Prop????

[helimech2003]

Hi there all. let me start by saying this is by far the best boat forum i've run across. i've been lurking for a litle bit soaking up as much info as possible, and you guys have been a great help,but i've got a real head scratcher here.

i bought a 1981 Hammond 21ft about 2mos ago. i got three props with the deal. stainless even. one is a four blade 14 3/4X 17p that i havent tried yet. the second one is a 14"X25p ported and the one on the boat is a 14 3/4X 21p. with the 21p thats on the boat now has a great holeshot but once planed out its running around 50ish (on the speedo) at 6000rpm. it would prob turn faster if i held it wide open but when it hit 6k i pulled back. its not a sudden increase like its cavitating/venting it justs keeps climbing.

my buddy said to throw the 25p on it and that will fix it but im afraid it'll tear the outdrive apart........

on the diameter i think its right. i know the pitch is right. i can measure em tomorrow

what do you guys think

 

[tpenfield]

Re: Prop????

I think I'd like to know the type of engine that we are talking about.

this reminds me of the childhood story of Goldie Locks and the 3 Bears . . . one prop is too hard, one prop is too soft and the 3rd one is just right.

I think you have the '3rd' one on the boat right now.

21 foot boat, going 50 mph at 6000 rpm . . . you are living the dream. I think the 25P is gonna send you downhill . . . probably not enough engine to turn it . . . the 4 blade 17P would be great for water sports, but not much else.

Maybe a 22P would be ideal, but that is not part of the mix.

 

[steelespike]

Re: Prop????

Get us the information in "read this first" at the top of the page and if you can get the speed by gps it's better.
The calculator says the tach is kidding us.At 6000 it should be running about 85.
50 mph indicates about 3500 rpm which seems low. These figures use 1.32 ratio and a guess of 6% slip, so could vary some.

 

[helimech2003]

Re: Prop????

sorry about that..it's a mercruiser 260 and an alpha1. i kinda figure the 4blade would be good for watersports and not much else as previously mentioned but i would like to have a prop to make some high speed runs from time to time. not real sure on the weight of the boat. i know that my 160,000mi V-6 F-150 pulls it with no problem on a tandem trailer. as for weight in the boat, it's usually just me and the wife and a buddy, and sometimes a small cooler

ill try and get a gps on it this weekend

could it be that the tach is just lying to me?

 

[Philster]

Re: Prop????

25P is a lot of prop. I/O boats that run 65-70+ MPH tend to have 25p 3B props. You don't have enough engine to spin a 25P. Ain't no way.

6000 RPMs is way over-revving, so I would want to know how true the tach is, too. On the surface, she sounds like a boat built for a 21p, but if she is light and the deadrise isn't much, I can imagine a 23P and not more than that.

 

[steelespike]

Re: Prop????

Is the tach fooling you;
read the post above your question. I think its fibbing big time!

 

[tpenfield]

Re: Prop????

Is the tach fooling you;
read the post above your question. I think its fibbing big time!

based on the 6000 RPM, I was thinking it was a jacked up outboard. . . I think a Mercruiser 260 at 6K RPM would be in pieces.

 

[helimech2003]

Re: Prop????

well. i got a automotive DMM with an inductive pickup so im off to the lake for the weekend to test them all. ill let ya all know what i find and thanks for all the help.

of and if it helps any the hull design is similar to a fountain of the same year.

 

[helimech2003]

Re: Prop????

well i gps'd it with the 25p. 54mph at 4600rpm(per the tach. the DMM died) with three people and a full bag of gas.
with the 21p it gps'd at 58mph but the tach was reading almost 5500rpm.
i'm starting to think the tach is lying. with the 25p it just dosen't sound right.
so i'm off to buy a new tach. thanks again for the replies.

 

[Bondo]

Re: Prop????

Ayuh,.... That motor is supposed to be propped to 4400/ 4800 rpms at Wot....

 

[steelespike]

Re: Prop????

Yes a new tach. 4600-1.32 ratio-25"pitch-6% slip=74 mph.