NEED SPECS TO GET ME ON PLANE

mdborn

Recruit
Joined
Jul 22, 2002
Messages
2
I need help! I'm running a Sunbird 198 Sport Cuddy with an OMC 2.5 (120 hp) and OMC outdrive. This boat is forever coming to plane. When I first leave the dock and throttle down it will run about 3,000 rpm. When it does finally plane, it will run 42-4400 rpm and will really fly across the water. I had a 13.5 X15 prop, then tried a 12.7 X 19 and it did even worse. The 13.5 is the biggest I can put on it and still clear the outdrive. Will a 4 blade Comprop help? What can I do? I don't believe this boat should have to have trim tabs to get it out of the water. I did put the big fin on the outdrive though. I'm lost. Thanks for any help.<br /><br />Mdborn
 

suzukidave

Petty Officer 1st Class
Joined
Jul 1, 2000
Messages
387
Re: NEED SPECS TO GET ME ON PLANE

If that is a 20 foot boat you are a little underpowered so you are going to have to compromise between time to getting on plane and top speed. Trim tabs would help but are not essential. What you should do first is find a prop that gives you the hole shot you want and then decide if it's speed is dissatisfying. Only if it's too slow do you need to get into anything fancier than a new prop.<br /><br />You are heading in the wrong direction with prop diameters. Don't worry them at all - a half inch here or there in diameter is not a factor on performance.<br /><br />What you care about is prop pitch and the rated WOT rpm for your motor. To set up the boat properly you have to find out what the recommended operating rpm range is for your motor and then try different props to see how they perform in that power range. They key is to get a prop that allows your boat to rev wide open to just below the recommended rpm limit and no more. <br /><br />One rule is that each drop of an inch in prop pitch gives you better hole shot (start up planing), but reduces top speed, while a higher pitch prop reduces your hole shot (think greater resistance in the water means longer time to build up to RPM) but yields higher top speed in the end (because for each prop turn a 13" pitch prop will push the boat 1 inch more than a 12").<br /><br />Therefore you were going the wrong way with a 19" prop. You should have been going down to a 13 or 14" pitch to improve holshot.<br /><br />The other rule to consider is that each change of an inch in prop adds or subtracts about 2-300 rpm. If you overdo the pitch change in either direction you will either over-rev your motor and damage it with too low a prop pitch or drop your maximum rpm out of your motor's power band with too big a pitch and actually reduce your top speed and your hole shot (likely what happened with the 19" prop).<br /><br />So try a lower pitch prop moving only one or two inches at a time. I wouldn't bother with a 4 blade yet. Also, make sure you try different trim set-ups with each prop.
 

blifsey

Senior Chief Petty Officer
Joined
Jan 17, 2002
Messages
769
Re: NEED SPECS TO GET ME ON PLANE

Suzukidave, good explanation :cool: I've read several prop posts and I think a light bulb finally went on. Is it fair to compare prop pitch as having reverse effect as rear end gearing on a car (lower gear ratio has more top end but less low end accelaration)? If not, I need to turn that bulb back off :( <br /><br />Thanks,<br />BL...
 

mdborn

Recruit
Joined
Jul 22, 2002
Messages
2
Re: NEED SPECS TO GET ME ON PLANE

You're right Suzikidave on the prop pitch. The 19" pitch experiment clearly proved that. And I would love to be able to stay with the larger 13.5 and drop to a 13 or 14 pitch, but all the mags I've read say that only those three props are available for that set-up. Any idea where I can get a 13.5 X 14 or 13 for a 13 tooth shaft with a 4" hub? I'd love to try one of those. I also read where it is better to just throw it on her when taking off instead of slowly building up R's to get to plane. That makes sense too I guess. I'll try it next time out. Thanks for the response. You all keep the ideas coming.<br /><br />Mdborn
 

suzukidave

Petty Officer 1st Class
Joined
Jul 1, 2000
Messages
387
Re: NEED SPECS TO GET ME ON PLANE

BLifsey, I agree with the rear end gearing analogy. <br /><br />MDBorn have you talked to Michigan Wheel and Ace Propellor (both on the net)? They have greater selection than local dealers and, will mail order you props at competitive prices. Once again, just forget about an exact diameter for your new prop - you just want a prop that fits with the pitch you need -- diameter is not exact and different pitched props for the same motor will have different diameters to optimize performance. I cannot believe there is nothing below a 15" prop for this outdrive. Normally they will go way down for heavy commercial installations.
 

Bob_VT

Moderator & Unofficial iBoats Historian
Staff member
Joined
May 19, 2001
Messages
26,082
Re: NEED SPECS TO GET ME ON PLANE

The larger props normally will fit if you sacrifice and remove the trim tab. This is probably the point where the larger prop will hit.<br /><br />Bob
 
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