Prop Calculator, Rigging Questions and help???

[scott lambert]

Hey guys try hard not to laugh here but could someone help me figure my efficiency of my prop & engine set up, as well as if my rigging sounds close. Not sure what I’m doing wrong on the prop calculator but something does not seem to be coming out right when I enter it into the calculator. I didn't want to post on the board that I usually post on, as all I would get is greif about trying to max out my combo.

Here’s what I can share….. I have a Carolina Skiff 17’ dlx flat bottom, center console fishing boat that I’m hoping to get close to 40 mph out of. The boat comes out of the hole good and rpms seem just a touch high….not a large rooster tail at WOT but seems to have just a little higher spray than it should be when trim all the way down at full throttle (it’s not spinning the prop out during acceleration but does have a 4.5’ rooster tail spray to the starboard side of the boat at WOT). I do not have the engine all the way down on the transom, It’s in the 3rd hole about 2” inches to full drop but the cavitation plate sits about ¾ of an inch above the bottom of the boat's hull , I can and do trim the engine up a little to get my max mph, I trim it up until it starts to porpoise, then bump it down a click. Last thing to share is the engine does have a whales tail, I think if it didn't she would spin the prop with current set up.

I will bold what I have been entering on the calculator.......

Engine -1992 60 hp OMC (not 100% sure of gear ratio – Think they are 2.4 to 1, someone tell me if I’m incorrect?)

Prop 13.5 x 15 (I’ve been entering 15 on the calculator, is that correct?)

RPM’ showing 6100 at WOT

GPS Speed – 33.5 mph

Could someone tell me what they are coming up with on the caculator and thoughts on my current rigging set up...

Thanks for any and all advice……..

[Fastjeff57]

I've seen a few of those boats. Was stunned to note its flat bottom--good for minimizing roll, I suspect. They appear to be hard riding boats in a chop, however.

That rooster tail is indicative of the type of prop you're running. Read the attached and you'll see what I mean, and what to do about it.


Jeff

[zul8tr]

Here is a prop calculator I have used

http://continuouswave.com/cgi-bin/propcalc.pl

With the 6100, 15 pitch, 2.4 gear ratio and zero slip the calculated speed is 36 mph. The zero slip is not possible as some slip is always there but it gives an indication of upper limit. Slip is variable and depends on the setup and local water conditions, etc. You can back calculate the slip if you measure your speed with a gps and then use the calculator to determine the slip knowing all the other data.

The gear ratio you note might be in error so research it. I do not know what it would be. If you can't find it you can count the rotation of the prop to one flywheel revolution. As an example if the flywheel turns once and the prop turns 1/2 a revolution the gear ratio is 2:1. You will have to estimate the amount of rotation of the prop in your case it will turn less than 1 rev for 1 flywheel rev. But this should give you an idea if the 2.4:1 is correct.

[scott lambert]

Jeff, as far as the spary goes I'm only getting it to the starbord side of the engine, does that sound like cavitation due to engine being to high vs the prop style. I don't think I'm spinning the prop to the point it blows out.

[Fastjeff57]

The starboard side? That's a bit weird, for a right hand prop should make the water blow up on the port side.

Jeff

[LittleCharger]

You have the proper gear ratio of 2.42.

[dog74lover]

Here is a prop calculator I have used

http://continuouswave.com/cgi-bin/propcalc.pl

With the 6100, 15 pitch, 2.4 gear ratio and zero slip the calculated speed is 36 mph. The zero slip is not possible as some slip is always there but it gives an indication of upper limit. Slip is variable and depends on the setup and local water conditions, etc. You can back calculate the slip if you measure your speed with a gps and then use the calculator to determine the slip knowing all the other data.

The gear ratio you note might be in error so research it. I do not know what it would be. If you can't find it you can count the rotation of the prop to one flywheel revolution. As an example if the flywheel turns once and the prop turns 1/2 a revolution the gear ratio is 2:1. You will have to estimate the amount of rotation of the prop in your case it will turn less than 1 rev for 1 flywheel rev. But this should give you an idea if the 2.4:1 is correct.

I'm actually interested in using this as a resource down the road.