Bad prop causing slow speed? NOW WITH PHOTOS

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
This is something I've wondered about for the last couple seasons. I've got a 1970's vintage 16ft deep V fiberglass hull with an 80hp Merc and a 17" pitch prop, and at WOT (nearly 5800rpm) I can only get about 25-27mph on a good day - GPS verified.

The boat/motor/fuel/gear/passengers etc weighs in about 2000-2100 Ib. The prop looks to be in decent shape, no dings or chips etc. The anti-ventilation plate sits about 1" above the bottom of the hull. I have great hole shot and the boat pops out of the water onto plane very fast, but the top end seems slower than what I was expecting. The boat is rated at 110hp on the name plate.

Any thoughts? I don't NEED more speed but it would be nice to have if it was available?
 

86 century

Ensign
Joined
Sep 8, 2009
Messages
986
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Is there a point that you gain rpm with little to no speed gain. If so you are most likely getting ventalation.
Could be as simple as a damaged prop or your eng could be mounted to high.

What is the max recomended rpm for your eng.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

What year is the motor? I think it is rated for 5500 rpm.
I think we need to varify your tach.Borrow a shop tach or get a Tinytach.
Your tach may have a switch on the back note its position and operate it a few times to clean the contacts.
Be sure it is on the right setting.
To turn 5800 and only do 27 you would have 33% slip possible but not likely.
High normal slip is about 10 or 12%
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

OK, motor is 1971 and I think max RPM is about 5400 but I've read elsewhere that these motors will run to 6000. With the 17" pitch prop I can max it out at about 5800 with just me in the boat on a light chop.

This is the 2nd tach I've installed and they both registered the same rpms.

The prop when I installed it two seasons ago was freshly rebuilt and nicely painted. Now, after two seasons, it looks like it has been sandblasted. The paint is all blotchy and probably 60% of the paint is missing. Is it possible that the prop is cavitating badly?

Anyways, it's going to be awhile before I can get out on the water. It's a little hard still here.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Does the boat respond to up trim?
or
Does it lay down flat like it is stuck to the water?
Does the prop have any cup?
Can you post a couple of close pictures?
You'll have to use a host site.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Yes the boat does respond to trim. I can adjust trim to raise and lower the bow. Not extreme amount of range but it does respond.

Not sure about the cup on the prop. How can I tell a cupped prop from a standard prop? Dumb question ?

I will take a couple photos. Why can't I post them directly on this site? I've done that in the past.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Give the pictures a try worse that happens it won't work.
If you look carefully at edge of the blades you may notice
a slight cup shape.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Took a few photos of the prop. I don't believe it is cupped.

DSCF0620.JPGDSCF0621.JPGDSCF0622.JPGDSCF0623.JPGDSCF0624.JPG
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

One more photo. Wondering if this affect is the result of cavitation? Could that affect the performance of the prop? Is it possible that it is "slipping" more than it should?

DSCF0625.JPG
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Cavitation usually occurs on the back of the blades in confined low pressure areas,
Kind of looks like someone ran a file over it.Yours looks like simple loss of paint.
Its possible the rough paint could aggravate slip a little, imo not 33%. If it was mine I'd take some really fine sandpaper 300-400 and remove the remaining paint.Before going any farther varify your tach.
A Tinytach is reasonable,easy to install and will work on any motor. Be sure to indicate you have a multi cylinder motor as it rerquires a
special antenna wire. You may be able to borrow a shop tach.
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

A simple solution would be to simply switch to a stainless prop. I would go general purpose, something like the Quicksilver Silverado. It sounds to me like your pitch is correct. Don't expect a huge gain in top speed but 1-3 mph could be seen. If the prop you have is the root of your problem then you might see a larger top speed gain but don't be too dissapointed if you don't.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

OK so it doesn't look like cavitation is a problem, just a bad paint job. I'm still thinking there is something keeping me from getting more speed out of this thing. Maybe I'm wrong?

It will be 4 to 6 weeks before I am back on the water to do any verification of the tach. Ice is just starting to melt now.

