Prop Calculator, Rigging Questions and help???

Joined
Sep 1, 2012
Messages
6
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.

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 on the engine sits about ? of an inch above the bottom of the boat, 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 in the caculator and thoughts on my current rigging set up...

Thanks for any and all advice……..
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: Prop Calculator, Rigging Questions and help???

Don't know your gear ratio.

Yes on the 15

6100 says you need more pitch to cut the rpms down....200 drop per inch is the numbers we use on here

The calculator will tell you how 33.5 stacks up. You will have to factor in some slip and 10-15% ought to be plenty on that boat

3/4 inch above the water at WOT is fine if you have the prop to handle it....you didn't specify what you had.

Somebody put the tail on there. Question is did it help or hurt your performance.

That engine on that boat is no show stopper....don't expect miracles. I ran one once with a 60 Merc down in Lafitte, LA. (rent boat) and it ran out real nice. It was propped shallow (low pitch) but with the coastal marsh down there you had to have a fast hole shot to get out of the mud before you tore up your prop. Obviously at WOT it (the engine) screamed...no tach.

The bottom is flat with the sponsons and all and as a result you have a large wetted area. This translates to drag and drag costs you mph.

My 2c,

Mark
 

Campylobacter

Chief Petty Officer
Joined
Aug 4, 2007
Messages
503
Re: Prop Calculator, Rigging Questions and help???

I found a ratio of 2.42 and max theoretical speed of 36@6100 RPM, so 33.5 if only 7% slip. You could go up to a 17 and gain a few MPH at the top end (assuming you can still turn at least 5500), but the hole shot would suffer.

Looks pretty good to me.
 

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Prop Calculator, Rigging Questions and help???

If you want more speed, put a 17" pitch on it. You will still be well within the recommended rpm range with the 17.
 
Joined
Sep 1, 2012
Messages
6
Re: Prop Calculator, Rigging Questions and help???

Thanks for the help guys......

I thought with the small spray I was getting on the startboard side of the engine was cavation. :confused:
 

etracer68

Ensign
Joined
Oct 11, 2009
Messages
906
Re: Prop Calculator, Rigging Questions and help???

The spray may be coming from the tail on the OB, or a transducer or speed pick-up mounted on the transom. Just my .002
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: Prop Calculator, Rigging Questions and help???

The spray may be coming from the tail on the OB, or a transducer or speed pick-up mounted on the transom. Just my .002

We only know what we are fed as info. The rest we just have to guess. Nice job.

Mark
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop Calculator, Rigging Questions and help???

You should try to get it right without the whale tail.
The tail could be sucking up a couple of mph.
 
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