Looking to change props

[sparetime]

I'm looking to go from an aluminum prop to a SS prop. I've got a 90 HP Mariner with a 21p on it.  The problem I'm having is that the boat is porpoising at WOT trimmed out. I've been told that the prop is loosing bite because it is an alum prop and the blades are flexing. Is this or could this be correct? So I want to try a SS prop but I don't want to just start throwing props at it. It has great hole shot and gets on pad in about 3 seconds even when loaded and wells are 1/2 to 3/4 full. RPM's stay right around 5000 in the summer months and 5200 in the winter. And what the manual is saying I'm at the proper max RPM range. What do you guys think?

[ChampioNman]

 What boat is it? What is the prop to pad measurement? Sometimes raising/lowering the motor a bit will correct the porpoising issue also moving some of the load around on the boat will also fix it. I'd look at the same pitch in SS and try that. See if your local Merc dealer has a try and buy before you make the purchase. You could also consult Mercury's prop selection chart for your motor/hull combo.

[sparetime]

It is a 1985 bass master 16'. Not sure what the pad to prop measurement is. By the way how exactly do I measure it? From the very bottom of the hill where the drain plug is located or from the very back lip where it steps up right behind the plug? And what part of the prop do I measure from? Centerline, bottom, or top?

[ChampioNman]

 It's pretty simple to get the correct measurement.  Level the trailer and then trim motor down so the cavitation plate is level with the boat. Use a small level for this. Once that is set. Measure from the ground to the very bottom of the pad, just below the drain plug. Record that measurement, then measure from the ground to the center of the prop shaft or the center of the gear case bullet. Subtract that measurement from the hull measurement that will be the prop to pad measurement.

[Ron Fogelson]

Level your boat on the trailer using the jack and a level at the rear of the boat.  You want to find the lowest spot on the bottom not the step up above the plug.  Once the boat is level trim you motor down until level rest the level on the lip between the center and lower unit "cavitation plate".  Once boat and motor are level measure from the pad to the ground and than measure from the center of the prop nut to the ground the difference is the prop to pad measurement above or below.

I don't know what your max RPM's are but 5,000 sound like the low end of the upper limits.  What did you find out is the operating range?

Also when a boat is porpoising it could be from the prop not being able to carry the load, not having enough RPM's, not having the right prop, yes an alum prop will flex but if you can get on pad and have a good hole shot IMHO it's not flexing enough to be the reason for the porpoising.

When going from an alum prop to a SS prop you will have to drop down in size IMHO with the RPM's your telling us so far.

You can also just try trimming down a bit to see if it helps or lower your motor if you have a jackplate.

[sparetime]

Sorry for the length in delay for responding. Ive been doing some digging to find more info. My manual says 4500-5000 rmps operating range. did a prop to pad measurement and I came up with 5 and 7/8" below pad. That seems too deep to me just from reading other chatter, but I really have no clue. I have been trying to find out what the boat weighs but all I can come up with is 850 pounds dry weight does that include the engine weight? So if I add fuel,Tackle, person(s),ect ;I'm adding another 600 pounds. That would put me around 1450. Then if I have to add the motor,I'd figure another 600 pounds, so realistically I'd say the boat is around 2100-2150 pounds sitting in the water ready to blast off. Oh and when I do trim down, it does stop slightly.  So now what do you guys recommend I do? Hope this helps you guys help me out. I really want to get this fixed.

[Ron Fogelson]

I'm sure the blades are flexing, aluminum props are known for that but,,,,,,,, they are not flexing back and forth making you boat porpoise; rather there is flex on the blades under a load.  Like during the hole shot.  Sounds more like the prop isn't able to carry the load of the boat like it should.

5 and some change below pad is deep for my boat but I just don't know anything about yours sorry.  Coming up some might let you spin the same size SS prop and keep your RPM's the same but typically going from aluminum to SS you drop about 200 RPM's and if your running max and just over max now that will help save your motor from red lining.

Do you have a local dealer close by that you can talk with?  I'd recommend getting to know the set up guy there if you can and start asking about props and what they recommend.

So do you have a jackplate?  If so you can try just coming up and running it and see if not trimming as high with the motor raised reduces the porpoising   

[sparetime]

No jack plate, but I can bring it up two bolt holes. Which that = about an inch per hole. Also what does setting the motor back do exactly? Just wondering in case I decide to install a jack plate.

[Ron Fogelson]

Not only will a jackplate help with the ability to make quick adjustments to motor height it changes the pivot point of the boat; think of a seesaw and how it would act if you moved the pivot point 6,8,10 or 12 inches one way or the other.

Also with some hull designs an additional set back of the motor can provide better, undisturbed water flowing over the prop giving it more bite. 


IMHO without ever being in/around your boat I truly think you need to talk with a local marine dealer and work with their set up guy/team.  Often if you have a good relationship with one you can get dialed in very close and than use different props to find the right one not just for your boat but for how you want it to run based on how you have it loaded.   

[sparetime]

Thanks Ron for all the help