I took a quick look at prop options and there are a ton of features. Cupping, raking, 3 or 4 blade etc etc. How does one go about choosing the optimum prop for a given boat? What is the advantage of a stainless prop over the aluminum?

One of the running issues that I have is that the boat does not "cruise" well. If I drop rpm down below 3500 the boat just settles into the water and slows right down. Seems I have to run at 3/4 throttle or higher to keep it on plane. That consumes a lot of fuel. Can prop choice improve the planing efficiency or would I be better off investing in smart tabs?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

You must be in a real winter.Here in NY state the St Lawrence river didn't freeze. Many local folks just left their boats in the water.
Usually early december is the cut off.
From your description of your boats response it seems it is normal
is in good sound shape.
Did it do 27 mph when the prop was fresh? and at what wot rpm?
What is the speed at 3500 just before it falls off plane?
I think the motors wot rpm rating is 5200-5500. If it was turning 5800 it would sound like it.
I think the actual rpm is around 4000 so when you reach 3500 from your 5800 your probably.really at 2000 or 2500.
Its any wonder it won't stay on plane. That sounds like an abbott and Costello routine.lol.
Check it is runninmg on all cylinders I assume it is a 4 cylinder.Be sure the throttle opens all the way and the carbs are syncronized.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Still have between 6-9" of ice in the big bay. Ice fishing huts don't have to be off the lake until Mar 15 and realistically full ice out usually happens early April. Probably will happen sooner this year because of the abnormally warm temps.

Like I mentioned, top speed at wot (5600-5800) is max 27mph and that is under ideal conditions with just me in the boat. At around 3500 rpm the speed is about 18-20 mph. If I'm pulling a wakeboarder, I run at 4200 rpm and 22 mph.

Motor runs strong. I can pull a 250Ib skier out of the water on a slalom ski. I'm really not sure about the accuracy of the tach. My untrained ear tells me its running in the upper rpm range. Once I'm back on the water I will definitely hook up another tach for comparison.
 

H20Rat

Vice Admiral
Joined
Mar 8, 2009
Messages
5,199
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

One more photo. Wondering if this affect is the result of cavitation? Could that affect the performance of the prop? Is it possible that it is "slipping" more than it should?

yeah, not cavitation. It would take about 4x as much horsepower for cavitation to even come close to becoming an issue. It takes some fairly specific conditions and A LOT of horsepower for a prop to actually cavitate for long enough to damage anything.

That prop actually looks to be in very good shape. The paint chipping off is common, and won't impact performance much, if at all.
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

yeah, not cavitation. It would take about 4x as much horsepower for cavitation to even come close to becoming an issue. It takes some fairly specific conditions and A LOT of horsepower for a prop to actually cavitate for long enough to damage anything.

That prop actually looks to be in very good shape. The paint chipping off is common, and won't impact performance much, if at all.

I agree, you don't know what it was painted with after it was cleaned up. You also don't know why it was repainted. I've made more than one aluminum prop look really good but still be whacked out when it runs. For what you're doing and the size of the engine and boat your top end isn't all that far off. I have a 4 stroke 60 on my 16' JetCraft and it does about 32 tops but I also have a fairly flat V and a Delta Pad. In my opinion, the engine is on the low end of power for the boat which explains why you drop off plane. You simply don't have the HP to keep the stern lifted. A 115 would make a world of difference and give you about a 35 mph top end. Rather than wasting time and money on props you might consider looking into a decent used 100 or 115.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Looks like I'll just accept that 25-27 mph is the best I can get with what I've got. I'm not interested in a larger motor and a larger fuel bill :).

Thanks for all the info. I will confirm my tach once back on the water.
 

Star

Chief Petty Officer
Joined
Sep 21, 2010
Messages
481
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Have you weighed your boat lately, May be your foam is retaining water!
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Bad prop causing slow speed?

Re: Bad prop causing slow speed?

Nope, that's not an issue. This old timer has the floatation foam UP on the underside of the decking around the gunnels. No foam in the bottom of the boat what-so-ever. This boat is DRY as a bone.
 
Top