still beating the dead horse!!!!

[buttonhook]

I know it has been covered alot but. I'm going to raise my motor today and this is not a easy undertaking. drilling new holes etc. and I'm wanting to get it as close as possible the first time. (background info) 15'-16' fiberglass tri-hull with a flat bottom at the stern. 4" of set back, 1988 20" shaft 28 hp spl johnson, stingray hydofoil, 9.25 X 12 alum prop(could change prop), no power tilt or trim. I know that I could move it to the bottom of the hull but with the set back do I need to move it up more? And what is the gear ratio of this motor?(trying to figure my prop slip)<br />5000 rpm sweet spot and 19-21 mph on the speedo.<br />it seems it should not have that much slip and if I changed the motor height and prop pitch would it slip less.<br />Sorry for this long post. it is the first boat I've even owned and the former owner though he was McGyever! I'm just wanting to get it right the first time.

 

[JB]

Re: still beating the dead horse!!!!

There must be a question in there somewhere, but for the life of me I don't see it. <br /><br />Try again and make it simple for me.<br />

 

[buttonhook]

Re: still beating the dead horse!!!!

JB,<br />I sorry I was giving so much information that my question got lost in the words.<br /><br />What I want to know it with that set up how high do I need to raise my motor?

 

[JB]

Re: still beating the dead horse!!!!

Aha! Now I see.<br /><br />With a 4" setback I wouldn't raise the AV plate more than 1.5-2" above the bottom.<br /><br />Others may disagree.<br /><br />Good luck.

 

[buttonhook]

Re: still beating the dead horse!!!!

thank you,<br />I will see if that works!! If the storms ever stop!!<br /><br /> Also the gear ratio on my engine is .57 or(12:21). I am getting 19-21mph out of it with a 9.25 X 12 prop. do you think I have too much prop and it is slipping? this engine get it's peak power at 5000 rpm's.<br /><br />I did some calulations and 5000 rpm's=5000 fpm (with a 12" pitch prop) and at .57 ratio thats<br />8771.92 fpm.(with 0% slip) thats like 526,315.20 fph or 99.68 mph!!??<br />Now I know my math is bad but......thats like 80% slip!! that can't be right can it?????

 

[JB]

Re: still beating the dead horse!!!!

Hmmm. My math comes up with 32.4mph with zero slip, Buttonhook.<br /><br />5,000rpm X .57 gear ratio = 2850 prop rpm.<br /><br />2850 prop rpm X 12" pitch = 34,200" per min.<br /><br />34,200" / 12" = 2850' per min.<br /><br />2850'/min X 60 = 171,000' per hour.<br /><br />171,000' per hour / 5280 = 32.4 mph<br /><br />If you are getting 20mph, that is a 37% slip. Seems like too much slip to me.<br /><br />Have you verified that your tach is accurate? How are you measuring your speed?

 

[buttonhook]

Re: still beating the dead horse!!!!

with just water pessure speedo I don't have a tach.<br />if I went down a pitch or two would that help with the slip? and maybe increase speed.<br />would going to stainless help any with it.<br />I know I'm not hitting 5000 rpm by the sound of the engine.<br />So my math is that bad! <br />I thought something was up with the numbers!<br />i should have X by .57 not / by it.

 

[JB]

Re: still beating the dead horse!!!!

Borrow a GPS to verify/calibrate your pitot speedo. They are very inaccurate unless calibrated.<br /><br />Don't mess with your prop until you have a borrowed or new tach on it and verify WOT RPM.<br /><br />If you find that you do have a WOT RPM problem let us know and we can help you resolve it.<br /><br />Good luck.

 

[buttonhook]

Re: still beating the dead horse!!!!

will do!!!<br />I'm also going to try out the new set up with the motor raise first that may slove my problem.<br />I know I'm going to have to have a new prop. I found a hairline crack in it this past weekend.<br /><br />Thank again!!!!

 

[buttonhook]

Re: still beating the dead horse!!!!

ok lets try this again,<br />I rasied the motor and took it out the boat picked up 2-3 mph over the prev. set up but now when I go to WOT. the prop slips and the engine will rev. and it will come off plane. the prop will slip until I back the throttle off to 3/4 or so.<br />I'm guessing the prop is cavitating. Is that because the pitch is too steep to hold the water with the weight of my boat.(Like cars on wet pavment) I know it is not the hub slipping as I dont have a hub that I can see. the prop apears to have a solid spline connection with the shaft. gears on gears.

 

[JB]

Re: still beating the dead horse!!!!

Howdy, Hook.<br /><br />Your prop is almost surely slipping or ventilating.<br /><br />I don't know of any outboard that has neither shear pin nor rubber hub (Not really the hub. It is a rubber sleeve with a splined metal hub). I am almost certain that your engine uses a rubber protected prop.<br /><br />You said that your prop is entirely under water. Is that at drift or at full plane? If it can grab air at plane it will ventilate.<br /><br />How have you got the trim angle adjusted? Is the AV plate parallel with the bottom? If it has too much out angle it can promote ventilation.<br /><br />You are getting there. Let us know about the above questions.<br /><br />Good luck.

 

[buttonhook]

Re: still beating the dead horse!!!!

I'll take the prop off and check for the rubber insert. but the motor didn't do that when at original height 6" lower.<br /><br />At full plane the av plate is underwater in fact the stingray is still under water even when set 3" higher than what I have it set now. <br /><br />The av plate is as close to paralell as i can get it with the hull and 1/2" above bottom. I tried one hole either way. (1st) bow too high couldn't get on plane, (2nd) bow too low buried it in the water. I'm sure that is at the correct angle.<br />So I think I'm getting there I just need the correct prop I think.<br /><br />Will a cupped ss prop of the same pitch (9.25 X 12) solve this problem?? opinion??? Or do I need to go to a 9.25 X 11 or 9 X 10?<br /><br />btw,<br />the alum prop that I have now does have some minor dings in it. but I don't think it has any major damage. EXCEPT FOR THE HAIRLINE CRACK!!

 

[buttonhook]

Re: still beating the dead horse!!!!

ok the prop does have a hub but it is not rubber in appears to be (bronze?). but it looks ok and is tight.

 

[buttonhook]

Re: still beating the dead horse!!!!

UPDATE!!!!!!<br />It seems as if the prev. owner hit something with the prop and instead of buying a new one he just ground it down. so instead of it looking like a mickey mouse ear it looked like a weed sheading prop with alot of schew. I bought the new prop and will try it out this weekend.<br /><br />wish me luck!

 

[buttonhook]

Re: still beating the dead horse!!!!

Update,<br />I got the new prop installed 9.25 x 11 it runs alot better i get 23 mph (indicated) with two people,cooler,full livewell, and all the gear. I have a tiny tach installed and I get 5600 rpms with that load about as heavy as it will get maybe 100-150# more if both kids are with us(there is no change in speed with kids either). the range for my motor is 4500-5500 with peak power at 5000 <br /><br />Do I need to increase the pitch of the prop too a 12 or 13 pitch to lower the rpms some???<br /><br />How do you calibrate the water pickup speedo?????