Went up 10hp, should I keep the same pitch?

[Rick K]

Hi,
This is a little long, but I want to give as much info as possible.

Was wondering if someone might be able to answer a question on prop pitch... Read all the stickies, and did my research, but looking for someone to confirm.

Running a 15 foot Starcraft Jet, closed bow runabout. Approx weight with 2 people and gear, maybe 700-800 lbs(?)

Had a '76 Evinrude 25hp with a 9x10 prop that did very well. Up on plane quickly, decent top end, all around good prop. Wasn't a speed demon, but did very well for that little motor.

Swapped up to a '79 Evinrude 35hp that has a 10x13 prop. Boat doesn't plane very quickly, engine seems to take forever to get up to top rpm's, etc., and doesn't plane as well as the old 25 did. It is definately faster than the 25, but again, seems to take quite awhile to get up there.

Question: Should I go down to a 10 pitch prop to match what I had, or drop down only 2 points, to an 11 picth. From what I've researched, you should try going up or down in 2 pitch increments Will the 10 pitch on the 35 give the same performance and the 25 did (perhaps a bit better given 10 more hp?), or will it just scream the motor and not really do much at all?

I'm thinking to go to the 11, as the 10 might be a bit drastic. Tiny Tach is on order to check the r's, etc.

My goal is to not have the fastest boat around but would like to gain the best top end I can. I understand about propping out to match the rated rpms, but was thinking someone here might have some practical experience.

Any insight is appreciated.

Thanks
Rick

 

[steelespike]

Re: Went up 10hp, should I keep the same pitch?

Wait for the tach and get it right.In the mean time try to get the motor as high as possible.Just lay some sticks on top of the transom to support the motor till you tighten the clamps. Be sure the clamps have a good grip and it pumps water.I believe the motor came with a 10 1/2 X 11 with a 13" being the biggest at the time.

 

[Rick K]

Re: Went up 10hp, should I keep the same pitch?

Thanks for the reply Steelspike. On the engine height, the cav plate is maybe 3/4" below the hull. I could stick a piece of 1x2 under the motor but that's about it before I run out of grip for the clamps. The bracket on the 35 doesn't seem as "deep" as the older 25.

Didn't really think about that little bit of drop making too much of a difference.

No problems waiting on the tach. I have a laser tach, but imagine trying to use THAT when your banging down the river!

Thanks again,
Rick

 

[jingjing]

Re: Went up 10hp, should I keep the same pitch?

Had a '76 Evinrude 25hp with a 9x10 prop that did very well. Up on plane quickly, decent top end, all around good prop. Wasn't a speed demon, but did very well for that little motor.

Swapped up to a '79 Evinrude 35hp that has a 10x13 prop. Boat doesn't plane very quickly, engine seems to take forever to get up to top rpm's, etc., and doesn't plane as well as the old 25 did. It is definately faster than the 25, but again, seems to take quite awhile to get up there.??

 

[steelespike]

Re: Went up 10hp, should I keep the same pitch?

I forgot to mention the 13 probably would be a little sluggish it most likely is intended for a relatively light boat. The motor heigh probably won't affect things much at the speeds you will go but could add some speed and rpm I've seen 2 inches add 1 or 2 mph under ideal conditions in setups similar to yours.

 

[Rick K]

Re: Went up 10hp, should I keep the same pitch?

Thanks steelspike... Me and my buddy tried out my laser tach this morning and it showed 4400-4600 rpm on average. The tach is pretty accurate, so I have no reason to doubt the reading.

I KNEW the motor sounded sluggish... Tiny Tach should be here in a day or two, so I will confirm.

I'll get her right sooner or later, just a matter of time...

Rick

 

[Rick K]

Re: Went up 10hp, should I keep the same pitch?

Well, I threw a 10.5x11 prop on her tonite (after dealing with carb issues) and checked her out. A MAJOR improvement out of the hole and plane-time. According to Tiny Tach I'm between 5400-5600, but staying the 5400 range.

I DO have to raise the motor on the transom a bit though. I'm almost 2" below where I need to be, or am I ok with this?

Is raising this up going to make that much of a difference? I've got room for about an inch, then I'm out of transom...

Thanks

 

[Triton II]

Re: Went up 10hp, should I keep the same pitch?

Is raising this up going to make that much of a difference? I've got room for about an inch, then I'm out of transom...

Thanks

I'd raise her the 1" - it really does make a difference in my experience. Obviously getting the cav plate level with the bottom of the keel is ideal, but 1" too low is way better than 2" too low.

TII

 

[Rick K]

Re: Went up 10hp, should I keep the same pitch?

I figured as much. Seems there's a few different schools of thought on this, but I do agree, it should be even with the keel. I know I can do the jackplate thing and all that, but I think for an inch, I'll live with it!

Thanks for the reply...
Rick

 

[Rick K]

Re: Went up 10hp, should I keep the same pitch?

I'd raise her the 1" - it really does make a difference in my experience. Obviously getting the cav plate level with the bottom of the keel is ideal, but 1" too low is way better than 2" too low.

TII

Didn't get the opportunity to raise the motor yet, but, I had a carb-flooding issue so I had to rebuild the carb. Motor runs 100% better, and has much more power, etc. Now the boat's fishtailing when ever I come out of the hole real fast.

What causes this? Is this that "tourqe steer" thing? My boat ('61 Starcraft Jet - 15') is by NO means a race boat or a speed demon. The boat seemed to lose "traction" for lack of a better word.

I always thought that if the motor was too low, you'd get drag, etc., but this appears to be just the opposite.

I see they make a small stabelizer (similar to a dolfin) that corrects this. Is it worth the investment?

Thanks