a chart to help start a prop choice???

[bob johnson]

I got a new boat..20.5 foot Lowe Ultra Roughneck Mod V.
70 " at the chine...with a 12 degree dead rise.

the hull weighs 790 lbs now..I bought a center console and a storage-helm seat to install myself..and i removed the rear bench seat.( i am repositioning the level flotation to the sides at the rear of the boat)

( the factory CC models weigh in at about 1100, but I should be less than that with no decking on my rig)

I will have flotation pods welded onto the back of this boat to let it float even shallower than it does.

I am aslo going to insatll a hydraulic jack plate so i can motor out of the shallow flats.

I dont care too much for top end, what eve rit is, that is fine, because it will be mostly a duck boat.

I have two motors that may end up on this boat

1988 evinrude 70 hp with a 13 -3/4" X 15 stainless steel prop( motor 264 lbs)

and

1997 evinrude 90hp dont know what prop is on it now( motor 325lbs)

so the boat is kinda light for its length...but I may have as many as 3 guys in it...

I will have the gas tank up front 12 gallons
I will have two marine batteries under the CC
3 guys would be about 600 lbs 100 lbs of additional gear and dog

Id like to be able to raise the jack enough to motor out of the shallows even if the prop is a little out of the water. i think i read someplace they make a CLEAVER style prop for that.

I dont need a performance WHOLE SHOT..i am not racing anyone to any place and usually it is too rough out to go at cruise speed..

so what prop specs am i needing to look at and what TYPES will help me with what my goals are

cupped ,
uncupped
ss
aluminum
cleaver
lots of pitch

smaller dia ( so as to not bang bottom)

the boat will be used around rocks often as well

dont need to get it nailed...just the choices to ponder over.

I dont want to buy three or four props and play mix and match..

I figure i might not be able to pin it down at less than 2 prop trials

thanks

bob

 

[QC]

Re: a chart to help start a prop choice???

I dont want to buy three or four props and play mix and match..

Check out the loaner program here, as I think that's exactly what you're gonna have to do . . .

 

[Silvertip]

Re: a chart to help start a prop choice???

There is only "one" right prop and that's the one that allows the engine to operate within the engine manufacturers recommended wide open throttle RPM band and performance will be whatever that prop provides. Now I know you'll say "but I won't be operating at wide open throttle -- at least not very much!" Well, if the prop is wrong at wide open throttle with what you consider an average load it is wrong all across the rpm band as well. That's just the way it is. So install the motor, use whatever prop you have, add the load you will normally run with and go wide open throttle noting maximum rpm as you adjust the jack plate. If the engine tends to over rev you increase pitch (1 inch drops rpm by 150 - 200 rpm). The converse is also true. When you start messing with diameter to run skinny water, you are in danger of running the engine without water as the water pickups will not be able to intake any water. Propeller diameters are designed to be used on a specific range of engines so you can't just plop a 10 inch prop on an engine that has a gearcase designed for a 13 inch prop. Besides, the skeg still extends below the prop and if you bang the skeg you will bang the prop regardless what diameter you are running. Wish I had a simple answer for you but prop selection is a crap shoot if you have reasonable test data to go with. With nothing to go by its anybody's guess.

 

[bob johnson]

Re: a chart to help start a prop choice???

well the prop loaner program is the way to go...as long as I dont nick it ..

you know I am just putting out feelers for ideas...I totally realize the final choice has to come from running the boat at WOT.

but there are choices still to be covered..like props that some hunters use that break the waters surface..

I have run my 50 trimmed way up so I was throwing a rooster tail and going about 2 mph max...in shallow water and i still got water to the pick up...I think with a jack plate taking my motor straigh up I can STAY level and still have water reaching my pickup but get the prop up higher..

i heard there was a prop design made for that type of use anyone here heard of it...

is there a design called a CLEAVER?

how about cupped or no cupped...is there a consensus on that?

I will probably install a water pressure guage, since i have the capacity to drive my motor up and out of the water... and ill need to be able to monitor that with out looking back for the tell tale discharge

thanks for your guys help.

I know we all have to figure stuff out on our own in the end...but the forum here cut a lot of work out and make better use of our time

bob