Which prop? HELP!

[speedrulez]

This is a bit long, sorry. I have a 2006 20' maycraft center console with a 140 jonson/suzuki 4 stroke. When I bought it it had a 14X20 prop. the motor was in the 3rd hole on the mount. When I had it in for service the tech told me I needed a new prop because I wasn't getting the right rpms. I was getting approx. 5000 rpms. Specs say it needs to be 5800-6200. I was getting 35 mph at the time I bought it. The tech ended up putting a 13.75 X 15 on it and lowering the motor to the 1st hole. Now I cant even get 30 mph out of it, and I am at 4500 to stay on plane. The tech says the motor ventilates with other props and higher positions on the mount. The motor seems too low to me. I am close to my wits end here. Any thoughts? Thanks .

Chris

 

[steelespike]

Re: Which prop? HELP!

I'm no expert but thats screwy.You should be able to run in the same hole with the right prop.If you had no venting problem with the original setup you might even be able to raise the motor more with the right prop or even the original.Raising the motor alone might give you a couple of hundred rpm.
Generally speaking a Stainless Steel prop or a prop with some cup will combat venting.Be sure the replacement props are the same or better technology.Come on guys help this guy out.

 

[speedrulez]

Re: Which prop? HELP!

Can I raise it myself? What is involved? The tech said the 2 water pickup strainers under the boat may be causiing some venting. Can this be true? I did notice some venting with the original prop but only on hard turns from a dead stop giving hard throttle. Oh yeah, the original prop was a 21 not a 20.

Chris

 

[steelespike]

Re: Which prop? HELP!

Again I'm not an expert:: If you were getting a little venting with your old prop then you were at about the max for that prop.But the addition of some cup will help control venting .Unfortunately with that prop the cupping will further reduce your rpm.A ss prop will also help but it will also reduce your rpm.But by selecting the right pitch probably around 15 in.will increase your rpm.Hopefully someone will be along with more info.You might look on the net for a boat test with similar motor and boat and note the prop used.
I'll do a search myself as well.

 

[Crownie2]

Re: Which prop? HELP!

On the Suzuki site they have a test of a 21' Center Console and it is running a 14 x 18 3 blade prop at 37.1 mph at 6,200 rpm's. By the way it was mounted one hole up.

Something to look at...

Bob

 

[Dhadley]

Re: Which prop? HELP!

Whatever else the "tech" worked on should be checked too. Obviously he has no clue as to what he's doing.

 

[speedrulez]

Re: Which prop? HELP!

Yeah, I say "Tech" because it takes too long to type "the idiot who I overpaid to screw up my boat." I saw on Suzuki's website their boat tests with the 140 and the Triumph 210 seemed close to my boat.

 

[Dhadley]

Re: Which prop? HELP!

Reducing pitch to gain rpm is a last resort. The amazing thing is that he lowered the pitch a ton which should have put you over on rpm and then to cover his "guess" he lowered the motor which will decrease rpm.

Lowering pitch and lowering the X dimension only decreases efficiency.

What I was hinting at before was, you may want to have his other work double checked too. Changing the prop and the X dimension are one thing but if he "adjusted" anything else like timing, that may have something to do with why the motor is now struggling.

 

[speedrulez]

Re: Which prop? HELP!

Jeez, who am I gonna take it to??? I took it to the highest recommended service facility around here prior to this guy when I was having some issues with stalling. 3 weeks later he claimed I had a birds nest it the cowl. Said he put it on the dyno. He lied. I took it to the next closest certified Johnson service center on the list, and he said I had a cross threaded gas gap and water in the tank. He is the one that did the prop work also. I am thinking that none of them in the Baltimore/DC area know what they are doing.