over propped and top end

gorco

Cadet
Joined
Aug 21, 2006
Messages
24
What affect does being over propped have on top end speed?

I ask because I am only getting 35mph (4200rpms) with my setup.

My setup is setup is: 18' flats boat with a 1990 115hp Mariner. The prop is a Laser II 20p SS. I've calculated the boat/motor/gas and me should weigh between 1800 and 2000lb.

I realize changing to a 17" prop will get me higher rpms, but will it have much affect on top end speed?

Based on speed calculators on the internet, my boat has the potential of high 40s.

Is an oversized prop stealing 10+ mph from me?

Also, the boat is currently in line for a tune up. I'm guessing there maybe a problem with it.

Thanks
Steve
 

WillyBWright

Fleet Admiral
Joined
Dec 29, 2003
Messages
8,200
Re: over propped and top end

To get Hp you need RPMs. At 4500, you aren't getting 115Hp. What is the max Hp rating for the boat?
 

gorco

Cadet
Joined
Aug 21, 2006
Messages
24
Re: over propped and top end

The boat is custom built. based on transom width and length, USCG rates it at 150+. The designer recommends a 90, but said a 115 is acceptable.

Thanks
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: over propped and top end

To answer your question about being over propped -- prop pitch can be high enough that the engine does not have enough power to twist it at the engines maximum recommended RPM. Sort of like a truck trying to climb a steep hill. At some point the driver needs to downshift. Boats don't have transmissions so they are essentially in high gear all the time. That means prop selection is a compromise between hole shot, cruise, and top end. You pick which performance characteristics you like by selecting a prop with various characteristics that suit your needs. Rake, diameter, pitch, cupping, stainless vs aluminum, three blade/four blade/five blade are just some of those characteristics. Therefore, the prop is probably costing you top end. By going down in pitch, the engine can now rev higher and quite possibly get it "over the hump" so to speak so the boat may actually increase speed. The net effect is dependent on lots of factors such as hull design, engine tuning, engine set up, and prop selectioni etc.
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: over propped and top end

Sounds like the motor is running way too deep. Does the hull have a pad bottom (The keel will look like a ski)? Do you have a jackplate?
 

gorco

Cadet
Joined
Aug 21, 2006
Messages
24
Re: over propped and top end

Thanks for your replies.

Silvertip, what you say makes sense to me. I hadn't thought of it that way.

Dhadley, the cav plate is level with the bottom of the hull. I've tried lifting the motor 1 and 2 holes without much/if any result. No pad or jack plate.

My next steps will be based on what the mechanic finds...if anything.

Thanks again,

steve
 

gorco

Cadet
Joined
Aug 21, 2006
Messages
24
Re: over propped and top end

for what it's worth, the cause of the low speed ended up being a plugged main jet on the bottom carb. once fixed, the boat now hits 5200rpms and 40mph easily.

I'm now searching to buy a 22" pitch prop to keep my rpms between 4700 and 5200.
 
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