prop pitch vs top speed

[ron7000]

trying to figure out if I overpower my raft, what max speed could be.<br /><br />For a 25hp motor the prop finder lists a 10" diameter prop on average with max pitch of 15".<br />Does anyone know what the oem prop pitch is on the 10-15-25hp outboards?<br /><br /><br />

 

[Solittle]

Re: prop pitch vs top speed

Your post is very confusing. I have little clue on what information you are seeking. <br /><br />OEMs offer different pitch props for different applications - check the OEM web sites for the engine and boat you are considering or ask the dealer for a recomendation. <br /><br />Who knows what the max speed would be for an overpowered raft??

 

[shep70057]

Re: prop pitch vs top speed

That table is rather nice......

 

[Booner]

Re: prop pitch vs top speed

quote: by ron 7000<br />__________________________________________________<br />trying to figure out if I overpower my raft, what max speed could be.<br />__________________________________________________<br />ron I’ll take a guess at what you’re asking. The outboard you have is a 25 HP and the boat lets say max recommended HP on your boat is 50 HP take the outboard that you have and divide by the recommended HP rating on the data plate of the boat. i.e. 25 div by 50 and you get 50%. A percentage under 70% would be considered underpowered and a rating above 100% is over powered. The pitch of the prop in your selection is determined by the RPM”S that your outboard can spin the prop. For this you will need a tach. You will want to be as close to the Max rpm’s that the manufacturer recommends.

 

[ron7000]

Re: prop pitch vs top speed

I'm overpowering it with the 25, it is a 10' rib rated for 15. But a yamaha 25 is lighter than a 4-stroke 15, so.... 25 it's gonna be.<br /><br />I just don't know what pitch prop that size outboard comes with, and how high up in pitch I can go and still get 5000 rpm minimum out of the motor.<br />With the 1960's 18hp on it now, I get 27mph on the gps however I have no idea what pitch prop is on it nor the gear ratio. Just trying to get a ballpark for how much mph I'll get vs cost of new motor and if it's worth it.<br /><br />FYI:<br />running a stiletto 4-blade 20" pitch prop on my 19' I was gps'ing 41.5mph with the tide, wide open, at 4900 rpm. Against the tide it was 39-40mph. Outdrive ratio is 2:1. Did some math and it seems to suggest that efficiency number should be around 0.91, or 10% prop slip if you want to think of it that way, at top end. I don't know how it'll change at slower speeds.<br /><br /><br />Also, gear ratio of the yamaha 15 and 25 outboards is 2.08:1

 

[SCO]

Re: prop pitch vs top speed

Its probably marked on the hub somewhere what the pitch is. I looked at the yamaha site performance tests yesterday, and calculated pitch efficiencies and got 90% at the top end too, dropping to about 80 to 85% at 3500 to 4000 rpm ranges. The worst is at rpms where you have not climbed onto plane as you would expect and low speeds. Youll probably get at least 90% at the top end since the 10 foot hull isnt providing much resistance to the 25hp motor. So, just calculate your top speed. <br />If you have a gear ratio of 2.08 per motor spec or just by counting the prop revolutions per engine revolution, and you think 5000 is max rpm, and have say pitch of 21, then the calculation is :<br />top speed=.9x(5000/2.08)x21x60/12/5280<br /><br />Disclaimer: What are you doing man? A small boat with a big engine is squirrelly. Make sure you have a short lanyard attaching yourself to the kill switch.

 

[ron7000]

Re: prop pitch vs top speed

lanyard schmanyard,<br />it's already squirly <br /><br />the max pitch available looks to be 15", so that tops out at 35mph,<br /><br />unless I can find a way to get one custom made or repitched

<br /><br />thanks for the idea

 

[SCO]

Re: prop pitch vs top speed

You know, that motor, if new, is rated 5000 to 6000. You could overspeed with that motor I bet so calculate with the 15 pitch at 6000 at 95% efficiency, and you might get 39 mph. I think it is easy for them to repitch a prop.

 

[SCO]

Re: prop pitch vs top speed

They in the old days used to race these little boats. I bet they figuresd out a lot of ways to pitch props and gear motors (lower gear ratio) to be able to go faster.

 

[4.3sunbird]

Re: prop pitch vs top speed

figured I'd bump this back up <br /><br />and after sitting here racking my brain I figured out the math used <br /><br />engine rpm divided by gear ratio<br />then multiply that number by 60<br />then multiply that by pitch<br />then divide that by 63360 <br />the answer would be your speed at 100% efficiency<br /><br />to get a more realistic speed multiply the mph you came up with by .90 to .95 depending on style prop you have <br /><br />I figured written out like this might help some dummy's like myself

 

[stevens]

Re: prop pitch vs top speed

And here is the same calculation in a spreadsheet:<br /><br /> Speed-pitch calculation <br /><br />Click on the tabs at the bottom to switch between knots, mph and KMph. Also, at the bottom, you can enter your own custom ratios, rpms and pitch.<br /><br />Hope this is useful for any of you. Feel free to download and amend as you see fit.