Correct Prop Pitch? Updated: with pictures

[Basscat 1]

I am wondering about what size prop pitch I should use on my boat.<br />Here is the situation.<br />Motor 85hp Johnson with tilt and trim.<br />Prop pitch I think is 19'<br />Boat, 15' tri-hull bass boat.<br />I am also using one of these on the motor, SE SPORT 300 <br />The reason I am asking is with the current setup, I am only getting about 4000 rpms at wot with a top speed of 40mph. Motor is running great with good compression, so I dont believe it is an engine problem.<br />I am not sure if the tachometer is reading correct because it is used.<br />Thanks.

 

[Barnacle_Bill]

Re: Correct Prop Pitch? Updated: with pictures

First step is to verify your tach reading. What year is the motor?

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

1974 is the year. I plan on replacing the tach with a new OMC tach.

 

[moderator1]

Re: Correct Prop Pitch? Updated: with pictures

If the 40mph is GPS, your RPM would be about 5000, which isn't enough.<br />Need to verify type of prop, and pitch.<br />GPS for speed?<br />Alot of variables here.

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

Yes the the GPS speed and boat speedometer says 40 mph, so I know that is accurate. I thought the same thing, I had to be turning more RPM's than 4000 at a speed of 40 mph.<br />The boat is charging at 14.5 volts, so thats not an issue.<br />I am going to replace the tach first and then go from there. Like I said, the one I am using now is used, Im not even sure what brand it is. I have it set on 6-p. The only name or writing I can see on the tach is it has the letter "D" on the front.<br /><br />If I am not mistaken, the prop pitch should be stamped on the rear of the prop, correct?

 

[walleyehed]

Re: Correct Prop Pitch? Updated: with pictures

Yea, on the rear usually, or atleast a P/N...

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

Update:<br />I changed the tachometer and put on a new OMC tach. I couldnt find any numbers on the prop what so ever, so I took it to the local "mom and pop" (literally) boat shop. They couldnt tell me the pitch either. I am 99% sure that the prop man I had rebuild my prop a couple of years ago said it was a 19" prop.<br />Anyhow, my rpm's did go up to 5000 at wot (verifying a bad tach before) and still a top speed of 40mph.<br /><br />The question is, should I change to a 17" prop, and if so will my speed increase or decrease?

 

[Dhadley]

Re: Correct Prop Pitch? Updated: with pictures

If it were mine I'd play with set up to gain rpm before going to a smaller prop. At 5K youre at about 12% slip which is very believable. <br /><br />Suggestion -- take the prop to a dealer and have him give it to the prop guy to verify pitch. They usually come to each dealership twice a week. Give it to him at the beginng of a week so you get it back at the end of the week.<br /><br />I think it probably is a 19. If it were a 17 now youd be at about 1.5% slip. Thats not so believable.

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

If it were mine I'd play with set up to gain rpm before going to a smaller prop.

How? <br />Lets say it is a 19, I am not sure what to do to gain more rpm's unless I change to a smaller prop?

 

[Dhadley]

Re: Correct Prop Pitch? Updated: with pictures

To gain rpm we consider going to a smaller pitch only as an absolute last resort. Is the motor all the way down on the transom? Have you tried going up?

 

[Dhadley]

Re: Correct Prop Pitch? Updated: with pictures

Lets assume that you went to the same style / brand 17" prop and gained the expected 400 rpm. Using the same 12% slip youd be at about 38.5 -39 mph.<br /><br />Lets say the with the 17 you somehow got to 6000. Again with the 12% slip youd still be around 43. <br /><br />But lets say that you worked with the set up and the 19 and got to 5500-5600. Once more with 12% slip you'd be around 45 mph. Remember we're looking for 5800.

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

Is the motor all the way down on the transom? Have you tried going up?

Could you explain this in a little more detail? I have not changed this since I got the boat, however I know this motor did not come on the boat. I have read on here somewhere when somebody was talking about the motor setup and something about the cavitation plate should be 1-2 inches above or below the bottom of the boat (I dont really recall)<br />I am need some schooling when it comes to exactly what you are talking about.<br />If need be, I may be able to provide some pictures of the boat.<br />Thanks.

 

[Dhadley]

Re: Correct Prop Pitch? Updated: with pictures

The location of your AV plate depends on the technology built into your prop and the shape its in now. The better the prop, the higher you can mount the motor. That mounting position is whats refered to as the "X" dimension. With any of todays props the AV plate can be an inch or so above the bottom. If you have a step in the hull, it can go higher.

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

Not to sound to stupid, ( as I said I need schooling here) but I need to clarify some things. AV plate is? I think I know what you are talking about, but I am not for sure. (sorry)<br /><br />When you say step in the hull, you are refering to the very bottom of the hull. Example, mine has a portion of the hull that is below the stern (up under) where the water flows into the live well and the drain plug is.<br />So if I am understanding you, with the motor fully down, the AV plate could be up to 2 inches above the lowest portion of the stern?<br /><br />The prop is in good shape, like I said I had it rebuilt by shop with more than 30 years exp.

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

Bump^

 

[CalicoKid]

Re: Correct Prop Pitch? Updated: with pictures

The AV plate is the Anti-Ventilation Plate, the wide horizontal plate just above the prop. It prevents the prop from sucking large amounts of air from the surface basically. There will be an adjustable trim tab mounted to the underside of the AV plate.<br /><br />Looking down your keel from front to back, does the keel run even all the way to the bottom of the transom? If so yours is not a stepped hull and you'll want your motor's AV plate an inch or so higher than the bottom of the transom.<br /><br />If your keel rises or steps up a couple inches at a point 1 to 2 feet ahead of the transom then it is a stepped hull. This design allows water to begin rising sooner after being displaced by the hull so the prop can run higher and still in clean water.<br /><br />Do you have power trim? You call your boat a trihull... Is it shaped like an 'M' underneath with a blunt bow, or are the side "hulls" much shorter than the center one and higher up?

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

CalicoKid, thanks for the response. I do have power trim. <br />I have decided to post pictures of the hull to make things a little more clear, however that will have to wait until next week. As soon as I take the picture I will post it.

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

The first picture shows the type of hull I have.web page <br /><br />This picture shows the rear of the boat. What I am wandering about, is the "step" that you were talking about Dhadley?<br />web page <br /><br /><br />This picture shows that I can only raise the motor one more hole up. I believe that will raise the AV just above the back of the boat.<br /> web page <br /><br />This picture shows the type of fin that I am using. The reason I added this in the first place is that I had a problem with boat " <br />skipping or porpousing (sp?).<br /> web page

 

[Dhadley]

Re: Correct Prop Pitch? Updated: with pictures

Yep, thats a step. If it were me I'd go up that last hole just to see what happens. IE, see how the set up reacts to less drag. (When you do that get some Nylock nuts and longer bolts. That scares me -- and I'm fearless...)

 

[Basscat 1]

Re: Correct Prop Pitch? Updated: with pictures

Will do boss, and let you know how it works.<br />(those are lock washers, but nylon would probably be better)<br />Thanks.