prop tuning query

[Butt Ugly]

Boat is a 17.5' K&C with a mercruiser 140. The prop that was on it when I bought it last fall was from a bigger boat. I was only able to get 3600 rpm WOT and it would go about 21knots. Merc manual says the 140 should be spinning at 4200-4600 rpm WOT. The boat was very very slow to get up on plane. Bow was way up for far too long, and I hated how long I couldn't see in front of me till it did plane out.

Went to the local prop shop, and the fellow there gave me a prop that he said should be close with the info I gave him, and if I let him know what I get out of it, he would tune it from there. This new prop will only get about 3300-3400rpm at WOT, but I now get 25knots out of it, and it climbs up onto a plane much better, and really quite nicely.

The boat will only be used to get out and go fishing, no tubing or skiing. I would prefer economy over speed. I like the 25knots and the nice gradual rise to plane in a few seconds that it gets now.

I am much happier with this prop, and 25 knots is probably plenty enough speed for me, and too much more would probably start to scare me. But it's still missing about 1000rpm at WOT. Don't know if this should concern me. What should I do? What would you do?

I'm still learning, and figured some of you more experienced boaters would help steer me in the direction of what I should be doing. Do I leave it alone, or should it still be tuned for better and the missing 1000rpm? Will more rpm reduce fuel economy, or is it better for it?

Thanks to all who reply.....

 

[Degree of Freedom]

The ideal prop (most efficient...correct pitch and diameter) gives you the specified WOT range with the throttle all the way down. It sounds like you are using props with way too much pitch (possibly wrong diameter. The pitch required depends on the boat/engine combo. My 18' Sea Ray with the mercruiser 3.0L runs ideal with a 21 pitch prop (46 mph). Any less pitch, and it over revs (wasting horsepower)...I had the opposite problem to you.
There is a rule-of-thumb...don't quote me on the actual number, a prop shop should know this...500 rpm per inch of pitch.


Too much pitch and the engine is working harder to give less performance (bad for the engine), too little pitch and the engine can over rev (bad for the engine)...try to get the right one.


http://sites.mercurymarine.com/portal/page?_pageid=126,48572,126_49259&_dad=portal&_schema=PORTAL

 

[Butt Ugly]

Nice website for prop selection. Just wish it had more selections for my boat. Year is 1978, pre-alpha, and probably 1.98:1 gears if I'm reading the seloc manual correctly. I guess mercury doesnt figure there are any of these still out there.

The prop I was given to try is stamped 48 78120 A4 19P and looks to measure about 14" diameter.

 

[Degree of Freedom]

where are you located? I've got an older 17 pitch that you could give a try if you want. It's not a true 17 as I had the blades cupped trying to bring my revs down, but I'll likely never use it again so might be worth a try. I'm in Nanaimo.

 

[Butt Ugly]

Hi Backatit, If you look to the east across the water, that's me waving at you. Halfmoon bay! Sunshine coast.

I was back to the prop guy today. He gave me another to try with less pitch to raise the rpms. Apparently the rule of thumb is 200rpm per inch, so saying that, going from the 19 pitch to a 15 it should increase another 800 rpm. We'll see when I get it back out to run this prop.

He didn't have this pitch in a reconditioned prop, so he sold me a new one. As long as I don't ding it up, if it's still not right, he'll take it back. As well as the rpm and knots, he would like to know what vacuum the motor pulls at WOT. I could have given him this info on the other props too had I known it would've helped. He also echoed what you said that with the low rpm, the engine is really working hard, and that's bad for the motor.

Thanks for your imput with this. If it weren't for the water between us, I'd try your prop too and see what it gets. I'll reply back to this post with my new figures once I give it a go.

 

[Degree of Freedom]

Good luck, sounds like you're on the right track.

...by the way, is the boat butt ugly..or is that you

 

[marula]

Butt ugly, backatit has you on the right track, in addition to working the motor extra hard with too large of a pitch there is the safety aspect. If your dragging your butt extra long trying to get on plane, you can get yourself in trouble with an large following sea and a wave crashing your stern. Offshore props grab a lot of water and throw you up on plane quick, protected water can take a little more pitch for top end and economy.

 

[Despy]

The mechanic is right, lugging the motor down drops the vacumn way down and all fuel economy goes out the window.A motor turning 3500 rpm and running with less load will get better economy than one working hard at 3000.

 

[Butt Ugly]

quote:Originally posted by Backatit

...by the way, is the boat butt ugly..or is that you

Ohhhh! That stings!

As Frank Zappa said "To all you beautiful people out there,.... there's alot more of us ugly motherf#@$&%s than you!"

of course, I am a beautiful guy! [8D]

I have since removed the vinyl top and replaced it with a hardtop, so maybe I should change the name to just "Ugly". Pretty easy to do, as I own a sign shop.

 

[Butt Ugly]

Well, I am getting pretty stumped by this prop thing! I put on the brand new 15 pitch prop tonight and took it out for a run. RPM came back up alittle but only getting 3600 with this one. 23 knots, and the vacuum guage was reading zero wide open. If I backed down the throttle just a touch to 3500 rpm, the vacuum came up to about 3 or 4. This prop will come up to a plane just as nice as the 19 pitch did.

I have no idea where to go from here. I will let the prop shop know about these new figures with this new prop and hopefully he has some idea what to try next.

I think I will replace the tach just on the off chance that it is not reading correctly. Not sure what else I should do. I am open to any and all suggestions.

Just to keep up with the progress, here are my details with the different props:

Old prop 1, about 16", 15 pitch, 3600rpm, 21knots, slow to plane
Recond. prop 2, about 14", 19 pitch, 3300rpm, 25knots, good to plane
Brand new prop 3, box says 15 1/4", 15 pitch, 3600rpm, 23knots, good to plane, but zero vacuum at wide open.

 

[Degree of Freedom]

huh...that boat with that engine should easily exceed 30 knots. My Dad had the same era, same size Double Eagle with a Volvo (same hp) and it would do 35 mph. 15 1/4" seems kind of big...but I'm no expert. 15" pitch is almost flat...it should be revving like crazy. Might come down to going with the one you're most happy with, and not worrying about it being perfect.

 

[MyEscape]

Take the time and call Reg or Todd @ Advance Marine (Volvo / Merc Dealer and top notch mechanics) in Campbell River trust what they say they know what there talking about or North Island Propeller in Campbell River. They are all friendly and very helpful and will know what you really need for the best all round prop for your boat and motor.

Cheers ME

 

[esora]

Only a suggestion, is the tach and speedo correct?

 

[explorer185]

Also give Victoria Propellor a call, they have a toll free number and are located in Nanaimo. I have called and they give very good advice over the phone on different pitches and sizes.

 

[Last Chance]

If it were me, I would try 17" pitch, 14.5" diameter. The large diameter 15.25" should NOT be needed on that size boat.