Prop/speed/rpm doesn't compute..help!

[nofear1006]

24ft pontoon w/2 23" tubes w/2005 Yamaha 150hp 4 stroke, approx. 3000lbs. w/ 2people and full tank. Final drive ratio is 2.0. Started with 14 3/4 x 17" prop, was getting 4700 to 4800 @ wot, approx. 27mph. Switched to 15 1/4 x 15" and got expected 4000rpm increase to 5100 to 5200 @ wot, approx. 29mph. According to calculations seen on the website this should give me 21% slip. Obviously way to high, not getting much help from our dealer, says that puts me in the rpm range. Checked some Yamaha performance bullitens... same motor, same 15" prop...their numbers showed 38 - 39mph at 5800rpm. Only difference is the boats tested were tritoons. Is it possible that I am getting so much drag that this is the cause of the low rpm performance? Should I go to a 13" prop? Not many 13" prop options for a Yamaha 150. For what it's worth, this is the first time the dealer has put a 150 on a regular pontoon. Lucky me the guinea pig. Also, can trim the motor up and get a couple hundred rpms but speed drops 1-2 mph. Any help would be appreciated...

 

[rodbolt]

Re: Prop/speed/rpm doesn't compute..help!

dont know but ya need to prop it to 5800-6000 for max life. speed is a secondary concern.<br /> I dont see to many pontoons but your engine speed is still 800 RPM low.

 

[Ray Neudecker]

Re: Prop/speed/rpm doesn't compute..help!

Yes too much drag, actually a V-4 2 cycle would most likely have been faster. Too much motor and weight. Try adding a Bob's Machine Shop tail on and see if you can find a tail lift prop like a small four blade with the small hub. You are going to have to pick up that weight on the tail before you get any effiency.

 

[rodbolt]

Re: Prop/speed/rpm doesn't compute..help!

its part of a problem of the inherent torque curve difference between a 2 cycle and a 4. the final drive ratio stayed about the same as did prop design. neither really work well with a 4 stroke curve. but for now its what we have. it also comes from they try to make a motor that can reliably push a sled like yours and a flats boat and a barge or about anything else someone tries. its all a compremise. sometimes it works better than others. but to design and build an engine to specifically fit your toon would render it cost prohibitive. so now its time to play with the avalible props that are on the market. but you do need to get the WOT up to 58K or so.<br /> maybe in the future we will get some better ratios and props. until then we are stuck.

 

[ivar]

Re: Prop/speed/rpm doesn't compute..help!

21% slip on a 3000lb pontoon, doesn't sound too bad to me... Have you checked with the boat manuf. if they can provide you with some performance data?

 

[stinkylinky]

Re: Prop/speed/rpm doesn't compute..help!

Originally posted by rodbolt:<br /> ya need to prop it to 5800-6000 for max life.

Rodbolt, <br />Does this RPM theory appy to all applications. I have a Yam F115 (on fish/ski) that makes 5300 - 5500 WOT. How does this slightly lower RPM affect max life??

 

[rodbolt]

Re: Prop/speed/rpm doesn't compute..help!

yamaha reccomends all F motors to be propped at or near the upper limit. I try to prop them so with a medium load and a half tank of fuel I can bump or almost bump the rev limiter. it takes some of the combustion pressure and temp load off the engine thus reducing the chance of detonation. most F yams dont have a knock sensor. the new 250 does but its a whole different breed. same as going up a grade in 5th gear. the motor lugs but it will do it. without a knock sensor it will most likly rattle all the way up though. in a boat ya just cant hear it. the load on a car running flat is only about 10% of what the engine can do. on a boat its 100% most the time. only we cant shift gears to select the operating range. we have to rely on a fixed ratio and a fixed propellor and hope.<br /> as in Hope it dont blow up today.<br /> the 2 strokes make a power input to the crank throw with every down stroke. the 4 stroke is every other down stroke. with similar bores and strokes you tell me which will have a peakier HP and torque curve. its all about how much force the piston caan exert on the crank throw(lever) without self destructing. for a quick exercise lets go drag racing with top fuel. elapsed time is 4 .5 sec. RPM is fixed at 8K RPM. assuming the same RPM at launch as at shut down how many times did each piston fire? and we still burned 5 gallons

 

[nofear1006]

Re: Prop/speed/rpm doesn't compute..help!

rodbolt- are you saying that you would go as low as 11" pitch to obtain higher rpms? Very little 11" and 13" pitch options out there. Only 11" prop I could find for my application is from Piranha propellers. Any opinion on them? Thanks for the help

 

[nofear1006]

Re: Prop/speed/rpm doesn't compute..help!

Ray - went to Bob's Machine shop website. Are you referring to the stabilizer plate? I noticed that they had the lifting props you were referring to but nothing in the pitch I need to fit my application. Do you know of any other outlets? Thanks for the help.

 

[Ray Neudecker]

Re: Prop/speed/rpm doesn't compute..help!

Yes that is it. Can be a big help on your type problem. Look at the V-4 props in a four blade. Hydromotive Marine or Performance Propellor in Soddy Daisy, Tn. may can help.

 

[rodbolt]

Re: Prop/speed/rpm doesn't compute..help!

some applications the 4 blade helps others it hurts. we are having some success using the 4 blade props of 33 grady white's with pairs of F225's. even with vented props the F225's struggle to get that big camper on plane and keep it there.

 

[ivar]

Re: Prop/speed/rpm doesn't compute..help!

My dealer, which sells Honda/Tohatsu/Yamaha, agrees totally with Rodbolt, he has sold quite a few Honda 75/90/115HP used in commercial applications, they have to be propped just right to keep them alive.

 

[SCO]

Re: Prop/speed/rpm doesn't compute..help!

on the yAMAHA WEBSITE THEY HAVE PERFORMANCE BULLITANS, and(sry about the caps) a lot of tests on pontoon boats. It gives speed at various rpms,prop pitch, motor gear ratio , so you can look and calculate what kind of prop efficiencies thqay are getting on similar pontoons and motors as a check. <br /><br /> http://www.yamaha-motor.com/outboard/products/lifestylehome/home.aspx

 

[nickcrawford]

Re: Prop/speed/rpm doesn't compute..help!

Originally posted by rodbolt:<br /> some applications the 4 blade helps others it hurts. we are having some success using the 4 blade props of 33 grady white's with pairs of F225's. even with vented props the F225's struggle to get that big camper on plane and keep it there.

 

[nickcrawford]

Re: Prop/speed/rpm doesn't compute..help!

Originally posted by rodbolt:<br /> some applications the 4 blade helps others it hurts. we are having some success using the 4 blade props of 33 grady white's with pairs of F225's. even with vented props the F225's struggle to get that big camper on plane and keep it there.

I have been looking at buying a '02 Grady 33.. I like the idea of 4 strokes but is the twin 225 option just to small for this boat?

 

[JB]

Re: Prop/speed/rpm doesn't compute..help!

Nick, you need to post your question in the correct forum as a new topic. Tagging it on like this is called hijacking. Not a good way to get responses.<br /><br />This thread is about props, and it is in the wrong forum.<br /><br />I suggest that you start a new topic with your question in the Boats forum.