speed difference in props

[steelhead1]

i am currently running a 13 1/4x17 stainless and at full throttle i am barely getting the bottom of my motors rpm range 5000-6000.its top end speed is 35-37 mph.to get my rpms up which i am told is better on the motor i would have to drop to a 15 pitch.how much top end speed would i lose doing this or is it ok if you are at the bottom of the rpm range?<br />thank you<br />scott

 

[Hooty]

Re: speed difference in props

Why don't you do some trimming with the engine before you start re-proping? Try raising the engine up higher on the transom (X dimension). This gets more engine out of the water and lets the engine spin up higher.<br /><br />c/6<br /><br />Hooty

 

[ob]

Re: speed difference in props

Lowering pitch to a 15" would incerase wot rpms around 250 and drop mph minimally with the increase in rpms.However I am at a loss to give specific advise not knowing what motor or boat is in question and height of motor on transom with respect to the anti-ventilation plate relation to bottom of boat transom.

 

[steelhead1]

Re: speed difference in props

ob,it is a 120 evinrude pushing a 22ft starcaft islander aluminum boat.i had the motor raised so it is at its highest point on the gil bracket to be safe from what the marina said.<br />thanks

 

[rickdb1boat]

Re: speed difference in props

steelhead1, Just to jump in for a moment, <br />According to the Michigan site, you should be running a 13-3/4 X 15 pitch, for what it's worth. I would drop to the 15" if it were me.

 

[ob]

Re: speed difference in props

Follow Ricks advice.He's right on.Overpropping will cause undo strain on motor and shorten its life.I'm guessing you won't lose much if any top end and gain a handfull of throttle response.Also recommend using OMC props as they, by design, better match the torque curves of your motor.

 

[steelhead1]

Re: speed difference in props

thank you for the advice guys.<br />scott

 

[Jack Shellac]

Re: speed difference in props

Just as a reference, I have a 90HP Johnson. With a 13 3/4 x 15 SS prop, it turns 5600 RPM. With a 13 3/4 x 17 SS prop, it turns 5000 RPM.

 

[steelhead1]

Re: speed difference in props

jack,did u lose any top end speed going to the 15 pitch over the 17.you did gain 600 rpm's that is a lot.

 

[Jack Shellac]

Re: speed difference in props

I lost about 6MPH going to the 15". My motors rated at 4500-5500RPM. I use the 15" for holeshot when the kids are in town and want to ski. Otherwise, I keep the 17" on it for running to the fishing spots. I'm going to try venting the 17" as a lot of the guys on here have gotten good results with it. BTW, the boat is an 18' deep-V, about 2000lbs loaded up.

 

[papasage]

Re: speed difference in props

how do you vcenta prop

 

[papasage]

Re: speed difference in props

vent a prop

 

[BillP]

Re: speed difference in props

Steelhead1,<br />Reducing diameter and keeping the same pitch is another option. The smaller diameter will be more sensitive to loads but speed may not suffer as much because of the rpm gain. Your prop man should have told you this and should know what numbers to use.

 

[Forktail]

Re: speed difference in props

I've got a boat that has a better top speed (and overall performance) with a 15 instead of the 17. The 15 has less pitch, but it is cupped more and therefore gives me more trim (bow lift) without cavitation. The increased rpm and more lift gives more speed. I guess it really depends on the boat.

 

[Jack Shellac]

Re: speed difference in props

There's alot of good information already on here about venting props- much better than I could give you. Do a search under venting or take a look at www.marinemechanic.com

 

[steelhead1]

Re: speed difference in props

thanks guys for your replys and help.<br />scott