I need help with a prop please!!

[Sjason835]

I just got a 1990 Larson all american 190 with mercruiser 4.3L with a 3 blade 19p prop. When I'm wide open it runs about 5900-6000 rpm, and only about 34mph. I know that this is way wrong. From what I understand I should be running between 4400-4800rpm. I was thinking about going to a 4 blade 23p prop, but I don't want to have to buy more than one prop if I get it wrong.so any help at all would be greatly appreciated.

 

[Bondo]

Re: I need help with a prop please!!

I just got a 1990 Larson all american 190 with mercruiser 4.3L with a 3 blade 19p prop. When I'm wide open it runs about 5900-6000 rpm, and only about 34mph. I know that this is way wrong. From what I understand I should be running between 4400-4800rpm. I was thinking about going to a 4 blade 23p prop, but I don't want to have to buy more than one prop if I get it wrong.so any help at all would be greatly appreciated.

Ayuh,..... Welcome Aboard,.... IF it's spinnin' a 19" prop to 6 Grand,....

It's got the Wrong drive ratio...

Verify that the tach is correct, 'n find the drives ratio, either by numbers stamped on the upper housing, aft, portside,...
Or pull it, 'n turn the shafts, countin' the revolutions...

 

[rav221]

Re: I need help with a prop please!!

I would make sure the tach is operating properly, that seems way out of kilter. I don't think the prop is your problem. I'd check that gear ratio and make sure tach is reading accurately.

 

[jestor68]

Re: I need help with a prop please!!

Take the prop to a shop and have them check the hub for slippage. A 19' prop is not out of line for that set up; a 19 ft, 3000 lb(wet) boat with a 4.3L depending on the gear ratio.

The Mercury prop calculator says at 5700 rpm with a 2.0 ratio, you should be doing 43 mph. Actually, their calculator only goes to 5700 rpm for that engine. I guess they figure after that, it'd be about to come apart.

I would get that prop inspected before I did anything else. If the hub is slipping, you cannot know where you are. It might work OK at cruising speed and only start slipping under higher loading. Is this your boat?

 

[Sjason835]

Re: I need help with a prop please!!

Thank you all for the help. I'm going to get the prop inspected tomorrow. The prop slipping really sounds possible. That's pretty close to my boat. I think that one has a bigger motor thou. For some reason thou when I had 10 people in the boat it got up to 39 mph, and ran about 5000 rpm. I will keep you posted thanks again.

 

[Sjason835]

Re: I need help with a prop please!!

Also I did look at at the drive gear ratio. 1.84.

 

[Maclin]

Re: I need help with a prop please!!

I second and third the motions to verify the RPM's as first step.

 

[jestor68]

Re: I need help with a prop please!!

For what it's worth, the Mercury prop selector comes up with a 21 in 3 blade and a 20 in 4 blade for a boat of that size, weight, engine.

Since no prop selector can account for all the variations in hull design, that is just an approximation. Since your motor is not new, the 19" still sounds reasonable.

 

[Tail_Gunner]

Re: I need help with a prop please!!

I just got a 1990 Larson all american 190 with mercruiser 4.3L with a 3 blade 19p prop. When I'm wide open it runs about 5900-6000 rpm, and only about 34mph. I know that this is way wrong. From what I understand I should be running between 4400-4800rpm. I was thinking about going to a 4 blade 23p prop, but I don't want to have to buy more than one prop if I get it wrong.so any help at all would be greatly appreciated.

Your tach is off and use a gps for speed boat tachs and speedo's hmmm lie like a rug..Seriously there both not correct.

http://trailtech.net/tto_tach-hour.html

Umm mod's that would be a great little addition to your inventory

 

[Sjason835]

Re: I need help with a prop please!!

A little update. I found a hole in my bellows, and the bilge pump didn't work. So after I was on the lake for a while and the boat took on some water the prop worked better. I was told it was proped for more weight. And that's why the r's were so high but came back to where they should be after I took on 2 hours of water. Replaceing the bellows and gimble bearing, but I want to know if I should do the u joint at the same time? Thanks for all the info.

Ps. I got a new tach and the rpm were about 5300 Not the 5900-6000 it was showing.

 

[kmarine]

Re: I need help with a prop please!!

I would replace th u joints, they got wet & will eventually need replacment anyway. this way you will be reliable for many years of trouble free service. Should only be about $40 per bearing.

 

[spdracr39]

Re: I need help with a prop please!!

5300 is still too fast. Don't run at WOT till you get it down to or below 4800.

 

[Sjason835]

Re: I need help with a prop please!!

Ok bellows, gimbal bearing, and u joint all replaced. Took me most of the day, but I got it. Put a new 4 blade 21" prop on and now I'm running 4700 rpms at 44mph. Things are starting to look up. Thanks again for all your help

 

[steelespike]

Re: I need help with a prop please!!

Ain't it nice when things come together?