New prop, wrong results

[Bumperlt12]

So the little anode that goes above the prop back out and tore up the prop. There isn't any legible markings on the old prop.

Boat is a 91 Searay 220da 24', with a 5.0 and alpha. Boat weighs 4k dry nearest i can figure.

Today I put on the new 14.25 x 17 prop. There was 40 gallons of fuel, and 20 gallons of water, along with 600 lbs of human bodies.

Before, 7 kts would be at anout 1800 RPM. After the prop got tore up it increased to about 2200 RPM to reach the same 7 kts.

Before, Max RPM was 4800, but i don't know speed because i didn't have GPS at that point. After the prop got tore up, it would still hit 4800 RPM, and i think the max speed I saw was 27 kts or so via gps.

NOW, with the new prop, it really struggled to get on plain, and maxed out at 4000 RPM, and only going 23 kts via gps.

I don't think the prop is slipping cause in my mind the RPMs would be higher. So I assume I got the wrong damn prop. I didn't do enough research I guess. I read on this forum a guy that had my same boat with my same motor and was running a 14.25 x 17 and said it was perfect, 32 kts @ 4800 RPM. This obviously wasn't my result.

Please help I want to get back out on the water without guzzling all this extra fuel running at higher than necessary rpm.

Here is the only markings I can find on the torn up prop

 

[airshot]

Re: New prop, wrong results

Never assume that because it worked for someone else it will work for you. If your old prop turned the max rpm recomended for your motor then you should be replacing it with the same dia and pitch. What did you change?

Airshot

 

[Bumperlt12]

Re: New prop, wrong results

The old prop has a lot of pitting and I can't identify anything.

Got this off Searay's website http://www.searay.com/boat_graphics/electronic_brochure/Company1729/1C1_25_74DEIP4PW0M.pdf

Looks like searay recommends 14.5 x 17. Mine is 14.25 x 17, I feel like even if it were 14.5 it wouldn't be a huge difference

 

[steelespike]

Re: New prop, wrong results

It is essentially the same prop,should produce the same type numbers.This assumes the old prop hadn't been changed to a lighter pitch.
Did you look under the prop nut for numbers?
Is your bottom clean and smooth?
A prop shop can measure it for you.
Saw a trick on Pawn Stars sprinkle some talc on the number and smooth it with your finger.
Use a magnifying glass and led light from different directions.Usually low down shone
across the numbers.
Last 2 numbers are usually the pitch.Looks like 15 to me.Pitch is the key number . With a heavy boat you would lean toward a larger diameter if available. The 14 1/4" diameter shouldn't make that much difference.

 

[Bumperlt12]

Re: New prop, wrong results

I didn't know there was numbers under the nut.. I'll take a look today when I go to the boat. I'm regretting not bring the old prop home with me now.

Bottom is definitly clean and smooth.

I'll see what i can identify on the old one. thanks.

 

[jestor68]

Re: New prop, wrong results

It appears that your old prop's markings indicate a 15 pitch.

 

[JoLin]

Re: New prop, wrong results

Yeah, that marking at the end looks like '15 P' to me, too.

 

[Bumperlt12]

Re: New prop, wrong results

Well I brought the old prop home and the area that shows info under the nut is even worse than the side. The only other marking I can find is Y559, and its on the side similar to the pictures marking, but much larger.

Maybe I've stared at the marking too long but I just don't see anything definitive. I'm going to take it to a local prop shop and see if they can tell me more.

I have another prop but it's 14.25x21, which would give me an even worse problem.

 

[Starcraft5834]

Re: New prop, wrong results

I see you have a 24ft boat,, two things,,, consider losing the anode and go with a flat one,,, and put a 4 blade Solas 15p 14.75 in on your boat..

I have a 20ft cuddy, w/ 3.0 merc sterndrive,,had a 3 blade, lousy hole shot, 4200 WOT,, bout 35mph,, changed it to 4 blade mentioned above, HUGE difference in hole shot, up on plane in less than 10 seconds, at 4500 WOT, bout 32 with 1000 lbs of bodies in boat.., 5 adults in boat.. consider 4 blade,, its a very smart move for a larger boat such as yours,,,

 

[Bumperlt12]

Re: New prop, wrong results

Okie dokie-

Do went to my local prop shop and told them what was going on. They gave me a 15.25x15.

So said 15.25 x 15, The max rpm is correct at 4500-4800 depending on wind, however at that RPM I'm only going 23-24 knots (via gps), which is 27 mph. I have read other people doing near 35 mph at max rpm.

As far as cruising speed, to go 18 knots (20.7 mph) I have to have the RPM at 4000, whereas I've read people cruising at 22-24 mph, at 2500-3000 RPM.

Just pulled the boat out of the water today and getting the right prop is top priority because cruising at 4k RPM is guzzling gas like an alcoholic with alcohol.

I still have the 14.25 x 17, I am considering reinstalling it and checking results at the lake by my house. When I put it on before, I was on my belly at the marina, perhaps I did something wrong.

I'm calling the prop shop tomorrow to see if I can exchange the 15.25 x 15 for something different, anyone have a ballpark prop size I should start with?

 

[Bumperlt12]

Re: New prop, wrong results

double post

 

[schalmat]

Re: New prop, wrong results

That last number looks like a 19 p to me.look closley at the top of the number,,more rounded like a 9 .

 

[steelespike]

Re: New prop, wrong results

As I recall you weren't making rpm with the 17" you replaced the prop with. You surely won't make rpm with a 19.
It seems to me that for what ever reason your motor is lacking power.Perhaps a compression check is in order.
Note the condition of the plugs as you remove them.

 

[Bumperlt12]

Re: New prop, wrong results

Alrighty-

Put the 14.25 x 17 prop back on and took it down to the lake. I was able to get 35 mph at 4500 rpm and cruised at 22-23 mph at 3200 RPM. Do these numbers seem right? I'm inclined to say yes. Maybe the prop was spinning on the shaft before?

However, there was no cooler or anyone on board but me, and the water was calm.

Before, when it wouldn't plane, there was 4 people on board, 2 coolers, and rough water. So I guess I'll leave this prop on and wait for a rough water kind of day.

 

[Bumperlt12]

Re: New prop, wrong results

Finally had some people on my boat and sure enough it wouldn't plane again.

So I took the prop back and they gave me a 16x16 to try.

With the 16x16 I am at much lower rpm in both low speed (5.3 kts @ 1200) and cruising (21 kts @ 3000). However, The maximum rpm I can achieve is 4200 rpm (30 kts) (3800 with 7 people on board). The low speed and cruise numbers are right where I want them, but I'm affraid that because i can't reach the right RPM, that the motor might be working too hard.

Anyone?

 

[Bumperlt12]

Re: New prop, wrong results

Should I be concerned about the motor working too hard?

 

[Howard Sterndrive]

Re: New prop, wrong results

you have a power problem. All this prop switching is like taking your car out of D and driving around in 2nd gear because the engine is sick.

A prop shop can measure the pitch of your old damaged prop - if that worked right before, get the same pitch and then check the boat's fuel and fuel system and ignition and clean the flame arrester. Basically tuneup and visual inspections. And check compression as was suggested. That's part of the annual maintenance anyway.

How old are things like the fuel filter and spark plugs?

 

[Bumperlt12]

Re: New prop, wrong results

Compression is 155psi -/+ 5-8psi in all cylinders.

I just swapped the spark plugs, and changed the oil and fuel filters when I changed the oil a couple weeks ago.