Prop question

[Manimal]

Ok, here we go. I consider myself a novice boating enthusiast although I have owned several boats (I know how to use them properly, just not all the technical stuff).

I have a 15ft 6in bass boat with a 1988 Johnson seahorse 70hp. I know that the prop is not original because I have the original off as a spare. Nor did I replace the prop, it was on there when I got the boat.

What am I looking for? A good, cheap (preferable aluminum) new prop, but I have no idea about things like pitch, ventilation and the such. I have done some reading on the internet but I am still confused as to which one is right for me.

I use the boat for fishing in local bass tournaments. I need a decent hole shot but I want to maximize top end. Which one do I get?

Also worth mentioning: With the prop I have now, when starting off it takes a while to get on plane. But just before it does, as the bow is lowering, engine rpm's skyrocket and it seems the prop is almost out of the water. Once the bow settles, off I go like a rocket. Is there a problem there?

 

[mpsyamaha]

Re: Prop question

what size and pitch is the prop you have on the boat now? what is you wot rpm? this is important if you want advice on a new prop.

do you have power trim and tilt? if so is the motor trimmed all the way in for hole shot?

 

[Manimal]

Re: Prop question

I wasnt sure about all the info that was needed till after I read some of the other posts about the same topic.

RPM--I have no idea, dont have a tach
current prop-- again, no idea. The oem prop says 19 on it, but its not on the boat.
I do have power trim and tilt, and yes, I do keep it all the way in for taking off.

I guess mainly what I am trying to find out is what prop size did the motor come with originally? I have tried to find out, but cant seem to find the info on the web. I would be happy with just that, an original prop size because that should cover what I am after.

Any thoughts on a composite prop? I noticed that they are much cheaper (probably for a reason) but am not sure about durability. I dont have much money to spend, so a ss prop is probably out of the question.



Thanks for the help, I know I am a novice and probably am not able to get all the info you would need to help me, sorry.


Update: The only markings I can find on my currently installed prop is K15, dont know if that is a part number or pitch or what?

 

[Silvertip]

Re: Prop question

Your engine did not come with a prop. The manufacturer doesn't know whether the engine is being installed on a barge, pontoon, fast hull or what so they leave that up to the dealer as he is installing the motor. A light fast boat can run higher pitched props than a barge for example which requires a low pitch prop. So without RPM readings, we can't help you other than to suggest that since it originally had a 19P prop, you should use that as a starting point. That prop is very typical for that engine and boat combo. Measure the diameter of the prop which should be in the range of 13-1/4 - 13-3/4 inches. Measure from the tip of one blade to the center then multiply by 2.

 

[Manimal]

Re: Prop question

Ok, the prop that is installed now is stamped K15 and measures 13 1/4. The motor does seem to run at wot a little high in the rpms (my own uneducated opinion).

What do you think a 13 1/4 with 19 would do?

When viewed, there does seem to be some extra room between the edge of the prop and the upper plate, so, is a longer bladed prop better?

And, I am working on finding a cheap tach that I can install myself. I know that will help this whole process.

Thanks

 

[Silvertip]

Re: Prop question

Diameter of a prop is a function of its design. No -- you do not buy a prop by diameter. You buy a prop for your engine -- diameter will take care of itself. Props are designed for a given gearcase size so get too large and the blades collide with things. I can only guess what a 19P prop will do as we have no idea what your baseline numbers are. My 75 HP Merc as a 19P prop on a bigger boat and it runs 40 MPH. Since you have a few less ponies and about the same gearing, I would expect a 19P would put you in that same range provided setup is proper and the engine is in a proper state of tune, the hull is not fouled and you are not lugging around a couple hundred pounds of water soaked foam. The K15 you mention means nothing to me. Look for numbers behind the prop nut and washer.

 

[steelespike]

Re: Prop question

In your hunt for a tach give the tiny tach a look.easy install and can be used on any similar motor.no worry about poll selection.

 

[Manimal]

Re: Prop question

Ok, thanks for the possible location of numbers for the prop, I will pull it off today and take a look. As for the tach, any idea where I can get that? My local boat dealer kinda has a corner on the market around here in Oklahoma so he feels he can charge whatever he wants. The cheapest tach I found at his shop was around $80 (installing myself).

One more question. How do y'all feel about composite props? I have read good and bad about them. I know I will get the best out of a ss prop, but money is an issue for me. I found a pirhana that you can change the blades for pitch or breakage, but am unsure if I would be better with an aluminum. Any thoughts? (collisions with debris or rocks is not an issue where I run the boat).

Thanks again everyone.

 

[steelespike]

Re: Prop question

Check tiny tach on the net. They are a little odd but easy install and about
$40.As far as composite props take a look at the pro pulse a 4 blade that can be changed between two pitches.Other wise I think I would use a composite for a back up only.

 

[Silvertip]

Re: Prop question

Get a "real" tachometer. You can get them right here on iBoats. Another source for new "surplus" units is Greatlakesskipper.com