (same diameter in my case but that can be varied)from a Minnesota-friendly 19 and 17 to a 15. This makes it run between 5100-5500 RPM max, depending on weight distribution and where my trim tabs are set. My engine makes max h.p. at 5500 RPM. I don't have a speedo and it matters not to me but I'm going over 30 mph but not much at the highest lakes. I have my 17 and 19 pitch props in case I want to try Lakes Powell or Mead (3,700 and 1,200' respectively)this winter. I don't want to change jets as I have a great range of water available to me with my current tuning. The most I could spin was around 2,500 RPM with the original jetting/prop when I first brought my "barge" home from Minnesota.

Hopefully this was helpful to lucky7.

Kevin

[lucky7]

Surface:

I looked for trim tabs and see nothing fitting that description. This motor came from Colorado. Thanks for being patient with me, I'm a true rookie. Just got a great deal on a boat and couldn't pass it up.

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lucky7

[lucky7]

Surface:

I just saw a picture of trim tabs and I do not have any on my boat.

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lucky7

[Surface Tension]

OK no tabs. Whats the elevation of the lake your boating in. You should ask the previous owner what elevation he used the boat at too.

[lucky7]

We were at 4600 ft. This is a rebuilt 305. The old motor was a 327.

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lucky7